NSF MIMO Interference Coordination Project

NeTS: Small: Practical Strategies for using MIMO to Mitigate Interference in High-Density Uncoordinated Wireless Networks

NSF Project Number: CNS–1319455

People

Prof. Douglas Blough
Prof. Raghupathy Sivakumar
Dr. Chao-Fang Shih (Project alumnus, currently with Mimosa Networks)
Dr. Luis Cortes-Pena (Project alumnus, currently with Harris Corporation)
Dr. Ramya Srinivasan (Project alumnus, currently with Intel)

Goals

Internet traffic demand has grown remarkably in the last couple of decades, and it is predicted to increase even further in the coming years. Internet users are increasingly relying on WiFi for the last mile connectivity. This has produced a situation where, in many locations, the density of WiFi access points is extremely high. In these areas, the numerous access points and large number of users all share the scarce bandwidth of the unlicensed bands on which WiFi operates. One approach that is beginning to be considered to deal with this problem is cooperation between multiple access points (APs) serving overlapping areas. Cooperation allows more intelligent scheduling of transmissions among all competing users and also permits the use of cooperative multiple-input multiple-output (MIMO) transmission techniques, which can produce much higher aggregate throughput compared to non-cooperative techniques. This research project addressed two key technologies required to make cooperative-MIMO-based WiFi a reality: scalable and high performance cooperative MIMO techniques and scheduled WiFi. The primary goal of this project was to make major leaps forward in addressing the critical problem of limited bandwidth in dense wireless networks, with an emphasis on the use of MIMO capabilities and access point cooperation.

Accomplishments

Accomplishments included contributions in two main areas: (1) access point (AP) cooperation with multi-input multi-output (MIMO) communications, and (2) achieving scheduled operation in wireless networks that use distributed contention.

In the area of cooperative MIMO for WiFi access points, the project participants developed a number of novel concepts and techniques that permit high performance networking while preserving computational efficiency and fairness up to a large number of users, which is necessary for the techniques to be applied in practical settings. Specific concepts and techniques developed and evaluated in this area include:

The concept of interference-aware fairness and its application to transmission scheduling across cooperative APs. Prior notions of fairness did not account for the interference that occurs when multiple users are communicating with multiple APs in close-by locations. The research from this project demonstrated that classical notions of fairness could actually result in very high performance penalties on certain users in this setting. Research in the project demonstrated how the interference-aware fairness concept can be used to maintain high overall performance and achieve good fairness among users despite interference from near-by transmissions.
New iterative algorithms for weighted sum-rate maximization with cooperative MIMO across multiple APs with fast convergence. Unlike prior algorithms, these algorithms have the ability to activate and deactivate individual links from one iteration to the next, which allows them to converge quickly to a high-performing solution even when the number of users is very large. The new algorithms maintain network performance very close to that of prior algorithms but typically converge to a solution within 10 iterations versus 100 or more iterations required for prior algorithms, making the new algorithms practical for network settings with a large number of users.
New scheduling algorithms for cooperative MIMO multi-AP networks. These algorithms extend the fast converging maximum sum rate algorithms that choose a set of active users in one slot to a full schedule of user transmissions across multiple slots. In the cooperative MIMO multi-AP setting, these algorithms are the first to optimize sum rate across multiple slots while achieving given fairness criteria among users over a complete transmission schedule.

The project's research on scheduled WiFi aims to improve network performance in WiFi networks by providing controllability and predictability in channel access. Scheduled WiFi has many potential uses, among them being that it is required for the cooperative MIMO multi-AP techniques proposed in the project. Following are some of the key outcomes of the scheduled WiFi research:

A new media access control (MAC) protocol, called Rhythm, which provides centralized control and scheduling of WiFi networks, with the properties of i) low protocol overhead, ii) work conservation of non-backlogged nodes, and iii) robustness to partial connectivity scenarios.
A second new MAC protocol, called LWT, which achieves scheduled WiFi in a purely distributed fashion. LWT provides better backward compatibility for existing network devices, because it works with legacy nodes and does not require network topology information.
A mechanism, called Switch, that provides a lightweight control plane for WiFi transmitters and receivers that can be used during ongoing communications. Switch can be used to improve scheduled WiFi performance and it can also i) extend the range of WiFi carrier sense, ii) provide early collision termination, and iii) improve the efficiency of WiFi backoff.

