Page 410 - Complete Wireless Design

Communications System Design

Communications System Design 409

Figure 9.10 The RF2418 IC from RFMD.

ohm matched outputs. And in the case of the RF2418 RFIC, its buffer may be
completely bypassed for an even larger power consumption savings, with the
IF output now going into the input of a high-impedance filter stage.

9.4.4 System issues
Within communications systems there are certain inherent general design
problems that the engineer should be aware of:
1. The Class A or Class AB power amplifier of a microwave transmitter in
multipoint service (where many clients are served by a single central hub
transceiver site) should be designed to transmit minimal additive wide-
band noise. This wideband noise effectively lowers the SNR at the central
hub receiver when in this multipoint environment, since there may be
hundreds of client transmitters that are on at the same time—whether
modulated or not.
2. Adjacent channel interference (ACI) problems can be alleviated by employ-
ing different polarizations or different frequencies in adjacent sectors in a
multisectorized hub antenna environment, by using sectorized hub anten-
nas with proper side and back lobe rejection, by avoiding excessive hub
transmitter power level outputs.
3. Amplitude ripple and tilt across a wideband digital channel must be min-
imized to a level not to exceed 0.5 dB for QAM and QPSK so as to decrease
BER degradation.
4. Such esoteric impairments as in-channel tones, transient bursts, or TDMA
timing issues can create crippling cochannel interference or decreased
BER. In-channel tones and transient bursts can be lessened by redirecting
the client antenna (if possible), by using a more directional antenna, or by

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