Executive Summary : | It is expected that there will be a growing demand for mobile services in vehicles, high‑speed trains, and even in aircraft. The degree of mobility (i.e., speed) support required will depend upon the specific use case. For example, the speed can go up to 500 km/h in bullet trains and up to 1000 km/h in airplanes. High Doppler spread and multipath propagation observed in such applications result in a doubly dispersive channel, which significantly degrades its estimation, and consequently the bit error rate (BER) of orthogonal frequency division multiplexing (OFDM) scheme, which multiplexes symbols in the time-frequency (TF) domain. The recently-proposed orthogonal time-frequency space (OTFS) scheme, instead multiplexes symbols in the delay-Doppler (DD) domain, wherein a highly time-varying channel appears almost time-invariant. Therefore, OTFS systems offer a significantly lower BER than that of OFDM systems over vehicular speeds ranging from 30 km/h to 500 km/h. Furthermore, the DD channel is sparse, which can be exploited for i) enhancing the accuracy of channel estimation schemes, and ii) reducing the complexity of OTFS receivers. Unfortunately, the OTFS waveform, after interacting with the DD channel, results in a twisted convolution, which radically increases receiver complexity and poses significant challenges in designing channel estimation schemes. Most of the existing OTFS literature either use a dedicated pilot frame for channel estimation or place data and pilot in the same frame and insert zeros for mitigating the interference between them. This makes the existing channel estimation schemes spectrally efficient. The twisted convolution increases the size of the effective OTFS channel matrix, which in turn increases OTFS receiver complexity. Therefore, channel estimation and low-complexity receiver design have been at the centre of OTFS literature. The aim of this project is, therefore, to design and analyse low-complexity zero-forcing (ZF) and minimum mean square error (MMSE) receivers and spectrally efficient channel estimation schemes for practical pulse-shaped multi-user multiple-input multiple-output (MIMO)- and massive MIMO-OTFS systems, which, to the best of our knowledge, the existing OTFS literature has not done yet. To check the real-time performance of the designed receivers and the channel estimation schemes in the presence of practical impairments, this project would also be conducting real-time experiments by implementing a point-to-point MIMO-OTFS system on FPGA based software-defined radio (SDR). |