EXTENDING IEEE 802.11 (Wi-Fi) RANGE: EXPERIMENTAL VALIDATION AND FUNDAMENTAL PERFORMANCE LIMITS DETERMINATION

Abstract

This thesis demonstrates the use of UNII-3 (Wi-Fi) frequencies, 5.725 - 5.825GHz, in setting up long distance point-to-point links, capable of providing broadband Internet in rural areas. Although this frequency band was initially intended for indoor wireless local area networks (WLAN), its lack of licensing and inexpensive off-shelf networking devices has prompted many researchers and technology enthusiasts to extend its use to outdoor settings. This work seeks to experimentally verify that Wi-Fi radios whose MAC protocol is based on TDMA channel access mechanism overcome the fundamental challenges associated with CSMA based radios when used to implement long distance point-to-point links. A long distance point-to-point test-bed model that uses high-gain directional antennas, which may be replicated to provide broadband Internet access in rural areas particularly in developing countries, is developed. Five such long-distance point-to-point links of varying distances have been set up, the longest distance being 24.3 kilometers. Performance characteristics of such links are carried out, and their behavior analyzed at varying link parameters. This thesis seeks to determine what the best link parameters (channel width, Modulation and Coding Scheme (MCS), packet size and transmit power) are for links of varying lengths as well as the effect of each on the achievable throughput. Further, the interrelationship of throughput and packet loss is sought. The results obtained confirm that a TDMA based Wi-Fi radios are able to reliably deliver high throughput in long distance point-to-point links, the only mandatory requirement being a clear line of sight with at least 60% Fresnel zone clearance. Channel bonding enables utilization of 40MHz wide channels and when tested over long distance point-to-point links it delivers high throughput, as high as 100Mbps. This performance is superior in comparison to the 20MHz-wide channels when used over the same scenario, of long distance point-to-point links. Higher modulations and coding schemes indices, MCS 13, 14 and 15, deliver the highest throughput in comparison to the lower indices; their performance, however, begin to crumble with increase in distance. MCS 14 and 15 perform abysmally in links of lengths exceeding 10km. It was expected that the links would withstand external Wi-Fi interference, by shifting the channel of use dynamically. However, it is observed that, the link performance in the presence of an interferer is inferior, by far, to when there is no interference.