International Journal of Wireless Information Networks, Vol. 3, No. 3, 1996

Wireless Loops: What Are They? Donald C. Cox 1'2

Several loop applications of wireless technology are aimed at reducing the cost of deploying communications services ranging from telephone to wideband video. In these applications, wireless links replace a portion of a wireline loop from a central location (a central office or cable headend) to a subscriber. The replacement of labor-intensive wireline technology by complex mass-produced integrated electronics in wireless transceivers is projected to reduce the overall cost of the resulting loop. These wireless loop applications attempt to provide existing communications services or small modifications to existing communications services. A different interpretation of a wireless loop makes use of low-power digital radio technology to provide the last thousand feet or so of a loop. Low-power low-complexity wireless loop technology in small base units can be integrated with network intelligence to provide the fixed-infrastructure network needed to support economical personal communications services (PCS) to small, lightweight, low-power personal voice and/or data communicators. Low-complexity communicators can provide many hours of "talk time" or data transmission time and perhaps several days of standby time from small batteries ( < 1.5 oz). Because this application of wireless loop technology can reduce the inherent costs in several parts of a wireline loop, it has the potential to provide convenient widespread PCS at less costs than providing telephone services over conventional wireline loops. This low-power wireless loop application does not fit into any existing communications system paradigm. Wireless technology with tetherless access and wide-ranging mobility, e.g., the personal access communications system (PACS), does not fit the accumulated wisdom of the wireline telephony paradigm. It also does not fit the paradigm of existing cellular radio that has sparsely distributed expensive cell sites, and it is not targeted at fixed video services as is wireless cable. Because a significant change in thinking is required in addressing this new low-power low-complexity widespread wireless loop paradigm, its large economic advantages and service benefits have not yet been embraced by many of the existing communications providers, who appear to be more comfortable pursuing the better-known paradigms of video using wireless cable, or of cellular radio in the guise of high-tier PCS, or in the guise of rapid economical deployment of telephone services in developing nations. This paper discusses the inherent economic advantages and service benefits of low-power low-complexity wireless loop technology integrated with network intelligence aimed at providing economical low-tier PCS to everyone. KEY WORDS: PCS; PCN; wireless loops; low-tier; personal communications.

several different applications o f wireless technologies have been included under the umbrella o f wireless loops. These different applications, which may be provided by a local telephone c o m p a n y , a cable television c o m p a n y , or a competing local service provider, include

1. I N T R O D U C T I O N One o f the earliest references to wireless loops was in 1986 [1]; that was followed by Ref. 2. Since then t Department of Electrical Engineering, Stanford University, Stanford, California 94305-4055. -'Correspondence should be directed to Donald C. Cox, Department of Electrical Engineering, Durand 305, Stanford University, Stanford, California 94305-9515.

a. The use o f cellular radio technology to provide fixed telephone service rapidly in d e v e l o p i n g countries [3] 125 1068-960519610700-0125$09.50109 1996PlenumPublishingCorporaliun

126

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b. The use of specially designed point-to-multipoint radio technology to provide fixed telephone service to rural subscribers in sparsely populated regions [4] and to subscribers in urban and suburban areas c. The use of specially designed wideband pointto-multipoint microwave and millimeter-wave technology ("wireless cable") to distribute television and video services to fixed subscribers [5] d. The use of specially designed digital cordless telephones to provide wireless PBX and wireless CENTREX services to large businesses [6-8] e. Variations on the original proposal [1, 2] to use low-power digital radio for the last 1000 ft or so of a telephone or data loop [7, 9-17] After reviewing a "telephone" loop, we briefly discuss the first four wireless loop applications noted above, and then finish with a discussion of application (e) and its relationship to personal communications systems (PCS), both low-tier and high-tier [18-20].

2. A T E L E P H O N E " L O O P " A telephone loop, in general, provides the circuit from switching equipment to a subscriber's telephone, fax machine, or data terminal. In earlier times, a loop was usually a pair of copper wires or a single wire with an earth-ground return path. More recently, part of a loop circuit often is multiplexed with other circuits onto glass fiber, twisted-pair copper lines, coaxial cable, or radio links. Although loops to residential subscribers are similar in many aspects to those to business telephones, there are often some differences in details.

copper feeder cables or multiplexed onto digital lines, e.g., T1 lines, carried either by copper wire pairs or by optical fiber, which are sometimes further multiplexed to carry a number of Tls. Some distance from a CO, the feeder bundle is broken down into smaller bundles of circuits (loops) and further distributed in distribution cables which may still contain digitally multiplexed circuits on copper wire pairs or optical fibers. Along the distribution cables and/or at their ends, the individual circuits are separated into individual drops for the loop ends. In the past, loop ends have been carried on separate copper wire pairs to individual subscriber sets in houses or apartment buildings. More recently, loop ends of optical fiber or coaxial cable have begun to be installed for broadband services, 3 e.g., video. This loop architecture is depicted in Fig. 1. Several points about fixed loops are important to emphasize, even though they may appear obvious. We note first that almost every residence in the United States has at least one telephone loop terminating in it. A second point is that telephone service provided over these loops is considered economical. Providing this economical service is a financially successful business that others besides the "historical" telephone companies are trying to enter. Another important point is the distribution of costs for loops. The cost per subscriber or per loop in the highly shared feeder portion is the lowest since loop costs are distributed over many subscribers in the feeder cables. The cost per loop in the less shared distribution portion, although higher than in the feeders, is still significantly lower than the cost of the nonshared drops at the loop ends. Thus, the loop cost per subscriber tends to be dominated by the nonconcentrated dedicated loop ends, i.e., by the oftenreferred-to expensive "last mile" or "last kilometer." In low-population-density rural regions, the costly loop ends sometimes become "last tens of miles" or "tens of kilometers."

2.1. Residential Loops Typical residential subscriber loops start from central offices (COs) as bundles of circuits in many-pair

3Coaxial (coax) and hybrid fiber/coax cable television (CATV) systems also provide high-bandwidth "'drops" into residences.

Wireless Loops: What Are They? 2.2. Business Loops Loops to business telephones, fax machines, and data terminals for small dispersed businesses are similar to residential loops. However, such loops in high-subscriber-density larger businesses in large buildings often have very short loop ends and may terminate on a private branch exchange (PBX) switch in place of a CO. Although the details of such large business loops may appear quite different, their costs still tend to be dominated by nonshared loop ends. An additional factor in business loop expenses results from the frequent moving of personnel that occurs often in some businesses. The cost of rearranging circuits to accommodate moving or " c h u r n " can be significant when rearrangements are frequent.

3. A P P L I C A T I O N O F W I R E L E S S TECHNOLOGY TO WIRELESS LOOPS Several wireless technologies have been proposed or introduced as loop technologies in attempts to reduce costs in different population densities or to compete with existing or proposed fixed-loop technologies or service providers.

