A new connectivity strategy for wireless mesh networks using dynamic spectrum access

The introduction of Dynamic Spectrum Access (DSA) marked an important juncture in the evolution of wireless networks. DSA is a spectrum assignment paradigm where devices are able to make real-time adjustment to their spectrum usage and adapt to changes in their spectral environment to meet performance objectives. DSA allows spectrum to be used more efficiently and may be considered as a viable approach to the ever increasing demand for spectrum in urban areas and the need for coverage extension to unconnected communities. While DSA can be applied to any spectrum band, the initial focus has been in the Ultra-High Frequency (UHF) band traditionally used for television broadcast because the band is lightly occupied and also happens to be ideal spectrum for sparsely populated rural areas. Wireless access in general is said to offer the most hope in extending connectivity to rural and unconnected peri-urban communities. Wireless Mesh Networks (WMN) in particular offer several attractive characteristics such as multi-hopping, ad-hoc networking, capabilities of self-organising and self-healing, hence the focus on WMNs. Motivated by the desire to leverage DSA for mesh networking, this research revisits the aspect of connectivity in WMNs with DSA. The advantages of DSA when combined with mesh networking not only build on the benefits, but also creates additional challenges. The study seeks to address the connectivity challenge across three key dimensions, namely network formation, link metric and multi-link utilisation. To start with, one of the conundrums faced in WMNs with DSA is that the current 802.11s mesh standard provides limited support for DSA, while DSA related standards such as 802.22 provide limited support for mesh networking. This gap in standardisation complicates the integration of DSA in WMNs as several issues are left outside the scope of the applicable standard. This dissertation highlights the inadequacy of the current MAC protocol in ensuring TVWS regulation compliance in multi-hop environments and proposes a logical link MAC sub-layer procedure to fill the gap. A network is considered compliant in this context if each node operates on a channel that it is allowed to use as determined for example, by the spectrum database. Using a combination of prototypical experiments, simulation and numerical analysis, it is shown that the proposed protocol ensures network formation is accomplished in a manner that is compliant with TVWS regulation. Having tackled the compliance problem at the mesh formation level, the next logical step was to explore performance improvement avenues. Considering the importance of routing in WMNs, the study evaluates link characterisation to determine suitable metric for routing purposes. Along this dimension, the research makes two main contributions. Firstly, A-link-metric (Augmented Link Metric) approach for WMN with DSA is proposed. A-link-metric reinforces existing metrics to factor in characteristics of a DSA channel, which is essential to improve the routing protocol's ranking of links for optimal path selection. Secondly, in response to the question of “which one is the suitable metric?”, the Dynamic Path Metric Selection (DPMeS) concept is introduced. The principal idea is to mechanise the routing protocol such that it assesses the network via a distributed probing mechanism and dynamically binds the routing metric. Using DPMeS, a routing metric is selected to match the network type and prevailing conditions, which is vital as each routing metric thrives or recedes in performance depending on the scenario. DPMeS is aimed at unifying the years worth of prior studies on routing metrics in WMNs. Simulation results indicate that A-link-metric achieves up to 83.4 % and 34.6 % performance improvement in terms of throughput and end-to-end delay respectively compared to the corresponding base metric (i.e. non-augmented variant). With DPMeS, the routing protocol is expected to yield better performance consistently compared to the fixed metric approach whose performance fluctuates amid changes in network setup and conditions. By and large, DSA-enabled WMN nodes will require access to some fixed spectrum to fall back on when opportunistic spectrum is unavailable. In the absence of fully functional integrated-chip cognitive radios to enable DSA, the immediate feasible solution for the interim is single hardware platforms fitted with multiple transceivers. This configuration results in multi-band multi-radio node capability that lends itself to a variety of link options in terms of transmit/receive radio functionality. The dissertation reports on the experimental performance evaluation of radios operating in the 5 GHz and UHF-TVWS bands for hybrid back-haul links. It is found that individual radios perform differently depending on the operating parameter settings, namely channel, channel-width and transmission power subject to prevailing environmental (both spectral and topographical) conditions. When aggregated, if the radios' data-rates are approximately equal, there is a throughput and round-trip time performance improvement of 44.5 - 61.8 % and 7.5 - 41.9 % respectively. For hybrid links comprising radios with significantly unequal data-rates, this study proposes an adaptive round-robin (ARR) based algorithm for efficient multilink utilisation. Numerical analysis indicate that ARR provides 75 % throughput improvement. These results indicate that network optimisation overall requires both time and frequency division duplexing. Based on the experimental test results, this dissertation presents a three-layered routing framework for multi-link utilisation. The top layer represents the nodes' logical interface to the WMN while the bottom layer corresponds to the underlying physical wireless network interface cards (WNIC). The middle layer is an abstract and reductive representation of the possible and available transmission, and reception options between node pairs, which depends on the number and type of WNICs. Drawing on the experimental results and insight gained, the study builds criteria towards a mechanism for auto selection of the optimal link option. Overall, this study is anticipated to serve as a springboard to stimulate the adoption and integration of DSA in WMNs, and further development in multi-link utilisation strategies to increase capacity. Ultimately, it is hoped that this contribution will collectively contribute effort towards attaining the global goal of extending connectivity to the unconnected

