Abstract
We investigate an adaptive modulation method for simultaneous voice and data transmission over fading channels. The proposed scheme applies a different switching threshold-carrier-to-noise ratio (CNR) boundary between different modulation indices-according to the required bit error rate (BER). Specifically, it starts to transmit voice with binary phase shift keying (BPSK) when the CNR allows BPSK transmission with the target BER for voice. When the CNR is improved to support M-ary quadrature amplitude modulation (QAM) with the target BER for data, it begins to transmit both data and voice with the same BER target. Compared with the adaptive hybrid BPSK/M-AM (amplitude modulation) scheme (see Mohamed-Slim Alouini et al., IEEE J. Select. Areas Commun., vol.17, p.837-50, 1999), the proposed method has the same P/sub out/ (outage probability) for voice, and slightly higher P/sub out/ for data. However, the spectral efficiency in high CNR is improved significantly. For example, in a Rayleigh fading channel with an average CNR of 30dB, the proposed scheme is able to transmit about 4.5 bits/s/Hz of data while adaptive hybrid BPSK/M-AM can send only 3.5 bits/s/Hz of data at a BER less than 10/sup -4/.
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