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# Calculating Path Loss, RF Power and Bit Rates

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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 ? = 0.25. The receiver has an implementation margin of 2.0 dB.

a. Calculate the path loss for this link in dB.

b. Calculate the RF power at the input to the receiver in dBW and the CNR in dB. Ignore any losses on the link.

c. How many bits are transmitted by each symbol in a 256 QAM signal?
What is the bit rate of the 256-QAM signal?

d. The river estuary is tidal causing the surface level of the water to rise and fall. At certain times of day, reflections from the water surface cause deep fading of the received signal. The modulation at the transmitter is changed to QPSK in deep fades. What is the new bit rate on the link?

e. State two ways in which the effect of multipath fading can be reduced on this link. Explain how each of the methods can reduce the depth of fading caused by multipath.

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#### Solution Summary

This solution provides assistance calculating the path loss, RF power, and bit rates.

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## Microwave line of sight link transmits signal a distance

(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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