Two-Stage ICI Suppression in the Downlink of Asynchronous URLLC

Applying the concept of virtual cell together with open-loop communications has been recently shown to achieve ultra-reliable and low-latency communication (uRLLC) in vehicular networks. Nevertheless, losing perfect synchronization due to proactive communications creates difficulty in mitigating multiple access interference (MAI). Multiple carrier frequency offsets (CFOs) resulting from different oscillators at different access points (APs) incur serious inter-carrier interference (ICI) to further complicate downlink MAI. Asynchronous multiuser detection (MUD) with ICI-Whitening was shown leading to satisfactory performance, but the whitening scheme needs the covariance matrix of ICI that is practically hard to obtain for downlink receivers. We develop a two-stage ICI suppression method to resolve this challenge. The first-stage processing is Pseudo-ICI-Whitening (P-ICI-W) or its simplified version Pseudo-Truncate ICI-W (PT-ICI-W), which does not rely on the estimation of ICI covariance and is suitable for asynchronous downlink. In terms of post-processing signal-to-interference-plus-noise ratio (SINR) and bit-error rate (BER), our proposed mechanism can approach ICI-Whitening. The second-stage processing is based on the recently proposed generalized linear minimum mean square error (LMMSE) projection to further cancel some ICI terms. Moreover, our proposed mechanism is compatible with space-time-block-coded signals, namely Alamouti coding and Complex Interleaved Orthogonal Design (CIOD), to yield more reliable proactive wireless communications.