MIMO stands for ‘multi-input, multi-output’, where input refers to the number of transmitter antennas and output refers to the number of receiver antennas (see figure below). SISO mode (single input, single output) is the classical mode of communication, where there is one antenna transmitting and one antenna receiving. There are also Multiple Input/Single Output systems (MISO), such as transmit or spatial diversity systems, where more than one transmitting antenna delivers data to a single receiver antenna. There are also the Single Input/Multi Output systems, which are used in traditional cellular uplinks, where the device has one transmit antenna but there is receive diversity enabled by more than one antenna port at the base station.
MIMO uses multiple transmit antennas and multiple receive antennas. Multi-antenna configurations have been around for years, but with advances in signal processing and silicon, MIMO is now economically possible in many small form factor devices such as handsets and data cards. All practical LTE devices support MIMO, as required by the 3GPP standard.
While initial LTE networks use downlink 2×2 MIMO (where there are two transmit antennas and two receive antennas), future LTE systems will use downlink 4×2 or even 4×4 MIMO and even higher dimensions of antenna configurations. LTE Advanced systems will have the nominal ability to use up to 8×8 MIMO downlink antenna configurations and up to 4×4 MIMO in the uplink, although actual device implementations supporting these modes may take some years to come to market.
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For MIMO to work, a rich scattering environment (with many different paths between transmitter and receiver) as well as a high signal-to-noise ratio (SNR) are needed. Rather than being a detriment to network performance, a multi-path environment is actually exploited by MIMO processing to increase the capacity or the coverage of the network. The key is that each path must be independent and look different to the receiver. The differences in the multipath are used to create orthogonal communication channels analogous to the orthogonal spreading codes in CDMA-based systems. In addition to being required for the higher orders of modulation, such as 16-QAM and 64-QAM, a high SNR is also required to properly exploit the MIMO wireless channel, and to allow MIMO systems to algorithmically separate the multiple spatial transmission paths, which overlap one another in frequency and time.
You can read the rest of this article in the September issue of Electronic Specifier Design by clicking here.
