Performance of Secondary Radios in Spectrum Sharing with Prioritized Primary Access

Research and discussion into more efficient utilization of frequency spectrum has led to a paradigm shift in future allocation and utilization of spectral resources in the frequency band of licensed (primary) systems. Secondary usage of licensed spectrum is being considered as a viable solution to the problem of inefficient usage of spectral resources. In this paper, we model and analyze the performance of the secondary system in dynamic spectrum access by secondary users employing an overlay approach of sharing spectral resources with a primary system. Our model assumes the operation of primary system is completely unaffected by the introduction of a secondary system, i.e., primary users have priority in transmission over secondary users. Each primary user is randomly assigned an available frequency slot for transmission by the primary system controller. Secondary users are required to sense across the same spectrum to identify any unoccupied frequency slots for transmission. During its transmission, if it detects a primary user accessing the same frequency slot, the connection will be forced to drop. Using a three-dimension Markov chain model, this paper presents an analytical approach to study the secondary system performance under a given primary traffic. Two important parameters that are indicative of the grade-of-service (GoS) of the secondary system, namely, secondary call blocking probability and secondary call dropping probability are computed using the model presented. The analysis provides an insightful study about how the secondary system capacity is determined by its GoS requirement. We use the same analytical approach to show that when the secondary call dropping probability is the cause of low capacity, by suitably reserving a number of frequency slots for primary users' access, the secondary system capacity can be improved significantly