Wireless Local Area Network Devices Research Articles

An Adaptive WLAN Interference Mitigation Scheme for ZigBee Sensor Networks

We propose an adaptive interference avoidance scheme that enhances the performance of ZigBee networks by adapting ZigBees' transmissions to measured wireless local area network (WLAN) interference. Our proposed algorithm is based on a stochastic analysis of ZigBee operation that is interfered with by WLAN transmission, given ZigBee and WLAN channels are overlaid in the industrial, scientific, and medical (ISM) band. We assume that WLAN devices have higher transmission power than ZigBee devices. Then, the high transmission power of WLAN devices causes the capture effect when WLAN and ZigBee transmit simultaneously. On the other hand, ZigBee performs backoff during clear channel assessment (CCA) operation if the WLAN is transmitting its frames on the channel. We adopt a widely used WLAN queueing/transmission model that is based on a Markov chain concept. We model a ZigBee device's operation using the Markov chain that includes WLAN interference statistically derived from the WLAN queueing/transmission model. Our proposed algorithm is evaluated in a simulated ZigBee network in the presence of varying WLAN interference. Numerical results show that our WLAN interference mitigation scheme finds the ZigBee control parameters, among a candidate set, which enhances ZigBee network performance compared to the conventional ZigBee operation.

Electromagnetic field strength in proximity of WLAN devices during data and video file transmission

Wireless local area network (WLAN) devices emitting in the 2.4 GHz band may raise questions concerning human exposure mainly when a person stands near an antenna. Proximity far-field level variation with distance, in the range 40 cm to 1 m is investigated experimentally in order to extract peculiarities of the radiated field in comparison with theoretical predictions. Electric field strength depends on the method of measurement (not yet standardised for quasi-stochastic signals) and on some key characteristics of transmission. The field level proved to differ from theory: it is lower than predicted and it does not always follow the inverse law with distance. The time-amplitude and the duty-cycle methods of weighting to obtain a realistic field level allowed highlighting the dependence of the field strength on the traffic direction, on the type of the transmitted file and on the actual data rate and its statistical distribution. In the proximity of access points, downloading conducts to the highest fields, whereas near laptops, uploading does this. A data file transfer implies a higher level of exposure than a video file transfer, but one should consider the data rate dispersion. The proximity field does not significantly depend on the distances between the WLAN devices.

Design of compact antenna for 2.4/4.9/5.2/5.8‐GHz WLAN

ABSTRACTThe design of a compact antenna for wireless devices in the 2.4/4.9/5.2/5.8‐GHz wireless local area network (WLAN) application is presented. The antenna consists of a U‐element with microstrip‐fed on one side of the substrate and an L‐element connected to ground on the other side. The U‐element generates a high‐frequency band at around 5.5 GHz for the 4.9/5.2/5.8‐GHz WLAN bands. The same U‐element, by forming a loop element with the L‐element and the ground, also generates a low‐frequency band at 2.44 GHz for the low‐WLAN band. The two radiating elements together have a compact size of 12.6 × 9 mm2 and the whole antenna has a very small size of only 25 × 25 mm2, including the ground plane. Computer simulation and measurement are used to study and optimize the antenna. For practical use of the design, the antenna is also designed on a mobile‐phone printed‐circuit board and studied using computer simulation and measurement. A simulation model of the feeding cable is included in simulation. Simulated and measured performances show good agreements. Finally, the effect of mobile phone housing is also studied using computer simulation. All results show that the antenna can cover all the 2.4/4.9/5.2/5.8‐GHz WLAN bands; hence the proposed antenna is a very promising candidate for practical WLAN devices. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:1360–1366, 2014

Exposure levels due to WLAN devices in indoor environments corrected by a time-amplitude factor of distribution of the quasi-stochastic signals

With the development of radiofrequency technology, radiating quasi-stochastic signals like the wireless local area networks (WLAN), a proper procedure of exposure level assessment is needed. No standardised procedure exists at the moment. While channel power measurement proved to overestimate the field strength, weighting techniques were proposed. The paper compares the exposure levels determined by three different procedures, two of them correcting the field level by weighting. Twenty-three experimental cases of WLAN traffic load are analysed in an indoor environment in controlled conditions. The results show the differences obtained when the duty cycle (DC) method is applied comparatively with the application of weighting based on an amplitude-time correction. Significant exposure level reductions of 52.6-79.2 % from the field determined by frequency domain method and of 36.5-72.8 % from the field determined by the DC weighting method were obtained by time-amplitude method. Specificities of weighting factors probability density functions were investigated and regression analysis was applied for a detailed characterisation of this procedure.

