04.01.08
WirelessWeek
By: Elliot Drucker
Wireless Balloons
The economics of commercial wireless services generally do not favor environments of low population densities. The area of service coverage that can be provided by even the tallest cell towers is perhaps a couple thousand square kilometers. That may sound like a lot of terrain, but there are vast stretches, even in America, where total population in such an area cannot justify the cost of providing service except perhaps along highway corridors.
Until recently, the only alternative to terrestrially based networks for more general wireless service in thinly populated areas has been mobile satellite systems like Iridium and Globalstar. But mobile satellites, too, have problems with economics, as evidenced by bankruptcies of the original incarnations of both these carriers.
If neither terrestrial nor satellite-based wireless systems are practical for covering sparsely populated regions, how about something in between? Space Data, a privately held company based in Chandler, Ariz., is betting that free-floating balloons operating at high altitude, but far below the realm of satellites, are the answer. Its “SkySite” system will suspend miniaturized wireless base station equipment from hydrogen-filled latex balloons launched at intervals from multiple sites along the edge of regions to be covered.
Above the Weather
The balloons, which are similar to widely used weather balloons, will then ascend rapidly to an altitude between 20,000 and 30,000 meters, far above any weather and where winds are stable and predictable. Once there, each SkySite base station will activate and begin to provide coverage over a huge area. Each balloon will remain aloft for a day or two, during which it and its coverage will slowly drift across the network service area.
At the end of its life (limited by ballast or power source exhaustion), or when it reaches the far edge of the network service area, the balloon will burst and its two tethered 6 lb payloads – the base station electronics, antennas and power-generating equipment – will parachute to earth. In flight and during descent the payload determines its precise location using GPS and reports it to the SkySite network operations center.
At first blush, the economics of wireless service provided by SkySites seem preposterous. After all, terrestrially deployed base stations are anticipated to operate for years, and their capital costs are amortized accordingly. Now along comes Space Data intending to make money using base stations that last only a day or two.
But a key to its business model is that the majority of launched payloads will be recovered, refurbished and reused for later launches. Space Data is currently working on a steer-able parachute it hopes will bring descending payloads to locations that allow easier recovery.
1-Shot Balloons
Miniaturized wireless base stations intended for indoor, and even residential, service are now available from a number of manufacturers at relatively low cost. The balloons themselves are a 1-shot deal, of course, but they are cheap. In fact, Space Data manufactures its own balloons at a cost of around $50 each. By using hydrogen rather than much more expensive helium for buoyancy, and by contracting launches to people who already live in optimal locations, per-launch costs are kept low.
Economics aside, can commercial wireless service really be provided from high altitude balloons? The most obvious technical question, and perhaps the one easiest to answer, concerns range. Won’t path loss be excessive over the 120 km maximum service range anticipated for SkySite service? Not really.
Path Loss
Typical cellular networks operate fine with a path loss between base station and handset of up to 150 dB. At 850 MHz, free space path loss over 120 km is only about 133 dB. The key term here is “free space.” In most wireless networks, base station path loss is mainly provided by urban “clutter” and building penetration. From 30 km up, a SkySite will be line-of-sight to pretty much everyplace within its service area. Path budgets will allow only a few decibels for light building penetration, but then again there aren’t many big buildings in the sorts of areas the SkySite network is intended to serve.
It turns out that digital air interface protocols provide more problems for SkySite than path loss. The primary objective of SkySite is to provide voice service to the hundreds of millions of users with conventional GSM and CDMA 1X-RTT handsets. Normally, GSM operates to a distance of only about 35 km, limited by the maximum timing advance that can be assigned to handsets. 1X-RTT doesn’t have any such hard constraints to basic operating range but it does have timing limits that define the maximum difference in distance between a handset and any two base stations involved in a handoff. Space Data believes that it has viable work-arounds for these limitations that will be transparent to conventional handsets and their users.
So, if the range issue is nailed down, how about power? Typical terrestrial base stations consume many kilowatts. Just the transmitted power alone usually accounts for hundreds of watts. How will Space Data provide that kind of power given a total payload weight of around 12 lbs? First, each SkySite will operate on only a few channels. Remember, it’s only serving areas of extremely thin traffic beyond the reach of terrestrial networks. At altitudes, there will be essentially zero relative wind, so light weight, high gain transmit antennas can be realized using foil attached to thin Mylar sheets. This serves to reduce required power amplifier output to a watt or so per channel, and at such output levels amplifier efficiency can be reasonably high. At any rate, Space Data feels confident that it can find enough juice to keep each SkySite operating for a day or two.
What about backhaul? Fixed ground stations with high gain dish antennas that track each SkySite will allow high bandwidth links to operate at low power.
It’s clear that Space Data will have to overcome a lot of challenges to succeed with its SkySite system, but so far its track record is pretty impressive. It already offers balloon-based telemetry data services over a huge swath of the southwestern United States, and it won a $49 million Air Force contract for tactical battlefield communications over vast areas. Providing cellular-style phone service will be much more complex, but Space Data is confident it can get there.
Drucker is president of Drucker Associates. He may be contacted at edrucker@drucker-associates.com.