3.1. Cellular Radio Cellular radio technology is being applied as wireless loop technology, particularly in developing nations, e.g., Hungary, and sometimes in low-population-density rural areas. This application makes use of existing or slightly modified cellular mobile radio technology and derives significant economic benefit from the large and growing worldwide cellular radio market [19]. This application is somewhat " b l u r r y " as a loop application because installations simultaneously provide "fixed telephone" loops and mobile communications using the same base stations and interconnecting infrastructure. As a loop technology, cellular radio tends to replace the fixed distribution and drop, although in some cases wire loop ends may be installed from a fixed transceiver at the subscriber end of a wireless loop to a telephone handset. One might consider that "feeder" and some "distribution" exists as the "backhaul" from a mobile (telephone) switching office (MTSO or MSO) to base stations, and that the MTSO replaces a CO. Point-topoint microwave radio links often are used to connect base stations to MTSOs so an entire system can be installed without using any fixed cable. Some advantages of cellular in this loop application are that it can be deployed quickly over large regions without installing large

127 amounts of fixed cable and drop, it may be more economical than fixed cable if fixed installations would disturb many existing structures or if the density of subscribers is low, and it can provide desirable mobile services using the same facilities. In some places, copper is valuable as scrap, so copper cable may not remain in place after it has been installed. The use of elevated antennas with gain at fixed subscriber locations can provide extended range and can reduce channel impairments from multipath propagation. Of course, the need for handoff is small for fixed users, so handoff load on switching equipment is also greatly reduced. Some of the "cellular loop" installations use analog FM cellular technology. Cellular loop installations that use new digital cellular technology experience speech degradation from low-bit-rate speech encoding. The quality of lowbit-rate digital cellular speech is discussed in Section 4.1.

3.2. Point-to-Multipoint Rural Radio Several radio systems have been designed specially to provide fixed telephone services in low-populationdensity rural areas [4, 21-24]. Examples of these are an NEC system [21] installed in Australia [22] and the International Mobile Machines (IMM) Ultraphone [23, 24] system installed in a few places in the United States and worldwide. These systems use fixed elevated directional antennas at subscriber locations and do not provide "handoff" on active channels. Licensing requirements are often different from country to country, making a universal technology difficult. For example, the NEC system deployed in Australia does not fit within the channel bandwidth allocated for rural radio in the United States, i.e., Basic Exchange Telecommunications Radio Service (BETRS) [4]. Rural radio systems usually replace part of distribution and part of a very long drop. In rural areas, highly multiplexed feeders are seldom identifiable. Shared distribution cable or point-to-point microwave radio may run from a CO to base stations at different locations, or base stations may serve as repeaters between CO-located base stations and subscribers, At the subscriber end, a drop or several drops are run from the transceiver near the antenna to one or more subscriber sets. While the cost of providing rural loops can often be reduced by these rural radio systems, the costs are usually still significantly higher than loop costs in more densely populated urban and suburban areas. Also, since the numbers of such rural subscribers are generally small, these radio technologies have not benefited from the economics of scale that have benefited the cellular radio technologies. It would appear that adaptation of lower-cost

128 cellular technology could result in significantly lowercost wireless loops for telephony in low-population-density rural regions. However, higher-bit-rate speech encoding than is used in digital cellular technologies would be required to meet the speech quality needs of a wireless loop technology. The issue of speech quality for low-bit-rate digital cellular speech is discussed in Section 4.1. Special point-to-multipoint fixed wireless technologies also have been designed for use in urban and suburban areas, e.g. IONICA.

3.3. Point-to-Multipoint Video Distribution Point-to-multipoint wireless technologies, sometimes referred to as "wireless cable," are being proposed as an approach for providing interactive video services to homes [5, 25, 26]. This application, in contrast to many other voice-oriented wireless loop applications, requires large transmission bandwidths. Potential lower costs, compared with other technologies being used or proposed for video distribution to homes, are the motivation for this wireless application. As such, wireless point-to-multipoint video distribution will have to compete with existing coaxial cable and fiber/coax video distribution by CATV companies, with directbroadcast satellites, and with fiber and fiber/coax systems being installed or proposed by telephone companies and other entities [5]. Another competitor is proposed asymmetric digital subscriber line technology, which uses advanced digital signaI processing to provide high-bandwidth digital distribution over twisted copper wire pairs. Point-to-multipoint wideband wireless loop technology is aimed at replacing distribution and part of a drop in a subscriber loop. The equivalent of feeder cable will be required to connect central program origination, switching, and control to the equivalent of base stations. At the subscriber end, a short drop may be required from an elevated antenna to the TV set. In the United States two widely different frequency bands are being pursued for video distribution. These bands are at 28 GHz for local multipoint distribution systems or services (LMDS) [5, 25] and 2.5 to 2.7 GHz for microwave or metropolitan multipoint distribution systems (MMDS) [26]. The goal of low-cost video distribution is based on the low cost of point-to-multipoint line-of-sight wireless technology. However, significant challenges are presented by the inevitable blockage by trees, terrain, and houses, and by buildings in heavily built-up residential areas. Attenuation in rainstorms presents an additional problem at 28 GHz in some localities. Even at the 2.5-GHz MMDS frequencies, the large bandwidth required for distribution of many video chan-

Cox nels presents a challenge to provide adequate radio-link margin over obstructed paths. From mobile satellite investigations it is known that trees can often produce over 15 dB additional path attenuation [27]. Studies of blockage by buildings in cities have shown that it is difficult to have line-of-sight access to more than 60% of the buildings from a single base station [28]. Measurements in a region in Brooklyn, NY [29], suggest that access from a single base station can range from 25% to 85% for subscriber antenna heights of 10 to 35 ft and a base station height of about 290 ft. While less blockage by houses could be expected in residential areas, such numbers would suggest that greater than 90% access to houses could be difficult, even from multiple elevated locations, when mixes of one- and two-story houses, trees, and hills are present. In regions where tree cover is heavy, e.g., the eastern half of the United States, tree cover in many places will present a significant obstacle. Heavy rainfall is an additional problem at 28 GHz in some regions. In spite of these challenges, the lure of low-cost wireless loops for video distribution is attracting many participants, both service providers and equipment manufacturers.

3.4. Digital Cordless Telephones for Wireless PBX and CENTREX Digital cordless telephones for wireless PBX and CENTREX are aimed at loops in the business environment in contrast to the technologies discussed in the earlier sections that are aimed at residential applications. Digital cordless telephone replaces part of the distribution and the drop with a wireless link. Fixed distribution and sometimes feeder cable is used to connect distributed base stations to a PBX or to a CO for CENTREX. Economics is not the only motivation for these applications, unlike the residential technologies discussed earlier. While some savings in installation and when moving connections are expected from using wireless loops, a strong motivation for this business application is mobility of users. Perhaps the best-known digital cordless telephone technology is Digital European Cordless Telephone (DECT) [6, 8, 19, 20]. The cordless telephone, second generation (CT-2) [6, 19, 20], its enhanced version (CT-2+), Personal Handiphone System (PHS) [19, 20], and Personal Access Communications System (PACS) [18-20] or its compatial Unlicensed Band alternative (PACS-UB) [18] also can be used in this application.

3.5. Cordless Telephones While not often thought of as such, cordless telephones can be considered to be "loop end" technology.

