  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.
