personal communication network

Technologies on the Horizon 1 echnologies on the Horizon” introduces readers

of IEEE Communications Magazine to leading- “T edge applications of communications technologies. The column’s intent is to provide more than a tutorial-this regular feature aims at an informative discussion of system alternatives, technology limits, and issues critical to successful deployment of new communications capabilities.

tember focused on/Pechnology choices and approaches to Personal Communications Networks (PCNs)-networks that allow mass communication between users who are, in general, mobile. In that column, Ray Steele of the Universi- ty of Southampton, England, discussed current plans for PCNs in Great Britain and examined the relationship between widespread PCN availability and current cellular mobile telephony.

As noted in the introduction to September’s column, en- gineers who want a clear view of technologies close to the horizon are at a disadvantage, for objects close to the horizon are subject to a variety of optical distortions caused by unusual temperature distributions between viewer and ob- ject. These optical effects are not unlike the selective im- pressions that the nonspecialist may get from the heated rhetoric of trade newspapers or technical advertisements.

To make it easier for the nonspecialist reader to develop a clear perspective on current directions and key challenges (technical or other) for PCNs, “Technologies on the Hori- zon” this month provides another expert’s point of view. Don Cox, past recipient of the IEEE Morris E. Leeds Award and other professional society recognition for his research in radio and vehicular communications technology, expresses a personal view on the relationship between low- power digital cordless technology and some alternatives: analog cordless, digital mobile radio, and cellular mobile

The first “Techn logies on the Horizon” column in Sep-

radio. He emphasizes, in particular, PCN power requirements, circuit quality, level of network integration, and access to network intelligence (e.g., call forwarding, call transfer, and personal number calling).

Column readers are encouraged, of course, to compare Don Cox’s views with those expressed by Ray Steele in September, or with the ideas outlined in the references provided by either author. Both technologists, with many years’ experience in radio technology and PCNs, have at- tempted to frame key questions and issues in the current technology and policy debate for a wide audience with definite stakes in the outcome. Those stakes, in simple terms, are: Which users will benefit from new communication technologies, when can network deployment start, how fast will it proceed, and what will the costs be?

In the vocabulary of radio technology, those of you who read and think about Don Cox’s and Ray Steele’s views, or who take time to look into the references for either column, will be gaining the benefits of “macroscopic diversity.” By processing signals from multiple experts, you will be increasing the signal-to-noise ratio in your own understanding of personal communications.

The column editor thanks readers for their enthusiastic response to the September column and welcomes additional comments, suggestions, and other contributions that will advance the horizon of communication technologies. Corre- spondence should be sent to:

Howard L. Lemberg Bellcore 445 South Street Morristown, NJ 07962- 19 I O USA

Howard L. Lernberg

D. C. Cox nterest in “Personal Communica- tions’’ in the United States and I worldwide has increased to a level

unexpected a year ago. Recently, the term Personal Communication Net- work (PCN) has become a telecommuni- cation industry buzzword. Along with this increased interest has come consid- erable confusion as to what constitutes personal communications or a PCN. In many contexts, personal communications is taken to encompass a wider range of communications capabilities than those represented by current analog cellular mobile radio technology, or even by second generation digital cellular mobile radio technology. One view of an extended concept of personal communications is discussed in the following sections of this article.

Britain has licensed a number of wireless communications networks, all of which are aimed at providing some aspects of personal communications [I] . Britain has two high-power’ vehicular cellular mobile radio networks that will evolve to the second generation pan- European high-power digital technology that has been standardized by the Group Speciale Mobile (GSM). They have four operators of phone-point or telepoint service based on low-power2 CT-2 digital cordless telephone technology. In addition, Britain has licensed three con- sortia to build versions of PCNs based on the GSM digital mobile radio technology. It was this licensing of PCNs in Britain in 1989, and its very short implementation time schedule, that fueled the recent high interest in PCNs in the U.S.

and elsewhere. However, Steele, in a recent “Technologies on the Horizon” article [ 2 ] , noted that “Britain is not so much getting novel PCN systems ... but essentially a second GSM pan-European system.” He goes on to note “it may be a lost opportunity.” The U.S. should profit from this experience.

Other technologies are also providing or will provide some aspects of personal communications. These include the high-power vehicular cellular mobile radio systems in North America and

Herein. “high-power” refers to average transmitted power by user sets on the order of 1 w.

Zc‘Low-power7’ refers to average transmitted power by user sets on the order of I O rnW.

8 November I990 - IEEE Communications Magazine

Japan that will soon evolve to digital technology (note: the Telecommunica- tions Industry Association-TIA-and the Cellular TIA-CTIA-in North America are standardizing a Time- Division Multiple Access-TDMA- digital technology for the North Ameri- can Cellular Mobile Radio Industry, and a competing spread spectrum, i.e., Code-Division Multiple Access- CDMA-system, is also being explored); the low-power advanced analog cordless telephone technology integrated with Private Branch Exchanges (PBXs) and in use in Japan; and the low- power digital cordless telephone technology, DECT, being standardized in Europe for use with PBXs and telepoints. What is evident from looking at these technologies is that they are pro- ceeding down two completely separate evolutionary paths, high-power vehicular cellular mobile radio technology and low-power pedestrian cordless telephone technology. It should not be sur- prising that the high-power vehicular mobile telephone technology that has been evolving for about 45 years is more advanced than the low-power pedestrian cordless telephone technology that has been evolving for less than 15 years. The different equipment requirements for these two evolutionary paths have been articulated before [3-61, but they have recently been argued even more el- oquently by Steele [ 2 ] .

