| WLAN(Wireless Local Area Network) technology is being widely used to provide indoor wireless internet access in many traditional sectors ranging from corporate, exhibition, family life, retail, catering, warehouse, and etc. Meanwhile, with carriers' continuous progress in wireless city project and rapid growth of mini net books and Wi-Fi-enabled smart phones, outdoor WLAN hot spots infrastructure has also advanced considerably in the recent years. On its way to becoming a “wireless city”, Shanghai government initiated a free Wi-Fi program “i-Shanghai”, aims to have a large coverage of free outdoor Wi-Fi hot spots to be built by local mobile carriers in public places. These places include parks and green leisure space administrated by Shanghai Municipal Afforestation & City Appearance and Environmental Sanitation Administration and Shanghai Municipal Tourism Administrative Commission, transport hubs by Shanghai Transportation and Harbor Administration, and CBDs and shopping zones by the commissions of commerce at the district level.Unlike the indoor WLAN environment, an outdoor WLAN environment always suffers signal disturbance. Also, due to the varied and complex paths for signal transmission, which can also be usually blocked by the surroundings, conventional WLAN technology is often difficult to provide high-quality coverage. The outdoor hot spot solution we adopted right now is to intensify the power of the 500 mw outdoor AP(Access Point) and transmit the signal from a directional outdoor antenna tabular. However, its performance is still not so good because of the increased noise produced by itself. Several types of new outdoor WLAN technologies have emerged in the recent years, including smart antenna and MIMO(Multiple-Input Multiple-Output), while the strength of these new technologies are always be grounded in different outdoor conditions, and there is still a gap between theoretical expectations and the practice. Given that carriers tend to invest greatly, while at the same time, are always short of a thorough research, the purpose of this paper is to outline and provide solutions to the key challenges of adopting smart antenna and MIMO on outdoor WLAN coverage building-up, and serves as a strategic suggestion to the carriers on selecting and adopting different technology and infrastructure in light of different situations.This paper, first of all, describes the WLAN-related protocols and modulation techniques, and then on this basis, further elaborates the WLAN radio frequency range and speed. Next, it gives analysis on outdoor WLAN network's capacity and coverage requirements based upon the WLAN network system structure, by taking the methods of spatial link computational analysis, frequency planning and interference attenuation analysis, and capacity calculation model approach. After this, it identifies the main strengths of smart antenna and MIMO in outdoor circumstances in terms of coverage, speed, throughput, and anti-interference by building a mathematical model. Finally, after a series of theoretical analysis and practical field testing, the paper provides an objective and concrete observation on the performance of smart antenna and MIMO in outdoor circumstances, and then makes suggestion on how the carriers should select and adopt different methods to meet the different demand by the much varied outdoor WLAN situations in the next phase, conquering the challenges in outdoor WLAN network coverage, thus ensuring the smooth running of Shanghai's wireless city project. This will also result in not only improved uplink & downlink, expanded coverage, decreased number of conventional AP; but also lowered the blocking probability of interference, increased system capacity; and eventually cut off the expense on transmission line, power equipment and site establishment by co-locating the communication networks. |
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