Base station cooperation is recognized as a key technology for future wireless cellular communication networks. Considering antennas of distributed base stations and those of multiple terminals within those cells as a distributed multiple-input multiple-output (MIMO) system, this technique has the potential to eliminate inter-cell interference by joint signal processing and to enhance spectral efficiency in this way. Although the theoretical gains are meanwhile well-understood, it still remains challenging to realize the full potential of such cooperative schemes in real-world systems. Among other factors, such as the limited overhead for pilot symbols and for the feedback and backhaul, these performance limitations are related to channel and synchronization impairments, such as channel estimation, feedback quantization and channel aging, as well as imperfect carrier and sampling synchronization among the base stations. Because of these impairments, joint data precoding results to be mismatched with respect to the actual radio channel and the gains of base station cooperation are limited. In order to analyze the signal distortion and the interference among the multiple users, which are caused by mismatched data precoding, it is required to model and investigate impairment effects isolatedly. Therefore, a signal model is provided for base-coordinated orthogonal frequency division multiplexing (OFDM) transmission with channel and synchronization impairments, and closed-form expressions are derived for the mobile users’ signal-to-interference ratio (SIR). Analytical results are numerically verified and lead to practical system requirements. Based on channel modeling and outdoor measurements, inter-site distance limitations for interference-free and time-synchronous transmission are also investigated. It is further discussed how to synchronize distributed base stations by using commercial oscillators locked to externally provided references, e.g. signals provided by Global Positioning System (GPS). Mitigation techniques including adaptive feedback compression and channel prediction are developed. Adaptive feedback compression keeps the channel mean square error (MSE) below a threshold and achieves more than order of magnitude overhead reduction. Doppler-delay base channel prediction reduces the delay-based MSE by 10 dB, approximately. For evaluating purposes, the spatial channel model extended (SCME) as well as channel data from outdoor measurements are used. Multi-cellular simulations reveal that the users’ SIR can be enhanced by 10 dB on average. Practically this means that larger feedback delays, higher mobilities and a larger number of users can be supported in coordinated multi-point (CoMP), compared to than previously believed.
|School:||Technische Universitaet Berlin (Germany)|
|Source:||DAI-C 81/1(E), Dissertation Abstracts International|
|Keywords:||Base station cooperation|
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