We consider a dense small cell (DSC) network where multi-antenna small cell base stations (SBSs) transmit data to single-antenna users over a shared frequency band. To enhance capacity, a state-of-the-art technique known as noncoherent joint transmission (JT) is applied, enabling users to receive data from multiple coordinated SBSs. However, the sum rate maximization problem with noncoherent JT is inherently nonconvex and NP-hard. While existing optimization-based noncoherent JT algorithms can provide near-optimal performance, they require global channel state information (CSI) and multiple iterations, which makes them difficult to be implemeted in DSC networks.To overcome these challenges, we first prove that the optimal beamforming structure is the same for both the power minimization problem and the sum rate maximization problem, and then mathematically derive the optimal beamforming structure for both problems by solving the power minimization problem.The optimal beamforming structure can effectively reduces the variable dimensions.By exploiting the optimal beamforming structure, we propose a deep deterministic policy gradient-based distributed noncoherent JT scheme to maximize the system sum rate.In the proposed scheme, each SBS utilizes global information for training and uses local CSI to determine beamforming vectors. Simulation results demonstrate that the proposed scheme achieves comparable performance with considerably lower computational complexity and information overhead compared to centralized iterative optimization-based techniques, making it more attractive for practical deployment.

翻译：我们考虑一个密集小蜂窝网络，其中多天线小蜂窝基站通过共享频段向单天线用户传输数据。为提升容量，采用一种称为非相干联合传输的先进技术，使用户能够接收来自多个协调基站的数据。然而，非相干联合传输下的和速率最大化问题本质上是非凸且NP难的。虽然现有基于优化的非相干联合传输算法能够提供接近最优的性能，但它们需要全局信道状态信息并进行多次迭代，这使其难以在密集小蜂窝网络中实际部署。为克服这些挑战，我们首先证明功率最小化问题与和速率最大化问题具有相同的最优波束赋形结构，随后通过求解功率最小化问题从数学上推导出两类问题的最优波束赋形结构。该最优结构能有效降低变量维度。基于此最优波束赋形结构，我们提出一种基于深度确定性策略梯度的分布式非相干联合传输方案以最大化系统和速率。在该方案中，每个小蜂窝基站利用全局信息进行训练，并基于本地信道状态信息确定波束赋形向量。仿真结果表明，与基于集中式迭代优化的技术相比，所提方案以显著降低的计算复杂度和信息开销实现了可比性能，因而更具实际部署吸引力。