Analysis And Optimization Of Energy Efficiency In Heterogeneous Networks

Heterogeneous cellular networks (HetNets) are to be deployed for future wireless communications to meet the ever-increasing mobile traffic demand. However, the dense and random deployment of small cells and their uncoordinated operation raise important concerns about energy efficiency. On one hand, stochastic geometry has become a major tool of modeling and analyzing HetNets. On the other hand, base station (BS) cooperation and discontinuous transmission (DTX) scheme can serve as effective technologies to improve the energy efficiency of overall systems. Accordingly, based on the framework of stochastic geometry, this thesis mainly discusses the impact of BS cooperation and random DTX scheme on the energy efficiency of HetNets.Firstly, based on user-centric geometric BS clustering model, this thesis studies energy-efficient BS cooperation in HetNets. We derive the ergodic rate expression in a K-tier HetNet using tools from stochastic geometry. Furthermore, we formulate a power minimization problem with a minimum ergodic rate constraint and derive a closed-form approximate result of the optimal cooperative radii. Simulation results verify the effectiveness of theoretical analysis and also show a good accuracy of the closed-form approximate results. Meanwhile, we show that deploying a two-tier HetNet consisting of macrocell BSs and small cell BSs is more enery-saving compared to a traditional macro-only network.Secondly, note that there still exist some problems in our above work on BS cooperation. For instance, the impact of BS cooperation on the network energy efficiency has not been studied, the impact of slow-varying fading (e.g. shadowing) on the cooperative cluster has not been taken into account, and we have made an unrealistic assumption that all the network tiers have an identical path loss exponent and so forth. Therefore, this thesis next proposes a received signal strength (RSS)-based BS clustering model and further studies cross-tier BS cooperation in a more general scenario. We derive the average user rate expression in a K-tier HetNet using tools from stochastic geometry. Furthermore, we formulate a power minimization problem with a minimum average user rate constraint and derive the optimal RSS thresholds under certain approximation. Simulation results verify the effectiveness of theoretical analysis and also show a good ac-curacy of the approximate results. Finally, simulation results show that the proposed RSS-based clustering model is more energy-saving than the geometric clustering model.Finally, this thesis studies the impact of the random DTX scheme on local delay and energy efficiency in HetNets. Using a stochastic geometry based model, we derive the local delay and energy efficiency in a general case and obtain closed-form expressions in some special cases. These results give insights into the effect of key system parameters, such as path loss exponents, BS densities, signal-to-interference ratio (SIR) threshold and mute probability on the system performance. We also provide the low-rate and high-rate asymptotic behavior of the maximum energy efficiency. It is analytically shown that it is less energy-efficient to apply random DTX scheme in the low-rate regime. In the high-rate regime, however, random DTX scheme is essential to achieve a finite local delay and higher energy efficiency.