We study the problem of unsourced random access (URA) over Rayleigh block-fading channels with a receiver equipped with multiple antennas. We propose a slotted structure with multiple stages of orthogonal pilots, each of which is randomly picked from a codebook. In the proposed signaling structure, each user encodes its message using a polar code and appends it to the selected pilot sequences to construct its transmitted signal. Accordingly, the transmitted signal is composed of multiple orthogonal pilot parts and a polar-coded part, which is sent through a randomly selected slot. The performance of the proposed scheme is further improved by randomly dividing users into different groups each having a unique interleaver-power pair. We also apply the idea of multiple stages of orthogonal pilots to the case of a single receive antenna. In all the set-ups, we use an iterative approach for decoding the transmitted messages along with a suitable successive interference cancellation technique. The use of orthogonal pilots and the slotted structure lead to improved accuracy and reduced computational complexity in the proposed set-ups, and make the implementation with short blocklengths more viable. Performance of the proposed set-ups is illustrated via extensive simulation results which show that the proposed set-ups with multiple antennas perform better than the existing MIMO URA solutions for both short and large blocklengths, and that the proposed single-antenna set-ups are superior to the existing single-antenna URA schemes.

翻译：我们研究了瑞利块衰落信道下配备多天线接收机的无源随机接入（URA）问题。本文提出了一种包含多阶段正交导频的时隙结构，其中每个导频从码本中随机选取。在所提出的信令结构中，每个用户使用极化码对其消息进行编码，并将其附加到所选导频序列上以构建传输信号。因此，传输信号由多个正交导频部分和一个极化编码部分组成，并通过随机选择的时隙发送。通过将用户随机划分为具有唯一交织器-功率对的不同组，进一步提升了所提方案的性能。我们还将多阶段正交导频思想应用于单接收天线场景。在所有设置中，我们采用迭代方法解码传输消息，并配合合适的连续干扰消除技术。正交导频的使用和时隙结构提高了所提方案的精度并降低了计算复杂度，使得短块长度实现更加可行。通过大量仿真结果展示了所提方案的性能，表明在短块长度和长块长度下，多天线设置均优于现有MIMO URA方案，且所提单天线设置优于现有单天线URA方案。