A Space-Time-Frequency Spreading CDMA Scheme for the UAV's Downlink

We have designed a space-time-frequency spreading code-division multipleaccess (SITS-CDMA) framework for the Umanned Aerial Vehicle's(UAV's) downlink orthogonal frequency-division multiplexing (OFDM) system employing multiple transmit and receive antennas(MIMO).Firstly,we analyse the UAV's downlink multi-user structure and corresponding the linear minimum mean-square error(MMSE) detection algorithm.Secondly, we present the spreading code design criterias of maximal sapce-time-frequency diversity gains and equal multipleaccess interferences over users, and conclude the spreading code design structure according to the signal-to-interference-noise ratio(SINR) of MMSE detector in conformity to the above design criterias,we find that the fraction codes corresponding the static child channels except for the different users are orthogonal. Not only affording the form of multi-access but also achieving the maximal diversity gains,the spreading code is optimized to match the sapce-time-frequency features of MIMO-OFDM systems.