  The parallel fading channel, which consists of finite number of subchannels,
is very important, because it can be used to formulate many practical
communication systems. The outage probability, on the other hand, is widely
used to analyze the relationship among the communication efficiency,
reliability, SNR, and channel fading. To the best of our knowledge, the
previous works only studied the asymptotic outage performance of the parallel
fading channel which are only valid for a large number of subchannels or high
SNRs. In this paper, a unified performance metric, which we shall refer to as
the outage exponent, will be proposed. Our approach is mainly based on the
large deviations theory and the Meijer's G-function. It is shown that the
proposed outage exponent is not only an accurate estimation of the outage
probability for any number of subchannels, any SNR, and any target transmission
rate, but also provides an easy way to compute the outage capacity, finite-SNR
diversity-multiplexing tradeoff, and SNR gain. The asymptotic performance
metrics, such as the delay-limited capacity, ergodic capacity, and
diversity-multiplexing tradeoff can be directly obtained by letting the number
of subchannels or SNR tends to infinity. Similar to Gallager's error exponent,
a reliable function for parallel fading channels, which illustrates a
fundamental relationship between the transmission reliability and efficiency,
can also be defined from the outage exponent. Therefore, the proposed outage
exponent provides a complete and comprehensive performance measure for parallel
fading channels.
