In this paper, we consider M-ary phase shifting keying (MPSK) and M-ary quadrature amplitude modulation (MQAM) data symbol burst transmission for adaptive decode-andforward (ADF) relay systems over quasi-static Rayleigh fading channels. A burst has pilot symbols and data symbols. From pilot symbols, we can estimate the channel gain and detect each relay node's transmission mode for ADF schemes. At first, we focus on the derivation of the received signal to noise ratio's (SNR's) probability

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... ty function (PDF) for ADF relay systems, in which error events at relay nodes and channel estimation errors are considered. Then, we derived the averaged channel capacity as an approximated closed-form for an arbitrary link signal-to-noise ratio (SNR) for different modulation orders. We verified its accuracy by comparison with simulation results. 150 Copyright ⓒ 2015 SERSC environment. By comparing the derived numerical results and simulation results, the accuracy of analytical approach is verified. Consequently, we can find that our derived analytical expression is a very tractable form, and can be used as a tool to verify the effects of a burst transmission on the averaged channel capacity.