Traditional Land Mobile Radio Systems
The purpose of this section is to describe the systems and technologies traditionally used in the provision of the different types of services described in Fundamentals of Wireless Communications. It also deals with the limitations of these systems in order to provide the reader with a better understanding of the impetus for the development and deployment of modern systems described in Modern Wireless Systems and Trends which follows. The distinction between "traditional" and "modern" systems is an arbitrary one, but it does provide a means for understanding the latest developments in the field. Because the wide-spread deployment of two-way mobile data systems is a fairly recent phenomenon, they will only be discussed in Modern Wireless Systems and Trends.
A. Paging
A paging system with dial-in capability consists of a terminal and a radio distribution network. The terminal, which contains computer logic, answers the incoming line and, based upon the telephone number dialed, matches that number to the corresponding pager address stored in memory. If the service is other than tone-only, the terminal prompts the caller for some additional information (e.g., a voice message if it is a tone-voice service, a telephone number if it is a numeric display service, or free form text for an alphanumeric display service).
In its simplest form, the radio distribution network may consist of a single transmitter and associated antenna mounted on a tower, building, or mountain (or a combination of these). In this simple configuration, the terminal merely puts the message into the correct format or protocol for transmission and conveys it to the transmitter where it is broadcast over the coverage area. The pager address may be broadcast as a sequence of bits (ones and zeroes) using digital modulation. The address is followed by the message itself if it is other than the tone-only service. The paging receivers (pagers) in the coverage area listen for their address and, if they "hear" their unique address, they are activated and the trailing message is delivered. As explained in more detail below, a modern paging system may employ a network of multiple transmitter locations allowing an extension of the geographic service area.
B. Single Frequency Dispatch
The simplest type of two-way dispatch system uses a single frequency slot (i.e., an unpaired channel) for both transmitting and receiving. The parties at each end of the conversation (e.g., a dispatcher and a mobile unit in the field) take turns talking. This is sometimes referred to as the push-to-talk/release-to-listen (PTT/RTL) mode. The base station consists of a simple transmitter/receiver combination (a transceiver), an antenna mounted on the building or tower, a transmission or feedline (normally coaxial cable) for connecting the transceiver to the antenna, plus, perhaps, some accessories such as a dispatch console. These simple dispatch systems typically operate in the analog mode and employ Frequency Modulation (FM).
These conventional, single frequency dispatch systems are still used extensively at both low-band and high-band. Two fundamental disadvantages of such systems are (a) that they mix higher power base stations and lower power mobile units in the same frequency slot which exacerbates interference problems when the channel is shared among multiple systems, as is often the case, and (b) that they do not permit the use of repeaters (as described below), nor do they allow for full-duplex operation. Other disadvantages of these systems will be described in Section III.D.
C. Two-Frequency Dispatch/Community Repeater
As explained earlier, over the years, as low-band and high-band became more congested, the FCC regularly allocated (or reallocated) additional spectrum at higher frequencies for use by the land mobile radio services. This meant that a simple, single frequency dispatch system with the antenna on the user's premises often did not provide adequate range, especially for communicating with much lower power portable radios that were gaining in popularity. This led the FCC to organize the bands to permit the assignment of paired frequency slots for each channel. Thus, other than for one-way paging, paired frequency assignments are available for dispatch systems in the UHF and 800/900 MHz radio bands.
This arrangement allows the higher power stations to transmit on one frequency of the pair, and the lower power mobile units on the other, thus eliminating the power disparity and reducing interference. It also allows the deployment of mobile relay or community repeater systems. In community repeater systems, a relatively high power transceiver and associated antenna are placed on a tall tower, building or mountain top that offers good coverage over a wide area. The mobile units transmit on one frequency which is picked up by the receiver in the unit on the mountain/building top and then simply retransmitted or repeated on the other frequency of the pair. This special type of transceiver with the receiver and transmitter hooked "back-to-back" is called a repeater. With such a system, the dispatcher can communicate with his/her mobile units via the repeater using a low power transceiver located on his/her premises. The latter is known as a control station. Thus, the repeater relays all of the low power signals, enabling both control stations and mobile units to communicate with one another over a wide area.
These systems typically operate in the PTT/RTL mode, although full-duplex operation is possible when communicating to and from the repeater (e.g., when telephone interconnection is provided at the repeater site). In addition to wide coverage, repeater systems offer other advantages. For example, because a repeater simply retransmits what it "hears," it is very easy for several customers with different fleets and their own control stations to share the use of a single repeater. This means that individual users with a small number of mobile units do not have to construct their own dedicated, high power/high antenna site facilities. The systems also allow the use of low power mobile and portable units with corresponding reductions in cost. Like the single frequency dispatch systems, these two-frequency, repeater systems operate in the analog mode and employ FM modulation.
D. Disadvantages of Single Frequency and Paired Frequency Repeater Systems
The two types of two-way dispatch systems discussed thus far exhibit several basic limitations. These limitations are briefly described in the paragraphs which follow.
1. Undisciplined Access to a Shared Channel
As explained earlier, there are not enough channels available to allow each user to have a dedicated channel of his or her own. Thus, except in the case of the very largest users, the channels must be shared among a number of such users. For example, a particular UHF channel might be shared among five companies, each with a control station and five to ten, or even more, mobile/portable units each. Because each individual user may not know when another user is going to transmit (i.e., push the PTT button), users will occasionally interfere with one another. As congestion grows, the situation gets worse and there may be several users each waiting to transmit and each unaware of the others. Moreover, some less polite users may get impatient, and transmit even if someone else is on the channel. Thus the inevitable consequence is delays, interference, and lost messages, especially when there are unaffiliated users on the channel. In short, access to the channel is not disciplined.
2. Limited Addressing Capabilities and Lack of Privacy
Both of the described systems operate as giant party lines in which each user is generally able to hear the conversations of other users. Some systems are equipped with relatively simple devices that allow messages to be directed to a particular mobile or group of mobile units. However, these addressing systems are rather rudimentary and, while they help reduce the amount of chatter to which a particular user has to listen, they do nothing to protect the user against casual eavesdropping by other people sharing the channel or by individuals (including competitors) employing simple scanning receivers. Moreover, they typically do not have enough unique addresses to allow the creation of large, networked systems.
3. Severe Channel Congestion in Some Areas/Services
Even with the creation of new land mobile radio bands, channels are often very congested during peak periods, especially in major urban areas like New York and Los Angeles. This often produces excessive delays in accessing the channel.
4. Inefficient Use of the Spectrum Resource
Despite the heavy loading, at any instant of time, there may be some channels in the area that are unused or lightly used because, for example, the peak usage of the different channels may not coincide. However, with the simple systems just described, access is limited to only one channel or only a handle of manually selected channels. It is clear that it would be more efficient for many channels to be put into a common pool and then drawn from the pool to carry conversations on an as-needed basis. In other words, in technical terms, the channels are not efficiently used because they are not "trunked."
Moreover, because they use high power and high antenna sites, a single conversation -- say between a control station and a mobile unit a few miles apart -- precludes the use of a channel over a very wide geographic area. For example, a system providing coverage over a 20 mile radius may preclude the assignment of the same channel for some 70 miles. In short, the systems are not spectrally efficient because the channels are not reused intensively in a given area. Finally, the channels are not efficiently used because a single voice conversation with a nominal bandwidth of 3 kHz occupies a 25 kHz frequency slot in the radio spectrum.
E. Multichannel Trunked Radio Systems
Multichannel trunked systems, such as those operated by very large private organizations on a private basis and by third-party providers on a commercial basis, are designed to provide more disciplined access to the channels and to allow for the message-by-message sharing of a pool of channels. They operate on an FDMA basis. In a modern multichannel trunked mobile radio system, this is accomplished through the use of computer logic which assigns channels from a pool and recovers them at the end of a transmission or message. Thus a modern trunked mobile radio system consists of a collection of repeaters, each operating on one of a pool of multiple channels (typically from five to 20), and under computer logic control.
The FCC did not require the standardization of trunked radio systems and a number of proprietary systems have emerged. In one popular system, one of the available channels in the pool is set aside as a digital signaling or control channel. All of the end-user mobile units and control stations monitor the control channel and make requests and receive instructions on it. When a user indicates that he or she wants to send a message by pushing the PTT button, the mobile unit or control station sends out a burst of digital information on the control channel which identifies the individual mobile unit or fleet of mobile units with which the user wants to communicate. The computer logic at the repeater site finds an idle channel in the pool and sends back a burst of digital information telling the individual mobile unit or fleet of mobile units to all move to the selected idle channel. Once the control station/mobile units have arrived on the idle channel, the user who originated the transmission can begin to talk. In a modern system, this whole process takes less than one-half second.
