Effect of channel mismatch on AMC in asymmetric TDD/OFDM system with pre-equalization in downlink

In time division duplex (TDD) system, channel information extracted from the uplink signal can be used for channel pre-equalization of the downlink signal as well as for selecting the appropriate modulation and coding scheme (MCS) set. In asymmetric TDD systems which have more than one consecutive downlink slots, if pre-equalization is applied in the downlink, the signals to be transmitted in all downlink slots have to be pre-equalized with the same channel weights computed from the nearest uplink signal. This results in channel-mismatch in subsequent downlink slots and the transmission performance is severely degraded. Since the optimum signal-to-noise power ratio (SNR) thresholds to switch the MCS set are different in each slot due to the channel-mismatch, use of the same SNR thresholds in all downlink slots may severely degrade the system performance. Based on this observation, this paper investigates the impact of channel mismatch on adaptive modulation and coding (AMC) in asymmetric TDD systems with pre-equalization in the downlink. Moreover, application of correlation dependent slot-wise thresholds and CRC based adaptive threshold control scheme are considered to alleviate the effect of channel mismatch and compensate channel state information (CSI) estimation error.