Spectrum Access Of The Femtocells Based On Confidence Factor Weighted Cooperative Spectrum Sensing In Cognitive Massive MIMO System

Massive Multiple-Input Multiple-output (Massive MIMO) technology is achieved by mounting a large antenna array at the macrocell (MC) base station (MBS), and effectively exploits the channel reciprocity using time-division duplex (TDD) scheme. This technology can bring tremendous improvements in throughput and energy efficiency, and has a good robustness as well as low cost hardware implementation. Nevertheless, the concentration of the energy means a lot of blank areas of spectrum in the regions far away from MBS and MUs. In order to exploit these areas effectively, cognitive massive MIMO technology is proposed and widely studied, which deploys a large number of small range femtocells (FCs) in massive MIMO system, and enables significant increase in system capacity by spectrum reuse (SR) of the FCs.The research on the access scheme of FCs is an important problem in cognitive massive MIMO technology, i.e. when and how the FCs can access the spectrum. The typical access schemes of FCs include the time division duplex (TDD) access scheme, the reverse time division duplex (RTDD) access scheme and the hybrid division duplex (HDD) access scheme, these schemes have different advantages and disadvantages. Based on these schemes above, this thesis proposes a hybrid time division duplex (HTDD) access scheme for FCs, and then exploits a confidence factor based weighted cooperative spectrum sensing (CFWCSS) technology to improve the detection performance of FCs in this HTDD access scheme. Furthermore, this thesis presents the theoretical analysis and simulation on the performance of the proposed scheme, and compares it with other schemes. The main research work of the thesis is as follows.Firstly, an HTDD access scheme for FCs is proposed. In the traditional access schemes, the FCs, which detect the MBS or one of the MUs is active, must abandon the SR opportunity, this reduces the spectrum sharing probability of system. Considering the different communication conditions of FCs caused by the difference of distribution and environmental, we propose an HTDD access scheme for FCs in this thesis. This scheme makes full use of the distribution characteristics of FCs, i.e. FCs can flexibly adopt the access scheme, which causes the smallest effects on MC, according to the energy detection results in both the macro uplink (UL) and downlink (DL) time slots. This thesis presents the concrete implementation process of this HTDD access scheme, and analyzes the throughput. Simulation results show that the proposed scheme can significantly improve the system throughput.Secondly, a CFWCSS scheme is proposed. Considering the poor detection reliability of a single FC and the interference superposition problem of multiple FCs in realistic situations, this thesis proposes a CFWCSS scheme. At first, according to the signal-to-noise ratio (SNR) of all the inter-user channels for each FCs in the massive MIMO system, we can group all the FCs into clusters dynamically and make them perform cooperative spectrum sensing to update the confidence factors. Then, each FC will have more confidence to the detection result from itself or the other FCs according to the confidence factors. This thesis presents the concrete dynamical grouping process of this CFWCSS scheme, and analyzes the performance by utilizing information theory and probability statistics as mathematical tools. Simulation results show that the proposed scheme can significantly improve the spectrum sharing probability and throughput of FC-tier while only slightly sacrificing MC-tier throughput.Finally, the problems, e.g. the threshold optimization, to be solved in cognitive massive MIMO technology and future research topics are summarized.