Seven LTE MIMO modes for the downlink

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MIMO Mode 1

Single antenna port: Port 0: This ia analogous to most current wireless systems where a single data stream (code word) is transmitted on one antenna and received by either one (SISO – single input single output) or more antennas (SIMO – single input multiple output, received diversity).

MIMO Mode 2

Transmit diversity: This mode involves the transmission of the same information stream on multiple antennas (LTE support the option of two or four antennas). The information stream is coded differently on each of the antennas using so-called “Space Frequency Block Codes”.

Unlike the Alamouti “Space time block codes” where data sysmbols are repeated in time, SFBC data symbols over different subcarriers on each antennas.

This mode is used in LTE by default for the Common Channels as well as for control and broadcast channels. Since it is a “single-layer transmission” it does not improve the peak rate.

Rather the signal quality becomes more robust and lower signal to interference plus noise ratio (SINR) is required to decode the signal.

MIMO Mode 3

Open loop spatial multiplexing (OL-SM): In this case, two information streams (two code words) are transmitted two or more antennas (up to 4 in LTE). There is not explicit feedback from the user equipment (UE), although a “Transmit Rank Indication” (TRI) transmitted by the UE is used by the base station to select the number of spatial layers.

As multiple code words are transmitted, OL-SM provides much better peak throughput than transmit diversity. It is also simpler to implement and is considered to be on of the main modes of MIMO to be implemented in LTE systems.

MIMO Mode 4

Closed loop spatial multiplexing (CL-SM): similar to OL-SM, two information systems are transmitted over two code words from N antennas (up to 4).

The difference is “pre-coding matrix indicator” PMI which is fed back from the handsed to the base station. This feedback mechanism allows transmitter to pre-code the data to the optimize transmission over the wireless channel so the signals at the receiver can be easily separated into the original streams.

This mode is expected to be highest performing mode of MIMO in LTE.

MIMO Mode 5

Multi-User MIMO> This mode is similar to CL-SM, but the information streams are targeted at different terminals. Hence, multiple users share the same resources.

While each user experience the same data rate, the overall network data rate is improved. It is expected that linear antenna arrays be used for this mode in practice as opposed to multiple spatially distributed antennas.

MIMO Mode 6

Close loop Rank 1 with pre-coding: this case represents the scenario when a single code word transmitted over a single spatial layer. Many consider this case to be a fall back scenario of CL-SM and it has been associated with beamforming.

MIMO Mode 7

Single antenna port: Port 5: This is a beamforming mode where a single code word is transmitted over a single spatial layer. A dedicated reference signal forms an additional antenna port (ie port 5) and allows transmission from more than 4 antennas. The terminal estimates the channel quality from the common reference signals on antennas 1-4, Linear antenna arrays are expected to be used for this mode.