February 25th, 2008 by Robert Heath
It’s rare that you see the term MIMO these days without OFDM (orthogonal frequency division multiplexing) following close behind. I dare say that MIMO-OFDM, as it’s called, is the most prevalent embodiment of MIMO. Several systems use MIMO-OFDM including IEEE 802.11n, IEEE 802.16, as well as the forthcoming 3GPP2 UMB (ultra mobile broadband) and 3GPP LTE (long term evolution). Why is there a marriage between MIMO-OFDM?
To understand, some background in OFDM is in order. OFDM is a digital modulation technique, first proposed in the 1960’s (in an analog form) and developed in the 1980’s and early 1990’s into the form most commonly used today. The main application of OFDM is to simplify equalization of the multi-path propagation channel at the receiver. Multi-path arises from multiple different propagation paths between the transmitter and receiver, each with a different path delay, phase shift, and attenuation. This creates distortion in the received signal. Wide bandwidth signals are especially susceptible to multi-path since their large bandwidth implies smaller sample spacing. Consequently, using wider bandwidth signals (typically 1MHz or more, but this depends on the coherence bandwidth of the channel) will require some form of equalization at the receiver.
Just like in audio systems, an equalizer is used to restore attenuated frequency components. In systems that do not use OFDM, some form of inverse convolution (a type of filter) or nonlinear detector is required for equalization. OFDM, through a specially constructed transmit signal involving a cyclic prefix, is able to simplify equalization by operating directly in the frequency domain.
MIMO is unique in that it can exploit multi-path to enhance system capacity. Unfortunately, this capacity benefit comes at the expense of more challenging equalization. In a MIMO system, the receiver must deal not only with multi-path but also co-antenna interference, which is created from the transmissions from all transmit antennas. Thus equalization in MIMO systems is even more challenging than in single antenna systems. Thus MIMO benefits even more from efficient equalization techniques like OFDM. This is the core reason for the union between MIMO-OFDM in most practical systems.
There are other more technical benefits in the connection. Perhaps the most important is that efficient rate allocation is possible through the use of adaptive modulation and coding. It is also possible to share the bandwidth more efficiently through the use of OFDMA (A for access), where users are allocated different parts of the spectrum but the equalization benefits are still obtained. There are also challenges, most inherited from OFDM. These include tighter synchronization requirements and a high peak-to-average-power-ratio. Alternatives such as single carrier frequency domain equalization (SC-FDE) have been suggested to solve this problem but has yet to see commercial success. Thus it appears that MIMO-OFDM will continue its dominance, at least for the next five years.