Ubiquitous Mesh Networking: application to mobile communication and information dissemination in a rural context

ICT has furthered the social and economic development of societies but, rural African communities have lagged behind due to issues such as sparse population, low household income, a lack of electricity and other basic infrastructure that make it unattractive for telecommunication service providers to extend service provision. Where the service is available, ubiquitous service coverage has not translated into ubiquitous access for individuals because of the associated costs. A community-wide WMN offering VoIP using fixed telephone handsets has been deployed as a viable alternative to the cellular service provider. The effectiveness of this WMN VoIP service springs from the mobile phone usage statistics which showed that the majority of calls made are intra-community. This dissertation has been an effort towards improved communication and access to information for the under-served communities. Key contributions include, mobile VoIP support, translation gateway deployment to make textual information accessible in voice form via the phone, IP-based radio for community information dissemination. The lack of electricity has been mitigated by the use of low-power devices. In order to circumvent the computational challenges posed by the processing and storage limitations of these devices, a decentralised system architecture whereby the processing and storage load are distributed across the mesh nodes has been proposed. High-performance equipment can be stationed at the closest possible place with electricity in the area and connectivity extended to the non-electrified areas using low-power mesh networking devices. Implementation techniques were investigated and performance parameters measured. The quality of service experienced by the user was assessed using objective methods and QoS correlation models. A MOS value of 4.29, i.e. very good, was achieved for the mobile VoIP call quality, with the underlying hardware supporting up to 15 point-to-point simultaneous calls using SIP and the G.711 based codec. Using the PEAQ algorithm to evaluate the IP-based radio, a PEAQ value of 4.15, i.e. good, was achieved. Streaming audio across the network reduces the available bandwidth by 8Kbps per client due to the unicast nature of streaming. Therefore, a multicast approach has been proposed for efficient bandwidth utilization. The quality of the text-to-voice service rendered by the translation gateway had a PESQ score of 1.6 i.e. poor. The poor performance can be attributed to the TTS engine implementation and also to the lack of robustness in the time-alignment module of the PESQ algorithm. The dissertation also proposes the use of the WMN infrastructure as a back-haul to isles of WSNs deployed in areas of interest to provide access to information about environmental variables useful in decision making

Exploring RSSI Dependency on Height in UHF for Throughput Optimisation

This paper considers exploiting the unique outdoor propagation characteristics of the Ultra High Frequency (UHF) band to optimise wireless network deployments. The relationship existing between signal strength and antenna height in UHF band is analysed. Received signal strength increases steadily with an increase in receiver antenna height up to about 8.5 m above ground, which can be explained in part by the resulting effect of Fresnel zone and obstacle clearance such as typical house height in the area. When raised beyond 8.5 m further signal strength gain stifles, possibly due to effects of multi-path fading. The contribution of this paper is firstly, the implication of Received Signal Strength Indicator (RSSI) dependency on height and secondly, the consideration of throughput corresponding to RSSI thresholds

A Correlation between RSSI and Height in UHF Band and Comparison of Geolocation Spectrum Database View of TVWS with Ground Truth.

