Wireless communication systems employing multiple-input and multiple-output (MIMO) provide significant benefits over single chain communication systems. A MIMO system is characterized by the use antenna arrays for the transmitter and receiver to provide either increased data throughput or increased range. In addition to the obvious benefits of using more chains to transmit more data, MIMO systems often feature improved resistance to fading or other multipath effects. Further, the use of multiple transmit antennas allows beamforming techniques to selectively produce constructive interference and boost signal strength at the receiver.
Implementations of MIMO systems include Wireless Local Area Networks (WLANs) adhering to the IEEE 802.11(n) standard. This amendment sets a target data rate of at least 100 Mbps at ranges greater than its predecessors, such as 802.11(a) or (g), a goal it seeks to achieve using MIMO technologies. However, the 802.11(n) standard also calls for continued compatibility with legacy systems. As such, an 802.11(n) receiver can be expected to receive packets conforming to multiple protocols. For example, when communicating with another 802.11(n) device, a receiver will receive high throughput packets that require the simultaneous operation of multiple receive chains. On the other hand, when communicating with a legacy device, it will be receiving packets formatted for 802.11(a) or (g) that do not provide a high throughput mode.
Another important aspect of wireless communication systems, particularly with regard to mobile or other battery powered applications, is efficiency. The performance advantages represented by the use of MIMO technologies come at the cost of increased power consumption. By maintaining multiple receive chains in an on state, the rate of power drain for a MIMO receiver can be considerably greater than a legacy receiver. The increased power consumption is most detrimental when the receiver is in search mode as it is simply waiting incoming communications. Since no data is being transmitted while the receiver is waiting for packets, the increased energy use associated with providing power to multiple receive chains is not offset by the increased throughput or performance that results when a link has been established and multiple steams of data are being exchanged.
Current strategies for reducing the power consumption of a MIMO receiver generally rely on a handshake with the transmitter. For example, one scheme involves the transmitter sending a request to send (RTS) signal to initiate a multi stream packet delivery, which is then acknowledged by a clear to send (CTS) signal from the receiver. As will be appreciated, this approach reduces throughput by allocating bandwidth to the handshake process. Further, typical implementations of the scheme result in the receiver remaining in multiple chain mode for a given period of time after receipt of the multi stream RTS.
Therefore, it would be desirable to provide systems and methods for wireless communication that allow a single receive chain to be employed to search for transmitted packets and multiple receive chains to be employed when receiving such packets. In particular, it would be desirable to provide such systems and methods that switch from single chain operation to multiple chain operation as a transmitted packet is being received. It would also be desirable to provide these functions with requiring a handshake between the transmitter and receiver. This invention accomplishes these and other goals.