IEEE 802.11 system design

Many wireless LAN (WLAN) systems are densely used in conference rooms and cafes owing to the widespread use of mobile access points (APs) and smart phones with tethering functions. Under this situation, duplicate (the same) channels are often used in the 2.4-Ghz band owing to a limited bandwidth. In addition, adjacent channels interfering with other channels are also used. Then, if the WLAN uses a channel easily affected by adjacent channels, the throughput of the WLAN can decrease. As a result, it is possible that the fairness of the WLAN will decrease. Therefore, this study proposes a switching method for media access control (MAC) to improve the throughput fairness in an adjacent channel interference environment. The proposed method attempts to apply a modified MAC method to a WLAN under a dense environment in which the throughput is appropriately decreased. From the simulation results, it was found that the proposed method can improve the throughput fairness. Therefore, we confirmed the effectiveness of our method in an adjacent channel interference environment.

The effect of terminal movement on the performance of the IEEE 802.11 g wireless LAN (WLAN) system is evaluated using a measurement set-up including a radio channel simulator. The evaluation is based on laboratory measurements of WLAN PC cards in different simulated radio environments. In the measurements, two different radio channel models are used; the exponential channel model and the UMTS vehicular channel model. The measurement results indicate promising operation of IEEE 802.11 g WLAN systems as such. However, the use of different packet sizes has a significant effect on the system behaviour. With large packets the terminal is more likely to experience channel estimation errors than with small packets. This is due to the fact that the IEEE 802.11 g receiver estimates the channel only once per frame, and uses this estimation over the entire frame. Based on the measurement results we suggest a modification to the medium access control (MAC) layer operation that overcomes this problem: the use of optimized fragmentation.

Wireless local area network (WLAN) systems are typically designed to provide fixed or nomadic access. Using WLAN systems for military purposes raises the question of how WLAN terminals can be utilized in a fully mobile outdoor environment. In these environments, there typically exists multipath propagation with a large delay spread in the radio channel. This paper investigates the effect of terminal movement on the physical layer performance and capacity of IEEE 802.11b WLAN systems in a realistic outdoor mobile environment. Several wireless network interface cards are measured in laboratory conditions using a simulated radio channel. The measurement results indicate that WLAN systems can operate in mobile outdoor environment up to a certain mobile speed, and there are significant differences between equipment from product vendors

The study of the impact of a self-similar traffic, with respect to that of a Poisson classical one, on ad-hoc Wireless LAN (WLAN) systems is performed. The self-similar traffic generation is obtained by using a chaotic system, with structural parameters varied in order to have different self-similarity degrees and activity factors, and to permit a correct comparison with the Poisson traffic. The WLAN systems considered are based on polling MAC protocols and on different packet delivery strategies. In the performance analysis, obtained by means of simulations, some simple models for channel impact, retransmission strategy and terminal mobility are taken into account.

We propose a wireless ATM LAN system, which operates in the 5 GHz band at a channel rate of 25 Mb/s, to provide customer premises network (CPN) users with multimedia services. For the development of the wireless ATM LAN system, the system level requirements are described, and an architecture of the wireless ATM LAN system to support multimedia communications is proposed. In addition, we consider the technical issues to realize the wireless ATM LAN system.

We propose a new method for energy saving to transfer packet data in a wireless local area network (WLAN). The core of this method is based on load-sharing. For this reason, we propose a mathematical framework for minimizing power dissipation when transferring MAC (medium access control) protocol data unit (MPDU) using CSMA/CA. Further, we present a load-sharing algorithm enabling energy efficient transfer of MPDU. Finally, we implement the algorithm considering a typical scenario for a WLAN system using the 802.11 specification and direct sequence spread spectrum (DSSS) at the physical layer. The extensive simulation results show that power reduction of the excess power is 10-50% resulting in total power saving from 5-15%.

