When David Tse's mobile phone drops out in the middle of a conversation, he doesn't curse his wireless carrier. According to the Department of Electrical Engineering and Computer Sciences professor, this perceived problem, called channel fading, is the key to building high-bandwidth wireless networks. Indeed, Tse's "opportunistic communication" technology is at the core of Qualcomm's High Data Rate (1xEV) wireless network set to launch commercially at this year's World Cup in Korea.
"Wireless communication research is undergoing a complete change in viewpoint," Tse says.
A wireless frequency band has a limited amount of bandwidth, or space available for data transfer. Within this band are channels, essentially communications paths wide enough to permit a transmission to pass through. Due to reflections off objects in the environment, there are multiple signal paths between the transmitter and receiver. It's the constructive and destructive interference between these signal paths that cause channels to fade.
And how can channel fading possibly be reframed as a feature instead of a bug? The trick, Tse says, is "to see the glass as half full rather than half empty."
"In a large system with users fading independently, there is likely to always be a user with a very good channel," Tse explains. "The total throughput can be maximized by always serving the user with the strongest channel."
Simply put, a user should only be able to send and receive transmissions when he or she has a good signal. If the channel is fading, the bandwidth is best diverted to someone with a better signal.
To exploit channel fluctuations, each wireless device is equipped to measure its own signal strength and transmit that information to a base station. (Data such as this can be "prioritized" so that even with a weak signal the signal measurement gets through.) At the base station, this channel strength data is processed and Tse's innovative scheduling algorithm acts as a traffic cop to determine when data should be transmitted to each user. To be fair, users with a continuously strong signal are prevented from hogging all the resources - the algorithm schedules a user when the channel quality is high relative to the user's own average. This scheduling algorithm is already at work on the downlink of Qualcomm's High Data Rate wireless network helping enable up to 2.4 Mbps of bandwidth in a standard channel.
Tse's most recent innovation is even more surprising. When the channels are clear, he adds some chaos. To create channel fading when there is none, Tse introduces random interference into the system causing the channels to fluctuate a lot and often. This is done by adding multiple antennas at the base station transmitting the same signal but with the relative phase of the radio waves varying psuedo-randomly over time.
"You're increasing your capacity basically by creating opportunities when nautre gives you none," Tse says.
While Tse's "opportunistic communication" theories are beginning to yield real world applications, he's also looking at other novel approaches to wireless networking. For instance, he's exploring ad-hoc networking where multiple wireless devices create their own peer-to-peer networks using each other as base stations.
"My laptop may say to your laptop 'hey, I'm not near a base station so will you send this data along for me?'" Tse explains. Or, he adds, a group of mobile phones could create their own ad-hoc wireless network in an emergency situation where, perhaps, cellular base stations are down.
"You could even imagine a bunch of nodes cooperating to form an array of antennas to create and exploit channel fading," he says.
David Tse's Home Page
Qualcomm High Data
Rate Network