Although the North American effort on high-power second-generation digital cellular mobile radio started well after the GSM effort, it proceeded rapidly and may yet result in digital mobile radio equipment available within the same time frame. With one exception, interest in other personal communications approaches in the U.S. has lagged behind interest elsewhere. The exception is the pioneering applied research effort at Bellcore on the use of low- power digital radio to access the intelligent local exchange network and provide the low-power radio access needs of widespread personal portable communications [3-71. The Bellcore effort, on behalf of the regional telecommunications companies divested from AT&T, has been ongoing since 1984, and has advanced to preliminary generic requirements with the issue of FA-TSY- 001013 in March 1990. Since 1989, however, there have been many filings with the FCC in the U.S. for experimental licenses to try various aspects of personal communications ranging from CT-2 telepoint technologies imported from Britain to medium-power3 spread

3“Medium-power” refers to average transmitted power on the order of 0.1 W.

spectrum access technologies for widespread overlaid communications networks as proposed by Millicon/PCN America [8] [9]. Serious consideration of the aspects of personal communications has been made particularly timely by the issuance of a Notice Of Inquire (NOI) on personal communications services by the U.S. FCC (Docket No. 90- 3 14, released June 28, 1990). This NO1 provides the U.S. with an opportunity to profit from Britain’s earlier PCN activi- ties, as discussed by Steele [2]. The U.S. should aim toward an implementation of personal communications that includes appropriate separate radio systems that are optimized to provide voice and moderate-rate data services to low- power small shirt-pocket-size personal communicators operating on dedicated frequencies. These low-power radio access systems should be separate from the second-generation high-power digital vehicular-optimized cellular mobile radio systems now evolving.

A Personal Communications Vision

A vision of personal communications should embrace the integration of several communication concepts, approaches, or systems into one interconnected and interworking network. This integrated network should support several different tetherless communications devices optimized for their specific environments and should also include wireline communications. The vision of personal communications should include the ability for a person to initiate or receive calls (voice) or other information (data, fax, etc.) anywhere, at least anywhere within areas having reasonable population densities or along highways interconnecting such areas. Sparsely populated areas may eventual- ly be covered by highly specialized mobile satellite systems that should also be integrated into the overall interworking network. Note that this concept of an integrated interworking network does not imply that all subnetworks, systems, or elements of this overall network are owned or operated by the same business entity. However, the efficient interconnecting of subnetworks will require standardization of interfaces and protocols. The personal communications vision should include tetherless access to the interworking network within large buildings, shopping malls, airports, automobiles, trains, and airplanes, as well as in residences. The integrated interworking network for personal communications should have sufficient capacity to meet the communications needs of everyone. It should be possible for a person to direct calls to either an-

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November I990 - IEEE Communications Magazine 11

other specified person via a personal number or a specified place via a place number.

Current wireline communication networks provide excellent, high- quality place-to-place calling capability. With intelligent network-based features such as call forwarding and call transfer, intelligent wireline networks are providing some relief from requiring a person to be reached at a particular place. Cel- lular mobile systems are providing the capability of communicating with vehicles over increasingly large areas, albeit often at a circuit quality somewhat less than that of wireline circuits. These mobile systems also provide limited capability for communicating with hand- carried or briefcase-contained portable cellular phones. Paging provides a means for alerting a person that some- one wants to communicate with him or her, and can provide limited one-way messages. Systems that provide limited capability to place voice calls from airplanes are evolving, albeit again with decreased voice circuit quality. Cordless telephones, when integrated with the “originate only” telepoint or phone point concept, as pioneered in Britain, are beginning to extend the convenience of low-power pedestrian-based wireless communications away from home. The integration of cordless telephones with PBXs is also extending this convenience to the business environment. The capabilities and limitations of these communications approaches are discussed in more detail in [3].

What is clear from worldwide activi- ty aimed at personal communications is that the following are needed: more than one wireless network-access technology, better integration of the intelligence among subnetworks or systems, and more attention to the quality of the communications circuits being provided by various wireless access technologies. Key missing ingredients are first, a tetherless access system that can provide wireline-quality voice and information communications to very small shirt- pocket-size lightweight (<0.5 lbs.) low- power personal communicators anywhere within areas of reasonable population densities, and perhaps from within trains, busses, and commercial airplanes; second, an intelligent network feature that can locate and route voice or data calls to a person via a personal number, whether that person can be best reached on a low-power personal communicator, a high-power vehicular cellular mobile phone, or a wireline telephone, if the called person wishes to receive the call (a screening provision must be provided in the network intelligence to permit the called person to route calls to another person or to voice or data storage if the person does not

want to receive that call at that time); and third, the integration of intelligent network features over all the network access technologies to implement the general vision of personal communications. As discussed by Steele [2] and throughout this article, a radio system to provide access to small shirt-pocket-size lightweight low-power tetherless personal communicators is very different from a radio system to provide access to high- speed, wide-ranging, high-power vehicular mobile sets.

Low-Power Tetherless Access for Personal Communications

This section discusses issues associ- ated with providing tetherless communications to low-power shirt-pocket-size personal communicators for voice and moderate-rate data.

Power Consumption Issues High-quality, economical communi-

cations to personal communicators could be provided by using low-power digital radio as an access technology for the intelligent local exchange network [3-71. Low-power exchange-access digital radio could be integrated with network intelligence to provide convenient widespread portability.

