Quality of service (QoS) provisioning is one of the most important criteria in newly emerging UWB WPANs, as they are expected to support a wide variety of applications from time-constrained, multimedia streaming to throughput-hungry, content transfer applications. Recently, the WiMedia Alliance has announced the PHY/MAC specification together with the common radio platform for UWB WPANs. The WiMedia MAC uses the contention-free reservation-based access, which supports isochronous traffic with distributed reservation protocol (DRP), and the contention-based access, which provides service differentiation for best effort and variable-rate real-time traffic with prioritized channel access (PCA), similar to the enhanced distributed coordinated access (EDCA) in IEEE 802.11e. In this paper, we conduct a comprehensive theoretical analysis and show that with the currently recommended parameter setting, EDCA cannot provide adequate QoS. In particular, without responding to the system dynamics (e.g., taking account of the number of active stations in each class), EDCA cannot allocate bandwidth in a deterministic proportional manner and the system bandwidth is underutilized. After identifying the deficiency of EDCA, we propose an enhanced QoS-provisioning framework for the PCA scheme that dynamically controls channel access to adjust allocated bandwidth depending on the change of application's bit rate, i.e., variable-rate real-time traffic. We show that in this framework, 1) variable-rate real-time traffic is adaptively guaranteed of deterministic bandwidth via a contention-based reservation access method; 2) best effort traffic is provided with deterministic proportional QoS; and 3) moreover, the bandwidth utilization in PCA is maximized. By deterministic proportional QoS, we mean that the proposed protocol will maintain the ratio, rj, of per-station throughput attained by a station of class j to that attained by a station of class 1 at the targeted value, and yet, the real amount of network bandwidth is changing according to the current available network bandwidth. We have also validated and evaluated the QoS provisioning capability and practicality of the proposed PCA framework both via simulation and an empirical study with the Multiband Atheros Driver for WiFi (MADWifi) Linux driver for Wireless LAN devices with the Atheros chipset.

Recently, there has been an increasing interest in developing 60 GHz wireless personal area networks (WPANs) for short-range high-speed wireless communications. Both industrial and standardization organizations such as IEEE 802.15.3c and IEEE 802.11ad are making considerable efforts to bring the very high data rate (up to several Gbps) WPANs into reality. In this work, we develop an NS-2 extension for simulating 60 GHz WPANs, based on which we conduct a comprehensive simulation study to investigate the impacts of antenna directivity and CSMA/CA operation modes on the throughput of 60 GHz WPANs. Observations from extensive simulation results provide valuable insights and guidelines in designing future MAC schemes for 60 GHz WPANs with directional antennas.

Cognitive radios provide means to improve the spectrum utilization by allowing opportunistic usage of allocated spectrum today if they are not utilized by the primaries. We first outline that there are two types of cognitive radios (S.N. Shankar et al., 2005), namely, static and dynamic cognitive radio. Then we explain the role of physical (PHY) and medium access control (MAC) layer of the cognitive radio and outlines the interaction to perform optimal operation. We model this total interaction by means of a queueing system and analyze the performance. It was found that dynamic cognitive nodes exhibit higher statistical multiplexing gain and lower delay compared to static cognitive radio network.

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.

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.

Static spectrum allocation prohibits radio devices from using spectral bands designated for others. As a result, some bands are under-utilized while other bands are over-populated with radio devices. To remedy this problem, the concept of spectrum agility has been considered so as to enable devices to opportunistically utilize others' spectral bands. In order to help realize this concept, we establish an analytical model to derive performance metrics, including spectrum utilization and spectrum-access blocking time in spectral-agile communication systems. We then propose three basic building blocks for spectral-agile systems, namely spectrum opportunity discovery, spectrum opportunity management, and spectrum usage coordination, and develop protocols for each blocks. These protocols are integrated with the IEEE 802.11 protocol, and simulated using ns-2 to evaluate the protocol overhead. The simulation results show that our proposed protocols can improve the throughput of an IEEE 802.11 wireless LAN by 90% for the simulated scenarios, and the improvements matched well our analytical model. These results demonstrate the great potential of using spectrum agility for improving spectral utilization in an efficient, distributed, and autonomous manner

The 11 papers in this special issue focus on adaptive, spectrum agile and cognitive wireless networks. Some of the topics covered include: fundamental performance aspects of spectrum sharing; spectrum sensing capabilities in cognitive radios; detection and estimation approaches to spectrum sharing; MAC design in opportunistic spectrum access; and competitive spectrum sharing and resource allocation using concepts from game theory and pricing.

