MCS and its role in 5G NR

Jyothi
4 min readFeb 12, 2024

Modulation and coding schemes define the combination of the modulation and channel coding used to transmit over the air.

The purpose of MCS is to adapt to varying channel conditions and achieve a balance between data rate and reliability.

Modulation is the process of encoding digital data into analog carrier signals for transmission. 5G NR uses 5 types of modulation schemes:

· Binary Phase shift keying (BPSK)

· Quadrature phase shift keying (QPSK)

· 16 Quadrature amplitude modulation (16 QAM)

· 64 Quadrature amplitude modulation (64 QAM)

· 256 Quadrature amplitude modulation (256 QAM)

Modulation order is defined as the number of symbols representing digital data in a single modulation symbol. The below table shows the Modulation order of each modulation scheme:

Higher order modulation allows more bits to be transmitted in a modulation symbol, providing higher data rates but requiring a higher SNR for reliable reception. MCS in 5G NR is essential for dynamically adapting the modulation and coding schemes based on current channel conditions. This adaptive approach ensures that the uplink/downlink transmissions are optimized for both higher data rates and reliable communication in radio environments

Channel coding

Channel coding involves adding redundant information to the transmitted data. It is useful for error detection, and correction at the receiver. It helps to mitigate the effects of channel impairments. Channel coding schemes are a combination of error detection, correction, and rate matching. Channel coding schemes include turbo codes, LDPC codes, and polar codes.

The coding rate is the ratio of original information bits and the total number of bits, useful in determining the amount of redundancy added.

The below table shows the channel coding schemes used for transport channels/control information in 5G NR:

3GPP 38.214

gNB passes the MCS value in PDCCH to UE part of DCI 0_0,0_1,1_0/1_1. This value ranges from 0 to 31. 3GPP 38.214 defines the MCS Index tables for PUSCH and PDSCH:

· Table 5.1.3.1–1 is 64 QAM table,

· Table 5.1.3.1–2 is 256 QAM table,

· Table 5.1.3.1–3 is Low Spectral Efficiency (SE) table,

· Table 6.1.4.1–1 is MCS Index table PUSCH with transform precoding and 64QAM (64QAM TP MCS table 4)

· Table 6.1.4.1–2 is MCS Index table 2 PUSCH with transform precoding and 64 QAM (Low SE TP MCS table 5). These tables define modulation order, target code rate and spectral efficiency.

MCS Index table selection for PDSCH

Section 5.1.3.1 explains the MCS table selection for PDSCH.

gNB can allocate resources to UE either using PDCCH or part PDSCH configuration when using SPS config.

· Case 1: PDSCH Config MCS table IE is configured as qam256 and DCI 1_1 scrambled with C_RNTI → 256QAM MCS Index table

· Case 2: PDSCH Config MCS table IE is configured as qam64LowSE and UE not configured with MCS-C-RNTI and UE scheduled using UE specific search space and C-RNTI → LowSE MCS Index table

· Case 3: UE is configured with MCS-C-RNTI and CRC scrambled with MCS-C-RNTI → Low SE MCS Index table

· Case 4: SPS-Config and no MCS table configured in SPS-Config and PDSCH Config MCS table IE is configured as qam256 → 256QAM MCS Index table

· Case 5: SPS-Config MCS table = qam64LowSE → LowSE MCS Index table

· Case 6(default): 64 QAM MCS Index table

MCS Index table selection for PUSCH

· Case 1: PUSCH-Config MCS Table = qam256 and DCI 0_1 scrambled with C-RNTI/SP-CSI-RNTI → 256 QAM MCS table 2

· Case 2: PUSCH Config MCS table IE is configured as qam64LowSE and UE not configured with MCS-C-RNTI and UE scheduled using UE specific search space and C-RNTI/SP-CSI-RNTI → If no transform precoding, LowSE MCS Index table 3 ; else LowSE TP MCS Index table 5

· Case 3: UE is configured with MCS-C-RNTI and CRC scrambled with MCS-C-RNTI → If no transform precoding, LowSE MCS Index table 3 ; else LowSE TP MCS Index table 5

· Case 4: UL Config grant MCS table = qam256 and CRC scrambled with CS-RNTI or no CRC when using UL configured grant → 256QAM MCS Index table

· Case 5: UL Config grant MCS table = qam64LowSE and CRC scrambled with CS-RNTI or no CRC when using UL configured grant → If no transform precoding, LowSE MCS Index table 3 ; else LowSE TP MCS Index table 5

· Case 6 (default): If no transform precoding, 64 QAM MCS Index table 3 ; else 64 QAM TP MCS Index table 4

MCS reserved values:

In MCS index tables, some indices are reserved. These indicate retransmissions.

TBS calculation steps for these cases are mentioned in Section 6.1.4.2:

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