We derive a novel uplink-downlink duality principle for optimal joint precoding design under per-transmitter power and information constraints in fading channels. The information constraints model limited sharing of channel state information and data bearing signals across the transmitters. The main application is to cell-free networks, where each access point (AP) must typically satisfy an individual power constraint and form its transmit signal using limited cooperation capabilities. Our duality principle applies to ergodic achievable rates given by the popular hardening bound, and it can be interpreted as a nontrivial generalization of a previous result by Yu and Lan for deterministic channels. This generalization allows us to study involved information constraints going beyond the simple case of cluster-wise centralized precoding covered by previous techniques. Specifically, we show that the optimal joint precoders are, in general, given by an extension of the recently developed team minimum mean-square error method. As a particular yet practical example, we then solve the problem of optimal local precoding design in user-centric cell-free massive MIMO networks subject to per-AP power constraints.

翻译：针对衰落信道中发射机端存在每发射机功率约束与信息约束的场景，本文推导出一种新型的上行-下行链路对偶原理用于联合预编码优化设计。信息约束描述了信道状态信息与数据承载信号在发射机间共享的局限性。该原理主要应用于无蜂窝网络，其中每个接入点（AP）通常需满足个体功率约束，并利用有限协作能力形成其发射信号。本文提出的对偶原理适用于由经典硬界给出的遍历可达速率，可视为Yu与Lan在确定性信道下先前结果的一种非平凡推广。这一推广使我们能够研究超越先前技术所涵盖的簇级集中式预编码简单情形之外的信息约束问题。具体而言，我们证明最优联合预编码器通常由近期发展的团队最小均方误差方法的扩展形式给出。最后，作为一个实用化特例，我们解决了面向每AP功率约束的用户中心式无蜂窝大规模MIMO网络中最优局部预编码设计问题。