Current Wireless Local Area Network Research Articles

Performance Analyses of Uplink MU-OFDMA Hybrid Access MAC in IEEE 802.11ax WLANs

Future wireless local area networks (WLANs) are expected to have numerous stations (STAs) with diverse Quality of Service requirements. IEEE 802.11ax is the current WLAN standard proposed to meet these requirements. This article analyzes the uplink-multiuser-orthogonal frequency division multiple access-hybrid access (HA) mechanism of the 802.11ax system under error-prone channel conditions. We propose an enhanced Markov chain model for analyzing such a WLAN system. By solving the system model, we derive vital systems performance metrics, such as aggregate and per-STA throughput, random access (RA) capability packet loss probability, mean delay, jitter, fairness, and worst-case delay bounds. Our analysis also considers different physical (PHY) and medium access control (MAC) layer aspects, such as channel bandwidth, the number of resource units (RUs), packet aggregation feature, fading, path-loss, modulation and coding schemes (MCS), etc. Furthermore, we have studied the MCS link adaptation-based dynamic HA process where the RUs are dynamically adapted between scheduled access and RA mechanisms. Additionally, to capture the role of PHY and MAC layer parameters on multi-Basic service set scenarios, we have integrated the Markov chain-based modeling with the stochastic geometry-based model. Our analytical model has been validated against extensive simulations, and the tradeoff between the metrics is studied.

Servicing delay sensitive pervasive communication through adaptable width channelization for supporting mobile edge computing

Over the last fifteen years, wireless local area networks (WLANs) have been populated on large variety of pervasive devices hosting heterogeneous applications. Pervasive edge computing has accelerated more distributed network applications for these devices, eliminating the round-trip to help in achieving zero latency dream. However, These applications require significantly variable data rates for effective functioning, especially in pervasive computing. The static bandwidth of frequency channelization in current WLANs strictly restricts the maximum achievable data rate by a network station. This static behavior spawns two major drawbacks: under-utilization of scarce spectrum resources and less support to delay sensitive applications such as voice and video.To this point, if the computing is moved to the edge of WLANs to reduce the frequency of communications, the pervasive devices can be provided with better services during the communication and networking. Thus, we aim to distribute spectrum resources among pervasive devices based upon delay sensitivity of applications while simultaneously maintain the fair channel access semantics of medium access control (MAC) layer of WLANs. Henceforth, ultra-low latency, efficiency and reliability of spectrum resources can be assured.In this paper, two novel algorithms have been proposed for adaptive channelization to offer rational distribution of spectrum resources among pervasive edge nodes based on their bandwidth requirement and assorted ambient conditions. The proposed algorithms have been implemented on a real test bed of commercially available universal software radio peripheral (USRP) devices. Thorough investigations have been carried out to enumerate the effect of dynamic bandwidth channelization on parameters such as medium utilization, achievable throughput, service delay, channel access fairness and bit error rate. The achieved empirical results demonstrate that we can optimally enhance the network wide throughput by almost 30% by using channels of adaptable bandwidths.

A Self-Scrutinized Backoff Mechanism for IEEE 802.11ax in 5G Unlicensed Networks

The IEEE 802.11ax high-efficiency wireless local area network (HEW) is promising as a foundation for evolving the fifth-generation (5G) radio access network on unlicensed bands (5G-U). 5G-U is a continued effort toward rich ubiquitous communication infrastructures, promising faster and reliable services for the end user. HEW is likely to provide four times higher network efficiency even in highly dense network deployments. However, the current wireless local area network (WLAN) itself faces huge challenge of efficient radio access due to its contention-based nature. WLAN uses a carrier sense multiple access with collision avoidance (CSMA/CA) procedure in medium access control (MAC) protocols, which is based on a binary exponential backoff (BEB) mechanism. Blind increase and decrease of the contention window in BEB limits the performance of WLAN to a limited number of contenders, thus affecting end-user quality of experience. In this paper, we identify future use cases of HEW proposed for 5G-U networks. We use a self-scrutinized channel observation-based scaled backoff (COSB) mechanism to handle the high-density contention challenges. Furthermore, a recursive discrete-time Markov chain model (R-DTMC) is formulated to analyze the performance efficiency of the proposed solution. The analytical and simulation results show that the proposed mechanism can improve user experience in 5G-U networks.

