Link Adaptation in LTE

Mobile wireless reception conditions vary greatly over frequency and time as described in Section 1.8. In order to cope with these circumstances and guarantee best possible QoS, a procedure is implemented known as Adaptive Modulation and Coding (AMC). AMC controls and changes transmission parameters to achieve a defined Transport Block Error Rate (BLER) of below 10%, in order to keep retransmissions in a suitable range. This is done by adapting the modulation scheme, in LTE on the shared channels between QPSK and 64QAM, and the Forward Error Correction (FEC) coding rate.

Different modulation schemes make the bit detection more robust against noise and other distortion caused by the wireless channel. Figure 1 shows the applied modulation scheme for the LTE shared channels. Most robust transmission is achieved by mapping just 2 bits to each modulation symbol as seen with QPSK, resulting in four stages. A large distance between modulation points as seen with QPSK allows a higher probability of the correct decision at the receiver even with noisy reception conditions. Both 16QAM and 64QAM map 4 and 6 bits respectively to one modulation symbol used with better wireless channel conditions to achieve a higher data throughput. It is to find the best compromise between the modulation scheme and code rate for a given channel quality. LTE defines a list of MCS combinations and just signals an MCS index.

 
Figure 1: Different QAM schemes used with LTE and the number of bits mapped to each scheme

The data modulated with the different modulation schemes to subcarriers needs to be protected against transmission errors. LTE defines a turbo de-/encoder with trellis termination of a native code rate of one-third. The turbo coder adds redundancy bits to the data, which makes it possible to correct some bit errors. The code rate is a fraction of source data rate to resulting protected data rate; thus, a code rate of one-third encodes 1 bit into 3 bits. Other code rates are needed in order to optimize the trade-off between protection and efficiency. This is done by puncturing the native coded bit stream to a higher (less protection) code rate by deterministically leaving out coded bits, or by deterministically repeating coded bits if a smaller code rate is desired (more protection).

Additionally, one parameter being controlled is the UL transmit power. UL power control is implemented to deal with the near–far effect. Figure 2 shows an UL scenario with a near–far effect compared to a DL scenario without power differences between user signals as they are equally attenuated because the mix of the signal is transmitted from one position (eNB). This occurs when a user is close to the base station (near) and another user is far away from the base station, introducing a higher power path loss which leads to a lower receive power of the signal of the cell edge user. All UL receive signals should have equal power in order to have the same analog-to-digital converter saturation of each signal to reduce the quantization noise of users with low received signals, reducing inter-subcarrier interference between the users. This happens with imperfect UL synchronization within real-life scenarios.

 
Figure 2: Near–far effect occurring in uplink direction, compared to equal signal strength reception in downlink

UL TPC commands are sent via designated DCI formats 3 and 3A. DCI 3 and 3A are differential power control commands for PUCCH and PUSCH transmission in steps of decibels. DCI 3 is a 2bit assignment as opposed to DCI 3A which is a single bit command. These dedicated DCIs with TPC commands are only used when there is no data to be transmitted to the UE; otherwise, the TPC command is transmitted embedded in other control information on the PDCCH for this UE. An initial 3-bit TPC command is embedded in the RAR message. The different TPC commands are listed in Tables 1 and 2.

Table 1: Mapping of TPC command field in DCI format 1A/1B/1D/1/2A/2/3 to δ_PUCCH values. Reproduced with permission from © 3GPP^™ 

TPC command field in DCI format 1A/1B/1D/1/2A/2/3	δPUCCH (dB)
0	-1
1	0
2	0
3	3

Table 2: Mapping of TPC command field in DCI format 3A to δ_PUCCH values. Reproduced with permission from © 3GPP^™ 

TPC command field in DCI format 3A	OPUCCH (dB)
0	-1
1	1

UEs report their received channel quality to the eNB by transmitting a CQI value. The CQI value represents either a wideband receive quality as a scalar or a more detailed report about frequency sections (sub-bands) as a vector. CQI reports are transmitted periodically or aperiodically configured by higher layers. Sub-band CQI reports indicate the receive quality of each sub-band relative to the wideband average with four steps: worse, equal, better, and much better. The UE reports the best M sub-bands compared to the average channel quality with the best M method as depicted in Figure 3.

 
Figure 3: CQI illustration with sub-bands and best M reporting. Reproduced with permission from Nomor

The UE should derive a MCS scheme from the above measurement information, which is indexing 1 MCS out of 16 to suit the target BLER of below 10%. This enables differentiation between high- and low-cost handsets which use more or less expensive RF hardware and/or a more sophisticated IQ signal processing engine.

Recapitulating, LTE link adaptation uses various stacked link adaptation techniques, securing transmission, or in order to make it more effective using different control loop delays regarding the process's dimension. The following items summarize the LTE link adaptation functions with their responsiveness:

Adaptive frequency-selective scheduling: Assign frequency resource to UEs on a 1 ms basis, which provides each UE with the individual best reception quality. AMC: Obtain the most efficient modulation and FEC code rate in order to balance retransmissions vs. maximization of throughput. HARQ: Multiple retransmission process using prior transmission to increase the correct decoding probability. TPC: Provides UL power control in order to minimize multiple user interference.