Error floor of pi/4-DQPSK in mobile radio channels

We analyze the error floor of pi/4-DQPSK in mobile radio channels due to the intersymbol interference caused by the time dispersion of the channel and compare it to results for GMSK. We first analyze a two-delay channel with fixed (but arbitrary) sampling instant; for optimum sampling, the error probability BER depends only on the mean rms delay spread S normalized to the symbol length T: BER=k (S/T)(2), where the proportionality constant k depends on the filtering. For two-branch selection diversity, the BER goes with (S/T)(4), and is about a factor 3 better for BER-driven diversity than for RSSI-driven diversity. In a next step, we analyze the care of adaptive sampling, and find that the error probability can be reduced as compared to fixed sampling. However, in contrast to MSK, ii is not possible to completely eliminate the error floor even for ''basic'' pi/4-DQPSK. Finally, we analyze the N-path channel with fixed sampling time. We find that here, the BER depends not only on the delay spread, but also on the shape of the delay power profile.