Wireless Loops: What Are They? The wireless link from a cordless telephone base unit to a handset can be viewed as an "extension" to the end of a loop. Since the cordless base unit connects to an existing copper-pair loop end, no economic benefit is derived from this wireless application; i.e., the entire conventional drop or loop end is still required. Since cordless telephones cost more than "corded" telephones, this wireless loop technology costs more than the nonwireless "corded" alternative. Why then have cordless telephones enjoyed long-term and increasing popularity? In the United States alone, they continue to sell well over 10 million a year, and over 60 million are in use [30]. The obvious answer is the tetherless limited mobility that is provided by cordless access to the loop end. The tetherless, but very limited mobility, is worth the extra cost to many telephone users. A lesson learned during the early introduction of cordless telephones should be noted and remembered as we evolve wireless loop technology. That lesson is the need for speech quality as good as that provided by wireline telephones. Many early cordless telephones had speech quality that was not as good as wireline telephones. They were purchased by the millions because of the desire for the limited mobility they provided, but they also were discarded by the millions because of their poor quality. When the cordless industry started to produced high-quality cordless telephones, their sales and usage increased dramatically, and they became "necessities" where they had once been novelties. This hard-learned lesson should not be forgotten. However, the cellular radio industry is learning this lesson all over again [31], as it attempts to convert to digital systems that have poorer speech quality than existing analog FM systems. Even with large incentives, customers are reluctant to adopt these speech-inferior technologies. Cordless telephone users frequently complain about their limited range of mobility. They express desire at least to be able to carry them to visit neighbors and not miss expected calls. In the United States, the introduction of higher-power spread-spectrum cordless telephones using the unlicensed Industrial Scientific and Medical (ISM) band at 902 to 928 MHz has aimed at increasing the usable range between a cordless handset and base unit. Another interesting experiment in increasing the mobility of cordless telephones has been the CT-2 phonepoint originated in the United Kingdom [6, 20, 30]. This attempt to provide limited mobility with highspeech-quality wireless loop ends to public cordless base units failed in two attempts to introduce it in the United Kingdom. However, it succeeded in Singapore, Hong Kong, and other East Asian cities (there were over 160,000 subscribers to CT-2 phonepoint service in Hong

129 Kong [20] in mid-1994). It appears that increased, but still limited (incomplete coverage, no handoff, etc.), wireless access to loop ends is attractive only in some environments.

3.6. Discussion The wireless loop applications noted in Sections 3.1-3.3 are aimed at reducing the cost of providing services that conventionally are provided by fixed wireline loop technologies. The applications noted in Sections 3.4 and 3.5 are aimed at providing limited mobility by low-power, low-complexity, low-cost wireless access at loop ends. It seems reasonable, considering these two objectives, that it should be possible to tailor a lowpower low-complexity wireless loop technology that could provide widespread tetherless mobility at lower costs than equivalent fixed services using fixed wireline loop technologies. Such wireless loop technology is the subject of the next section.

4. L O W - P O W E R DIGITAL RADIO AS A UBIQUITOUS WIRELESS SUBSCRIBER LOOP The wireless loops proposed in Refs. 1 and 2 are somewhat different from the wireless applications discussed in the previous section. The application stated in Refs. 1, 2, and 9-12 is to provide wireless access by using low-power demand-assigned digital radio links for the last several hundred to a thousand feet or so of distribution loops in residential areas and large buildings. It is proposed that management of the radio links and of the mobility of wireless users be done using intelligence in the networks. The wireless technology and system proposed [7, 11, 15, 20, 32] for this wireless loop application are aimed at providing the benefits of cordless telephones without having their mobility limitations, and, in addition, at providing moderate-rate data, and solving the wireless privacy issue by encrypting the digital radio links. New encryption techniques compatible with low-complexity handsets have been evolved to provide the proposed privacy [33, 34]. The wireless loop technology is configured to be able to provide these benefits at costs that potentially are less than the costs of fixed wireline telephone costs.

4. I. Required Attributes of Low-Power Wireless Loops Required attributes of cordless telephones that are included in the wireless loop application of Refs. 1 and

130 2 are (a) economical subscriber sets and low network costs for providing services, (b) high speech quality, and (c) long battery life, both while active and in standby (waiting for a call). The long battery life is achievable in small, low-power, low-complexity, lightweight, pocket-carried communicators that have small lightweight batteries. With loop-end wireless technologies widely deployed and having handoff capability, service availability would be continuous and widespread over large regions, i.e., throughout urban and suburban areas, and places where people congregate, but perhaps not extending into rural areas having low population densities. 4 That is, mobile voice and data services could be provided over large contiguous regions. The low-power, low-complexity pocket communicators, both voice and data, are distinguishable from hand-held sets for use in cellular systems because they have much longer usage times ( > 6 h "talk time" and/or data transmission time and several days of standby time) while using significantly smaller batteries ( < 1.5 oz). It should be noted that no existing or proposed technology for use in cellular mobile systems (high-tier PCS [18-20]), either digital or analog, can achieve this type of hand-held pocket-set performance because of their high maximum transmitter power and, for the digital technologies, their very high signal-processing complexity. Also, no existing or proposed digital cellular technology can provide the high wireline speech quality of 32 kb/s ADPCM while operating in acoustically noisy environments. 5 The PACS wireless loop technology [7, 20, 32] has a wireless link format and protocols that will support speech transmission rates of 8 kb/s and 16 kb/s as well as 32 kb/s. However, the need for high speech quality and low complexity drives initial implementation to 32 kb/s. With advances in speech processing, lower rates may become acceptable. The poor speech quality of the digital cellular (hightier PCS) technologies [31] is a direct result of attempts to maximize cell site (base station) capacity. In maximizing this particular capacity, extremes of signal processing are incorporated for speech compression to minimize the transmitted bit-rate-per-voice-circuit and for reducing interference during times of speech inactivity. a However, see Section 5.3 where possible extension of this type of system into low-population-densityregions is discussed. 5Claims of "equivalent quality" have been made recently for new 13-kb/s coders for GSM and IS-95. However, one prevalent characteristic of digital cellular evolution has been overoptimistic claims that have not been met when reality sets in [7. 20]. Therefore. it would be prudent to be reluctant to embrace these new claims until more evidence is accumulated. Excessive transmission delay still is incurred in 13-kb/s systems.

