March 7th, 2008 by Robert Heath

It is clear that MIMO is having a huge impact on wireless communication. If you are reading this, you probably already realize that MIMO can mean different things to different people. Traditionally, MIMO implied spatial multiplexing, but nowadays it may mean any number of different transmission techniques including space-time block coding, spatial multiplexing, hybrid approaches, closed loop approaches, etc. In this first of a series of posts I summarize current MIMO modes of operation in several different standards. The idea is to highlight the diversity of MIMO concepts currently being developed, no pun intended.

This post focuses on IEEE 802.11n, the next generation wireless local area networking (LAN) standard. Near standards compliant IEEE 802.11n products are already available, though beware of potential compatibility issues with the final standard. It makes sense that wireless LAN was the first commercial realization of MIMO, even though work on IEEE 802.11n started much later than other standards. The reason is that IEEE 802.11n just considers the physical and MAC layers in a relatively simple network configuration. It is also easier to incorporate multiple antennas on laptops and PCs versus small handsets.

Now I summarize some of the key MIMO features in IEEE 802.11n. There were several advancements in the MAC as well but I will focus on physical layer techniques. Thanks due to Bob Daniels who is diligently following the standard’s development.

Number of Antennas: Two transmit and two receive chains, called (2×2 operation) is mandatory with up to four transmit and four receive chains (4×4 operation) being optional. Up to eight antennas can be supported using antenna selection.

Multiplexing Modes Supported: Requires support for one and two spatial streams, with optional support for three and four streams. Naturally supporting more streams requires more antennas.

Diversity Modes Supported: Cyclic delay diversity is required. This is a very effective transmit diversity technique that uses cyclic shifts to make the spatial channel look equivalently like a channel with extra multi-path. Several optional modes are possible including beamforming, compressed beamforming (a type of limited feedback), space-time block coding (across two OFDM symbols), and antenna selection.

Hybrid Modes Supported: Several different hybrid modes (where there may be two or more streams and three or more antennas) are optionally supported. For example, 2 spatial streams with one stream space-time block coded for a total of three space-time-streams, two spatial streams with both streams space-time block coded for a total of four space-time-streams, three spatial streams with one stream space-time block coded for a total of four space-time-streams; and virtually any linear precoding technique.

Closed Loop MIMO Operation: Several different channel-aware precoding methods are supported in the standard. For example, limited feedback of channel state information is supported with varying levels of quantization. Implicit feedback is supported where channel reciprocity and calibration are used to derive forward channel state information from the reverse channel. Beamforming feedback with or without clustering and varying levels of quantization is supported along with compressed beamforming feedback, along with recommended modulation and coding scheme feedback (allowing for adaptive modulation decisions to be made at the receiver).