Quality of service (QoS) provisioning is one of the most important criteria in newly emerging UWB-operated WPANs, as they are expected to support a wide variety of applica- tions from time-constrained, multimedia streaming to throughput- hungry, content transfer applications. As such, the Enhanced Distributed Coordinated Access (EDCA) mechanism has been adopted by MultiBand OFDM Alliance in its UWB MAC proposal. In this paper, we conduct a rigorous, comprehensive, theoretical analysis and show that with the currently recommended parame- ter setting, EDCA cannot provide adequate QoS. In particular, without responding to the system dynamics (e.g., taking into account of the number of active class-i stations), EDCA cannot allocate bandwidth in a deterministic proportional manner and the system bandwidth is under-utilized. After identifying the deficiency of EDCA, we propose, in compliance with the EDCA-incorporated UWB MAC protocol proposed in (15) (20), a framework, along with a set of theoretically grounded methods for controlling medium access with determin- istic QoS for UWB networks. We show that in this framework, 1) real-time traffic is guaranteed of deterministic bandwidth via a contention-based reservation access method; 2) best-effort traffic is provided with deterministic proportional QoS; and moreover, 3) the bandwidth utilization is maximized. We have also validated and evaluated the QoS provisioning capability and practicality of the proposed MAC framework both via simulation and empirically by leveraging the MADWifi (Multiband Atheros Driver for WiFi) Linux driver for Wireless LAN devices with the Atheros chipset.

One of the reasons for the limitation of bandwidth in current generation wireless networks is the spectrum policy of the Federal Communications Commission (FCC). But, with the spectrum policy reform, open spectrum wireless networks, and spectrum agile radios are set to drive next general wireless networks. In this paper, we investigate continuous-time Markov models for dynamic spectrum access in open spectrum wireless networks. Both queueing and no queueing cases are considered. Analytical results are derived based on the Markov models. A random access protocol is proposed that is shown to achieve airtime fairness. A distributed version of this protocol that uses only local information is also proposed based on homo egualis anthropological model. Inequality aversion by the radio systems to achieve fairness is captured by this model. These protocols are then extended to spectrum agile radios. Extensive simulation results are presented to compare the performances of fixed versus agile radios.

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.

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.

Wireless multimedia services currently suffer due to limited bandwidth. One of the reasons for this limitation was the Federal Communications Commission's (FCC) spectrum regulation policy. But, with the advantage of open spectrum wireless networking the bandwidth constraint is minimized. In this paper, a Markov model is presented and used to analyze the unlicensed band access. Then a random access scheme is proposed which provisions the existing etiquettes with more efficiency and fairness. Later a homo egualis society model based access scheme is built to implement the random access scheme in a distributed fashion. Simulations are performed to demonstrate the accuracy of this Markov model and to show the efficiency and fairness of the proposed schemes.

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.

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.

No abstract available.

IEEE 802.11e medium access control (MAC) is an emerging supplement to the IEEE 802.11 wireless local area network (WLAN) standard to support quality of service (QoS). The 802.11e MAC defines a new coordination function, namely hybrid coordination function (HCF), which is based on the centrally controlled and contention-based channel accesses. In this paper we evaluate, via simulation in OPNET, the throughput performance of the new 802.11e MAC HCF, and compare it with the 802.11 legacy MAC. We also study the scheduler implementation included by the IEEE 802.11e draft standard. Finally, we evaluate the effect of the "bad link" on the capacity of the network. Different stations in an IEEE 802.11 network can operate with different physical transmission rates based on their distance from the access point. A station transmitting at a lower physical transmission rate brings down the performance of the whole WLAN network. Through our simulation study, we conclude that the transmission opportunity (TXOP) of the IEEE 802.11e can enhance the system performance and provide fairness in time.