If all of the channels are busy, the call requests are placed in a queue and handled on a "first-in, first-out" basis. At the end of the conversation, the channel is returned to the pool and mobile units and control stations in the fleet go back to monitoring the control channel again. All of this is done automatically and all channels are available to all users. Note that the conversation channels are only used for the duration of the call.
Trunked mobile radio systems can be used for placing and receiving ordinary telephone calls by interconnecting the transceivers at the repeater site with the Public Switched Telephone Network (PSTN). Thus, if interconnection is provided on the system, when a user wants to engage in a telephone call rather than a dispatch call, the conversation is appropriately routed to/from the PSTN.
Trunked mobile radio systems overcome many of the disadvantages associated with the operation of individual repeaters such as Community Repeaters. First, they provide disciplined access to the channel which prevents people using the system from intentionally or unintentionally causing interference to other users. Second, because the audio output of the receiver portion of the mobile/portable radio is only activated when the unit's individual or group address is received, casual eavesdropping is eliminated. Moreover, because the assigned channel jumps around from conversation to conversation (or even from transmission to transmission), it is also more difficult for other people to monitor the conversations of a particular user. Third, the waiting time to access a channel is greatly reduced because, unlike in a single channel system, if any channel in the pool is idle, it can be immediately assigned for use in a conversation. Fourth, the radio spectrum is used much more efficiently because more mobile units can be accommodated per channel. For example, a modern trunked system providing dispatch service on a pool of say 20 channels can provide excellent service (i.e., short waiting times) with an average of well over 100 users per channel. As a rough estimate, a trunked system with a reasonable number of channels can provide about three times the capacity of untrunked channels for the same grade of service (i.e., average delay to access a channel).
The principal disadvantage of a trunked radio system of the type just described is that a single, point-to-point conversation between a control station and a mobile unit or between a mobile unit and the PSTN via the repeater site occupies a valuable radio channel over a very wide geographic area. In other words, little frequency reuse is employed, which means that the systems are spectrum inefficient for one-to-one calling. Another disadvantage in congested areas is that each end of the voice conversation occupies, typically, a 25 kHz channel because the systems use ordinary FM, analog modulation. In addition to these spectrum efficiency concerns, the wide coverage provided by the high power repeaters can cause difficulties in serving small, very low power portable units operating from within buildings and other difficult to serve locations. In other words, the imbalance in transmitter power may make it difficult for the portable unit to "talk-back" to the receiver at the repeater site. These disadvantages are being addressed in third generation trunked mobile radio systems that are described in Section V.
F. Cellular Mobile Radio Systems
Cellular mobile radio systems get their name from the notion of dividing a large geographic area (e.g., an entire metropolitan area) into a series of small, hexagonal shaped cells. The hexagonal cell was chosen as a conceptual tool because its shape roughly approximates the circular coverage of a base station and because, when they are fitted together, they completely cover the area. Unlike the large geographic areas associated with high power dispatch systems of the type described above, the area covered by an individual cell is much smaller -- typically they have a radius of coverage from two to eight miles. Relatively low power base station transmitters and receivers (transceivers) with relatively low antennas are placed in each cell and connected by wirelines to the central switching computer called a Mobile Switching Center (MSC). The MTSO/MSC is, in turn, connected to the PSTN. The relatively low power/low antenna heights "match" the coverage to the area of the cell.
The base stations communicate with the mobile and portable telephones in their respective cells. Because the coverage areas or cells are small, the same set of frequencies in one cell can be used in a distant cell within the same metropolitan area. Thus, a single conversation occupies a channel over only a small geographic area, and the same channel can be reused for another conversation in another cell within the same metropolitan area. Within the cells, access to the network is provided on an FDMA basis that is conceptually similar to methods used in the trunking systems described above. That is, the channels are trunked so that the same efficiency gains associated with trunking are obtained.
In order for cellular systems to function with acceptable intra-system interference, there has to be a way of controlling interference from one cell to another. Historically, and in most systems today, this was accomplished by dividing the available channels into separate blocks and assigning different blocks to different cells arranged in, for example, a seven-cell pattern. Within the pattern, adjacent cells use different blocks of channels, and this pattern is repeated over the geographic area in such a way that the reuse of channels occurs in cells that are separated sufficiently in distance to limit the interference to acceptable levels.
A key concept associated with cellular mobile radio systems is that a startup system covering a large metropolitan area can be built with large cells and, as demand develops, these large cells can progressively be divided into smaller cells. When the cells are large, a particular channel may only be used a few times (e.g., three times), while in a more mature system it might be used ten or more times. Thus, the system becomes progressively more spectrally efficient -- i.e., more and more users are accommodated in the same amount of spectrum. To summarize, in a cellular system, increased spectrum efficiency comes from both trunking and extensive geographic reuse of channels. In the Community Repeater systems described earlier, reuse of a channel may be precluded over an entire metropolitan area, and they are spectrally inefficient in that sense. Moreover, in a cellular system, the small cells not only provide increased spectrum efficiency, they also provide better coverage to small, low power portable units since users are never far from a base station.
Because the cells are small, it is quite likely that, during the time span of an individual call, a user in a vehicle will move from the coverage area of one cell into the coverage area of another. Hence, besides setting up, maintaining, and tearing down calls, the MSC must manage call handoffs from cell-to-cell as the mobile units move throughout the coverage area. Note that while cellular systems are particularly efficient in terms of their use of spectrum for one-to-one calls, they are not particularly effective or efficient in handling one-to-many, fleet dispatch calls. This is true for two reasons. First, the setting up of a call on a cellular system is more complex and time consuming, hence making it inefficient for handling calls of very short duration as is typical of dispatch calls. Second, the individual mobile units comprising a fleet may be scattered over numerous cells requiring the use of multiple base stations and multiple channels to handle a simple one-to-many transmission.
Like the other types of land mobile radio systems described before, early cellular systems employed analog FM modulation and FDMA as the access technique. In the United States, the corresponding standard for the modulation, access technique, and other interfaces and protocols is known as the Advanced Mobile Phone System (AMPS). Modern wireless systems are being deployed using digital modulation and TDMA or CDMA as the access technique as explained in more detail in Section IV. The motivation for the shift to digital is primarily to extract more capacity from the existing spectrum space, to capture economies of scale, and to achieve increased functionality.
G. Cordless Telephones
One of the less publicized but important developments in wireless technology is the rapid growth in ordinary cordless telephones. Historically these telephones operated on an unlicensed basis and utilized very low transmitter powers. Hence their range is very limited. In the United States, the FCC set aside just ten, paired frequency channels for use by cordless telephones, and the quality of these analog systems often leaves something to be desired. Nevertheless, because they connect to the user's own telephone line, involve no "air-time" charges, are inexpensive, and offer mobility within or near a home or business, they have proven extremely popular. Some observers have pointed to this popularity as an indicator that the public has a high demand for wireless services.
Modern Wireless Systems and Trends
The purpose of this section is to describe new and evolving wireless telecommunications systems that are being developed and deployed (a) to take advantage of recent advances in hardware and software elements used in wireless systems, (b) to overcome the limitations associated with the traditional land mobile radio systems identified in Traditional Land Mobile Radio Systems, and (c) to be responsive to changing end user demands and an increasingly competitive wireless telecommunications marketplace. As noted in the introduction, the focus of this section is on technological and service trends in the medium term -- i.e., developments that can accurately be foreseen based upon current developments in infrastructure and end user systems and equipment. Since many of these advancements are being enabled by advances in hardware and software elements used in wireless systems, the section begins with a review of such advancements.
A. Advances in Enabling Technologies
On June 25, 1995, the Federal Communications Commission and the National Telecommunications and Information Administration established the Public Safety Wireless Advisory Committee (PSWAC) to provide advice on wireless communications requirements for public safety agencies through the year 2010. As part of the PSWAC process, a Technology Subcommittee was established to, among other things, identify emerging technologies that might serve to meet the needs of these agencies. The deliberations of PSWAC's Technical Subcommittee included presentations from nearly 20 organizations including manufacturers, service providers, organizations engaged in research and development, and users. In addition, many organizations with relevant knowledge and experience participated directly in the Technology Subcommittee's work. Although the work of the subcommittee related primarily to public safety communications, its efforts included an extensive review of advances in enabling technologies that are basic to all forms of wireless communications -- not just public safety applications.
