Shivendra S. Panwar

CoopMAC: A Cooperative MAC for Wireless LANs

Due to the broadcast nature of wireless signals, a wireless transmission intended for a particular destination station can be overheard by other neighboring stations. A focus of recent research activities in cooperative communications is to achieve spatial diversity gains by requiring these neighboring stations to retransmit the overheard information to the final destination. In this paper we demonstrate that such cooperation among stations in a wireless LAN (WLAN) can achieve both higher throughput and lower interference. We present the design for a medium access control protocol called CoopMAC, in which high data rate stations assist low data rate stations in their transmission by forwarding their traffic. In our proposed protocol, using the overheard transmissions, each low data rate node maintains a table, called a CoopTable, of potential helper nodes that can assist in its transmissions. During transmission, each low data rate node selects either direct transmission or transmission through a helper node in order to minimize the total transmission time. Using analysis, simulation and testbed experimentation, we quantify the increase in the total network throughput, and the reduction in delay, if such cooperative transmissions are utilized. The CoopMAC protocol is simple and backward compatible with the legacy 802.11 system. In this paper, we also demonstrate a reduction in the signal-to-interference ratio in a dense deployment of 802.11 access points, which in some cases is a more important consequence of cooperation

Video transport over ad hoc networks: multistream coding with multipath transport

Enabling video transport over ad hoc networks is more challenging than over other wireless networks. The wireless links in an ad hoc network are highly error prone and can go down frequently because of node mobility, interference, channel fading, and the lack of infrastructure. However, the mesh topology of ad hoc networks implies that it is possible to establish multiple paths between a source and a destination. Indeed, multipath transport provides an extra degree of freedom in designing error resilient video coding and transport schemes. In this paper, we propose to combine multistream coding with multipath transport, to show that, in addition to traditional error control techniques, path diversity provides an effective means to combat transmission error in ad hoc networks. The schemes that we have examined are: 1) feedback based reference picture selection; 2) layered coding with selective automatic repeat request; and 3) multiple description motion compensation coding. All these techniques are based on the motion compensated prediction technique found in modern video coding standards. We studied the performance of these three schemes via extensive simulations using both Markov channel models and OPNET Modeler. To further validate the viability and performance advantages of these schemes, we implemented an ad hoc multiple path video streaming testbed using notebook computers and IEEE 802.11b cards. The results show that great improvement in video quality can be achieved over the standard schemes with limited additional cost. Each of these three video coding/transport techniques is best suited for a particular environment, depending on the availability of a feedback channel, the end-to-end delay constraint, and the error characteristics of the paths.

Connectivity properties of a packet radio network model

A model of a packet radio network in which transmitters with range R are distributed according to a two-dimensional Poisson point process with density D is examined. To ensure network connectivity, it is shown that pi R/sup 2/D, the expected number of nearest neighbors of a transmitter, must grow logarithmically with the area of the network. For an infinite area there exists an infinite connected component with nonzero probability if pi R/sup 2/D>N/sub 0/, for some critical value N/sub 0/. It is shown that 2.195 >

Cooperative wireless communications: a cross-layer approach

This article outlines one way to address these problems by using the notion of cooperation between wireless nodes. In cooperative communications, multiple nodes in a wireless network work together to form a virtual antenna array. Using cooperation, it is possible to exploit the spatial diversity of the traditional MIMO techniques without each node necessarily having multiple antennas. Multihop networks use some form of cooperation by enabling intermediate nodes to forward the message from source to destination. However, cooperative communication techniques described in this article are fundamentally different in that the relaying nodes can forward the information fully or in part. Also the destination receives multiple versions of the message from the source, and one or more relays and combines these to obtain a more reliable estimate of the transmitted signal as well as higher data rates. The main advantages of cooperative communications are presented

A cooperative MAC protocol for wireless local area networks

In this paper, a novel idea of user cooperation in wireless networks has been exploited to improve the performance of the IEEE 802.11 medium access control (MAC) protocol. The new MAC protocol leverages the multi-rate capability of IEEE 802.11b and allows the mobile stations (STA) far away from the access point (AP) to transmit at a higher rate by using an intermediate station as a relay. Two specific variations of the new MAC protocol, namely CoopMAC I and CoopMAC II, are introduced in the paper. Both are able to increase the throughput of the whole network and reduce the average packet delay. Moreover, CoopMAC II also maintains backward compatibility with the legacy 802.11 protocol. The performance improvement is further evaluated by analysis and extensive simulations.

