[5G PHY] Introduction to 5G Modulation and Coding Strategy (MCS)

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I work for an internationally renowned terminal manufacturer and am responsible for modem chip research and development.
In the early days of 5G, he was responsible for the development work related to the terminal data business layer and core network. He is currently leading the research on 6G computing network technology standards.


The blog content mainly focuses on:
       5G/6G protocol explanation,
       computing power network explanation (cloud computing, edge computing, terminal computing),
       advanced C language explanation,
       Rust language explanation

1. Introduction to 5G Modulation and Coding Strategy (MCS)

MCS: Modulation and Coding Scheme
RB: Carrier (Radio Bearer)
RE: Resource Element (Rsource Element)
BLER: Block Error Rate (Blocker Error Rate)

       MCS defines the number of useful bits that can be carried within a symbol. A symbol is defined as an RE, and MCS defines the number of bits available for transmitting data in each RE . MCS depends on the signal quality in the wireless link. If the signal quality is better, the more bits can be used to transmit data in one symbol; if the signal quality is poor, the MCS is lower, then the number of bits that can be used to transmit data in one symbol is The number of data bits is smaller.

       In other words, we can say that the value of MCS depends on BLER, which is usually defined as a threshold of 10%. In order to keep BLER not exceeding this value in different wireless environments, gNB will allocate an MCS according to the link adaptation algorithm , and sent to the terminal through DCI signaling on the PDCCH channel , for example, DCI 1_0, DCI 1_1.

MCS mainly defines the following two points:

• modulation;
• Code rate

2. Modulation

       Modulation defines how many bits can be carried on an RE, regardless of whether they are bits used to transmit valid data or bits used for parity checking . 5G NR supports QPSK, 16QAM, 64QAM and 256QAM modulation. QPSK can transmit 2 bits per RE, 16QAM can transmit 4 bits, 64QAM can transmit 6 bits, and 256QAM can transmit 8 bits. These 16, 64 and 256 are the so-called QAM modulation orders. The number of bits for each modulation order can be calculated using the following formula:

3. Code rate

       The code rate can be defined as the ratio of the number of effective bits to the total number of transmitted bits (ie, effective bits + redundant bits) . These redundant bits are added for forward error correction (FEC). In other words, it is the ratio of the number of bits of information in the upper layer of the physical layer to the number of bits mapped to the PDSCH in the lower layer of the physical layer. We can also say that it is a measure of the redundant bits added by the physical layer. Lower bit rates mean more redundant bits. The relevant formulas are as follows:

4. MCS table in the agreement

• When gNB or UE does not support 256QAM, or in a poor wireless environment, 256QAM decoding fails and gNB needs to allocate QPSK modulation order, you can use the 64QAM table;
• When allocating 256QAM in a very good wireless environment, the 256QAM table can be used;
• Low spectrum efficiency (Low SE) 64QAM table is suitable for applications that require reliable data transmission, such as URLLC applications. This table includes MCS with low spectral efficiency, i.e. reduced coding rate and increased channel coding redundancy.

4.1 64QAM table

4.2 256QAM table

4.3 Low SE 64QAM table

4.4 How to choose which table to use

• gNB notifies the terminal to select a specific MCS table through a combination of RRC signaling and physical layer signaling (for example, RNTI);
• The MCS table is configured through the PDSCH-Config and SPS-Config (mcs-Table) parameters in RRC signaling. This parameter can then be further modified through RRC signaling;
• When the physical layer uses dynamic RNTI (this RNTI is scrambled using CRC and belongs to the PDCCH payload) selection, for example, switching between C-RNTI and MCS-C-RNTI can affect the selection of the MCS table;

Through the following example, we can show the selection process of MCS tables. The initial MCS is configured through RRC signaling, and can subsequently be controlled through physical layer signaling only:

• Assume that a UE has been configured with the parameter PDSCH-Config in which the value of mcs-Table is qam256, and is assigned a C-RNTI and an MCS-C-RNTI;
• If the terminal receives a PDSCH resource allocation indicated by DCI 1_1 (scrambling using C-RNTI), the terminal will select the 256QAM table;
• If the same terminal receives a PDSCH resource allocation indicated using DCI 1_0 (scrambling with C-RNTI), then the terminal will select the 64QAM table;
• If the same terminal receives a PDSCH resource allocation indicated using DCI 1_1 or DCI 1_0, scrambled by MCS-C-RNTI, then the terminal will select the Low SE 64QAM table;

5. Summary

• MCS defines the number of effective bits per symbol;
• The selection of MCS is based on the wireless environment and BLER;
• MCS is modified based on gNB’s link adaptation algorithm;
• MCS information is provided to UE through DCI;
• 5G NR supports QPSK, 16 QAM, 64 QAM and 256 QAM modulation of PDSCH;
• NR defines approximately 32 MCS indexes (0 ~ 31), of which 29, 30, and 31 are reserved for retransmission;
• The 3GPP 38.214 specification provides three tables for PDSCH MCS, namely 64QAM table, 256QAM table and Low SE 64QAM table;

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