Delay-Aware LTE WLAN Aggregation in Heterogeneous Wireless Network

The scarcity of spectrum turns the next generation wireless network increasingly heterogeneous. Recently, much attention has been focused on exploiting the unlicensed spectrum to leverage traffic burden from the cellular network. Specified in 3GPP Release 13, long term evolution (LTE) wireless local area networks (WLAN) aggregation (LWA) promises an effective approach for the LTE WLAN interworking and unlicensed spectrum utilization. The LWA could improve the data rate by the licensed and unlicensed carriers aggregation. In contrast to the previous studies, where the LWA is always chosen to boost the peak data rate without considering the user payment, we jointly consider the LWA with Wi-Fi offloading in this paper, aiming to strike the balance between user payment and quality of service (QoS) requirement. We formulate the multi-slot modes selection as a finite-horizon Markov decision problem and propose a Delay-Aware LTE WLAN Aggregation (DLWA) algorithm to obtain the optimal transmission modes strategy. Due to the computational complexity of sequential decision problem, we exhibit the threshold structure of LTE usage and develop a low complexity algorithm based on this structure. Moreover, imperfection in LWA backhaul is considered and analyzed in this paper. Simulation results show the DLWA algorithm and threshold-based DLWA algorithm could guarantee the QoS requirement with lower user payment compared with current LTE WLAN interworking schemes.

Low latency IR-HARQ for multiband WLAN system

This paper proposes a low latency incremental redundancy hybrid ARQ (IR-HARQ) scheme for a multiband wireless local area network (WLAN) system. The proposed scheme can reduce a retransmission delay and ensure backward compatibility with WLAN devices by using a clear to send (CTS) frame of a current WLAN system and rate compatible low-density parity-check (RC-LDPC) codes. Additionally, it can minimize the retransmission delay by selecting an optimal retransmission data length on the basis of the accumulated mutual information (ACMI) for available frequency bands. The results of computer simulations show that the multiband WLAN system using the proposed scheme has shorter retransmission delay and higher system throughput than the current WLAN system.

Research on the Virtual Access Points in the Wireless Local Area Network

The management of the access points in the establishment of the current wireless local area network is inde- pendent and scattered. The builder must log into the access points with the artificial method to set the identification, the operating channel and the authentication code. The client stations have to scan each channel for finding out the pre-set identification and then cooperating the process of the authentication for the access network. These processes may take several seconds and greatly degrade the Internet experience, especially for the users who frequently move among different coverage of access points. Additionally, if users desire high throughput and ubiquitous network accessed by distributing high-density access points, the performance of whole network does not grow as expected due to the ill management of in- dependent access points. In this paper, we extend the software-defined network (SDN) into the access network so that ac- cess points are managed by a central controller to deal with the issues, such as load balance, interference of channel allo- cation and handover. After the users can register an account and select the configuration for the server in advance, the configuration of the group can be directly adopted to enter the network. The server can create its special virtual access points in the neighboring access points, and then directly provide the service of the client side. When the client side moves, the virtual access points will move into other tangible access points with the moving of the client sides. As to the client sides, it has the same access points connection without observing whether the procedure of changing the hand exists.

Design of Interference-Resilient Medium Access for High Throughput WLANs

The proliferation of wireless communications systems poses new challenges in terms of coexistence between heterogeneous devices operating within the same frequency bands. In fact, in case of high-density concentration of wireless devices, like indoor environments, the network performance is typically limited by the mutual interference among the devices themselves, such as for wireless local area networks (WLANs). In this paper, we analyze a protocol strategy for managing multiple access in wireless networks. A network of sensors colocated with the WLAN terminals forms a control layer for managing the medium access and scheduling resources in order to limit collisions and optimize the WLAN data traffic; this control layer is based on a low-power wideband technology characterized by interference robustness, like CDMA (code division multiple access) or UWB (ultra-wideband) for sensors. In this work, we perform an analytical and simulative performance study of the saturated throughput, showing numerical results for the UWB-IR (Impulse Radio) sensors case and highlighting the advantage that can be provided particularly in very high capacity systems, which constitute the necessary evolution of current WLAN versions.

