Research On Key Techniques Of Full Duplex Systems Under I/Q Imbalance

Signals are allowed to be sent and received at the same time and frequency in full-duplex communication systems, which effectively improve the spectral efficiency and throughput. Full-duplex communication, as one of the key technologies in the fifth generation mobile communication system(5G), has some shortcomings, such as self-interference of the receiver. Hence, the self-interference cancellation technique has become a hot research topic recently.The key techniques of the full-duplex systems are focused on in this dissertation, where three aspects are studied, including the self-interference cancellation, the estimation and compensation of I/Q imbalance, and full-duplex two-way relaying. And the specific contents and contributions are as follows:Firstly, a self-interference cancellation scheme based on multi-stage cascade structure is proposed, according to the existing self-interference cancellation technique and “co-frequency co-time co-antenna” full-duplex researches, where the intermediate frequency(IF) self-interference cancellation is added, besides the existing circulator isolation, the radio frequency(RF) self-interference and the baseband cancellation. Due to its simple structure, the proposed scheme can be easily implemented in practice. The hardware circuit of IF cancellation is designed and the test results are also given. The results show that the IF cancellation structure can further mitigate the self-interference signal in the full-duplex system.Secondly, an algorithm is proposed for the estimation and compensation of I/Q imbalance, in the full-duplex systems with I/Q imbalance. Maximum likelihood signal model is established, and I/Q imbalance parameters of the receiver and transceiver are joint estimated, separated and then compensated, based on conventional transceiver I/Q imbalance signal model and the variable substitution. The advantages of the algorithm are as follows: 1) I/Q imbalance parameters estimation makes full use of the inherent self-interference signal in full-duplex system; 2) I/Q imbalance parameters of the transmitter and receiver can be obtained, respectively, which is conducive for signal compensation before sending and signal correction after receiving; 3) the transceiver I/Q imbalances are compensated respectively, from the external system, the transceiver does not suffer from the problem of I/Q imbalance; 4) The operation process of I/Q imbalance parameter estimation and compensation is quite simple, which is conducive for the realization of the algorithm. The results show that the proposed algorithm can not only accurately estimate the I/Q imbalance parameters, but also effectively compensate the I/Q imbalance at both ends of the transmitter and receiver, which improves the overall signal quality and the system performance.Finally,the outage of the full-duplex two-way OFDM relay system in the presence of the I/Q imbalance and residual self-interference(RSI) is studied, and the closed form expression of outage probability is derived. Analysis shows that the I/Q imbalance and RSI are two main factors that affect the outage probability, which means the outage performance is deteriorated, when the I/Q imbalance increases or RSI power improves. The simulation results verify the correctness of the theoretical analysis. Simulation results also show that, the outage performance of the two-way full-duplex OFDM relaying system cannot be optimized by reducing RSI, in other words, I/Q imbalance becomes the bottleneck of the whole system performance, and I/Q imbalance correction is necessary for improving the system performance, especially when the signal-to-noise ratio(SNR) is relatively high.