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.