Spatial Multiplexing

Spatial multiplexing is a transmission technique in MIMO wireless communication to transmit independent and separately encoded data signals, so-called streams, from each of the multiple transmit antennas. Therefore, the space dimension is reused, or multiplexed, more than one time.

In spatial multiplexing different signals or data bits are transmitted through several independent (spatial) communication channels by multiple antennas and at the same time the receiving side also use multiple antennas for receiving signals-this way increase the date transmission rate which is in direct proportion to the number of antennas used for both transmission and receiving purpose. The higher the number of antennas, the higher the number of data transmission rate.

Multiple antennas are used to provide diversity gain (receive and transmit diversity )and increase the reliability of wireless links. With channel knowledge at the transmitter, multiple transmit antennas can also provide a power gain via transmit beam-forming. Multiple transmit antennas are used to induce channel variations, which can then be exploited by opportunistic communication techniques. The scheme can be interpreted as opportunistic beam-forming and provides a power gain as well.

Spatial multiplexing requires MIMO antenna configuration. In spatial multiplexing, a high rate signal is split into multiple lower rate streams and each stream is transmitted from a different transmit antenna in the same frequency channel. If these signals arrive at the receiver antenna array with sufficiently different spatial signatures, the receiver can separate these streams, creating parallel channels for free. Spatial multiplexing is a very powerful technique for increasing channel capacity at higher Signal to Noise Ratio (SNR). The maximum number of spatial streams is limited by the lesser in the number of antennas at the transmitter or receiver. Spatial multiplexing can be used with or without transmit channel knowledge.