Page 397 - Complete Wireless Design

Communications System Design

396 Chapter Nine

P power, in dBm, delivered by the transmitter into its own
t
antenna
G gain, in dB, of the transmitter’s antenna
t
G gain, in dB, of the receiver’s antenna
r
PL free-space path loss, in dB, between the transmitter and
dB
receiver antennas
5. Now, confirm that the transmitter is outputting enough power to overcome
the free-space path losses and to account for the desired fade margin,
and/or that the receiver will have a low enough NF for the desired SNR and
enough gain for proper output signal strength into the modem or detector:
P P L G G L L RX
OUT t p t r t r dB
where P power at the receiver’s output into the modem or detector,
OUT
dBm
P transmitter’s power output into its own antenna coax, dBm
t
L free-space path loss as calculated above, dB
p
G transmitter’s isotropic antenna gain, dBi
t
G receiver’s isotropic antenna gain, dBi
r
L loss of the transmitter’s coax, dB
t
L loss of the receiver’s coax, dB
r
RX gain, dB, of the receiver section itself (including conversion
dB
and filter losses, and the front end and IF amplifier gains)
The following formula will tell the designer the signal strength output of
the receiver stage when the signal strength at the receiver’s antenna is
known:
P P G L RX
OUT SIG dB; r dB
where P signal strength, in dBm, available from the receiver’s output
OUT
into the modem or detector input
P power, in dBm, picked up by the receiver’s antenna from the
SIG
transmitter’s antenna
G gain, in dBi, of the receiver’s antenna
dBi
L loss of the coax cable, in dB, between the receiver’s antenna
r
and its front end
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
The following formula will give the designer the signal-to-noise ratio at
the output of the receiver stage:
SNR P ( 174 10 log (BW ) NF L RX )
OUT N r dB
where SNR signal-to-noise ratio present at the output of the receiver, dB

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