Transmission strategies and coexistence mechanisms for secondary users in shared spectrum

Abstract

With the explosive growth of mobile traffic, a demand for additional radio resource has been increased. Given the limited quantities of available spectrum for wireless communication systems, a framework of spectrum sharing has been considered in many regulatory domains such as FCC, ECC, and Ofcom[1,2]. Especially TV white space[3,4,5]. Secondary users (SUs) to be allowed for utilizing these bands have to protect primary users (PUs) that are granted the license with high priority on occupancy of a channel. To this end, in case of TVWS, SUs shall access a geo-location TV database for the purpose of obtaining a list of available channel and the allowable maximum level of transmission power at the certain position. For protection of radar system on 5 GHz band, SUs should have functionalities on dynamic frequency selection and transmission power control not to make a harmful interference into radar system in the vicinity of SUs. Additionally, because there would be multiple SUs on the same channel over the shared spectrum, SUs also take into account the coexistence with other SUs which can be either the same or different type of systems. For this reason, listen-before-talk (LBT) based channel access mechanism (CAM) is generally specified in the specifications of communication system (e.g. CSMA/CA in IEEE 802.11 for Wi-Fi) or regulatory requirements. In European regulatory domain, two types of CAMs for 5GHz unlicensed band are defined. With the advent of LTE in unlicensed (LTE-U) band, the CAM particularly has become important issue due to the coexistence between Wi-Fi and LTE-U. In this dissertation, we investigate transmission strategies for SUs with multiple antenna on the TVWS under the condition of limited transmission power and analyze two types of channel access mechanism specified by European regulatory domain. In Chapter 2, it is considered that the transmission power of white space device (WSD) using multiple transceiver antennas on TVWS is restricted by the location probability based geo-location database. Also, it is assumed that channel estimation error is present at WSD receiver which exploits zero-forcing linear processing. Under these conditions, we analyze the achievable throughput. Based on the analytic results, we propose a dynamic WSD transmission strategy that WSD transmitter first determines the number of active transmit antennas according to the WSD operational parameters obtained from the geolocation database. For further improving the WSDs performance, the WSD receiver can select the active transmit antennas based on instantaneous CSI at receiver (CSIR) and report their indices to the WSD transmitter with a very low feedback overhead. Here, to reduce the computational complexity, it is proposed that a low-complexity antenna selection method requires a single matrix inversion operation. In Chapter 3, two types of channel access mechanisms on 5 GHz unlicensed band, frame based channel access mechanism (FBCAM) and load based channel access mechanism (LBCAM), are analyzed in terms of normalized network throughput. The analytic expressions on CAMs are derived based on the Markov chain model. In case of FBCAM, since exact expression is not derived as a closed form due to its inherent mechanism, approximate solution with closed form is also provided. From simulation results it is shown that the provided analytic expressions are validated. For further study, it is considered that the transmission strategies in terms of minimization of bit error rate (BER) performance for WSD on TVWS and the CAM with enhancing the fairness on 5GHz band. Similar as maximization of achievable throughput, the BER performance of MIMO WSD could be affected on the number of transmit antennas under the restriction of allowable transmission power level. In this perspective, there will be the investigation on transmission strategies based on an analytic results on BER performance. With regard to CAM, FBCAM could provide an outstanding network throughput performance. However, it is not ensured that the fairness between SUs is achieved due to the fxed frame period. Therefore it is necessary to enhance the fairness without significant loss of network throughput. The conclusions on dissertation will be provided after further studies on above subjects.