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Common term for an access system that uses a wireless link to connect subscribers and can be both a substitute and a complement to copper wire in the local loop is called WLL (Wireless Local Loop) or FWA (Fixed Wireless Access). Initial stages of WLL system developments (during early 1990s) were for voice services and progressed onto data and broadband services towards the latter part of the 1990s . The fixed radio access network architecture is generally similar to cellular systems, with a radio base station providing service to an area around it. Subscribers receive service through a radio unit linked to the PSTN via the local base station. As illustrated in Figure 1(a), a basic WLL system's customer end equipment, the fixed radio access unit (FAU), consists of an antenna, a transceiver unit, and a processor subsystem. The latter converts signal between the form suitable for radio transmission and that for the customer premises equipment. The signal between the transceiver unit and the telephone handset is an analog signal carried via a copper pair. The unit is powered by a battery and a charger system. The system configuration usually results in creating a 48-V dc rail-based analog connection to the customer premises equipment.
Figure 1(a) Regular WLL system components at the subscribers' house, and (b) outline of a WLL system. (Source: )
Fixed access units communicate with the nearest base station. Sometimes, in order to place base stations in a more flexible manner to provide good coverage, the base station functions are separated into a radio node controller (RNC) and several transceiver units (TRXs). At the nearest local exchange, a transceiver pair between the exchange and the base station or RNC forms a duplex connection via possible options, such as a T1/E1/SDH link or a fiber-optic link. This completes the last mile in fixed wireless access systems. The WLL network manager carries out maintenance and subscriber management functions. The complete system is illustrated simply in Figure 1(b).
WLL systems require minimal planning and can be deployed quickly. Construction costs are minimal, and there is no need for rights of way for buried cable.WLL systems can help eliminate the backlog of orders for telephone service, which is estimated at over 50 million lines worldwide . Being principally a fixed service, the location of the residential subscribers is known. Hence, a WLL system provides user coverage at a lesser cost and is likely to support higher transmission rates, support broader bandwidth services, and promise a wider range of future services, including both entertainment and packetized data services, than cellular systems. Key factors preventing the rapid growth of WLL systems include the lack of common worldwide frequency allocations and technical standards.
Wireless Access in general refers to a wireless "method of access" and/or "means" to deliver telecommunication services to the end users (i.e. customers of network operators). Wireless access typically covers both mobile and non-mobile applications. This definition matches the definition of "wireline access" which is the same except that it is using other technologies, such as copper cable and/or fiber optics.
In general, Wireless Access is an alternative means to implement access to an operator's network. In this context Wireless Local Loop (WLL) means typically use of Wireless methods for implementation of traditional Public Telecom Operator (TO) local loops, i.e. using radio access and frequency spechum as a conduit instead of e.g. copper or fiber. Traditionally, point to multipoint radio can be considered as an example of WLL.
Technology wise, it is important to realize that different wireless methods i.e. different radio access standards/technologies (GSM, D ECT etc.) differ in their technical/economical "capabilities" for WLL. For comparison, the same applies to different wireline technologies (copper, fiber, etc.).
Wireless technology allows developing countries to quickly advance their existing telephone network into the 21st century. A wireless local loop (WLL) system uses radio technology to provide reliable, flexible, and economical local telephone service in place of traditional copper wireline. A WLL is sometimes called a "fixed cellular system."
From the service provider's prospective, the key benefits of WLL are low capital costs, fast network deployment, and lower maintenance costs, clearly attractive considerations. Also, the process of building a WLL system does not require precise knowledge of the user's location, adding flexibility to planning and deployment of the system. WLL networks have been proven to have the capability to function as core communications systems in times of disaster; for disaster recovery, service providers have the option of rapidly deploying a WLL system during an emergency. WLL systems can also be used as redundant backup systems for existing wireline networks. This way, communications downtime caused by natural disasters such as floods, earthquakes, hurricanes, and so on can be kept to a minimum.
WLL technology is also gaining popularity in the Asian and Latin American countries for providing telephone services in sparsely populated rural areas. A WLL is ideal as a startup telephone system that can be moved around to suit current needs. A WLL eliminates many problems and costs inherent in wireline loop systems. 