An investigation into the Received Signal Strength Indica- tor (RSSI) dependency on receiver antenna height in UHF band is con- ducted. The results show a high correlation between RSSI and height on channels with high signal strength. There is approximately 2.5 dBm RSSI gain per 1 m increase in height above ground up to 8.5 m. From 8.5 m to 12 m, there is no consistent observable increase in RSSI. Further- more, the geolocation spectrum databases (GLSD) view of white space in the television band is compared with the ground truth. Results show signal presence on some of the channels indicated free by the spectrum database. These findings imply that an increase in transmission range of UHF links can be achieved by increasing receiver height. White space devices using A GLSD should additionally require spectrum scanning to determine clear channels

A Road Map for Wireless Mesh Routing with DSA

Dynamic Spectrum Access (DSA) provides a new opportunity for last-mile and rural connectivity. However, the nature of DSA and varying environmental conditions means that desired spectrum is not always available. To that end, we have deployed a testbed using both DSA-enabled TV White Space devices and fixed spectrum WiFi (5GHz) in parallel to support communication between and within townships. Using this testbed, we will develop new algorithms to support wireless mesh routing in DSA networks

Performance Analysis of Dual 5 GHz WiFi and UHF TV White Space Network Links

Commonly used WiFi is known to be ill-suited for penetrating vegetation and buildings and non-line-of-sight conditions. Television white space (TVWS) operates in ultra-high frequency (UHF) bands that overcome many of the penetration and line-of-sight challenges found in the 2.4 GHz and 5 GHz bands normally used by WiFi. The aim of this study is to report on the performance of WiFi technology in the 5 GHz band and the TVWS technology in the 600 MHz UHF TV band as well as a combination of both radios in two different scenarios, short-range clear line-of-sight, and non-line-of-sight conditions. A number of performance metrics, such as estimated throughput, bitrate, signal strength, noise, transmit power, transmit error, packet loss, and round trip time, are compared for varied distances and increasing levels of vegetation in the propagation path. Both TVWS and WiFi experiments showed increased sensitivity to noise as channel widths increased with TVWS being particularly susceptible to noise in nearby channels from powerful TV transmitters. Aggregating the WiFi and TVWS radios proved to have the best performance improvements when the WiFi and TVWS links had similar throughput in line-of-sight conditions

Experimental Analysis of 5 GHz WiFi and UHF-TVWS Hybrid Wireless Mesh Network Back-Haul Links

This paper reports on the experimental analysis of hybrid back-haul links comprising WiFi operating in the 5 GHz and Ultra High Frequency Television White Space bands. Possible link permutations are highlighted. Performance results show that overall network optimisation requires a combination of frequency division and time division duplexing

Head to Head Battle of TV White Space and WiFi for Connecting Developing Regions

TV White Space networks are gaining momentum worldwide as an important addition to the suite of wireless protocols available for connecting developing regions. However, there has been no thorough investigation of scenarios where TV White Space performs better or worse than alternative low-cost wire- less technology such as WiFi. This paper analyzes the performance of 5 GHz WiFi links and TV White space links using down-converted WiFi, typically used as wireless backhaul for poorly connected regions, in different scenarios including line-of-sight links and links obstructed by trees and structures. The experiments make use of 802.11a/b/g WiFi and TV White Space equipment that down- converts standard 802.11 a/b/g WiFi from the 2.4 GHz band into the UHF band. The paper finds that 5 GHz links outperformed TVWS where clear line-of-sight is available and point-to-point links are required. TVWS however is a clear choice where there are obstructions and where wider coverage is needed. Some interesting observations on the negative effect of TV transmissions in adjacent channels a few channel-hops away from the channel being used for TVWS are also provided