With the rapid development of wireless communication networks, the fourth generation (4G) has emerged to move beyond the limitations of the third generation (3G), with increased bandwidth, enhanced quality of services, and reduced costs of resources. In this paper, we discuss the key technologies of 4G and focus on surveying the ongoing research in 4G communications in China. The 4G technologies under investigation include multicarrier transmission, multiple antenna techniques, carrier aggregation, relays, cognitive radio, distributed antenna systems, network convergence, and network self-optimization. These technologies are the building blocks of 4G and contribute largely to the requirements of International Mobile Telecommunications Advanced (IMT-Advanced). For each technology, its benefits, research topics, and some existing approaches are examined. Finally, we briefly discuss the challenges and future research issues in 4G systems.
TIAN JinFeng 1,2 , ZHENG XiaoYing 2,3 , HU HongLin 1,2 YOU XiaoHu 1,3 1 Shanghai Research Center for Wireless Communications (WiCO), Shanghai 200335, China
The feedback delay can severely affect the quality of the channel state information at the transmitter （CSIT） which is fed back from the receiver. The outdated CSIT will cause large performance loss in the transmit beamforming systems. The effect of variable feedback delay on the capacity of transmit beamforming systems over Rayleigh fading channels is studied. First, the case of fixed feedback delay is considered and a closed-form expression of system capacity is derived. Based on the results of fixed delay, the delay following certain distributions in variable delay case is assumed and the closed-form expressions of capacities are derived. The closed-form expressions show that the capacity is significantly affected by the statistical characteristics of the feedback delay. The obtained results provide an analytical insight into the effects caused by variable delay on the system capacity.
针对长期演进(long term evolution,LTE)系统小区边缘用户,提出一种主同步信号的快速捕获方法.该方法首先在LTE网络可用频段内接收信号,然后对接收到的信号进行自身滑动镜像相关.检测到明显峰值后,在峰值点进行主同步信号的本地相关,从而获取主同步信号索引.分析和仿真结果表明,该方法的运算复杂度低,适用于小区间保持时间同步的多小区网络,尤其是小区边缘用户主同步信号的快速检测.
