Spectral efficiency of packet transmission with adaptive uncoded signaling compared to channel capacity

The throughput performance (spectral efficiency) versus SNR of packet transmission using uncoded signalling is compared to channel capacity. It is shown that the uncoded M-QAM signalling is 8.25 /spl plusmn/ 0.5 dB away from capacity over a wide range of block lengths L. For m-chip spread spectrum, the corresponding value is 10.5 dB. This result may be useful for evaluating the benefit of adaptive signalling in various system contexts without the need to refer to a specific air interface.