Spectrum Utilization and Congestion of IEEE 802.11 Networks in the 2.4 GHz ISM Band

Wi-Fi technology, plays a major role in society thanks to its widespread availability, ease of use and low cost. To assure its long term viability in terms of capacity and ability to share the spectrum efficiently, it is of paramount to study the spectrum utilization and congestion mechanisms in live environments. In this paper the service level in the 2.4 GHz ISM band is investigated with focus on todays IEEE 802.11 WLAN systems with support for the 802.11e extension. Here service level means the overall Quality of Service (QoS), i.e. can all devices fulfill their communication needs? A crosslayer approach is used, since the service level can be measured at several levels of the protocol stack. The focus is on monitoring at both the Physical (PHY) and the Medium Access Control (MAC) link layer simultaneously by performing respectively power measurements with a spectrum analyzer to assess spectrum utilization and packet sniffing to measure the congestion. Compared to traditional QoS analysis in 802.11 networks, packet sniffing allows to study the occurring congestion mechanisms more thoroughly. The monitoring is applied for the following two cases. First the influence of interference between WLAN networks sharing the same radio channel is investigated in a controlled environment. It turns out that retry rate, Clear-ToSend (CTS), Request-To-Send (RTS) and (Block) Acknowledgment (ACK) frames can be used to identify congestion, whereas the spectrum analyzer is employed to identify the source of interference. Secondly, live measurements are performed at three locations to identify this type of interference in real-live situations. Results show inefficient use of the wireless medium in certain scenarios, due to a large portion of management and control frames compared to data content frames (i.e. only 21% of the frames is identified as data frames)

Removing non-stationary noise in spectrum sensing using matrix factorization

Spectrum sensing is key to many applications like dynamicspectrum access (DSA) systems or telecom regulators who need to measure utilization of frequency bands. The International Telecommunication Union (ITU) recommends a 10 dB threshold above the noise to decide whether a channel is occupied or not. However, radio frequency (RF) receiver front-ends are non-ideal. This means that the obtained data is distorted with noise and imperfections from the analog front-end. As part of the front-end the automatic gain control (AGC) circuitry mainly affects the sensing performance as strong adjacent signals lift the noise level. To enhance the performance of spectrum sensing significantly we focus in this article on techniques to remove the noise caused by the AGC from the sensing data. In order to do this we have applied matrix factorization techniques, i.e., SVD (singular value decomposition) and NMF (non-negative matrix factorization), which enables signal space analysis. In addition, we use live measurement results to verify the performance and to remove the effects of the AGC from the sensing data using above mentioned techniques, i.e., applied on block-wise available spectrum data. In this article it is shown that the occupancy in the industrial, scientific and medical (ISM) band, obtained by using energy detection (ITU recommended threshold), can be an overestimation of spectrum usage by 60%

Effective Scheduling for Coded Distributed Storage in Wireless Sensor Networks

A distributed storage approach is proposed to access data reliably and to cope with node failures in wireless sensor networks. This approach is based on random linear network coding in combination with a scheduling algorithm based on backpressure. Upper bounds are provided on the maximum rate at which data can be reliably stored. Moreover, it is shown that the backpressure algorithm allows to operate the network in a decentralized fashion for any rate below this maximum

Nitrogen losses from two grassland soils with different fungal biomass.

Nitrogen losses from agricultural grasslands cause eutrophication of ground- and surface water and contribute to global warming and atmospheric pollution. It is widely assumed that soils with a higher fungal biomass have lower N losses, but this relationship has never been experimentally confirmed. With the increased attention for soil-based ecosystem services and sustainable management of soils, such a relationship would be relevant for agricultural management. Here we present a first attempt to test this relationship experimentally . We used intact soil columns from two plots from a field experiment that had consistent differences in fungal biomass (68 ± 8 vs. 111 ± 9 μg C g-1) as a result of different fertilizer history (80 vs. 40 kg N ha-1 y-1 as farm yard manure), while other soil properties were very similar. In the greenhouse, the columns received either mineral fertilizer N or no N (control). We measured N leaching, N2O emissions and denitrification from the columns during 4 weeks, after which we analyzed fungal and bacterial biomass and soil N pools. We found that N2O emission and denitrification were lower in the high fungal biomass soil, irrespective of the addition of fertilizer N. After fertilizer addition, N leaching in low fungal biomass soil showed a 3-fold increase compared to the control (11.9 ± 1.0 and 3.9 ± 1.0 kg N ha-1, respectively), but did not increase in high fungal biomass soil (6.4 ± 0.9 after N addition vs. 4.5 ± 0.8 kg N ha-1 in the control). Thus, in the high fungal biomass soil more N was immobilized. An additional experiment with 15N–labelled mineral fertilizer, showed a 2-fold higher immobilization of 15N into microbial biomass in the high fungal biomass soil. However, only 3% of total 15N was found in the microbial biomass 2 weeks after the mineral fertilization. Most of the recovered 15N was in the plants (approximately 25%) or in the soil organic matter (approximately 15%). Our main experiment confirmed the assumption of lower N losses in a soil with higher fungal biomass. The additional 15N experiment showed that higher fungal biomass is probably not the direct cause of higher N immobilization, but rather the result of low nitrogen availability. Both experiments confirmed that higher fungal biomass can be considered as an indicator of higher nutrient retention in soils

