Research On The Technologies Of Spectrum Sensing And Subcarrier Power Allocation In Cognitive OFDM

Currently, the fixed spectrum allocation policy leads to the insufficient utilization of radio spectrum resources. Cognitive radio (CR) is regarded as the research hotspot in wireless communications recently, which provides spectrum sensing and opportunistic spectrum resource utilization. Orthogonal frequency division multiplexing (OFDM) has unique advantages such as higher spectrum efficiency, higher transmission rate, and defeating frequency selective fading effectively. It becomes the physical layer (PHY) key technique in future mobile communications, OFDM-based CR has the ability of allocating subcarriers and powers flexibly in multiuser scenario. In cognitive OFDM system, secondary users (SUs) fast and efficient spectrum sensing directly influences the enhancement of spectrum utilization and validity of resource allocation. Wideband spectrum sensing based on compressive sensing (CS) overcomes the drawbacks of traditional narrowband spectrum sensing that cannot meet the spectrum demands for SUs. It overthrows the traditional Nyquist sampling theorem which has limitation of sampling rate, and has prosperous application prospect.Multiuser cooperative wideband spectrum sensing techniques based on CS theory in cognitive OFDM are mainly investigated in this dissertation. On the basis of it, subcarrier power joint allocation and optimization methods in cognitive OFDM are mainly studied. Specific research works are presented as follows.The research background and the significance of research work are introduced in chapter 1. In this chapter, the key technologies in cognive OFDM are briefly investigated. The current research progress of CS theory and CRN wideband spectrum sensing based on CS, as well as multiuser multiresource joint allocation and subcarrier power joint optimization based on rate adaptation (RA) criterion in cognitive OFDM are illustrated. Meanwhile, the research work and structure arrangement of this dissertation are also presented in this chapter.The basic principle of OFDM with IFFT/FFT implementation as well as OFDM guard interval (GI) and cyclic prefix (CP) are illustrated in chapter 2. On the basis of it, cognitive OFDM system model is introduced. Cognitive transceiver framework based on OFDM modulation and multiuser resource allocation model in cognitive OFDM are all presented.CR cooperative wideband spectrum sensing methods based on OFDM are studied in chapter 3. Firstly, the dissertation introduces distributed compressive sensing (DCS), with two joint sparse models (JSM-1 and JSM-2) presented. On the basis of the traditional SU spectrum sensing strategies, two improved distributed multiuser cooperative spectrum sensing methods based on DCS are proposed. Aim to the problem of single SU unreliable spectrum detection and high computational complexity of multiuser cooperative spectrum sensing, the improved method reduces the reconstruction complexity significantly with reliable spectrum detection. Furthermore, an improved distributed multiuser cooperative wideband spectrum sensing method based on sparsity matching pursuit is proposed in the case of signal sparsity is always unknown for SU receiver. Simulation and performance analysis are presented to validate the proposed schemes. These two methods both reduce computational complexity under the guarantee of spectrum detection performance, which enhances real-time and accuracy performance of wideband spectrum detection. It also provides certain basis for further research of Multi-resource allocation in cognitive OFDM.Multiuser subcarrier power joint optimization methods in cognitive OFDM are studied in chapter 4. Based on RA criterion, two schemes of multiuser subcarrier power joint optimization are proposed respectively. To avoid the problem of system capacity degradation caused by peer power assignment in the process of subcarrier allocation, an improved subcarrier power joint allocation algorithm based on channel capacity is proposed under the conditions of total transmit power constraint. The scheme not only achieves larger system capacity, but also decreases iteration numbers and reduces algorithm complexity obviously. In addition, according to different subchannel capacity requirements and SU rate fairness ratio in cognitive OFDM, subcarrier power joint allocation scheme is proposed with adjustable system capacity and rate fairness for SUs. Through the selection of best rate fairness control parameter, the scheme obtains approximate maximum cognitive system capacity while keeping rate fairness between each SU.Conclusion and further work prospect are given in chapter 5. This dissertation is the initial exploration of wideband cooperative spectrum sensing and subcarrier power joint allocation in multiuser cognitive OFDM. Research results provide significant meaning for the further related works.