Part 2
Wireless Broadband Networks Handbook: 3G, LMDS & Wireless Internet
Chapter 7: Local Multipoint Distribution Service (LMDS) Design Technology
November 19, 2001
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Performance
This
part of the chapter presents actual measurement results showing the performance
of DECT when combined with LMDS radio equipment. The goal of the experiment is to
study the actual BER performance and frequency tolerance of the integrated
system. Figure 7-15 shows the setup for the reference experiments, which
consist of connecting the DECT modulator and demodulator back to back without
going through the LMDS system.27 The BER analyzer generates
pseudorandom data. The data from the BER analyzer are modulated and then
subsequently demodulated by the DECT demodulator. The demodulated data are then
fed back to the BER analyzer. Figure 7-16 shows BER versus frequency offset
over selected received power levels.28 As expected, it can be seen
that frequency offset tolerance is higher for a higher received power level.
For voice applications, a BER of better than 10-3
is appropriate. Figure 7-16 shows that the frequency tolerance ranges from 30
kHz at a received power of -69.3 dBm to approximately 100 kHz at
received power of -57.3 dBm. This result indicates that an expensive
frequency drift compensation circuit is not needed.
In addition to the frequency tolerance measurements, you also should
verify the performance of DECT after it has been integrated into the LMDS
system. In particular, you should verify that indeed the special frequency
drift compensation circuit is not needed. Figure 7-17 shows block diagrams for
the downstream and upstream paths for the integrated system, respectively.29
These block diagrams are simply the reference block diagrams (see Figure 7-14)
added to the appropriate frequency translators for downstream and upstream
operations. The LMDS mm/rf system employs a free-running DRO with no phase lock
loop to covert signal to/from the LMDS frequency band. Figure 7-18 shows the
BER performance of the integrated system for various values of Eb/No.30 The
reference curves are obtained using the reference setup in Figure 7-14. It was
found that the performance degradation is acceptable. There were 3 and 4.25 dB
of degradation at a BER of 10-6 for the downstream and upstream paths,
respectively.
An approach to provide telephony over LMDS was presented in this
part of the chapter. The telephony solution is designed using current PCS
technology. A product based on the European DECT standard was presented. The
advantage of the proposed telephony solution is that by using DECT or other
existing PCS technology, the cost of product development and time to market can
be reduced. In addition, the LMDS architecture effectively can extend the range
of existing PCS systems while integrating them to a wireless broadband
infrastructure. A service provider therefore potentially can reduce
infrastructure cost by using only one tall antenna tower at the headend to
deliver integrated services (telephony, high-speed data, and video) to
customers within a 2-km radius at the LMDS frequency band as opposed to
delivering only voice services at the PCS band using several other antenna
towers.
LMDS/LMCS Market Drivers
Since completion of the FCC spectrum auctions, local multipoint
distribution services (LMDS) have been proposed for the delivery of a wide
range of services. A point-to-multipoint radio access system is capable of
providing services ranging from voice to high-speed data (up to 155 Mbps) and
serving customers ranging from small to large businesses. In this brief
overview, let's take a look at the LMDS market drivers, where it can
satisfy a market need, the characteristics of the market, and its impact on the
technology.
LMDS: A Transport System
As
mentioned previously, LMDS differs from ordinary transport systems in the way a
train differs from a pipeline. Both are transport systems, but a pipeline can
transport only one product from one place to another. A train, on the other
hand, can transport many different products over the same infrastructure. LMDS,
implemented with a multiservice protocol such as Asynchronous Transfer Mode
(ATM), can transport, among others, voice, Internet, ethernet, video, computer
files, and transaction data.
It is the multipoint radio technology, combined with the appropriate
protocol and access method, that gives LMDS the tremendous potential to
transform society. With transport technologies such as fiber in place, how can
a newcomer compete? LMDS has some overwhelming advantages: reliability; as a
transport system, LMDS can be engineered to provide 99.999 percent
availability, rivaling that of the best fiber backbones; and speed of
deployment. Once a hub is installed (a matter of days), new customers can be
added in a matter of hours, future proof that LMDS provides data rates from T1
to OC-3c per user interface.
Managed Investment
An LMDS hub can provide service to all
buildings visible from the hub site. Physical
technologies such as copper or fiber require individual rights of way to each
building, as well as physical placement of the transport medium.
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