Transmission Efficiency And Security Analysis Of Wireless Communication Systems Based On A Statistical Physics Approach

Transmission efficiency and security is of great importance to modern communication,new generation wireless networks are required to supply services with high speed, high re-liability and enough security for mobile users. As a measurement criteria of informationtransmission rate, channel capacity plays a significant role in the area of communicationtransmission efficiency. Otherwise, the aim of communication security is ensuring informa-tion transmitted by sender can be reached the receiver without eavesdropping by a illegalthird-part. The thesis focuses on analyzing the two problems related to CDMA and MIMOwireless communications. One is the transmission efficiency and the other is transmissionsecurity. Based on analyses of the above two problems, within the framework of a statisticalphysics approach, this thesis works on the following four main subjects.In the first part of the thesis, we utilize frequency selective channel model and focuson analyzing channel capacity of CDMA communication system. In this part, the effects ofsystem parameters, such as signal-to-noise ratio and system load, on the channel capacityare studied. When channel estimation is applied to the receiver. It is difficult to avoid errorin the process of channel estimation. As an example, MMSE is used to analyze the impactof channel estimation error on the channel capacity in the thesis. The main results of thispart are: (a) if the frequency selective channel is looked as a transverse filter, the channelcapacity is reduced when the numbers of the taps of the filter are increased; (b) When channelestimation is applied at the receiver, the channel capacity loss is more serious when thevariance of channel estimate error is increased.The second part aims at analyzing the security of CDMA communication system, andthe analyzing method of this thesis is a statistical physics approach, which is firstly intro-duced to analyze the security of wireless communication system. Unlike the traditionalmethods to analyzing the security of CDMA communication system, the physics layer secu-rity is taken into account in the thesis. The traditional methods are based on cryptographyand the security is considered above the physics layer. The following facts motivates us toexplore the physics layer security of CDMA communication system. This is to say, classicalcryptosystems assume that the channel between encryption machine and decryption machineis perfect and error-free. However, The assumption is not always true in wireless commu-nication because of noise and other interference in wireless channel. Thus, the concept of generalized Gaussian code-division multiple-access wiretap channel and a related reasonablesecurity criterion are proposed. The analyzed results are validated by the numerical simula-tions. The main results of this part are: (a) The analyses with large limit system based onstatistical physics show that the CDMA wiretap channel is equivalent to a single-user Gaus-sian wiretap channel when any active user in CDMA communication system transmits theinformation in the wiretap channel. (b) The transmission rate can be increased with reducingthe number of the active users in the system when some security condition is satisfied. (c)Increasing the length of the spreading codes is beneficial to improving the security of CDMAcommunication systemCorresponding to the analysis of CDMA channel capacity, the third part studies that ofMIMO wireless system. Unlike CDMA wireless communication system in the first part ofthis thesis, multi-antenna technique is employed in MIMO wireless system and the channeltransfer function is much complicated than that of CDMA wireless system. As a result, Thispart focuses on the effect of MIMO channel properties, such as the numbers of antenna,scattering objects, on the channel capacity of MIMO communication system. The mainresults of this part are: Adding more antennas at transmitter can improve the channel capacitywhen the number of antennas at the receiver are fixed. Moreover, rich scattering objects arehelpful for increasing the channel capacity.To date, much research has focused on the role of multi-antenna technique in enhancingthe throughout and robustness for wireless system. In the last part, the security of MIMOwireless system is chosen as a research start point, and the role of multi-antenna techniquein security of MIMO wireless system is explored. The main results of this part are: (a)Up to now, research on AWGN wiretap channel has always indicated that the transmitterand receiver can communicate with each other with perfect security. By contrast, this thesisshows that the eavesdropper can make the secrecy capacity reach to zero with the help ofmulti-antenna technology, even if noise is introduced in the eavesdropping process. This isto say, previous results about AWGN wiretap channel is difficult to hold when multi-antennasare employed in the system; (b)In order to make the secrecy capacity reach to zero, the rationof number antennas of wiretapper and legal receiver must be satisfied with some condition,when noise is introduced by eavesdropper, which is analyzed in the thesis quantitatively;(c) Adding more antennas at legal receiver is of great benefit to improving system security.However, to the transmission part Alice, it is not the same as the above case, because addingmore antennas at the transmitter benefits both legal receiver Bob and wiretapper Eve. The above four parts are all studied under the same framework of a statistical physicsapproach. The reason that statistical physics can be successfully applied in the above investi-gations, simply speaking, is that the information entropy and mutual information has a closerelation with the concept of entropy in statistical physics. So the connections between chan-nel capacity and the concept of statistical physics can be found when some conditions aresatisfied. Similarly, because security criterion is associated with mutual information betweenthe sender and wiretapper, the security analysis can be related with statistical physics.