Personal portable radio sets, i.e., personal communicators, must be lightweight and small. To be lightweight requires small batteries. Long service time between battery recharging requires low power consumption. Also, conversely, high-power electronics require weight and bulk for heat sinks, etc., in addition to heavy batteries. This requires that a set be optimized for low power, not power-controlled for low power during some of its usage. If a set ever has to operate at a high average power output, it must have the heat sinks, shielding, filtering, and battery capacity to operate at the highest level that will ever be required; that is, it is not adequate for a small, economical personal communicator to use power control to reduce transmitted power to a few milliwatts in a set designed to operate at power levels of the order of 1 W. For example, portable sets made for vehicular cellular mobile systems may decrease transmitter power from the average level of I W to the milliwatt level, but low-power personal communicators should be designed for a maximum average transmitter output power of the order of I O mW, and be able to decrease power output down to sub-milliwatt levels. T h u e will remain a discrepancy in the power capabilities of about two orders of mag- nitude between portable sets designed

for use in systems optimized for vehicular users and low-power personal communicators designed for use in systems optimized for pedestrian or stationary users. As discussed in [3], separate high- power systems optimized for vehicles and low-power systems optimized for pedestrians will continue to coexist. Low-power personal portable sets are incompatible with high-power vehicular sets because of the difference in levels of cochannel interference produced. Be- cause of this, low-power portable sets in coexistence with high-power vehicular sets require separate frequency assign- ments to avoid serious interference problems. Separate frequency bands and high-powerllow-power system ar- chitectures are tantamount to two separate systems. These different needs have been recognized by an Interim Working Party (IWP 8/13) of Study Group 8 of the International Radio Consultative Committee (CCIR) [ 121, and such different needs are implied by the existence of FCC NO1 No. 90-314. A separate dedicated spectrum is needed for each system in order to ensure high-quality service to both groups of users.

As Steele pointed out [2], for a small low-power personal communicator, not only must the transmitter power be kept low, but the power consumption of the other circuits must also be minimized. Customized digital signal processing techniques implemented in Applica- tion-Specific Integrated Circui ts (ASICs) are the key to high-quality digital radio links. However, digital signal processing must be used sparingly to avoid excessive power consumption.

Power consumption in Complemen- tary Metal Oxide Semiconductor (CMOS) ASICs is proportional to the number of operations performed per unit time, and is thus proportional to computational complexity. Extensive manipulation of many high-precision numbers, particularly multiplications and matrix operations, consumes significant power. Functions that require complex digital processing are thus to be avoided [2]. Examples of such functions include computationally intensive fop ward error correction, bit-rate com- pressed speech, and multipath delay equalization. The goal of a radio link architecture should be to minimize portable-set power consumption [7]. A long-term goal should be for total portable-set average power consumption in the range of 100 to 200 mW or less when the set is actively providing a communications circuit, dropping below 5 mW when in standby between calls.

The high transmitter power needed in vehicular mobile systems reduces the incentive to minimize the power consumption of other system functions.

This factor, along with the strong desire to maximize the number of radio circuits provided at an expensive cell site within a given frequency bandwidth, has led to the inclusion of functions that require complex signal processing in the proposed second-generation vehicular digital cellular mobile technologies. Complex low-bit-rate speech coding (about 8 kb/s), and high-complexity error correction coding are included in the pan-European GSM TDMA system, in the North American CTIA TDMA system, and in the proposed spread- spectrum (CDMA) system. High-complexity delay equalization is included in the mobile radio TDMA systems, and multiple correlator processors are included in the CDMA system. These complex power-consuming techniques are not included in the advanced digital cordless telephone technologies, e.g., CT-2 or DECT, which have emphasized the minimizing of power consumption, or in low-power local exchange access technology proposed for personal communications [5] [7]. Low-power approaches achieve high system capacities by using more fixed radio sites that are relatively inexpensive and unobtrusive, rather than by using complex signal processing to increase the capacity of expensive cell sites while both reducing circuit quality and increasing portable set complexity. Thus, even when power- controlled to lower transmitter power levels, portable sets used in digital vehicular cellular mobile systems will con- sume considerably more power than the less complex personal communicators used in systems that are optimized to minimize their power consumption.

Circuit Quality Issues Another important issue regarding

tetherless access to local exchange networks and vehicular cellular mobile radio systems is the quality of speech and data circuits. Speech coding at rates below 32 kbls must make compromises among speech distortion, processing delay, and complexity (i.e., power consumption). Any improvement in one of these characteristics is achieved only by a significant sacrifice in the other two. Also, low-bit-rate speech coders general- ly do not transmit voiceband data well. While the best data access for personal communications ultimately will be via direct digital transmission (i.e., without a voiceband modem) of the data over the digital radio link, it is inevitable that voiceband modems will be used initial- ly. Thus, it appears desirable to start low-power radio access services with 32 kb/s speech coding over a flexible radio- link architecture [3] [5] [7]. A flexiblear- chitecture will permit straightforward evolution to lower bit rates when they

12 No\(ernber 1990 - IEEE Communications Magazine

become practical for this application, and when direct digital data access has evolved sufficiently to render voiceband modems unecessary.

