When LTE was introduced in Release-8 it had
7 transmission modes that were increased to 8 in Release-9. Earlier, I posted an
R&S whitepaper on the different Transmission modes (10K+ views already) that listed transmission modes till TM 8. In Release-10 (LTE-A) 3GPP Introduced a new transmission mode, TM 9. TM9 is designed to help reduce interference between base stations to maximise signal stability and boost performance. The new TM-9 enables the enhancement of network capabilities and performance with minimum addition of overhead. TM9 is designed to combine the advantages of high spectrum efficiency (using higher order MIMO) and cell-edge data rates, coverage and interference management (using beamforming). Flexible and dynamic switching between single-user MIMO (SU-MIMO) and an enhanced version of multi-user MIMO (MU-MIMO) is also provided.
A new Downlink Control Information (DCI) format - known as format 2C - is used for TM9 data scheduling. Two new reference signals are defined in TM9: Channel State Information Reference Signal (CSI-RS) and Demodulation Reference Signal (DMRS). The first is used from the UE to calculate and report the CSI feedback (CQI/PMI/RI), while the latter is an evolution - providing support for more layers - of the UE specific reference signal that is already used for beamforming in Rel-9, and is used for signal demodulation. TM-9 is particularly smart as it can detect when a mobile device is being used and send a different type of signal that is optimal for a mobile device (variable DM-RS – demodulation reference signals). This maximises the efficient use of the base station and guarantee’s a decent data rate for users.
Early results in
SK Telecom press release are positive with a claimed 10-15% increase in data rates in locations where there was known inter-cell interference.
I also looked into couple of books and here is one explanation from An Introduction to LTE by Chris Cox.
To use eight layer spatial multiplexing, the base station starts by configuring the mobile into a new transmission mode, mode 9. This supports both single user and multiple user MIMO, so the base station can quickly switch between the two techniques without the need to change transmission mode.
The base station schedules the mobile using a new DCI format, 2C. In the scheduling command, it specifies the number of layers that it will use for the data transmission, between one and eight. It does not have to specify the precoding matrix, because that is transparent to the mobile. The base station then transmits the PDSCH on antenna ports 7 to 7 + n, where n is the number of layers that the mobile is using. The maximum number of codewords is two, the same as in Release 8.
The mobile still has to feed back a precoding matrix indicator, which signals the discrepancy between the precoding that the base station is transparently providing and the precoding that the mobile would ideally like to use. Instead of using the PMI, however, the mobile feeds back two indices, i1 and i2. Both of these can vary from 0 to 15, which provides more finely-grained feedback than the PMI did and in turn improves the performance of the multiple user MIMO technique. The base station can then use these indices to reconstruct the requested precoding matrix.
Embedded below is an extract from Google books for Lte-Advanced Air Interface Technology By Xincheng Zhang, Xiaojin Zhou
