  In this paper, we derive a closed-form expression for the combiner of a
multiple-input-multiple-output (MIMO) receiver equipped with a
minimum-mean-square-error (MMSE) estimator. We propose using
variable-bit-resolution analog-to- digital converters (ADC) across radio
frequency (RF) paths. The combiner designed is a function of the quantization
errors across each RF path. Using very low bit resolution ADCs (1-2bits) is a
popular approach with massive MIMO receiver architectures to mitigate large
power demands. We show that for certain channel conditions, adopting unequal
bit resolution ADCs (e.g., between 1 and 4 bits) on different RF chains, along
with the proposed combiner, improves the performance of the MIMO receiver in
the Mean Squared Error (MSE) sense. The variable-bit-resolution ADCs is still
within the power constraint of using equal bit resolution ADCs on all paths
(e.g., 2-bits). We propose a genetic algorithm in conjunction with the derived
combiner to arrive at an optimal ADC bit allocation framework with significant
reduction in computational complexity.
