N. Sai Shankar

Enhancements to Cognitive Radio Based IEEE 802.22 Air-Interface

The IEEE 802.22 standard for wireless regional area network is the first standard for cognitive radio that tries to harness the idle or under-utilized spectrum allocated for TV bands. Two major challenges that are faced by IEEE 802.22 are (i) the issue of self co-existence and (ii) the hidden incumbent problem. In this paper, we discuss these two challenges and provide enhancements to the existing IEEE 802.22 air-interface. We use a graph theoretic technique and propose utility graph coloring for allocating spectrum to different IEEE 802.22 base stations so that they can co-exist. The allocation is done such that objectives such as throughput maximization, proportional fairness, and complete fairness are met. We also propose the use of dynamic multiple broadcast messages that resolves the contention among various consumer premise equipments and alleviates the hidden incumbent problem. Through simulation results, we show that the proposed techniques increase the system spectrum utilization and reduce connection set-up delay.

Spectrum agile radio: capacity and QoS implications of dynamic spectrum assignment

Radio spectrum is very scarce today because a considerable amount of the spectrum is set aside for licensed wireless applications. With the rapid growth of wireless technologies, spectrum scarcity has become a serious problem as more and more wireless applications compete for very little spectrum. On the other hand the licensed spectrum allocated to applications like television, cellular telephony and public safety show very little usage over time at different geographical locations. The evolution of newer technologies has been seriously impaired because of current regulatory constraints on the operation of these networks in licensed spectrum, such as TV bands, and is being addressed by FCC through its recent rule making. With the goal of ubiquitous communication in mind, we look into spectrum agile radio, a new technology enabled by such FCC rule making, and study its advantages over conventional radios. In this paper, we first show the utilization achievable by agile radios through simple analysis. Then we will outline two types of agile radios and derive their maximum capacities. Then we will go ahead and derive the rules that increase the spectrum utilization using agile radios. We then highlight how spectrum agile radio impacts quality of service as defined in conventional sense

Mobility support enhancements for the WiMedia UWB MAC protocol

The WiMedia Alliance is currently specifying a distributed medium access control (MAC) protocol for ultra wide band (UWB) communications in the wireless-personal-area-network (WPAN) environment. Even though this MAC protocol includes basic mechanisms to provide mobility support, such support can be improved via coordination among devices. In this paper, we propose two enhancements for mobility support in the WiMedia MAC protocol. Our enhancements are based on the beacon period (BP)-merging algorithm of the WiMedia MAC protocol with a new distributed coordination. Two cases of mobile WPANs are studied to demonstrate the effectiveness of the proposed enhancements

Self-coexistence among interference-aware IEEE 802.22 networks with enhanced air-interface

Abstract IEEE 802.22 is a cognitive radio-based Wireless Regional Area Network (WRAN) standard that allows opportunistic access to idle or underutilized sub-900 MHz TV bands by unlicensed (secondary) networks. Though most of the standard has been laid out, there is still no consensus on the channel access policies for the uncoordinated secondary networks. Hence, the possibility of interference always exists. Moreover, in the absence of any control channel, the problem of establishing a connection becomes even more challenging, more so in the presence of hidden incumbents. In this paper, we address the above-mentioned self-coexistence problem among the IEEE 802.22 networks and provide novel solutions to improve the IEEE 802.22 air-interface. We use an interference-aware graph-theoretic technique and propose Utility Graph Coloring (UGC) for allocating spectrum to different IEEE 802.22 base stations such that they can coexist with the least interference, thereby maximizing the system spectrum utilization. We also consider allocation fairness among the networks in terms of minimal fairness, proportional fairness, and complete fairness. With the spectrum allocated to the IEEE 802.22 networks, we propose enhancements to the IEEE 802.22 MAC layer to maximize spectrum usage efficiency. In particular, we make use of aggregation and fragmentation of channel carriers, dynamic multiple broadcast messages, and aggressive contention resolution. Through simulation experiments, we show how the proposed techniques can increase the spectral efficiency and spectrum utilization, and still maintain fairness. We show that the spectral efficiency obtained with UGC is more than three times compared to the existing standard. The average number of collisions among the IEEE 802.22 enabled devices is significantly reduced, resulting in low connection setup delay, enhanced system performance, and higher spectrum allocation for data transmissions.

Optimal packing of VoIP calls in an IEEE 802.11 a/e WLAN in the presence of QoS constraints and channel errors

In this paper, we study the performance of VoIP over an IEEE 802.11e network. The contribution of this paper is to understand the advantages and limitations of IEEE 802.11e enhanced distributed coordination channel access (EDCA) as well as HCF coordinated channel access (HCCA) for carrying VoIP traffic. We also study the influence of the backoff window and TXOP size on the capacity of VoIP calls that can be admitted. We provide a closed form solution for the maximum number of VoIP calls that can be admitted subject to the QoS constraints of packet loss and delay. We analyze the HCCA with a simple scheduler and decide the optimal TXOP allocation for VoIP calls. The scenario consisted of all calls originating from a basic services set (BSS) and destined for outside the BSS. For this scenario, we evaluate via simple analysis and simulation, the capability of the IEEE 802.11e MAC to support VoIP in the presence of errors.

