Wi-Fi over water

01 Jan 2006

Hong Kong already has a reputation as one of the most wired (and wireless) cities in Asia, but one of its outlying residential areas is determined to be one of the most connected spots in the territory - including its ferry route.

Discovery Bay (DB), a township established in 1989 and developed by Hong Kong Resort Co on the east coast of Lantau island, is populated mainly by professionals who rely on IT technology in one form or another. As such, in late 2004 HKR began responding to demands for Wi-Fi by establishing access points at the two piers used by its ferry service between DB and Central District on Hong Kong Island. Since then, HKR has covered the entire public plaza and even the local beach with Wi-Fi signals.

In September 2005, HKR extended its Wi-Fi coverage from the ferry piers to the ferries themselves. The access portion is straightforward enough - two Wi-Fi APs onboard, one for the upper deck and one for the lower. What's more interesting is the backhaul connection, which isn't a satellite link (as First Mile - who, for the record, is a DB resident - initially guessed when trying out the service several months ago) or even pre-standard 802.16e WiMAX, but a point-to-multi-point OFDM-based WLL system.

Challenges on the sea

The proprietary system from Proxim comprises a subscriber unit for the ferry and a series of base stations established along the 16-km ferry route. The onboard APs connect to the subscriber unit, which connects to the nearest base station, providing a 2-Mbps backhaul link in the 5.8-GHz band. The system also hands off the backhaul signal from station to station as the ferry moves along the route.

With the system still in the beta phase, the backhaul network is currently just a few base stations on either end of the ferry route, which amounts to only 75% coverage of the full route. However, Samuel Leung, senior IT manager for HKR, says that HKR intends to establish more base stations along the route on coastal sites and nearby islands such as Tsing Yi and Peng Chau to fill in coverage gaps. PCCW - which has been supplying the links between the base stations and the Internet - will help HKR establish those sites. The service is scheduled for a full launch early this year, and like DB's other public Wi-Fi hotspots, it will be offered free of charge.

Leung says the company considered a number of technologies for the backhaul and even entertained using existing 3G coverage at one point. 'In the end we decided it was just too expensive,' he says.

Leung also acknowledges that designing a terrestrial-based backhaul network for a maritime service has had its fair share of challenges. Apart from obvious things like site acquisition and power supplies for base stations, one challenge is that the ferry path isn't as predictable as, say, a railway or a highway.

'On a ferry you have more leeway to move around, and they can deviate hundreds of meters off course,' Leung says. 'We had to make sure that the cells could provide a signal at least 400 meters on either side of the route. We also found things like tidal variations can be a factor too.'

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