D2D Enabled Cooperation in Massive MIMO Systems With Cascaded Precoding

In frequency-division duplexing massive multiple-input multiple-output (MIMO) systems, cascaded precoding can convert the high-dimensional physical channel into a low-dimensional effective channel, such that traditional channel estimation and limited feedback can be used. The inner precoder in cascaded precoding depends on the spatial covariance matrix of the massive MIMO channel. If the users are close to each other, they may share the same inner precoder because the similar surrounding environments will result in the same spatial covariance matrix. In this case, user cooperation can be achieved among those users by sharing receiver information through device-to-device (D2D) links. We propose a decoding codebook-based scheme to reduce the amount of information that needs shared, which can achieve D2D enabled cooperation without the need of channel state information. We also investigate two practical schemes for sharing the receiver information.