Heterogeneous Cellular Networks

Coordinated Scheduling and Spectrum Balancing in Multicellular Networks

In this work we consider a multi-cell OFDMA-based wireless network with full spectral reuse and we study the problem of co-channel interference mitigation via base station coordination in the downlink channel. Assuming that the cluster of coordinated base stations can only share channel quality measurements in real time but not user data symbols, we address the problem of jointly selecting the set of co-channel users and the power allocation across tones in order to maximize the weighted system sum-rate subject to per-base station peak power constraints. Since the optimal solution of this problem is NP-hard, several low-complexity sub-optimal scheduling and spectrum balancing algorithms are presented and discussed, each one requiring a different level of coordination among base stations and a different feedback bandwidth. Finally, numerical examples are provided to assess the performance of the proposed strategies.


Coordinated Beamforming in Downlink Multicellular Networks

In this work, we consider a multi-cell wireless network with universal frequency reuse and treat the problem of co-channel interference mitigation in the downlink channel. Assuming that each access point serves multiple single-antenna mobiles via space-division multiple access, we propose to jointly optimize the set of linear beam-vectors across a cluster of coordinated cells: the objective function to be maximized is the weighted system sum-rate subject to per-base-station power constraints. After deriving the general structure of the optimal beam-vectors, a novel iterative algorithm is presented which attempts to solve the Karush-Kuhn-Tucker (KKT) conditions of the nonconvex primal problem. Simulation results are provided to assess the performance of the proposed algorithm.


Interference Mitigation through Coordinated Scheduling and Power Control

Inter-cell interference mitigation is a key challenge in next generation wireless networks which are expected to use an aggressive frequency reuse factor and a high-density base station deployment to improve coverage and spectral efficiency. In this work, we consider the problem of maximizing the weighted sum-rate of a wireless cellular network via distributed base station coordination. We present two novel algorithms which iteratively optimize the transmit power level and the user assignment at each base station based on some limited local information. Both algorithms converge to a solution where no base station can unilaterally modify its status to improve the overall weighted sum-rate. Numerical studies are carried out to assess the performance of the proposed schemes in a realistic system based upon the IEEE 802.16m specifications. Simulation results show that the proposed algorithms achieve a significant rate gain over uncoordinated transmission strategies for both cell-edge and inner users.


FERMI: A Centralized Resource Management System for Small Cell Networks

The demand for increased spectral efficiencies is driving the next generation broadband access networks towards deploying smaller cells (pico, micro) with sophisticated air interface technologies (Orthogonal Frequency Division Multiple Access or OFDMA). The expected dense deployment of small cells however, makes interference and hence resource management both critical and extremely challenging. In this work, we design and implement one of the first resource management systems, FERMI, for OFDMA-based small cell networks that ICIC feature in these deployments. As part of its design, FERMI (i) provides resource isolation in the frequency domain (as opposed to time) to leverage power pooling across cells to improve capacity; (ii) uses measurement-driven triggers to intelligently distinguish clients that require just link adaptation from those that require resource isolation; (iii) incorporates mechanisms that enable the joint scheduling of both types of clients in the same frame; and (iv) employs efficient, scalable algorithms to determine a fair resource allocation across the entire network with high utilization and low overhead. We have implemented FERMI on a prototype four-cell WiMAX small cell testbed and show that it yields significant gains over conventional approaches.



RADION: A Distributed Resource Management System for Small Cell Networks

Next generation wireless networks (i.e., WiMAX, LTE) provide higher bandwidth and spectrum efficiency leveraging smaller (femto) cells with orthogonal frequency division multiple access (OFDMA). The uncoordinated, dense deployments of small cells however, pose several unique challenges relating to interference and resource management in these networks. Towards addressing these challenges, we propose RADION, a distributed resource management framework that effectively manages interference across small cells. RADION’s core building blocks enable small cells to opportunistically find the available resources in a completely distributed and efficient manner. Further, RADION’s modular nature paves the way for different resource management solutions to be incorporated in the framework. We implement RADION on a real WiMAX small cell testbed deployed in a typical indoor setting. We extensively evaluate two solutions integrated with RADION, both via prototype implementation and simulations and quantify their performance in terms of quick and efficient self-organization.


ProBeam: A Practical Multi-Cell Beamforming System for Small Cell Networks

Small cells form a critical component of next generation cellular networks, where spatial reuse is the key to higher spectral efficiencies. Interference management in the spatial domain through beamforming allows for increased reuse without having to sacrifice resources in the time or frequency domain. Existing beamforming techniques for spatial reuse, being coupled with client scheduling, face a key limitation in practical realization, especially with OFDMA small cells. In this context, we argue that for a practical spatial reuse system with beamforming, it is important to decouple beamforming from client scheduling. Further, we show that jointly  addressing client association with beamforming is critical to maximizing the reuse potential of beamforming. Towards this goal, we propose ProBeam – a system for multicell beamforming and client association in OFDMA small cell networks. ProBeam incorporates two key components - a low complexity, highly accurate SINR estimation module that helps determine interference dependencies for beamforming between small cells; and an efficient, low complexity joint client association and beam selection algorithm for the small cells that accounts for scheduling at the small cells without being coupled with it. We have prototyped ProBeam on a WiMAX-based network of four small cells. Our evaluations reveal the accuracy of our SINR estimation module to be within 1 dB, and the reuse gains from joint client association and beamforming to be as high as 115% over baseline approaches.


