We investigate an adaptive modulation method for simultaneous voice and data transmission over fading channels. The proposed scheme applies different switching threshold(carrier to noise ratio (CNR) boundary between different modulation index) according to required bit error rate (BER).Specifically, it starts to transmit voice with binary phase shift keying (BPSK) when the CNR allows BPSK transmission with target BER for voice. When the CNR is Improved to support Mary quadrature amplitude modulation (QAM) with target BER for data, it begins to transmit both data and voice with the same BER target. Compared with adaptive hybrid binary shift keying(BPSK)/M-amplitude modulation (AM) modulation scheme [5],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 Rayleigh fading channel with an average CNR of 30 dB, 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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