To achieve a long run-time for battery-operated portable electronic devices that incorporate wireless transceivers, efficient power management of the radio is a critical requirement. The power management function of IEEE 802.11 wireless local area networks (WLANs) allows stations (STAs) to operate in the doze mode so that their power consumption is significantly reduced. Hence, efficient algorithms to manage when and how often a STA enters and exits doze mode are crucial to battery-operated STAs. We address this problem by developing a novel model for stochastic analysis of timer-based power management in infrastructure IEEE 802.11 WLANs. Based on this model, the probabilities that a STA is active, idle, or dozing are derived, and the power consumption of the STA, number of frames buffered, and average delay per frame are obtained. These results enable an efficient power management algorithm that optimizes the idle timer and doze duration at the STA and the frame buffer at the access point. Moreover, similar statistics for the basic power management method in the IEEE 802.11 standard are derived as a special case of the proposed timer-based power management scheme. Numerical results are presented to demonstrate the effectiveness of the proposed algorithms.

This work studies RF (radio frequency) harvesting of electromagnetic energy from intentionally generated RF signals in the 2.4 GHz ISM band as power supply for a battery-less (back-)scattering transponder. In addition to such a transponder, our implementation uses two regular, unmodified mobile telephones, the first one for transmitting WLAN (Wireless Local Area Network) signals and a quasi-continuous Bluetooth (BLE) advertising signal, the second one for receiving the scattered signals from the transponder. The transponder accumulates energy from the incident WLAN and BLE signals for powering a micro-controller, which in turn controls the scattering and modulating of the incident BLE signal. The scattering process applies a 4 MHz frequency translation and frequency shift keying for information transfer such that the scattered signal is detectable by the BLE receiver of the second mobile telephone. The present study implements the WLAN signals as data stream and examines the coexistence of this data stream and the BLE advertising signals as well as the effects of alternative data transfer protocols and different WLAN transmission frequencies. Results show that the User Datagram Protocol (UDP) provides high duty cycle transmissions at an operational range of approx. 2 cm without interfering the scattering process. Thus, the UDP data stream enables an improved wireless power transfer from the transmitting telephone to the battery-less transponder, which enables a significant range extension and encourages further endeavors in this direction.

In this paper, we propose a robust symbol timing synchronization method for IEEE 802.11n wireless LAN (WLAN) systems. Symbol timing synchronization using a preamble is affected by the timing of the packet detection and automatic gain control (AGC) completion. However, conventional methods assumed that symbol timing synchronization starts at the beginning of the preamble, which is typically invalid in 802.11n systems. Also, the cyclic shift diversity (CSD) scheme is adopted for achieving spatial diversity and avoiding undesired beamforming in 802.11n standard. However, it induces a pseudo multipath problem which generates multiple-peaks of the cross-correlation. In order to solve these problems, a novel symbol timing synchronization method which is not affected by the pseudo multipath problem is devised. This method estimates a coarse time-offset of short training symbols (STSs) using the cross-correlation and detects a coarse boundary between the last STS and GI which is prior to long training symbols (LTSs) using maximum normalized correlation (MNC)-based algorithm and dividing the preamble into units of STS length. At last, to improve precision of boundary detection, the fine timing of GI start is refined by a verification stage using periodicity of STS. Simulation results show that the proposed method has the SNR gain of 2.4 dB and 3.2 dB compared to the conventional method at the rate of 1% synchronization failure probability in 2TX-2RX and 4TX-4RX WLAN systems, respectively.

In 1999, IEEE supplemented the 802.11 standard for wireless local area networks (WLANs) in order to provide for higher pay load data rates. This higher-speed physical layer (PHY) extension in the 2.4 GHz band, approved as document IEEE 802.11b, specifies two advanced modulation schemes (namely CCK and PBCC) for the direct sequence spread spectrum (DSSS) technology, thus enabling data rates of 5.5 and 11 Mbps in addition to 1 and 2 Mbps. Together with the optional Short Preamble mode (instead of the conventional Long Preamble), an 802.11b WLAN may transmit in seven different modes1. Based on the fundamental contention-based access mechanism — the distributed coordination function (DCF), which allows two different handshaking techniques —, we compare the performance of the resulting transmission modes. To this end, we adapt a previously published stochastic Petri net model and evaluate it by simulation.

