Research On Key Technologies In OQAM-OFDM Communication Systems

The offset quadrature amplitude modulation-orthogonal frequency division multiplex-ing (OQAM-OFDM) technique has been listed by IMT-2020as one of the key physical layer candidates in the fifth generation mobile communication systems (5G), since the OQAM-OFDM has much lower out-of-band radiation than the OFDM. However, in OQAM-OFDM communication systems, the interference caused by wireless channels and non-perfect re-construction is harmful to the system performance. Considering the above two types of interference, this dissertation investigates two major techniques, cyclic prefix (CP) and pro-totype filter design, in OQAM-OFDM systems, and applies the OQAM-OFDM to femtocell networks to mitigate the interference between mobile equipments.First, this dissertation concentrates on the inter symbol interference (ISI) caused by wireless channels and investigates the cyclic prefix (CP) insertion problem in OQAM-OFDM communication systems. This dissertation investigates the properties of existing CP based OQAM-OFDM communication systems and analyzes their power spectral densi-ties (PSDs). This dissertation shows that the transmitted CP-OQAM-OFDM signal after the CP insertion is equivalent to a truncated signal obtained by inserting CP and cyclic suffix to the input data symbols in the OQAM-OFDM communication system, implying that the PSD of the CP-OQAM-OFDM may be affected by the truncation. Based on this investigation, This dissertation proposes an Extended CP-OQAM-OFDM (ECP-OQAM-OFDM) commu-nication system without truncating the OQAM-OFDM output signal, which has nearly the same PSD as the original OQAM-OFDM signal. This dissertation also compares three CP based OQAM-OFDM systems with windowed CP-OFDM communication systems. Simu-lation results show that, CP based OQAM-OFDM communication systems perform better than windowed CP-OFDM communication systems considering both the PSD and bit error rate (BER) performances, and the newly proposed CP based OQAM-OFDM communication system has the best PSD performance in existing CP based OQAM-OFDM communication systems. Then, this dissertation concentrates on the ISI and inter carrier interference (ICI) caused by non-perfect reconstruction and investigates the prototype filter design problem in OQAM-OFDM sysetems. This dissertation formulates a problem of direct optimization of the filter impulse-response coefficients to minimize the stopband energy and constrain the total ISI/ICI power. Since the number of variables in the optimization problem is too large, this dissertation greatly reduces the number of variables in the optimization prob-lem by proposing a linear transformation and approximation method, meanwhile, the error caused by the approximation is constrained to an acceptable threshold. Based on the above linear transformation and approximation, this dissertation utilizes the Branch and Bound algorithm to obtain the optimal solution of the approximated optimization problem. Sim-ulation results show that, the optimized filters obtained with the proposed method achieve significantly lower stopband energy than those with the frequency sampling and windowing based techniques.Finally, this dissertation concentrates on the interference between mobile equipments caused by the multi-system coexistence, applies the OQAM-OFDM to the femtocell networks, and investigates the interference mitigation method. This dissertation proposes utilizing the low out-of-band radiation property of the OQAM-OFDM signal and asyn-chronous transmission to mitigate the interference between neighboring microcells, and proposes the frequency and cell partitioning method to mitigate the interference between the macrocell and microcells. In the proposed frequency and cell partitioning method, the downlink and uplink frequency bands are both divided into four non-overlapping parts. The macrocell and microcell are divided into three or four regions, with one part of frequency band allocated to one region. With the frequency and region partitioning methods, each user and the interfering users are separated with certain distance, which can greatly mitigate the interference. Moreover, This dissertation theoretically analyzes the existing interference and average rate performance. Simulation results show that, the proposed methods can significantly mitigate the interference between mobile equipments and improve the average network data rate.