What is a Phased Array?
- A phased array is a system for focusing and triangulating the signals of any kind of propagating wave. This mainly applies to wireless technology and communications. It uses a minimum of 4 antennas and overlaps their broadcasts in specific ways so as to strengthen the signal in one direction and lets the rest dissipate into the surrounding environment. This is also applied to incoming signals. The incoming signals are timed and the relative times that each antenna received the signals is used to mathematically calculate the direction of the incoming signal. It is essentially a super precise method of triangulation.
What is 802.11b?
- 802.11b is a IEEE protocol pertaining to wireless communications in the 2.4 ghz bandrange. It is being adopted by community networking efforts because it is cheap, effective and public domain. It is capable of transferring data at 11mbps, and has ranges of up to 12 miles. A basic directional setup can be purchased for about $300. An 'access point' with omni-directional antenna will run around $500-$1000 (or more).
Why are Phased Arrays useful for wireless networking?
- 802.11b has recently been enabled with bridging, so that any device connected to the network should be able to talk to any other device connected to the network regardless of direct line of sight to the final recipient. The reason phased arrays are useful is their inherent directionality. They can track the location of any transmission made to them and transmit information in fairly specific directions. I propose that an 802.11b phased array could be built by anyone using 4 ordinary omnidirectional antennas in a specialized mounting frame, and either 4 wireless card connections to a PC running intelligently designed software, or a dedicated 'smart box' that provided the same functionality.
What are the limitations of phased arrays is wireless networking?
- precision. Omnidirectional Antennas are made cheaply and cover a large area, so there is little concern for the level of precision in the device. Also the cards may introduce random delays for various processes. It is critical for the application that the data be maintained relevant to the current time.
What would it take to implement a phased array in a wireless network?
- I envision it as being possible to require at a minimum, 4 omnidirectional antennas, 4 coaxial cables, 4 wireless network cards and a computer smart enough to be able to see all 4 and use them appropriately. Worst case is requiring a proprietary setup with 4 custom omnidirectional antennas, 4 precision coaxial cables and a custom built smartbox to run the whole shebang.
What know-how is required?
- Wireless networking inside and out. Some strong physics and math. Any other wireless technology experience may be beneficial. 802.11b protocol, and probably device implementation of that protocol. Its hard to make something work for you if you don't understand it quite well. Knowing about coaxial cable will be quite useful. Hardware, firmware, and software programing will be essential to effective implementation. And a little marketing never hurt anyone.
What can we know about 4 antennas randomly placed?
- Well my initial idea was to bounce signals between the local antennas and, using only that information, determine your arrays topology, so it didn't matter if you had them aligned perfectly, the system would adjust around them. But if the topology has to include a random, non linear section of coaxial cable, this process is a lot more difficult. I hope that there is some way to bounce a signal off the antenna from 'inside' so that the coaxial cables induced delay can be subtracted/added and thereby factored out of the equations. The other option I see is to force the user to use very precise coaxial cables of all the same length and factor out their known propagation delay.
How can this be used to make a wireless router?
- The array keeps track of client locations, and addresses. Whenever it recieves a signal it does the router thing and list the address in some sort of table. Whenever it gets a signal bound for a device in range, it sends it only to the appropriate recipient and avoids cluttering up the airwaves quite as much. A cool side benefit of having the arrays is that you can use triangulation and any baseline to measure and/or plot your network. Spiffy eh?
Brain Dump by ColinDabritz Here were a few useful links. First, a great visual example of phased arrays (shockwave) http://www.explorescience.com/activities/Activity_page.cfm?ActivityID=49 and the always useful google search: http://www.google.com/search?query=phased%20array&num=10
Commercial Phased Array Options
ArrayComm - combines adaptive antenna technology with a wireless network architecture optimized to deliver high speed Internet at an affordable price.
Adaptive Broadband - uses a rooftop transceiver with an antenna and an environmentally sealed box containing the radio, modem, MAC and Ethernet components which convert the wireless 5GHz from the cell site into the standard Ethernet delivered to the subscriber's PC.
BeamReach Networks - likes OFDM with adaptive beamforming antennas. It was given an FCC license for nationwide coverage at 2.3 Ghz. It gives operators non-line-of-sight, consumer-installable CPE, high data rates, and the ability to deploy large cells, serving a large number of customers with fewer base stations.
Flarion - plans to introduce PCMCIA modem cards for laptop computers and use IPV6. Their flash-OFDM technology, originated by Bell Labs, was developed as a means for mobile operators to offer IP-based broadband communications at low cost. It "is the first cellular link layer to be designed to truly mobilize the Internet, and is unencumbered by the architectural, protocol, and commercial legacies of the circuit-switched past". Partners include Cisco, Compaq, GTRAN, PacketVideo, Philips and others.
Iospan - developing "the first, mass-deployable fixed wireless broadband solutions that provide Non-Line-of-Sight (NLOS) delivery of high-speed residential and business IP services".