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# Examination of a digital radio link

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A digital radio link is designed to carry a T1 rate digital signal (1.544 Mbps). The transmitter and receiver have ideal RRC bandpass filters with roll off factor (r) = 0.2.

a. What RF bandwidth must the radio link provide if the modulation is binary phase shift keying (BPSK)?

b. The link is unchanged but the modulator and demodulator in the transmitter and receiver are changed to transmit and receive QPSK. The link has the bandwidth you found in part (a) above. What is the maximum bit rate that the link can carry using QPSK modulation?

c. If the filters in the transmitter and receiver are changed for ideal RRC filters with roll off factor (r) = 0.5, what is the maximum symbol rate for this link?

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#### Solution Preview

(a) Bit rate (R) = Symbol Rate {SR} x Bits/Symbol {m}

R = m*SR (1)

Since BPSK is only two levels (M = 2) we get the number of Bits/Symbol (m) determined from (2) as

m = log2{M} (2)

m(BPSK) = log2(2) = 1

Therefore symbol rate for the BSPK case, using (1), is

SR (BPSK) = R/m(BPSK) = 1.544/1 = 1.544 MSymbols/s

The Symbol rate of a modulation set occupies a Bandwidth that is the real RF Bandwidth (B) reduced by a factor of (1+r) where (r) represents ...

#### Solution Summary

Examination of a digital radio link and determination of some of its parameters including RF bandwidth, bit rate and sysmbol rate for known parameters calculated

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## A number of RF link parameters are determined from information about the link provided

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.

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