IEEE 802.11 e medium access control (MAC) is an emerging supplement to the IEEE 802.11 wireless local area network (WLAN) standard to support quality-of-service (QoS). The 802.11e MAC is based on both centrally-controlled and contention-based channel accesses. In this paper, we evaluate the contention-based channel access mechanism, called enhanced distributed coordination function (EDCF), in comparison with the 802.11 legacy MAC. The ECDF provides differentiated channel access to frames with different priorities. We also consider an optional feature of the EDCF, called contention-free burst (CFB), which allows multiple MAC frame transmissions during a single transmission opportunity (TXOP). Through our simulation study, we conclude that the EDCF can provide differentiated channel access for different traffic types. Furthermore, the CFB is found to enhance the EDCF performance by increasing the overall system throughput and achieving more acceptable streaming quality in terms of frame losses and delays.

We present a scheme for reliable multicast based on the combination of correction codes and iterative polling. The receivers are polled for the number of lost packets in a series of request with increasing reply probability. Receivers that have a lower loss number than the ones that replied in the previous poll are excluded from the pole. This exclusion prevents an implosion of responses to the sender, while the increase in reply probabilities eventually discloses the maximum number of packets lost by a receiver in the multicast group. This information is used to determine an adequate transmission of correction code, such that the majority of receivers can repair their - possibly different - losses. We analyze how the reply parameters can be chosen such that the amount of feedback satisfies given statistical bounds. The approach is based on estimating the number of receivers above a certain loss level, and does not make assumptions about the loss characteristics of the network.

The Medium Access Control (MAC) scheme proposed by DAVIC/DVB, IEEE 802.14 and DOCSIS for the upstream channel of Hybrid Fiber Coaxial (HFC) access networks is based on a mixable contention-based/contention-less time slot assignment. Contention-less slots are assigned by the head end to end stations according to a reservation scheme. Contention-based slots are randomly accessed by active terminals without any preliminary allocation, so that 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 novel contention resolution mechanism and compare its performance with that of existing procedures. The proposed procedure is termed as static arrival slot mechanism. In this mechanism, one slot in each frame is exclusively reserved for new arrivals that wish to access the channel using contention resolution, and at least one slot is reserved for resolving their contention if there was one in the arrival slot. The performance of the proposed mechanism is evaluated through analysis and simulation. The results show that the proposed mechanism outperforms existing contention resolution procedures under heavy traffic.

The emergence of wireless and personal communications has brought random access protocols for packet radio networks back to the research fore fronts. Most of the protocols are based on ALOHA protocol. Unfortunately this protocol is inherently unstable and requires sophisticated schemes to stabilize it. Another class of random access schemes, called limited sensing or stack algorithms, has been proposed that is stable and allows for the dynamic incorporation of new stations into the network. Indeed, after the second generation networks supporting voice service only, the third generation systems, UMTS (Universal Mobile Telecommunication System), should offer more services with quality. We analyze the possibility to apply the tree random access protocol for the W-CDMA (wideband-code division multiple access) part in the UTRA (UMTS Terrestrial Radio Access) radio interface proposition. The analytical model uses a modified version of the tree protocol which can be represented by a simple Geom/G/1 queue. We analyze this simple Geom/G/1 queue to predict the performance of the new random access protocol. It Is found that random access procedure is very stable and has higher channel capacity and lower access delay than the conventional slotted ALOHA access. It was also inferred that the protocol is insensitive to the way in which the tree runs, either breath first search or depth first search, thus giving the implementation flexibility. Also, simulation results are presented and show the predominance of this protocol compared with the conventional tree access mechanisms.