MIMO OFDM pilot designs, CSI acquisition, and adaptive transmissions in wireless systems

Systems which utilize multiple input-multiple output orthogonal frequency division multiplexing (MIMO OFDM) for current and emerging wireless standards benefit from its advantages in terms of resource adaptation, simplicity in equalization, and rate and reliability improvement. Realizing most of these advantages will require channel state information (CSI). Thus, reliable CSI acquisition is crucial which is typically accomplished by transmitting pilots. MIMO OFDM pilot designs have been studied, but advances in technology have caused new considerations for designing these reference signals. In-phase and quadrature (I/Q) imbalances, practical device imperfections whose impacts are more pronounced as semiconductor technology moves towards smaller scale, can have a significant impact on CSI estimation performance. Some works have addressed pilot designs considering I/Q imbalance but not in an optimal fashion in terms of estimation performance and pilot overhead. We propose new pilot designs for MIMO OFDM systems with frequency dependent and independent I/Q imbalances that meet optimal pilot conditions which we derive to maintain optimal estimation performance while minimizing overhead. Another emerging concept is the use of relay and multi-point to multi-point (MP-MP) systems for transmission of signals. How these are incorporated into the communication system will significantly affect CSI acquisition algorithms, and previous works have limited scopes. We provide pilot designs, channel estimators, and CSI acquisition/sharing algorithms for many various relay and MP-MP transmission strategies which use minimal overhead while maintaining nearly optimal estimation performance. Additionally, we present a new adaptive relay transmission scheme which utilizes an initial CSI estimate of a time varying channel to switch between relays, transmit antennas, subbands, and MIMO transmission modes to enhance reliability of the relay link. For this adaptive scheme we implement temporal power and bit loading for the cascaded source-relay-destination link with/without inter-hop power allocation to further enhance the performance. The simulation results demonstrate an improved performance in the overall bit error rate (BER) and throughput.