ECE researchers' innovations improve wireless performance
Electrical and Computer Engineering Professor Akbar Sayeed and graduate student Ke Liu have developed novel iterative algorithms that significantly simplify signal processing and improve performance of antenna arrays used in wireless communications systems.
It was long thought that each wireless transmission required a separate frequency. Then in 1998, Lucent Technologies' Bell Labs introduced BLAST (Bell Labs Layered Space-Time) technology. Multiple antenna systems coupled with signal processing allowed multiple transmissions to occupy the same frequency. Employed at a communications base station, antenna arrays for both transmitting and receiving provide increased capacity and reliability in communication between the tower and the user.
In an array of four transmission and four reception antennas, each transmission antenna can send a separate signal. But on the receiving end, all four receiving antennas get a copy of an individual signal. To separate the four interfering copies at the receivers and balance the signal to noise ratio, BLAST uses a sequential signal processing technique where the first antenna signal is estimated, computed and then applied to the estimation of the second signal, which is computed and applied to the third and so on. Each estimated signal is progressively more stabilized.
Sayeed and Liu identified an opportunity for improvement in this process. The BLAST system's first signal estimation is based on a very random signal and never takes full advantage of all four signal copies. In addition, if the first signal estimation were wrong, the error would propagate rather than resolve the signal, creating a bottleneck.
By adding an iteration to the process, Sayeed and Liu found they could dramatically improve performance. Their solution takes the fourth signal estimate and applies it in reverse order.
"The advantage in this case is that we are starting off with the strongest, most stabilized estimate off the transmitted signal," says Sayeed. "Now your initial estimates are much better and you can actually improve the performance by running a reverse feedback loop. It almost sounds like magic but we see improvements in performance like five or six decibels, sometimes more."
The teams says the technique could be applied to existing systems with relatively few modifications of original equipment.
Managing multipath channels
In a related patent, Sayeed, Electrical and Computer Engineering Professor Barry Van Veen and graduate student Eko Onggosanusi have invented a method of characterizing and managing the multiple channel paths generated when antenna arrays are used at the transmitter and/or receiver.
The technology effectively partitions the overall channel into a series of nonoverlapping subchannels. Van Veen says it's as if a large pipe were filled with a number of smaller pipes of varying sizes. Each smaller pipe has an individual capacity and characteristic that can be paired with the needs of the signal to be sent. Most importantly, the subchannels do not interfere with one another.
Consider the nine pairings possible in an array of three transmission antennas and three receiver antennas. The inherent distortion between any pair of antennas is different from every other pair. The distortion is often due to reflections off structures located between the receiver and transmitter, which cause different components of the signal to arrive at different times. Varying types and levels of distortion mean each channel will have varying transmission capabilities.
The team's invention allows all of these factors to be considered in choosing over which subchannel(s) information should be sent. By weighing the options, information can be sent using the least transmission power to get the best error performance at the other end or to maximize data rate.
"In reality, it's not that I'm going to use any one antenna on either side," says Van Veen. "I use all three to form a new synthetic transmit antenna that optimally couples into the distortion of the channel by weighting the physical antennas differently. At the physical antennas of the receiver, I then take a weighted combination of the signals to optimally capture the energy in the transmitted signal. So there are actually infinite combinations that one could achieve with different weightings. Because we've identified the optimal sets of weightings that result in noninterfering subchannels, we can send multiple messages in parallel and therefore increase the overall data rate while using minimum transmit power."