  This paper investigates user cooperation in massive multiple-input
multiple-output (MIMO) systems with cascaded precoding. The high-dimensional
physical channel in massive MIMO systems can be converted into a
low-dimensional effective channel through the inner precoder to reduce the
overhead of channel estimation and feedback. The inner precoder depends on the
spatial covariance matrix of the channels, and thus the same precoder can be
used for different users as long as they have the same spatial covariance
matrix. Spatial covariance matrix is determined by the surrounding environment
of user terminals. Therefore, the users that are close to each other will share
the same spatial covariance matrix. In this situation, it is possible to
achieve user cooperation by sharing receiver information through some dedicated
link, such as device-to-device communications. To reduce the amount of
information that needs to be shared, we propose a decoding codebook based
scheme, which can achieve user cooperation without the need of channel state
information. Moreover, we also investigate the amount of bandwidth required to
achieve efficient user cooperation. Simulation results show that user
cooperation can improve the capacity compared to the non-cooperation scheme.
