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Examination of a digital radio link

A. A digital radio link operates at a frequency of 75 MHz and uses ideal RRC filters with
roll off factor r = 0.2 in the transmitter and receiver. The modulation used on the link is BPSK and the bit
rate is 100 kbps. The 75 MHz transmitter has an output power of 1.0 W.
Calculate the bandwidth occupied by the transmitted radio signal.

b. At the receiver of the radio link described in part (a) above, the CNR measured in clear air
conditions is 15.5 dB. The link has an implementation margin of 0.5 dB. Calculate the approximate BER at the
receiver output and choose an appropriate value from the following.

3 × 10-5 2 × 10-7 4 × 10-10 6 × 10-12 0

c. The modulation on the link is changed to Gray coded QPSK and the bit rate is increased to
200 kbps. The output power of the 75 MHz transmitter is increased to 2.0 W. All other
parameters of the link are unchanged. Calculate the approximate BER at the receiver output.

Solution Preview

a. Answer: 120 kHz

Here use occupied bandwidth B = (1+r)*SR where r is the roll off given and SR is the symbol rate for BPSK. Note
also that the SR for BPSK is just equal to the bit rate R. So in the specific case

SR = R = 100 ksps

B = (1+0.2)*100 = 120 kHz

b. Answer: 0.

The only reference I can find is by extrapolating the BER performance graphs for BPSK in reference [1].

First we note that the actual CNR hitting the demodulation stage is CNR - Implementation margin or

CNR = 15.5 - 0.5 = 15 ...

Solution Summary

A digital radio link operates at a frequency of 75 MHz and uses ideal RRC filters with roll off factor r = 0.2 in the transmitter and receiver. The modulation used on the link is BPSK and the bit rate is 100 kbps. The 75 MHz transmitter has an output power of 1.0 W. From this information the bandwidth occupied by the transmitted radio signal is determined.

At the receiver of the radio link described in part (a) above, the CNR measured in clear air conditions is 15.5 dB. The link has an implementation margin of 0.5 dB. The BER at the receiver output is determined

The modulation on the link is changed to Gray coded QPSK and the bit rate is increased to 200 kbps. The output power of the 75 MHz transmitter is increased to 2.0 W. All other parameters of the link are unchanged. The new approximate BER at the receiver output is determined from this information.

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