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Multipath expands radio frequency bandwidth

Posted: 17 Mar 2003 ?? ?Print Version ?Bookmark and Share

Keywords:rf bandwidth? wireless data service? mimo technology? wireless system? spectrum?

Managers of IT departments expect the demand for wireless data services to increase by 250 percent over the next two years, according to recent reports. The applications identified were those needed by traveling business people who require broadband access to corporate intranets while away from the office. This demonstrates a trend for enhanced data services from future mobile wireless networks.

At present, 3G networks are designed to offer up to 2Mbps data rates. Multiple-input, multiple-output (MIMO) technologies, however, can boost those data rates to 14.4Mbps and potentially even higher. This is achieved by utilizing the available spectrum in a more-efficient manner with an array of antennas at both transmitter and receiver.

Spectrum is a limited resource in any wireless system. Recently, there has been a considerable amount of work done using multiple antennas to more efficiently utilize the available spectrum. One of the simplest methods of achieving this is to use selection diversity at the receiver, where the receiver selects the antenna that has the highest receiver sensitivity. On the transmit side, multiple antennas can be used for space-time coding for transmit diversity. This provides additional margin in the communication link budget and can reduce the transmit power, increase the system range or increase network capacity.

A further step is to utilize intelligent antennas (IAs) for beam forming to increase system capacity significantly by reducing the level of interference between users. IA is already being deployed into mobile wireless-infrastructure products. Multiple-antenna communication systems are here to stay, and as the cost of integrated electronics for RF and baseband processing decreases, multiple antennas will quickly become mainstream in wireless systems.

While multiple antennas used in wireless systems to date have had an impact on system capacity and receiver sensitivity, there is a significant opportunity for wireless communications to increase throughput over a given channel.

MIMO systems use an array of transmit and receive antennas for enormous gains in spectral efficiency by exploiting a rich multipath fading environment. The systems split a single user's data stream into multiple substreams and use an array of transmit antennas to simultaneously transmit the streams into the same frequency band. While the signals would interfere with one another in a single-antenna system, it is possible to use the scattering of those signals to enhance, rather than degrade, transmission accuracy. The scattered paths use spatial signatures to create separate, parallel subchannels.

At the receiver, an array of antennas detects the multiple transmitted substreams. Using the MIMO technique, the rate of transmission is increased in proportion to the number of antennas used to transmit the signal. Furthermore, previous techniques to orthogonalize channels, like CDMA, can be laid on top of MIMO systems to ensure the bandwidth can still be a shared resource. A MIMO system can be added to a 3G system in a seamless manner, boosting the data-carrying capacity of the network without impacting the other 3G services.

Targeting wireless standards

CDMA-MIMO is currently being considered for the 3GPP and 3GPP2 mobile wireless standards. The high speed downlink packet access (Hsdpa) standard for the universal mobile telecommunications system (UMTS) uses the same 2.1GHz spectrum as regular UMTS but allows the use of higher constellations and shorting spreading codes to achieve high data rates to the portable device. A working group is currently proposing a MIMO-Hsdpa standard.

One of the major factors holding back the standardization of MIMO systems is the complexity and performance of the receiver design. The computational complexity of a MIMO receiver increases dramatically with the number of antennas in the system and the type of constellation used. In an asymmetric standard like Hsdpa, the receiver must be embedded into a portable device like a PDA or PCMCIA card. The cost and power consumption of the receiver is therefore critical to enable widespread commercial deployment.

- David Garrett & Chris Nicol

Bell Laboratories

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