Proposals have appeared recently for the use of pilot symbols to mitigate the effects of rapid fading in mobile communications. Unlike the more familiar pilot tone systems, pilot symbol assisted modulation (PSAM) does not affect the transmitted pulse shape or the peak-to-average power ratio, and implementation is straightforward. This paper puts PSAM on a solid analytical basis, a feature missing from previous work. It presents closed form expressions for the BER in BPSK and QPSK, for a tight upper bound on SER in 16QAM, and for the optimized receiver coefficients. The error rates obtained are lower than for differential detection for any combination of SNR and Doppler spread, and the performance is within 1 dB of a perfect reference system under slow fading conditions, and within 3 dB when the Doppler spread is 5% of the symbol rate.
A modulation system is proposed that continuously adjusts its data rate in response to signal strength variations in a fading channel. The optimum variation of data rate with channel conditions is determined, and includes the effects of feedback channel time delay, the interval between rate changes, and restriction of the number of allowable rates. Application of these results to a full duplex situation allows determination of the optimum fraction of the data stream to be devoted to service information. Comparison of this scheme with diversity transmission on the basis of error probability and bandwidth utilization reveals a reduction on the order of 14 dB in transmitter power for a typical duplex link.
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