Strategies for wireless network control with applications to LTE
Xiang Xu, Gerd Ascheid, Rudolf Mathar · RWTH Publications (RWTH Aachen) · 2015
The 4th generation (4G) mobile cellular network aims at providing high data rate, low latency wireless links and ubiquitous connectivity. To meet these demands, sophisticated network control methods are required. Since the 4G mobile cellular system is constituted of many advanced techniques, the Long-Term Evolution (LTE) standards are established to provide unified technical specifications and thus ensure compatibility. Following the LTE standards, various strategies for wireless network control on both link level and network level are presented and discussed in this dissertation.First, to engage systematic analysis of wireless networks, different channel models are reviewed. Based on two existing modeling methodologies, namely, geometry based stochastic model and deterministic ray-launching, a semi-stochastic channel model is derived. The semi-stochastic channel model first uses a geometric description of the propagation environment to calculate propagation paths for radio waves, and then employs stochastic procedures to calculate the channel impulse response for multiple-input multiple-output (MIMO) channels. Hence, the semi-stochastic model can improve modeling accuracy by knowledge of the propagation environment, while keeping randomness for Monte-Carlo simulations. When comparing with measurement data, the semi-stochastic channel model shows better modeling accuracy than the WINNER model.Second, the feedback information from mobile stations (MS) and its influence on LTE systems are studied. Since base stations (BS) need channel state information to facilitate adaptive modulation and coding schemes and manage radio resources, the MSs have to measure the signal-to-interference-plus-noise-ratio (SINR) and send the compressed channel quality indicator (CQI) to the BSs. To compensate for temporal variation, prediction schemes of the SINR are investigated. The statistics of SINR in a multi-cell network are derived analytically for slowly moving MSs. Furthermore, a simple approximation of the autocovariance function of the SINR is given. Since different prediction schemes show different behavior for the MSs moving with different speed, that optimal prediction schemes are chosen to adapt to the speed. In addition, by assuming the prediction noise follows a Gaussian distribution, bandwidth efficiency of LTE systems with imperfect CQI feedback is obtained considering both cases with and without hybrid automatic repeat request (HARQ). Further investigations show that a biased estimator may provide higher throughput than an unbiased one. Finally, transmit power control for heterogeneous LTE networks based on CQI is addressed. To provide pervasive coverage to indoor users, femtocells are introduced as a part of the heterogeneous network structure. Due to the shared frequency spectrum among femtocells and macrocells, co-channel interference is inevitable. Conventional interference suppression methods usually require full knowledge of the network structure or depend on the accuracy of the pathloss model. The presented power control scheme takes only the feedback CQIs as input. By differentiating service types of users and applying different quality of service (QoS) constraints, the transmit power of femtocells can be managed in a self-organizing fashion. The self-organizing power control does not need prior information about the network structure and thus is easy to implement. It shows superior performance compared to conventional methods with respect to both capacity and coverage.