![]() ![]() ![]() ![]() It does this by using several ‘subreceivers’ called fingers. The delay profile of the channel provides multiple versions of the transmitted signal at the receiver.Ī rake receiver is a radio receiver designed to counter the effects of multipath. The multipath is nothing but the same signal arriving to the receiver through different paths and hence with different delays or phase shifts. Effectually, the diversity techniques can provide significant performance improvement for multipath. Since we have already learnt about these diversity techniques in the last chapter, these are not elaborated here. Both space diversity and time diversity are effective in this case. Multipath being a local phenomenon, and time-dependent too in some cases, use of diversity technique helps in mitigating the multipath effect. Since the multipath scenario changes with time, it is effective to use adaptive equalizers for mitigating the multipath effects more efficiently ( Chakrabary and Datta, 2007). Effectively, equalizers render the frequency response flat over the total bandwidth. The actual waveform of the transmitted signal is thus preserved and any additional group delay and phase delay between different frequency components, introduced by the channel are cancelled out. Then, assuming that the channel remains invariant, it compensates the channel effects on the signal by introducing reverse effects while the signal passes through it. The equalizer estimates the channel response as a function of time, which is possible since the receiver knows both the expected signal and that actually received in distorted form. When equalizers are used, first a known signal, also called training signal, is transmitted. In telecommunication, equalization is the reversal of distortion incurred by a signal transmitted through a channel. This may be corrected using an equalizer. Multipath fading can be viewed as transmission through a linear time varying system. The following methods are useful in mitigating the multipath effects. Multipath fading is a feature that needs to be taken into account when designing or developing a radio communications system. Rajat Acharya, in Satellite Signal Propagation, Impairments and Mitigation, 2017 8.2.3 Multipath Mitigation The antenna effectively gets gain from multipath while nearly eliminating its bad effects. The spider legs are twisted into many different angles, forming curves, all carefully calculated to either accept or recombine multipath signals or to cancel them out. Underneath its radome, this antenna looks like a spider that has died in agony, lying on its back. When it is not possible to eliminate multipath, a special antenna exists that seems to effectively minimize multipath signals. Multipath is a major problem and should be eliminated where possible by careful positioning of the antennae. This has the effect of creating artificial distance for the good signal, reducing its effectiveness and sometimes making the multipath signal the only one seen by the receiving antenna. Out-of-phase signals can effectively reduce the quality when they combine with a good signal at the receiving antenna. Additionally, the simple fact of the delay can cause signals to arrive out of phase. Thomas Norman CPP, PSP, CSC, in Integrated Security Systems Design (Second Edition), 2014 Cancellation Lossesīecause multipath signals arrive not only later but also often out of phase (not oriented the same as the transmitting antenna- pure vertical or pure horizontal), some signal cancellations can occur. ![]()
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