 iBUS: Integrated Scheduler and Beamformer for Uplink in Small Cells

Addressing interference in next generation OFDMA femtocells forms an important component in leveraging their increased network capacity. We observe that beamforming has a unique and complementary advantage in this regard compared to existing resource isolation mechanisms. However, before realizing its true potential for interference mitigation across femtocells, one needs to integrate beamforming with scheduling within each cell. This essential first step forms our main focus in this work. Unlike downlink, we show that the integration of beamforming with uplink scheduling faces an interesting trade-off between beamforming gain on the one hand and the power pooling gain resulting from joint multi-user scheduling on the other hand. This in turn makes the uplink scheduling problem even hard to approximate, to address which we propose algorithms that are simple to implement, yet provably efficient with a worst case guarantee of half. We implement our solutions on a WiMAX femtocell platform integrated with an eight element phased array beamforming antenna. Evaluations from both prototype and trace-driven simulations indicate that our solution provides significant gains of over 40% compared to an omni-directional femtocell.


PRESTO: IP-over-60 GHz Wireless with Switched Beamforming

The multi-Gbps throughput potential of 60 GHz wireless interfaces make them an attractive technology for next-generation gigabit wireless links with potential application to small cell backhaul. For increased coverage, and improved resilience to blockage, beamsteering with high gain directional antennas is emerging to be an integral part of 60 GHz radios. However, the real-world performance of these state-of-the-art radios in has not previously been explored well in open literature. To this end, in this paper, we address the following open questions: how do these radios perform in line-of-sight (LOS) and non-line-of-sight (NLOS) locations? how sensitive is performance to factors such as node orientation or placement? how robust is performance to human-body blockage and mobility? Our measurement results using a first-of-its-kind experimental platform (called Presto), show that, contrary to conventional perception, state-of-the-art 60 GHz radios perform well even in NLOS locations, in the presence of human-body blockage and LOS mobility. While their performance is affected by node (or more precisely, antenna array) orientation, simply using a few more antenna arrays and dynamically selecting amongst them shows potential to address this issue.


Relevant Publications

ProBeam: A Practical Multicell Beamforming System for OFDMA Small-cell Networks
The 14th ACM International Symposium on Mobile Ad Hoc Networking and Computing (MobiHoc 2013)
pp. 147-156, 2013
Jongwon Yoon, Karthikeyan Sundaresan, Mohammad A. Khojastepour, Sampath Rangarajan, Suman Banerjee

A Resource Management System for Interference Mitigation in Enterprise OFDMA Femtocells
IEEE/ACM Transactions on Networking
21(5): pp. 1447-1460, 2013
Mustafa Y. Arslan, Jongwon Yoon, Karthikeyan Sundaresan, Srikanth V. Krishnamurthy, Suman Banerjee

A Distributed Resource Management Framework for Interference Mitigation in OFDMA Femtocell Networks

The Thirnteenth International Symposium on Mobile Ad Hoc Networking and Computing (MobiHoc 2012),
pp. 235-244
Jongwon Yoon, Mustafa Y. Arslan, Karthikeyan Sundaresan, Srikanth V. Krishnamurthy, Suman Banerjee

Design and Implementation of an Integrated Beamformer Design and Uplink Scheduler for OFDMA Femtocells
The Thirteenth International Symposium on Mobile Ad Hoc Networking and Computing (MobiHoc 2012)
pp. 215-224, 2012
Mustafa Y. Arslan, Karthikeyan Sundaresan, Srikanth V. Krishnamurthy, Sampath Rangarajan

Experimental Characterization of Interference in OFDMA Femtocell Networks
IEEE Infocom 2012
pp. 3238-3242
Mustafa Y. Arslan, Jongwon Yoon, Karthikeyan Sundaresan, Srikanth V. Krishnamurthy, Suman Banerjee

On 60 GHz Wireless Link Performance in Indoor Environments
The13th Passive and Active Measurement Conference (PAM 2012)
pp. 147-157, 2012
Xiaozheng Tie, Kishore Ramachandran, Rajesh Mahindra

FERMI: A Femtocell Resource Management System for Interference Mitigation in OFDMA Networks
17th Annual Conference on Mobile Computing and Networking (MobiCom 2011)
pp. 25-36, 2011
Mustafa Y. Arslan, Jongwon Yoon, Karthikeyan Sundaresan, Srikanth V. Krishnamurthy, Suman Banerjee

Weighted Sum-Rate Maximization in Multi-Cell Networks via Coordinated Scheduling and Discrete Power Control
IEEE Journal on Selected Areas in Communications
29(6): pp. 1214-1224, 2011
Honghai Zhang, Luca Venturino, Narayan Prasad, Peilong Li, Sampath Rangarajan, Xiaodong Wang

Coordinated Linear Beamforming in Downlink Multi-Cell Wireless Networks
IEEE Transactions on Wireless Communications
9(4): pp. 1451-1461, 2010
Luca Venturino, Narayan Prasad, Xiaodong Wang

Coordinated Scheduling and Power Allocation in Downlink Multicell OFDMA Networks
IEEE Transactions on Vehicular Technology
Luca Venturino, Narayan Prasad, Xiaodong Wang

Efficient Resource Management in OFDMA Femto Cells
ACM MobiHoc 2009
pp. 33-42
Karthikeyan Sundaresan, Sampath Rangarajan