Cooperative vehicular communication systems with physical layer security and noma techniques

Koşu, Semiha
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Graduate School
In recent years, with mobile communication systems development, higher bandwidths and higher data rates are required for individual users. Moreover, in the next-generation wireless communications (5G+), with the emergence of smart cities, many autonomous vehicles and infrastructures are expected to connect. In addition to these numerous connections, it must provide ultra-reliable and low-latency communication (URLLC), which is also necessary for next-generation wireless communications. There are studies examining system performance in vehicle-to-vehicle (V2V) and vehicle-to-everything (V2X) communication systems in the literature. The inter-vehicle environment requires lower antenna heights, cost, and hardware complexities due to the high mobility of vehicles compared to other traditional mobile environments. Also, fading environments in inter-vehicle systems are different from those of stationary users in the literature. Besides, this inter-vehicle fading medium is assumed to be the product of channels in the conventional fading medium and is called the cascade channel model. Therefore, the cascade channel model has an adverse effect on overall system performance. However, some techniques have been studied in the literature which improves V2V system performance. Cooperative communications and receive diversity techniques are considered as a potential solution for inter-vehicle communication systems. When vehicles are not close enough to each other, the signal may be transmitted over relay nodes, increasing the source coverage area enabling cooperative communications. On the other hand, multiple antenna systems are used to combine signals in the receiver to increase the reliability of the system. The diversity technique at the receiver, which reaches the optimum result by maximizing the received signal-to-noise ratio (SNR), is considered to be the maximum ratio combining (MRC) technique that corresponds to the sum of all SNR values received at the destination. Thus, system performance is improved compared to the use of a single antenna, and the coverage area of the source node is increased with the help of a relay. Also, the relay can use different transmission protocols while transmitting the information of the source. In the decode-and-forward (DF) relaying protocol, the transmitted signal is decoded in the relay first, and then an estimated version of symbol is transmitted to the target node. In the amplify-and-forward (AF) relaying protocol, the signal received on the relay is amplified and then transmitted to the destination. Unlike DF relaying protocol, the noise component is also amplified and sent to the destination in this transmission technique as a disadvantage. In this thesis, a comparison of DF and AF relaying protocols are studied, assuming that all nodes are mobile in the system. Also, the channels between all vehicle nodes are designated as cascade Rayleigh fading. It is also assumed that the relay is placed co-linearly and with equal distance between source and destination. Moreover, the relay is equipped with a multi-antenna and applies the MRC technique. Results are provided in terms of bit error probability (BEP) versus SNR values. Accordingly, the increasing number of antennas have improved system performance for both AF and DF relaying protocols. As a result, it is shown that the obtained mathematical expressions are consistent with the Monte-Carlo simulation results. With the tremendous increase in mobile devices in recent years, the continuous broadcast feature of mobile nodes has become a fundamental problem for ensuring security in the system. Therefore, information can become available even to illegitimate listeners. In the open system interconnection (OSI) model, as the physical layer is critical, it is crucial to provide security and transmitting secure information to other layers. Jammer and eavesdropper are the two main types of physical layer attacks studied in the literature. In jamming attacks, the jammer deliberately generates a noise, causing the received signal to be distorted at the destination. However, in eavesdropping attacks, the eavesdropper intercepts the confidential information transmitted to the destination. In all types of attacks, the secrecy capacity of the general system decreases. However, physical layer security (PLS) techniques focusing on increasing system security performance are studied in the literature. For instance, a secret key generation is a PLS technique that increases system security by using the randomness of channels. In this method, the secret key is generated based on the channel state information (CSI) between the legitimate users. Therefore, the data is kept confidential since the illegal user fails to predict the key, even empowering them with high power. In this thesis, the eavesdropper is equipped with multiple antennas for a realistic scenario and applies the MRC technique. Moreover, the eavesdropper receives the broadcasted information from both source and relay in the proposed vehicular communication system. The channel models between all mobile nodes are assumed as the cascade Rayleigh fading channel. The secrecy capacity in this system is calculated by subtracting the eavesdropper's capacity from the destination node's capacity. As an evaluation criterion, the secrecy outage probability (SOP) is calculated first. SOP gives the expression when the secrecy capacity falls below a particular threshold value. Moreover, the probability of positive secrecy capacity (PPSC) means that the instantaneous secrecy capacity is always greater than zero is examined. For system performance, it is observed that when the number of receiver antennas of the eavesdropper increases, SOP increases, and PPSC decreases. Finally, the theoretical analyses of SOP and PPSC are verified by Monte-Carlo simulations. In wireless communication networks, several multiple access methods are drawing attention, such as frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). These orthogonal multiple access (OMA) techniques share the same resource and allow multiple users to work simultaneously in a limited spectrum based on frequency, time, or code. In other words, mobile users can access a limited number of the spectrum simultaneously in these techniques. However, spectrum scarcity is encountered in next-generation wireless networks due to users' need for high data rates and limited resources. At this point, the non-orthogonal multiple access (NOMA) technique could be a promising technology for future wireless networks in terms of providing high spectral efficiency and ensuring fairness between users. The basic concept of NOMA is to allocate different power to users and enable them to work on the same resource block (frequency, time, or code). Besides, NOMA can be classified into two categories, power-domain and code-domain. In power-domain NOMA, the signals of current users are superimposed at the base station (BS) and broadcasted towards the users to decode their signals. The transmitted signal is decoded at the users using the successive interference cancellation (SIC) method, starting from the strong user with better channel quality conditions. Unlike traditional OMA techniques, weak users with poorer channel quality are allocated more transmission power in NOMA, while stronger users with better channel quality are allocated less transmission power. This power allocation can considerably compensate for the trade-off between the quality of service of the system and user fairness. In this thesis, the cooperative power-domain downlink NOMA system is studied. The BS communicates with two vehicles via the relay node and operates in half-duplex (HD) mode. Also, relay transmits the signal of the source to the users by applying the AF relaying protocol. Since both relay and users have high mobility, the channel corresponding to link BS and relay is subjected to Rayleigh fading. In contrast, the channels between relay and users are considered as double Rayleigh fading. Since transmitted signals of each user are superimposed, the SIC method helps to decode these signals. It is assumed that the signal of weak user is correctly decoded on the strong user's channel. In other words, the SIC technique is performed perfectly. Additionally, system performance is evaluated in terms of outage probability and ergodic capacity. In both analyzes, the results are provided using different system parameters (power allocation, distance and transmission power of the BS) for the users. Besides, the overall system performance is also taken into account. Finally, the numerical results are consistent with the Monte-Carlo simulation results.
Thesis (M.Sc.) -- İstanbul Technical University, Graduate School, 2021
Anahtar kelimeler
Wireless communication systems, Computer networks