Simulation of Third Generation CDMA Systems - Virginia Tech
Simulation of Third Generation CDMA Systems - Virginia Tech
Simulation of Third Generation CDMA Systems - Virginia Tech
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The Rayleigh waveform is generated at a sampling rate equal to the chip rate.Samples for higher sampling rate is constructed by interpolation. This reduces the simulationtime significantly at the expense <strong>of</strong> an injection <strong>of</strong> a small amount <strong>of</strong> noise. The noisegenerated as a result <strong>of</strong> interpolating the Rayleigh waveform is insignificant as shown inFigure 3.7.The Rayleigh waveform can be generated at the symbol rate rather than at the chiprate. This will reduce the simulation run time even more. Even for a relatively large Dopplerspread <strong>of</strong> 223 Hz which corresponds to a mobile speed <strong>of</strong> 120 km/h, the interpolation noiseremains below –30 dB for the worst case scenario <strong>of</strong> a sampling factor <strong>of</strong> 8 and a spreadingfactor <strong>of</strong> 256. This noise does not cause any measurable change in the BER.3.2.4 Addition <strong>of</strong> NoiseThe Additive White Gaussian Noise (AWGN) added at the front end <strong>of</strong> the receiver isgenerated by a Gaussian random number generator. The variance <strong>of</strong> the noise distributiondepends on the Signal to Noise Ratio (SNR) or E Nb0at the receiver front end. The noisevariance is also a function <strong>of</strong> the spreading factor, signal amplitude and sampling rate i.e. thenumber <strong>of</strong> samples per chip. The calibration <strong>of</strong> noise is discussed in the following section.3.2.4.1 Calibration <strong>of</strong> NoiseLet us assume for the time being that no pulse shaping is employed. Figure 3.8a and3.8b show the chip and the double sided noise spectrum.37
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