EEE 802.11 Medium Access Control(MAC) is proposed to support asynchronous and time bounded delivery of radio data packets in infrastructure and ad hoc networks. The basis of the IEEE 802.11 WLAN MAC protocol is Distributed Coordination Function(DCF), which is a Carrier Sense Multiple Access with Collision Avoidance(CSMA/CA) with binary slotted exponential back-off scheme. Since IEEE 802.11 MAC has its own characteristics that are different from other wireless MAC protocols, the performance of reliable transport protocol over 802.11 needs further study.

In recent years, wireless Internet service providers (WISPs) have established Wi-Fi hotspots in increasing numbers at public venues, providing local coverage to traveling users and empowering them with the ability to access email, Web, and other Internet applications on the move. In this paper, we observe that while the mobile computing landscape has changed both in terms of number and type of hotspot venues, there are several technological and deployment challenges remaining before hotspots can become an ubiquitous infrastructure. These challenges include authentication, security, coverage, management, location services, billing, and interoperability. We discuss existing research, the work of standards bodies, and the experience of commercial hotspot providers in these areas, and then describe compelling open research questions that remain.

We have built a network, called the CHOICE network, which globally authenticates users and then securely connects them to the Internet via a high-speed local area wireless network. Our network provides easy-to-use, individualcentric, service-oriented wireless Internet access in places other than the traditional corporate offices and homes. Our architecture is hardware and protocol agnostic and is built on an easily deployable software module called the Protocol for Authorization and Negotiation of Services or PANS. PANS provides authorization, access, privacy, security, policy enforcement, quality of service (QoS) and accounting. In this paper, we describe PANS in detail. We discuss our system design and operation, implementation, and performance. We evaluate PANS and show that it is scalable and secure. Our network has been deployed and is operational at a local mall in Bellevue, Washington.

In this paper, we consider the problem of joint syuchronization and channel estimation for orthogonal frequency division multiplexing (OFDM) systems. A new algorithm is proposed that estimates the channel and symbol timing simultaneously by using a technique based on maximum-likelihood (ML) theory and the generalized Akaike information criterion (GAIC). Finally, we demonstrate the performance of our algorithm by simulation results.

The dawning of the 21st century has seen unprecedented growth in the number of wireless users, applications, and network access technologies. This trend is enabling the vision of pervasive, ubiquitous computing where users have network access anytime, anywhere, and applications are location-sensitive and contextaware. To realize this vision, we need to extend network connectivity beyond private networks, such as corporate and university networks, into public spaces like airports, malls, hotels, parks, arenas, etc. -- those places where individuals spend a considerable amount of their time outside of private networks.

Abstract—We present a narrowband interference (NBI) canceller that suppresses spectral leakage in an orthogonal frequency-division multiplexing (OFDM)-based system caused by a narrowband (NB) signal. In our scenario, we assume that the spectrum of the NB signal is within the spectrum of the OFDM signal. This can be the case, e.g., on digital subscriber lines (DSL) and in new unlicensed frequency bands for radio transmission. The canceller makes linear minimum mean-square error estimates of the spectral leakage by measuring the NBI on a few modulated or unmodulated OFDM subcarriers. It uses a model of the NB signal’s power spectral density as a priori information. Using frequency invariant design it is possible to cancel NBI from signals that are changing their frequency location with significantly reduced complexity overhead. The operational complexity of the canceller can be lowered by using the theory of optimal rank reduction and using the time-bandwidth product of the NB signal. Analytical performance evaluations, as well as Monte Carlo simulations, also show that without perfect a priori information this canceller can suppress the spectral leakage from a strong NB signal (e.g., with equal power as the OFDM signal) to well below the background noise floor for typical applications where it causes negligible signal to noise ratio and symbol error rate degradation. Index Terms—Digital subscriber lines (DSL), discrete multitone (DMT), industrial–scientific–medical (ISM) band, orthogonal frequency-division multiplexing (OFDM), radio frequency interference (RFI), wireless local area network (WLAN). I.

Abstract: Mobile devices in the IEEE 802.11 wireless local area network (WLAN) have the ability to transmit data frames at one of four transmission rates 1Mb/s, 2Mb/s, 5.5Mb/s and 11Mb/s. This is because the commercial WLAN transceivers are equipped with several modulation schemes. According to the characteristics of the modulation scheme, a higher transmission rate will result in a smaller transmission range and longer time consumption on data frame transmission. If the channel environment is relatively clear and the transmission distance is short, one should choose a higher transmission rate for data transmission to maximise channel utilisation. On the contrary, a lower transmission rate should be selected to minimise the frame loss and frame error probabilities if the bit error rate is high. Therefore, the problem of choosing a proper transmission rate to accommodate a varying environment is a new and valuable problem in the wireless LANs. To our knowledge, it is very difficult and impractical to formalise an indoor environment since the channel status is quite unstable and unpredictable. Instead, we propose an adaptive rate

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A low complexity antenna diversity architecture with a new combining technique suitable for Orthogonal Frequency Division (OFDM) based systems is introduced in this paper. For a two branch antenna diversity, this structure requires only one FFT at the receiver and allows optimum maximum ratio combining. For systems with a higher number of antennas, the number of required FFTs when using such technique is reduced by half as compared to the conventional case where a di#erent FFT is used for each receive antenna. As a result a considerable reduction in processing complexity is obtained especially for systems with high number of subcarriers. This method can also be seen as a way of increasing the number of antennas in OFDM transceivers without any major complexity increase. Thus, a considerable increase in diversity gain can be achieved. I. Introduction Orthogonal Frequency Division Multiplexing (OFDM) transmission techniques have been introduced and proposed for high data rate communica...

In this paper, deliberate level clipping and Turbo-coding are combined to achieve an Orthogonal Frequency Division Multiplexing (OFDM) transmission system with a low Peak-to-Average power Ratio (PAR) and a good perfor-mance. Using the linear approximation technique, we first modify the metric computation for the Turbo-decoding in order to consider the distortion effects of the nonlinearity, caused by the Cartesian clipping. The linear approximation of the nonlinear device is based on the Minimum Mean Square Error (MMSE) criterion. By exploiting the linear model, the receiver calculates a modified metric considering the effects of the nonlinearity. Also, this paper introduces a modified Turbo-decoder which simultaneously performs the data estimation and signal reconstruction. In other words, the Turbo-decoder iteratively recovers the clipped signal by using the estimated data, and then improves the data estimation by using the newly recovered signal. Numerical results are presented showing an improvement in the performance of the OFDM transmission system over the nonlinear channel, an increase in the efficiency of the High Power Amplifier (HPA), and/or an expansion of the transmitter coverage area. 1