Long-range Wi-Fi

Main article: Wi-Fi

Long-range Wi-Fi is used for low-cost, unregulated point-to-point connections, as an alternative to cellular networks, microwave or satellite links. The use of the term "long range wifi" as depicted on this page for extreme ranges is not in any way endorsed by the Wi-Fi Alliance and is not in any way tested or certified by the Wi-Fi Alliance for interoperability or performance.

Introduction

Since the development of the Wi-Fi radio standard, great leaps in the technology's abilities have been made. In one area, range, Wi-Fi has been pushed to an extreme, and both commercial and residential applications of this Long Range Wi-Fi have cropped up around the world. It has also been used in experimental trials in the developing world to link communities separated by difficult geography with little or no connectivity options.

Applications

Business

• Provide coverage to a large office or business complex or campus.
• Establish point-to-point link between large skyscrapers or other office buildings.
• Bring Internet to remote construction sites or research labs.

Residential

• Bring Internet to a home if regular cable/DSL cannot be hooked up at the location.
• Bring Internet to a vacation home or cottage on a remote mountain or on a lake.
• Bring Internet to a yacht or large sea-faring vessel.
• Share a neighborhood Wi-Fi network.

Large-scale deployments

The Technology and Infrastructure for Emerging Regions (TIER) project at University of California at Berkeley, in collaboration with Intel, utilizes a modified Wi-Fi setup to create long-distance point-to-point links for several of its development projects in the developing world. This technique, dubbed Wi-Fi over Long Distance (WiLD), is used to connect the Aravind Eye Hospital with several outlying clinics in Tamil Nadu state, India. Distances range from five to over fifteen kilometers with stations placed in line of sight of each other. These links allow specialists at the hospital to communicate with nurses and patients at the clinics through video conferencing. If the patient needs further examination or care, a hospital appointment can then be scheduled. Another network in Ghana links the University of Ghana, Legon campus to its remote campuses at the Korle bu Medical School and the City campus; a further extension will feature links up to 80km apart.

Increasing range in other ways

See also 802.11 non-standard equipment

Specialized Wi-Fi channels

In most standard Wi-Fi routers, the three standards, A, B and G, are enough. But in long-range Wi-Fi, special technologies are used to get the most out of a Wi-Fi connection. The 802.11-2007 standard adds 10 MHz and 5 MHz OFDM modes to the 802.11a standard, and extend the time of cyclic prefix protection from 0.8 µs to 3.2 µs, quadrupling the multipath distortion protection. Some commonly available 802.11a/g chipsets support the OFDM 'half-clocking' and 'quarter-clocking' that is in the 2007 standard, and 4.9 GHz and 5.0 GHz products are available with 10 MHz and 5 MHz channel bandwidths. It is likely that some 802.11n D.20 chipsets will also support 'half-clocking' for use in 10 MHz channel bandwidths, and at double the range of the 802.11n standard.

802.11n (MIMO)

Preliminary 802.11n working became available in many routers in 2008. This technology works by using multiple antennas to target one or more sources to increase speed. In tests, the speed increase was said to only occur over short distances rather than the long range needed for most point to point setups.

Power increase or receiver sensitivity boosting

A rooftop 1 Watt WiFi amp,feeding a simple antenna

Another way of adding range uses a power amplifier. Commonly known as "range extender amplifiers" these small devices supply usually around ½ Watt of power to the antenna. Such amplifiers may give more than five times the range to an existing network. Each 6dB gain doubles range, and in the case of the popular Linksys WRT54G The RadioLabs 2.4 GHZ range extender amplifier this increases the stock power of the WRT54G from +18dBm (63 mW) to +27dBm (500 mW), a 9dB (8x) increase - enough for perhaps tripling the range. These power amplifiers offer a tempting enhancement that can easily be added to existing networks in places where law allows. Aside from legal issues (significant in some countries), they however may cause interference and channel swamping to other WiFi users. The alternative techniques of selecting a more sensitive WLAN adapter (some are quite "deaf") and more directive antenna should hence be initially considered.

