Optimal Rate Allocation and SINR Control in Wireless Networks

Team: Navid Ehsan and Rene Cruz

Summary: In a wireless ad-hoc network, we propose an optimization problem to maximize the average horizontal throughput. We show that in a dense network the optimal SINR is asymptotically zero. We then study a fixed rate allocation and a variable rate allocation policy in a low interference regime. We derive expressions for the average horizontal throughput as a function of system parameters and the transmission rate (or the set of available transmission rates). We then optimize over the set of transmission rates. We plan to extend our results to the case of directional antennas and study the optimal choice for the transmission and reception beam-width.

Publications: N. Ehsan and R. L. Cruz, "On the optimal SINR in random access networks with spatial reuse", CISS 2006, Princeton University.

N. Ehsan and R. L. Cruz, "Optimal rate allocation in wireless ad-hoc networks", Allerton Conference on Communication, Control, and Computing, 2006.

Network Coding for Wireless Networks

Team: Rathinakumar Appuswamy, Massimo Franceschetti, and Kenneth Zeger

Summary: We study coding over networks with broadcast-mode transmissions, there each node transmission corresponds to sending the same message on all the outgoing links of the node. We study the performance of non-linear and linear network codes for energy efficient multicast problems in directed networks with capacitated links. We also consider coding for multiple unicast problems on undirected graphs with interference, and corresponding rate and energy gains over routing strategies.

Publications: R. Appuswamy, M. Franceschetti, and K. Zeger, "Optimality of Linear Codes for Broadcast-Mode Multicast Networks", IEEE International Symposium on Information Theory (ISIT) Seattle, Washington, July 2006.

Scheduling Policies for Sharing Network Resources

Team: Ehsan Ardestani, Tara Javidi and Massimo Franceschetti

Summary: Network resources can be aggregated to share different data streams and improve the overall performance of the system. An example of application is data aggregation for broadband access points, where data traffic can be split among different Internet access points. We approach the problem from a stochastic network perspective, studying the delay-throughput constraints in different models of network queues.

Publications: N.A.

Towards a decentralized operation of complex networks.

Team: Nikhil Karamchandani, Massimo Franceschetti and Rene Cruz

Summary: Recent scaling laws and capacity results for wireless networks have focused on asymptotically large networks. However, for large networks, routing schemes which require sychronization over the entire networks and/or complete knowledge of the network topology are impractical. Our research focuses on developing decentralized strategies to operate networks and achieve various objectives such as high data rates, and low transmission delays in presence of fading.

Publications:
N. Karamchandani M. Franceschetti,  "Scaling laws for delay sensitive traffic in Rayleigh fading networks ", IEEE Globecom Conference, 2007.  

Optimal Operating Point in MIMO channels with Bursty Traffic

Team: Somsak Kittipiyakul, Tara Javidi

Summary: We consider a bursty source and a MIMO channel. Given the Zheng-Tse's diversity-multiplexing tradeoff of the MIMO channel (for large SNR), we find the optimal multiplexing gain that optimizes the end-to-end loss probability which is defined by the delay bound violation probability as well as the channel decoding error probability. We have extended the model and analysis to a MIMO multiplex access channel. In this case, the system brings together the four types of gains: diversity, spatial multiplexing, space-division multiple-access, and statistical multiplexing gains. Current research involves extending the analysis to other types of gains such as time-diversity and cooperative diversity.

Publications:
S. Kittipiyakul and T. Javidi, "Optimal operating point for MIMO multiple access channel with bursty traffic," submitted to Trans. of Wireless Communication.

S. Kittipiyakul and T. Javidi, “Optimal operating point in MIMO channel for delay-sensitive and bursty traffic,” IEEE International Symposium on Information Theory (ISIT), Seattle, Washington, July 2006.

Space-Time Duality in Multiple Antenna Channels

Team: Kaushik Chakraborty, Massimo Franceschetti

Summary: The concept of information transmission in a multiple antenna channel with scattering objects is defined from physical principles. It is shown that the amount of information that can be transported over the channel to the receiver depends on both the spatial and the frequency bandwidth of the radiating system composed by the transmitting antennas and the scattering objects. The spatial information content can be quantified in a similar fashion than its temporal counterpart, by reducing the inverse problem of field reconstruction to a communication problem in space, and determining the relevant communication modes of the channel by rigorously applying the sampling theorem on the field’s vector space. The consequence is a space-time information duality principle arising in the computation of the capacity of the radiating system.