The Chosen System (Wireless Local Loop)
3.1 Access Technologies
3.1.1 Competing Technologies for Access
The evolution of basic voice telephony towards broadband services has developed the following types of access techniques:
â€¢ Third generation mobile;
â€¢ Fixed/mobile integration;
â€¢ Microwave video distribution;
â€¢ Fiber-optic distribution;
â€¢ Internet telephony;
â€¢ Digital broadcasting;
â€¢ Satellite systems;
â€¢ Power-line communications.
The challenge is to figure out the least cost and most efficient solution for a given set of boundary conditions.
Figure 3.1 Competing technologies for access. 
Figure 3.1 shows the main contending access technologies. A point of interest in this figure is that in the evolutionary process, traditional technologies are moving both from right to left as well as left to right. The concept of convergence predicts that a range of different media, including voice, data, and video, will be transmitted along the same path, possibly in an integrated form. 
3.1.2 Alternatives for Wireless Access
While DSL systems, for example, rest heavily on already existing infrastructure, radio transmission allows rapid installation and is particularly attractive for new carriers entering the market and for thinly populated areas. Expected features and capabilities of such systems are described in. This evolution along several wireless technologies is illustrated in Figure 3.2. Satellite-based wireless access is intended to serve sparsely populated remote regions or for specific services, such as banking or retail networks. Wireless LANs and personal area networks (PANs) are indoor short-range wireless access schemes. Along all these axes, the evolution is towards a unified body of broadband access systems generally envisaged as the fourth generation wireless systems.
When examining the position of WLL in this multidimensional competing scenario, it is seen to have broadly, two different roles:
â€¢ As a technology for basic voice provision. This application scenario may be the only practical means of connections for reducing waiting lists or as an alternative or supplement to basic voice provision (POTS).
â€¢ As an advanced access method providing broadband access.
While the first item is mostly relevant to developing countries and high-capacity hot spot applications, the second is becoming increasingly important due to emerging multimedia communications needs as outlined in other chapters of this text.
Figure 3.2 Different wireless access alternatives: (a) convergence, and (b) capabilities.
3.2 WLL Technologies Primarily for Voice Provision
3.2.1 Fixed Cellular Technologies in WLL Applications
Due to the basic architecture of cellular and WLL systems are the same, it is convenient to adapt cellular technologies for WLL applications. However, key differences that exist between the two environments are summarized in Table 3.0.
Mobility (subscriber location, hand off, roaming) must be supported
Mobility is not an essential requirement
Line of sight between the mobile and the base station does not exist
Line of sight can be achieved by suitable base station and antenna spacing
Poorer quality than wireline service is acceptable
Quality must be equivalent to wireline services
Table 3.0 Comparison between Cellular Environment and WLL Environment.
First generation WLLs are based on analog cellular technologies, which provide good access techniques for medium- to low-density fixed applications. These operate in the same 900-MHz frequency band and are based on FDMA/FDD. However, the key drawback is that these are optimized for mobility rather than local loop service with low-bit-rate voice coding. Thus, quality equivalent to wireline service is hard to achieve.
The next generation of this type of WLL is based on the digital cellular standards that use TDMA and CDMA. CDMA-based WLLs have gained wide popularity. Furthermore, to providing higher voice quality than analog systems, digital WLL systems are able to support higher speed data services. However, digital cellular technology provides coverage areas that are usually smaller than with analog.
3.2.2 PCS Technologies in WLL Applications
Most of the WLL systems make use of the standardized cordless telecommunications systems such as cordless telephone-2 (CT2), digital European cordless telephony (DECT), personal wireless telecommunications (PWT), personal access communications system (PACS), and personal handy phone system (PHS). This low mobility, low-power wireless communications systems are generally referred to as personal communications systems (PCS). Developed for microcellular environments, their coverage is typically several hundred meters. However, with fixed elevated antennas and other enhancements, their range can be extended to several kilometers for WLL systems. These systems operate in the 1,800- to 1,900-MHz frequency bands. Their suitability for WLL applications is examined in .