In its final report, the Technology Subcommittee identified and described advances in nine enabling technologies: digital integrated circuits, RF generation devices, source coding, modulation, multiple access techniques, error correction coding, constraints on the use of various bands, backbone system elements, and performance modeling and verification. The following discussion summarizes and builds upon the work of the Technology Subcommittee.
1. Digital Integrated Circuits
In its report, the PSWAC Technology Subcommittee observed that the fundamental technology thrust through the year 2010 will continue to be, as it has been in the recent past, that of semiconductor technology. This fundamental technology thrust is a "two-edged sword." On the one hand, it increases the need for various computer-based services and, hence, increases the demand for radio spectrum to accommodate them. On the other hand, increased semiconductor capabilities make possible improved information compression techniques and more efficient modulation techniques that facilitate more efficient use of the radio spectrum resource and, hence, at least partially offset the need for additional spectrum. These same advances in semiconductor technology also fuel increases in the "intelligence" or computer processing power residing in the wireless network itself and in the end user equipment used to access the network. The Technology Subcommittee quantified past improvements in memory devices, microprocessors, and computer systems and projected them into the future. The impact of these trends in semiconductor technology is analyzed in more detail in later sections.
2. RF Generation Devices
The PSWAC Technology Subcommittee included batteries, oscillators, and antennas under this heading. In terms of battery technology, the Technology Subcommittee observed that batteries required to operate portable communications equipment are usually heavy, provide limited operating time, and can be expensive. It went on to note, however, that a number of developments in battery technology are alleviating this situation. These involve new technologies such as nickel-metal-hydride and lithium-ion batteries as well as zinc-air batteries that draw oxygen from the atmosphere to extend their life. It also noted that improvements were being made in wireless systems to conserve on the use of battery power. These include more efficient RF power amplifiers and more efficient "sleep" or standby modes.
In terms of oscillators, the Technology Subcommittee noted the ability to place more communications channels with a given amount of spectrum depends upon both the transmission bandwidth and the stability of the oscillator. In other words, if the oscillator drifts, for example, to changes in temperature, then more guard space must be provided between channels to prevent spillover or interference from one channel into the next. The Technology Subcommittee quantified past improvements in oscillator stability and the impact on spectrum efficiency. These improvements in oscillator stability, coupled with the advances in semiconductor technology described above, permit the design of radio equipment with the ability to operate on a frequency agile fashion on literally hundreds of different channels.
In terms of antennas, the Technology Subcommittee, among other things, noted the development of "smart antennas." Essentially, these antennas utilize microprocessor technology to electronically (rather than physically) steer the radio beam at the transmitter and/or receiver site. Such antennas can be used to reduce interference and improve performance by, for example, allowing the base station antenna to track a low power portable unit. The Technology Subcommittee noted that, while such techniques have been used in military systems for some time, they have not been widely used in commercial systems because of cost considerations. They noted, however, that this was likely to change with attendant improvements in semiconductor technology.
3. Source Coding
In Section II, waveform coding and voice coding were both described in the context of voice communications. The term source coding is a general term for techniques that take into account the known characteristics of the information source and receptor (e.g., the human ear or eye) to reduce the amount of information that must be sent over the communications channel. Typically these techniques work by removing redundant information or information that cannot be utilized because of limitations in the receptor. Voice coding, as described earlier, is one example of source coding, but analogous techniques can be used to compress other types of signals as well -- including images (e.g., the transmission of facsimile messages or still pictures) and both slow scan and full motion video. Once again, it is beyond the scope of this report to delve into the details of these techniques but, as pointed out in the PSWAC Technology Subcommittee report, increasingly powerful digital signal processing integrated circuits will facilitate the introduction of more powerful and effective methods for reducing the amount of information that must be transmitted on a communications channel. The source content and compression capabilities of present day technology and expected gains in compression due to algorithmic advances and/or semiconductor technology gains are summarized in Appendix C of the PSWAC Technology Subcommitee report.
4. Modulation
Another technique for increasing the amount of information that can be transmitted in a given amount of bandwidth is to improve the modulation efficiency. As alluded to earlier, modern digital systems use various combinations of frequency, amplitude and phase modulation as well as other techniques to increase the number of bits per second that can be transmitted over a given channel. Implementation of these techniques is facilitated by the improved performance of digital signal processing integrated circuits. Modern wireless systems can achieve modulation efficiencies of over one bit per second per Hertz, even in the severe multipath/fading environment that is typical of mobile communications in an urban environment.
5. Multiple Access Techniques
Different channel access methods, including FDMA, TDMA, CDMA, and TDD, were discussed briefly in Section II. The PSWAC Technology Subcommittee report notes that these methods have specific strengths and weaknesses. It goes on to state that (a) FDMA is employed in narrowest-bandwidth, multi-licensed channel operation, (b) TDMA is employed in exclusive license use, moderate bandwidth applications and (c) CDMA is employed for widest-bandwidth applications in both single systems such as cellular mobile radio systems as well as uncoordinated and/or unlicensed applications (e.g., unlicensed, wireless local area networks). As described earlier, TDD is employed to achieve full-duplex operation in a single (unpaired) radio channel. While the Technical Subcommittee report briefly describes the advantages and disadvantages of each of these methods, it does not project any fundamental breakthroughs that would radically change or add to this basic set of multiple access techniques.
6. Error Correction Coding
As pointed out in the PSWAC Technology Subcommittee report, in a digital communications system, the objective is to maximize the ability of the receiver to successfully decode digitally encoded messages. In other words, the objective is to deliver without error the exact sequence of ones and zeros that was transmitted. The report states that:
A simplistic method to improve reliability is to send [the digital] messages more than once. This has the serious disadvantage of increasing transmission time by the number of times the message is repeated. More efficient methods uses [sic] error control techniques that add bits to the data stream in a precise fashion. The extra bits, however, are placed in a precise mathematically-prescribed pattern at the transmitter end such that complementary circuitry in the receiver can tell when an error has occurred, and determine what the correct bit value should be.
There are two types of error control techniques -- simple error detection and forward error correction. Error detection is typically employed in data communications applications in concert with protocols that use a return channel to automatically request the retransmission of corrupted data. Forward error control provides the ability to detect and correct digital messages even in the presence of transmission errors. Forward error control is particularly useful for applications like voice where retransmission is not practical. While at first it may seem counter-intuitive that adding redundant bits would actually improve total performance, the increased throughput and/or decreased error rate associated with the use of sophisticated error control techniques more than compensates for the increase in transmission overhead. Such techniques are useful, for example, when improved performance cannot otherwise be achieved because of restrictions on maximum transmitted power. It should be obvious that, once again, these error control techniques are facilitated by the availability of increasingly powerful digital signal processing integrated circuits as described earlier.
7. Software Programmable Radios
The PSWAC Technology Subcommittee report contains a section entitled "Constraints on using various bands" but it is primarily devoted to software programmable radios. A software programmable radio is a radio in which functions or applications are configured under software control and in which the function or application, in whole or in part, is implemented in software resident in the radio. As pointed out in the Technology Subcommittee report, a software programmable radio requires the information or signal be presented in a digital format for processing. For example, even when receiving a digital radio signal, a traditional radio may perform many functions (e.g., filtering and demodulating the received signal) using analog components before the digital signal (i.e., the basic ones and zeros comprising the message) is actually extracted. In contrast, in a software programmable radio, these same functions may be accomplished using digital signal processing techniques. This requires, of course, that the received signal (including the carrier and the accompanying modulation) first be converted to the digital format.
Proponents claim a number of advantages for software programmable radios. One of the major advantages is that a number of different radios with different characteristics can reside or be hosted on one hardware platform, thus reducing the total amount of equipment required. For example, in principle, one could change the type of modulation used or the bandwidth of the channel employed by the radio as simply as one changes from a word processing program to a spread sheet program on a personal computer. In other words, such radios could facilitate multimode operation. In addition, the radio could be upgraded without changing the hardware, thereby increasing its performance at low cost. Similarly, software programmable radios could be used to facilitate the creation of multiband radios, and combining the multimode and multiband capabilities could facilitate interoperability among different radio systems. For example, it is conceivable that this high degree of functionality could be used to allow an end user to roam from one type of system -- say one employing TDMA as an access technique -- to another type of system -- say one employing FDMA as an access technique. This would be accomplished by a change in software on a single hardware platform (radio) that allowed operations in both locations.
