Coordinated multipoint or cooperative MIMO is one of the promising concepts to improve cell edge user data rate and spectral efficiency beyond what is possible with MIMO-OFDM in the first versions of LTE or WiMAX. Interference can be exploited or mitigated by cooperation between sectors or different sites. Significant gains can be shown for both the uplink and downlink. A range of technical challenges were identified and partially addressed, such as backhaul traffic, synchronization and feedback design. This article also shows the principal feasibility of COMP in two field testbeds with multiple sites and different backhaul solutions between the sites. These activities have been carried out by a powerful consortium consisting of universities, chip manufacturers, equipment vendors, and network operators.

Channel models are important tools to evaluate the performance of new concepts in mobile communications. However, there is a tradeoff between complexity and accuracy. In this paper, we extend the popular Wireless World Initiative for New Radio (WINNER) channel model with new features to make it as realistic as possible. Our approach enables more realistic evaluation results at an early stage of algorithm development. The new model supports 3-D propagation, 3-D antenna patterns, time evolving channel traces of arbitrary length, scenario transitions and variable terminal speeds. We validated the model by measurements in a coherent LTE advanced testbed in downtown Berlin, Germany. We then reproduced the same scenario in the model and compared several channel parameters (delay spread, path gain, K-factor, geometry factor and capacity). The results match very well and we can accurately predict the performance for an urban macro-cell setup with commercial high-gain antennas. At the same time, the computational complexity does not increase significantly and we can use all existing WINNER parameter tables. These artificial channels, having equivalent characteristics as measured data, enable virtual field trials long before prototypes are available.

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It is common to use channel models such as the 3GPP spatial channel model (SCM), the WINNER model or ray tracing to evaluate multiple-antenna multiple-user techniques in wireless communications. Cross-polarized antennas can enhance the channel rank and thus the throughput of such systems especially in case of a line-of-sight (LOS) connection. This requires an exact model of the polarization characteristics. To increase the accuracy of the existing channel models, we propose a new method that predicts the polarization state of a microwave link based on findings in the field of optics. We verified the method by cross-polarized multiple-input-multiple-output (MIMO) measurements at 2.6 GHz with 16 transmitters and ten receivers in an urban macrocell environment under strong LOS conditions in downtown Berlin, Germany. Comparisons of simulation and measurement results show that the coefficients of the polarized LOS channel can be predicted very well by the new method. Measured capacities at 10-dB signal-to-noise ratio (SNR) were in between 14.2 and 19.1 b/s/Hz-values that can be predicted by the channel model with more than 90% accuracy. This increase in modeling accuracy is an important feature for many applications such as heterogeneous networks, space-to-ground satellite communications, and cooperative communications.

Abstract-Coordinated multi-point (CoMP) is a new class of transmission schemes for interference reduction in the next generation of mobile networks. We have implemented and tested a distributed CoMP transmission approach in the downlink of an LTE-Advanced trial system operating in real time over 20 MHz bandwidth. Enabling features such as network synchronization, cell-and user-specific pilots, feedback of multicell channel state information and synchronous data exchange between the base stations have been implemented. Interferencelimited transmission experiments have been conducted using optimum combining with interference-aware link adaptation and cross-wise interference cancellation between the cells. The benefits of CoMP transmission have been studied over multi-cell channels recorded in an urban macro-cell scenario.