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13 Current and Future Services Using Wireless Local Loop (WLL) Systems Dong Geun Jeong and Wha Sook Jeon 13.1 Introduction In the telephone networks, the circuit between the subscriber's equipment (e.g. telephone set) and the local exchange in the central office is called the `subscriber loop' or `local loop'. Traditionally, the copper wire has been used as the medium for local loop to provide voice and voice-band data services. Since 1980's, the demand for communications services has increased explosively. There has been a great need for the basic telephone service, i.e. the plain old telephone service (POTS) in developing countries. On the other hand, in the industrialized countries, the demand for high-speed data and multimedia services at home and/or office has increased continuously. These requirements have been a motivation for innovation in local loop. There are two remarkable challenges in local loop technologies. One is mainly from the expansion of landscape in service types. According to drastic growth of Internet, to access Internet at home (or office) became a usual lifestyle today. Moreover, to enjoy multimedia services at home will not be strange in near future. These services require broadband local loop systems. To deal with this situation in short term, the digital subscriber line (DSL) technologies, including high-bit-rate DSL (HDSL), asymmetrical DSL (ADSL), and very-high-bit-rate DSL (VDSL), have been studied and developed [1]. Another technical advance, on which we will focus in this chapter, is the wireless local loop (WLL) adopting radio as the transmission medium. WLL is often called the radio local loop (RLL) or the fixed wireless access (FWA). And WLL services are also referred to as the `fixed cellular services'. WLL has many advantages from the viewpoints of the service providers and subscribers [2±8]: . The WLL approach significantly speeds the installation process since it can eliminate the wires, poles, and ducts essential to wired networks. Thus, WLL systems can be rapidly developed, easily extended, and are distance insensitive. Since WLL is a quick start for startup systems, wireless access is viable means to meet the high demand for POTS in many developing countries. 271 Wireless Local Loops: Theory and Applications, Peter Stavroulakis Copyright # 2001 John Wiley & Sons Ltd ISBNs: 0±471±49846±7 (Hardback); 0±470±84187±7 (Electronic) . The operations and maintenance are easy and the average maintenance time per subscriber per year is short (40 min compared to 2.2 hr for wireline). . Using advanced digital radio technologies, WLL can provide a variety of data services and multimedia services as well as voice. . Among radio systems, WLL enjoys the merits of fixed system: using high-gain direc- tional antennas, the interference decreases. This reduces the frequency re-use distance, increases the possible number of sectors in a sectored cell, and increases, in turn, the system capacity. Since WLL is a kind of radio system, it is natural that the WLL technologies have been affected by wireless mobile communication technologies. In fact, as will be shown later, most of WLL systems are developed according to the standards (or their variants) for mobile systems. In terms of multiple access technique, they have adopted the fre- quency-division multiple access (FDMA), the time-division multiple access (TDMA), the code-division multiple access (CDMA), or their hybrids. Basically, almost all of wireless systems or multiple access techniques can be used for WLL. However, it is also true that there exist some technologies or systems that have comparative advantages in a certain WLL environment. The reference frame for compari- son between systems is given by the service requirements in a specific service area. In this chapter, we investigate WLL services. The insight into WLL services gives: . as mentioned above, the reference for comparison between several systems or technol- ogies available for WLL, and . some prospects of future WLL services that can be a motivation for further study and development of systems. This chapter is organized as follows. Section 13.2 gives an overview of a typical architecture of WLL systems. Section 13.3 explains the WLL service requirements in developing and developed countries. Section 13.4 outlines the services provided by the representative systems available today or soon to be available. Section 13.5 discusses some further considerations for future WLL services with the relevant technologies. Finally, the chapter is concluded with Section 13.6. 