A Slotted Polar Random Spreading Scheme for Massive MIMO Unsourced Random Access

As a new massive multiple-input multiple-output (MIMO) unsourced random access scheme for wireless channels, this paper proposes a slotted polar random spreading scheme. In the proposed scheme, the transmission frame is partitioned into several slots and each user independently selects one slot for message transmission. The above-mentioned message is divided into three parts: The first part indicates which slot is selected while the second one corresponds to pilot and spreading sequence mapping. The remaining part is polar encoded and spread over the selected slot. On the receiver side, a low-complexity activity detection (AD) is composed of the energy detector (ED) and covariance-based maximum likelihood detector (CB-MLD). Using the AD's output along with estimated channel frequency responses, the demodulator provides soft information on the coded bit sequences for single-user polar decoders that can extract the message bit sequences. For further improvement with low-complexity, successive interference cancellation (SIC) is employed as a multiuser detector. Computer simulations for 100-bit transmission per message demonstrate that the proposed scheme can outperform the conventional state-of-art scheme called FASURA, with a large number of active users, and requires lower complexity.