MIMO and spatial multiplexing
Reflections and multipath wireless transmission affect the integrity and quality of radio frequency (RF). As we discussed, in the previous article, the remedy for multipath issues was to use it to produce the best-quality signal from the active combination of good-quality signals. Going a step further, it became apparent that a system could be made much more reliable by using combinational algorithms in concert with multiple radios on both the transmitting and receiving sides of the system. This is the essence of the IEEE 802.11n standard.
IEEE 802.11a/b/g are single in, single out (SISO) systems using antenna diversity to combat the effects of multipath. Multiple in, multiple out (MIMO) is a technology taking advantage of an AP’s multiple radio arrangement to use multipath and increase system reliability and performance. In newer APs, up to eight separate radios can be contained in one unit. To review, the nomenclature used for these arrangements follows the form: Tx X Rx: RC, which translates to the number of transmitters, X, which translates to the number of receivers: number of radio chains: 3X3:3 means the AP has 3 transmitters, 3 receivers, and is capable of 3 spatial streams or radio chains. It is possible to have an odd number of transmitters, an even number of receivers, and an incongruent number of radio chains; most systems use an equivalent number of transmitters and receivers and corresponding radio chains. Radio chains are the connections between a given transmitter and receiver pair. The 802.11ac standard allows up to eight radio chains.
The use of multiple radios offers the possibility of having multiple users on one AP, a technology that is being implemented in 802.11ac APs and called multiple user MIMO, or MU-MIMO. Conceivably, up to eight separate users will be able to be simultaneously connected to a given AP, rather than each waiting for an opportunity to associate and transmit in turn. This will greatly increase throughput and allow greater user density for a given AP.
A technique that takes advantage of MIMO is called spatial multiplexing, or spatial diversity multiplexing. Using this technique, an AP will use multiple radios to transmit separate segments of a message to a receiver, effectively increasing throughput. Using this technique, multiple unique streams of data can be sent between the transmitter and receiver. As noted above, the number of radio chains, or streams, corresponds to the number of unique data streams that are possible in a given AP. Spatial multiplexing is different from the technique of antenna diversity, in which multiple antennas are used to determine which will receive or transmit the best signal. This technique was used in pre-802.11n systems to deal with the effects of multipath.
A technique used to improve system reliability is called space time block coding (STBC). STBC is a type of transmit diversity that will not increase throughput, though it does improve the receiving systems’ ability to receive data at a lower signal-to-noise (SNR) ratio. STBC is used when there are more transmit radio chains than receiver chains. Essentially, the same information is sent over multiple radios to one receiver. STBC improves the receiver’s sensitivity and is used primarily between 802.11n devices; it cannot be used for communication to legacy systems.
Another transmit diversity technique is called cyclic shift diversity (CSD). CSD allows pre-802.11n devices to realize STBC benefits. CSD allows legacy frame data to be transmitted over multiple radio chains. Data is transmitted over multiple streams with each stream being subjected to a cyclic delay to avoid having the transmissions arrive at the receiver at the same time. Without CSD, the receiver would simply treat the data as multipath and discard it entirely.
Transmit beamforming (TxBF) is an advanced technique that allows an AP to use multiple transmit antennas as a phased array. A phased array is an antenna system that controls the phase and amplitude of data transmissions to an antenna array to shape the transmitted data into a directional beam. TxBF is typically used when there are more transmitters than spatial data streams. TxBF requires that the location of the receiver is generally known. The transmitter, called the Beamformer, relies on data received from the receiver, or Beamformee, to optimally adjust the phase and amplitude of the transmitted data to the greatest effect. The result is vastly improved SNR, received data amplitude, and range. A future segment describing the 802.11ac standard will contain an expanded discussion of this technique.
– Daniel E. Capano, owner and president, Diversified Technical Services Inc. of Stamford, Conn., is a certified wireless network administrator (CWNA). Edited by Chris Vavra, production editor, CFE Media, Control Engineering, firstname.lastname@example.org.
www.controleng.com/blogs has other wireless tutorials from Capano on the following topics:
Propagation revisited: Multipath
The management, control, and data planes
www.controleng.com/webcasts has wireless webcasts, some for PDH credit.
Control Engineering has a wireless page.