Energy-Efficient Heterogeneous Cellular Networks with Spectrum Underlay and Overlay Access

In this paper, we provide joint subcarrier assignment and power allocation schemes for quality-of-service (QoS)-constrained energy-efficiency (EE) optimization in the downlink of an orthogonal frequency division multiple access (OFDMA)-based two-tier heterogeneous cellular network (HCN). Considering underlay transmission, where spectrum-efficiency (SE) is fully exploited, the EE solution involves tackling a complex mixed-combinatorial and non-convex optimization problem. With appropriate decomposition of the original problem and leveraging on the quasi-concavity of the EE function, we propose a dual-layer resource allocation approach and provide a complete solution using difference-of-two-concave-functions approximation, successive convex approximation, and gradient-search methods. On the other hand, the inherent inter-tier interference from spectrum underlay access may degrade EE particularly under dense small-cell deployment and large bandwidth utilization. We therefore develop a novel resource allocation approach based on the concepts of spectrum overlay access and resource efficiency (RE) (normalized EE-SE trade-off). Specifically, the optimization procedure is separated in this case such that the macro-cell optimal RE and corresponding bandwidth is first determined, then the EE of small-cells utilizing the remaining spectrum is maximized. Simulation results confirm the theoretical findings and demonstrate that the proposed resource allocation schemes can approach the optimal EE with each strategy being superior under certain system settings

プライマリシステムの干渉制限を考慮した周波数共用のためのリソース割り当てに関する研究

In wireless communications, the improvement of spectral efficiency isrequired due to the shortage of frequency resource. As an effectivesolution, spectrum sharing has been attracted attention. A cognitiveradio is promising technology for realization of spectrum sharing. Inthe spectrum sharing, cognitive user (secondary user) has to protectlicensed user (primary user) according to the interference constraint.However, conventional metric of interference constraint cannot avoidlarge performance degradation in primary system with widely rangeof Signal to Noise Ratio (SNR) such as a cellular system. Additionally,conventional interference constraints do not considers schedulingbehavior in cellular system. In order to solve these problems, thispaper proposes novel metric of the interference constraint whichsupports the widely SNR region of the primary system, so calledcapacity conservation ratio (CCR). The CCR is defined as the ratio ofthe capacity of the Primary receiver without interference from thesecondary transmitter, to the decreased primary capacity due tointerference. Proposed interference constraint based on CCR canprotect primary capacities over the widely SNR region. In addition,scheduling behavior of the primary system can be protected by usingproposed interference constraint. In addition, we propose transmitpower control schemes: exact and simplified power control. The exactpower control can satisfy requirement of interference constraintwithout large margin; however, transmit power cannot be derivewithout numerical analysis. In contrast, transmit power isclosed-form solution in the simplified power control with satisfyingthe interference constraint. Finally, this thesis proposes the resourcescheduling under the interference constraint. Proposed schedulingachieves the high throughput and high user fairness in the secondarysystem without increasing feedback information compared withconventional algorithm.現在、無線通信において周波数リソース不足が深刻な問題となっており、抜本的な対策技術としてコグニティブ周波数共用が注目されている。本論文では、周波数共用において既存システムの周波数帯を他システム（2 次システム）が二次利用するために干渉制限指標及びリソース割り当てに関する研究を行った。一つ目の研究では、既存システムに与える与干渉状態の評価指標について提案を行い，幅広い通信品質の既存システムを保護可能な干渉制限について評価を行った．評価ではシステムのリンクが静的モデルおよび動的なリソース配分で変更される動的モデルを用いた．二つ目の研究では，その干渉制限達成可能な送信電力制御の検討を行った。送信電力制御を行う際に，外部からチャネル情報の一部のみが得られると仮定し，確率的に変動するフェージング要素について所望のアウテージ確率を満足できるように数値解析を行い，厳密設計および簡易設計について提案を行った．三つ目の研究では、既存システムが複数端末に対して無線リソースをスケジューリングするモデルへと拡張し，2 次システムが干渉を回避しつつ，効率的リソース割り当てに関する検討を行った。電気通信大学201

Inter-cell collaborative spectrum monitoring for cognitive cellular networks in fading environment

We propose a novel inter-cell power allocation for multi-carrier cognitive cellular networks. The proposed scheme incorporates the network-wide primary service communication activity into sub-channel power allocation. To model the primary service activity we define sub-channel activity index (SAI). SAI is then evaluated through a simple yet efficient collaborative spectrum monitoring scheme with very low signaling overhead. Corresponding to a secondary user transmission over a sub-channel, a utility function is defined which is a decreasing function of SAI, and an increasing function of the sub-channel achievable rate. Optimal power allocation is then formulated to maximize the total secondary base station (SBS) utility, subject to SBS transmit power, and primary system collision probability constraints. The sub-optimal solutions to the non-convex optimization are then obtained utilizing dual decomposition method. Comparing with a cognitive cellular network with no signalling among the SBSs, where SBS adopts equal sub-channel power allocation, simulation results indicate a significant gain on the achievable rate. We further compare the rate performance with an ideal system in which perfect interference channel state, and spectrum sensing information are available at the SBS and a combination of underlay and overlay access techniques are adopted. Comparing to the ideal system, the proposed method requires significantly lower signaling overhead while its rate performance closely follows the ideal access

Energy-Efficient Heterogeneous Cellular Networks with Spectrum Underlay and Overlay Access

IEEE In this paper, we provide joint subcarrier assignment and power allocation schemes for quality-of-service (QoS)-constrained energy-efficiency (EE) optimization in the downlink of an orthogonal frequency division multiple access (OFDMA)-based two-tier heterogeneous cellular network (HCN). Considering underlay transmission, where spectrum-efficiency (SE) is fully exploited, the EE solution involves tackling a complex mixed-combinatorial and non-convex optimization problem. With appropriate decomposition of the original problem and leveraging on the quasi-concavity of the EE function, we propose a dual-layer resource allocation approach and provide a complete solution using difference-of-two-concave-functions approximation, successive convex approximation and gradient-search method. On the other hand, the inherent inter-tier interference from spectrum underlay access may degrade EE particularly under dense small-cell deployment and large bandwidth utilization. We therefore develop a novel resource allocation approach based on the concepts of spectrum overlay access and resource efficiency (RE) (normalized EE-SE trade-off). Specifically, the optimization procedure is separated where the macro-cell optimal RE and the corresponding bandwidth is first determined, then the EE of small-cells utilizing the remaining spectrum is maximized. Simulation results confirm the theoretical findings and demonstrate that the proposed resource allocation schemes can approach the optimal EE with each strategy being superior under certain system settings