Wireless Connectivity: ISM, WLAN, WMAN, Bluetooth et al. 7

Published:2011/8/18 2:49:00 Author:Amy From:SeekIC

Stefan Tauschek

Spatial multiplexing

Antenna diversity is aimed exclusively at improving results at the receiver end, and it can be used even if the transmitter end has only one antenna. However, if multi­ple antennas are used al both ends of the path, even more refined technical tricks can be used to improve the air interface.

It’s clear that when radio waves in the centimeter-wave range are used for data communications inside a build­ing, closely adjacent locations con have quite different data transfer characteristics. This knowledge can be used to cleverly transform the problem of multipath reception into a major benefit. If al least two antennas are used for both transmission and reception, it can be assumed that the two transmit-receive antenna pairs do not use the same path through the air, but instead two different paths. In theory, these two transmit-receive paths con actually be regarded as two separate channels, despite the fact that they use the same carrier frequency, so they con transfer different information and thus double the bandwidth {Figure 11). In practice, or course, the two paths do not behave exactly like two mutually isolated channels. Nevertheless, a significant increase in bandwidth can be obtained using this technique, which is called ’spatial multiplexing’.



Combination therapy

There’s a very good chance that MIMO will be included in the upcoming 802.11 WLAN standard, although the standard is unlikely to be approved before 2007. Never­theless, manufacturers of WLAN chip sets, such as Atheras, are already anticipating the new standard with new generations of ICs incorporating ’pre-802.11n’ features. For instance, the AR5005VA chip set has all the technical capabilities necessary to allow it to be made 802.11-compliont by simply updating the driver software after the standard has been firmly defined. The chip set has Iwo RF front ends, so the MIMO tricks described above can be implemented using two antennas for the transmit and receive functions. A data rate of more than 100 Mbit/s can already be achieved using the AR5005VA, which is enough to allow even HDTV video to be transferred over a wireless network.

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