| Just the same as each new generation system, to achieve higher transmission rate and better QoS (Quality of Service) performance, cellular mobile communication system adopts many latest technologies when it evolves to the 4th generation currently. Among them, Coordinated Multi-Point (CoMP) Transmission/Reception technology attracts lots of attentions. Some of the backgrounds to apply CoMP are powerful backhaul transmission ability, widely used distributed antenna technology, shrinking cellular size and severe inter-cell interference. Based on multi-user multiple-input multiple-output (MU-MIMO) and virtual MIMO techniques, coordinated communication can be realized within multiple base stations (BSs) and multiple mobile stations (MSs).The cellular mobile communication systems nowadays are mostly based on orthogonal frequency-division multiplexing (OFDM) technology so that intra-cell interference can be neglected. In order to further improve the system throughput, to lower inter-cell interference, which is inherent to the cellular systems, is one of the main issues for research. Traditional approaches mostly mitigate interferences from neighboring sites by partitioning resources, e.g. frequency, time slot, or space resources, for different sites. However, full spectral reuse can allow the system to achieve higher network capacities and spatial resource utilization efficiency if the inter-cell interference is effectively managed. CoMP is based on the transmission of cooperative BSs by reusing the entire frequency spectrum, where the potential interference across cooperative BSs will act as assist transmission. Additionally, CoMP can strengthen the covering of the dead zone.Thanks to mature precoding techniques and widely application of multiple antennas, MU-MIMO is exploited in cellular network for the first time. The precoded signals of multiple users served by the same BSs are transmitted in orthogonal subspaces without interfering each other. With virtual MIMO techniques, cooperative BSs communicate with users via a virtual MIMO antenna array to take the advantage of the spatial diversity.The research background and the contributions of this thesis are based on CoMP system. Four research points are envolved and each of them is detailed discussed in the thesis.The first research point is multi-cell antenna calibration problem.Channel information of MSs is required to be shared among multiple coordinated BSs. Take the downlink for example. In order to precode for MSs, the cooperative BSs need to be aware of the exact downlink channel information. Generally, there are two ways for BSs to achieve this:through the feedback from the MSs; by predicting the exact downlink channel information from the uplink using the channel reciprocity of downlink and uplink in time-division duplex (TDD) system. Channel reciprocity can be realized due to the fact that when occupying the same frequency band, signal experiencing the same fading in the space between downlink and uplink channels if the time interval between them is less than the channel coherence time. The latter one doesn’t need feedback and is delay free. From this point of view, there is a natural advantage to apply CoMP in TDD system.Actually, channel reciprocity can only apply to wireless space propagation channel. In hardware realization aspect, the analog circuits in transmitter and receiver would act as non-linear amplifiers to the signal, and the coefficients of this effect are closely related to the state of the art of the hardware itself and properties of the surroundings (e.g. temperature and humidity and etc.). So the coefficients of the circuits are time-variant. This effect is equivalent to adding a distortion to the transmission channel in terms of the signal processing. In addition, the effects usually would not be the same to downlink as that to uplink channel, so that the channel reciprocity might be affected.In realistic system, every antenna located in either BS or MS has independent receiver and transmitter chains which can be seen as analog amplifiers. Thus, a complex magnification coefficient will be multiplied to each antenna which would alter the property of the transmission to simulate the effect of each chain. This coefficient varies according to the environmental changes. A procedure, therefore, which is called "antenna calibration", should be performed to compensate the mismatch between the receiver and transmitter chains, i.e., to bring in extra coefficients factitiously to reconcile the properties between receiver and transmitter chains to assure the validity of the channel reciprocity.Traditional works mainly focused on the mismatch and calibration within a single cell. In single cell case, calibrations are performed only among multiple antennas in each single BS or MS. However, mismatches among multiple BSs or MSs may also need to be concerned due to joint processing of a group of BSs in CoMP system. Accordingly, the antenna calibration problem in CoMP is even more complicated than traditional system, and it is what this research point focuses in.In this thesis, a theoretical model is set up and a complete overview of the antenna calibration problem is given.Besides, the impact of un-calibrated cases in BSs and MSs and corresponding properties of calibration from the perspective of linear precoding in CoMP system is specifically analyzed. A novel practical concept is proposed which can realize inter-BS antenna calibration within a group of BSs. In this scheme, the feedback amount is minimized while it can still achieve high practicality.The second research point is multi-cell optimal downlink beamforming algorithm with per-base station power constraints.The objective of the problem is usually multi-cell downlink throughput maximization, which is a constrained nonconvex