13.2 WLL System Architecture Since WLL systems are fixed, the requirement for interoperability of a subscriber unit with different base stations is less stringent than that for mobile services. As a result, there exist a variety of standards and commercial systems. Each standard (or commercial system) has its own air-interface specification, system architecture, network elements, and terminology. Moreover, although the network elements in different systems have same terminology, the functions of the elements may differ according to systems. In this section, we present a conceptual (and typical) architecture of WLL systems (see Figure 13.1). The fixed subscriber unit (FSU) is an interface between subscriber's wired devices and WLL network. The wired devices can be computers or facsimiles as well as telephones. Several systems use other acronyms for FSU such as the wireless access fixed unit (WAFU), the radio subscriber unit (RSU), or the fixed wireless network interface unit (FWNIU). 272 Current and Future Services Using Wireless Local Loop (WLL) Systems FSU FSU BSC PC TelephoneFax BTS BTS Switch Air Interface Figure 13.1 Typical architecture of WLL FSU performs channel coding/decoding, modulation/demodulation, and transmission/ reception of signal via radio, according to the air-interface specification. If necessary, FSU also performs the source coding/decoding. We will discuss some air-interface speci- fications later, but detailed description of them is beyond the scope of this chapter. When a dummy telephone set is used, FSU may perform dial-tone generation function for users so as not to be aware of WLL system. FSU also supports the computerized devices to be connected to the network by using voice-band modems or dedicated data channels. There are a variety of FSU implementations. In some types of commercial products, an FSU is integrated with handset. The basic functions of this integrated FSU are very similar to those of handset for mobile communications, except that it does not have a rich set of functions for mobility management. Another example of FSU implementation is a high-capacity, centralized FSU serving more than one subscriber. Typical application of this type of FSU can be found in business buildings, apartment blocks, and the service area where some premises are located near by (see Figure 13.2). FSU is connected with the base station via radio of which band is several hundreds of MHz or around 2 GHz. Since WLL is a fixed service, high-gain directional antennas can be used between FSU and the base station, being arranged by line-of-sight (at least, nearly). Thus, WLL signal channel is a Gaussian noise channel or strong Rician channel (not a Rayleigh fading channel) [7]. This heightens drastically the channel efficiency and the capacity of the system. The base station is implemented usually by two parts, the base station transceiver system (BTS) and the base station controller (BSC). In many systems, BTS performs channel coding/ decoding and modulation/demodulation as well as transmission/reception of signal via radio. BTS is also referred to as the radio port (RP) or the radio transceiver unit (RTU). A BSC controls one or more BTSs and provides an interface to the local exchange (switch) in the central office. An important role of BSC is to transcode between the source codes used in wired network and that at the air-interface. From the above roles, a BSC is often called the radio port control unit (RPCU) or the transcoding and network interface unit (TNU). WLL systems do not need to offer mobile services basically, even if some systems provide limited mobile services. Thus, for example, there is no home and visitor location register (HLR/VLR) in a WLL system and its overall architecture may be simpler than that of the mobile systems. WLL System Architecture 273 Wireline Network telephone telephone computer FSU FSU BTS Figure 13.2 Fixed subscriber unit serving multiple subscribers As one can easily guess from Figure 13.1, the WLL services depend not only on the functionality of FSU, BTS, BSC, and air-interface specification but also on the service features provided by the switch in the central office. For example, when WLL is used as a telephony system, there are the basic telephony services (e.g. call origination, call delivery, call clearing, emergency call, etc.) and the supplementary services (e.g. call waiting, call forwarding, three-way calling, calling number/name delivery, etc.). In addition, as in the wired systems, the features such as custom calling features, Centrex features, custom local area signalling services (CLASS) can be supported by the switch [5,7]. If the air-interface provides a transparent channel to the switch, these service features depend totally on the switch functions. So, we hereafter focus on the air-interface specifications related to WLL services rather than the service features by the switches. 