Investigation of Channel Reciprocity for OFDM TDD Systems
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This dissertation investigates the assumption of channel reciprocity in orthogonal frequency division multiplexing (OFDM) systems using time-division duplex (TDD) access. Within TDD systems uplink and downlink transmission share the same channel, and so channel state at the transmitter (CSIT) can be inherently obtained through uplink channel estimation assuming that the channel is reciprocal and static over a few packet transmissions. For both closed-loop SISO-OFDM (single-input single-output) and MIMO-OFDM (multiple-input multiple-output) systems, the availability of CSIT enables the transmitter to apply adaptive modulation and coding (AMC) to improve throughput or signal processing and precoding algorithms in order to obtain a spatial diversity and/or multiplexing gain. This results in improved performance as compared to open-loop MIMO systems in which the channel state is not known at the transmitter. However, signi cant deviations between transmitter and receiver channel state information may result in degradation of performance, as precoding at the transmitter will be based on erroneous channel state information. In this work, we observe the assumption of channel reciprocity using a real-time OFDM-PHY FPGA prototype wireless communications system and we look at possible factors that contribute to deviations between uplink and downlink channel estimates. We also look at common linear precoding schemes to compensate for channel non-reciprocity. Of all the possible factors that contribute to channel reciprocity deviations, we nd that the dominant factor comes from imperfections in the RF front-end components which result in signi cant channel phase response deviations across subcarriers between the uplink and downlink.