Page 411 - Complete Wireless Design

Communications System Design

410 Chapter Nine

tracking down and eliminating the interferers (if possible). Timing is a
control issue, and will normally not be your concern.
5. Frequency stability is quite critical for the upstream channel, especially
when wideband cable modems are employed as the modulator/demodula-
tors. In these cases, stability must be better than ±12 kHz to catch a stan-
dard cable modem’s preamble (or the link will not lock up), as well as to
maintain the BER over time with no automatic reboots. A longer preamble
that can tolerate a frequency spec of ±25 kHz is possible with some cable
modems.
6. Group delay ripple (GDR) must be no more than 75 ns for QAM-64, and
less than 200 ns for QPSK, as increased GDR increases ISI. Simple ana-
log design considerations (mainly for the system’s bandpass filters) and
digital adaptive equalizing will keep GDR in spec.
7. Increased system linearity, through hardware design and/or the appropri-
ate SSPA back-off at the hub and client transmitter, will lessen BER
degradation of a digital signal by decreasing intermodulation distortion
levels.
8. Multipath causes phase cancellation, which creates both amplitude and
phase distortions of the desired signal. This decreases the received signal’s
strength, thus decreasing SNR and increasing BER. It also creates unde-
sirable amplitude notches or slopes, causing increased ISI. Increasing the
gain, and thus the directionality, of the antennas; proper location of the
hub transceiver (such as not placing it in front of a tall building or moun-
tain); and using equalizers will all mitigate, but cannot eliminate, multi-
path problems.
9. Near/far receiver issues can mean some compromises in transmitter and
receiver design. Depending on how close a client is to the hub, a close-in
receiver may become saturated by the constant power output of the hub
transmitter in a multiclient environment. This can be significantly
reduced by using receiver front-end attenuators, utilizing the natural
shadowing effect created by a high-mounted hub antenna (allowing the
close-in receiver to be under the main lobe of the transmitter’s antenna),
employing different gain antennas for the close-in client receiver to that of
the farther out client receiver, or using a switchable front end that will
permit the LNA to be bypassed if the client receiver is too close to the hub
transceiver.
10. Phase noise created by the nonperfect nature of a real LO will degrade the
BER and increase the ISI. A digital wideband client receiver, for example,
should possess a phase noise spec 85 dBc/Hz at 10 kHz, or better, to
lessen BER degradation of the incoming signal.
Assorted issues. The hub antenna in a multipoint system environment must
be located on as high an area and/or tower as practical. This will allow the cov-

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