Approaching V-BLAST Capacity With Adaptive Modulation and LDPC Encoding

This paper presents a practical implementation of the vertical Bell Laboratories layered space-time (V-BLAST) type system, in which the multiple-input multiple-output (MIMO) open-loop capacity can be approached with conventional scalar coding, using adaptive modulation with appropriate channel codes, e.g., low-density parity-check (LDPC) codes and optimum successive detection (OSD). The density evolution technique is employed to determine the maximal achievable rate of an LDPC code for each transmit antenna for a given channel realization at a given signal-to-noise ratio. Numerical results will show that the average sum rate of the adaptively modulated LDPC encoded system is quite close to the V-BLAST capacity with both rate and power adaptations. Considering the performance degradation caused by error propagation due to the imperfect feedback and relatively long decoding delay in the OSD, we use parallel interference cancellation followed by minimum mean square error filtering in the bit error rate (BER) performance simulation. Simulation results will show that a target BER of 10^-5 can be achieved by the optimally designed LDPC codes. To simplify the code design, we replace the LDPC codes, optimally designed for each channel realization, with rate-compatible punctured LDPC codes, at the cost of a slight sum rate loss. If the fading process is nonergodic, the outage capacity corresponding to a given outage probability is used to measure the channel performance. As an example, we design the LDPC codes for an adaptively modulated 2times2 V-BLAST system to approach its outage capacity for a given outage probability.