A general comparison of these low-power systems with cellular systems for WLL applications shows the following advantages of the former :
â€¢ Superior voice quality with ADPCM used in PCS systems than with the lower bit rate encoding techniques used in cellular systems;
â€¢ Ability to provide data rates in multiples of 32 kbit/s, whereas cellular systems provide only 9.6 or 14.4 kbit/s in basic systems;
â€¢ Ability to provide much higher capacity in dense urban areas;
â€¢ Operation in the 1,800- to 1,900-MHz bands means they avoid interference from the more crowded 900-MHz cellular band;
â€¢ Simplicity and low cost. These are results of a simpler infrastructure that does not need support for mobility and frequency planning.
3.2.3 The DECT System
Sponsored by the European Union and developed originally as a standard for cordless domestic and business systems and for limited mobility in the public market, DECT was intended to supersede the cordless telephony (CT) series of standards. The DECT subscriber units were low cost and low complexity in comparison with cellular systems such as GSM, with system performance optimized for confined indoor use. PWT is a DECT-based system developed in the United States for unlicensed PCS applications. PWT-E (PWT-enhanced) is a version for licensed PCS. These systems have been widely used in WLL applications around the world.
Figure 3.3(a) DECT system architecture, and (b) call processing example (outgoing call).
Figure 3.3(a) shows the architecture of the DECT system. PWT is similar. DECT is an interface between a fixed part (FP) and a portable part (PP). The FP has three major components. The radio fixed part (RFP) terminates the air interface. The central system, the radio node controller (RNC), provides cluster controller functionality, managing a number of RFPs. The DECT radio system is expected to work in conjunction with a wireline network. The interworking unit (IWU) provides all of the necessary functions for the PWT/DECT radio system to interwork with the attached network (e.g., the PSTN, ISDN, PLMN or a packet-switched network). The PP interfaces the subscriber premises equipment to the DECT air interface.
Figure 3.3(b) illustrates outgoing call processing in DECT. When the phone goes off hook, the subscriber premises FAU transmits a physical channel request to the RFP, and the allocation is confirmed. An authentication request is sent from the RNC, and when acknowledged by the FAU, the off-hook signal is sent to the local exchange. Dial tone is then received at the subscriber, followed by the dialed digits from the subscriber, via the channel formed. Subsequent processing is similar to the PSTN. 
DECT is designed to operate in the 1,800- to 1,900-MHz frequency band, with flexibility to use other bands close by. It is based on TDMA/TDD. There are 10 carriers spaced at 1,728 kHz. Each carries 12 duplex TDMA channels at an aggregate rate of 1,152 kbit/s. Therefore, a total of 120 channels are available. The modulation method is Gaussian FSK (GFSK), and 32- kbit/s ADPCMis used for voice coding. Table 3.1 summarizes the parameters of the DECT system and compares the important parameters of additional systems with those of DECT. The normal cell radius for DECT is several hundred meters.
Frequency band (MHz)
Number of carriers
Carrier spacing (kHz)
Channel bit rate (kbit/s)
Maximum rate (kbit/s)
11 Ã- 32
7 Ã- 32
2 Ã- 32
Number of slots/frame
12 + 12
4 + 4
Standards Institute (ANSI)
Spectral efficiency (b/s/Hz)
Table 3.1 Parameters of DECT, PACS, and PHS. 
As depicted in Figure 3.4(a), the time slot structure of a DECT RF channel consists of 24 time slots in 10 ms. Twelve slots are defined for base-to-subscriber transmission, and twelve are defined for subscriber-to-base transmission. The full-duplex channel between the FP and the PP consists of a pair of time slots on a single RF channel. Each TDMA time slot (burst) consists of fields for synchronization, signaling, speech data, and error checking. Figure 3.4(b) shows the 10 DECT carriers and the formation of 120 channels.
Figure 3.4 DECT channels: (a) time slot structure within an RF channel, and (b) formation of RF channels.
3.3 Other Cordless/PCS Systems
While DECT was well deployed in Europe and some parts of Asia, PHS (a digital System operating in the 1.9-GHz PCS band) has been successful in Japan. PACS, also operating in the 1.9-GHz band, is based on Bellcore's wireless access communications system (WACS) and on Japan's PHS. Detailed information and a comparison of these technologies are found in . General comparison of the systems, through underlying technologies and implementation instances, can be summarized in Table 3.2.
