Advances in space-time processing techniques open up mobile apps

STP signifies the signal processing performed on a system consisting of several antenna elements, whose signals are processed adaptively in order to exploit both the spatial (space) and temporal (time) dimensions of the radio channel. STP techniques can be applied at the transmitter, the receiver or both.

When STP is applied at only one end of the link, smart antenna techniques are used. When STP is applied at both the transmitter and the receiver, multiple-input, multiple-output (MIMO) techniques are used. Smart antenna and MIMO technologies have emerged as the most promising area of research and development in wireless communications, promising to resolve the traffic capacity bottlenecks in future high-speed broadband wireless access networks.

Until recently, almost all STP technology development has been related to base stations and access points, and not to mobile handheld devices. This was because of the computationally intensive STP algorithms and the limited battery and processing capabilities of handheld mobile devices. Now, with advancements in low-power mobile device technology and ground-breaking innovation in STP techniques, this technology can be applied to mobile devices.

Smart antenna techniques are a viable means of increasing the spectrum efficiency, range and reliability of wireless networks. Systems that exploit smart antennas usually have an array of multiple antennas only at one end of the communications link — for example, at the transmit side, such as multiple-input, single-output (MISO) systems; or at the receive side, such as single-input, multiple-output (SIMO) systems. Most conventional smart antenna systems use a concept known as beamforming, where the signal energy is focused in a particular direction — usually toward the receiver — to increase the received signal-to-noise ratio (SNR). Narrow antenna beams also reduce interference, improving signal to interference noise ratio (SINR) and thus increasing spectrum efficiency. Other smart antenna schemes improve the link quality by exploiting the diversity gain offered by multiple transmit antennas.

In a multipath environment, the received power level is a random function of the user's location and, at times, fading occurs. When multiple antenna elements are used, the probability of losing the signal altogether decreases exponentially with the number of de-correlated signals (or antennas). The diversity scheme common in current SIMO (or MISO) wireless LAN (WLAN) systems uses a simple switching network to select the antenna that yields the highest SNR out of an array of two antennas.

MIMO systems can turn multipath propagation — usually a pitfall of wireless transmission — into an advantage for increasing the user's data rate.

Quality, not quantity

Diversity-based and smart antenna schemes do not increase the data rate — they simply improve the link quality and range. In contrast, the capacity of MIMO communication systems, in which an antenna array is used at both the transmitter and the receiver, far exceeds that of conventional smart antennas.

In a multipath fading environment, the transmitted signal is scattered by various objects such as walls, buildings, trees and mountains before reaching the receiver. MIMO antenna techniques, coupled with space-time processing, exploit rich scattering environments by sending independent data streams out of all the transmit antennas simultaneously and in the same frequency band.

A high-rate bit stream at the transmitter is decomposed into independent bit sequences, which are then transmitted simultaneously using multiple antennas. The signals are launched and naturally mixed together in the wireless channel as they use the same frequency spectrum. Receive and transmit antennas must be sufficiently separated in space and/or polarization to create independent propagation paths. At the receiver, after having identified the mixing channel matrix through training symbols, the individual bit streams are recombined to provide the enhanced data rate signal.

For example, a MIMO-based WLAN 802.11a system with four transmit and four receive antennas shows a fourfold capacity gain up to 216 Mbits/second (4 x 54 Mbits/s), which can be shared by multiple hotspot users. This type of MIMO technique is called spatial multiplexing (SM).

The performance improvement of MIMO systems can be applied in two ways, depending on the environmental conditions experienced by the mobile device.

On one hand, when the channel conditions and SNR are favorable, the SM technique is used to increase the data rate. In this case, the receiver expends some ( if not all, depending on the STP algorithm used) of its degrees of freedom on extracting the multiple signals rather than providing diversity against fading.

On the other hand, at farther distances, multiple transmit and receive antennas are used to provide diversity and array gain for increased range. A link adaptation algorithm, usually residing in the media-access controller (MAC) processor, provides the switching between the diversity and SM modes of operation, depending on the channel conditions. Being able to adapt to the surrounding environment is a necessity for a robust implementation.

As discussed, an M-fold (where M is the number of antennas on the transmit and receive ends) MIMO system can yield up to an M-fold capacity increase over that of a single-input, single-output (SISO) system, depending on the propagation channel conditions and STP technique implemented.

When signals are coherently combined at the receiver using techniques such as maximal ratio combining (MRC), the average received SNR increases by 10*log10(M), where M is the number of receive antennas. For a four-antenna solution, we can see a 6-decibel improvement.

Diversity gain

On a mobile device, antenna diversity can be provided by spatial, polarization and angle diversity. For spatial diversity, because the mobile device is typically surrounded by scatterers, an antenna spacing of only 6 cm at 2.4 GHz is required for low fading correlation, allowing for multiple spatial diversity antennas, particularly at higher frequencies, such as 2.4 GHz. Furthermore, dual-polarization antennas can be co-located or placed close together with low fading correlation, as can antennas with different patterns — for angle or direction diversity.

Gains from diversity are large in multipath fading channels. At a nominal 10e-3 BER, for example, second-order diversity (two diversity branches) yields 10 dB and sixth-order diversity yields a 15-dB gain over SISO systems, assuming binary phase-shift keying (BPSK) modulation, uncorrelated Rayleigh flat fading channels and MRC. This gain can be directly translated to higher data rates and/or a larger coverage area.

When MRC is used, interferers can be effectively suppressed (attenuated). However, when a strong interferer is present or when a spatial multiplexing user is to be canceled, it is desirable to use a minimum mean square error (MMSE) algorithm to minimize the error between each desired signal and its estimate, thereby maximizing SINR.

In interference-canceling MIMO systems, it is better to have more receiving antennas than transmitting antennas. For example, if the number of transmitting antennas in a MIMO system is M, the preferred number of receiving antennas is 2M in order to cancel one interfering spatial multiplexing user with M independent data streams. Each interfering multiplexing data stream is seen at the interference-canceling MIMO receiver as a separate interferer. Therefore, M antennas are used to cancel the interference, and the remaining M antennas are used to demultiplex the desired data streams and achieve diversity gains.

The first evidence of commercially successful small-form-factor multiple antenna technologies can be found in Japan with NTT Docomo's personal digital cellular (PDC) and 3G Foma handsets, as well as in current 802.11 WLAN systems that use two diversity antennas at the receiving end. As described earlier, this technique does not increase the maximum data rate nor significantly extend the range of operation. However, it is a clear sign that multiple antenna technology is finally entering the consumer product markets, and doing so in small-form-factor devices.

The most significant challenge in making STP commercially feasible is to make the technology affordable. In order to do this, both the signal processing algorithms and radio hardware must be implemented in a cost-effective manner. Solutions that can simultaneously integrate these aspects into next-generation silicon will become the key enabling technology for current and future generations of wireless systems.

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