MIMO and OFDM

MIMO is applicable to all kinds of wireless communication technologies. However, the combination of MIMO and OFDM (Orthogonal Frequency Division Multiplex) has the following advantages. OFDM is adapted for multi-path propagation in wireless systems. The length of the OFDM-frames is determined by the Guard Interval (GI). This Gurad Interval restricts the maximum path delay and therefore the expansion of the network area. MIMO also uses the multi-path propagation.

OFDM is a wideband system with many narrowband sub-carriers. The mathematical MIMO channel model is based on a narrow band non-frequency selective channel. The latter is supported by OFDM as well. Fading effects in wideband systems normally occur only at particular frequencies and interfere with few sub-carriers. The data is spread over all carriers, so that only a small amount of bits get lost, and these can be
repaired by a forward error correction (FEC). OFDM provides a robust multi-path system suitable for MIMO. At the same time OFDM provides high spectral efficiency and a degree of freedom in spreading the time dimension of Space-Time Block Codes over several sub-carriers. This results in a stronger system based on the principle described previously

MIMO Standards:

Table 1 gives an overview of all current MIMO standards and their technologies. It is clear to see, that with the exception of 3GPP Release 7, all standards work with OFDM. The advantages of OFDM can obviously be linked to MIMO.

Table 1 MIMO Standards and the corresponding technology :

Standard Technology
WLAN 802.11n OFDM
WiMAX 802.16-2004 OFDM/OFDMA
WiMAX 802.16e OFDMA
3GPP Release 7 WCDMA
3GPP Release 8 (LTE) OFDMA
802.20 OFDM
802.22 OFDM