13.3 WLL Service Requirements The communication service requirements depend heavily on the socio-economical situ- ations of the service areas. In general, the WLL services required in developing countries and/or regions can differ from that in developed ones. 13.3.1 Developing Countries/Regions In many developing countries and/or regions, the infrastructures for basic telephone services are still insufficient. Accordingly, a lot of population in these areas has not been served with even POTS. This reduces the opportunity for the people to acquire information and, further, deepens the problem of information inequality. 274 Current and Future Services Using Wireless Local Loop (WLL) Systems For these areas, the emphasis points of WLL service requirements can be summarized as follows: . In terms of service coverage, a wide area should be covered within relatively short period. . Especially, for the regions with dense population, a high-capacity system is indispens- able. Here, the capacity means the available number of voice channels for given bandwidth. . On the other hand, there may exist wide areas with sparse population. For these service areas, if a small population with low traffic load resides near by, a centralized FSU serving more than one subscriber can be a solution (see Figure 13.2). . The service fee per subscriber must be low so as to offer the universal service. For this, a high-capacity system is again needed and the cost of system implementation and operation should be low. . The system should be implemented rapidly so that the services might be launched quickly. In choosing systems, the possibility of the rack of social overhead capitals (e.g. loads or electronic power) in some areas also should be considered [9]. As a trade-off to fulfil the requirements of high-capacity with low service fee, a medium-quality and relatively low data rate of channel (typically, up to 16 kbps) may be unavoidable. Using this channel, only voice and/or voice-band low-speed data com- munications are possible. However, the service requirements to the advanced services (e.g. high-speed data and broadband communications) will arise after (or with) the penetration of POTS. Therefore, at the initial choice and installation of WLL system, the service provider should take into account the future evolution of system to provide advanced services. 13.3.2 Developed Countries/Regions In the developed countries and/or regions, the service requirements contain not only POTS but also other advanced services. It is usual that more than one local switching service providers and cellular mobile service providers coexist in these service areas. We examine the WLL service requirements from the standpoint of each service provider. WLL provides a means to establish local loop systems, without laying cables under the ground crowded with streets and buildings. Thus, WLL is regarded as one of the most attractive approaches to the second local switching service providers. Unfortunately from the second providers' perspective, there are one or more existing providers (i.e. the first providers) who have already installed and operated wireline networks. To meet the increasing and expanding users' service requirements for high-speed data and multimedia services as well as voice, the first providers try to evolve their networks continually (for example, using DSL technologies). The second providers, entering the market in this situation, should offer the services containing competitive ones in terms of service quality, data rate of channel, and supplementary services, etc. That is, the WLL channel of the second provider should be superior to or, at least, comparable with the first operators' one in quality and data rate. Therefore, WLL should provide toll quality voice and at least medium-speed data corresponding to the integrated services digital network (ISDN) basic rate interface (BRI, 2B  D at 144 kbps). In addition, to give subscribers a motivation to WLL Service Requirements 275 Table 13.1 Comparison of public wireless communications services [4] WLL Only Service Cellular Mobile Service (including High-Tier PCS) Mobile/WLL Bundled Service Network Elements Local Exchange/BSC/BTS FSU MSC/BSC/BTS Mobile handset MSC/BSC/BTS Mobile handset and FSU Subscriber Unit FSU with wireline feeling (e.g. dial-tone) Mobile handset Mobile handset, FSU with wireline feeling Services No (or low) mobility, Medium- to high-speed data, Wireline-like supplementary services High mobility, Low- to medium-speed data Low/High mobility, Low- to medium-speed data, Wireline-like supplementary services migrate to the new provider, the service fee of the second provider needs to be lower than that of the first operators. Even to the first local switching service providers having wireline networks, WLL can be a useful alternative for their network expansion. Most countries impose the universal service obligation (USO) upon the first operators. In this case, WLL can be considered as a supplementary means to wireline networks, for covering areas with sparse population, e.g. islands. The first service requirement for this application of