Effects of filtering on the performance of QPSK and MSK modulation in D-S spread spectrum systems using RAKE receivers

Direct-sequence spread spectrum systems utilizing RAKE receivers provide inherent diversity for in-building mobile CDMA radio. A new compact expression for the SNR of the decision variable of a RAKE receiver is derived for generalized offset quadrature carrier modulation in this multipath environment. This expression also applies to BPSK, QPSK, and MSK modulation. The novelty of the derived expression is that it includes the effects of filtering. Filtering must be used to eliminate out-of band radiation and can also be used to shape the in-band spectrum, It is shown that bandlimiting filters have a significant effect on the SNR. A comparison of bandlimited O-QPSK and MSK reveals that, under the same co-user load, MSK has a lower SNR-0.917 times that of O-QPSK. However, MSK has an offsetting advantage of being a constant envelope-type modulation that can be very efficiently generated using direct modulation of a power RF oscillator. It is further shown that the O-QPSK SNR can be increased by a factor of 1.63 by whitening the in-band spectrum.<
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