TDD versus FDD
TDD results in simpler implementation than FDD. It has the advantage of needing only one RF channel (and equipment) for each call. However, TDD requires stringent time synchronization between all transmitters and receivers, with imperfections leading to severe loss of capacity. Maintaining time synchronization becomes more difficult as the coverage area becomes larger. FDD has the advantage of having lower bit rates in each of the two directions and thus needing less equalization to combat multipath fading.
Delay spread Delay spread
Delay spread in multipath environments places an upper limit on transmission rate. DECT is a high-bit-rate system designed for indoor and microcellular environments and can tolerate delay spreads up to about 90 ns. For WLL applications, the operation can be extended up to 300 ns with diversity. PACS and PHS use lower bit rates and can operate in delay spread environments up to 260 ns and 1,000 ns without and with diversity, respectively. PACS has diversity built into the standard. As a result, the possible cell size is smallest in DECT. Some DECT implementations adopt larger cell sizes at the cost of deactivating every other time slot.
PACS has the smallest frame size. These results in fewer delays in error correction in high bit-error-rate environments compared to DECT and PHS. Requirements for echo cancellation increase with frame length.
Channel selection and handoff is done almost exclusively by the subscriber units in DECT and PACS. In PHS, the base stations are also involved, and handoff occurs when performance becomes unacceptable. In the case of DECT and PACS, handoff is based on using the best channel available, which may lead to excessively frequent handoffs unless performance thresholds are also incorporated. The DCA technique used in DECT and PHS is superior to the QSAFA technique in PACS.
In PHS, a subscriber in a particular cell can communicate only with its base station. In DECT and PACS, the subscriber can access neighboring base stations. Therefore when all time slots in a particular cell are occupied, transmission can continue with neighboring cells. This reduces the blocking probability
In PACS and PHS, one time slot in each frame is used as a control channel. In DECT, control information is embedded into the traffic channels. Therefore, DECT has a better channel efficiency.
Two PHS portable units in close proximity can carry direct two-way communications. This reduces the load on the central switch and is particularly advantageous in indoor environments.
PACS is the only system that allows dual mode FDD/TDD operation in licensed/unlicensed bands. This makes it operable via dual-mode terminals in both unlicensed, private, indoor environments (e.g., wireless PABX) and public licensed wireless access systems.
Table 3.2 Summary and Comparison of DECT, PACS, and PHS 
In summary, no single low-power wireless system is ideally suited for WLL applications. The most suitable system can be chosen on the specific conditions prevailing in the environment. For low-traffic environments, PACS performs best due to its larger cells. The number of base stations per square kilometer in PACS would be significantly lower than other systems. In suburban areas, where capacity is an issue in addition to coverage, DECT has better performance. In urban areas, all three systems have similar performance, and the designer can increase capacity by reducing the cell size. The performance of PACS and PHS is better in terms of the ability to do this. However, DECT can be enhanced with interference-reduction techniques. Its superior dynamic channel allocation algorithm is an advantage in these environments. The same conclusions apply for both DECT and its derivatives, PWT and PWT-E.
One major technical problem with WLL systems based on these standards was the effect of multipath propagation, which makes radio planning extremely difficult. In a WLL application with common cell sizes of over 5 km, delay spreads of a few hundreds of nanoseconds are present. The technology should cater to such delay spreads. With the absence from the key PCS systems of technologies such as equalization, their ability to provide optimum performance in a complex radio environment is poor. Efforts to realize better WLL systems led to the implementation of proprietary systems.
3.3.1 MMDS and LMDS
Figure 3.5 Architecture of LMDS and MMDS systems.
Figure 3.5 shows the architecture of MMDS and LMDS systems. These systems employ a point-to-multipoint broadcast downlink with possibilities of either integrated or independent point-to-point uplink. Operation of MMDS/LMDS in an area will normally require a cluster of cells with separate base stations for collocated transmitter/receiver sites. Interference in adjacent cells would be avoided by using differently polarized antennas in adjacent cells. The service provider beams signals to a single point in multiple dwelling units or commercial buildings, and the signals are then distributed to individual tenants. One of the base station sites will serve as the coordination center and connect the cells to external networks. Inter-cell networking may be implemented using fiber or short hop radio relay connections. Co-location with mobile base stations allows for infrastructure sharing. 