Cox The resulting distortion in the 8-kb/s digital speech yields mean opinion scores (MOS) on the order of 3.2 in acoustically quiet environments compared to the MOS of 4.1 for 32-kb/s ADPCM, which is required for users to judge the speech good [7]. In acoustically noisy environments, i.e., in noisy rooms or vehicles or along noisy streets, the speech compression techniques convert noise into strange speechlike sounds that are subjectively annoying. Tandeming speech coding [7, 20] and errors in transmission further degrade the already poor quality speech. After years of touting "near toll quality" for their 8-kb/s speech, advocates of CDMA for cellular and high-tier PCS have recently changed to speech encoding at about 14 kb/s to attempt to make their speech quality acceptable! Of course, this change decreases the base station capacity directly by a factor of about 2/3, but this fact is seldom noted. While increasing the speech bit rate should improve quality, the new compression algorithms must be evaluated in "realworld" acoustically noisy environments with tandem encoding and transmission errors, in order to ensure their suitability. These compression techniques still require highly complex digital signal processing that will significantly reduce the "talking time" of battery-operated subscriber sets and they still incur excessive transmission delay. Another speech-quality-degrading feature of hightier PCS technologies is the large (near 1/4 s) round-trip transmission delay resulting from speech compression, bit interleaving, and other high-complexity signal processing used in both the T D M A and CDMA cellular technologies. With high-tier PCS technology on both ends of a voice circuit, the delay is about that of a satellite circuit! The impact of transmission delay on user acceptance of telephone circuits has been evaluated in the marketplace. Synchronous-orbit U.S. domestic satellites were used for "long-distance" circuits in the 1970s and 1980s. However, the unacceptably long transmission delay contributed significantly to the removal of long-distance circuits from these satellites. Delay also has been a significant contributor to the moving of intercontinental telephone circuits from satellites to undersea cable. The digital cellular technologies have accepted more speech impairments, distortion and delay, on one end of a communications circuit than were allocated in the past to the entire end-to-end wireline telephone network. Thus, since they leave a significantly negative balance for impairments in their supporting infrastructure networks, the high-tier PCS technologies are not suitable for wireless loop applications. In contrast, lowpower low-complexity wireless loop technology using

Wireless Loops: What Are They?

131

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32-kb/s speech coding can provide highly acceptable wireline-quality speech in small lightweight pocket communicators that provide many hours of "talk time" and data transmission time. These issues have been covered in more detail in Refs. 7, 15, 20, and 32. Providing continuous widespread coverage in urban and suburban areas and places where people congregate will cover most of the people in most countries, perhaps 80% or 90%, or so, in the United States. Moderate speed mobility also will cover most communications needs, since almost all people are in moderate- to low-speed mobility environments most of the time, e.g., within or around houses in residential areas (their own or neighbors'), within buildings (offices, industrial, commercial, shopping malls, airports, etc.), or as pedestrians along streets or within buildings. These wireless loop applications might leave highways between cities and other high-speed roads to be covered by cellular mobile systems. With over 100 million vehicles in the United States alone, providing premium service to vehicles will continue to provide an expanding market for vehicular cellular mobile systems. As discussed in Section 5.3, low-complexity wireless loop technology can be extended readily to provide coverage of many of these vehicular mobile (high-tier PCS [20]) environments. 4.2. Loop Evolution and Economics It is interesting to note that several of the wireless loop applications discussed in Section 3 are aimed at reducing cost by replacing parts of wireline or CATV loops with wireless links between transceivers. The economics of these applications are driven by the replacing of labor-intensive wireline and cable technologies with mass-produced solid-state electronics in transceivers. Figure 2 depicts the wireless loop application of Refs. 1 and 2. The economic impact of the evolution of this application from a residential wireline loop (Fig. 1) is the topic of this section.

Consider first a cordless telephone base unit as discussed in Section 3.5. The cordless base-unit transceiver usually serves one or, at most, two handsets at the end of one wireline loop. Now consider moving such a base unit back along the copper-wire-pair loop end a distance that can be reliably covered by a low-power wireless link [10, 11], i.e., several hundred to a thousand feet 6 or so, and mounting it on a utility pole or a street light pole. This replaces the copper loop end with the wireless link. Many additional copper loop ends to other subscribers will be contained within a circle around the pole having a maximum usable radius of this wireless link. Replace all of the copper loop ends within the circle with cordless base units on the same pole. Note that this process replaces the most expensive parts of these many loops, i.e., the many individual loop ends, with the wireless links from cordless handsets to "equivalent" cordless base units on a pole. Of course, being mounted outside will require somewhat stronger enclosures and means of powering the base units, but these additional costs are considerably more than offset by eliminating the many copper wire drops. It is instructive to consider how many subscribers could be collected at a pole containing base units. Consider, as an example, a coverage square of 1400 ft on a side (the PACS [18, 20] will provide good coverage over this range, i.e., for base unit pole separations of about 1400 ft, at 1.9 GHz). Within this square will be 45 houses for a 1 house/acre density typical of low-housing-density areas, or 180 houses for a 4 house/acre density more typical of high-density single-family housing areas. These represent significant concentration of traffic at a pole. Because of the trunking advantage of the significant number of subscribers concentrated at a pole, they can share a smaller number of base units, i.e., wireless base unit transceivers, than there are wireless subscriber sets. Therefore, the total cost compared with having a 6See also Section 5.30.

132 cordless base unit per subscriber also is reduced by the concentration of users. The technology proposed earlier for this wireless loop application [1, 2, 7, 9-18, 20] has been standardized by the U.S. Joint Technical Committee (JTC) for PCS as the low-tier PCS technology, PACS [18, 20]. A single PACS transceiver will support simultaneously eight TDMA channels or circuits at 32 kb/s (or 16 at 16 kb/s or 32 at 8 kb/s) [18, 20]. Of these, one channel is reserved for system control. The cost of such moderaterate transceivers is relatively insensitive to the number of channels supported; i.e., the cost of such an 8-channel (or 16 or 32) transceiver will be significantly less than twice the cost of a similar one-channel transceiver. Thus, another economic advantage accrues to this wireless loop approach from using time-multiplexed (TDMA) transceivers instead of single-channel-pertransceiver cordless telephone base units. For an offered traffic of 0.06 Erlang, a typical busyhour value for a wireline subscriber, a seven-channel transceiver could serve about 40 subscribers at 1% blocking, based on the Erlang B queuing discipline. From the earlier example, such a transceiver could serve most of the 45 houses within a 1400-ft square. Considering partial penetration, the transceiver capacity is more than adequate for the low-density housing. 7 Considering the high-density example of 4 houses/ acre, a seven-channel transceiver could serve only about 20% of the subscribers within a 1400-ft square. If the penetration became greater than about 20%, either additional transceivers, perhaps those of other service providers, or closer transceiver spacing would be required. Another advantageous economic factor for wireless loops results when considering time-multiplexed transmission in the fixed distribution facilities. For copper or fiber digital subscriber loop carrier (SLC), e.g., T1 or high-rate digital subscriber line (HDSL), a demultiplexing/multiplexing terminal and drop interface are required at the end of the time-multiplexed SLC line to provide the individual circuits for each subscriber loopend circuit, i.e., for each drop. The most expensive part of such an SLC terminating unit is the subscriber line cards that provide per-line interfaces for each subscriber drop. Terminating a T1 or HDSL line on a wireless loop

7The range could be extended by using higher base unit antennas, by using higher-gaindirectional (sectored) antennas, and/or by increasing the maximum power that can be transmitted. The topic of increasing range is taken up again in Section 5.3.

