Parimal Parag

Resource Allocation and Quality of Service Evaluation for Wireless Communication Systems Using Fluid Models

Wireless systems offer a unique mixture of connectivity, flexibility, and freedom. It is therefore not surprising that wireless technology is being embraced with increasing vigor. For real-time applications, user satisfaction is closely linked to quantities such as queue length, packet loss probability, and delay. System performance is therefore related to, not only Shannon capacity, but also quality of service (QoS) requirements. This work studies the problem of resource allocation in the context of stringent QoS constraints. The joint impact of spectral bandwidth, power, and code rate is considered. Analytical expressions for the probability of buffer overflow, its associated exponential decay rate, and the effective capacity are obtained. Fundamental performance limits for Markov wireless channel models are identified. It is found that, even with an unlimited power and spectral bandwidth budget, only a finite arrival rate can be supported for a QoS constraint defined in terms of exponential decay rate

A subcarrier allocation algorithm for OFDMA using buffer and channel state information

We propose a new subcarrier allocation algo- rithm for Orthogonal Frequency Division Multiple Access (OFDMA) that gives fair allocation of capacity to multiple users with different channel and traffic characteristics. This is achieved by utilizing buffer state information and mea- sured traffic statistics in addition to channel state feedback. Multiuser diversity gains are achieved in the proposed al- gorithm by scheduling across both time (buffering) and fre- quency (subcarriers). Simulation results are shown to illus- trate (a) improved capacity allocation and throughput, and (b) larger admissible traffic when compared to the existing algorithms.

Quality of Service Analysis for Wireless User-Cooperation Networks

A wireless communication system in which multiple users cooperate to transmit information to a common destination is considered. The traffic generated by the users is subject to a stringent quality of service requirement, which is defined in terms of the asymptotic decay-rate of buffer occupancy. The performance of this communication system is analyzed, and the corresponding achievable rate-region for the two-user scenario is identified. A simple user-cooperation scheme that improves performance is proposed. This cooperative scheme is shown to significantly enlarge the achievable rate-region of the service constrained communication system, provided that the quality of the wireless link between cooperating users is better than the individual connections from the users to the intended destination. Numerical results further indicate that the gains of cooperative strategies can be substantial. This suggests that cooperation allows for a fair distribution of the wireless resources among active users.

Content-aware caching and traffic management in content distribution networks

The rapid increase of content delivery over the Internet has led to the proliferation of content distribution networks (CDNs). Management of CDNs requires algorithms for request routing, content placement, and eviction in such a way that user delays are small. We abstract the system of frontend source nodes and backend caches of the CDN in the likeness of the input and output nodes of a switch. In this model, queues of requests for different pieces of content build up at the source nodes, which route these requests to a cache that contains the requested content. For each request that is routed to a cache, a corresponding data file is transmitted back to the requesting source across links of finite capacity. Caches are of finite size, and the content of the caches can be refreshed periodically. Our objective is to design policies for request routing, content placement and content eviction with the goal of small user delays. Stable policies ensure the finiteness of the request queues, while good polices also lead to short queue lengths. We first design a throughput-optimal algorithm that solves the routing-placement-eviction problem. The design yields insight into the impact of different cache refresh policies on queue length, and we construct throughput optimal algorithms that engender short queue lengths. We illustrate the potential of our approach through simulations on different CDN topologies.

Queueing Analysis of a Butterfly Network for Comparing Network Coding to Classical Routing

Network coding has gained significant attention in recent years as a means to improve throughput, especially in multicast scenarios. These capacity gains are achieved by combining packets algebraically at various points in the network, thereby alleviating local congestion at the nodes. The benefits of network coding are greatest when the network is heavily utilized or, equivalently, when the sources are saturated so that there is data to send at every scheduling opportunity. Yet, when a network supports delay-sensitive applications, traffic is often bursty and congestion becomes undesirable. The lighter loads typical of real-time traffic with variable sources tend to reduce the returns of network coding. This work seeks to identify the potential benefits of network coding in the context of delay-sensitive applications. As a secondary objective, this paper also studies the cost of establishing network coding in wireless environments. For a network topology to be suitable for coding, links need to possess a proper structure. The cost of establishing this structure may require excessive radio resources in terms of bandwidth and transmit power. Bursty traffic together with structural cost tend to decrease the potential benefits of network coding. This paper describes how, for real-time applications over wireless networks, there exist network topologies for which it may be best not to establish a network structure tailored to network coding.

