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    Ananalysis of a LOS Microwave link & link budget parameters

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    A microwave line of sight link has a single hop with the following parameters:

    Transmit power 5 W
    Transmit and receive antenna gain 38 dB
    Length of link 40 km
    Transmission frequency 10 GHz
    All losses 3 dB
    Receive system noise bandwidth 30 MHz
    System noise temperature in clear air 600 K

    a. Calculate the path loss Lp for the link. Remember to put R in meters.

    b. Calculate the power received Pr, by the receiving terminal.

    c. Calculate the noise power, N, in the receiver, in dBW. Hence find the CNR in the receiver in clear air conditions.

    d. Multipath can cause the signal to fall by 40 dB. What is CNR in the receiver when this happens? Can the link maintain BER < 10^-6 using QPSK modulation with an implementation margin of 1.0 dB?

    e. How can multipath fading can be mitigated in a line of sight (LOS) link? Describe one method.

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    Solution Preview

    (a) Calculation of the path loss (LP) for the link:

    Path loss L in this example is the sum of the free space loss LFSL and the other link losses LA. The total link loss is summarized by (1).

    L = LFSL + LA (1)

    The free space loss is given in terms of the operating wavelength of the transmission (Lamda) and the distance of the transmission (r) by (2) where Lamda and r are measured in m.

    LFSL = 20*log10{4pi*r/Lamda} (2)

    Putting in values:

    r = 40 km = 4 x 10^4 m

    Lamda = c/f where c is the speed of light, c = 3 x 10^8 m/s, f is the frequency f = 10 GHz

    Lamda = (3 x 10^8)/(10^10) = 0.03 m

    Therefore using (2) the free space loss is determined as

    LFSL = 20*log10{(4pi x 4 x 10^4)/0.03} = 144.5 dB

    As other total losses are LA = 3 dB, using (1) we determine the total link loss as

    L = 144.5 + 3 = 147.5 dB

    (b) Calculation of the power received (Pr) at the receiver:

    Power received can be deduced by performing the power link budget and thus the power received is given by (3) as the dB sum of the power transmitted{Pt + Gt}, the gain of the receive site antenna Gr minus the total path loss L. Note that Pt is the actual dB power transmitted and Gt is the gain of the transmit antenna in the direction of edge coverage.

    Pr = {Pt + Gt} + Gr - L (3)

    Transmit power is given ...

    Solution Summary

    A microwave LOS link is examined to determine the link budget paramters including the path loss, the power received by the receiver, the noise power in the receiver and the CNR. The effects of multipath interference on reducing CNR at the receiver is then explored using typical paramters. Such reduced CNR in multipath conditions is then explored in terms of its affect in maintaining a suitable BER above 10^-6. A method to mitigate against multipath fading in LOS conditions is then described