Higher gain antennas and adapter placement

Specially shaped antennas can be used to increase the range of a Wi-Fi transmission without a drastic increase in transmission power. High gain antenna may be of many designs,but all allow transmitting a narrow signal beam over distances of several kilometers,usefully often nulling out nearby interference sources. A popular low-cost home made approach increases WiFi ranges by just placing standard USB WLAN hardware at the focal point of modified parabolic cookware (see "www.usbwifi.orcon.net.nz/rvwifi.jpg"). Such "WokFi" techniques typically yield gains of 12-15dB over the bare system- enough for line of sight (LOS) ranges of several km and improvements in marginal locations. N.B. Although often low power,cheap USB WLAN adapters suit site auditing and location of local signal "sweet spots". As USB leads incur none of the losses normally associated with costly microwave coax & SMA fittings,just extending a USB adapter (or AP etc) up to a window,or away from shielding metal work and vegetation,may dramatically improve the link.(See "www.usbwifi.orcon.net.nz/wifiledge.jpg")

Protocol hacking

The standard 802.11 protocol stacks can also be modified to make them more suitable for long distance, point-to-point usage, at the risk of breaking interoperability with other Wi-Fi devices and suffering interference from transmitters located near the antenna. These approaches are used by the TIER project (see "Rethinking Wireless in the Developing World").

Obstacles to long-range Wi-Fi

Methods that stretch the range of a Wi-Fi connection may also make it fragile and volatile, due to mundane problems including:

Landscape interference

Obstacles are among the biggest problems when setting up a long-range Wi-Fi. Trees and forests degrade the microwave signal, and rolling hills make it difficult to establish line-of-sight propagation.

In a city, buildings will impact integrity, speed and connectivity. Steel frames partly reflect radio signals, and concrete or plaster walls absorb microwave signals significantly, but sheet metal in walls or roofs may efficiently reflect Wi-Fi signals, causing an almost total loss of signal.

2.4 GHz interference

Microwave ovens in residences dominate the 2.4 GHz band and will cause "meal time perturbations" of the noise floor. There are literally hundreds of other sources of interference that aggregate into a formidable obstacle to enabling long range use in occupied areas: baby monitors, wireless cameras, remote car starters, DECT and residential wireless phones, Bluetooth products to name just a few.

Phones

Many cordless phones in the US and Canada use the 2.4GHz frequency, the same frequency at which Wi-Fi standards b, g and n operate. This can cause a significant decrease in speed, or sometimes the total blocking of the Wi-Fi signal when a conversation on the phone takes place. There are several ways to avoid this though, some simple, and some more complicated.

• Buy/Use wired phones.
• Buy 5.8GHz or 900MHz phones, commonly available today.
• Use VoIP/WiFi phones; these share the WiFi base stations and participate in the WiFi contention protocols.
• Test several different Wi-Fi channels to avoid the phone channels.

The last will sometimes not be successful, as numerous cordless phones use a feature called Digital Spread Spectrum. This technology was designed to ward off eavesdroppers, but the phone will change channels at random, leaving no Wi-Fi channel safe from phone interference.

Car alarms

Certain car manufacturers use the 2.4GHz frequency for their car alarm internal movement sensors. These devices broadcast on 2.45GHz (between channels 8 and 9) at a strength of 500mW. Because of channel overlap, this will cause problems for channels 6 and 11 which are commonly used default channels for Wi-Fi connections. Because the signal is transmitted as a continuous tone, it causes particular problems for Wi-Fi traffic. This can be clearly seen with spectrum analysers.

Longest unamplified Wi-Fi link

279 km Wifi link with DD-WRT and openwrt Read here

Achieved by: Fundación Escuela Latinoamericana de Redes, Latin American Networking School

Abstract

• Pico del Águila- El Baúl Link.
• Central frequency: 2412 MHz
• IEE 802.11 (Wi-Fi), channel 1, 22 MHz. bandwidth
• Wireless routers: Linksys WRT54G, OPEN-WRT firmware at el Águila and DD-WRT firmware at El Baúl.
• Length: 279 km.
• Parabolic dish antennas were used at both ends, recycled from satellite service.
• At El Aguila site an aluminum mesh reflector 2,74 m diameter, center fed, at el Baúl a fiberglass solid reflector, offset fed, 2,4 x 2,74 m. At both ends the feeds were 12 dBi Yagis.
• Linksys WRT54g routers fed the e antennas with short LMR400 cables, so the effective gain of the complete antenna is estimated at about 30 dBi.
• As far as we know, this is the longest range attained with this technology, improving on a previous US record of 125 miles achieved last year in U.S. The Swedish space agency attained 310 km but using 6 watt amplifiers to reach an overhead stratospheric balloon

External Links

Long-Distance 802.11b Links: Performance Measurements and Experience (PDF)