| Along with the wide spread of various wireless devices, especially with the advent of user configurable intelligent devices, such as cognitive radios, the security threats of malicious jamming, detection, and interception is no longer limited to military communications. With the majority of today's transactions and communications relying heavily on wireless networks, security has become the key enabler for present and future high speed wireless networks. Patching or add-on security maybe effective in short term, but is far from adequate for addressing the needs on wireless security and can greatly complicate the communication systems. In this dissertation, we focus on the fundamental study of developing a spectrally efficient and secure wireless system by exploiting multiple diversity techniques.;First, we propose a more efficient space-time coding scheme based on the Alamouti scheme. Through bit-level inspection, our investigation reveals that the Alamouti scheme is a low efficient code and there is an opportunity for spectral efficiency enhancement. Unlike most of the existing methods which are designed to maximize the diversity or rate for space-time block codes, the proposed spectrally efficient Alamouti scheme aims to improve the code efficiency of the Alamouti codes, while achieving excellent transmit diversity and retaining the decoding simplicity. The code efficiency of the proposed scheme is enhanced by transmitting more information bits than redundancy bits per Alamouti block. Although the main focus of the proposed scheme is to improve the code efficiency for two transmit antennas, the same ideas can be extended in a straightforward way to the Alamouti codes with more than two transmit antennas.;Second, we propose an innovative spectrally efficient, jamming-resistant wireless scheme by exploiting the joint space-time and frequency diversity. Recently, the collision-free frequency hopping (CFFH) system, which is based on the orthogonal frequency division multiple access (OFDMA) framework and the secure subcarrier assignment scheme, was proposed as a spectrally efficient anti-jamming system. In this research, we investigate the security features of the CFFH system and propose to enhance the inherent security of CFFH through joint space-time and frequency diversity. More specifically, (i) we analyze the limitations of the CFFH system and propose a new subcarrier assignment scheme based on secure permutation. The new algorithm is designed to ensure that malicious users cannot predict or repeat the hopping pattern of the authorized users and hence cannot launch follower jamming attacks; (ii) We improve the performance of the CFFH system under random jamming, by enhancing the system diversity through space-time coding, and introduce the space-time coded collision-free frequency hopping (STC-CFFH) system. The proposed STC-CFFH is found to be particularly powerful in eliminating both channel interference and hostile jamming interference. Our analysis indicates that the proposed scheme is both highly efficient and very robust under various jamming scenarios.;Third, we investigate the use of quasi-orthogonal space-time block codes (QO-STBCs) to mitigate jamming noise. The combination of constellation rotation QO-STBC and OFDM can exploit multipath diversity resulting in excellent performance under frequency-selective fading. Moreover, such systems must be robust against jamming interference, especially partial-band noise jamming. Hence, proper analytical tools are needed to assess the performance of QO-STBC-OFDM in the presence of jamming. In this research, (i) we derive analytical expressions for the exact pairwise error probability (PEP) of the QO-STBC-OFDM system using the moment generating function (MGF); (ii) We calculate the exact PEP under various situations, and derive the closed-form expressions and union bound for the bit error probability (BEP). Our simulation results show that the union bound is tight. |
