Partial Equivalent Channel Inversion Based Precoding with Mode Adaptation in Multiuser MIMO Systems

In downlink multi-user MIMO systems, Block Diagonalization (BD) is a well known linear precoding scheme that eliminates inter-user interference. However, BD requires precoded pilot symbols or feed-forwarding to coordinate precoder information from base station to users. Pre-coded pilot symbols, specially user specific pilots, have an overhead and complexity that grows with the number of users; on the other hand, feed-forward links are limited in capacity. In this paper, a new technique called Partial Equivalent Channel Inversion (PECI) based BD is proposed. PECI avoids the need for coordination of precoder information by combining BD with an inversion of a part of the equivalent channel. The performance of PECI based BD is evaluated analytically and demonstrated using various simulations. Even though PECI based BD is sub-optimal, simulations show that the sum rate loss is very small and decreases with increase in spatial correlation. In order to improve the performance of PECI, a power allocation based mode selection algorithm, which requires forwarding only the number of streams allocated to a user, is also proposed. PECI is a general method that is also applicable to other precoding schemes in single and multi-cell precoding.