Cox transceiver eliminates all per-line interfaces, i.e., all line cards, the most expensive part of a SLC line termination. Thus, the greatly simplified SLC termination can be incorporated within a TDMA wireless loop transceiver, resulting in another cost savings over the conventional copper-wire-pair telephone loop end. The purpose of the previous discussions is not to give an exact system design or economic analysis, but to illustrate the inherent economic advantages of lowpower wireless loops over copper loop ends and over copper loop ends with cordless telephone base units. In an earlier published economic analysis using somewhat different parameters [35], Duet found that wireless loop ends were more economical than copper loop ends when subscribers used low-power wireless handsets. Another proposed configuration of this low-power wireless loop technology uses a fixed transceiver mounted on a house to provide a wireless loop end to a conventional fixed telephone [35, 36]. Because of the need for an additional transceiver for the loop ends at each subscriber house, Duet's analysis found that this fixed wireless loop configuration was less attractive economically than the configuration that used only wireless handsets. For the fixed-loop configuration, sometimes copper and sometimes wireless provided the best economics. It should be noted that the addition of a cordless telephone to a fixed wireless loop results in two extra transceivers compared to the configuration with a fromthe-pole direct to a cordless-handset wireless loop end. Other economic analyses since those of Duet have indicated similar results, i.e., economics of wireless loop with wireless handsets are favorable compared with wireline telephone, and wireless loops that only replace the copper wire drop are sometimes favorable and sometimes not, depending on the conditions considered. Rizzo and Sollenberger have also discussed the advantageous economics of PACS wireless loop technology in the context of low-tier PCS [18]. The discussions in this section can be briefly summarized as follows. Replacing copper wire telephone loop ends with low-complexity wireless loop technology like PACS can produce economic benefits in at least four ways. These are a. Replacing the most expensive part of a loop, the per-subscriber loop-end, with a wireless link b. Taking advantage of trunking in concentrating many wireless subscriber loops into a smaller number of wireless transceiver channels c. Reducing the cost of wireless transceivers by time multiplexing (TDMA) a few (7, 15, or 31)

Wireless Loops: What Are They? wireless loop circuits (channels) in each transceiver d. Eliminating per-line interface cards in digital subscriber line terminations by terminating timemultiplexed subscriber lines in the wireless loop transceivers In addition to the economic benefits of wireless loop access, the further highly desirable wireless-access communications features of both small-scale and largescale mobility [20] can be provided to the wireless loop subscribers by integrating the wireless loop access with intelligence in the infrastructure network [1, 2, 7, 9-18, 20]. This added mobility feature results in some additional complexity in network-control intelligence, but this complexity is in computer-like technology at centralized locations, and the cost of this technology is rapidly decreasing with the decreasing cost of very large scale integrated circuits (VLSI).

4.3. Relationship of Wireless Loops to Wireless Personal Communications From the discussions in the previous sections it should be obvious that the wireless loop application proposed in Refs. 1 and 2 is aimed at an architecture and control structure that can provide low-tier personal communications services [18, 20, 32] (PCS) economically. The integration of wireless access/fixed loop distribution/switched network intelligence can be optimized so that each part provides only the PCS functions for which it is best suited [10-17]. Like cordless phones, wireless links (loops) can provide flexible tetherless connections to fixed distribution. Wire or fiber can provide the dense widespread distribution for connecting wireless transceivers to network switching and intelligence. Intelligence in the network can provide switching, wireless loop control, and management of mobility at all scales, i.e., handoff for continuous small-scale mobility, and identification, registration, and routing for large-scale mobility [37]. This integrated low-tier PCS approach has been stadardized by the U.S. JTC as the PACS standard [18, 20, 32, 38]. The wireless technology for this wireless loop/low-tier PCS application is based on frequency-division duplex (FDD), time-division multipleaccess (TDMA), and autonomous quasi-fixed frequency assignment. It has been optimized to provide the userrequired attributes discussed in Section 4.1 while providing the significant economic advantages discussed in Section 4.2. The discussion in Section 4.2 is persuasive that the inherent cost of providing widespread low-tier

133 PCS using such wireless loop access integrated with network intelligence should ultimately be lower than the cost of providing wireline telephone. PACS technology is aimed at minimizing complexity and cost of both subscriber sets (personal communicators) and the supporting fixed-distribution network as discussed throughout this paper and in more detail in references cited. As other examples, frequency-division duplex does not require the synchronization of transmissions [39, 40] from base station transceivers as would the alternative of the time-division duplex used in the personal handiphone system (PHS) and digital European cordless telephone (DECT), two other technologies being considered for low-tier PCS [20]. Autonomous quasi-fixed frequency assignment, while not providing the potentially high base station capacity of dynamic channel assignment (DCA), does not incur the radio link architecture and control complexities 8 required to implement effective DCA [40, 41]. Low-power low-tier PCS technology can provide the needed system capacity by close spacing of many low-complexity low-cost base station transceivers interconnected by fixed distribution, i.e., the many economical wireless loops discussed in Section 4.2. It should be noted that the immense strength of frequency-reusing systems is their ability to provide almost unlimited capacity by reducing the separation between base stations. For example, considering the same base stationcircuit capacity, the total circuit capacity (or Erlang capacity) of a frequency-reuse system will be increased by a factor of 64 when reducing the base station Spacing from 2 miles to 1/4 mile! Such a tremendous increase in capacity cannot be obtained by any tweaking of technology, regardless of the complexity or sacrifice in speech quality that may be incorporated into the design compromises. These capacity issues are discussed in more detail in Ref. 20. It should be noted that in interconnecting a grid of base stations with fixed-distribution lines of copper or fiber, the required number of interconnecting lines scales directly with the base station spacing while the system capacity scales with the square of the spacing. This can be seen in Fig. 3, which depicts base station transceivers as Xs connected by copper or fiber distribution. Prototype PACS technology has been extensively tested in laboratories and in many wireless loop environments, including fixed copper replacement [36] and SHigh-speed handoffis readily supported by autonomousquasi-fixed frequency assignment, whereas high-speed handoffis more difficult to implementwith DCA.

Cox

134

distribution

fe~~er

Fig. 3. Interconnection of wireless loop base station transceivers arranged on a square grid using fixed copper or fiber distribution. The X's indicate base station locations.

low-tier PCS. Many early trials are noted and discussed in Ref. 7. Other successful trials using equipment made by Motorola and by Hughes Network Systems have been run by Southwestern Bell (SBC) and by US West. A highly successful trial using equipment from several manufacturers was run in Boulder, Colorado, in the autumn of 1995.

5. OTHER ISSUES Several issues that are sometimes raised regarding the wireless loop basis for low-tier PCS are briefly discussed in the following sections.

5.1. Cordless Telephone at Home and Cellular Away The use of special "cordless-telephone-like" base units at home for access by a cellular handset while a subscriber is at home has been suggested and used in several market trials and is in use in several areas of the United States. This approach makes use of existing cellular handset technology and cellular systems while away from home. Inherent, but unstated assumptions are that people will always have a wireline drop into their house, that wirelines access will always be more economical than wireless because of experiences with cellular service rates, and that people will always want a cellular handset as their portable away-from-home wireless phone. These assumptions are rooted in current communications paradigms and are not necessarily true.