Interleaving of bits over several tens of milliseconds and strong error correction codes are employed in all proposed second-generation digital cellular mobile systems. These techniques help reduce transmission errors in the rapidly fading radio channels caused by vehicular motion at highway speeds, but the techniques are significantly less effec- tive in the slowly varying channels that result from slow pedestrian motion. A voice-circuit-quality penalty, incurred from strong interleaving and computa- tion for error correction, is long transmission delays, of the order of many tens of milliseconds. A circuit with such delays on both ends, i.e., two mobile end users, could experience over half the overall delay of a synchronous satellite circuit. This delay will be perceived as a circuit-quality degradation by many users.

Radio has a reputation for providing poor-quality communication circuits in approaches like cordless and mobile telephones, but radio provides some poor circuits only because those radio systems have been statistically designed to include a significant number of poor- quality circuits. For example, the design criterion for cellular mobile coverage for vehicles, considering radio propagation, receiver sensitivity, and cochannel interference, has been to provide good or better service in 90% of a service area during the busy hour of an average business day [ I O ] . While this is significantly better than earlier mobile system designs based on median coverage con- tours, it does not provide the large number of high-quality circuits expected of wireline networks; that is, by design, 1 in I O radio circuits for vehicular users in a cellular mobile system will be judged to have a less-than-good circuit quality! Portable-set users with lower-power transmitters, who often try to use the sets inside buildings, experience a significantly larger percentage of less-than- good circuits [4]. Second-generation vehicular cellular mobile system designs have not aimed at increasing the percentage of good-quality circuits. A low- power exchange-access digital radio system for personal communications must be designed to provide circuit quality more nearly equivalent to that expected from wireline services. Good-or-better circuits over 99% or even more of a service area is a necessary design objec- tive.

Integration of Low-Power Personal Communica- tions with Other Systems

In the context of personal communications, it is sometimes stated that one portable personal communicator should function in all tetherless access environments, e.g., in the vehicular cellular mobile environment as well as in the pocket phone environment of low-power tetherless access. This is an expression of a desire for “one size fits all.” Howev- er, as in clothing, the requirement for a one-size-fits-all personal communicator results in a poor fit for most because of the very different requirements noted earlier. Where minimum size, weight, and power consumption are paramount for personal communicators, these are not as important for vehicular use. On the other hand, where a communicator- like handset may be all right for the oc- cupant of a chauffeured limousine or a passenger of an automobile, a hands- free capability built into the automobile is often more desirable for the driver. The ability to cover widely ranging vehicles over wide areas is a definite asset of the high-power radio technology used in cellular mobile radio systems optimized for vehicles [3].

Perhaps a more reasonable approach to providing ubiquitous personal communications is to continue to expand cellular mobile systems optimized for vehicular communication, and to de- ploy new tetherless access systems optimized to support low-power shirt- pocket-carried personal communicators. The user’s identity could be contained either in memory in the personal communicator set, or in a “smart card” inserted into the set. When entering an automobile, the small personal communicator or card could be inserted into a receptacle in a vehicular cellular mobile set installed in the a u t ~ m o b i l e . ~ The user’s identity would then be transferred to the cellular mobile The cellular mobile set could then initiate a data exchange with the cellular mobile system,

41n the near future, new automobiles will probably be sold with vehicular cellular mobile sets included as customer options, or even as standard equpment in upscale mod- els.

51nserting the small personal comrnunica- tor in the vehicular set would also facilitate charging the personal communicator’s battery.

indicating that the user could now receive calls at that mobile set. This information about the user’s location would then need to be exchanged between the cellular mobile network and the exchange network intelligence so that calls to the user could be correctly routed. Note that in this approach the radio sets are optimized for their specific environments, high-power vehicular or low- power pedestrian, and the network access and call routing are coordinated by the interworking of network intelligence. This approach does not compro- mise the design of either radio set or radio system, and places the burden on network intelligence technology, a technology that benefits from the enormous- ly large and rapid advances in computer technology.

The approach described above is consistent with what has actually hap- pened in other applications of technology in significantly different environments. For example, consider the case of audio cassette tape players. Pedestrians often carry and listen to small portable tape players with lightweight headsets, e.g., a Walkman.@ When one of these people enters an automobile, he or she often removes the tape from the Walkman and inserts it into a tape player installed in the automobile. The automobile player has speakers that fill the car with sound. The Walkman is optimized for a pedestrian, whereas the vehicular mounted player is optimized for an automobile. Both use the same tape, but they have separate tape heads, tape transports, audio preamps, etc. They do not attempt to share electronics. In this example, the tape cassette is the information-carrying entity similar to the user identification in the personal communications example discussed earlier. The main points are that the information is shared among different devices, but the devices are optimized for their environments and do not share electronics. Similarly, a high-power vehicular cel,lular mobile set does not need to share oscillators, synthesizers, signal processing, or even frequency bands or protocols with a low-power pocket-size personal communicator. Only the information identifying the user and where he or she can be reached needs to be shared among the intelligence elements, e.g., routing logic, databases, and common channel signaling [3] [5], of the infrastructure networks. This information

“Walkman is a Registered trademark of the Sony Corporation.

14 November 1990 - IEEE Communications Magazine

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exchange between network intelligence functions can be standardized and coordinated among infrastructure subnetworks owned and operated by different business entities, e.g., vehicular cellular mobile radio networks and intelligent local exchange networks. Such standardization and coordination are the same as are required to pass intelligence among local exchange networks and in- terexchange carrier networks.

