Wireless Net may require 'smart antennas'
WEST LAFAYETTE, Ind. Just as people hear better with two ears than with one, future wireless communications devices may have two or more antennas so they can outperform conventional, single-antenna versions.
These "smart antennas," when combined with sophisticated signal processing techniques, may be especially critical in preventing Internet traffic jams as an increasing number of people use wireless devices to download files from the Web. The use of multiple antennas may enable a new generation of cellular communications equipment to better access the Internet and download large amounts of data, including video files, says Michael Zoltowski, a professor in Purdue University's School of Electrical and Computer Engineering.
In the United States, the number of people downloading data with wireless devices is expected to surge from the current 3 percent of the online population to 78 percent over the next year, according to Cap Gemini America Inc., an information technology and management consulting service.
The performance of wireless devices, such as cell phones and laptop computers, is plagued by interference. But equipping future devices with two or more antennas would drastically reduce the interference, increasing reception accuracy by as much as 100 times and enabling three times as many wireless users to operate within the same frequency band. The antennas are referred to as smart because they are able to reject the interference and compensate for the "multipath effects" caused by signals reflecting off buildings and other structures, Zoltowski says.
"The problem will be when numerous users want to download from the Web through wireless links at the same time," he says." The interference created will choke the wireless Net connection."
Texas Instruments Inc., which is partially funding research at Purdue to develop the technology, is testing a prototype cell phone that has two antennas. The second antenna is a patch-like strip instead of the standard whip antenna. A similar setup might also be used for laptop computers.
"We want a user to be able to fire up the laptop and, while in a car or on a train, with no wire connections, be able to download information from the Web," Zoltowski says. "That's the goal of the next generation of cellular communication systems."
Cellular communications depend on a series of "base stations" that provide service to separate regions called cells, which are arranged in a sort of honeycomb pattern spanning large geographical areas. The performance of these systems is hindered by two types of interference that foul up a widely used technique critical to the economical functioning of cellular communications. That technique, called code division multiple access commonly referred to in the wireless business as CDMA makes it possible for many users to operate on the same frequency band at the same time.
One type of interference is caused by the multipath effects that result when signals bounce off buildings or mountains on their way to and from cellular base stations. For CDMA systems to work without interference, the signals for numerous users must be transmitted in a precise sequence with exact timing. But delayed echoes of the signals are created when they bounce off objects, fouling up this delicate sequence and causing them to interfere with each other. It's a problem called "multi-user access interference." Having more than one antenna would help correct the problem.
The other type of interference in CDMA systems occurs while the user approaches the boundary of two adjacent cells and has to be "handed off" from one base station to the next.
"When I am at the edge of a cell, two base stations are talking to me simultaneously," Zoltowski says. "While I am attempting to listen to one of them, the other one is interfering. Having two antennas allows the receiver to distinguish between the two base station signals because they are arriving from different directions, in much the same way that having two ears gives us the ability to determine from which direction sound is coming."
The performance of wireless systems could be further improved by switching from the conventional technology used to deal with multipath effects, referred to as a "Rake receiver," to sophisticated techniques that attempt to restore the delicate timing and sequence of the codes that are used to transmit data. Using these techniques, called "space-time equalization," and equipping devices with more than one antenna, would increase the number of users able to operate simultaneously in each frequency band.
Theoretically, CDMA should be able to accommodate up to 64 users in each band.
"But the reality is, because of these kinds of interference effects, they can only allow about 20 users in a given band, or, at most, a third of the potential number," Zoltowski says.
Computer simulations have shown that using two antennas and space-time equalization would enable the simultaneous use of all 64 channel codes in each frequency band.
"Just for one user to download video files even close to real time requires a large bandwidth for that user," Zoltowski says. "Given limited spectrum allocation by the Federal Communications Commission, the key technological challenge is how to deal with many users trying to download information from the Web wirelessly at the same time in the same cell or geographical area."
Having more than one antenna also would increase the accuracy with which wireless systems receive the codes. The accuracy can be increased by about 100 times when two antennas are used instead of one. However, adding more antennas poses new challenges, because they will increase power consumption for portable equipment.
Zoltowski says some of that increased power consumption will be offset by power savings that accrue because of the reduced interference.
The research was funded by the National Science Foundation and by Texas Instruments' Digital Signal Processing University Research Program.
Source: Michael Zoltowski, (765) 494-3512, firstname.lastname@example.org
Writer: Emil Venere, (765) 494-4709, email@example.com
Purdue News Service: (765) 494-2096; firstname.lastname@example.org
NOTE TO JOURNALISTS: An electronic or hard copy of the research paper referred to in this news release is available from Emil Venere at the Purdue University News Service, (765) 494-4709, email@example.com
Simple MMSE Equalizers for CDMA Downlink to Restore Chip Sequence: Comparison to Zero-Forcing and Rake
Thomas P. Krauss (graduate student), Michael D. Zoltowski and Geert Leus (Research Assistant)
This work focuses on the forward link in a CDMA based multi-user communication system experiencing frequency dependent multi-path fading. The mobile handset is assumed to have two antennas, either spatially separated or having diverse polarizations. Previous work on linear equalizers to restore orthogonality of the Walsh-Hadamard channel codes in this scenario led to a comparison of zero-forcing (ZF) equalizers and the RAKE receiver. In this paper a chip-rate MMSE equalizer is derived that minimizes the MSE between the synchronous "sum signal" of all the users from a given base station and the equalized chip sequence; this equalizer is followed by correlation with the desired user's spreading code times the base-station's long code. In this MMSE derivation, the sum of chip sequences of all the users is modeled as an i.i.d. random sequence. This leads to a "simple" MMSE equalizer that does not depend on the Walsh-Hadamard spreading codes, or the long code, currently employed at the base station. For further improvement, the "best" equalizer delay is chosen to minimize the MSE. Theoretically, the MMSE equalizer approaches the RAKE receiver at low SNR and the ZF equalizer's average performance is dominated by a very small percentage of "bad channels" with close to common zeroes, while the MMSE equalizer is resistant to this pathological noise gain problem. The end result is that the MMSE's average BER performance is dramatically better than that of both ZF and RAKE (even though only moderately better than ZF for most well behaved channels).