Experimental Investigation of Co-channel and Adjacent Channel Operations of Microwave Power and IEEE 802.11g Data Transmissions

SUMMARY We discuss the division of radio resources in the time and frequency domains for wireless local area network (WLAN) devices powered with microwave energy. In general, there are two ways to avoid microwave power transmission (MPT) from influencing data communications: adjacent channel operation of continuous MPT and WLAN data transmission and co-channel operation of intermittent MPT and WLAN data transmission. Experimental results reveal that, even when we implement these methods, several problems arise because WLAN devices have been developed without supposing the existence of MPT. One problem clarified in our experiment is that adjacent channel operation at 2.4 GHz does not necessarily perform well owing to the interference from MPT. This interference occurs regardless of the frequency separation at 2.4 GHz. The other problem is that intermittent MPT could result in throughput degradation owing to the data rate control algorithm and the association scheme of the WLAN. In addition, the experimental results imply that a microwave energy source and a WLAN device should share information on the timings

Cooperative Carrier Signaling: Harmonizing Coexisting WPAN and WLAN Devices

The unlicensed ISM spectrum is getting crowded by wireless local area network (WLAN) and wireless personal area network (WPAN) users and devices. Spectrum sharing within the same network of devices can be arbitrated by existing MAC protocols, but the coexistence between WPAN and WLAN (e.g., ZigBee and WiFi) remains a challenging problem. The traditional MAC protocols are ineffective in dealing with the disparate transmit-power levels, asynchronous time-slots, and incompatible PHY layers of such heterogeneous networks. Recent measurement studies have shown moderate-to-high WiFi traffic to severely impair the performance of coexisting ZigBee. We propose a novel mechanism, called cooperative carrier signaling (CCS), that exploits the inherent cooperation among ZigBee nodes to harmonize their coexistence with WiFi WLANs. CCS employs a separate ZigBee node to emit a carrier signal (busy tone) concurrently with the desired ZigBee's data transmission, thereby enhancing the ZigBee's visibility to WiFi. It employs an innovative way to concurrently schedule a busy tone and a data transmission without causing interference between them. We have implemented and evaluated CCS on the TinyOS/MICAz and GNURadio/USRP platforms. Our extensive experimental evaluation has shown that CCS reduces collision between ZigBee and WiFi by 50% for most cases, and by up to 90% in the presence of a high-level interference, all at negligible WiFi performance loss.

Heterogeneous uncoordinated deployment of WLANs: an evolving problem for current and future Wi‐Fi access

SUMMARYThis article addresses the problem of uncoordinated heterogeneous deployment of 802.11 wireless local area networks (WLANs). It is expected that such deployments by different WLAN owners and WiFi providers will become a challenging problem that limits the network performance and quality of service of wireless users. We present results of a real case study that show a need for coordination among WLAN devices in order to avoid current and future problems. We provide potential solution directions. A special focus is given to channel assignment and coordinated channel access problems. Our results show that a new paradigm for designing WLAN devices seems to be crucial. Copyright © 2012 John Wiley & Sons, Ltd.

Characterization of the indoor/outdoor radio propagationchannel at 2.4 GHz on Qatar University campus

This technical report presents the site-specific signal strength measurement results for path loss, shadowing, and fading in the 2.4GHz band under typical harsh environment (high temperature 40-50 C and humidity 80-90%). We used spectrum analyzer Rohde & Schwarz FSH8 and InSSIDer, free software for wireless local area networks (WLANs). Measurements were taken in indoor and outdoor environments at various locations at different times of the day. An empirical channel model has been derived from these measurements that characterizes the indoor-outdoor wireless channel. This report provides information that would be useful for the design and deployment of wireless mesh network in Qatar University. For a radio communication system, the channel describes how the electromagnetic propagation of a transmitted signal provides that signal at the receiver. In a mobile communication system, the channel changes according to the movement of the communicating entities and other objects that have an effect on the electromagnetic fields at the receiver. In the last decade, most of the indoor wired networks have been replaced by wireless networks. These networks can also provide outdoor connectivity inside the campus areas. WLANs based on IEEE 802.11 are largely deployed to provide users with network connectivity without being tethered to a wired network.Wireless networks can provide nearly the same services and capabilities commonly expected with wired networks. Like their wired counterparts, IEEE 802.11 has been developed to provide large bandwidth to users located in indoor and outdoor campus environments and are being studied as an alternative to the high installation and maintenance costs incurred by traditional additions, deletions, and changes experienced in wired LAN infrastructures. Because of the unlicensed spectrum availability, IEEE 802.11 WLAN devices operate in the ISM (Industrial Scientific Medical) band at 2.4GHz or 5GHz. For an accurate planning of indoor/outdoor radio networks the modeling of the propagation channel is required.