Low-power personal portable communications could be pr vided to occu- pants of airplanes, trap’( and busses by installing compatible radio access ports inside these vehicles. The ports could be connected to high-power vehicular cellular mobile sets or to special air-ground mobile communications sets. Intelli- gence between the internal ports and mobile sets could interact with cellular mobile networks or air-ground networks in one direction and with personal communicators in the other direction to exchange user identification and route calls to and from users inside these large vehicles. Radio isolation between the low-power units inside the large metal vehicles and low-power systems outside the vehicles can be ensured by using windows that are opaque to the radio frequencies.

Network Infrastructure for Low-Power Tetherless Personal Communications Infrastructure Alternatives

Several approaches have been proposed for providing personal communications via low- or medium-power access to neu infrastructure networks that overbuild or overlay wireline local exchange networks. These proposals include: the British PCN initiatives that Steele [2 ] s t a t e s have become “essentially ... a GSM pan-European Sys- tem,” as discussed earlier, the Millicoml PCN America proposal [9] that includes medium-power spread-spectrum access technology, and suggestions by cellular mobile carriers that they could build such infrastructure networks for various access technologies. Steele [2] also suggested the use of optical or radio LANs interconnected with mobile switching centers.

An attractive alternative [3-71 is the use of low-power digital radio as an access technology to the ubiquitous intelligent local exchange network. This network is currently dominated by wireline access technology. This local exchange network approach would facilitate the providing of intelligent network services to radio access customers as personal communications services [3], and would

hasten the evolution of the personal communications vision discussed earlier. For example, call-forwarding techniques could be used to route calls to personal communicators anywhere. Call transfer could be automated to switch active calls from one radio port to another as the quality of radio circuits changes, because of either interference from newly set-up calls or user motion. Such call transfer is analogous to the call hand-off done by cellular mobile switching machines. Personal number calling service, which is being explored as an intelligent network service for wireline access customers, could provide a similar service for the low-power radio access customers. Personal number calling requires a routing look-up in a database similar to that needed for 800 service.

Using low-power digital radio access to existing local exchange networks to provide tetherless personal portable comunications as exchange network services has the following potential benefits:

Much of the infrastructure is already in place-copper and fiber distribution networks, digital central office switches, and intelligent network capabilities. This provides more rapid availability and deployment, since j t requires only the addition of radio ports, central-office-related electronics, and software modifications, and offers easy expansion within the area served by a modified central office.

*This approach is more economical when compared to other approaches, because it avoids overbuild and du- plication of feeder facilities and switching offices.6 Inexpensive out- door radio ports could be attached to utility poles on utility right-of- ways. It offers easy evolution to truly ubiquitous service compared with other approaches, encourages national and international standards that are required for ubiquitous service, and avoids isolated islands with incompatible protocols and modulation formats. Work toward industry standardization has started with the issuance, for industry comments [ 1 11, of the Bellcore FA-TSY-00 10 1 3. Ubiquity provides new service op- portunities, including airports, shopping centers, railroad stations, na- tionwide (worldwide?) use, and universal emergency call capability.

6The overbuild network arrangement of cellular mobile radio with separate Mobile Telephone Switching Offices (MTSOs) did not result from sound economic or technical considerations [ 151 [ 161, but was the result of an earlier legal requirement for complete structural separation of 850 MHz vehicular mobile radio systems.

Local exchange carriers are already obligated and organized to provide economical common carrier communication services to mass markets.

Can Cellular Radio Provide PCN?

As discussed throughout this paper, a vision of personal communications in- volves concepts and technologies in addition to those of existing and evolving vehicular cellular mobile radio. Howev- er, one sometimes hears the statement, “Cellular will do PCN.” This statement by itself is not sufficiently specific for discussion. However, three possible specific interpretations of the statement rephased as questions are discussed below.

Interpretation I: Can cellular mobile radio technology provide personal communications?

It should be clear, from the discussion by Steele [2] and throughout this paper, that both existing analog cellular mobile technology and planned next- generation digital cellular mobile technology are optimized for high-power vehicular applications. Neither is suited for small, low-power, shirt-pocket-size, long-usage-time personal communicators. Thus, the answer to Interpretation I is that high-power cellular mobile radio technology is well suited to providing the vehicular communications functions needed in a personal communications vision, but such high-power technology is not suitable for the widespread low-power tetherless access needed to support small shirt-pocket personal communicators.

Interpretation 11: Can the radio spectrum allocated for cellular mobile radio adequately provide personal communications?

Given the amount of spectrum currently available to cellular radio carriers to support both vehicular users and low- power pocket personal communicators, the,answer is “very unlikely.” The capacity of cellular mobile systems is stressed in several places today. A short time ago, cellular carriers were actively seeking more spectrum to enable them to serve their expanding customer base. In the near future when automobiles come factory-equipped with cellular phones, current spectrum will be even more severely stressed. There could eas- ily be 100 million cellular-mobile- equipped vehicles in the U.S. by early in the next century. An interim working party of CCIR Study Group 8 has recently recommended the need for 170 MHz for vehicular use and a separate 60 MHz for low-power tetherless access.

