8
Fixed Wireless Access Systems
The goal of this chapter is to review the main techniques used for Wireless Local Loop
(WLL) including the Multichannel Multipoint Distribution Service (MMDS), and the Local
Multipoint Distribution Service (LMDS). We also present the main aspects, advantages and
disadvantages, and applications of these techniques. The chapter also deals with the wireless
local loop subscriber terminals, Wireless Local Loop Interfaces to the Public Switched
Telephone Network (PSTN), and the IEEE 802.16 standards on Broadband Wireless Access.
Then a final section is given to summarize the main points presented in this chapter.
8.1 Wireless Local Loop versus Wired Access
Fixed Wireless Access (FWA) systems, which can also be called Wireless Local Loop (WLL)
systems, are intended to provide primary access to the telephone network; that is, wireless
services supporting subscribers in fixed and known locations. In general, WLL is a system
that connects subscribers to the public switched telephone network (PSTN) using radio
signals as a substitute for copper transmission media for all or part of the connection between
the subscriber and the switch. This may include cordless access systems, proprietary fixed
radio access, and fixed cellular systems. There are two alternatives to WLL: narrowband and
broadband schemes. Narrowband WLL offers a replacement for existing telephone system
while broadband WLL can provide high speed voice and data service. Some authors call
WLL, Radio In The Loop (RITL), Fixed-Radio Access (FRA), or Fixed Wireless Access
(FWA) [1,2].
It is expected that the global WLL market will exceed 202 million subscribers by the year
2005. Much of this growth will be in the developing countries where over half the world’s
population lacks Plain Old Telephone Service (POTS). This approach is cost effective and
can save burying tons of copper wire. WLL networks can be deployed very quickly and in a
cost-effective manner. This is a key advantage in a market where multiple service providers
are competing for the same user base. In developed countries, WLL will help unlock compe-
tition in the local loop, enabling new operators to bypass existing wireline networks to deliver
traditional and data access [3,4] (Figure 8.1).
WLL systems have a number of advantages over wired systems to subscriber local loop
support. Among these are [1–10]:

Figure 8.1 A wireless local loop configuration
ate demand for service is apparent. Service for a town or city, for example, can begin area by
area as the WLL base stations are deployed. This distinction between mobile and fixed
applications does not in itself imply much difference in the technology, but other dissimila-
rities associated with network deployment do. It is important to note that the capacity needed
for a WLL system is different from that for a fixed WLL system. A mobile system’s base
stations must provide adequate capacity to support worst-case traffic, while a fixed system’s
base stations must only provide the capacity needed to support a known number of subscri-
bers. In a fixed system, the Quality of Service (QoS) may need to be better, and the traffic
generated per subscriber may be higher due to lower charging rates than those for premium
mobile service and the different usage patterns of homes and offices. Therefore, the ideal fixed
WLL system should be fully scaleable and modular in order to be able to add any necessary
additional capacity to the base stations. This allows the redistribution of additional capacities
among existing base stations. Also, this means that base stations can be redeployed as needed
in order to meet new demands in traffic. There is also a difference in the nature of coverage of
WLL and wired access loop applications. While in the former applications a mobile system
must effectively provide communications to all areas within signal range of the base station, a
wired access system can assume that the subscriber terminal has been positioned to obtain the
best possible signal.
In a fixed subscriber environment, a terminal is oriented for the greatest signal strength
upon installation, and, if needed, a directional antenna pointing to the nearest WLL base
station can be used to improve the signal quality in terms of the carrier-to-interference ratio or
range extension. Furthermore, a fixed subscriber terminal will not experience the same
magnitude of fading effects seen by a mobile terminal.
Due to the differences between fixed and mobile propagation environments, the transmit
power levels of a fixed WLL system can be reduced compared to that of a mobile system,
assuming the same range of coverage and all other variables hold constant. Directional
antennas may be used at the base station to further improve the system’s link margins if
the fixed subscribers are localized [5–7].
8.2 Wireless Local Loop

The main drawback of MMDS systems compared to LMDS systems is that they offer much
less bandwidth. Due to this drawback, it is expected that MMDS services will be used mainly
for residential subscribers and small businesses [1–6].
8.2.2 Local Multipoint Distribution Service (LMDS)
The Local Multipoint Distribution Service (LMDS) is the broadband wireless technology
used to deliver voice, data, Internet, and video services in the 25 GHz and higher spectrum. It
is considered a relatively new service. Due to the propagation characteristics of signals in this
Wireless Networks232
Table 8.1 Frequency bands allocated by the United States FCC for fixed wireless communications
bands
Frequency range (in GHz) Application
2.150–2.162 Licensed Multichannel Distribution Service (MDS),
2 bands of 6 MHz each
2.4000–2.4835 Unlicensed Industrial, Scientific, and Medical (ISM)
2.596–2.644 Licensed Multichannel Multipoint Distribution Service (MMDS),
8 bands of 6 MHz each
2.650–2.656 Licensed MMDS
2.6620–2.6680 Licensed MMDS
2.6740–2.6800 Licensed MMDS
5.7250–5.8750 Unlicensed National Information Infrastructure (ISM-UNII)
24.000–24.250 Unlicensed ISM
24.250–25.250 Licensed
27.500–28.350 Licensed LMDS/Block A
29.100–29.250 Licensed LMDS/Block A
31.000–31.075 Licensed LMDS/Block B
31.075–31.225 Licensed LMDS/Block A
31.225–31.300 Licensed LMDS/Block B
38.600–40.000 Licensed
frequency range, LMDS systems use a cellular-like network configuration. In the United
States, 1.3 MHz of bandwidth has been allocated for LMDS to deliver broadband services in

Fixed Wireless Access Systems 233
Figure 8.2 An example of Local Multipoint Distribution Service (LMDS)