Multi-User Pre-Processing in Multi-Antenna OFDM TDD Systems with Non-Reciprocal Transceivers

The combination of OFDM with joint pre-processing in adaptive multi-antenna systems offers both an ease of equalization in frequency-selective channels and keeping the signal processing at the mobile stations simple. In addition, the spatial dimension can be efficiently exploited to ensure high system throughput. With the utilization of higher-order modulation the performance of the system is highly sensitive to multiple access interference and nonorthogonal subchannels due to hardware impairments or insufficient adaptation to the current channel conditions. A further source of error in TDD systems are the non-reciprocal transceivers inhibiting the baseband-to-baseband channel reciprocity required for accurate channel state acquisition based on the uplink channel estimate. In this paper, measurement results of a low-cost hardware-based calibration are presented and the drawbacks are discussed leading to the utilization of a recently introduced relative calibration. The latter is applied to an OFDM system and achieves or at least approximates the baseband-to-baseband reciprocity. Thus, it enables the link adaptation using the uplink channel state information. Furthermore, preliminary hardware implementations of the relative calibration running on a real-time system show accurate results.