16 November 1990 - IEEE Communications Magazine

Faster Than FDDI! Fiber Optic It(ansmisslon Modules

The current suggestion of adequate cellular mobile spectrum is based on expected increases in the number of radio circuits that can be provided within a given bandwidth at a cell site by applying new digital technologies. Increases by factors of 3 to 20 over existing analog technology have been projected for various digital technologies. Even such large factors would not be adequate to meet the demand of both low-power personal communicators and vehicular mobiles. There will likely be more users of personal communicators than of vehicular sets, and personal communicator users are likely to present three to six times the demand of mobile users, i.e., closer to the 0.06 to 0.12 Erlangltelephone of wireline telephones than the 0.02 Erlanglmobile of vehicular users. As discussed earlier, the trade of large increases in cell-site and user-set complexity for cell-site circuits is only pertinent when considering large, expensive cell sites. This is not the best trade when small, inexpensive low-power radio ports attached to utility poles are considered. As also discussed earlier, a large portion of the projected increase has been obtained at the expense of voice circuit quality.

With good equipment design, analog mobile sets can provide circuit quality about equivalent to 24 to 32 kbls digital speech coding within the existing 30 kHz cellular mobile channel spacing. Thus, the digital technologies using 8 kbls coding achieve a factor of 3 to 4 increase in channels at the expense of voice quality7 (distortion and transmission delay) and complexity (power cosumption). In fact, at 8 kb/s coding rates, it is more usual to measure intelligibility of coders than speech quality, as is the practice at 16 kb/s and above. While intelligibility is an adequate measure for military communication and perhaps even for special mobile services, such as police, fire, and taxi dis- patch, it is not an adequate measure for mass-market public communications. This 8 kbls approach is not acceptable for low-power tetherless access for personal communicators, which will need to provide service with voice quality more like wireline telephones. The approach may even turn out to provide voice circuit quality that is not acceptable to vehicular users.

The highest cell-site capacity estimates (factors of 20) are also based on

7Voice quality comparable to 30 kHz FM could possibly be obtained with 16 kbk coding if high enough complexity, with its required high power consumption, were implemented. This may be satisfactory for high- power vehicular applications, but the power penalty is excessive for low-power personal communicators.

optimistic theoretical estimates of factors for highly complex spread-spectrum systems that incorporate multiple levels of power control, precise cell-site synchronization, and “soft handoff” [ 131. When it is noted that variations in power control with standard deviations of as little as 1 or 2 dB could result in capacity decreases of as much as 30% or more and that soft handoff imperfec- tions could result in similar decreases, the likelihood of realizing such high theoretical estimates is small. The lack of correlation between multipath fluctua- tions on uplinks and downlinks is likely to degrade the effectiveness of soft handoffs, and thus decrease their contri- bution to theoretical cell-site capacity.

Further capacity decrease will result from the interaction of power control with multipath power outside of the correlator time resolution cell that is used for power control. A member of the communications community once wrote [ 141, “The mystique of spread- spectrum communications is such that commercial enterprises, as well as aca- demia, are often attracted by the novelty and cleverness of the technique.” The mystique is even greater this time around as ever-increasing complexity is added to address the inherent high sus- ceptibility of spread spectrum systems to the near-far problem. Such high complexity significantly decreases the likelihood of achieving theoretical expecta- tions in real-world environments.

The large-scale complex multiple- feedback dynamic system also raises concern for overall system reliability and stability. In order to achieve significant cell site capacity, the system must be delicately balanced with tight feedback power control, and multiple cell site and vehicle set interaction (soft handoff). Precise synchronization of radio ports (cell sites) to better than 1 ps , as required for soft handoff in these spread spectrum systems, is a particularly difficult problem for the many thou- sands of radio ports connected to many widely separated interconnected central office switching machines that would be needed to serve the millions of users of low-power access systems. The margins and techniques needed to guarantee dynamic stability may further reduce the theoretically expected capacity. Where- as some modest capacity increase may be achieved for high-power vehicular users to relieve spectrum congestion without building additional expensive cell sites, such a complex, highly syn- chronized, dynamically balanced technology is not well suited for simple, inexpensive low-power personal communicators and tetherless access radio ports that must provide near-wireline- quality voice circuits.

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In both second-generation TDMA and spread-spectrum vehicular cellular mobile systems, an increase in the number of radio circuits within a given bandwidth at a cell site results from the use of directional antennas to divide the coverage area from a cell site into angu- lar sectors. However, small inconspicu- ous antennas on utility poles or street lights are required for low-power tetherless access ports to minimize the aesthetic environmental impact of the radio ports, particularly in residential areas. The large size of directional antennas needed for sectorization are gen- erally incompatible with the aesthetic environmental requirements of the small low-power access ports.

Thus, cell-site capacity increases projected as a result of sectorization are not appropriate for low-power tetherless access systems.

As noted before, for low-power tetherless access systems, high overall system capacity is better achieved by the use of many small, inexpensive radio ports attached to utilily poles to cover a region than by following the vehicular cellular mobile system approach of using a few highly complex, expensive cell sites. It should be noted that for a given amount of radio spectrum bandwidth, a factor of 4 increase in overall system capacity can be realized by decreasing the radio port spacing by half.

Even with technological advances, current cellular mobile radio spectrum is likely to be overstressed when only serving vehicular users whose traffic demand per user is relatively low (0.02 Erlanglmobile). Separate new dedicated spectra will be needed for a tetherless acces digital radio system to serve many tens of millions of low-power personal communicators whose traffic demand is likely to be significantly higher, like that of wireline telephone users (0.06 to 0.12 Erlangltelephone).

Interpretation 111: Can the current cellular mobile carriers use new low- power technology and new dedicated spectrum to provide personal communications?

