Future of Wireless Systems Lies with Next-Gen Technology called MIMO, Say UCLA Researchers

Date: June 11, 2004
Contact: Chris Sutton ( chris@ea.ucla.edu )
Phone: 310-206-0540

In homes and offices nationwide demand is growing for reliable high speed connections that can support high performance wireless communications, from faster multimedia networks to more reliable cellular phone connections. Researchers in one UCLA lab, led by electrical engineering professor Babak Daneshrad, are working on what could be the best solution yet; a next generation wireless networking technology known as MIMO.

Engineering students Stephan Lang (seated) and Raghu Rao talk shop with electrical engineering professor Babak Daneshrad.

MIMO, or multiple input, multiple output technology, is a communications technique that uses multiple antennas to send and receive wireless signals, allowing more data to be transmitted without increasing bandwidth. This is accomplished by communicating along parallel spatial channels at the same time and in the same frequency. The result is clearer wireless video conferencing, more reliable broadband Internet connections and crisper cellular phone calls.

“Traditionally, when you wanted to send more data, you used more bandwidth,” explained Daneshrad, who operates his lab in the UCLA Henry Samueli School of Engineering and Applied Science. “But spectrum is an expensive, limited resource. MIMO systems allow operators to provide broadband services within the current spectrum that they have purchased from the FCC.”

All wireless devices use a particular part of the radio spectrum. Air traffic control radars, for example, operate between 960 and 1,215 megahertz, and cell phones between 824 and 849 megahertz. As a growing number of wireless devices enter the consumer market, the spectrum becomes more congested every year. MIMO has the potential to expand radio capacity and relieve the burden on existing bandwidth.

In the late 1990s, theoreticians at Lucent’s Bell Labs and Stanford University discovered that the random scattering radio signals experience as they travel from transmitter to receiver can be exploited to improve transmission accuracy. Individual pieces of data could be sent out on unique paths, overlaid on each other within the available bandwidth and then separated, or decoded, once the data arrived at the receiver. Since several transmissions could occupy the same frequency band, the theorists learned, spectrum could be used very efficiently and with greater capacity than using single channels.

Daneshrad, who designed high-speed indoor and outdoor wireless data communications systems at AT&T Bell Laboratories before joining the UCLA faculty in 1996, says that while the concept of MIMO has been proven through theoretical and simulation-based studies, the real challenge is turning the concept into a reality.

“You need to look at how to implement it, build it efficiently and cost effectively, within a reasonable power budget,” said Daneshrad.

Rapid progress in MIMO VLSI research has been somewhat slowed, according to Daneshrad, by a traditional disconnect between the digital IC researchers, who are primarily interested in working at the transistor level, and the communications experts, who are most interested in high level algorithmic definition and evaluation.

“That disconnect doesn’t exist at UCLA,” said Daneshrad. “We work closely with the Integrated Circuits and Systems Laboratory as well as the Communication Systems Laboratory. The underlying theme in our work is to successfully bridge the gap between theoretical and experimental aspects of wireless data communications.”

This close collaboration has produced tangible benefits. While most universities and research labs have theoreticians who design simulations or develop testbeds, UCLA is one of the few research institutions also working on real time VLSI circuit development. The main reason for the scant attention elsewhere, explains Daneshrad, is that the processing power required to realistically test the theories behind MIMO is “horrendous.”

“To do what we want to do would take about 20 high-end DSP chips. A cell phone runs on only one low-end DSP chip,” said Daneshrad. “So it is roughly 200 times more processing power than a cell phone.”

Daneshrad and his students are designing and testing their own ASICs (Application-Specific Integrated Circuits), which they hope will provide the processing power needed to make MIMO a viable technology for next generation mobile communication systems.

“Our chips have demonstrated that all the needed advanced algorithms can be incorporated in a highly power efficient ASIC solution,” said Daneshrad.

The key is to design ASIC architectures that minimize power consumption while supporting high data rates and while being sufficiently agile to enable MIMO communications in multiple standards. And Daneshrad is not interested in a solution that only works in a lab; he argues MIMO must be tested with today’s mobile technology devices in mind.

“You can use a whole suitcase of FPGA chips, use 20 fans to cool them and test out the algorithms, but we want a more realistic form factor. It has fit to into a portable device,” said Daneshrad.

In addition to low-power chip design, MIMO must be coupled with a modulation scheme such as QAM, DSSS-CDMA or OFDM, which stands for Orthogonal Frequency Division Multiplexing. OFDM has become the modulation of choice for next generation broadband data communications.

When a person speaks into a cell phone, her analog voice data is converted into a digital data – a series of 1s and 0s – and then compressed so it takes up less transmission space. Sophisticated modulation schemes are needed to convert analog information into digital, compress it, and then convert it back again while maintaining an acceptable level of voice quality.

Daneshrad uses OFDM, which is also commonly used in wireless LAN standards like 802.11a. The scheme operates through a divide-and-conquer approach, dividing bandwidth into many smaller subchannels which are placed arbitrarily close to each other. This makes OFDM a very efficient method for transmitting signals in multipath wireless channels.
Daneshrad is confident that MIMO technology represents the future for wireless communications systems, from next generation cellular systems to home and office multimedia networks.

“Over time, all wireless systems will go by way of MIMO, and that horizon might be in the next seven to 10 years,” said Daneshrad.