Dynamic Multiuser Sub-Channels Allocation and Real-Time Aggregation Model for IEEE 802.11 WLANs

Wireless local area networks (WLANs) are becoming increasingly popular due to the recent availability of affordable devices providing multiple and high rate capabilities. New PHY and MAC layer enhancements have been introduced in different IEEE 802.11 WLAN standards. These improvements have given birth to a wide range of bandwidth channels (up to 160 MHz) and very high transmission data rates (> 1 Gbps) to keep up with current and up-coming multimedia applications. The MAC layer in current WLANs randomly allocates the entire channel to only one user as a single resource, which, unfortunately, reduces the efficiency of WLANs. One way to significantly improve WLAN performance is to reduce the channel width and create many sub-channels. Based on the user's channel conditions and QoS requirements, the PHY layer resources can be dynamically allocated to several users at the same time. In this paper, we shall present a novel sub-channels access approach, titled MU-Access, with new features suitable for the forth-coming high-speed MIMO-based WLAN products. We will evaluate its performance compared to state-of-the-art systems, using NS-3 under a variety of network conditions. Our experiments will then demonstrate that our proposed scheme enhances the performance of IEEE 802.11 devices with a much higher throughput.

WLAN Security Issues and Solutions

Wireless network is widely used in many sectors due to ease of installation, flexibility, mobility, reduced cost-of-ownership, and scalability. Wi- Fi network can be accessed with laptops, mobile phones, cameras, game consoles and many other increasing numbers of consumer electronic gadgets. Wireless Local Area Network (WLAN) technology has changed the way people communicate and share information by eliminating the boundaries of distance and location. However, WLAN have some security threats- Denial of Service, Spoofing, Eavesdropping, Man-In-The-Middle etc. In this report, we will discuss about the various WLAN (IEEE 802.11) security standards - WEP, WPA, WPA2 (IEEE 802.11i - 802.1x, PPP and EAP). The solutions /products to prevent wireless networks from the described threats have also been brought to limelight for current and future WLAN security.

The Study of WLAN Cell Radius for Outdoor Coverage in Wireless Digital City Deployment

Firstly, the technical features and frequency characteristics of IEEE802.11x protocol in wireless local area network (WLAN) are introduced. The functional relationship of path loss and coverage distance in dense urban and suburb is shown in figures. The uplink and downlink radio power budget of current WLAN outdoor coverage is calculated. Secondly, calculation and comparison of WLAN pass loss, link budget and cell coverage radius is done for dense urban and suburb. Thirdly, hot spot area WLAN outdoor coverage signal strength is simulated in dense urban for several situations by using simulation software. The results of simulation validate the accuracy of the calculation for uplink/downlink power budget, path loss in WLAN 2.4GHz band and WLAN cell coverage radius. The cell coverage radius for dense urban and suburb is 300∼500m and 1000∼1200m respectively. Finally, a valuable hot spot area WLAN outdoor coverage deployment suggestion is given: high power WLAN BTS/AP and high gain antenna with 4 sectors deployment will improve the uplink/downlink power budget; enlarge radio coverage in hot spot area. The analysis in this paper is significant for the deployment of wireless digital city.

Optimal Configuration of 802.11e EDCA for Real-Time and Data Traffic

The enhanced distributed channel access (EDCA) mechanism of the IEEE 802.11e standard provides quality-of-service (QoS) support through service differentiation by using different medium-access-control (MAC) parameters for different stations. The configuration of these parameters, however, is still an open research challenge, as the standard provides only a set of fixed recommended values that do not take into account the current wireless local area network (WLAN) conditions and, therefore, lead to suboptimal performance. In this paper, we propose a novel algorithm for EDCA that, given the throughput and delay requirements of the stations that are present in the WLAN, computes the optimal configuration of the EDCA parameters. We first present a throughput and delay analysis that provides the mathematical foundation upon which our algorithm is based. This analysis is validated through simulations of different traffic sources (both data and real time) and EDCA configurations. We then propose a mechanism to derive the optimal configuration of the EDCA parameters, given a set of performance criteria for throughput and delay. We assess the effectiveness of the configuration provided by our algorithm by comparing it against 1) the recommended values by the standard, 2) the results from an exhaustive search over the parameter space, and 3) previous configuration proposals, which are both standard and nonstandard compliant. Results show that our configuration outperforms all other approaches.

Preamble-Assisted Estimation for Frequency-Dependent I/Q Mismatch in Direct-Conversion OFDM Receivers

In direct-conversion orthogonal frequency division multiplexing (OFDM) receivers, the impact of frequency-dependent I/Q mismatch (IQ-M) with carrier frequency offset (CFO) must be considered. A preamble-assisted estimation is developed to circumvent the frequency-dependent IQ-M with CFO. The results of a simulation and an experiment show that the proposed method could provide good estimation efficiency and enhance the system performance. Moreover, the proposed scheme is compatible with current wireless local area network standards.

