simul - Simulating Digital Communication Systems Digital...

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Simulating Digital Communication Systems Digital communication systems are always simulated via a discrete, baseband equivalent model of the passband continuous time system. That is, we consider a system where the carrier is removed by means of a complex down-conversion to baseband, and then we sample the system at some rate in excess of the symbol duration T. If we sample everything at exactly the rate T, for two-dimensional signal sets one can regard the simulation as a direct implementation of the signal space model of the communications system. That is, every interval T we send a complex signal over a channel, which attenuates by some factor and adds noise in each dimension. The demodulator then forms decision variables (the received vector components). In some circumstances we would like to model effects such as intersymbol interference or frequency selective fading. In this case, we can model the impulse response of the channel as an FIR filter, with the delays corresponding to T or to some shorter interval depending upon the application. By using a shorter interval such as T/2 or T/4 it is possible to pass the signals through an interpolation filter on the other side to model for example timing and phase estimation error effects. Fractionally spaced equalizers additionally enable proper synthesis of the phase without requiring use of a matched filter. Verification of Accuracy of a Simulation The most difficult aspect of a simulation is the creation of a proper model for the system. There are many points at which factor of 2 errors and the like can creep into model parameters, and thus it is always worthwhile in constructing a simulation to compare individual blocks and subsystems to known analytical results. For example, a simulation might be created to determine the error probability over a Rician-fading channel with arbitrary diversity order, Doppler spread,
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This note was uploaded on 03/29/2011 for the course EECS 100 taught by Professor Dun during the Spring '11 term at CSU Channel Islands.

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simul - Simulating Digital Communication Systems Digital...

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