FOR WIRELESS OPERATORS, the introduction of bandwidth- intensive, rich media applications means that more subscribers will begin to consume increasing amounts of data packets. To conquer the resulting capacity issues, operators can acquire more spectrum channels and deploy more sites. Yet these approaches are both inefficient and costly. In "A Practical Guide to WiMAX Antennas: MIMO and Beamforming Technical Overview," Motorola asserts that mobile WiMAXin conjunction with multi-antenna techniquesprovides a capable platform on which current and future wireless-access service needs can be addressed.

According to this five-page white paper, the key is to leverage multiple antenna techniques at the base station and end-user device in addition to sophisticated signal processing. With the names of many multi-antenna techniques being thrown around as buzzwords these days, it is helpful to have the varieties explained as they are in this paper. Multiple-input multiple-output (MIMO) approaches can be used to reference any multi-antenna technology. Textbook MIMO configurations are represented as either "Open- Loop" techniques, which are most commonly used, or the "Closed-Loop" methods that are known as beamforming. With Open-Loop MIMO, the communications channel does not use explicit information regarding the propagation channel. Closed-Loop MIMO differs in that the transmitter collects information regarding the channel to optimize communications to the intended receiver.

The note also details MIMO Matrix A and Matrix B. With Matrix A, one data stream is replicated and transmitted over multiple antennas. Each redundant data stream is encoded using a mathematical algorithm, dubbed the space time block code (STBC). Such coding reduces the risk of self-interference while allowing the receiver to distinguish between multiple signals. MIMO Matrix A is mainly used to enhance system coverage.

For its part, Matrix B seeks to increase channel capacity. Here, the signal to be transmitted is split into multiple data streams. Each data stream is then transmitted from a different base-station transmit antenna, which is operating in the same time-frequency resource allocated for the receiver. In a multipath environment, the multiple signals will arrive at the receiver-antenna array with spatial signatures that allow the receiver to discern the multiple data streams.

An ideal WiMAX system employing MIMO techniques will support both of these matrixes. To further increase system coverage and capacity, some WiMAX systems rely on beamforming. By using arrays of transmit and receive antennas, beamforming antennas control the directionality and shape of the radiation pattern. Both constructive and destructive interference are leveraged. As a result, the radiation pattern is steered and formed to provide an optimal radiation pattern.

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