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 suboptimal, 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.