Sanjay Shakkottai

Cross-layer design for wireless networks

In this paper, we propose a structure of the DLC (data link control) protocol layer, which consists of the functional component, with radio resource channel allocation method. It is operated by the state of current traffic volume for the efficiency of radio resource utilization. Different adequate components will be taken by the current traffic state, especially fraction based data transmission buffer control method for the QoS (quality of service) assurance

Scheduling algorithms for a mixture of real-time and non-real-time data in HDR

High Data Rate (HDR) technology has recently been proposed as an overlay to CDMA as a means of providing packet data service to mobile users. In this paper, we study various scheduling algorithms for a mixture of real-time and non-real-time data over HDR/CDMA and compare their performance. We study the performance with respect to packet delays and also average throughput, where we use a token based mechanism to give minimum throughput guarantees. We find that a rule which we call the exponential rule performs well with regard to both these criteria. (In a companion paper, we show that this rule is throughput-optimal, i.e., it makes the queues stable if it is feasible to do so with any other scheduling rule). Our main conclusion is that intelligent scheduling algorithms in conjunction with token based rate control provide an efficient framework for supporting a mixture of real-time and non-real-time data applications in a single carrier.

FlashLinQ: a synchronous distributed scheduler for peer-to-peer ad hoc networks

This paper proposes FlashLinQ - a synchronous peer-to-peer wireless PHY/MAC network architecture for distributed channel allocation. By leveraging the fine-grained parallel channel access of OFDM, FlashLinQ develops an analog energy-level based signaling scheme that enables SIR (Signal to Interference Ratio) based distributed scheduling. This new signaling mechanism and the corresponding allocation algorithms permit efficient channel-aware spatial resource allocation, leading to significant gains over a CSMA/CA system with RTS/CTS. FlashLinQ is a complete system architecture including (i) timing and frequency synchronization derived from cellular spectrum, (ii) peer discovery, (iii) link management, and (iv) channelaware distributed power, data-rate and link scheduling. We implement FlashLinQ over licensed spectrum on a DSP/FPGA platform. In this paper, we present performance results for FlashLinQ using both implementation and simulations.

Rethinking information theory for mobile ad hoc networks

The subject of this article is the long standing open problem of developing a general capacity theory for wireless networks, particularly a theory capable of describing the fundamental performance limits of mobile ad hoc networks. A MANET is a peer-to-peer network with no preexisting infrastructure. MANETs are the most general wireless networks, with single-hop, relay, interference, mesh, and star networks comprising special cases. The lack of a MANET capacity theory has stunted the development and commercialization of many types of wireless networks, including emergency, military, sensor, and community mesh networks. Information theory, which has been vital for links and centralized networks, has not been successfully applied to decentralized wireless networks. Even if this was accomplished, for such a theory to truly characterize the limits of deployed MANETs it must overcome three key roadblocks. First, most current capacity results rely on the allowance of unbounded delay and reliability. Second, spatial and timescale decompositions have not yet been developed for optimally modeling the spatial and temporal dynamics of wireless networks. Third, a useful network capacity theory must integrate rather than ignore the important role of overhead messaging and feedback. This article describes some of the shifts in thinking that may be needed to overcome these roadblocks and develop a more general theory.

On combining shortest-path and back-pressure routing over multihop wireless networks

Back-pressure-type algorithms based on the algorithm by Tassiulas and Ephremides have recently received much attention for jointly routing and scheduling over multihop wireless networks. However, this approach has a significant weakness in routing because the traditional back-pressure algorithm explores and exploits all feasible paths between each source and destination. While this extensive exploration is essential in order to maintain stability when the network is heavily loaded, under light or moderate loads, packets may be sent over unnecessarily long routes, and the algorithm could be very inefficient in terms of end-to-end delay and routing convergence times. This paper proposes a new routing/scheduling back-pressure algorithm that not only guarantees network stability (throughput optimality), but also adaptively selects a set of optimal routes based on shortest-path information in order to minimize average path lengths between each source and destination pair. Our results indicate that under the traditional back-pressure algorithm, the end-to-end packet delay first decreases and then increases as a function of the network load (arrival rate). This surprising low-load behavior is explained due to the fact that the traditional back-pressure algorithm exploits all paths (including very long ones) even when the traffic load is light. On the other-hand, the proposed algorithm adaptively selects a set of routes according to the traffic load so that long paths are used only when necessary, thus resulting in much smaller end-to-end packet delays as compared to the traditional back-pressure algorithm .

