On the Exact Distribution of Mutual Information of Two-user MIMO MAC Based on Quotient Distribution of Wishart Matrices

TL;DR

This paper derives exact formulas for the distribution of mutual information in a two-user MIMO MAC over Rayleigh fading channels, enabling precise performance analysis and effective Gaussian approximations.

Contribution

It provides the first exact closed-form expressions for the PDF and CDF of mutual information based on quotient Wishart matrices, improving analysis accuracy.

Findings

Exact distribution formulas match simulations closely.

Gaussian approximation is effective for outage probability estimation.

Adding antennas improves capacity and reduces outage probability.

Abstract

We propose the exact calculation of the probability density function (PDF) and cumulative distribution function (CDF) of mutual information (MI) for a two-user MIMO MAC network over block Rayleigh fading channels. So far the PDF and CDF have been numerically evaluated since MI depends on the quotient of two Wishart matrices, and no closed-form for this quotient was available. We derive exact results for the PDF and CDF of extreme (the smallest/the largest) eigenvalues. Based on the results of quotient ensemble the exact calculation for PDF and CDF of mutual information is presented via Laplace transform approach and by direct integration of joint PDF of quotient ensemble's eigenvalues. Furthermore, our derivations also provide the parameters to apply the Gaussian approximation method, which is comparatively easier to implement. We show that approximation matches the exact results remarkably well for outage probability, i.e. CDF, above 10%. However, the approximation could also be used for 1% outage probability with a relatively small error. We apply the derived expressions to analyze the effects of adding receiving antennas on the receiver's performance. By supposing no channel knowledge at transmitters and successive decoding at receiver, the capacity of the first user increases and outage probability decreases with extra antennas, as expected.