WLL is the compatibility with and the transparency to the existing wireline network. On the other hand, the cellular mobile service providers can offer easily WLL services by using their existing infrastructure for mobile services. In this case, fixed WLL service may have competitiveness by combining with the mobile services. For example, these two services can be offered as a bundled service [4,7]. In addition, so-called `one-phone service' can be offered with an appropriate billing strategy. That is, with a single subscriber unit, a sub- scriber enjoys the fixed WLL services at home and the mobile services on the street. Table 13.1 gives a comparison between the WLL services using a dedicated network and the mobile/WLL bundled services. Note that the table also contains the pure cellular mobile services for the purpose of comparison. 13.4 Services with Currently Available Systems Now let us examine the WLL services provided with the systems that follow international (or domestic) standards and are commercially available today. We focus on the standards rather than specific products. Most of WLL systems are developed according to the standards (or their variants) for mobile systems. Among mobile systems, we in this chapter consider only the digital systems (the second- and the third-generation systems) although the analogue systems, e.g. the advanced mobile phone service (AMPS) systems, still are used in many regions. The mobile systems can be categorized into the low-tier and the high-tier systems. Low-tier systems support only low mobility in general. They can be characterized by low transmission power, small coverage with a base station, high subscriber density, and high circuit quality services. In comparison with high-tier systems, low-tier systems 276 Current and Future Services Using Wireless Local Loop (WLL) Systems provide more wireline-like services. There are several standards for low-tier systems. The representative examples are the personal access communications system (PACS), the digital enhanced cordless telecommunications (DECT), and the personal handy-phone services (PHS). These standards adopt the TDMA technology. The high-tier cellular systems support high mobility and can be characterized by the wider coverage with relatively low data rate. These systems include the second-generation digital cellular systems using 800 MHz band (e.g. IS-95 CDMA, IS-54 TDMA, and GSM) and their up-banded variations for the personal communications services (PCS) using 1.8 $ 2.0 GHz band (for example, ANSI J-STD-008 CDMA as an up-banded version of IS-95). Among the above-mentioned systems, we briefly outline PACS, DECT, and IS-95 CDMA systems. We also review a wide-band CDMA system recently commercialized. 13.4.1 PACS Services [8,10] PACS [11] is a hybrid standard of Bellcore's wireless access communications system (WACS) and Japanese PHS. In PACS, the basic unit of channel is a time slot per TDMA frame, which transports data at 32 kbps. PACS offers toll-quality voice using ITU-T G.726, adaptive differential pulse code modulation (ADPCM), at 32 kbps as the default coding scheme. Optionally, 16 kbps low-delay code-excited linear prediction (LD-CLEP) being defined as ITU-T G.728 can be used. For voice-band data, PACS provides 64 kbps pulse code modulation (PCM) connection (ITU-T G.711) by aggregating two time slots. This service is used to support all voice- band modems including 56 kbps modems. PACS supports circuit mode and packet mode data services. In addition, individual message service and interleaved speech/data service are also provided. . Circuit-mode data service: PACS offers reliable real-time data transport service using link access procedure for radio (LAPR). LAPR operating in a 32 kbps channel provides a data throughput of more than 28 kbps at wireline error rate (10 À6 ). . Messaging services: This is two-way point-to-point message service for large file trans- fer up to 16 Mbytes. The messages can contain text, image, audio, and video files. . Packet-mode data service: This is a shared, contention-based, RF packet protocol using a data sense multiple access contention mechanism. It supports FSU by using single time slot (32 kbps) or multiple time slots (up to 256 kbps) per TDMA frame. The applications being suitable over the PACS packet channel are wireless Internet access and mobile computing, etc. . Interleaved Speech/Data: It provides the ability to transmit both speech information and data information by using a single 32 kbps time slot. Data are transmitted during the silent period of voice. 