Hybrid adaptive/non-adaptive multi-user OFDMA systems in the presence of user-specific imperfect channel knowledge

In this article, a hybrid multi-user OFDMA scheme which considers different user demands regarding channel access and user-specific imperfect channel quality information (CQI) is analytically described and evaluated. The considered hybrid scheme offers two possible modes to serve the user: firstly, via a non-adaptive mode which applies a discrete Fourier transform precoding to exploit frequency diversity and, thus, does not require any CQI at the transmitter. Secondly, via an adaptive mode which performs an adaptive resource allocation and link adaptation. Since this adaptation to the channel is done based on CQI, the adaptive mode is prone to imperfect CQI which results from estimation errors or time delays. Hence, in case of user-specific imperfect CQI, the question arises which user shall be served adaptively or non-adaptively and which resource shall be allocated to which user such that the system data rate is maximized while fulfilling a certain target bit error rate (BER) and minimum required user data rate. Analytical expressions of the system performance considering imperfect CQI and different user demands are derived. Based on these expressions, algorithms determining which user is served adaptively or non-adaptively subject to the BER and minimum data rate constraints are developed. Simulations show the superiority of the hybrid OFDMA scheme in terms of achievable data rate and user satisfaction compared to conventional pure adaptive and non-adaptive OFDMA schemes in the presence of user-specific imperfect CQI.