The answer to this question depends on the outcome of regulatory proceed- ings. However, from the start, there is no more reason for the cellular carriers to provide services to low-power personal communicators than there is reason for other telecommunications carriers to provide such services. As discussed earlier, services to low-power personal communicators will require a high density of simple, inexpensive low-power tetherless access ports, the interconnec- tion of the ports through a dense distribution network of optical fiber andlor copper, and a large intelligent switching

network. Considering these needs, the intelligent local exchange carriers (i.e., telephone companies) is as well suited to providing the network infrastrucure [3] and intelligent network services as are the networks of the cellular mobile carriers. The local exchange network consists of dense distribution networks and many central office switching centers with a high penetration of network intelligence that provides intelligent network services to wireline access customers. In contrast, cellular mobile networks have very sparse distribution that connects

ily one or two Mobile Telephone

Switching Offices (MTSOs) in any metropolitan area to fewer than a hundred cell sites in most metropolitan areas. Perhaps 50% of the local exchange network will soon include the intelligence needed to support personal communications services to low-power personal communicators, but that is 50% ofa network that supports over 100 million wireline access lines in the U.S. This represents far more deployed network intelligence and distribution facilities than those contained in the sparse cellular mobile networks that currently support fewer than 5 million cellular mobile

Circle number 11

November 1990 - IEEE Communications Magazine 19

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sets in the U.S. As was noted earlier and in [3], intelligent network services provided to wireline access customers are equivalent to the services needed to provide personal communications to low- power personal communicators. Thus, in answer to Interpretation 111, local exchange carriers are as likely candidates as the cellular carriers.

Discussion The issues of circuit quality, system

capacity, complexity (power consumption), spectrum utilization, and system economics are very complex. They are made even more complex when very different applications are considered, e.g., low-power cordless telephones, low- power tetherless access for personal communicators, and high-power vehicular cellular mobile radio. Many at- tempts to address these issues suppress important considerations, e.g., circuit quality and complexity, and oversimpli- fy the comparison to only one figure of merit, e.g., cell-site capacity for a given amount of spectrum. One figure of merit does not adequately represent the many dimensions of the complex issues. Ex- amples of complex capacity issues are:

Example I . U.S. vehicular cellular mobile radio systems use large, complex, expensive cell sites with tall towers on expensive land. The over- riding next-generation digital thrust of operators of such systems is to maximize the number of voice circuits per cell site within their allocated spectrum. This thrust is further encouraged by the need to serve high- speed users moving over wide re- gions on streets and highways. Be- cause these systems are primarily optimized for vehicular users, and vehicles have a generous power source and adequate space for equipment, there is little concern for the complexity of the radio technology. Thus, trades for increased cell-site capacity are readily made for increased complexity and decreased circuit quality, even though larger system capacity, i.e., numbers of users per area in the same spectrum, could be accommodated with less complex technology and higher quality by decreasing the spacing between expensive cell sites. (A factor of 4 increase in system capacity results from decreasing the cell site spacing by half.) Example I I . At the other extreme is the cordless telephone. In this case, each base unit serves only one or two handsets, and trunking efficiency is very low. Since the handsets are pedestrian-carried and long usage time is desired away from a battery charger, power consumption is at a premium. This constraint, coupled with handset and base-unit economics, dictates IOW complexity in the technology. Its use with wireline networks makes voice circuit quality a

significant concern. System capacity in thjs approach is obtained by a high density of simple, very inexpensive base units. In next-generation digital approaches like CT-2 and DECT, the base units attempt to minimize interference by autonomous interference monitoring and avoidance, The economic consequence of low-base unit utilization is minimized because they are inexpensive, and the wireline access is also used for wireline telephones. One evolutionary step from cordless

telephone is telepoint, a step toward low-power tetherless-access personal communications. Consequences of this evolutionary step are increased expense for providing facilities for the individu- al telepoint base locations and increased densities of randomly located users in public places. Both of these consequences encourage the aggregating of base units into consolidated, multi-access base unit locations.

These examples can only suggest the complex nonlinear interactions among cell-site/base-unit locations; cost of electronics, real estate, and interconnecting network infrastructure; complexity/ power consumption; spectrum utilization/system capacity; and quality of service. Analytical optimization of all these many complex factors is intracta- ble.

Such optimization is like a chess game. In principle an exact solution ex- ists, but in practice complexity makes it impossible to obtain. Therefore, as in a chess game, the best optimizations result from applying the judgment ofa few highly experienced experts. Experts are considering the optimization of high- power vehicular digital cellular mobile ratio systems and low-power digital cordless telephone. However, as discussed throughout this article, the con- straints on complexity, power consumption, availability, voice and data quality, and radio-port and network infrastructure economics for widespread pedestrian-oriented low-power personal communications lie somewhere in between those for cellular mobile radio and cordless telephone. Therefore, it is logical to expect the optimized technology for such personal communications to lie between the technologies for those applications. Since the low-power personal communication application is pedestrian-oriented like cordless telephone, the optimum technologies for these low-power pedestrian applications can be expected to be closer to each other than to that for high-power vehicular cellular mobile radio.

Although progress has been made toward selecting a technology for widespread low-power tetherless-access personal communications [3-71 [ 111, it is now time to further refine the technolo-

(Continued on page 92) Circle number 10

20 Nwember 1990 - IEEE Communications Magazine

the analysis of some real networks. A model is presented where the asyn- chronous overrun problem is also taken into account. Furthermore, the paper shows the influence of the introduction of multiple priority classes for the non- real-time traffic on the total throughput of this type of message. Finally, it is shown that the difference between the values obtained under worst case assumptions are close to those obtained under best case assumptions; therefore, the method presented in this paper may be used to provide important guidelines so as to properly tune timed token pro- tocol parameters for each specific network installation.