The Technology Subcommittee report notes that the military is placing an increasing emphasis on digital radio technology because of its potential for lower cost reconfigurability for multimode/interoperable communications. The report goes on to describe a military project for developing a software programmable radio whose goal it is to develop and open system architecture for radio service and demonstrate interoperability of multiple and simultaneous waveforms (signals) across a frequency range of 2-2,000 MHz. The report cautions that software radios are now much more expensive than hardware-based radios, but that "over time, the cost of software enabling technologies will decrease" and that "within a few hardware generations, software radios will sufficiently leverage the economics of advancements in microelectronics and provide seamless communications at a vest-pocket and palmtop level of affordability and miniaturization."
8. Backbone System Elements
In this section of its report, the Technology Subcommittee focused on a number of issues that are unique to public safety communications. But the basic notion that developments in backbone systems have implications for wireless telecommunications is an important one. Backbone systems are telecommunications systems, or, perhaps more accurately, subsystems that are used to carry signals between the base station sites and the control points and/or switch locations. The developments include lower cost, more reliable point-to-point microwave and fiber optic equipment for self-provision of backbone networks as well as an increasing number of commercial providers. For example, Competitive Access Providers (CAPs) are emerging in many metropolitan areas offering highly reliable, ground-based fiber optic connectivity. As described in more detail later in this report, there is also an increasing interest in more tightly integrating wireline and wireless networks wherein the wireless provider would not only share backbone systems with the wireline network, but switching, control, and "network intelligence" as well.
9. Performance Modeling and Verification
Radio systems engineers have an increasingly powerful set of computer-based tools for modeling, simulating, and measuring the performance of wireless telecommunications networks. While these tools are well hidden from the end user, they are extremely important to the proper design and operation of increasingly complex wireless systems. For example, improved software programs for predicting the coverage of radio systems and for modeling the interference between and among such systems can lead to significantly improved performance as ultimately seen by the end user.
B. Advances in Wireless Systems
1. Paging
There are three major technological advances associated with radio paging systems -- the deployment of vast regional, national, and even international networks (including the use of simulcasting on a single frequency slot); the development and deployment of higher speed signaling techniques; and the development and deployment of two-way paging systems.
The deployment of paging systems covering large geographic areas has been made possible by the development of high-capacity terminals capable of controlling a large number of widely dispersed transmitters employing single or multiple frequencies and, often, offering a selection of coverage areas. Signals in nationwide and regional systems may be distributed from the central site containing the terminal to each market over satellite or terrestrial transmission facilities. In so-called "satellite" paging, signals are transmitted via satellite to very small aperture terminals (VSATs) located in each market served. Within a given market, a technique of simulcasting is typically used with multiple transmitters operating on a common frequency slot. Multiple transmitters are used both to extend the coverage area and to ensure good radio coverage, especially within buildings. With simulcasting, the transmitters and interconnection facilities are precisely controlled to prevent interference between the transmitters while still providing seamless coverage.
Another major advance in paging systems is the development and deployment of systems with protocols that operate at much higher speeds. Recent systems transmit one-way in a 25 kHz frequency slot at a speed of 6,400 bits per second (bps), a significant improvement over the prevalent standard of 2,400 bps. Moreover, the FCC recently allocated some new, wider paging channels (50 kHz) that allow transmission speeds as high as 112,000 bps. This not only provides for a vast increase in the number of pagers that can be supported per frequency slot or channel, it also allows for the provision of advanced services. For example, for many years, tone-voice services were discouraged in congested urban areas because the voice messages consumed a large amount of valuable airtime on the channel -- time that could otherwise be used to transmit a large number of tone-only or numeric pages. The development of high-speed paging protocols and voice compression are now allowing providers to offer voice paging on an efficient basis, while eliminating the need for operators to transcribe voice messages into alphanumeric messages.
In addition to allocating wider channels for paging, the FCC recently made available (via auction) paging channels with a paired response channel. The services to be provided on these additional channels are sometimes referred to as Narrowband Personal Communications Service. The pairing of an in-bound frequency slot with an out-bound slot allows paging providers to offer two-way paging or messaging services. The response channel may be used to simply acknowledge the receipt of a page, to return one of a number of preprogrammed responses, or to provide a more symmetrical, two-way messaging service. In the latter case, the customer unit may no longer be a pager, of course, but rather a laptop computer or personal digital assistant (PDA).
2. Two-way Mobile Data
A fundamental advantage of mobile data systems is that they offer a much higher effective information transfer rate than voice systems. Human beings, dispatchers for example, transfer information at relatively low rates, on the order of 100 to 150 words per minute maximum. If one assumes each word is an average of six characters long and each character requires eight bits to transmit, then the effective data rate is only about 120 bits per second. If the system uses ordinary 25 kHz dispatch frequency slots, then the spectral efficiency as measured in bits-per-second per Hertz of bandwidth is less than .005.
However, using modern techniques, it is now easy to send 4,800 bps in a 25 kHz channel and much higher rates are achievable -- as high as 25 kbps or more. Stated another way, even at 4,800 bps, sending information as data messages rather than voice messages increases the throughput and spectral efficiency by a factor of 40. At 25 kbps, the improvement could be as high as 200. Heavy users of mobile communications, overnight courier services for example, take advantage of this by loading several hundred mobile data terminal users on a single channel -- many more than can be accommodated using voice on a single channel.
Two-way voice dispatch and cellular mobile radio systems can be used for transmitting data messages by using modems and a "dial-up" connection. However, since these systems were optimized for voice communications, they are subject to limitations when used for sending data. Not only must the modem contend with a much more hostile channel with time varying characteristics such as multipath distortion, signal fading, noise, and interference, but also with mobile/portable customer equipment that contains special circuitry that enhances voice performance but constrains or complicates data handling. In addition, in an analog cellular mobile radio system, certain signaling functions are carried out over the voice channel itself. This is done by briefly interrupting the voice signal and sending a burst of digital signaling information. Such a burst of digital signaling is used, for example, to order the mobile unit to change channels during a handoff from one cell to another. The interruption is so short that it typically remains unnoticed to a voice user, but it can cause havoc with ordinary dial-up modems. Modem manufacturers have responded to the growing interest in using analog cellular systems for data communications by developing modems containing special protocols aimed at overcoming or at least minimizing these problems. In addition to these problems, there are often difficulties associated with connecting the modem to the mobile/portable unit. This is because the physical and electrical interfaces to the mobile/portable unit are not standardized and, in some cases at least, may not even contain desired electrical connections.
In any event, the use of circuit switched systems is not efficient for handling "bursty," computer data where the integrity of the data is critical to the service or application. In this part of the report, the focus is on specialized systems that have been optimized for the provision of two-way data services. A number of companies operate such specialized two-way mobile data systems either for their own private use or for the provision of service on a third-party basis. In certain respects, these two-way data systems can be looked at as extensions of the specialized packet switched networks that have evolved using terrestrial transmission facilities.
In these specialized systems, the message (i.e., collections of alphanumeric characters) to be sent is divided up into smaller units of information called packets. Each packet includes the address of the person or equipment to whom the message is directed as well as extra bits that are used to detect (and perhaps correct) errors in transmission. In contrast to circuit switching used for voice services, in packet switching there is no full-time direct connection between the two ends of the connection. Instead, these packets are sent from node-to-node through the network in store-and-forward fashion and, at the receiving end, the individual packets are reassembled into the original message.
There are a number of advantages to using this technique. First, because there is no "realtime" connection, each packet can be disassembled at each node and checked for errors. If an error has occurred, it can be corrected based upon the extra bits transmitted or by requesting the forwarding node to resend the packet that is in error. In this way, error free messages can be assured. Second, packets from different users can be commingled on the same link; that is, multiple users can share the same channel. If one unit has nothing to send, it transmits no packets and no channel capacity is used. When it does have something to send, its packets are interleaved with the packets being sent by other terminals. This technique is particularly well suited for "bursty" data traffic because capacity is only consumed when information is being sent.
The tradeoff with packet switching, of course, is that there are delays associated with getting on to a link and with disassembling the packet, checking and correcting errors, and reassembling the packet at each node. These delays make traditional packet switching unsuited for voice. However, delays of several seconds are perfectly acceptable in many data applications (e.g., in notifying a central computer that a package has been picked up). From a user standpoint, this delay is often more than offset by error free performance and increased efficiency in the use of network resources.
In a packet switched, two-way mobile data system, the region to be covered is divided up into small areas or cells, each served by a base station transceiver and associated antenna equipment. These cells are then interconnected to the fixed network using wireline or other facilities. The over-the-air signals are sent digitally (as described before) and, because channels with paired frequency slots are utilized, full duplex (i.e., simultaneous transmissions in both directions) is possible.
