Semi-blind approach to reception of short QAM data packets with pilot symbols

Coherent reception of signals with digital modulation requires information about the channel state, in particular, about the magnitude decay and phase offset introduced by the channel. Actually, receiving side usually estimates channel state using special pilot elements incorporated into the signal structure. Transmission of these redundant pilot signals requires extra power and time-frequency resource, that reduces spectral and energy efficiency of the communication system, so the number of pilots should be minimized. The paper describes two views on the reception of short data packets transmitted using quadrature amplitude modulation. On the one hand, coherent reception with semi-blind channel estimation is considered, namely, signal level and phase offset estimation is performed by blind algorithms using all symbols of the packet, and pilot symbols are only used to resolve ambiguity inherent for blind estimates of phase offset. On the other hand, the whole packet can be treated as a signal from a signal set suitable for noncoherent reception. High-correlation approximation for bit error probability is evaluated and compared with results obtained for coherent reception with channel estimation. Computer simulation showed that even for moderately short QAM packets bit error probability for semi-blind channel estimation is close to achievable limits for noncoherent reception of the packet as a whole with a loss about 0.4 dB.