This paper addresses the performance limits of a multi-band/multi-user frequency fading channels. We firstly study the optimal diversity-multiplexing trade-off of such systems when channel state information is available at the receiver only (CSIR). However, when channel state information is made available at the receiver and at the transmitter (CSIT), the transmitter knows the channel gains and thus will not send bits unless they can be decoded correctly. Accordingly, there is no more notion of capacity versus outage in this case where the transmitter sends bits that cannot be decoded. We introduce the notion of outage capacity and derive optimal outage-rate trade-off for some commonly used power allocations and conclude on their ability to optimize rate and/or outage with respect to classical uniform power allocation.