MIMO OFDM: Space Time Coding & Spatial Multiplexing
Our information society today is marked by an increasing need for mobility and permanent accessibility. At the same time, the desire for ever higher data transfer rates is also increasing. The greatest current challenge for future wireless communication systems is therefore to provide broadband mobile (or at least portable) data access with a quality of service (QoS) as high as possible.
The key role in mobile radio technologies is played by the effective bandwidth where only one limited bandwidth for data transmission is always available to each wireless service. However, spectrum is now a very scant commodity that should be exploited as efficiently as possible. The desired increase in communication speed requires the consistent utilization of the degrees of freedom on offer. This can be achieved through various multiple access (MA) methods.
MIMO Systems (Multiple Input Multiple Output)
The following resources are available: time, frequency, code and space. While the first three resources find application in Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA) and Code Division Multiple Access (CDMA) – often even in combination – in existing systems, the previously little noticed resource of space is now coming to the fore for the first time (Space Division Multiple Access – SDMA). It is accessed through the use of multiple antennas – both on the sender side and the receiving side. For years researchers around the world have been intently searching for these multiple-antenna systems, called MIMO systems (Multiple Input Multiple Output). "In" and "out" always refer to the transmission channel. MI stands for multiple sending antennas and MO accordingly for multiple receiving antennas.
MIMO and OFDM (Orthogonal Frequency Division Multiplexing)
OFDM is used in numerous wireless transmission standards nowadays (DAB, DVB-T, WiMAX IEEE 802.16, ADSL, WLAN IEEE 802.11a/g and the powerline technologies Home Plug AV or DS2 200, aka "Home Bone"). The OFDM modulation transforms a broadband, frequency-selective channel into a multiplicity of parallel narrow-band single channels. It has been shown that OFDM can be favourably combined with multiple antennas on the sending side as well as the receiving side to increase diversity gain and/or transmission capacity in time-varying and frequency-selective channels. The result are the MIMO OFDM systems now crowding the market.
Further information on WLAN Draft-N Tests