Recently, my daughter-in-law, an avid pocket-cellular-phone user along with her husband, stated, to my surprise, that if their cellular pocket phones were a little more reliable they might not continue their regular telephone service. Upon some questioning, she noted that "not-reliable" meant that the batteries did not last very long in use and could not be counted on. They did not need a wireline telephone because they were away from home so much that the cellular handsets were more useful. One of my cousins recently asked me how soon she would be able to completely replace her telephone (wireline) with her pocket phone (cellular handset). Such sentiments bring into serious question the assumption of continuing wireline service to all homes, particularly in the light of widespread low-power, low-complexity, long-battery-life, economical low-tier PCS based on the wireless loop technology discussed in Section 4. The discussion in Sections 3.6, 4.2, and 4.3 is persuasive in arguing that correctly optimized wireless loop technology can provide wireless-access-based low-tier PCS that is as economical as wireline telephone. The JTC-standardized PACS technology has been optimized to provide such economical low-tier PCS [18]. The assumption regarding "always a cellular handset" and the related issue of "people only want one handset" is based on the fact that the only currently available widespread wireless coverage is via cellular systems that require high-power handsets. But cellular handsets cannot provide the long battery life, and new digital cellular technologies cannot provide the speech quality required to meet the needs of pocket-portablephone users. These handset issues are discussed throughout this paper and in more detail in Refs. 20 and 32. Thus, the cordless-base-unit-at-home and cellularaway approach is at best an interim solution that makes good use of existing communications paradigms, i.e., low-cost wireline access at home, and widespread but battery-inefficient cellular access away. With low-power low-complexity economical wireless loop technology supporting widespread low-tier PCS in the future, this interim solution will no longer be viable.

5.1. Broadband Fiber/Coax, Wireless Cable, and Low-Tier PCS Wireless Loops Another mechanism is sometimes suggested for implementing the alternative of cordless base station at home and cellular away discussed in the previous section. That mechanism is the use of a "set top box" on a TV set to provide the network access for the cordless base unit. While broadband access for video is a highly likely continuing or expanding service to homes and

Wireless Loops: What Are They? businesses, its use as a wireless access port to a network is less obvious, and is based on the similar assumptions of cellular and wireline access costs discussed in the previous section. The significant issue to raise is why carry the wireless access point into each house. The persuasive economic arguments in Section 4.2 also will be applicable to broadband networks for moving of the wireless access point from a base unit in a house to a pole-mounted (or building-side-mounted) multiplexed and shared lowcomplexity wireless access port. Thus, even though broadband "drops" will undoubtedly be brought into houses for video, they will not provide an economic advantage over the wireless loop access for low-tier PCS described throughout this paper. The set top box approach also does not provide wireless loop access economically for low-tier PCS throughout neighborhoods.

5.3. Coverage, Range, Speed, and Environments Interest has been expressed in having greater range for low-tier PCS technology for low-population-density areas. One should first note that the range of a wireless link is highly dependent on the amount of clutter or obstructions in the environment in which it is operated. For example, the radio link calculations that result in the 1400-ft base station (radio-port) separation at 1.9 GHz used in the Section 4.7 example contain over 50dB margin for shadowing from obstructions and multipath effects [11, 42]. Thus, in an environment without obstructions, e.g., along a highway, the base station separation can be increased at least by a factor of 4 to over a mile, i.e., 25 dB for an attenuation characteristic of d-4, while providing the same quality of service, without any changes to the base station or subscriber transceivers, and while still allowing over 25-dB margin for multipath and some shadowing. This remaining margin allows for operation of a handset inside an automobile. In such an unobstructed environment, multipath RMS delay spread [43, 44] will still be less than the 0.5 /zs in which PACS was designed to operate [15]. Operation at still greater range along unobstructed highways or at a range of a mile along more obstructed streets can be obtained in several ways. Additional link gain of 6 dB can be obtained by going from omnidirectional antennas at base stations to 90 ~ sectored antennas (four sectors). Another 6 dB can be obtained by raising base station antennas by a factor of 2 from 27 feet to 55 ft in height. This additional 12 dB will allow another factor of 2 increase in range to 2-mile base station separation along highways, or to about 3000-ft separation in residential areas. Even higher-gain and taller antennas could be used to concentrate coverage along high-

135 ways, particularly in rural areas. Of course, range could be further increased by increasing the poWer transmitted. As the range of the low-tier PACS technology is extended in cluttered areas by increasing link gain, increased RMS delay spread is likely to be encountered. This will require increasing complexity in receivers. A factor of 2 in tolerance of delay spread can be obtained by interference-canceling signal combining [45] from two antennas instead of the simpler selection diversity combining originally used in PACS. This will provide adequate delay-spread tolerance for most suburban environments [43, 44]. The PACS downlink contains synchronization words that could be used to train a conventional delayspread equalizer in subscriber set receivers. Constantmodulus (blind) equalization will provide greater tolerance to delay spread in base station receivers on the uplink [20, 46] than can be obtained by interferencecancellation combining from only two antennas. The use of more base-station antennas and receivers can also help mitigate uplink delay spread. Thus, with some added complexity, the low-tier PACS technology can work effectively in the RMS delay spreads expected in cluttered environments for base station separations of 2 miles or SO.

The guard time in the PACS TDMA uplink is adequate for 1-mile range, i.e., 2-mile separation between base station and subscriber transceivers. A separation of up to 3 miles between transceivers could be allowed if some statistical outage were accepted for the few times when adjacent uplink timeslots are occupied by subscribers at the extremes of range (near-far). With some added complexity in assigning timeslots, the assignment of subscribers at very different ranges to adjacent timeslots could be avoided, and the base station separation could be increased to several miles without incurring adjacent slot interference. A simple alternative in lowdensity (rural) areas, where lower capacity could be acceptable and greater range could be desirable, would be to use every other timeslot to ensure adequate guard time for range differences of many tens of miles. Of course, the capability of transmitter time advance could be added to PACS in order to increase the range of operation. Such time advance is applied in the cellular TDMA technologies. Minor changes in link start-up procedures for the low-tier technology would be required to synchronize initially the uplink transmitter timing advance. Changes to incorporate time advance into a "highertier" PACS standard would be minor compared to the time-division-duplex (TDD) and link architecture changes made for the alternate standard unlicensed PACS [18], i.e., PACS-UB.

136 The synchronization, carrier recovery and detection in the low-complexity PACS transceivers will perform well at highway speeds. The two-receiver diversity used in uplink transceivers also will perform well at highway speeds. The performance of the single-receiver selection diversity used in the low-complexity PACS downlink transceivers begins to deteriorate at speeds above about 30 mi/h. However, at any speed, the performance is always at least as good as that of a single transceiver without the low-complexity diversity. Also, fading in the relatively uncluttered environment of a highway is likely to have a less severe Ricean distribution, so diversity will be less needed for mitigating the fading. Of course, more complex two-receiver diversity could be added to downlink transceivers to provide twobranch diversity performance at highway speeds. It should be noted that the very short 2.5-ms TDMA frames incorporated into PACS to provide low transmission delay (for high speech quality) also make the technology significantly less sensitive to high-speed fading than the longer-frame-period cellular technologies. 9 The short frame also facilitates the rapid coordination needed to make reliable high-speed handoffs between base stations.~~ Measurements on radio links to potential handoff base stations can be made rapidly, i.e., a measurement on at least one radio link every 2.5 ms. Once a handoff decision is made, signaling exchanges every 2.5 ms ensure that the radio Iink handoff is completed quickly. In contrast, the long frame periods in the high-tier (cellular) technologies prolong the time it takes to complete a handoff.

