To mitigate the strong interference from the Base Station (BS) to Device-to-Device (D2D) receivers when the downlink resource is shared by the D2D and cellular users, a novel null-space based precoding scheme is proposed. The proposed scheme takes into account the interference mitigation for D2D users when designing the downlink precoders of the BS. It allows the transmitted signal of BS to aim at the scheduled cellular user while lying in the interference-free subspace of the BS-D2D channel, so as to avoid the interference to the D2D receivers. To make the design more practical, the performance of the proposed scheme under imperfect channel estimations is further analyzed. Simulation results show that substantial gains in D2D throughput can be obtained by using the proposed precoding scheme, but such gain is largely dependent on the accurate channel state information of the BS to D2D receiver.%针对终端直通(D2D)用户复用蜂窝下行资源时，基站对D2D接收端干扰较严重的问题，该文提出一种基于Null-space的基站下行预编码策略。该策略通过在基站预编码时加入对D2D用户干扰抑制的考虑，使得基站在将下行波束对准其调度的蜂窝用户的同时，在 D2D 用户方向上形成零陷，从而有效地抑制基站对 D2D 用户的干扰。考虑到实际系统中基站难以获取完美信道信息，又进一步推导了信道估计误差对预编码性能的影响。仿真结果表明，该文提出的基于Null-space的预编码策略能有效提高D2D用户的吞吐量，但这种性能增益很大程度上依赖于基站到D2D用户信道估计的准确性。
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