Initial Results of a New Mobile Spectrum Occupancy Monitoring Network

In this paper we present results of the new monitoring network for spectrum governance. The network is based on the RFeye system of CRFS where the data is collected employing mobile monitoring vehicles. The measurement data, obtained from a frequency sweep between 10 MHz and 6 GHz, is further analyzed. For this purpose a tool is made, tailored for the needs of the spectrum regulator. Initial results are presented based on the measurement data showing the spectrum occupancy of various Dutch operators in the GSM downlink bands for different geographical areas. In addition, the statistics are provided with respect to the in-band power variation

Sharing your urban residential WiFi (UR-WiFi)

Cheap and ubiquitous broadband wireless access is what most of the operators are aiming for. This paper analyses an innovative proposal to extend the traditional fixed coverage offered by residential broadband into an urban wireless coverage using urban residential wireless fidelity (UR-WiFi) project. UR-WiFi assumes that by giving some incentive, the broadband customer would extend his surplus broadband and wireless bandwidth for public usage. The study performs a feasibility study of coverage, capacity and interference modelling based on different 802.11x technologies. In-depth technical analysis is complimented with detailed analytical and experimental data and is extrapolated on a countrywide basis. Based on results, we believe UR-WiFi provides a viable option for next generation broadband wireless access

Interference Measurements in IEEE 802.11 Communication Links Due to Different Types of Interference Sources

The number of wireless devices (smartphones, laptops, sensors) that use the 2.4 GHz ISM band is rapidly increasing. The most common communication system in this band is Wi-Fi (IEEE 802.11b/g/n). For that reason coexistence between Wi-Fi and other systems becomes more and more important. In this paper we have investigated the influence on Wi-Fi communication for different interference sources, i.e., wireless Audio/Video (A/V) transmitter, microwave and Bluetooth. A measurement tool has been developed to measure this influence both at the Physical (PHY) Layer and at the link layer to assess the overall Quality of Service (QoS). At link layer the tool allows to analyze the received packets types and sub fields; a sophisticated approach to analyze interference mechanisms compared to traditional packet sniffers that focus on throughput and packet error rate only. In addition, this tool allows to identify the type of interference source based on the occurring interference mechanisms at these lower two layers of the OSI protocol stack. The experimental results show severe impact of A/V transmitters which causes significant overall QoS degradation of WiFi communication in contrast to microwave and Bluetooth interference

Association of Earthworm-Denitrifier Interactions with Increased Emission of Nitrous Oxide from Soil Mesocosms Amended with Crop Residue▿ †

Earthworm activity is known to increase emissions of nitrous oxide (N2O) from arable soils. Earthworm gut, casts, and burrows have exhibited higher denitrification activities than the bulk soil, implicating priming of denitrifying organisms as a possible mechanism for this effect. Furthermore, the earthworm feeding strategy may drive N2O emissions, as it determines access to fresh organic matter for denitrification. Here, we determined whether interactions between earthworm feeding strategy and the soil denitrifier community can predict N2O emissions from the soil. We set up a 90-day mesocosm experiment in which 15N-labeled maize (Zea mays L.) was either mixed in or applied on top of the soil in the presence or absence of the epigeic earthworm Lumbricus rubellus and/or the endogeic earthworm Aporrectodea caliginosa. We measured N2O fluxes and tested the bulk soil for denitrification enzyme activity and the abundance of 16S rRNA and denitrifier genes nirS and nosZ through real-time quantitative PCR. Compared to the control, L. rubellus increased denitrification enzyme activity and N2O emissions on days 21 and 90 (day 21, P = 0.034 and P = 0.002, respectively; day 90, P = 0.001 and P = 0.007, respectively), as well as cumulative N2O emissions (76%; P = 0.014). A. caliginosa activity led to a transient increase of N2O emissions on days 8 to 18 of the experiment. Abundance of nosZ was significantly increased (100%) on day 90 in the treatment mixture containing L. rubellus alone. We conclude that L. rubellus increased cumulative N2O emissions by affecting denitrifier community activity via incorporation of fresh residue into the soil and supplying a steady, labile carbon source

Can the presence of plantain (Plantago lanceolata L.) improve nitrogen cycling of dairy grassland systems on peat soils?

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