Publications:
K. Chakraborty, M. Franceschetti "Maxwell Meets Shannon: Space-Time Duality in Multiple Antenna Channels", Allerton Conference on Communication, Control, and Computing, 2006.

Mechanism Design in Cellular Networks: Leveraging Forward Link for Efficient Reverse Link Allocation

Team: Jennifer Price, Tara Javidi

general wireless networks. It is well known that the economic notion of pricing can be used to design appropriate signaling mechanisms whose goal is to align the behavior of individual users with the global well being. If these signals do not represent a true cost to users, however, they offer little guarantee that users will exhibit the desired behavior in the face of a potential tragedy of commons situation. In addition, if the desired behavior is dependent on information held locally by Summary: The goal of this project is to design socially optimal and incentive compatible rate allocation schemes for the end-users (e.g. queue backlogs, QoS requirements, etc), users may not be truthful when revealing this information. We show that by leveraging users’ demand for forward link rate, it is possible to design an incentive-compatible signaling mechanism that ensures users will truthfully reveal their locally held information.

Publications:
J. Price, T. Javidi. “Leveraging Downlink for Regulation of Distributed Uplink CDMA.” IEEE Globecom, December 2006.

J. Price, T. Javidi. “Leveraging Forward Link for Optimal Reverse Link Allocation: An Incentive Compatible Approach.” Asilomar Conference on Signals, Systems and Computers, November 2006.

Team: Yoav Nebat, Rene Cruz

Summary: Relaying and cooperative communications are an area of current focus in the research and development communities. Extracting spatial diversity via route selection is a promising practical method for robust multi-hop communications. Both simple relaying networks and capacity scaling laws are investigated. 

Randomized access in multi-hop wireless communications.

Team: Yoav Nebat, Rene Cruz

Summary: Performance bounds for a simple Aloha-like access protocol in a distributed setting.
Practical distributed adaptive algorithms that approach the performance bounds.

Publications:
Y. Nebat, R.L. Cruz, L.B. Milstein, "Relay Selection Diversity: On the Near-Optimality of Store-and-Forward Routing," submitted for publication in IEEE Trans. Inf. Theory, August 2006.

Y. Nebat, "A Lower Bound for the Achievable Throughput in Large Wireless Networks under Fixed Multi-path Fading," IEEE SpaSWiN, Boston, April 2006.

Y. Nebat, S. Bhardwaj, "On the Capacity of Random Wireless Networks under Fixed Multi-path Fading," IEEE WiOpt, Boston, April 2006 (Best paper award).

Y. Nebat, M. Sidi, "Parallel Downloads for Streaming Applications – a Resequencing Analysis," Performance Evaluation, Vol.63, No.1, pp15-35, January 2006.

Y. Nebat, R.L. Cruz, "Routing, Cooperative Transmission and the Relaying Bound: the Effect of Multi-User Diversity," in the proceedings of the 39th annual conference on information sciences and systems, Johns Hopkins university, Baltimore, March 2005.

Throughput-Optimal Minimum Delay Routing for Wireless Ad-Hoc Networks

Team: Parul Gupta, Tara Javidi

We consider the problem of designing routing protocols for wireless ad-hoc networks. Lately, there has been a movement from the traditional routing approach, which identifies the best path to the destination (or at least theh best next hop!) before transmission to opportunistic approaches which make routing decisions adaptively, based on actual transmission outcomes. We compare the stable rate region (a.k.a network layer capacity region) of both the approaches and find, interestingly, that opportunistic routing schemes do not always fare better. Backpressure routing schemes are known to be throughput-optimal, i.e. achieve queue stability for all the rates in the capacity region, but compromise on delay instead. We analyze the various schemes closely and consider the design of a routing policy that achieves throughput optimality as well as minimum expected delay.

Team: Paolo Minero, Massimo Franceschetti

In a distributed control system the controller elements are not in a central in location but are distributed throughout the system with each component sub-system under the control of one or more controllers. The entire system may be networked for communication and monitoring. Such a system typically uses sensors for estimation, computers as controllers, and interconnections and protocols for communication. Stabilization and control become challenging tasks due to the time varying nature of the channel conditions. The achievable information rate over the channel must be high enough compared to the sytem's unstable modes to guarantee stability, but this rate is also likely to change unpredictably over time. Our aim is to develop a theory for control over communication channels that explicitly accounts for the randomness in the channel.

Publications:
P. Minero, M. Franceschetti, S. Dey & G. Nair, "Towards control over
fading channel", Proc. Workshop on Control over Communication Channels, IEEE, Limassol, Cyprus, April 2007.