Green WLANs: On-Demand WLAN Infrastructures

Enterprise wireless local area networks (WLANs) that consist of a high-density of hundreds to thousands of access points (APs) are being deployed rapidly in corporate offices and university campuses. The primary purpose of these deployments is to satisfy user demands for high bandwidth, mobility, and reliability. However, our recent study of two such WLANs showed that these networks are rarely used at their peak capacity, and the majority of their resources are frequently idle. In this paper, we bring to attention that a large fraction of idle WLAN resources results in significant energy losses. Thousands of WLANs world-wide collectively compound this problem, while raising serious concerns about the energy losses that will occur in the future. In response to this compelling problem, we propose the adoption of resource on-demand (RoD) strategies for WLANs. RoD strategies power on or off WLAN APs dynamically, based on the volume and location of user demand. As a specific solution, we propose SEAR, a practical and elegant RoD strategy for high-density WLANs. We implement SEAR on two wireless networks to show that SEAR is easy to integrate in current WLANs, while it ensures no adverse impact on end-user connectivity and performance. In our experiments, SEAR reduces power consumption to 46%. Using our results we discuss several interesting problems that open future directions of research in RoD WLANs.

Energy efficiency modelling for IEEE 802.11a distribution coordination function system without finite retry limits

Use of IEEE 802.11 wireless local area networks (WLANs) as an extension to existing wired networks, offering both mobility and portability in a residential or office environment, is growing at an unprecedented rate. One of the critical limitations of current WLANs is the limited energy storage of mobile devices, and the design of energy-efficient protocols for WLANs has therefore become an area of intensive research. An analytical framework to study the energy consumption and energy efficiency of IEEE 802.11a WLANs is proposed. The energy consumption by considering the interactions between IEEE 802.11a PHY and MAC layers is modelled. Simulation results demonstrate that the theoretical model is accurate in predicting the energy efficiency over a wide range of scenarios. In addition, the effects of different PHY and MAC layer parameters on energy efficiency of IEEE 802.11a WLANs are investigated, as are the effects of different parameters on energy efficiency.

WLAN Technologies for Audio Delivery

Audio delivery and reproduction for home or professional applications may greatly benefit from the adoption of digital wireless local area network (WLAN) technologies. The most challenging aspect of such integration relates the synchronized and robust real-time streaming of multiple audio channels to multipoint receivers, for example, wireless active speakers. Here, it is shown that current WLAN solutions are susceptible to transmission errors. A detailed study of the IEEE802.11e protocol (currently under ratification) is also presented and all relevant distortions are assessed via an analytical and experimental methodology. A novel synchronization scheme is also introduced, allowing optimized playback for multiple receivers. The perceptual audio performance is assessed for both stereo and 5-channel applications based on either PCM or compressed audio signals.

Bidirectional multimode-fiber communication links using dual-purpose vertical-cavity devices

This paper describes the development of novel multimode-fiber (MMF) communication links that achieve half-duplex bidirectional transmission over a single fiber through the use of 850-nm dual-purpose GaAs vertical-cavity optoelectronic devices. The dual-purpose devices are fabricated by focused-ion-beam micromachining (FIBM) of standard vertical-cavity surface-emitting lasers (VCSELs) and are able to be switched between operation as either efficient laser sources or as resonant-cavity-enhanced avalanche photodetectors. An avalanche multiplication factor of 10 is achieved at a reverse-bias voltage of 12.3 V. A 1.25-Gb/s half-duplex bidirectional data communication link is demonstrated and shown to allow error-free transmission of binary data over 500 m of standard 50-mum-core-diameter MMF. Receiver sensitivities for transceivers based upon these devices are predicted for the first time to be comparable with conventional data communication transceivers. Furthermore, a half-duplex analog bidirectional system is constructed, which permits the transmission of advanced RF modulation formats over MMF. Link characterization by error-vector magnitude (EVM) is described. The dual-purpose vertical-cavity devices are employed for bidirectional transmission of 32-symbol quadrature-amplitude-modulated (32-QAM) signals on a carrier frequency of 2 GHz, relevant to many current wireless local-area network and cellular standards. EVM as low as 1.3% rms is observed for a 600-m link of 62.5-mum-core-diameter MMF

Robust ICI Self-cancellation OFDM Receiver with Dynamic Phase and Amplitude Estimations

