  A unified analytical framework for optimum power allocation in the unordered
V-BLAST algorithm and its comparative performance analysis are presented.
Compact closed-form approximations for the optimum power allocation are
derived, based on average total and block error rates. The choice of the
criterion has little impact on the power allocation and, overall, the optimum
strategy is to allocate more power to lower step transmitters and less to
higher ones. High-SNR approximations for optimized average block and total
error rates are given. The SNR gain of optimization is rigorously defined and
studied using analytical tools, including lower and upper bounds, high and low
SNR approximations. The gain is upper bounded by the number of transmitters,
for any modulation format and type of fading channel. While the average
optimization is less complex than the instantaneous one, its performance is
almost as good at high SNR. A measure of robustness of the optimized algorithm
is introduced and evaluated. The optimized algorithm is shown to be robust to
perturbations in individual and total transmit powers. Based on the algorithm
robustness, a pre-set power allocation is suggested as a low-complexity
alternative to the other optimization strategies, which exhibits only a minor
loss in performance over the practical SNR range.
