On the Impact of Outdated Channel Information on the Capacity of Secondary User in Spectrum Sharing Environments

Conventional investigations on the capacity of a secondary link in spectrum sharing environments have assumed that a secondary user knows perfect channel information between the secondary transmitter and primary receiver. However, this channel information may be outdated at the secondary user because of the time-varying properties or feedback latency from the primary user. If the secondary user allocates transmission power using this outdated channel information, the interference power to the primary receiver will not satisfy the predetermined interference constraint. In this paper, we investigate the impact of outdated channel information between secondary and primary users in spectrum sharing environments. We begin by deriving the ergodic capacity of secondary user along with the optimum power allocation under the average received-power constraint. We also provide a closed-form expression for the ergodic capacity without interference from the primary transmitter, and the capacity bounds with interference from the primary transmitter. Moreover, we provide the power margin required to satisfy the interference outage probability at the primary user under the peak received-power constraint. Lastly, we derive the secondary user's ergodic capacity with and without interference from the primary transmitter. Comparisons done using simulations show the effects of the uncertainty of channel information and interference from the primary transmitter under both constraints.