The MMDS service, which has existed for some time, offers a maximum of 33 analog video channels in a total bandwidth of 500 MHz. These services had a competitive advantage in providing television to rural populations out of reach of cable and ordinary broadcast services. For this reason, MMDS is also referred to as wireless cable. The transmit power allowed for MMDS services allowed signals to be carried as far as 70 km from the transmitter to receivers within line of sight. MMDS requires terrestrial wired networks to communicate back to the headend (e.g., to select programming or use VCR-type controls on video-on-demand programming).
Current MMDS operators are looking to use digital compression techniques to increase the number of channels to around 200, making it competitive with presently available wired cable systems and satellite TV systems. With the evolution of digital technologies, typically with MPEG-2 encoding and complex modulation schemes, these systems may also provide two-way connectivity and transport to the transmitter from the headend using ATM or SONET networks, perhaps using TCP/IP protocols.
Where as MMDS was developed for analog TV distribution, the first digital systems evolved during the late 1990s, leading to LMDS. It originally supported Motion Picture Expert Group (MPEG) video transmission on several carriers with an 8- to 27-MHz spacing, each transporting about 40Mbit/s. LMDS services operate in the 27.5- to 31.3-GHz band. Typical LMDS applications now include all kinds of interactive services, using an extra return channel.
The LMDS, having cell radii of less than 12 km, can deliver two-way high-speed data, broadcast video, video-on-demand services, and telephony to residential areas. These systems have a total capacity of 34 to 38 Mbit/s per transport stream, giving high flexibility for inclusion of any type of data. The interactive channel capacity may range from a few kilobits per second to at least 25.6 Mbit/s. The LMDS transmitter should be sited at a high point overlooking the service area. The transmitter covers a sector typically 60Â° to 90Â° wide. Full coverage thus requires four to six transmitters. The streams transmitted contain 34-38 Mbit/s of data addressed to everybody in the coverage zone (television), subgroups, or individuals (typical communication is the Internet). The return channel is determined by the needs of the individual user and can be typically up to 8 kbit/s, with possibilities up to 25.8 kbit/s. A capacity comparison of LMDS with other competing broadband access technologies is provided in. 
The primary disadvantages of both MMDS and LMDS are CCI from other cells and limitations on coverage. Coverage issues are not as great a challenge with MMDS as they are with LMDS. Millimeter-wave radio signals do not penetrate trees and are susceptible to precipitation effects. Thus, line-of-sight propagation paths are required, making antenna placement on subscribers challenging. Even if the transmitter and receiver are placed at fixed points with line of sight, the influence of motion of traffic and foliage creates a hostile fading environment  and the references there in discuss propagation issues that post major impediments for LMDS services.
LMDS technology, first implemented in 2000, is expected to enhance development of broadband services such as e-commerce and distance education.
3.3.2 Multipoint-to-Multipoint Systems
As a solution to the propagation and coverage difficulties in point-to-point and point-to-multipoint BWA systems, multipoint-to-multipoint systems such as the one shown in Figure 3.6 have been developed.
Figure 3.6 Point-to-multipoint Internet radio operating system (IROS) scheme from Rooftop Communications.
Suppose it is desirable to extend the reach of a wireless system by letting every transceiver communicate with any other transceiver in the system. In such a system, multiple logical links exist between one receive/transmit point and its neighbors. Information would be forwarded through the network to the correct destination. Each link between two points may have different characteristics, such as transmit power, data rate, and reliability. All of these factors call for a new approach toward the physical medium access, network protocols, and even an overall operating system. A few such systems in operation and further research carried out in this area of FWA systems are outlined in .
3.3.3 IEEE 802.11 Wireless LANs
In June 1997, the IEEE approved international interoperability standard IEEE 802.11, specifying both physical and medium access control procedures for wireless extensions to LANs. Three physical layers, two in the 2.4-GHz ISM band using frequency-hopped spread spectrum (FH-SS) and direct-sequence spread spectrum (DS-SS) and one using infrared light (IR) were defined. All physical layers supported a data rate of 1Mbit/s and optionally 2Mbit/s. For multiple access, the carrier sense multiple access/collision avoidance (CSMA/CA), a distributed medium access control protocol, was adopted.
