D2D-Enabled User Cooperation in Massive MIMO

In this article, we investigate device-to-device (D2D) communication-enabled user cooperation in massive multiple-input multiple-output (MIMO) systems. In frequency-division duplexing massive MIMO systems, cascaded precoding has been adopted to reduce the overhead of channel estimation and limited feedback. The inner precoder in cascaded precoding depends on the spatial covariance matrix of the massive MIMO channel and can be the same for the users that are close to each other. When the users are close to each other, high-quality D2D links can be expected among the users and the users can form a virtual MIMO receiver via D2D links. Since the users may require different services, the required transmission rates of those users can also be different. To improve the overall user experience, utility functions are adopted in this article and two D2D-enabled user cooperation schemes, including the data-symbol-information exchanging and receive-signal-information exchanging, are proposed, which can allocate the overall transmission rate among the users adaptively in terms of the required transmission rates of different users. In this way, the overall utility function can be maximized.