The mobile units and base stations exchange packets comprising the messages as just described. In the in-bound direction, the base stations receive the packets and, based upon the address, route them toward a destination such as the customer's computer. The packets may be picked up by more than one base station, but the duplicate packets can be discarded using logic in the network. Similarly, in the out-bound direction, the mobile unit receives packets that are addressed to it and discards the rest. Error free performance over otherwise marginal radio links can be assured using the error checking/correcting techniques just described.
Another interesting advancement in the provision of two-way mobile data services is the development and deployment of the Cellular Digital Packet Data (CDPD) system by portions of the cellular industry. These systems, which -- as the name implies -- use the packet data approach just described, are overlaid on existing analog cellular networks in such a way that they can utilize a portion of the existing cellular infrastructure. The packetized data transmission occurs either during normal gaps in the usage of the cellular channels for voice communications or on separate cellular channels that have been set aside for such purposes. In either case, the normal analog cellular operations can continue without interference or interaction with the data traffic.
The CDPD system allows the carrier to exploit economies of scope or integration while providing customers with a wide coverage, packet data communications capability or a combination of normal cellular voice services and packet data services within a single device. A large number of cellular carriers support CDPD.
3. Two-way Dispatch
Major developments in the systems for providing two-way dispatch have occurred in the multichannel trunked radio systems described in Section III.E. As described there, compared to conventional repeaters, the early trunked systems offered greatly improved dispatch services by providing disciplined access to channels and much greater spectral efficiency. Subsequent developments in trunked systems include the use of more powerful signaling techniques, virtually unlimited addressing capabilities, smooth integration of dispatch and interconnected mobile telephone service, and multi-site networking capabilities.
The combination of greater addressing capabilities and more powerful signaling, for example, allows the creation of different communications groups among the units that comprise a customer's fleet. Thus, a dispatcher at a company that owns cement trucks and sand and gravel trucks can, at his or her option, communicate with the entire fleet, subfleets consisting of just the sand trucks, gravel trucks, or field supervisors, or, on a private basis, with individual units. Moreover, using high-speed signaling, the network can be dynamically reconfigured, thus creating what is often referred to as a software defined network. The ability to reconfigure networks dynamically is especially useful in emergency or other abnormal circumstances where field resources need to be redeployed to fit a particular circumstance.
Another major trend in trunked radio systems, like in telecommunications networks more generally, is the movement toward digital rather than analog transmission. Digital transmission has a number of advantages such as, for example, the ease of providing different services on a common platform. But a principal impetus for the use of digital rather than analog transmission in radio systems is that, when it is combined with voice compression and more efficient modulation techniques, it allows multiple voice conversations to be carried in the same frequency slot that formerly carried a single analog voice conversation. Typically this is done using the TDMA techniques described earlier, although FDMA and CDMA techniques are being proposed/deployed as well. One commercial TDMA-based system now being introduced in the United States allows six voice conversations to be carried simultaneously in a single, 25 kHz wide frequency slot or channel.
Another, somewhat less clear trend in trunked systems is toward more cellular-like configurations with smaller coverage areas for each base station. For one-to-one communications (e.g., mobile telephone service), this allows greater spectrum efficiency through increased frequency reuse. However, this approach is problematical for one-to-many (fleet dispatch) communications because it requires, in effect, separate calls to be simultaneously set up to each cell which contains a unit in the fleet. This may reduce both the technical and economic performance of the network in the dispatch mode. Thus a network configured with large cells provides better performance in fleet dispatch applications at the penalty of low spectrum efficiency in mobile telephone applications, while a network configured with small cells provides better spectrum efficiency (and hence greater capacity) in mobile telephone applications, but at the penalty of poorer performance in dispatch applications.
4. Two-way Mobile Telephone
Major developments in systems for the provision of wireless telephony have occurred on four fronts. First, in response to reduced regulatory restrictions and for other reasons, trunked, two-way dispatch systems have improved significantly in their ability to provide wireless telephone services. Second, due to rapid growth in the number of subscribers, evolving competition and other factors, the cellular systems operating in the 900 MHz region of the spectrum in the U.S. have changed significantly in technological terms. Third, the FCC has reallocated and assigned (through the use of auctions) a large block of new spectrum in the 1.9 GHz region for the provision of Personal Communication Services -- including mobile/portable telephone services. Fourth, there have been significant technological developments in cordless telephones and related devices using unlicensed spectrum -- including additional unlicensed spectrum in the 1.9 GHz region.
The developments in trunked two-way dispatch systems that allow them to provide wireless telephone services more effectively were described in Section IV.C, immediately above, and will not be addressed further here. Developments on the remaining three fronts are described in the paragraphs which follow.
a. CELLULAR
The most far-reaching trend in cellular mobile radio systems is the conversion from analog to digital transmission. In the U.S., the original AMPS systems operating in the 900 MHz band used FM modulation in frequency slots that are each 30 kHz wide. The standard that is associated with AMPS is known formally as IS-553. Thus, in AMPS, each direction of a two-way conversation uses a 30 kHz-wide slot -- a width that is almost ten times wider than the voice signal that is being transmitted.
The phenomenal growth of the cellular service, coupled with the difficulties and expense of moving to extremely small cells in order to increase capacity through frequency reuse, has put pressure on the service providers and their equipment vendors to develop more spectrum efficient transmission methods. Digital compression and transmission techniques were chosen as one method of achieving the needed increase in capacity. The situation was complicated by the need to maintain backward compatibility with the existing analog systems during at least a transition period, if not indefinitely.
Two incompatible digital standards have been adopted for the conversion of existing cellular systems to digital techniques in the U.S. The first divides existing frequency slots/channels using TDMA, while the second divides a contiguous set of channels (i.e., a block of existing cellular spectrum) using CDMA. The standard for the former, which is based on providing three time slots (and hence three simultaneous voice conversations) in each channel is known as IS-54 and the latter, for which proponents claim even greater capacity gains, is known as IS-95. Both standards are "dual mode" which means that a handset is capable of operating in either the digital mode or in the analog mode. This means that a customer using one of the new dual mode subscriber units can operate on either an analog system or a digital system. Thus, if the customer roams to a system (in a rural area, for example) that has not converted to the digital standard, he or she can still place and receive calls. Likewise, a cellular carrier can operate a mix of analog (IS-553) and digital (either IS-54 or 95) systems in order to continue to provide service to users who have not purchased digital subscriber units while still achieving greater capacity on the digital channels. Over time, as the percentage of analog-only subscriber units decreases, the percentage of channels devoted to the less spectrum efficient analog technology can be reduced. Note that, because the industry has not settled on a single, compatible digital standard, the analog channels are necessary to provide "a lowest common denominator" for roaming customers. Thus, in the absence of a common digital standard (or the development and full deployment of dual-mode TDMA/CDMA subscriber units), all providers must continue to provide analog support channels.
Besides the over-arching trend toward digital transmission and TDMA and CDMA as alternative access techniques, there are several developments worth mentioning. First, the industry has been able to develop standards that, among other things, facilitate handoffs between systems and the provision of automatic roaming. Automatic roaming, of course, refers to the ability of the customer to place and receive calls automatically as he or she moves across cellular systems owned by different providers and utilizing infrastructure equipment manufactured by different vendors. These developments are based, in part, on increased reliance on advanced signaling techniques associated with the emergence of digital transmission standards and with inter-switch communications based on common channel signaling.
Second, vendors have developed and providers have deployed microcells in order to provide better coverage inside buildings and other difficult to serve locations. Such microcells become increasingly important as low-power portable units (rather than higher power mobile units) proliferate and competition from new Personal Communications Systems operating in the 1.9 GHz band emerge. On a related point, cellular carriers and their equipment vendors have also developed handsets that operate on unlicensed cordless telephone frequencies in one mode and as ordinary cellular handsets in another mode. This means that the customer can use the unlicensed mode to place and receive calls without incurring airtime charges when in the vicinity of the cordless telephone base station (say, inside an office complex), while still having full cellular service when outside that range.
b. PCS
As mentioned earlier, one of the most important recent developments impacting on the provision of wireless telephone services in the U.S. is the reallocation of a large block of new spectrum for the provision of Personal Communications Services or PCS. One of the difficulties of dealing with PCS is that it has no single, clear definition. However, the ultimate concept or vision of a Personal Communications Service seems to revolve around the notion of the ability of customers to obtain personalized telecommunications services (e.g., to make or receive calls) at virtually any location at virtually any time. The essence of PCS seems to be a wireless service utilizing a common handset and a single telephone number (referred to as Personal Number Calling). Under this latter concept, customers would receive a single telephone number and that single number would no longer be associated with a physical location on the PSTN. Rather, intelligence residing with the network would be used to route calls to the individual regardless of his or her location.