QoS Signaling for Parameterized Traffic in IEEE 802.11e Wireless LANs

IEEE 802.11e Medium Access Control (MAC) is an emerging extension of the IEEE 802.11 Wireless Local Area Network (WLAN) standard to support Quality of Service (QoS). The IEEE 802.11e uses both centrally-controlled as well as contention-based channel access mechanisms to transfer data across the wireless medium. It also provides the mechanism to specify and negotiate the resource based on the users QoS requirement. This paper presents a MAC-level QoS signaling for IEEE 802.11e WLAN and addresses its interaction with higher layer signaling protocols including Resource ReSerVation Protocol (RSVP) and Subnet Bandwidth Manager (SBM). We also explain a novel way of setting up sidestream connections for direct station-to-station streaming within an 802.11e WLAN.

Integrated application MAC modeling for cross-layer optimized wireless video

Cross-layer optimization has been recently proposed for improving the performance of real-time video transmission over 802.11 WLANs. To perform cross-layer optimization realistic analytical models are necessary that arc able to accurately predict the interactions between layers, such that the parameters of the physical, MAC, transport and application layers are correctly identified. In this paper, we propose to modify existing IEEE 802.11 WLAN MAC models to explicitly consider how they impact and are impacted by the application layer in real-time video transmission systems. Specifically, we adapt existing MAC distributed coordination function (DCF) models to consider the actual arrival rates of video flow packets, as well as provide adaptive retransmission limit for video packets with different priorities. Conversely, the video application can adjust the maximum retransmission limit per packet and the video rate based on realistic 802.11 MAC models rather than on simplistic two-state Markov models for packet-losses. Consequently, these integrated MAC-application layer models can be used to assist the cross-layer optimization for real-time wireless transmission by realistically predicting the interactions between layers.

New fairness paradigms for wireless multimedia communication

Wireless devices currently operate in a non-collaborative manner that limits their performance and the overall communication system performance, as the competing stations do not always effectively exploit the available resources. We propose to fundamentally change the non-collaborative way in which wireless stations currently interact, by allowing them to exchange information and resources to improve the performance of wireless multimedia applications. In this paper, we introduce several new fairness concepts for wireless multimedia systems that allow information and resource exchanges among stations, and show their advantages as compared to existing fairness mechanisms used in wireline communications.

Interference Handling in UWB Versus 802.11n Networks

UWB and 802.11n have both been touted as technologies for next generation wireless networks. UWB exploiting a large bandwidth, and 802.11n utilizing MIMO, demonstrate high data rates in single links. However, in real world applications these two technologies will not operate as isolated single links, but in a network environment characterized by interference. Through the application of a formal optimization theoretic framework, we demonstrate that their relative network performance is very different from a physical layer comparison. The interference handling capabilities of UWB and 802.11n cause this significant network performance result. We explore practical effects that alter the perceived interference and so, relative network performance. Further, recommendations for the use of multiple antennas in 802.11n radios are presented.

A New Contention Resolution Procedure for HFC Access Networks and its Performance Evaluation

Bidirectional Cable TV networks using hybrid fiber coaxial (HFC) systems are good examples of broadcast environments where a contention resolution algorithm is needed in order to allocate the multiaccess medium among various customers. The medium access control (MAC) scheme, proposed by DAVIC/DVB, IEEE 802.14 and DOCSIS for the upstream channel of HFC access networks is based on a mixable contention-based/contentionless time slot assignment. Contention-less (CL) slots are assigned by the head-end (HE) to end stations according to a reservation scheme. Contention-based slots (CB) are randomly accessed by active terminals without any prelimanry allocation and so collisions may occur. To resolve contention the contention tree algorithm has been widely accepted by the DVB/DAVIC, IEEE 802.14 and DOCSIS standards for MAC because of higher throughput and lower access delay. In this paper we propose a simple modification to the existing protocol and analyze its performance. We propose to have one slot in the frame exclusively reserved for the new arrivals that wish to access the channel capacity using contention resolution and atleast one more slot reserved for resolving their contention if there was a contention in the arrival slot. This assumption simplifies the protocol to a queuing mechanism and we use the results of the queue to analyze the protocol. The queuing analysis method is used to determine the throughput of the channel and waiting times of the arbitrary customers. Furthermore, we present numerical results and compare that with simulations.