5G/NR, LTE: MCS and Modulation Order 调制方式的选择

MCS = modulation coding scheme 和 Modulation Order (Modulation depth， QPSK，16QAM，64QAM，256QAM) 是紧密相关的。在3GPP中已经定义了MCS调制等级 [TS 38.214] table 5.1.3.1。

Qm    Modulation Method
2    QPSK
4    16QAM
6    64QAM
8    256QAM
为什么有两个类似的表格
仔细观察，有两个不同的表格。这是因为要取决于UE是否支持256QAM。那么基站如何知道UE是否支持256QAM呢？UE会在开始的时候就会上报基站自己的能力。

If the UE is configured with higher layer parameter ul-256QAM-Table-r14, and if the PDCCH corresponding to the PUSCH transmission is located in UE specific search space, the UE shall determine the TBS index ( ITBSITBS) using ITBSITBS and Table 8.6.1-3

UE如何知道它使用的是64QAM呢？
[36.213] UE是否使用64QAM是需要高层进行配置的.

If the UE is capable of supporting 64QAM in PUSCH and has not been configured by higher layers to transmit only QPSK and 16QAM, the modulation order is given by Qm in Table 8.6.1-1. // this means UE should use 64QAM if MCS >= 21

If the UE is not capable of supporting 64QAM in PUSCH or has been configured by higher layers to transmit only QPSK and 16QAM, Q’m is first read from Table 8.6.1-1. The modulation order is set to Qm = min(4, Q’m ) // This means UE does not use 64 QAM。

但是调制方式在38.214中的定义与36.213不同。后续会继续跟进。

36.213 7.1.7 downlink - modulation order and transport block size determination
36.213 8.6.1 uplink - modulation order and redundancy version determination
38.214 5.1.3 downlink - modulation order, target code rate, redundancy version and transport block size determination
38.214 6.1.4 uplink - modulation order, redundancy version and transport block size determination