optimization problem and it needs some skills to solve this problem.So far, there have been plenty of works on the downlink beamforming for multi-antenna wireless systems in the past. A main tool to solve these problems is uplink-downlink duality. Authors in used Lagrangian duality to prove that for both the beamforming problem and the capacity region problem the duality of a multi-antenna downlink channel with per-antenna power constraints is an uplink channel with an uncertain and diagonally constrained noise.There are not many works on coordinated multi-cell downlink beamforming problem, which aims to maximize the throughput of the systems. Some greedy algorithms were proposed in the reference, which considered this problem under a graphic framework and solved the problem by separating interference reduction and resource allocation intuitively. In a reference, the uplink-downlink beamforming duality with SINR (Signal to Interference and Noise Ratio) constraints in the multi-cell environment was considered. In that work, it was shown that the optimal downlink beamforming in MISO (Multiple-Input Single-Output) system with single user per sub-channel can be the minimum mean squared error (MMSE) beamforming in the dual uplink. It also proved that the dual uplink problem and the primal downlink problem achieve the same optimal throughput.The dual uplink problem considered in this paper is a min-max mixed optimal problem. Since the dual problem is a nonconvex optimization, it is not easy to compute the globally optimal solution. The main contribution of this paper is a novel efficient algorithm for improving the sum rate of the system through finding the solution satisfying the necessary optimality conditions of dual problem. Numerical results show the convergence of the proposed algorithm.The third research point is the study of low complexity implementation of block diagonalization (BD) precoding.BD algorithm is one of the key precoding techniques for downlink MU-MIMO system. In this precoding algorithm, MU-MIMO channel is decomposed into multiple parallel independent single-user MIMO channels and precoding vector (or matrix) for a MS is computed in the null space of other MSs’channel matrices so that interference between data transmitted to these MSs is cancelled.However, the BD precoding in the previous researches requires implementations of SVD (Singular Value Decomposition) of channel matrices. When the dimension of channel matrix is large, for example, in the cooperative transmission scenario, the SVD based BD will suffer high computational complexity.In practice, the number of receiving antennas of the user equipment is usually limited due to size restraint. In the current evolution of 3G, two-receiving-antenna is a main antenna configuration of the downlink. Studies also show that when each user is equipped with only two antennas, system can achieve the target of ITU (International Telecommunications Union) downlink/uplink spectral efficiency requirement for the International Mobile Telecommunications-Advanced (IMT-Advanced) by the CoMP transmission and reception system.Motivated by this, in this thesis, a low complexity scheme is provided to substitute the procedure of SVD in BD precoding when the number of receiving antennas of MS is two. The proposed method for computing eigenvectors reaches the same effect as the SVD while its computational complexity is significantly reduced. For example, when the number of transmit antennas is 20, computational amounts of the proposed method for computing eigenvectors and the corresponding entire scheme are about 1500 and 53000 flops, respectively, while they are around 16000 and 78000 flops, respectively, in the SVD based scheme.The fourth research point is the clustering approach in CoMP system. In realistic systems, affordable amount of the overhead signals should be taken into consideration. While centralized jointly processing scheme may demand large amounts of interactive information between the central scheduler and each BS, distributed cooperative scheme requires communications among BSs. Due to the fact of significant improvement on data rate in the future mobile network, information exchanged in both centralized and distributed scheme will grow dramatically.Therefore, to reduce the overhead, the number of cooperative BSs will be limited and it will restrict the performance of CoMP. Our interest is in compromising cooperation approaches that maximize the capability of cooperation as much as possible meanwhile lower the complexity of system in order to achieve a smooth network evolution.Cooperation clustering is one key point to the transmission gain, which attracts lots of attention. The thesis only focuses on BS cooperation cluster, which is referred in particular as BS cooperation cluster in the followings. A BS cooperation cluster is defined as a set of cooperative BSs that serve the same group of MSs.In this thesis, the widely used network structure is adopted, i.e., sectorization hexagonal cell, and static clustering concept to investigate on centralized and distributed approaches. In centralized approach, transmit antennas are assembled in one site (or BS); while in distributed approach they are located in different sites. An acknowledged realization of static clustering is the cooperation among three nearby BSs. A semi-dynamic clustering approach in distributed clustering scheme that is named as "multi-layered clustering" is proposed, which aims at remedying the cooperation defects in static clustering with relatively low increment on complexity. For the centralized scheme, an evolution path is also provided to further improve the system performance. In addition, resource allocation is discussed and it will make our approach more feasible compared with the traditional one. |
