The problem of designing downlink transmission schemes for supporting heterogeneous ultra-reliable low-latency communications (URLLC) and/or with other types of services is investigated. We consider the broadcast channel, where the base station sends superimposed signals to multiple users. Under heterogeneous blocklength constraints, strong users who are URLLC users cannot wait to receive the entire transmission frame and perform successive interference cancellation (SIC) due to stringent latency requirements, in contrast to the conventional infinite blocklength cases. Even if SIC is feasible, SIC may be imperfect under finite blocklength constraints. To cope with the heterogeneity in latency and reliability requirements, we propose a practical downlink transmission scheme with discrete signaling and single-user decoding (SUD), i.e., without SIC. We carefully design the discrete input distributions to enable efficient SUD by exploiting the structural interference. Furthermore, we derive the second-order achievable rate under heterogenous blocklength and error probability constraints and use it to guide the design of channel coding and modulations. It is shown that in terms of achievable rate under short blocklength, the proposed scheme with regular quadrature amplitude modulations and SUD can operate extremely close to the benchmark schemes that assume perfect SIC with Gaussian signaling.

翻译：本文研究了支持异构超可靠低延迟通信（URLLC）和/或其他类型业务的下行传输方案设计问题。我们考虑广播信道场景，其中基站向多个用户发送叠加信号。在异质块长度约束下，与传统的无限块长度情况不同，作为URLLC用户的强用户由于严格的延迟要求，无法等待接收整个传输帧并执行连续干扰消除（SIC）。即使SIC可行，在有限块长度约束下也可能存在不完美SIC。为应对延迟与可靠性需求的异质性，我们提出了一种采用离散信号与单用户解码（SUD，即不进行SIC）的实用下行传输方案。通过精心设计离散输入分布以利用结构干扰实现高效SUD，并推导了在异质块长度与错误概率约束下的二阶可达速率，据此指导信道编码与调制设计。研究表明，在短块长度可达速率方面，采用常规正交幅度调制与SUD的所提方案性能极接近假设完美SIC与高斯信号的基准方案。