It should be emphasized that PCS encompasses not a single network, but a seamless web of interconnected networks that, as one observer defined it, is "an extension of and integration of current and emerging wired and wireless telecommunications network capabilities allowing communications with persons and supporting personal mobility." This vision of PCS is broad enough to encompass today's paging and cellular services, but the focus in this section of the report will be on the development, deployment and evolution of PCS in the newly reallocated spectrum in the 1.9 GHz region.
In the 1.9 GHz region, the FCC has reallocated a total of 140 MHz for PCS. It has divided this spectrum up into six licensed blocks of differing sizes, and they have labeled the blocks with the letters A through F. Blocks A, B, and C each contain a total of 30 MHz, while blocks D, E, and F contain 10 MHz each. Each of the blocks is split evenly into two parts separate sufficiently in frequency to allow full-duplex operation as explained previously. All six of these blocks, which total 120 MHz, are for licensed services. The Commission's rules allow the 10 MHz blocks to be combined with the 30 MHz blocks to create a maximum of 40 MHz of spectrum for one system. The remaining contiguous block of 20 MHz has been set aside for unlicensed operation and it has been further divided into two parts -- one for voice services and one for data services.
The technical rules that the FCC has established for the use of the newly reallocated spectrum for licensed PCS services are loose enough and non-specific enough to accommodate different types of PCS systems. Out of this flexibility, two basic types of PCS systems seem to be evolving. The two types are often referred to as "low tier" (or "low mobility") and "high tier" (or "high mobility").
The low tier systems can be thought of as evolving from today's cordless telephone technology and market. Like cordless telephones, these systems would provide wireless access to today's wired network while the user is on the customers' premises, but the same handset would also provide access to the wired network away from the home, office or factory. More generally, these low tier PCS systems are expected to be deployed in cases where low mobility is involved, wireline telephone quality voice is needed, customer usage approaches local loop usage levels, traffic density is high, low cost (suggesting low complexity) is desired, and service to low cost, light weight handsets with long battery life is required. Low mobility means that the system is only capable of handing off calls at relatively low speeds -- i.e., at walking speeds.
The high tier systems can be thought of as evolving from the existing cellular mobile radio technologies. In fact, many observers regard the developing high tier PCS systems as being cellular systems that operate in another region of the spectrum -- namely 1.9 GHz rather than 900 MHz. In keeping with the similarity between today's cellular and the evolving high tier PCS systems, the latter are expected to provide service where high mobility is involved, voice quality is somewhat less of a requirement, customer usage levels are more typical of today's cellular rather than ordinary wired telephones, traffic density is less, and the size, weight and cost of the handsets are somewhat less critical.
These different requirements for low tier versus high tier systems result in different network architectures or system designs. It is beyond the intended scope of this report to describe the various technical tradeoffs in detail, but a few of them will be suggested. The higher voice quality requirements for the low tier system, along with the need for low cost, light weight handsets, implies the use of less voice compression -- i.e., the use of waveform encoding and decoding. As described earlier, higher bit rate speech not only improves quality, but it also reduces the amount of computer processing power employed in the handset. A reduction in processing power, in turn, translates into a reduction in battery drain and a corresponding increase in battery life. On the other hand, higher bit rate voice is less efficient in the use of the radio spectrum resource and that, coupled with the need to provide service to very low power handsets in high usage, high traffic density situations, translates into the need for cells with much smaller radii than traditional cellular systems. Consequently, smaller cells, typically on the order of .25 miles or so, are needed both to increase the amount of frequency reuse obtained in the area and to allow service to the low power handsets. With such small cells, cell-to-cell handoffs become extremely difficult and work against the need to keep complexity and costs low. Thus, as explained above, low tier systems, which may compete with wired telephone systems, may only offer handoffs at pedestrian speeds. In addition, the small cells may make it less practical to provide seamless coverage over less densely populated areas.
The larger cells associated with traditional cellular systems allow the coverage of larger areas with fewer base stations and ease the handoff problem. But larger cells reduce the amount of frequency reuse obtained and imply the need for higher power handsets. The reduced spectrum efficiency can be compensated for by an increase in the amount of voice compression used, but at the expense of voice quality and battery life. However, the resulting voice quality may be more than adequate since the handsets are often used in vehicles and other noisy places where the extra quality is not noticeable or critical. Likewise, access to the vehicle's battery lessens the need to depend entirely on the battery in the handset.
These and other tradeoffs interact in complex ways with market considerations, and the details of how all of these tradeoffs will be made is far from certain at this point. However, two contenders have emerged for low tier system standards, four for high tier, and one for a "middle tier." One of the contenders for the low tier category is a system known as PACS. It is a combination of a system developed at Bellcore known as WACS and the Japanese Personal Handiphone (PHP) system. It uses TDMA, frequency division duplexing, a form of phase modulation, 32 kbps voice coding, and handsets with an average transmitter power of 12 milliwatts. The second low tier contender is based on the Digital European Cordless Telecommunications (DECT) standard. It uses TDMA, time division duplexing, a form of frequency shift modulation, 32 kbps voice coding, and handsets with an average transmitter power of 10 milliwatts.
One of the four contenders for the high tier systems includes a system based on the U.S. TDMA digital cellular standard (IS-54). It employs TDMA as the access method, frequency division duplexing, a form of phase shift modulation, approximately 8 kbps voice coding, and handsets with an average transmitter power of 200 milliwatts. A second contender for the high tier system is based on the U.S. CDMA digital cellular standard (IS-95). It employs CDMA as the access and modulation method, frequency division duplexing, variable voice encoding (8 and 13 kbps), and handsets with an average transmitter power of 200 milliwatts. The third system is based on the second generation European digital cellular standard known as GSM. This system, sometimes referred to as DCS-1900, employs TDMA, frequency division duplexing, a combination of phase/frequency modulation, 13 kbps voice coding, and handsets with an average transmitter power of 125 milliwatts. The fourth contender is also a CDMA system, but one which spreads the signals over a wider bandwidth. The single "middle tier" contender employs a combination of CDMA and TDMA as the access method, time division duplexing, a range of voice coding speeds, and handsets with an average transmitter power of 62 milliwatts.
It is almost certain that different standards for PCS will emerge in the U.S., not only between low tier and high tier systems, but within the two categories as well. What systems will actually be deployed and whether the industry will eventually converge on a single standard (or, perhaps, single standards for high tier and low tier systems) is still uncertain.
c. UNLICENSED TWO-WAY VOICE
In Section III.G., it was pointed out that today's unlicensed analog cordless telephones have proven enormously popular despite rather severe limitations in the number of channels available. The FCC recently expanded the number of channels available in the 49 MHz region of spectrum from ten to 20 and much more sophisticated cordless telephones using spread spectrum techniques have developed. The latter cordless telephones operate in the 900 MHz region of the spectrum. Recall that it is possible to construct handsets that include both cellular and unlicensed cordless telephone technology. As pointed out before, this allows the customer to avoid air time charges when he or she is at home or in the office.
Some manufacturers have extended the single line, cordless telephone technology to create wireless Key Telephone Systems (i.e., multiline "push button" telephone systems) and wireless Private Branch Exchanges (PBXs) for business applications using the unlicensed spectrum. These systems use cellular techniques, but with very small cells scattered over the office, factory, or campus to provide good coverage and higher capacity. These systems allow employees to make and receive telephone calls while away from their desks or other regular location and reduce the costs of the "moves and changes" that are associated with moving employees around due to growth or reorganizations.
The reallocation of an additional 20 MHz of spectrum for unlicensed uses in the 1.9 GHz PCS band will permit further expansion of wireless voice services, including wireless KTSs/PBXs. Of the 20 MHz of spectrum, the Commission has allocated 10 MHz for voice services as reported earlier. Systems in the new band will utilize a special industry developed "Listen before you talk" etiquette to avoid interference between nearby systems. Since the unlicensed band is in such close proximity to the licensed bands, it will facilitate the development of handsets that will operate in both the licensed and unlicensed portions of the PCS spectrum.
d. INTERRELATIONSHIP BETWEEN WIRELINE AND WIRELESS DEVELOPMENTS
One of the most important trends in the development of the wireline telephone network is movement to common channel signaling, or separating voice traffic from signaling traffic. Signaling involves the exchange of information associated with setting up, maintaining and taking down calls. In the wireline network, up until fairly recently, analog tones were used to convey signaling information which was carried on the same channel as the voice conversation. With common channel signaling, all of the signaling information associated with multiple conversations is digitized and handled on a common packet-switched subnetwork. Common channel signaling allows calls to be set up faster because the signaling itself is faster and more efficient since signaling does not consume conversation capacity on the circuit switched voice subnetwork. The specific common channel signaling system now being widely deployed is referred to as Signaling System 7 (SS7).
