Legacy user support in multiuser MIMO with vector perturbation precoding

We address backward compatibility issues when developing new mobile communication systems, where older “legacy” users do not support features of the newer system. With multiuser MIMO vector perturbation precoding, all active users have to support modulo detection of the perturbed receive signal. Since legacy users cannot undo this nonlinear operation, they must be kept spatially separated from the new users by provision of legacy zones. New users may also switch off advanced features and share the same resources. In any way, flexibility and performance of the system are limited by these constraints. This paper proposes a legacy mode enabling joint transmission to new and legacy users, whereby new users receive nonlinearly precoded transmission, but legacy users remain linearly precoded. We formulate this as a zero-element constraint on the perturbation vector and present a method incorporating it into finding the optimum perturbation. We present simulation results showing that transmitting to new and legacy users together works well, but does have an impact on system diversity. Providing higher flexibility for vector perturbation precoding, our legacy mode can help introducing advanced MU-MIMO systems more easily.