Page 412 - Complete Wireless Design

Communications System Design

Communications System Design 411

erage of the largest amount of client transceivers, since most nonbusiness
antennas will normally be located at a single-dwelling roof level—and may be
up to 15 to 20 miles away. This permits clearance for the Fresnel zone and
increased mitigation of hub multipath problems, with the antenna as the high-
est local structure.
The implementation margin (IM) is another important aspect of systems
design. IM is the decrease in SNR, and the corresponding increase in BER,
that occurs in a system from the design to the actual building of the radio. The
IM losses must be accounted for by increasing the required SNR of the radio
to compensate for this effect during the design phase. When all of these
modem and radio impairments in an imperfect practical wireless link are
added—excluding the fade margin—an IM of up to 6 dB is common in high-
data-rate radios. These hardware impairments can be caused by excessive
phase noise, amplitude errors, noisy carrier recovery, jitter, group delay vari-
ations, noise through excess bandwidth, nonlinearities, thermal noise, adja-
cent channel interference, frequency instabilities, etc.
The choice as to whether we should use frequency division duplex (FDD) or
time division duplex (TDD) in a radio system should obviously be addressed
early in the design cycle. FDD radios operate with separate transmit and
receive frequencies—isolated from each other by a duplexer filter—to allow
the radio to transmit and receive during the same time period. A TDD radio
utilizes the same frequencies for both transmit and receive, but employs a
high-isolation switch in the transceiver’s front end, near the antenna, to
switch between transmit and receive during different time periods. Normally
the choice as to which one to exploit will be dictated by the systems engineer
on the project, as will most of the other system specifications.

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