Another major trend in the wireline network is toward unbundling of stored-program control switching systems. Modern computerized or stored-program control circuit switches are composed of two basic parts -- the matrix where physical connections are made between circuits and a processor that contains the logic that controls the switching. In early stored-program control switches, the switch (matrix) and processor elements were tightly integrated, and this tight integration ultimately caused a number of disadvantages. The modern trend of separating the switch control from the lower level switching functions overcomes these disadvantages and allows entire networks to become programmable.
These advances in signaling and switching allow the creation of what is referred to as the "Intelligent Network." Computer processors and their associated databases are placed in the network where they can be accessed from the signaling channel. The system uses the calling and called numbers and other information to handle calls in special ways -- e.g., to route calls to different locations depending on the time of day and/or the originating location. One obvious application of these intelligent network capabilities is in providing customers with the ability to roam among different locations and even among different networks. For example, a switch can suspend a call briefly, look up the called number in a database, and then route the call to the intended recipient at another number and location on the same system (call forwarding). Similar techniques can be used in wireless networks to allow customers to roam from system to system. When a wireless customer carrying a portable telephone arrives in a distant city, for example, the visited system recognizes that the user is roaming and automatically notifies the user's home system that the handset is in the new area. This information is placed in a database called a home location register. Subsequently, when a call is placed to the user's number on the home system, the call is automatically routed to the customer in the distant city.
Due to regulatory as well as technological constraints, early wireless systems were built entirely separate from the wireline network, and it was only later that interconnection became possible between the two. This separateness carried over to early cellular radio systems but, more recently, there has been increased interest in the issues associated with interconnecting and even sharing the databases, processors, and common channel signaling associated with wireline and wireless networks of all types. Such seamless interconnection/resource sharing will allow the efficient provision of powerful new services such as personal number calling. With personal number calling, customers no longer have to have multiple telephone numbers for their home, office, cellular phone, fax machine and other lines, but rather they receive a single number that is not associated with a fixed, physical location on a network. Calls to that number will then be routed to the customer wherever they are and irrespective of the network by which they are currently being served. This will be done by using the databases, processors and signaling capabilities comprising the Intelligent Network to route the call. Thus there is a significant interrelationship between wireline and wireless developments in supporting the concept of personal mobility described earlier.
5. Unlicensed Two-way Data
Broadly speaking two-way wireless data communications can be divided into two categories. The first is the wide-area data systems of the type described in Section IV.B. The second category is local data systems that provide service over a more limited area such as an office, factory, or campus. These local systems are referred to as wireless Local Area Networks (LANs). Wired LANs have grown extremely rapidly in recent years as a way of interconnecting personal computers for, among other things, the exchange of electronic mail, the sharing of peripheral equipment such as printers and high-speed modems, and accessing large centralized files.
Wireless LANs, like wireless KTSs/PBXs in the voice world, allow more mobile employees (e.g., warehouse workers) to be connected into the organization's wired network and reduce the costs of moving employees from place to place within the business complex. There are a number of commercial products available today that operate in the unlicensed band in the 900 MHz region as well as other unlicensed bands near 2.4 and 5.7 GHz. Typically these systems employ packet techniques and some form of spread spectrum as the modulation and access technique. Efforts are now underway to create standards for wireless LANs.
Because of the wider bandwidths available and the frequency reuse that is possible with the short ranges involved, much higher data rates are possible than on the wide area, two-way wireless data systems described previously. As noted earlier, the FCC has allocated 10 MHz of spectrum in the 1.9 GHz region for unlicensed data applications and, due at least in part to certain interference issues in that band, has indicated a willingness to reallocate other spectrum for wireless LAN and similar applications.
6. Mobile Satellite for Voice and/or Data
This report has focused primarily on terrestrial based systems, and space does not allow a full discussion of satellite based systems. However, it should be noted that satellites in geostationary orbit have been used for many years to provide voice and data services to mobile terminals and, more recently, organizations have moved to deploy constellations of low earth orbiting (LEO) satellites that are suitable for communicating with relatively small, low power, handheld, subscriber equipment.
Thus, one can envision the day when a customer can obtain service virtually anywhere in the world using a common handset (or a different handset using a form of "smart card" associated with the particular user). Such a system would automatically select the lowest cost service that delivers the desired quality. So, for example, the user could access an unlicensed cordless telephone-like system when at home or at the office, a public low tier PCS system when in high density locations, a high tier PCS system (including cellular) when in a vehicle and/or less urban location, and a satellite-based system when in remote locations outside the range of all terrestrial-based networks.
C. Summary and Evaluation of the Medium Term Technological and Service Trends
The previous two subsections (IV.A and IV.B) provided, respectively, an overview of advances in enabling technologies and a description of short to medium term wireless telecommunications technology and service trends that take advantage of those advances in response to user demands and competitive marketplace pressures. The purpose of this subsection is to summarize the medium term trends and to evaluate them in terms of their implications for people with disabilities.
1. Summary of Medium Term Trends
Broadly speaking, the trends identified earlier can be divided into five categories -- improved coverage, integration of services, increased network-based functionality, increased functionality of end user equipment, and improved spectrum efficiency. These broad trends are discussed in more detail in the paragraphs which follow.
In evaluating the coverage of wireless telecommunications systems, it is common to speak in terms of both the breadth and the depth of coverage. The breadth of coverage refers to the outer limits of the geographic area over which service can be obtained. Depth of coverage refers to the ability to receive service at locations with those outer limits, e.g., within buildings, tunnels, and similar hard to reach locations. Because of the technological developments described earlier, competitive pressures, regulatory build out requirements, and other factors, both the breadth and depth of coverage of wireless telecommunications systems have increased significantly in the United States over the past decade. For example, today's cellular mobile radio systems provide coverage over 70 percent of the land area which translates into population coverage approaching 96 percent. Moreover, what were once independent wireless telephone, dispatch, paging, and two-way wireless data systems are being knit into larger networks that facilitate universal roaming. Intensifying competitive pressures and the development and deployment of new microcellular systems at one extreme and mobile satellite systems at the other, holds out a strong promise for the continuation of these trends toward improved coverage and universal roaming.
For a host of regulatory, technological, and economic reasons, wireless telephone, dispatch, one-way paging, and two-way data services in the past were supplied on separate networks. However, there is apparently a strong demand from users for integrated combinations of services. For example, some users may desire a combination of one-way paging and dispatch service, while another may want voice dispatch and mobile telephone service.
Fundamentally, there are two ways of responding to this demand for integrated services. A provider can offer multiple services on a common network or furnish the customer with end user equipment that is able to receive different services from separate networks. In other words, the integration can be provided either within the network or within the handset or other end user device. The former solution, integration within the network, has the advantage that it helps capture any network-based economies of scale or scope. Such economies can be important in networks that require the construction of expensive backbone networks over vast areas. On the other hand, because of the fundamental differences between the requirements for voice dispatch, wireless telephone, paging, and two-way data services that were discussed earlier, there are inevitable engineering compromises that arise in creating a "one size fits all" network. This may lead to a network that does not provide any one of the multiple services as effectively or efficiently as a standalone network dedicated to the provision of a single service. In such situations, it may be more effective to provide the desired integration within the end user equipment allowing the construction of separate networks to capture the economies of specialization associated with standalone networks customized to provide a single or more limited set of services.
Examples of both types of integration were provided earlier. Modern trunked radio systems have been designed to provide both dispatch and mobile telephone services on a common platform, and packet switching equipment has been added to cellular mobile radio services to render them more effective in providing two-way wireless data services. Modern cellular/PCS systems often combine mobile telephone service with a "short message service" which is similar to one-way, alphanumeric paging. These are examples of the service integration being provided on a common network. Combining an unlicensed cordless telephone with a cellular or PCS wireless telephone in a single handset is an example of providing integrated services on separate networks with the integration occurring in the handset. Combining a cellular mobile telephone unit with a satellite mobile telephone unit in a single mobile unit is another example. Given marketplace demands, the advances in enabling technologies described earlier and the increased competitive pressures associated with new entrants into the wireless market, it is likely that these trends toward integrated service provision will continue in the medium term.