13.4.2 DECT Services DECT standard [12] has been developed to replace the cordless telephone-2 (CT-2) standard. It originally supports small cells (radius of 100 $ 150 m) with pedestrian-speed Services with Currently Available Systems 277 mobility. To use DECT in WLL applications, one of the most important problems to be solved is to extend the maximum coverage of a fixed part (i.e. BTS). A solution is to use directional antennas, by which the maximum diameter of a cell can be extended up to several kilometers. For rural applications, the coverage can be extended by using repeaters at the expense of capacity [10]. The basic unit of channel in DECT is a time slot per TDMA frame, operating at 32 kbps. If data rates higher than 32 kbps are required, multiple time slots per frame are used. Otherwise, if the requested data rate is lower than 32 kbps, several FSUs can share a 32 kbps channel by skipping time slots. DECT offers toll quality digital speech and voice-band modem transparency either via a 32 kbps ADPCM codec (ITU-T G.726) or as a 64 kbps PCM (ITU-T G.711) bearer service [13]. As of 1997, DECT provides 480 kbps full duplex data transfer and basic ISDN access [1]. Since all user information is encrypted, there is confidentiality between the different users belonging to a same cell. DECT has signalling compatibility with basic ISDN and GSM. For more detailed aspects of DECT WLL, one can refer to [14]. 13.4.3 IS-95 CDMA IS-95-A [15] standard has been developed for a digital cellular system with direct sequence (DS) CDMA technology, operating at 800 MHz band. ANSI J-STD-008 [16] being an up- banded variation of IS-95 is a standard for PCS systems, operating at 1.8 $ 2.0 GHz band. Recently, IS-95-B [17] merges IS-95-A and ANSI J-STD-008. In DS-CDMA systems, each channel is identified by its unique spreading code. IS-95 based CDMA WLL can support two rate sets. A code channel (that is, a traffic channel) operates at maximum of 9.6 kbps with the rate set 1 or 14.4 kbps with rate set 2. Using rate set 1 (rate set 2), the system supports 8 kbps (13 kbps) Qualcomm's codebook excited linear predictive (QCELP) vocoder. This channel can be used for circuit-mode data transmission. Since a type of radio link protocol (RLP) [18] is used in this case, the effective data rate lowers at around 7.2 kbps for 9.6 kbps channel. The packet-mode data transmission is also possible, although it is not popular still nowadays. IS-95-B offers high-speed data services through code aggregation. In IS-95-B systems, multiple codes (up to eight codes) may be assigned to a connection. Thus, the data rate is maximum of 76.8 kbps using rate set 1 or 115.2 kbps using rate set 2. Since IS-95-B can be implemented without changing the physical layer of IS-95-A [19], it is relatively easy for the vendor of IS-95 WLL system to develop the IS-95-B WLL system. In mobile IS-95 systems, a sectored cell is designed with three sectors in usual. As mentioned above, in WLL systems, the antennas for BTS and FSU can be arranged by line-of-sight and this reduces interference from the other user. So, the CDMA WLL cell can be designed with six sectors [7]. This increases the frequency efficiency and the system capacity. 13.4.4 Service using Wide-band CDMA Systems The existing cellular systems (including the second-generation digital systems) have some limitations in supporting high-speed data or multimedia services because of its insufficient 278 Current and Future Services Using Wireless Local Loop (WLL) Systems maximum data rate per channel. An alternative technology to cope with this problem is the wide-band CDMA (W-CDMA). In comparison with the existing narrowband CDMA systems (e.g. IS-95 system with the spreading bandwidth of 1.25 MHz), W-CDMA sys- tems use higher chip rate for direct sequence spread spectrum and, thus, spread its information into wider spectrum bandwidth (typically, equal to or over 5 MHz). Thus, data rate per code channel in W-CDMA can be higher than that in narrowband system. Note that all of the major candidates for radio transmission technology (RTT) of the international mobile telecommunications-2000 (IMT-2000) systems have proposed W- CDMA for next-generation mobile communication systems (e.g. [20±22]). Therefore, to try to use W-CDMA systems for WLL application is natural to the second operators, entering the market newly. In this section, we explain the W-CDMA WLL services in Korea as an example. In Korea, the development and the standardization of W-CDMA systems both for mobile service and for WLL service are conducted simultaneously. As a result, several vendors have developed W-CDMA systems according to Korean WLL standard [23] and a second local switching service provider in Korea has a plan to start WLL service with these systems in 2000. WLL services with these systems are as follows. The downlink (from BTS to FSU) uses the band from 2.30 to 2.33 GHz and the uplink (from FSU to BTS) uses the band 2.37 $ 2.40 GHz. Thus, the bandwidth of each link is 30 MHz. The spreading bandwidth can be either 5 MHz or 10 MHz. For both spreading bandwidth, the information bit rates are 8, 16, 32, 64, and 80 kbps. For the case of 10 MHz spreading bandwidth, 144 kbps of