Research On Multi-channel Sepectrum Sensing In Cognitive Radios

Nowadays, cognitive radio is a key technology to solve the problem of spectrum scarceness, and spectrum sensing is the precondition to achieve such technology. In order to guarantee the transmission of licensed users and satisfy the rate requirement of cognitive users simultaneously, multi-channel spectrum sensing is required. So cognitive users can discover enough spectrum holes for spectrum access and can sense the change of spectrum activities. In my paper, I have studied multi-channel spectrum sensing based on random selection of channels, multi-channel sensing based on down sampling, and spectrum sensing in asynchronous OFDMA network, which gives some significant contribution to the research on spectrum sensing.Firstly, the problem of multi-channel spectrum sensing based on random selection of channels is studied. The system model of spectrum sensing is described, the asymptotic probability density fuction of the test statistic and the equivalent bipartite graph are analyzed, and belief propagation algorithm (BPA) is introduced, as a typical fusion algorithm in the bipartite graph. Three spectrum sensing methods are proposed then, whose basic idea is that cognitive users choose part of channels to sense randomly and infer the spectrum activities of all the channels by BPA. In method one, the fusion center starts data fusion only when enough sensing information is received. In method two, once sensing information is received, the fusion center starts data fusion and never detects the channels whose activities are already judged, which can decrease the sensing delay further. In method three, the sensing information in the above methods would be quantified, and only the reliable results would be chosen for data fusion, which can decrease the bandwidth constraint.Next, the problem of multi-channel spectrum sensing based on down sampling is studied. The system model and the signal model of licensed users in time domain are characterized, the signal model of licensed users in frequency domain when the frequency sampling rate doesn't satisfy the theorem of Nyquist Sampling is investigated, and the asymptotic probability density function of the test statistic and the equivalent bipartite graph of our model are analyzed. Then a kind of spectrum sensing method is proposed, whose basic idea is that the cognitive user chooses the frequency sampling rate according to a certain rule to sense the whole wide-band spectrum in every sensing process and infers the spectrum activities of all the channels in terms of BPA. The problem of the selection of sampling rate is mainly analyzed. And it proves that it is better to choose the frequency sampling rate which is a multiple of the channel bandwidth of licisend users and the value of multiple is a prime.Finally, the problem of spectrum sensing in asynchronous OFDMA network is studied, especially when fractional carrier frequency offset exists. The model of the received signal in each subcarrier and the effect to spectrum sensing induced by CFO are analyzed firstly. Then the signal is simplified, and a spectrum sensing method based BPA is presented, which can decrease the effect to spectrum sensing induced by CFO and false alarm. After that a simplified sensing method is proposed by advancing energy detection, which can reduce the complexity of computation and can be implemented on the hardware device easily. And the DSP implementation of the simplified method is introduced based on ADSP-TS201.