User demand for higher bit rates and international availability of the 2.4-GHz band has spurred the development of a higher speed extension to the IEEE 802.11 standard called IEEE 802.11b, providing a basic rate of 11 Mbit/s and a fallback rate of 5.5 Mbit/s to be used with the already standardized medium access control. Yet another physical layer option, which offers higher bit rates in the 5.2-GHz band intended for use in UNII devices, was standardized as IEEE 802.11a, offering data rates up to 54 Mbit/s using orthogonal frequency division multiplexing (OFDM).
3.4 Harmonization of Standards
ETSI, IEEE 802.11, and MMAC standardization groups have been closely coordinating with each other to harmonize the systems developed by the three fora. There are many similarities between IEEE802.11b and HIPERLAN systems, the main ones being the adoption of orthogonal frequency division multiplexing (OFDM) and link adaptation schemes in the physical layer. The different modes of operation in link adaptation are found in . MMAC also uses OFDM. IEEE802.11b and HIPERLAN systems can operate in infrastructure (centralized) or ad-hoc (direct) modes. Their main difference lies in the medium access control scheme. While IEEE802.11b has a distributed media access control protocol (CSMA/CA), HIPERLAN has a centralized, scheduled media access control protocol based on ATM/TDMA/TDD. MMAC supports both types of media access controls. 
3.4.1 IEEE 802.16 Wireless MANs
The IEEE has standardized broadband WLLs for use in, but not restricted to, the LMDS bands as IEEE 802.16 for initial target markets requiring 2 to 155 Mbit/s. The IEEE standard 802.16 Wireless MANTM, "Air Interface for Fixed Broadband Wireless Access Systems," was published in April 2002. It addresses the last mile connection in wireless metropolitan area networks, focusing on the efficient use of bandwidth in the region between 10 and 66 GHz, and defines a common medium access control layer that supports multiple physical layer specifications customized for the frequency of use. Between the physical and media access control layers, a transmission convergence (TC) sub-layer is defined, which forms the interface to different physical layers. This standard supports continuously varying traffic levels at many licensed frequencies (e.g., 10.5, 25, 26, 31, 38, and 39 GHz). The more recent IEEE802.16a standard does the same for the frequency band 2 to 11 GHz (licensed and unlicensed). This lower frequency band offers the opportunity to reach many more customers less expensively, although at generally lower data rates. This suggests that such services will be oriented toward individual homes or SMEs, where the higher frequency systems will be geared toward large corporate customers.
Whether the promise of BWA will materialize depends on its appeal to telecom operators from the perspective of deployment economics, where the critical factor is the ease of installation of subscriber units. The ultimate objective is for nonprofessional installation of integrated all-indoor subscriber units. Consequently, the physical layer has to mitigate the very tough impairments that characterize these nonline-of-sight environments. The IEEE802.16a standard emphasizes this requirement.
Many of the features in 802.16 have already been implemented in the BWA systems discussed in Section 5.3.5 and hence show the gradual evolution of a closely related set of standards. This standard sets the stage for widespread and effective deployment of BWA systems worldwide. [2-4]
3.5 The Medium Access Control Layer
The medium access control layer addresses the need for very high bit rates. Access and bandwidth allocation algorithms must accommodate a large number of terminals per channel with terminals that may be shared by multiple applications. The services required by the end users are varied in their nature and include legacy TDM voice and data, IP connectivity, and packetized VoIP. To support this variety of services, the 802.16 medium access control must accommodate both continuous and bursty traffic. Additionally, these services expect to be assigned QoS in keeping with the traffic types.
The 802.16 medium access controls provides a wide range of service types analogous to the classic ATM service categories, as well as newer categories such as guaranteed frame rate (GFR). It also must support a variety of backhaul requirements, including both ATM and packet-based protocols. Convergence sublayers are used to map the transport layer-specific traffic to a medium access control protocol that is flexible enough to efficiently carry any traffic type. The protocol is based on a request-grant mechanism that is designed to be scalable, efficient, and self correcting. The 802.16 access system does not lose efficiency when presented with multiple connections per terminal, multiple QoS levels per terminal, and a large number of statistically multiplexed users. It takes advantage of a wide variety of request mechanisms, balancing the stability of contentionless access with the efficiency of contention-based access .
