Study On Key Techniques In LTE-Advanced System

With the ever-increasing data demand for mobile radio communication services, the next generation wireless communication systems (i.e.4G) are expected to provide higher date and better quality of service. Recently, the next generation wireless communication systems are under discussing. It is necessary to investigate some new techniques to further improve the average spectral efficiency and cell-edge spectral efficiency, and improve the robustness of the mobility performance.The LTE-Advanced system is consistently considered as the forth generation wireless communication systems. It is known that the MIMO-OFDM technology is adopted in LTE-Advanced system. And the key technologies include the link adaptation, limited feedback, and3D MTMO technology, which highly affect the system throughput. It deserves much effort to dig the key thechnologies deeply to improve the system average spectral efficiency further and the UE’s throughput in cell edge coverage. Besides, the HetNets are dicussed in LTE-Advanced system and it is deemed as a promising and effective solution to further improve the system capacity. However, compared with the traditional homogeneous networks, UE will suffer more serious co-channel interference in HetNets, and thus, the mobility performance of UE will be deteriorated. In order to enhance the mobility robustness of UE, it is necessary to investigate some new techonologies. The rearch iterms in this paper include:enhanced link adaptation, SU/MU MIMO transmission,3D MIMO, and mobility enhancement in HetNets. The novelties in this paper are as follows.1) It is known that the dynamically changed inter-cell interference is exsited in closed-loop MIMO based wireless systems. The standard method to solve this issue is OLLA. However, the OLLA technology can only reduce the CSI mismatch from the long term statistic point of view. In this paper, an enhanced link adaptation scheme based on cooperative interference prediction is proposed to reduce the CSI mismatch issue. In this scheme, it utilizes the CSIs which can be shared within the cluster to predict the dynamically changed ICI and then adjust the SE at the BS side. With the dynamic ICI prediction, more proper MCS can be chosen and thus the accuracy of link adaptation can be improved. What’s worth noting that, based on joint scheduler, the overhead in the air interface doesn’t increase in the proposed scheme. 2) Because multiple users can share time and frequency resources simultaneously, the system throughput can be improved significantly when performing MU MIMO transmission, compared with SU MIMO transmission. However, the MS cannot be aware of the MU transmission candidates at the BS side. Thus, the MS only optimizes its own link when performing link adaptation. When BS performs the MU transmission, this CSI may be mismatched and then the system capacity could reduce. In this paper, we propose one novel CSI feedback method, which could improve the system throughput when the BS dynamically performs the SU/MU switching. In this scheme, BS can indicate the MS feeds back the SU-MIMO optimized or MU-MIMO optimized CSI. When the MU-MIMO optimized CSI is fed back, the high rank layer CSI can always contain the principle eigen vector of the fading chanel. Besides, the higher rank layer CSI can capture the inter-layer interference automatically. Due to instruction of the BS signaling, the performance of the SU-MIMO transmission is not compromised either.3) The3D MIMO technology has been attracted many attentions and is considered as a key technology to improve the spectral efficiency for the MIMO based wireless communication systems. Compared with the existed2D MIMO technology, in the3D MIMO based wireless system, the BS can exploit both the horizontal and vertical space to transmit signal and eliminate the inter-cell interference. However, the3D MIMO technology is still under discussing. In this paper, as a pioneer, we propose two schemes to support the3D MIMO transmission. The two schemes are based on the vertical beamforming gains compensated at BS side and MS side, respectively. In the two schemes, the CSI-RS ports are configured differently and thus different reference signals can be measured by MS. The two schemes can be implemented in LTE-Advanced system both. We give their pros and cons detailedly. Compared with the reference scheme in LTE-Advanced system, the system average SE and cell-edge SE can be improved significantly using the proposed schemes.4) It is difficult for the convertional homogeneous networks to meet the rapidly-expanding data demand for the wireless communication systems. One effective and cost-efficency method to increase the system capacity is to deploy the LPNs, which is named as HetNets. However, UE will suffer more serious interference in HetNets compared with it is in homogenous networks and the mobility performance will be deteriorated. In this paper, we propose two schemes to enhance the mobility performance for HetNets, which are based on the dynamically changed RSRP from pico cell and radio link quality, repectively. Importantly, the two schemes are complimant rather than contradict, and can be implemented together to further enhance the mobility performance.In this paper, for MIMO-OFDM bases LTE-Advanced system, we investigate and deeply dig four key technologies, that are link adaptation, SU-MIMO/MU-MIMO dynamical switching,3D MIMO, and mobility enhancements in HetNets. Based on the analysis, we propose some schemes and solutions accordingly. Accroding to the system level simulation, using the proposed schemes, it is shown that the system average spectral efficiency and UE’s throughput in cell edge coverage can be improved largely, and the mobility robustness of UE can be enhanced significantly.