LTE Transmission Modes Part 1 (Physical Layer Processing, TM1,TM2 and TM3)

In LTE (Long-Term Evolution), the transmission mode refers to the method by which data is transmitted between the base station and the mobile device. LTE supports multiple transmission modes, such as Single-Input Single-Output (SISO), Multiple-Input Single-Output (MISO), and Multiple-Input Multiple-Output (MIMO). These modes determine the number of antennas used for transmitting and receiving data, as well as the spatial processing techniques employed to improve transmission performance, spectral efficiency, and overall system capacity.

Prior to proceeding, it is advisable to gain an understanding of the DL-SCH transport channel and the PDSCH Physical Channel. If you are not acquainted with the concept of LTE channels, I suggest reading the article below first.

LTE Channel Concept and Introduction to Air Interface Protocols. (linkedin.com)

If we focus more on Physical Layer Processing, can be illustrate as below,

In practical scenarios, the channel matrix ([C]) is typically given as it is determined by the communication's channel conditions. We do not have the ability to change it arbitrarily. Therefore, the only way to achieve the maximum rank (indicating the best MIMO condition) is to modify either [P] or [B] in order to make the rank of the path matrix as close as possible to its maximum value. To accomplish this, knowledge of the channel matrix [C] is necessary. However, in the case of LTE downlink communication, the transmitter (eNB) is unable to directly obtain information about [C]; only the receiver can evaluate [C] based on the reference signal embedded in the received signal. Consequently, the UE (user equipment) must determine the optimal matrix for [P] or [B] and communicate this information to the transmitter (eNB). This is the purpose of the PMI (Precoding Matrix Indicator) and RI (Rank Indicator) report.

In this article we are going to dig into Transmission Mode 1 till Transmission Mode 3. In this 3 Transmission modes,

1. Down Link Channel Estimation will be done using -? CRS (Cell Specific Reference Signal)
2. Downlink Data Demodulation (PDSCH) will be done using - CRS (Cell Specific Reference Signal)
3. Uplink Data Demodulation (PUSCH)? - Will be done using – Uplink DMRS(Demodulation Reference Signal)

If you are not familiar with CSR pls go through the below article.

LTE CRS (linkedin.com)

RRC Setup Request includes the initial transmission mode.

1) TM 1 – Single transmit antenna

?This mode uses only one transmit antenna.

?

2) TM 2 – Transmit diversity

Transmit diversity is the default mode for Multiple-Input Multiple-Output (MIMO) systems. It involves transmitting the same information through different antennas, where each antenna stream employs distinct coding and frequency resources. This technique enhances the signal-to-noise ratio and enhances the robustness of the transmission.

In the context of LTE, transmit diversity serves as a backup option for certain transmission modes when spatial multiplexing (SM) cannot be utilized. It is also employed for transmitting control channels like PBCH (Physical Broadcast Channel) and PDCCH (Physical Downlink Control Channel). To achieve transmit diversity with two antennas, a frequency-based variant of Alamouti codes known as Space Frequency Block Codes (SFBC) is utilized. Meanwhile, for four antennas, a combination of SFBC and Frequency Switched Transmit Diversity (FSTD) is employed.

Following is the case where a single layer data stream gets transmitted by two antenna. Overall procedure is as follows. (TM2, TM6 is using this configuration).

Downlink - Layer Mapping

Below is the extract from TS 136.211 regarding the layer mapping for Transmit Diversity.

Below is the extract from TS 136.211 regarding the layer mapping for Transmit Diversity.

2 x 1 Tx Diversity, 2 Layer, 1 Codeword

The reason for 4x4 matrix for 2x1 Tx Diversity is based on following precoding matrix.

3) TM 3 – Open loop spatial multiplexing with CDD

In LTE (Long-Term Evolution) technology, TM3 (Transmission Mode 3) offers several benefits, including:

• Improved spectral efficiency: TM3 enables the efficient use of available spectrum, allowing for higher data rates and increased capacity within the LTE network.
• Enhanced performance in MIMO systems: TM3 supports multiple input, multiple output (MIMO) configurations, which can result in improved signal quality, better coverage, and higher data throughput.
• Flexibility in resource allocation: TM3 provides flexibility in allocating resources to users based on their specific transmission requirements, leading to optimized network performance and better user experience.
• Robustness in challenging environments: TM3 is designed to provide reliable communication in challenging radio environments, such as those with high levels of interference or fading.

Overall, TM3 plays a crucial role in maximizing the efficiency and performance of LTE networks, ultimately leading to improved user satisfaction and network reliability.

This mode supports spatial multiplexing of two to four layers that are multiplexed to two to four antennas, respectively, in order to achieve higher data rates. It requires less UE feedback regarding the channel situation (no precoding matrix indicator is included), and is used when channel information is missing or when the channel rapidly changes, e.g. for UEs moving with high velocity.

In addition to the precoding , the signal is supplied to every antenna with a specific delay (cyclic delay diversity, or CDD), thus artificially creating frequency diversity.

As per the 3GPP TS 136.213, in TM3 if the Rank Indicator is 1 Transmit Diversity will be use like in TM2 and otherwise(if RI is lager than 1) large delay CCD will be used. This is a open loop transmission method.

What is this CCD(Cyclic Delay Diversity) ?

Cyclic Delay Diversity (CDD) is a technique used in wireless communications to mitigate the effects of multipath fading and improve the reliability of a communication link. It involves introducing a specific delay in the transmission of signals through different antennas, which helps to create diversity in the received signals. By doing so, CDD can combat the impact of fading and enhance the overall performance of the communication system.

Very simply put, in CDD one antenna is transmitting the original copy of data and the other antenna is transmitting the cyclic shifted version of the original data as illustrated below (see how the yellow part to represent the cyclic shift).

If you represent the transmitted data in frequency domain, the original data and the cyclic shifted version can be represented as follows. As you see, the cyclic shift in time domain produce the phase shift for each symbols in frequency domain and it generate the same effect as frequency diversity.

CDD is applied differently for each transmission mode as shown in the following table.

From TS 136.211

CDD is implemented by applying two additional matrix (D, U) to channel matrix (W) in precoding process. CDD being applied in Precoding Step implies that real implementation of CDD is done in frequency domain and will be converted to Time Domain right before being transmitted through Antenna.

CDD part of precoding, the matrix are as follows. The D(i) matrix, it has all zero value except diagonal line. The values on diagonal line performs phase shift.

Large Delay CDD – frequency domain

?

• Every ”even” subcarrier will carry codeword 1 on ”layer 1” and codeword 2 on ”layer 2”
• Every ”odd” subcarrier will carry codeword 1 on ”layer 2” and codeword 2 on ”layer 1”

Large Delay CDD – spatial domain

Will discuss the CSI Reporting and TM4 in next article.

References:

1. TS 136.211
2. TS 136.213
3. www.sharetechnote.com
4. https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_application/application_notes/1ma186/1MA186_2e_LTE_TMs_and_beamforming.pdf