Code-Rate Selection, Queueing Behavior, and the Correlated Erasure Channel

This paper considers the relationship between code-rate selection and queueing performance for communication systems subject to time-varying channel conditions. While error-correcting codes offer protection against channel uncertainties, there exists a natural tradeoff between the enhanced protection of low-rate codes and the rate penalty imposed by additional redundancy. In the limiting regime where codewords are asymptotically long, this tradeoff is well understood and characterized by the Shannon capacity. However, for delay-sensitive communication systems and finite block lengths, a complete characterization of this tradeoff is not fully developed. This paper offers a new perspective on the queueing performance of communication systems with finite block lengths operating over correlated erasure channels. A rigorous framework that links code rate to overall system performance for random codes is presented. Guidelines for code-rate selection in delay-sensitive systems are identified. These findings are supported by a numerical study.

On the queueing behavior of random codes over a gilbert-elliot erasure channel

This paper considers the queueing performance of a system that transmits coded data over a time-varying erasure channel. In our model, the queue length and channel state together form a Markov chain that depends on the system parameters. This gives a framework that allows a rigorous analysis of the queue as a function of the code rate. Most prior work in this area either ignores block-length (e.g., fluid models) or assumes error-free communication using finite codes. This work enables one to determine when such assumptions provide good, or bad, approximations of true behavior. Moreover, it offers a new approach to optimize parameters and evaluate performance. This can be valuable for delay-sensitive systems that employ short block lengths.

Queueing analysis of a butterfly network

Network coding has gained significant attention in recent years as a means to improve throughput, especially in multicast scenarios. These capacity gains are achieved by combining packets algebraically at various points in the network, thereby alleviating local congestion at the nodes. The benefits of network coding are greatest when the network is heavily utilized or, equivalently, when the sources have infinite backlogs. However, if a network supports delay-sensitive applications, traffic is often sparse and congestion becomes undesirable. The lighter loads typical of real-time traffic with variable sources tend to reduce the returns of network coding. This work seeks to identify the potential benefits of network coding in the context of delay-sensitive applications. As a secondary objective, this paper also studies the cost of establishing network coding in wireless environments. For a network topology to be suitable for coding, links need to possess a proper structure. The cost of establishing this structure may require excessive wireless resources in terms of bandwidth and transmit power. Together, these effects decrease the potential benefits of network coding. For real-time applications over wireless networks, it may be best not to combine information at the nodes.

Service Routing in Multi-ISP Peer-to-Peer Content Distribution: Local or Remote?

The popularity of Peer-to-Peer (P2P) file sharing has resulted in large flows between different ISPs, which imposes significant transit fees on the ISPs in whose domains the communicating peers are located. The fundamental tradeoff faced by a peer-swarm is between free, yet delayed content exchange between intra-domain peers, and inter-domain communication of content, which results in transit fees. This dilemma is complex, since peers who possess the content dynamically increase the content capacity of the ISP domain to which they belong.

Code rate, queueing behavior and the correlated erasure channel

This paper considers the relationship between code-rate selection and queueing performance for communication systems with time-varying parameters. While error-correcting codes offer protection against channel unreliability, there is a tradeoff between the enhanced protection of low-rate codes and the increased information transfer of high-rate codes. Hence, there exists a natural compromise between packet-level error protection and information rate. In the limiting regime where codewords are asymptotically long, this tradeoff is well-understood and characterized by the Shannon capacity. However, for delay-sensitive communication systems and finite code-lengths, a complete characterization of this tradeoff is still not fully developed. This paper offers a new perspective on the queueing performance of communication systems with finite block-lengths operating over correlated erasure channels. A rigorous framework that links code rate to overall system performance for random codes is presented. Guidelines for code rate selection in delay-sensitive systems are identified.