FlashLinQ: A synchronous distributed scheduler for peer-to-peer ad hoc networks

This paper proposes FlashLinQ-a synchronous peer-to-peer wireless PHY/MAC network architecture. FlashLinQ leverages the fine-grained parallel channel access offered by OFDM and incorporates an analog energy-level-based signaling scheme that enables signal-to-interference ratio (SIR)-based distributed scheduling. This new signaling mechanism, and the concomitant scheduling algorithm, enables efficient channel-aware spatial resource allocation, leading to significant gains over a CSMA/CA system using RTS/CTS. FlashLinQ is a complete system architecture including: 1) timing and frequency synchronization derived from cellular spectrum; 2) peer discovery; 3) link management; and 4) channel-aware distributed power, data rate, and link scheduling. FlashLinQ has been implemented for operation over licensed spectrum on a digital signal processor/field-programmable gate array (DSP/FPGA) platform. In this paper, we present FlashLinQ performance results derived from both measurements and simulations.

Pathwise optimality of the exponential scheduling rule for wireless channels

We consider the problem of scheduling the transmissions of multiple data users (flows) sharing the same wireless channel (server). The unique feature of this problem is the fact that the capacity (service rate) of the channel varies randomly with time and asynchronously for different users. We study a scheduling policy called the exponential scheduling rule, which was introduced in an earlier paper. Given a system with N users, and any set of positive numbers {a n }, n = 1, 2,…, N, we show that in a heavy-traffic limit, under a nonrestrictive ‘complete resource pooling’ condition, this algorithm has the property that, for each time t, it (asymptotically) minimizes max n a n q̃ n (t), where q̃ n (t) is the queue length of user n in the heavy-traffic regime.

On the design of device-to-device autonomous discovery

This paper proposes a synchronous device discovery solution for ad-hoc networks based on the observations that time synchronization, along with an FDM based channel resource allocation, can lead to gains in terms of energy consumption, discovery range, and the number of devices discovered. These attributes are important for the success of proximity-aware networking, where devices autonomously find peer-groups over human mobility scales. In this paper, we develop the PHY and MAC protocols to enable autonomous device discovery. Using both simulations and stochastic-geometry based analysis, we validate our design, and argue that there can be significant gains over a conventional Wi-Fi based solution.

Scheduling real-time traffic with deadlines over a wireless channel

Recently, there has been widespread interest in the extension of data networks to the wireless domain. However, scheduling results from the wireline domain do not carry over to wireless systems because wireless channels have unique characteristics not found in wireline channels, namely, limited bandwidth, bursty channel errors and location-dependent channel errors.

On Combining Shortest-Path and Back-Pressure Routing Over Multihop Wireless Networks

Back-pressure based algorithms based on the algorithm by Tassiulas and Ephremides have recently received much attention for jointly routing and scheduling over multi-hop wireless networks. However a significant weakness of this approach has been in routing, because the traditional back-pressure algorithm explores and exploits all feasible paths between each source and destination. While this extensive exploration is essential in order to maintain stability when the network is heavily loaded, under light or moderate loads, packets may be sent over unnecessarily long routes and the algorithm could be very inefficient in terms of end-to-end delay and routing convergence times. This paper proposes new routing/scheduling back-pressure algorithms that not only guarantees network stability (through-put optimality), but also adaptively selects a set of optimal routes based on shortest-path information in order to minimize average path-lengths between each source and destination pair. Our results indicate that under the traditional back-pressure algorithm, the end-to-end packet delay first decreases and then increases as a function of the network load (arrival rate). This surprising low-load behavior is explained due to the fact that the traditional back-pressure algorithm exploits all paths (including very long ones) even when the traffic load is light. On the otherhand, the proposed algorithm adaptively selects a set of routes according to the traffic load so that long paths are used only when necessary, thus resulting in much smaller end-to-end packet delays as compared to the traditional back-pressure algorithm.