High throughput constant envelope pre-coder for massive MIMO systems

This study describes a high throughput constant envelope (CE) pre-coder for Massive MIMO systems. A large number of antennas (M), in the order of 100s, serve a relatively small number of users (K) simultaneously. The stringent amplitude constraint (only phase changes) in the CE scheme is motivated by the use of highly power-efficient non-linear RF power amplifiers. We propose a scheme that computes the CE signals to be transmitted based on box-constrained regression (coordinate-descent), with an O(2MK) complexity per iteration per user symbol. A highly scalable systolic architecture is implemented, where M Processing Elements (PEs) perform the pre-coding for a system with up to K=16 users. This systolic architecture results in a very high throughput of 500 Msamples/sec (at 500 MHz clock rate) with a gate count of 14K per PE in 65nm technology.