Orthogonal frequency division multiplexing (OFDM) has been applied in the current wireless local-area networks and digital video broadcasting since it is robust against the frequency-selective channels. However, there is still a crucial intercarrier-interference (ICI) problem due to Doppler effect, local frequency drift and sampling clock offset, associated with OFDM systems. Recently ICI self-cancellation schemes have been proposed to significantly reduce the ICI and empirically they greatly outperform the convolutional coding schemes adopted by the IEEE 802.11 standard. However, all existing ICI self-cancellation receivers are still sensitive to the phase and amplitude ambiguities due to the phase offset, the local oscillator frequency drift and the multipath reflections. Therefore, in this paper, a novel receiver design integrating the ICI self-cancellation with a proposed dynamic phase and amplitude estimation mechanism is introduced, which can well solve the ambiguity problem. The Monte Carlo simulation results show that our phase and amplitude estimators can greatly decrease the error probability for the final symbol detection in the ICI self-cancellation OFDM receivers.

Joint Phase/Amplitude Estimation and Symbol Detection for Wireless ICI Self-Cancellation Coded OFDM Systems

OFDM has been applied in the current wireless local-area networks and digital video broadcasting systems since it has the advantage over the conventional single-carrier modulation schemes when the frequency-selective fadings are present. Nevertheless, intercarrier-interference (ICI) due to Doppler frequency drift, phase offset, local oscillator frequency drift, and multipath fading will be a severe problem in OFDM systems. Previous ICI self-cancellation coding schemes can greatly reduce the ICI, but they are very sensitive to the phase ambiguity, which is due to the composite effect of the phase offset, the multipath fading and the local frequency drift. In this paper, the phase ambiguity and amplitude ambiguity problems in ICI self-cancellation coded OFDM receivers will be formulated and discussed. Then, a novel receiver which combines the current ICI self-cancellation coding techniques with a new expectation-maximization-based joint phase/amplitude estimation and symbol detection scheme is proposed. The outstanding performance of this proposed scheme is shown and compared with other existing methods at different noise levels through OFDM simulations.

Optimization of efficiency and energy consumption in p-persistent CSMA-based wireless LANs

Wireless technologies in the LAN environment are becoming increasingly important. The IEEE 802.11 is the most mature technology for wireless local area networks (WLANs). The limited bandwidth and the finite battery power of mobile computers represent one of the greatest limitations of current WLANs. In this paper, we deeply investigate the efficiency and the energy consumption of MAC protocols that can be described with a p-persistent CSMA model. As already shown in the literature, the IEEE 802.11 protocol performance can be studied using a p-persistent CSMA model (Cali et al. 2000). For this class of protocols, in the paper, we define an analytical framework to study the theoretical performance bounds from the throughput and the energy consumption standpoint. Specifically, we derive the p values (i.e., the average size of the contention window in the IEEE 802.11 protocol (Cali et al.)) that maximizes the throughput, p/sub opt//sup C/, and minimizes the energy consumption, p/sub opt//sup E/. By providing analytical closed formulas for the optimal p values, we discuss the trade-off between efficiency and energy consumption. Specifically, we show that power saving and throughput maximization can be jointly achieved. Our analytical formulas indicate that the optimal p values depend on the network configuration, i.e., number of active stations and length of the messages transmitted on the channel. As network configurations dynamically change, the optimal p values must be dynamically updated. In this paper, we propose and evaluate a simple but effective feedback-based distributed algorithm for tuning the p parameter to the optimal values, i.e., p/sub opt//sup E/ and p/sub opt//sup C/. The performance of the p-persistent IEEE 802.11 protocol, enhanced with our algorithm, is extensively investigated by simulation. Our results indicate that the enhanced p-persistent IEEE 802.11 protocol is very close to the theoretical bounds both in steady-state and in transient conditions.

A novel technique for quantitative performance evaluation of wireless LANs

The network performances of two commercial wireless local area networks (WLANs) are measured using a novel technique. Current methods of evaluating the performance of WLANs are largely based on file transfer operations using benchmark tests such as Novell's PERFORM3, AT&T's Read/Write, PC Magazine's Lab Series, Smart LAN Performance Test and others. Measurements obtained from these tests must specify the processor type, processor speed and network operating system in order that meaningful comparisons can be made. A simple technique introduced in this paper provides quantitative results in terms of throughput and response time measured at the medium access control sublayer. The method will be useful for evaluating the performance of current and future WLAN systems.