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    Microwave line of sight link transmits signal a distance

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    (1) A microwave line of sight link transmits a 10.0 GHz signal a distance of 25 km across a river estuary. The transmitter delivers 2 W to the transmitting antenna. The antennas at each end of the link have a gain of 36 dB. The receiver has a noise bandwidth of 20 MHz with rms noise power of -120.0 dBW. The transmitter and receiver have ideal RRC filters with alpha = 0.25. The receiver has an implementation margin of 2.0 dB.

    (2) The microwave link in the question above is modified to increase the capacity of the link by adding more channels to increase the transmitted bit rate.

    a. Four additional channels are added to the link using FDMA with 5 MHz guard bands between each channel. Each channel transmits a signal identical to the signal described in Question #1. What is the total RF bandwidth used by the LOS link?

    b. Under deep fade conditions on any channel, the modulation is changed to QPSK. When the CNR in the receiver is 20.6 dB, what is BER at the receiver output and the average time between bit errors?

    c. If the CNR falls to 15.6 dB, what is the new bit error rate on the link with QPSK modulation?

    d. The link could be modified to use TDMA to combine five bit streams. The bit rate of each bit stream is to be 100 Mbps in clear air conditions when 256-QAM modulation is used. (This is different from the bit rate in Question #1.) Ignoring any overhead bits added to the signals, what RF bandwidth is required to transmit the TDMA signal using ideal RRC filters with alpha = 0.25?

    e. The gain of the antennas must be increased when the TDMA system described in part (d) is used because the bit rate is higher than with FDMA and the noise power in the receiver is therefore higher. What gain must the antennas have, in dB, to obtain the same clear air CNR as in Question #1 part (b)? Which multiple access method would you recommend for this LOS link? Give your reasons.

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    Solution Preview

    Please see the attached document for complete and detailed response. As it is not exactly clear what the very last part of the question is asking (as it is not explicit enough) so I have assumed that it is asking to evaluate the most appropriate scheme for the link based on comparing FDMA(QPSK) and TDMA(256-QAM). As you can see it all depends on the type of traffic, QoS (BER), acceptable latency, bulk throughput required etc..

    The defining parameter in deciding which transmission and multiple access methodology is most appropriate is really ...

    Solution Summary

    A microwave line of sight link transmits signal a distance is examined.