Research On Multi-carrier Transmission Scheme In Cognitive Radio System

The emergence of cognitive radio technology has brought revolutionary changes for wireless communication. Non-licensed users detect the information of spectrum activities and access the idle spectrum opportunistically, which effectively improves the efficiency of spectrum utilization. Research on multi-carrier transmission scheme in cognitive radio system is very important. In this paper, we study and design the appropriate transmission scheme for cognitive radio system, which gives some significant contribution to the research and implement on transmission scheme of cognitive radio system.Firstly, the transmission scheme based on Non-Contiguous Orthogonal Frequency Division Multiplexing (NC-OFDM) is studied. A novel frame structure for cognitive radio communication system is given. The preamble is transmitted in every available subcarrier to mark the cognitive radio network. Each coding packet is only transmitted in one certain available subcarrier through continuous OFDM symbols. If some sub-carriers are severely affected by the channel, the corresponding coding packets will be lost but do not affect the decoding process. The details of each signal processing module in NC-OFDM transceiver is introduced in the paper. Especially, we focus on the problems of carrier frequency offset (CFO) estimation and active interference cancellation (AIC). A frequency domain estimation algorithm is adopted in the transmission scheme. The new model of active interference cancellation combined with power control is studied. We simplify the model to a least squares problem with constraints. The optimal solution is given and its performance is shown by simulation results.Secondly, the transmission scheme based on transform domain communication system (TDCS) is studied. In this paper, we propose a novel concept of variable rate transform domain communication system (VR-TDCS). To increase the efficiency of spectrum utilization, VR-TDCS transmits data symbols with variable M_ary modulations in different subcarriers instead of transmitting one data symbol in all the subcarriers in traditional TDCS. The power and bit allocation, which depend on the noises on the subcarriers, are also different with that of traditional TDCS. The theoretical bit error rate (BER) performance formula for MPSK modulation in VR-TDCS is derived in this paper. Based on the BER performance formula, an optimal algorithm for power and bit allocation in VR-TDCS is given to maximize the total bit rate or minimize the total power with a certain BER requirement.Finally, the hardware implementation of the transmission scheme in cognitive radio system is studied. The baseband signal processing hardware platform based on software defined radio (SDR) technology is introduced. Almost all the algorithms in physical layer are implemented in DSP. Based on ADSP-TS201, the relevant circuit diagram and the implementation of the simplified methods are described in detail.