Publications

The following publications were produced with partial or full support from this NSF award.

C.-F. Shih and R. Sivakumar, "Switch: Enabling Transmitter and Receiver Participation in Seamless Lightweight Control," Proceedings of the IEEE International Conference on Computer Communications (Infocom), 2017.
M. Ge and D.M. Blough, "High-Throughput and Fair Scheduling for Access Point Cooperation in Dense Wireless Networks," Proceedings of IEEE Wireless Communications and Networking Conference (WCNC), 2017.
M. Ge, J. Barry, and D.M. Blough, "Combined User Selection and MIMO Weight Calculation for AP Cooperation in Dense Wireless Networks," Proceedings of IEEE Wireless Communications and Networking Conference (WCNC), 2017.
M. Ge and D.M. Blough, "PBUS: Efficient User Selection for Block Diagonalization in Dense Wireless Networks," Proceedings of IEEE Global Communications Conference (Globecom), 2017.
C.-F. Shih, B. Krishnaswamy, Y. Jian, and R. Sivakumar, "Scheduled WiFi Using Distributed Contention in WLANs: Algorithms, Experiments, and Case Studies," Journal of Wireless Networks, Vol. 24, pp. 89–112, 2016.
C.-F. Shih, Algorithms and Protocols for Next Generation WiFi Networks, Ph.D. dissertation, Georgia Institute of Technology, 2017.
D.M. Blough, P. Santi, and G. Resta, "Interference-Aware Time-Based Fairness for Multihop Wireless Networks," Proceedings of IEEE International Conference on Computer Communications (Infocom), 2016.
C.-F. Shih, B. Krishnaswamy, and R. Sivakumar, "Rhythm: Achieving Scheduled WiFi Using Purely Distributed Contention in WLANs," Proceedings of IEEE Global Communications Conference (Globecom), 2015.
C.-F. Shih, Y. Jian, and R. Sivakumar, "Look Who's Talking: A Practical Approach for Achieving Scheduled WiFi in a Single Collision Domain," Proceedings of ACM International Conference on Emerging Networking Experiments and Technologies (CoNEXT), 2015.
L.M. Cortes-Pena and D.M. Blough "MIMO Link Scheduling for Interference Suppression in Dense Wireless Networks," Proceedings of the IEEE Wireless Communications and Networking Conference (WCNC), 2015.
Y. Jian, C.-F. Shih, B. Krishnaswamy, and R. Sivakumar, "Coexistence of WiFi and LAA-LTE: Experimental Evaluation, Analysis and Insights," ICC Workshop on LTE in Unlicensed Bands: Potentials and Challenges, 2015.
L.M. Cortes-Pena, J. Barry, and D.M. Blough, "Jointly Optimizing Stream Allocation, Beamforming and Combining Weights for the MIMO Interference Channel," IEEE Transactions on Wireless Communications, Vol. 14, pp. 2245–2256, 2015.
D.M. Blough, P. Santi, and G. Resta, "Interference-Aware Proportional Fairness for Multi-rate Wireless Networks," Proceedings of the IEEE International Conference on Computer Communications (Infocom), 2014.
D.M. Blough, P. Santi, and R. Srinivasan, "On the Feasibility of Unilateral Interference Cancellation in MIMO Networks," IEEE/ACM Transactions on Networking, Vol. 22, pp. 1831–1844, 2014.
L.M. Cortes-Pena, Optimizing Dense Wireless Networks with MIMO Links, Ph.D. Dissertation, Georgia Instiute of Technology, 2014.
C.-F. Shih and R. Sivakumar, "FastBeam: Practical Fast Beamforming for Indoor Environments," Proceedings of the IEEE International Conference on Computing, Networking and Communications (ICNC), 2014.
L.M. Cortes-Pena, J. Barry, and D.M. Blough, "Joint Optimization of Stream Allocation and Beamforming and Combining Weights for the MIMO Interference Channel," Proceedings of the IEEE Global Communications Conference (Globecom), 2013.

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