Research On The Coordinated Multi-Point Transmission Technology Based On The Distributed Multi-Cell System

The coordinated multi-point transmission (CoMP) technology in the distributed multi-cell system plays an important role in not only suppressing the inter-cell user interference, improving cell-edge user throughput and overall system throughput, but also avoiding a lot of data feedback, information exchange and processing burden sharing, as well as encoding and decoding with high computational complexity of the operation, so that the base station has a high distribution cooperative or flexibility. In the coordinated multi-point handoff technology of the distributed multi-cell system, selecting the appropriate combination of collaboration is the key to improve the quality of service of handoff, and promote efficiency and success of handoff. This study mainly involves CoMP technology in the distributed multi-cell system, including the coordinated beamforming based on the local channels, the coordinated beamforming based on the intra-cell local channels and the coordinated multi-point handoff technology of the distributed multi-cell system. The main contents and research results are as follows,(1) Based the local channels, a zero-forcing algorithm based on user selection algorithm is proposed. Conventional distributed beamforming schemes in multicell system often ignore two practical conditions, one is the coexistence of intra-cell interference (InCI) and inter-cell interference (ICI), the other is the number of users is larger than the number of base station (BS) antennas, we consider these two conditions and analyze the decentralized beamforming of the multicell system from the viewpoint of zero forcing (ZF). A novel user selection method based on ZF algorithm is proposed to divide the users into two categories, i.e., one of the categories is eligible to participate in ZF algorithm and the other is not. The proposed criterion of partitioning users is based on the degree of parallelity between local channels. In addition, each BS utilizes only the local channel state informations (CSIs) to derive its beamforming vector. The simulation results demonstrate that the proposed algorithm can achieve higher rates and are more robust against InCI and ICI than the traditional ones under the different number of transmitting antennas and users.(2) A distributed beamforming algorithm is proposed for downlink multicell network when the intra-cell local channel state informations (CSIs) are available at each base station (BS). The proposed algorithm includes three parts, firstly, an algorithm is proposed to estimate the direction vectors of the inter-cell local channels. The algorithm above utilizes a set of random orthogonal bases and the known training formula to get all basis vectors of orthogonal complement space of the inter-cell local channel vector, and these basis vectors can be used to derive the direction vector of the inter-cell local channel. Secondly, under the condition that the transmit antenna number of BS is less than the number of users, a proposition based on singular value decomposition of all local channel vectors is presented for the distributed beamforming causing minimum intra-cell interference (InCI) and inter-cell interference (ICI) toward other users. Thirdly, since the premise of the aforementioned proposition is that BS can obtain the amplitudes of the whole local channel vectors, a training algorithm is proposed with only intra-cell local CSIs at BS. This training algorithm exploits the unit matrix and unit transmit power to perform fixed steps of training process. In each step, the BS needs to observe the changes of another BS or user, then the amplitude of inter-cell local channel vector is estimated by the observations. Finally, simulation results demonstrate that the proposed algorithm can achieve higher user rate while offering more advantages to suppress InCI and ICI than the traditional ones.(3) Based on gradient iteration, a selection scheme of the coordinated group is proposed to efficiently complete the coordinated multi-point handoff in the distributed multi-cell system. To start with, a .metric function is established to restrict the ICI and maximize cell rate, it can not only maximize the rate of the target cell, but also control the interference to the remaining cells in coordinated group. This function depends on only local channel state information (CSI) and does not need additional CSIs. Secondly, a selection criteria with the metric function is proposed. This proposed criteria utilizes gradient iteration to maximize the metric function to improve sum rate of the cell, and the convergence of the proposed iterative algorithm is proved. Thirdly, a coordinated handoff scheme is proposed, in which the coordinated group is formulated based on both the overall sum rate performance of the target cell and the user fairness evaluated by variance. The simulation results reveal that, compared with traditional algorithms, the proposed gradient iteration algorithm could robust the cell sum rate, and the proposed coordinated handoff scheme could achieve higher handoff success rate and lower ping-pong handoff rate when the user mobile rate increases.