5.4. People Only Want One Handset This issue is often cited to justify the continued use of cellular pocket phones. It is based on the assumption that low-power low-tier PCS based on wireless loops would not be widespread. However, the one wireless handset that people want is the low-tier PCS handset discussed throughout this paper. With widespread contiguous low-tier PCS coverage, it will be the low-tier 9of course, with the long frame periods, the extensive interleaving, and the high redundancyerror correction encoding, the cellular technologies provide some mitigation of high-speed fading through "time diversity." The high complexity and degradation in speech quality from the large transmission delay are high prices to pay to mitigate high-speed fading, and the time diversity is ineffective at moderate to slow speeds. "~High-speed handoffs between PACS base stations have been demonstrated in Maryland and Colorado by several different equipment manufacturers.

Cox handset that people carry. A detailed discussion of this issue is contained in Refs. 20 and 32 and will not be repeated herein.

5.5. Distributed Antennas Distributed antennas are often proposed as a way to increase coverage and reduce required transmitter power. However, proposed distributed antennas are often much more than passive antennas interconnected with passive cable and are actually distributed low-complexity base stations, not just distributed antennas. The high loss of interconnecting cable makes the use of true distributed passive antennas impractical. (Superconducting cable with lossless dielectric would be required for such antennas.) Thus, proposed distributed antennas always include amplifiers for coaxial cable interconnections and sometimes electro-optic modulators and demodulators to make use of low-loss glass-fiber cable. The term "distributed antennas" is a misnomer for such technology since power is required at each active antenna location. One of the most difficult problems associated with small low-cost transceiver sites on utility poles or sides of buildings is the supplying of power. Once power is required, it becomes necessary to consider the alternative of antenna interconnection at baseband, i.e., the wireless loops discussed in Section 4. The economics of distributed-antenna technology with amplifiers for wireless loops do not appear advantageous compared with well-designed baseband wireless loop technologies [18]. At the fixed transceivers, the only difference is a baseband-to-radio-frequency (RF) and RF-to-baseband translation that is of little economic or power-consuming consequence when implemented in low-power integrated circuit technology. There is no need for percircuit processing at low-complexity wireless loop transceivers that use baseband digital interconnection. Thus, in the absence of economic low-loss (room temperature superconducting) transmission technology, true distributed antennas for wireless loop low-tier PCS applications remain an unfulfilled dream.

6. S U M M A R Y AND C O N C L U S I O N S Several loop applications of wireless technology are aimed at reducing the cost of deploying communications services ranging from telephone to wideband video. In these applications, wireless links replace a portion of a

Wireless Loops: What Are They? wireline loop from a central location (a central office or cable headend) to a subscriber. The replacement of labor-intensive wireline technology by complex massproduced integrated electronics in wireless transceivers is projected to reduce the overall cost of the resulting loop. These wireless loop applications, discussed in the Introduction and Section 3, attempt to provide existing communications services, or small modifications to existing communications services. Low-power digital radio technology can be used to provide the last thousand feet or so of a loop. Low-power low-complexity wireless loop technology in small base units can be integrated with network intelligence, i.e., data bases, signaling, control processors and switching, to provide the fixed-infrastructure network needed to support economical personal communications services (PCS) to small, lightweight, low-power personal voice and/or data communicators. These low-complexity communicators can provide many hours of "talk time" or data transmission time, and perhaps several days of standby time from small batteries ( < 1.5 oz). Because this application of wireless loop technology can reduce the inherent costs in several parts of a wireline loop as discussed in this paper, it has the potential to provide convenient widespread PCS at costs that are less than the costs of providing telephone services over conventional wireline loops. However, this low-power wireless loop application does not fit into any existing communications system paradigm. Wireless technology with tetherless access and wide-ranging mobility, e.g., the PACS, does not fit the accumulated wisdom of the wireline telephony paradigm. It also does not fit the paradigm of existing cellular radio that has sparsely distributed expensive cell sites ($1 million each), and it is not targeted at fixed video services as is wireless cable. Because a significant change in thinking is required in addressing this new low-power low-complexity widespread wireless loop paradigm, its large economic advantages and service benefits have not yet been embraced by many of the existing communications providers who appear to be more comfortable pursuing the better-known paradigms of video using wireless cable, or of cellular radio in the guise of high-tier PCS, or in the guise of rapid economical deployment of telephone services in developing nations. As the inherent economic advantages and service benefits of low-power low-complexity wireless loop technology integrated with network intelligence become better understood, this technology will become the technology of choice for providing low-tier PCS to everyone.

137 ACKNOWLEDGMENT I thank Ferdo Ivanek for his helpful comments on the draft of this paper.

REFERENCES I. D. C. Cox, Research toward a Wireless Digital Loop, Bellcore Exchange, Vol. 2, pp. 2-7, 1986. 2. D. C. Cox, W. S. Gittord, and H. Sherry, Low-power digital radio as a ubiquitous subscriber loop, IEEE Communication Magazine, pp. 92-95, 1991. 3. G. I. Zysman, Wireless networks, ScientiJicAmerican, Vol. 273, No. 3, pp. 68-71, 1995. 4. S. H. Lin and R. S. Wolff, A radio bridge to remote customers, Bellcore Exchange, Vol. 5, pp. 32-36, 1989. 5. B. W. Phillips, Broadband in the local loop, Telecommunications, pp. 37-42, November 1994. 6. D.J. Goodman, Trends in cellular and cordless communications, IEEE Communications Magazine, pp. 31-40, June 1991. 7. D. C. Cox, Wireless network access for personal communications, IEEE Communications Magazine, pp. 96-115, December 1992. 8. P. Motte, Wireless access: DECT in the loop, Microwave Journal, pp. 107-110, July 1995. 9. D. C. Cox, Universal portable radio communications, IEEE Transactions on Vehicular Technology, pp. 117-12 I, 1985. 10. D. C. Cox, H. W. Arnold, and P. T. Porter, Universal digital portable communications--a system perspective, IEEE Journal Selected Areas in Ct)mmunications, Vol. JSAC-5, pp. 764-773, 1987. I1. D. C. Cox, Universal digital portable radio communications, Proceedings IEEE, Vol. 75, pp. 436-477, 1987. 12. D. C. Cox, Portable digital radio communications--an approach to tetherless access, IEEE Communications Magazine, pp. 3040, July 1989. 13. G. G. Brusch and C. H. Butler, "A More Personal Kind of Communications,'" Bellcore Erchange, Vol. 7, pp. 18-23, 1991. 14. N. R. Sollenberger and A. Afrashteh, A remote port architecture for a portable TDM/TDMA radio communications system, Fourth Nordic Seminar on Digital Mobile Radio Communications, Oslo, Norway, paper 12.3, June 26-28, 1990. 15. D. C. Cox, A radio system proposal for widespread low-power tetherless communications, IEEE Transactions on Communications, Feb. 1991. 16. R. R. Goldberg and G. G. Brush, Getting ready for PCS, Telephony, pp. 24-26, Feb. 3, 1992. 17. H. W. Arnold et al., Wireless access techniques and fixed facilities architecture, Wireless '92, Calgary, Canada, July 8-10, 1992. 18. J. F. Rizzo and N. R. Sollenberger, Multitier wireless access, IEEE Personal Communications Magazine, pp. 18-30, June 1995. 19. IEEE Communications Magazine, Special Issue on Wireless Personal Communications, Vol. 33, January 1995. 20. D. C. Cox, Wireless personal communications: what is it?, IEEE Personal Communications Magazine, Vol. 2, pp. 20-35, April 1995. 21. T. Hiyama et al., Digital radio concentrator system (DRCS), NEC Research and Developmel,t, No. 76, pp. 24-35, January 1985. 22. V. Sargeant and J. Steele, Planning telephone networks for rural and remote areas using digital radio concentrator systems, Proc. I. E. Australian Engineering Conference, pp. 138-145, Canbena, Australia, 1981.