In order to accommodate the more demanding physical environment and different service requirements of the 2- to 11-GHz frequency range, the 802.16a standard upgrades the medium access control to provide automatic repeat request (ARQ) and support for mesh, rather than only point-to-multipoint architectures.
3.6 OFDM and OFDMA
The basic principle of OFDM is to split a high-rate data stream into a number of lower rate streams, which are transmitted simultaneously over a number of subcarriers. Because the symbol duration increases for lower rate parallel subcarriers, the relative amount of time dispersion caused by multipath delay spread is decreased. Intersymbol interference (ISI) is eliminated almost completely by introducing a guard interval (GI) in every OFDM symbol . The OFDM symbol is cyclically extended during the GI.
All subcarriers differ by an integer number of cycles within the symbol duration, which ensures orthogonality between them. In practice, the most efficient way to generate the sum of a large number of subcarriers is by using the inverse fast Fourier transform (IFFT). At the receiver side, the fast Fourier transform (FFT) can be used to demodulate all subcarriers.
Because of delay spread, the receiver sees a summation of time-shifted replicas of each OFDM symbol. As long as the delay spread is shorter than the guard time, there is no ISI or inter-carrier interference within the FFT interval of an OFDM symbol. The only remaining effect of multipath, the random phase and amplitude of each subcarrier, is estimated through the use of pilot symbols, and the carriers are coherently detected. In order to deal with weak subcarriers in deep fades, forward error correction is applied. 
A key parameter in OFDM is the guard interval (Tg). This provides robustness to RMS delay. The symbol duration (Ts) is selected by choosing an appropriate balance between the time and power spent on the GI. The subcarrier spacing is the inverse of the symbol duration. The total time spent in transmitting one OFDM symbol is Ts+Tg. An OFDM symbol consists of data as well as pilot signals for channel estimation. A large number of subcarriers are present, each carrying data using variable modulation types, from BPSK to 16-QAM, and variable convolutional coding schemes for error correction. Pilots are carried over a number of pilot subcarriers. OFDM is used in IEEE 802.11a, HIPERLAN, and IEEE802.16. In these implementations, all carriers are transmitted at once. The downstream data is time division multiplexed, and subscribers access the base station in the upstream through TDMA.
3.6.2 Orthogonal Frequency Division Multiple Access
One physical layer option for the IEEE802.16a standard for 2- to 11-GHz BWA systems is to use orthogonal frequency division multiple access (OFDMA). In this extension of OFDM, the subcarriers are grouped into subchannels, which are used in the downstream for separating data into logical streams. The subchannels use different amplitudes, modulation, and coding schemes to address subscribers with different channel characteristics. In the upstream, the subchannels are used for multiple access.
In order to mitigate the effects of frequency selective fading, the carriers of one subchannel are spread along the channel spectrum. The usable carrier space is divided into a number of NG successive groups, with each group containing NE successive carriers after excluding the pilot carriers. A subchannel has one element from each group allocated through a pseudorandom process based on permutations. Each subchannel therefore has NG subcarrier elements.
OFDMA allows for fine granulation of bandwidth allocation, consistent with the needs of most subscribers, while high consumers of upstream bandwidth are allocated more than one subchannel. A low upstream data rate is consistent with the traffic asymmetry, where the streams from each subscriber add up in a multipoint-to-point regime. In the downstream, all of the subchannels are transmitted together. In essence, OFDMA consists of different users sharing the spectrum, with each transmitting one or more subchannels. This can also be seen as a form of FDMA. With regards to interference, OFDMA subchannels constitute a form of FH-SS.
Broadband Fixed Wireless Access Systems
A key development in FWA systems in recent times is the extension of wireless LAN technologies for more general wireless access. Wireless LANs and broadband access systems being developed are the multimedia mobile access communication (MMAC) system in Japan, the broadband radio access networks (BRAN) family of systems in Europe, and the IEEE 802.11 in the United States. The evolutionary trends also show the development of hierarchical wireless networking environments, as illustrated in this section. 
