Impact of RF front-end impairments and mobility on channel reciprocity for closed-loop multiple antenna techniques

We focus on multiple antennas TDD OFDM-based WLANs. In this context, we assume that channel state information (CSI) is available at the transmitter (closed-loop techniques). In TDD systems, it is commonly assumed that the propagation channel is reciprocal, i.e. the same on the up link (UL) and the down link (DL). This implies that the channel estimated on the UL might be used as CSI at the transmitter for DL transmission. However, the RF front-ends contribute to the channel experienced, and their imperfections might be inconsistent with the reciprocity assumption. Based on observed characteristics of actual components, a model for these imperfections is introduced in order to quantify their effect on the performance of closed-loop techniques based on channel reciprocity. Simulation results for the TxAA and transmit selection diversity (TSD) schemes are provided in the context of a multiple transmit multiple receive (MTMR) IEEE 802.11a WLAN system with 4 transmit and one receive antennas. The impact of the delay between UL and DL is also considered in a mobility scenario.