"AnalysisISynthesis Techniques for Subband Image Coding," M. J. T. Smith and S. L. Eddins, IEEE Trans. on Acous- tics, Speech, and Signal Processing, vol. 38, no. 8, Aug. 1990.

Analysis/synthesis systems based on conventional FIR Quadrature Mirror Filters (QMFs) form the basis for most of the subband image coding systems re- ported in the literature. This paper ex- amines analysis/synthesis systems designed for low-bit-rate image coding, their impact on overall system quality, and their computational complexity. In addition, the theory, design, and implementation of both recursive and

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nonrecursive filtering systems are discussed. Methods are introduced that display advantages over conventional QMF-based approaches.

"Feedforward Transparent Tone-In- Band: Its Implementations and Applica- tions," A. Bateman, IEEE Trans. on Ve- hicular Tech., vol. 39, no. 3, Aug. 1990.

Implementation of the Transparent Tone-In-Band (TTIB) spectral manipulation technique is described, with particular attention given to the mecha- nism of feedforward subband recombin- ation. It is shown that transparency of TTIB can be maintained for a wide variety of input signal types, and extremely rapid and accurate subband recombina- tion can be achieved by correct choice of filter shape. Applications of TTIB are discussed, ranging from in-band signaling on public telephone lines through to dc offset compensation for direct con- version transceiver.

Solution to Puzzle No. 97

"The first nuclear age is drawing to a close, along with the cold war that it grew up with ... Yet, nuclear weapons will be around for a long time. One reason is that they can- not be uninvented. More countries can now make nuclear bombs than advanced micro- processor chips."

"The Nuclear Age," The Economist (3/10/90)

A. Tote B. How now brown cow C. Erbium doped fiber D. Neat E. Evanescent F. Wave G. Nerd H. Unit

I. Coherent optics J. Lanthanate

K. Earn L. Awes

M. Relish N. Ache 0. Ghoul P. Error Q. Thaw R. Hasn't S. Ease T. Ear U. Cyclic redundancy check v. ou t s

W. Nats X. Out of band signalling Y. Multiprogramming Z. Iota a. Slim protocols b. Twain

(Continued,fiorn page 20)

gy for that application, an application that is significantly different from the applications that have been previously considered; to allocate dedicated radio spectrum for that application; and to in- tegrate the network intelligence functions of vehicular cellular mobile radio networks, local exchange networks with low-power tetherless digital radio access and wireline access, and other emerging access networks to provide overall personal communications to everyone. As suggested by Steele and discussed herein, one size will not fit all.

References Special Issue on Mobile Communications, British Telecom Tech. J., vol. 8. no. 1, Jan. 1990. R. Steele, 'Deploying Personal Communi- cation Networks,- IEEE Commun. Mag., pp. 12-15, Sept. 1990. D. C. Cox, "Portable Digital Radio Communications-An Approach to Tetherless Access," IEEE Commun. Mag.,

D. C. Cox, H. W. Arnold, and P. T. Porter, "Universal Digital Portable Communica- tions-A System Perspective," IEEE J. on Se/. Areas in Cornmun.. vol. JSAC-5, pp. 764-773. June 1987. D. C. Cox, "Universal Digital Portable Radio Communications," Proc. of the IEEE. vol. 75. pp. 436-477, Apr. 1987. D. C. Cox, "Universal Portable Radio Com- munications," Proc. of the Nat'l. Commun. Forum, NCF '84, Chicago, IL, pp. 169- 174. Sept. 24-26, 1984. D. C. Cox, "A Radio System Proposal for Widespread Low-Power Tetherless Com- munications," IEEE Trans. on Commun., 1990. "FCC Approves Experimental Microcell Trial in DC,- Micro Cell News, Apr. 1, 1990. K. Bradsher, "A Phone in Your Pocket? Tryout Set for New Servce," New York Times, May 10, 1990. V. H. MacDonald, "The Cellular Con- cepts," BSTJ, vol. 58, pp. 15-42 (see p.29). Jan. 1979. FA-TSY-001013, issue 1, Bellcore. Mar. 1990. CClR Study Group 8 IWP 8/13, Report ME, July 14, 1989. A. Salmasi. "Talk on a Proposed CDMA Mobile Radio Systems,- CClR IWP 8/13 Meeting, Harrogate, England, July 4- 12, 1990. A. J. Viterbi, "When Not to Spread Spectrum-A Sequel." IEEE Commun. Mag., pp. 12-17, Apr. 1985. I. Dorros, "The New Future-Back to Technology." IEEE Commun. Mag., p. 59, Jan. 1987. R. Stoffels, "Cellular Arrives at Frozen North," T€&M, p. 68, July 15, 1987.

pp. 30-40, July 1989.

Biography Donald C. Cox is currently Division Manager

of Radio Research at Bellcore He has performed and directed research on mobile communications for more than 20 years and has been a pioneer in low-power personal portable radio systems Dr Cox is a Fellow of the IEEE and the American Asso- ciation for the Advancement of Science (AAAS), and has been Associate Editor of the IEEE Transac- tions on Antennas and Propagation

92 November I990 - IEEE Communications Magazine

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personal communication network