Joint uplink/downlink opportunistic scheduling for Wi-Fi WLANs

Recent advances in the speed of multi-rate wireless local area networks (WLANs) and the proliferation of WLAN devices have made rate adaptive, opportunistic scheduling critical for throughput optimization. As WLAN traffic evolves to be more symmetric due to the emerging new applications such as VoWLAN, collaborative download, and peer-to-peer file sharing, opportunistic scheduling at the downlink becomes insufficient for optimized utilization of the single shared wireless channel. Furthermore, without proper scheduling on the uplink, the downlink throughput gain diminishes proportionally to the increasing number of clients transmitting on the uplink. However, opportunistic scheduling on the uplink of a WLAN is challenging because wireless channel condition is dynamic and asymmetric. Each transmitting client has to probe the access point to maintain the updated channel conditions at the access point. Moreover, the scheduling decisions must be coordinated at all clients for consistency. This paper presents JUDS, a joint uplink/downlink opportunistic scheduling for WLANs. Through synergistic integration of both the uplink and the downlink scheduling, JUDS maximizes channel diversity at significantly reduced scheduling overhead. It also enforces fair channel sharing between the downlink and uplink traffic. Through analysis and extensive QualNet simulations, we show that JUDS improves the overall throughput by up to 127% and achieves both fairness between uplink and downlink traffic.

Channel Clustering and Probabilistic Channel Visiting Techniques for WLAN Interference Mitigation in Bluetooth Devices

Since Bluetooth and wireless local area network (WLAN) technologies both operate at the 2.4-GHz industrial, scientific, and medical (ISM) band, the two types of devices may suffer from mutual interference and performance degradations. In this paper, we propose two new techniques, channel clustering and probabilistic channel visiting, to effectively improve the existing coexistence and interference mitigation mechanisms. The channel clustering technique employs statistical pattern recognition to classify the status of Bluetooth channels more accurately. The probabilistic channel visiting is used to more equitably allocate the channel resources between Bluetooth and WLAN devices. The effectiveness of these techniques is quantified by simulations. Results show that both techniques are beneficial in improving the performance of the existing mechanisms.

Distributed Access Time Control for Per-Station Fairness in Infrastructure WLANs

SUMMARY Fairness among uplink and downlink TCP flows can notbe supported in infrastructure IEEE 802.11 Wireless LANs using the dis-tributed coordination function (DCF). In order to resolve this problem, wepropose the Distributed Access Time Control (DATC) scheme, in whicheach station controls the rates of its TCP flows based on channel accesstime. The DATC scheme can provide per-station fairness even when thenumber and direction of flows in each station are different. This schemecan be simply implemented in mobile stations without having to modifythe access points. The properties of the proposed scheme are investigatedand its effectiveness is verified through simulations. key words: wireless LAN, TCP, fairness 1. Introduction As the Internet has become an essential part of the daily lifeof an ordinary man, it must be accessible at any time andat any place. To satisfy such public demand, wireless localarea networks (WLANs) based on IEEE 802.11 standard [1]are widely deployed in public areas such as airports, cafes,hotels, etc. Consequently, due to the large number of peopleaccessing the Internet via WLANs [2], it should be ensuredthat such services be provided to users fairly.In the 802.11 standard, fairness is assured at the MediaAccess Control (MAC) layer by the Distributed Coordina-tion Function (DCF), which aims to give equal access op-portunity to all wireless devices. Meanwhile, the other MAClayer protocol, i.e., the Point Coordinate Function (PCF),gives more control to an Access Point (AP), but there areonly a few WLAN devices that support PCF. The issue offairness in WLANs has been studied extensively, but most ofwhich have focused on the enhancement of DCF for fairnessat the MAC layer [3]–[5]. Although these MAC layer fair-ness mechanisms canprovide fair access opportunity to eachstation, these solutions require changes at the MAC layerand are targeted for ad-hoc networks. Therefore, these so-lutions are impractical to be used in infrastructure WLANs.In infrastructure networks, it is necessary to provide fairnessamong stations that have numerous flows moving in differ-ent directions.Bottigliengo et al.[6] showed that the unfairness be-