  Beamforming is a widely used method of provisioning high quality wireless
channels that leads to high data rates and simple decoding structures. It
requires feedback of Channel State Information (CSI) from receiver to
transmitter, and the accuracy of this information is limited by rate
constraints on the feedback channel and by delay. It is important to understand
how the performance gains associated with beamforming depend on the accuracy or
currency of the Channel State Information. This paper quantifies performance
degradation caused by aging of CSI. It uses outage probability to measure the
currency of CSI, and to discount the performance gains associated with ideal
beamforming. Outage probability is a function of the beamforming algorithm and
results are presented for Transmit Antenna Selection and other widely used
methods. These results are translated into effective diversity orders for
Multiple Input Single Output (MISO) and Multiuser Multiple Input Multiple
Output (MIMO) systems.
