Page 389 - Complete Wireless Design

Communications System Design

388 Chapter Nine

1. Calculate the thermal noise power within a certain receiver’s bandwidth:

KTB 174 10 log BW
where KTB thermal noise power within the specified BW, dBm, and BW
receiver’s bandwidth, Hz.
2. Calculate the lowest sensitivity that would produce a signal at the specified
SNR at the receiver’s output:

SENS ( KTB SNR)
LOW
where SNR receiver’s desired output signal-to-noise ratio, dB, and KTB
thermal noise power, dBm.
3. The NF required to just meet the sensitivity specifications:

NF SENS ( SENS )
MAX LOW
where SENS required sensitivity of the receiver as specified and
SENS the sensitivity of the receiver at 0 dB NF (for the specified
LOW
SNR). NF maximum noise figure that the receiver can have and still
MAX
satisfy the sensitivity requirement of SENS
Example. If a receiver under design has been specified to have a sensitivity of
100 dBm, what is the maximum NF that we can permit the receiver to have (with
zero sensitivity margin) if the receiver’s bandwidth is 100 kHz?
First, calculate KTB 174 10 log 100,000 124 dBm.
Now calculate what the sensitivity of the receiver would be with 0 dB NF. Let’s say
the modulation chosen to be received will need a minimum of 18 dB SNR for a cer-
tain desired BER: 124 18 106
What is the maximum NF allowed for this receiver with zero margin?
The answer is: 100 ( 106) 6 dB.
In reality, a safety margin would have to be added to the above calculation
(the reasons for this are presented later). To add a safety margin, the NF of the
receiver would have to be reduced even further. To increase the sensitivity
margin by 3 dB, the NF would have to be lowered to 3 dB (6 dB 3 dB 3
dB). Depending on the radio service, more margin may be required, especially
if the implementation margin is taken into account (the implementation mar-
gin is the loss in system specifications that occurs from the design stage to the
actual implementation stage, as well as from mass production variances.

9.1.3 Receiver issues
As stated, the receiver’s NF, which is dominated by its first stages, determines
the sensitivity of the receiver at VHF and above. But since externally generated
noise and interference in metropolitan areas can reach high levels, an LNA

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