Cellular systems and WLL systems described here reside at typical data rates of tens of kilobits per second in wide area coverage, served by microcells and macrocells. Bearer services, which qualify as broadband access (i.e., hundreds of kilobits per second up to 2Mbit/s), are specified in third generation IMT-2000 systems for both fixed and mobile systems. There are two classes of broadband fixed wireless access (BWA) systems. One is the broadcast of television, with range of tens to hundreds of kilometers. The other is the wireless LANs (WLANs). Wireless LANs with 2- to over 50-Mbit/s transmission rates have coverage range of tens of meters.
The broadcast category is evolving towards multichannel, two-way fixed wireless communications, while the WLANs are evolving towards broadband outdoor systems with connectivity to ATM and TCP/IP transport networks. In the United States, the Federal Communications Commission (FCC) has set aside 15 bands for commercial BWA systems. Frequency allocations in other countries are very similar.
Known as the ISM band, the 2.4000- to 2.4835-GHz band is popular with operators because it is non-licensed and used with equipment manufacturers worldwide. Until recently, there was no question about using this band for communications, as the technology to overcome interference from ISM uses was not available. The arrival of spread spectrum technology for commercial communications opened up this band. Two more unlicensed bands span the frequency ranges 5.725 to 5.875 and 24.0 to 24.25 GHz. The former is known as the UNII band.
The ETSI project BRAN defines a family of high-performance radio access standards expected to be deployed in a hierarchical manner. Figure 3.7 shows how the three network categories might be deployed in business and domestic environments.
Figure3.7Scope of the different categories of wireless access systems defined by BRAN.
At 40 GHz, in addition to costly front-end technology, attenuation by precipitation is severe. The higher capacity offered at 40 GHz may compensate for these effects in the long run. These bands are expected to be shared among two or three licensees with 500 MHz to 2 GHz per licensee. The attractiveness of this possibility is driving these technology developments for fixed systems, despite propagation and other technical hurdles.
The flexible architecture applied in all BRAN standards defines physical and data link control (DLC) layers, which are independent of the core network. A set of core network specific convergence layers (CL) are placed at the top of the DLC layer. This allows BRAN systems to be used with a variety of core networks.
The advantages of broadband fixed wireless access system:
An increase in capacity as a result of frequencies being reused on a much localized level. Effectively, this is the equivalent of a microcellular approach on a conventional design, although these capacity gains could be offset by the need for each node to relay traffic.
An improvement in quality as a result of each link being short and hence having a high link budget.
A possible cost reduction in the subscriber unit as a result of the less demanding link budget. However, this may be offset by the additional complexity required to provide the repeater element needed within the mesh architecture.
An ability to replan the system without repointing subscriber antennas (e.g., in cases where subscriber numbers grow more quickly than anticipated).
A potentially nearly "infrastructure-less" deployment.
Against this needs to be balance the potential disadvantages:
Highly complex algorithms are required to manage the system and avoid "hot spots" which may be unstable and result in poor availability.
Different and novel medium access control (MAC) mechanisms may be required which will need development and add to the complexity.
The initial investment is relatively high since "seed nodes" have to be placed so that the mesh can form as soon as the first sub- scriber is brought onto the system.
Marketing issues may be problematic in that customers may not want to rely on nodes not in their control and not on their premises for their connectivity, and may not want their equipment to be relaying messages for others.
It is difficult to draw definitive conclusions at this point since many of the above variables are unknown. If the complexity and risk can be overcome, it seems highly likely this system will provide greater capacity than conventional systems for a given cost.
With open access interfacing and spin-off effects of cellular economies of scale, WLL has exiting potential as a new access instrument both for developing and developed countries. Although wireless has a promising role in future access network, it probably will not totally replace wireline access, but rather complement it in wide coverage area applications. Thus a combined wireline/wireless approach seems to be a promising approach for future access networks.
Various systems are being used or considered for providing fixed wireless access to the public switched telecommunications network. However, these systems are not optimized (either technically or economically) for fixed wireless access applications. The combination of a simple, low- power radio technology and an efficient network architecture promise to bring the cost of FWA systems for use in urban and suburban areas down significantly. Technologies currently under development by Bellcore and the CCIR should offer a cost effective approach to FWA in these areas.