information bit rate is also available. The WLL standard defines several options for voice codec: 64 kbps PCM (ITU-T G.711), 32 kbps ADPCM (ITU-T G.726), 16 kbps LD-CELP (ITU-T G.728), and 8 kbps conjugate structure algebraic-code-excited linear prediction (CS-ACELP, ITU-T G.729). However, the service provider seems to offer voice services using 16 kbps LD- CELP and 32 kbps ADPCM since those give toll quality of voice with adequate system capacity. As the voice-band data services, G3 facsimile and 56 kbps modem are planned. For packet mode data transmission, some dedicated channels, which are separated from voice channels, are provided. They are the packet access channels in uplink and the packet traffic channels in downlink. Using these channels, packet data services up to 128 kbps are offered. In addition, ISDN BRI is also provided. 13.4.5 Comparison of Services Table 13.2 summarizes the WLL services mentioned above. As shown in the table, most systems offer toll quality voice services and medium- to high-rate data services. The applications being suitable over this data channel are wireless Internet access, mobile computing, and file retrieval services. Among these systems, PACS and DECT, based on low-tier cellular system technol- ogies, seem to be suitable for urban and developed region since they support relatively high quality channel in the small ranges. On the other hand, the IS-95 CDMA systems being currently used (i.e. based on TIA/ EIA/IS-95-A [15] or ANSI J-STD-008 [16]) have higher capacity than TDMA systems [7] and support wider service range per BTS. Thus, CDMA seems to be more appropriate choice for rural area and for developing regions. In the developed countries, the IS-95 cellular mobile service providers can offer WLL services also, using same infrastructure. Services with Currently Available Systems 279 Table 13.2 Summary of WLL services PACS DECT IS-95 W-CDMA Voice Codec 32 kbps ADPCM (16 kbps LDCELP) 32 kbps ADPCM 64 kbps PCM 13 kbps QCELP (8 kbps QCELP) 32 kbps ADPCM 16 kbps LD-CELP Voice-Band Data voice-band modem up to 56 kbps voice-band modem transparency via voice channel 14.4 kbps (9.6 kbps) voice-band modem up to 56 kbps Data Service: Rate per Connection 32 $ 256 kbps up to 480 kbps* Basic ISDN access up to 115.2 kbps** (76.8 kbps) up to 128 kbps (144 kbps BRI) Coverage per BTS Small Small Large Medium to Large Capacity per Cell Medium Medium High High * as of 1997. ** when IS-95-B is used. ( ) denotes the rate set 1 for IS-95, or the optional items in others. In this case, the fixed WLL service and mobile service can be a bundled service in urban area as mentioned before. As another strategy, the provider may offer the mobile services in urban area and the WLL services in rural area. IS-95-B and W-CDMA systems taking advantages of state-of-the-art technologies can be used in any region, because of their high-capacity, wide service range per BTS, and high channel quality. A demerit of these systems is that the technology is not proven sufficiently still in commercial experiments. However, all of the systems discussed in this section cannot offer the local loops for future multimedia services such as video. One of the reasons is that these systems are originally based on the mobile systems technologies. We will review another alternative to satisfy these requirements in the next section. 13.5 Further Considerations for Multimedia Services For bandwidth hungry services, such as video-telephony or video-on-demand (VOD), the systems mentioned in the previous section are not sufficient. The constraint on capacity per channel can be relieved by migrating to higher frequency ranges and applying broad- band wireless systems [5]. In fact, WLL concept in wide-sense contains not only the systems mentioned in previous section but also the microwave multipoint distribution services (MMDS), the local multipoint distribution services (LMDS), the wireless asynchro- nous transfer mode (WATM), and the satellite access. In many countries, LMDS, which is often called the local multipoint communication system (LMCS) [24], is considered as a strong candidate for next-generation broadband WLL (B-WLL) services. The spectrum for LMDS differs from country to country but it is usually 20 $ 30 GHz band. The main purpose of LMDS in its beginning period is the 280 Current and Future Services Using Wireless Local Loop (WLL) Systems [...]... 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In DS-CDMA systems, each channel is. Since IS-95-B can be implemented without changing the physical layer of IS-95-A [19], it is relatively easy for the vendor of IS-95 WLL system to develop the IS-95-B WLL system. In mobile IS-95 systems,. codec: 64 kbps PCM (ITU-T G.711), 32 kbps ADPCM (ITU-T G.726), 16 kbps LD-CELP (ITU-T G.728), and 8 kbps conjugate structure algebraic-code-excited linear prediction (CS-ACELP, ITU-T G.729). However,