Optimal scheduling policies for a class of queues with customer deadlines to the beginning of service

Many problems can be modeled as single-server queues with impatient customers. An example is that of the transmission of voice packets over a packet-switched network. If the voice packets do not reach their destination within a certain time interval of their transmission, they are useless to the receiver and considered lost. It is therefore desirable to schedule the customers such that the fraction of customers served within their respective deadlines is maximized. For this measure of performance, it is shown that the shortest time to extinction (STE) policy is optimal for a class of continuous and discrete time nonpreemptive M/G/1 queues that do not allow unforced idle times. When unforced idle times are allowed, the best policies belong to the class of shortest time to extinction with inserted idle time (STEI) policies. An STEI policy requires that the customer closest to his or her deadline be scheduled whenever it schedules a customer. It also has the choice of inserting idle times while the queue is nonempty. It is also shown that the STE policy is optimal for the discrete time G/D/1 queue where all customers receive one unit of service. The paper concludes with a comparison of the expected customer loss using an STE policy with that of the first-come, first-served (FCFS) scheduling policy for one specific queue.

Supporting image and video applications in a multihop radio environment using path diversity and multiple description coding

This paper examines the effectiveness of combining multiple description coding (MDC) and multiple path transport (MPT) for video and image transmission in a multihop mobile radio network. The video and image information is encoded nonhierarchically into multiple descriptions with the following objectives. The received picture quality should be acceptable, even if only one description is received and every additional received description contributes to enhanced picture quality. Typical applications will need a higher bandwidth/higher reliability connection than that provided by a single link in current mobile networks. To support these applications, a mobile node may need to set up and use multiple paths to the desired destination, either simply because of the lack of raw bandwidth on a single channel or because of its poor error characteristics, which reduce its effective throughput. The principal reason for considering such an architecture is to provide high bandwidth and more robust end-to-end connections. We describe a protocol architecture that addresses this need and, with the help of simulations, we demonstrate the feasibility of this system and compare the performance of the MDC-MPT scheme to a system using layered coding and asymmetrical paths for the base and enhancement layers.

LayerP2P: Using Layered Video Chunks in P2P Live Streaming

Although there are several successful commercial deployments of live P2P streaming systems, the current designs; lack incentives for users to contribute bandwidth resources; lack adaptation to aggregate bandwidth availability; and exhibit poor video quality when bandwidth availability falls below bandwidth supply. In this paper, we propose, prototype, deploy, and validate LayerP2P, a P2P live streaming system that addresses all three of these problems. LayerP2P combines layered video, mesh P2P distribution, and a tit-for-tat-like algorithm, in a manner such that a peer contributing more upload bandwidth receives more layers and consequently better video quality. We implement LayerP2P (including seeds, clients, trackers, and layered codecs), deploy the prototype in PlanetLab, and perform extensive experiments. We also examine a wide range of scenarios using trace-driven simulations. The results show that LayerP2P has high efficiency, provides differentiated service, adapts to bandwidth deficient scenarios, and provides protection against free-riders.

Using layered video to provide incentives in P2P live streaming

In this paper, we design a distributed incentive mechanism for mesh-pull P2P live streaming networks. In our system, a video is encoded into layers with lower layers having more importance. The system is heterogeneous with peers having different uplink bandwidths. We design a distributed protocol in which a peer contributing more uplink bandwidth receives more layers and consequently better video quality. Previous approaches consider single-layer video, where each peer receives the same video quality no matter how much bandwidth it contributes to the system. The simulation results show that our approach can provide differentiated video quality commensurate with a peers contribution to other peers, and can also discourage free-riders. Furthermore, we also compare our layered approach with a multiple description coding (MDC) approach, and conclude that the layered approach is more promising, primarily due to its higher coding efficiency.

Full duplex cellular systems: will doubling interference prevent doubling capacity?

Recent advances in antenna and RF circuit design have greatly reduced the crosstalk between the transmitter and receiver circuits on a wireless device, which enable radios to transmit and receive on the same frequency at the same time. Such a full duplex radio has the potential to double the spectral efficiency of a point-to-point radio link. However, the application of such a radio in current cellular systems (3GPP LTE) has not been comprehensively analyzed. This article addresses the fundamental challenges in incorporating full duplex radios in a cellular network to unlock the full potential of full duplex communications. We observe that without carefully planning, full duplex transmission might cause much higher interference in both uplink and downlink, which greatly limits the potential gains. Another challenge is that standard scheduling methods which attempt to achieve the maximum capacity gain lead to a severe loss in energy efficiency. In this article, we identify new tradeoffs in designing full duplex enabled radio networks, and discuss favorable conditions to operate in full duplex mode. New scheduling algorithms and advanced interference cancellation techniques are discussed, which are essential to maximize the capacity gain and energy efficiency. Under this new design, most of the gain is achievable with full duplex enabled base stations, while user equipment still operates in half duplex mode.