Hi, I want to simulate a QPSK transmission through a multipath channel. I see that some people use 1 sample per symbol while others use 8 or 16. Could someone give me some hints about the advantages and disadvantages of each option? I'm thinking about...  whether the 1 sample per symbol option is valid if we assume ideal filtering, whether a doppler effect could be added in the future. As well, could someone give an expression to calculate the noise variance for a given Eb/No? To finish with  is Eb/No what some people call SNRper_information_bit? thanks Adam 

simulating QPSK modulation
Hi Adam, Typically the QPSK transmission (I and Q samples) should be passed through a Root Raised Cosine filter for bandwidth limiting (pulse shaping) and mitigating the effects of Inter Symbol Interference (for which a complementary Root Raised Cosine filter is implemented in the receiver to form a Matched filter). The effect of pulse shaping RRC filtering is to provide a roundness in the QPSK constellation diagram. The I and Q pulses are oversampled in order to provide continuous transition points, otherwise, just sampling at the symbol instants would give the constellation points only. These oversampled I and Q points can be used at the receiver for symbol timing recovery. Another advantage of oversampling the I and Q samples will be that at the receiver we can employ a loworder Lowpass antialias filter (as opposed to an ideal filter which will require sharp cutoff if we have 1 sample per I and Q) since we can use a higher sampling rate (at the oversampled frequency). But it is important that after sampling, the output should be downsampled at the symbolrate in order to ease processing in a Digital Signal Processor in the next stage. One way in which the noisevariance could be computed is by computing the variance in the I^2 and Q^2 samples over a period of time and movingaveraging it.The greater the period of time for estimation of variance (that is, the more the number of I and Q samples) the better would the estimate of noise be. Hope this information helps. Regards, Gopi mosse_adam <m...@yahoo.co.uk> wrote: Hi, Get a bigger mailbox  choose a size that fits your needs. 