Cognitive Radio (CR) is a novel concept for improving the utilization of the radio spectrum. It is a software controlled radio that senses the unused frequency spectrum at any time from the wide but congested wireless radio spectrum. This promises the efficient use of scarce radio resources. Orthogonal Frequency Division Multiplexing (OFDM) is a reliable transmission scheme for Cognitive Radio Systems [3] which provides flexibility in allocating the radio resources in dynamic environment. It also assures no mutual interference among the CR radio channels which are just adjacent to each other, making it one of the best schemes to be used in CR systems. Allocation of radio resources is a major challenge in cognitive radio systems. In a dynamic environment, many parameters and situations have to be considered which affect the total data rate of the system.udA Secondary users (CRUs/SUs) may coexist with the Primary user (PU) either on Conservative basis or on a more aggressive basis which allows secondary transmissions as long as the induced interference to the PU is below acceptable level. In this we have considered Uplink cognitive radio system heaving one PU coexists with M SUs and A Downlink of an Multi User Orthogonal Frequency Division Multiplexing CR system with one base station (BS) serving one PU and K SUs. We focused on the design on the design and analysis of subcarrier and power allocation scheme under imperfect CSI for cognitive OFDM systems. A two – step Algorithm for bit rate is proposed to obtain the (1) subcarrier allocation to secondary users and (2) bits, power allocation on subcarriers.udThe algorithms attempt to maximize the total throughput of the CR system (secondary users) subject to the total power constraint of the CR system and tolerable interference from and to the licensed band (primary users).
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