| Summary: | 碩士 === 國立中央大學 === 通訊工程學系 === 102 === The idea of cognitive radio (CR) has embodied concretely in hierarchical cellular systems by deploying an underlying microcellular system to reuse the underutilized spectrum licensed by a macrocellular system. The fundamental challenges for successfully realizing such hierarchical systems are to manage the intercell interference between the macrocell and microcell and to pursue the goal of maximizing the spectrum recycling efficiency. Recently, the idea of interference alignment has been emerged to utilize the spatial dimension offered by multiple antennas to overcome the interference problem and achieve the maximum sum rate performance.
In this thesis, we jointly consider antenna beamforming, power allocation, and multiuser scheduling for the secondary system to utilize the uplink spectrum of the primary cell and to concurrently serve multiple secondary users in the uplink. IA is applied to manage the interference from the secondary users to the primary cellular system in a transparent hierarchical cognitive radio (HCR) system. By change of variables, we can remove the IA constraint by absorbing it into the sum rate formula, and the original beamforming problem becomes solvable and the optimal solution can be gotten by using an iterative water-filling approach.
The iterative water-filling algorithm transform the multiuser sum rate maximization into a series of single-user sum rate maximizations for each user by regarding all other users’ signals as additional noise at each step. The algorithm finds the single-user water-filling covariance matrix for each step. The sum rate objective is increasing with each water-filling step and the sum rate will converge to a limit.
Furthermore, the two-stage user scheduling that makes use of multiuser diversity is investigated to further improve the sum rate performance and the fairness among users via PF rule and orthogonality.
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