Transmission Mode 9 (TM9) in LTE
In Release-10 (LTE-A) 3GPP Introduced a new transmission mode, TM 9. TM9 is designed to help reduce interference between base stations to maximize 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 maximizes the efficient use of the base station and guarantee’s a decent data rate for users.
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.
What are the benefits of using transmission mode 9.The answer to this question is, high cell edge data rates, better interference handling by base station, and therefore, improved coverage. TM9 can be combined with carrier aggregation and eICIC to create a highly spectrum efficient network.
TM9 is a new transmission mode introduced in 3GPP Release 10. In TM9, UE-specific RSs are introduced, and CSI measurement and data demodulation are performed by different RSs. Efficient and accurate closed-loop feedback is used to enable dynamic beamforming using multiple antennas. Single-user multiple-input multiple-output (SU-MIMO) and multi-user MIMO (MU-MIMO) are achieved to help exponentially improve both user experience and cell capacity.
UE-specific RSs and multiple-antenna technologies are also key technologies for 5G networks, representing the technological evolution trend in wireless networks.
In addition to UE-specific RSs, the compatibility of legacy non-TM9 terminals is also considered in TM9. Non-TM9 terminals can work properly on TM9-enabled networks. However, they cannot benefit from enhanced cell capacity and user experience brought by multiple-antenna technologies, as dynamic beamforming cannot be enabled. TM9-capable terminals can perfectly match TM9-enabled networks, helping to maximize network capabilities and achieve better user experience.
Although MU-MIMO cannot provide benefits for a single user by UE paring, user experience can still be notably enhanced through SU-MIMO. TM9 enables UE-specific dynamic beamforming to ensure the alignment of the strongest beam signals with UEs. TM9 also increases multiplexing gains by using multiple ports. The average user-perceived throughput of a single TM9 UE is over 25% higher than that of a non-TM9 UE.
TM9 is introduced to break through reference signal (RS) restrictions on antenna ports, promoting evolution of multiple-antenna technologies.
Before 3GPP Release 10, only the cell-specific reference signal (CRS) could be used over the LTE air interface to measure and report the CSI and to demodulate the data. When multiple-antenna technologies are used, signals from each antenna are transmitted along different paths. A group of CRS resources needs to be assigned to each antenna port to accurately measure the CSI for each antenna signal. A larger number of antenna ports results in increased air interface resource overheads. When two or four ports are used, the RS overhead accounts for 9.5% or 14.3% of the total, respectively. When eight or more antenna ports are used, the RS overhead will have a negative impact on the gains from the multiple-antenna technology. As a result, no more than four CRS ports can be used. In these circumstances, TM9 is introduced and two new RS types are specified: the CSI-RS and the DMRS. The first is used to measure and report the CSI, and the latter is used to demodulate the data. The CSI-RS consumes few air interface resources to measure and report the CSI. The DMRS is transmitted along with the data and is applied on demand. Therefore, the introduction of TM9 can break through RS restrictions on the number of antenna ports, enabling eight or more ports and promoting further evolution.
For common cells: TM9 enables downlink joint transmission (JT), increasing the downlink CEU-perceived throughput by 20% to 40%.
Cell edge user (CEU) experience is relatively poor due to the interference caused by neighboring cells. JT enables neighboring cells to simultaneously transmit data to CEUs for the purposes of improving CEU experience.
In the transmission modes prior to 3GPP Release 10, CEUs cannot identify RSs of neighboring cells by using CRSs. In this case, downlink JT cannot be achieved. TM9 uses UE-specific demodulation reference signals (DMRSs), which are transmitted along with data streams. In TM9, CEUs can successfully demodulate and combine the data streams of neighboring cells, effectively turning interference into gains while improving CEU experience.
Cell edge user (CEU) experience is poor due to the interference caused by neighboring cells. This can be greatly improved by using DL CoMP. CoMP makes use of joint transmission (JT) and interference coordination. JT enables base stations in neighboring cells to jointly transmit data to CEUs to obtain power gain and array gain. Interference coordination is used to adjust beam scheduling of UEs in intra-frequency neighboring cells to avoid interference to CEUs and improve CEU experience. The UE-specific CSI-RS in TM9 can be used to accurately measure the data streams sent by neighboring cells and to coordinate scheduling. According to the lab test results, TM9-based DL CoMP can increase the cell-edge throughput by 20% to 40%.
TM9 greatly improves beamforming and MU-MIMO capabilities on Massive MIMO networks. For example, if Rel-14 TM9 phones with 4 antennas are used, a 32T32R massive MIMO cell with TM9 enabled has up to 6.5-fold capacity increase over a 2T2R cell. Even in 2T2R and 4T4R network, TM9 can be used to significantly increase the gains for CEUs. TM9 fully utilizes the advantages of multiple-antenna technologies and improves user experience to a record high.