Flash-OFDM 'hops' wireless data communications into the main stream
By Laurie Ann Toupin
High speed. High capacity. Interactive. Affordable. Scaleable.
Such are the claims made by a new company, Flarion Technologies Inc., about their wireless data communication technology, flash-OFDM (Orthogonal Frequency Division Multiplexing), which forms the heart of the company's RadioRouter Networks.
Lucent Technologies launched Flarion (Bedminster, NJ) in February 2000 in order to commercialize flash-OFDM, an air interface technology and a network architecture developed at Bell Labs by Rajiv Laroia, the founder and CTO of Flarion.
"Ours is a new application of an existing technology," says Ray Dolan, president and CEO of Flarion. OFDM, a 20-year-old robust signal-processing scheme, is becoming increasing popular in the wireless world because of its ability to handle impairments such as noise, fading and multi-path interference. OFDM divides the available bandwidth into a number of discrete tones (frequencies) and users signal on different tones. For example, the Flarion system is designed for 5 MHz channels. Four hundred discrete tones are assigned to each channel, each at a slightly different frequency. OFDM makes the tones orthogonal to each other.
Using this signal-processing scheme as their base, Laroia and associates turned OFDM into a spread spectrum system by adding, "flash." "Flash refers to fast hopping between tones (frequencies)," says Laroia. In a cellular multiple-access environment, users are assigned a certain number of tones. "In flash-OFDM, we ‘hop' these users tones to randomize interference," says Laroia. "For example, say that a given user in a given time slot uses a certain tone. He will use a completely different tone in the next time slot. It is a pseudo random selection of tones from an available set."
There are inherent advantages to a spread spectrum technology in cellular environments, says Laroia. A spread spectrum system shares the available spectrum with all other cells and uses coding to deal with interference from other cells. For example, transmission by users in neighboring cells becomes interference in your cell. With flash-OFDM this interference between adjacent cells is averaged and its effect minimized. "This is important," Laroia says, "since spectrum is a precious resource."
In addition, spread spectrum provides the benefit of frequency diversity. Cellular channels have a tendency to fade out. Typically, the entire available spectrum does not fade but, for example, 2 megahertz of a 5-megahertz band may fade. Flash-OFDM quickly jumps the signal among the various tones within the 5 Mhz band. This converts the signal into a fast-moving, psuedo-random spread spectrum signal where only part of the signal fades but not all. With the part that gets through, users can still communicate.
Scalability
Today, erecting a cellular phone tower in a town or on a piece of property in the middle of no where can be quite controversial. And as wireless technology becomes ubiquitous, more towers will be essential. "You can't put a 50 meter tower every ½ mile. Nobody would allow this and it would not be economical to do so," says Laroia. Flarion developed the RadioRouter network so that each basestation can be deployed in form of a short monopole or attached to the side of buildings or other already existing structures (See Diagram). "This is ideal for dense network deployments with high data capacity," says Laroia. "Operators only need to install and activate a RadioRouter within their existing network, without the need for frequency or timing synchronization between the basestations, as required by other technologies."
Synchronization is not necessary, continues Laroia, because
Flarion technology uses OFDM as the air-interface and the enhancements made to it allow basestations to be independent of each other. "It is also one of the enabling factors that allows us to shift most of the networking intelligence from within the network (in traditional wireless systems) into the terminal of flash-OFDM based RadioRouter networks.
Appropriate for wireless data
Flarion's enhancements to the flash-OFDM network go beyond independent basestation operation. "We started with a clean slate and asked ourselves what was the best way to carry real-time, interactive data," says Laroia. This approach is important as data and voice communication requirements are not the same, he continues. Voice is a more of a continuous low bandwidth application (8 – 10 kBps) that tolerates occasional errors. Data on the other hand is oftentimes "bursty," but does not tolerate any errors, making retransmission over the airlink to correct errors a must for any efficient wireless data system.
Not only was the focus on data important says Laroia, but economics were also at the heart of the design. "The popularity of transmitting data is intimately tied to its cost," he says. If people worry about how much a transmission costs whenever they turn their device on, they will never use it."
Flarion's RadioRouter network system reflects this mentality. Third generation design is supposed to maximize bits per hertz per second per basestation. "Our philosophy is to maximize bits per second per hertz per dollar spent on infrastructure," says Laroia. "We wanted to minimize the cost of the infrastructure to the service providers to enable them to reduce the cost to the end user and enable broadband wireless data market to take off."
The Flarion technology supports high data rates at low latencies (minimized packet and delay losses) over a distributed all-IP wireless network. Flarion's Dolan foresees end-users with an always-on wireless IP connection that will allow real-time interactive and multimedia applications in a mobile environment.
Flash-OFDM looks for a niche
Presently, Flarion is in the throws of showing their technology to industry analysts. Ira Brodsky, president of Datacomm Research Company, says, "I don't know enough about them. Qualcomm [however] showed that a small company with a radically different solution can win. But it takes years of effort. Flarion must show that their technology offers significant competitive advantages. They must win over operators and manufacturers one by one. If they are going to succeed, they must either build bridges to their technology from CDMA or play in a different (and therefore smaller) space."
Flash-OFDM is not directly compatible with either CDMA or GSM/TDMA from the technical perspective, says Lars Johnsson, Director of Business Development for Flarion. "But for the wireless network operator who has investments in CDMA/TDMA and is operating voice networks, our flash-OFDM would allow him to enter the wireless data market with a very effective data network for mobile broadband."
This is like the airline industry, Johnsson continues. Initially people and cargo were transported on the same planes until air-cargo traffic exploded and required a dedicated system of cargo planes and routes that have different needs than people-planes. This analogy reflects the intersection of the wireless industry as it tries to address the emerging data market.
"As much as the industry seems to coalesce around two technology standards for third generation networks," says Dolan, "the industry is still looking for the ultimate solution with respect to high speed data." Technical trials during the fourth quarter of 2000 should help network operators determine if flash-OFDM could be that solution. Dolan expects Flarion's products to be on the market during the course of 2001.