Page 402 - Complete Wireless Design

Communications System Design

Communications System Design 401

with sufficient fade margin, to obtain this SNR (which is the value as required
by the receiver’s modem or detector in use at the output of the IF):
S 10 log BW 10 log [290 (10 NF/10 1) ] 198.6 G
A
where S entire receiver’s sensitivity, in dBm, with a 0 dB SNR at its
output
BW system’s IF bandwidth, Hz
NF receiver’s noise figure, dB
G receiver’s antenna gain, dB
A
Another factor that affects the noise of the receiver system is the antenna’s
orientation. If the receiving antenna is pointed more toward the sky (but not
toward the sun) in order to receive a signal from a transmitter that is placed
on a mountain top, as an example, then its noise temperature, and thus its NF,
will be less than if it were pointed at the ground (the ground has an approxi-
mate noise temperature of the above-mentioned 290 K). However, even if the
receiver antenna were pointed at the coldest region of space (to communicate
with a satellite transmitter), its sidelobe reception would still increase its
noise temperature.

9.4 The Complete System
9.4.1 Introduction
Complete communications systems design must take into account many vari-
ables, such as the link itself, general transmitter and receiver specifications,
type of modulation, data rate, and BER. Most of these issues have been or will
be covered in other sections of this book. This section, however, will concen-
trate on the overall technical issues that affect systems performance.

9.4.2 Wireless system design
At the beginning of any wireless design project, certain system parameters
must be established. In a digital data link, we would need answers to some of
the most basic of questions: What will be the frequency of operation? Will the
system need to be full duplex or half duplex? Will transmit and receive be sep-
arated in frequency [frequency division duplex (FDD)] or in time [time division
duplex (TDD)]? What is the bandwidth, modulation, fade margin, gain, BER,
SNR, phase noise, group delay, transmit power, and receiver NF? What num-
ber of conversion stages will we need for the transmitter and receiver sections?
What are the system’s required channels, frequency plans, frequency stabili-
ties, dynamic ranges, third-order intercept points, and frequency inversions?
Most of these specifications are interrelated, and must be answered, since dig-
itally modulated radio systems have to be especially designed for low levels of
phase noise, group delay variations, IMD levels, amplitude ripple and shape,

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