A very low-complexity on-off division multiple access (ODMA) scheme is proposed for $K$ -user non-orthogonal multiple access (NOMA) systems. At the transmission side, each user employs the same length- $m$ channel code whose coded bits, after modulation, are sent in a random time-hopping manner. Specifically, $m$ coded bits are randomly scheduled and sent using $n$ time slots with $n\gg m$ , i.e., only $m$ slots are used for signal transmission and the other $n-m$ slots are idle. The slot selection, referred to as an on-off pattern, is unique to each user, and it is the only means of user separation. Consequently, at each time slot only a very few users (i.e., 2 or 3) may simultaneously access the channel, leading to a super-sparse access system. Due to the sparse access property, a very low-complexity iterative multi-user decoding method can be implemented on an almost tree-like factor graph. Compared with existing iteratively decodable code division multiple access (CDMA) schemes, such as sparse-CDMA and interleave division multiple access (IDMA), ODMA does not rely on repetition (spreading) or user interleaving. In fact, we show that in using extrinsic information transfer (EXIT) analysis and simulation, idling is more effective than repetition in terms of enhancing the multi-user iterative decoding performance. By replacing repetition with idling, a remarkable multi-user decoding performance gain is achieved and, at the same time, the decoding complexity is significantly reduced.

中文翻译：

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超稀疏开关分多址：用空闲代替重复

提出了一种非常低复杂度的开关分多址 (ODMA) 方案，用于 $K$ - 用户非正交多路访问 (NOMA) 系统。在传输端，每个用户使用相同的长度—— 百万美元 信道代码，其编码位在调制后以随机方式发送 跳时方式。具体来说， 百万美元 编码位被随机调度并使用 $n$ 时隙与 $n\gg m$ ，即只有 百万美元 时隙用于信号传输，其他 $nm$ 插槽空闲。插槽选择，称为开关模式，对每个用户来说都是唯一的，它是用户分离的唯一手段。因此，在每个时隙，只有极少数用户（即 2 或 3 个）可以同时接入信道，从而导致接入系统超稀疏。由于稀疏访问属性，可以在几乎树状的因子图上实现非常低复杂度的迭代多用户解码方法。与现有的可迭代解码码分多址 (CDMA) 方案相比，例如稀疏 CDMA 和交织分多址 (IDMA)，ODMA 不依赖于重复（扩展）或用户交织。事实上，我们表明，在使用外在信息传输 (EXIT) 分析和模拟时，在增强多用户迭代解码性能方面，空闲比重复更有效。通过用空闲代替重复，实现了显着的多用户解码性能增益，同时显着降低了解码复杂度。