138 23. R, G. Saunders, Ultraphone-wireless digital loop carder system, Proceedings of National Communications Forum, Vol. 42, No. pp. 1860-1866. 24. C. E. Jones, Digital radio passes test in local loop, Telephone Engineer & Management (TE & M), pp. 85-87, May 15, 1989. 25. E. Keible, Broadband wireless systems, Stanford University Center for Telecommunications, Symposium on The Acceleration of the Wireless World, May 16, 1995, Stanford, CA. 26. C. Weseloh, Solutions for wireless communications, Stanford University Center for Telecommunications, Symposium, May 16, 1995, Stanford, CA. 27. J. Goldhirsh and W. Vogel, Mobile satellite system fade and statistics for shadowing and multipath from roadside trees at UHF and L-band, IEEE Transactions on Antennas and Propagation, Vol. 37, April 1989. 28. A. Ranade, Local access radio interference due to building reflections, IEEE Transactions on Connnunications, pp. 70-74, January 1989. 29. S. Y. Seidel and H. W. Arnold, Propagation measurements of 28 GHz to investigate the performance of local multipoint distribution services (LMDS), IEEE Globcom '95, 1995. 30. J. E. Padgett, T. Hattori, and C. Gunther, Overview of wireless personal communications, IEEE Communications Magazine, pp. 28-41, January 1995. 31. E. L. Andrews, When digital doesn't always mean clear, New York Times, p. C1, 4, June 26, 1995. 32. J. D. Gibson (ed.), The Mobile Communications Handbook, CRC Press and IEEE Press, Boca Raton, FL, pp. 209-241, 1996. 33. M. J. Belier, L. F. Chang, and Y. Yacobi, Privacy and authentication on a portable communications system, IEEE Journal Selected Areas Communications, Special Issue on Wireless Personal Communications--Part I, August 1993. 34. M. J. Belier and Y. Yacobi, Fully-fledged two-way public key authentication and key agreement for low-cost terminals," Electronics Letters, Vol. 29, No. 11, pp. 999-1001, May 27, 1993. 35. D. A. Duet, An investigation into the economic impact of lowpower digital radio in the telephone distribution plant," IEEE Globecom '89, Proceedings, pp. I, 377-I, 381, Dallas, TX, November 27-30, 1989. 36. A. J. Dagen et al., NYNEX Science Technology, Inc., Wireless loop telephone termination, Applied Microwaves and Wireless, pp. 46-62, 1994. 37. M.J. Beller, Call delivery to portable telephones away from home using the local exchange network," IEEE ICC'91, Denver, CO, June 1991. 38. C. I. Cook, Development of the air interface standards for PCS, IEEE Personal Communications Magazine, Fourth Quarter, 1994, pp. 30-34. 39. J. C-I. Chuang, Performance limitations of TDD wireless personal communications with asynchronous radio ports, IEE Electronics Letters, March 1992. 40. J. C-I. Chuang, N. R. Sollenberger, and D. C. Cox, A pilot based dynamic channel assignment scheme for wireless access TDMA/FDMA systems, International Journal of Wireless Information Networks (JWIN), Vol. 1, Issue 1, pp. 37-48, 1994. 41. J. C-I. Chuang, Performance issues and algorithms for dynamic channel assignment, IEEE GLOBECOM "92, Orlando, FL, Dec. 6-9, 1992. 42. D. M. J. Devasirvatham, R. R. Murray, H. W. Arnold, and D. C. Cox, Four-frequency CW measurements in residential environments for personal communications, Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC '93), Yokohama, Japan, pp. 201-205, Sept. 9-11, 1993.

Cox 43. D. C. Cox, Delay-Doppler characteristics of multipath propagation at 910 MHz in a suburban mobile radio-environment, IEEE Transactions on Antenna and Propagation, pp. 625-625, September 1972. 44. D. M. J. Devasirvatham, Radio propagation studies in a small city for universal portable communications, IEEE VTC '88, Philadelphia, PA, pp. 100-104, June 15-17, 1988. 45. P. Bill Wong and D. C. Cox, Low-complexity co-channel interference cancellation and macroscopic diversity for high capacity PCS, IEEE ICC '95, Seattle, WA, PP. 852-857, June 12-24, 1995. 46. Byoung-Jo Kim, private communications, Stanford University, August 1995.

Donald C. Cox is the Harald Trap Friis Professor of Engineering at Stanford and Director of the Center for Telecommunications. He did research at Bell Laboratories in the late 1960s and early 1970s on wireless mobile systems that still provides basic input to the design of cellular and personal communications systems. From the late 1970s to 1993 he led and was actively involved in pioneering wireless research, first at Bell Labs and then at Bellcore, that started and fueled the current explosion in wireless personal communications. He was the manager of all radio research at Bellcore for 10 years. He was instrumental in evolving this research into the Bellcore WACS specification that was combined with PHS into the PACS standard in the U.S. TIA/T1 JTC. For this pioneering work, he was awarded the IEEE 1993 Alexander Graham Bell Medal and the Bellcore Fellow Award, and was elected to the National Academy of Engineering. He also has done research in signal processing antenna arrays and satellite communications systems. Cox received the IEEE Morris E. Leeds award in 1985 and the Prize Guglielmo Marconi from Italy in 1983, and is a Fellow of the 1EEE, AAAS, and RCA. He holds 12 patents and has authored three award winning papers. Cox received a B.S. in E.E. in 1959, an M.S. in E.E. in 1960, and an Honorary Dr. of Science in 1983 from the University of Nebraska, and a Ph.D. from Stanford in E.E. in 1968. He was an R&D ot~cer in the U.S. Air Force from 1960 to 1963. He has authored or co-authored over 80 journal papers and 90 conference papers.