We discuss the problem of designing channel access architectures for enabling fast, low-latency, grant-free, and uncoordinated uplink for densely packed wireless nodes. Specifically, we study random-access codes, previously introduced for the AWGN MAC, in the practically more relevant case of Rayleigh fading, when channel gains are unknown to the decoder. We propose a random coding achievability bound, which we analyze both non-asymptotically and asymptotically. As a candidate practical solution, we propose an explicit iterative coding scheme. The performance of such a solution is surprisingly close to the finite blocklength bounds. Our main findings are twofold. First, just like in the AWGN MAC, we see that jointly decoding a large number of users leads to a surprising phase transition effect, where, at spectral efficiencies below a critical threshold, a perfect multi-user interference cancellation is possible. Second, while the presence of Rayleigh fading significantly increases the minimal required energy-per-bit, the inherent randomization introduced by the channel makes it much easier to attain the optimal performance via iterative schemes. We hope that a principled definition of the random-access model, together with their information-theoretic analysis, will open the road towards unified benchmarking and performance comparison of various random-access solutions for the 5G/6G.

中文翻译：

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无线上行链路的节能编码随机接入

我们讨论了设计信道接入架构的问题，以便为密集的无线节点实现快速、低延迟、免授权和非协调上行链路。具体来说，我们研究了之前为 AWGN MAC 引入的随机访问码，在实际更相关的瑞利衰落情况下，当解码器不知道信道增益时。我们提出了一个随机编码可实现性界限，我们对其进行非渐近和渐近分析。作为候选的实用解决方案，我们提出了一个显式迭代编码方案。这种解决方案的性能惊人地接近有限块长度界限。我们的主要发现是双重的。首先，就像在 AWGN MAC 中一样，我们看到联合解码大量用户会导致令人惊讶的相变效果，其中，在低于临界阈值的频谱效率下，完美的多用户干扰消除是可能的。其次，虽然瑞利衰落的存在显着增加了每比特所需的最小能量，但信道引入的固有随机化使得通过迭代方案获得最佳性能变得更加容易。我们希望随机接入模型的原则性定义及其信息理论分析将为 5G/6G 的各种随机接入解决方案的统一基准测试和性能比较开辟道路。通道引入的固有随机化使得通过迭代方案获得最佳性能变得更加容易。我们希望随机接入模型的原则性定义及其信息理论分析将为 5G/6G 的各种随机接入解决方案的统一基准测试和性能比较开辟道路。通道引入的固有随机化使得通过迭代方案获得最佳性能变得更加容易。我们希望随机接入模型的原则性定义及其信息理论分析将为 5G/6G 的各种随机接入解决方案的统一基准测试和性能比较开辟道路。