In basic terms, functionality reflects how well equipments or systems are designed or adapted to suit a particular function or use. From a wireless end user perspective, improvements in functionality can arise from two fundamental sources: improvements in the network and improvements in end user equipment. Other than the improved functionality associated with the greater breadth and depth of service coverage of wireless systems already described, most increases in wireless network functionality can be attributed to the increased incorporation of computer processing power with the network itself. As pointed out earlier, this increase in network based intelligence, coupled with more powerful signaling techniques, can be used, for example, to allow customers to dynamically reconfigure networks, thus allowing the creation of software defined networks. It is this same general concept that allows implementation of the personal number calling and enhanced levels of personal mobility -- all examples of network based improvements in functionality. With increased competition generated by new entrants in the wireless telecommunications marketplace, it seems safe to predict that over the medium term competitors will continue to use network based intelligence to create customized services in order to differentiate their services from those of their competitors.
Improvements in battery technology and higher levels of integration in digital integrated circuits have allowed cellular subscriber equipment to shrink in size to the point where units that will slip into a shirt pocket are now available. Increased digital processing power in the end user equipment (coupled with improved signaling techniques between the network and that equipment) has allowed end user equipment, for example, to assist the network in making handoff decisions in mobile telephone systems, thus improving the performance or functionality as seen by the end user. The technical (and economic) feasibility of allowing more processing power to be placed in the end user equipment facilitates improved functionality. Examples include the storage of frequently called numbers in cellular handsets and, more recently, the storage of voice messages in paging receivers. In essence, this means that the end user equipment can not only improve the basic performance of the network, but also be customized to meet the specific needs (e.g., working habits) of the customer. Improvements in display technology (e.g., in liquid crystal displays) allow numerical, text, and image messages to be offered visually. The same display technology can also be used to help the end user to "reprogram" the device to fit his or her needs. With some exceptions largely associated with proprietary systems, there are many more suppliers of wireless end user equipment than wireless infrastructure equipment and, accordingly, the market is highly competitive. While there are obviously some practical limits to how small subscriber units can be made and still be useful in certain applications, this intense competition coupled with the continued technological advances in digital integrated circuits, for example, means that the trends in improved functionality in end user equipment are apt to continue in the medium term.
Developments such as source coding and greater frequency reuse that lead to improved spectrum efficiency in wireless systems were discussed earlier. These developments, coupled with significant reallocations of spectrum for terrestrial- and satellite-based land mobile radio systems as a result of regulatory actions, suggest that spectrum scarcity may not seriously impede the further development of land mobile radio services (broadly defined) over the medium term.
2. Evaluation of the Medium Term Trends
With some exceptions noted later, the medium term trends in wireless telecommunications technology and services would appear to bode well for not only the general public, but disabled Americans as well. Like all consumers, Americans with disabilities will benefit from the improvements in the breadth and depth of service coverage that are being driven by competitive pressures and by technological advances ranging from advanced cordless telephone systems to the latest geostationary and low earth orbiting mobile satellite systems. Perhaps the most favorable trend for the disabled is the movement toward the provision of voice (both dispatch and mobile telephone), data, and image services on an integrated basis through integration in the handset, integration in the network, or both. This is a very favorable trend because a key concept in providing access to information technologies to persons with disabilities is to deliver the information in the most accessible form -- e.g., in text or image form for the deaf and in auditory or tacile form for the blind.
Moreover, because the wireless telecommunications networks themselves are becoming increasingly intelligent and programmable, it is increasingly possible for the network itself to respond to any special needs of the disabled subscriber. For example, because the network is, in effect, becoming increasingly programmable, it can (a) recognize from the address and/or service profile of the customer being called that the information should be delivered in a particular form and (b) change or translate the information (say from text to voice) in order to be able to deliver it in the most accessible form. It is important to note that this ability for the network to, first, recognize that the consumer's desire to have information in a particular format, to, second, make the translation in format if necessary and to, third, deliver the information in that format may have high value even for customers who are not disabled. For example, a sighted person driving to work may prefer to have waiting E-mail messages (or at least the "from" and "subject" lines ) delivered by voice to his or her cellular phone or pager, but, once at work, prefer to have them delivered to his or her personal computer connected to the office local area network.
The above illustrates an important principle long recognized by advocates of universal accessibility; namely, making networks or other facilities more accessible to the people with disabilities makes them more accessible for everyone. And, even better, the advancing technology and falling costs trends for wireless telecommunications networks and services reported herein mean that the "win-win" situation associated with universal accessibility can be more readily achieved now and in the future.
While the medium term trends in wireless telecommunications technology and services appear to bode well for not only the general public, but for disabled Americans as well, there are concerns in terms of universal access. Four particular concerns will be discussed in the following paragraphs:
First, in order to gain the benefits of more intelligent networks and terminals, they must be programmed or customized often by the subscribers themselves. Simple examples include instructing the network to forward incoming calls to a particular number during a certain part of the day or reprogramming an alphanumeric pager to deliver notification of an incoming text message by vibrating rather then beeping. In order to make these programmable features and functions accessible to the disabled, good design practices must be followed. For example, input-output devices that require good eye-hand coordination should be provided with alternative or supplemental modes which do not require such coordination. Note, once again, that reducing the eye-hand coordination required increases accessibility for everyone.
Second, the rapidly decreasing size of end user equipment (e.g., cellular telephones and pagers) may create accessibility problems for certain users. In other words, the competitive race to produce even smaller and lighter end user equipment inevitably puts pressure on the designer to decrease the size of keyboards, visual displays, and other built-in input-output devices. On a related point, the size, emphasis on battery life, and overall design of end user equipment such as cellular telephones can also create obstacles for hard of hearing or deaf people. These obstacles include difficulties in successfully coupling their hearing aids to the subscriber unit or in utilizing acoustic couplers to connect to their text telephones. In the case of text telephones, the strong emphasis on small size may also discourage manufacturers from providing built-in jacks to allow direct connection of such devices. The disabled, like all consumers, can benefit from the convenience associated with such decreases in size and weight of end user equipment, but if accessibility for, say, those with physical impairments cannot be provided directly as a result, then provisions should be made for the use of supplementary devices to accomplish the same purpose.
Third, one clear trend in both wired and wireless networks is a shift from analog to digital transmission. There are a host of benefits associated with this shift and it is these benefits that are driving the shift in both segments of the industry. In the wireless portion of the telecommunications industry, the trend has been coupled with digital voice compression. As explained in Section IV.A.3., the source coding used in voice compression systems uses computer processing power to remove redundancy from speech so that (a) fewer bits per second have to be transmitted to convey a voice signal and (b) the available bandwidth can be used more efficiently. Most operators of commercial wireless telephone systems have a strong incentive to employ digital voice compression because the lower bit rates translate into (a) more conversations in a given amount of spectrum -- i.e., more efficient use of the radio spectrum and greater capacity, and (b) more conversations per piece of radio equipment and/or radio site -- i.e., greater economies of scale. An unintended side-effect of employing voice compression is that it can cause problems for non-voice signals such as those emitted by fax machines, computer modems, and, especially important in the context of this report, text telephones. This is because the vocoders depend critically upon the input signal having the characteristics of the human voice.
There are a number of potential solutions to this problem, including techniques for bypassing the voice compression devices and thereby gaining direct access to the digital bit stream being transmitted or received. It is beyond the scope of this report to describe these techniques in detail, but it should be noted that such techniques are being developed to facilitate the high-speed transmission of fax and computer communications on a circuit switched basis as well. As engineers and their managers improve the data handling capabilities of their emerging digital networks, it is important -- indeed imperative -- that they also consider the needs of text telephone users.
Fourth, in the public policy arena, there is a strong and clear trend toward the FCC and other regulatory agencies becoming less involved in developing and promulgating standards in the wireless industry. This trend reflects, among other things, growing concerns about the time consuming nature of formal standards-making activities, the ability of the government -- no matter how well motivated or well intended -- to pick an appropriate standard in the face of rapid technology and market changes, and the tendency of standards to sometimes inhibit rather than promote technological advancements. This trend toward relying more upon private, voluntary standards and less upon governmentally mandated standards has a number of implications relating to the accessibility and usability of wireless telecommunications to people with disabilities. For example, instead of dealing with a single government agency in a formal proceeding addressing the establishment of a specific common standard, the disabled community will, most likely, have to participate in broader agency proceedings of more general applicability. At the same time, they will have to work with multiple private sector providers each using different technologies and proposing different solutions for improving accessibility and usability. This discussion illustrates that one unintended consequence of increased reliance on voluntary standards is that Americans with disabilities and the organizations that represent them will face a greater challenge in assuring such accessibility and usability even with the support provided by recent legislation.
Thus, while the medium term trends in wireless telecommunications technology and services generally bode well for disabled Americans, there are significant concerns that must be addressed if universal accessibility is to be achieved.