Chapter 4 - Wireless Local Area Networks

The authors have analyzed and compared performances of PN (pseudo noise) codes to find optimal PN codes which can be used in a DS/SS (direct sequence/spread spectrum) wireless LAN (local area network) system over a multipath fading channel. A wireless LAN system is susceptible to multipath fading that gives rise to intersymbol interference. The authors have analyzed and compared performances of Barker sequences with those of Gold sequences. As results, Barker sequences are better than Gold sequences for DS/SS wireless LAN systems, from the SNR and BER points of view.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Recently, IEEE 802.11 wireless LAN (WLAN) has emerged as a widely accepted technology for broadband wireless networking. A WLAN handoff operation, which occurs when a wireless station (STA) changes its association from one access point (AP) to another, has not been well understood so far since the mobility has not been a major concern for typical WLAN environments. However, the mobility within WLAN is becoming more and more important as the portable WLAN devices, e.g., voice over WLAN (VoWLAN) devices, emerge today. In this paper, we introduce an empirical approach to evaluate the 802.11 handoff process in a public WLAN service network, i.e., KT (Korea Telecom) NESPOT. We analyze the handoff delay by characterizing individual delay components in terms of protocol-oriented delays and network configuration-dependent ones. From the analysis, we identify that the user authentication-related procedure, especially, with delay terms not rooted in the employed protocols, could become a major delay in a public service WLAN, and hence should be reduced in order to achieve a faster handoff

Traffic diversion and load balancing are critical issues in Wireless Local Area Network (WLAN), such as Wi-Fi. When multi hop ad-hoc nodes are introduced between two WLANs, the performance becomes more inevitable, which brings new challenging issues, such as how to reduce the packet dropping rate, end-to-end delay, how to control traffic in more dense WLAN, how to take more advantage of multi hop relay nodes for better resource management. In this paper we proposed a traffic diversion scheme for high dense Infrastructure based WLAN. This proposed scheme consists of two parts. In the first part, we have focused on Access Point (AP) selection policy, which utilizes multi-hop adhoc nodes to divert traffic of high dense AP to less dense AP with non-overlapping coverage. In the second part, we provided a framework for routing scheme in multi hop ad-hoc nodes for exchange of information. Ongoing ns2 simulations demonstrate that the scheme that we proposed is more effective than traditional one in terms of packet drop and end to-end delay.

Recent demands on affordable, portable wireless communication and computation devices have resulted in exponential growth of wireless networks ranging from Wireless Local Area Networks (WLAN) and Wireless Wide Area Networks (WWAN) to Ad-Hoc and Sensor networks. The major goal of wireless communication is to allow users to communicate together and to have access to global network anytime anywhere. This has led to wide acceptance of infrastructure based cellular networks (WWANs) where mobile stations communicate with a centralized controller, often referred as Access Point (AP) that is connected to the wired networks. On the other hand, WLANs have appeared as dominant popular technologies in many venues including a local area such as an academic campus or an airport terminal. These wireless networks mostly rely on IEEE 802.11 Wi-Fi (Wireless Fidelity) technology and its various derived versions (i.e. 802.11a,b,g). IEEE 802.11 standard supports two operational modes: The infrastructure-based Wireless Local Area Networks (WLANs) and an infrastructure-less Ad-Hoc Networks. A WLAN (Conti, 2003) typically imposes the existence of an AP and normally is connected to the wired networks to provide internet access for mobile devices. Obviously, only one hop link is needed to communicate between mobile devices and AP. In contrast, there is no AP or infrastructure in Ad-Hoc networks. Any two stations can communicate directly when they are in the range of reception of each other. To this end, the stations may use multi-hop routing to deliver their packets to destinations. The ad-hoc protocols (Conti, 2003; Mohapatra & Krishnamurthy, 2005) are self-configured for address and routing in the face of mobility and the network topology may change in each configuration. The multi-hop wireless ad-hoc networks, or multi-hop wireless networks enable wireless networking in the environments where the wired or cellular connections are impossible, inadequate, or cost effective (e.g. battle field, disaster recovery, etc.). The popularity of internet over the last decades has resulted in rapid advancement of demanding applications. The Transmission Control Protocol/Internet Protocol (TCP/IP) (Stevens, 1994) is a well-known de facto protocol in developing today’s internet. Basically, TCP provides a connection-oriented and reliable end-to-end data delivery between two hosts in traditional wired networks. Since TCP is well tuned and due to its wide acceptance in internet, it is desirable to extend and adopt its functionality to wireless networks. On the