Page 398 - Complete Wireless Design

Communications System Design

Communications System Design 397

P signal strength, in dBm, available from the receiver’s output
OUT
into the modem or detector input
RX gain, in dB, of the entire receiver stage—including filters,
dB
conversions, and amplifiers—from the front end to the output
into the modem or detector stage
L loss of the coax cable, in dB, between the receiver’s antenna
r
and its front end (presented as a positive number only)
NF noise figure of the receiver, dB
BW 6-dB bandwidth of the IF (or, more accurately, the noise
N
bandwidth), Hz
The above calculations should give us an indication of the signal power, noise
figure, and gain required across a particular link.

Real-life path losses. Since free-space path loss in a microwave link will not be
the only loss encountered, other impairments must be added to this to estab-
lish a worst-case scenario within your link.
The path losses attributed to rain begin to dramatically increase at frequen-
cies above 10 GHz. A heavy rain shower can attenuate a 10-GHz signal by 2 dB
per kilometer (or more). As these frequencies increase, so do the losses. It is,
however, only the space between the transmitter’s and receiver’s antennas that
actually has this rainfall that will force such severe attenuation, so much of a
transmission path may in fact be clear. A wet snow has a similar attenuating
effect, while a dry snow will have little significance, even at the higher
microwave frequencies. Dense fog will attenuate a 10-GHz signal by up to 1 dB
per kilometer, with much smaller attenuation levels as the frequencies decrease.
Water vapor and atmospheric oxygen absorption create attenuation at 18 GHz
and above, with various high attenuation peaks at several microwave frequen-
cies. Fresnel zone clearance inadequacies, atmospheric reflection, and scattering
will also conspire to increase path losses. But not all impairments are continu-
ous attenuation mechanisms, especially the atmospheric effects causing reflec-
tion. Nonetheless, all such losses can be compensated for by increasing the
power from the transmitter, decreasing the receiver’s NF and increasing its
gain, and raising the transmitter and receiver antenna gain. The end effect we
want is to make sure that there is enough signal amplitude out of the IF of the
receiver to drive the modem or demodulator, and that that signal has the SNR
required for reliable demodulation at a low BER.

9.3.3 Will it work?
Consider a simple link budget analysis of the communications system and
path of Fig. 9.5.
A 40-km link, with no obstructions, is needed to operate dependably at 2.4
GHz with a transmitter/antenna combination that can output a 47 dBm
(50 W) EIRP signal with our modulation of choice. We find that the receiver
must, through all atmospheric conditions, maintain a signal into the detector

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