Depending on the modulation technique, each symbol stands for bits and each data point represents a different bit pattern.
QPSK, 16QAM, and 64QAM are used by LTE devices to modulate data and control information. The eNodeB supports all of these modulation techniques for the Down Link direction. However, 64QAM is optional in the Uplink direction.
Depending on the measured Signal to Noise Ratio (SNR), a modulation technique is chosen. If subscribers desire to avoid unsatisfactory data loss rates and are located away from the eNodeB, they must use more robust modulation schemes (lower throughput). Moreover, subscribers near the sector boundary may face inter-sector interference.
Each modulation scheme has a threshold of SNR. For example, let’s assume
- QPSK is associated with SNR 9 dB.
- 16QAM with 15 dB.
- 64QAM with 25 dB.
The eNodeB will instruct the UE to use QPSK modulation if the SNR decreases below 15 dB.
Measuring Signal Quality:
• Signal quality measured using Reference Signals
• eNodeB initially measures UE signal quality during network entry.
• The modulation selected for both DL and UL is controlled by eNodeB.
Using Reference Signals, both the eNodeB and the UE measure signal quality.
At a boosted power level, the Reference Signals carry a known (pseudo-noise) bit pattern. Based on the received Reference Signals, it performs channel equalization.
By calculating UE Reference Signals and by requesting Channel Quality Indication (CQI) feedback from the UE, the eNodeB can calculate the UE-transmitted signal quality.
The modulation and coding scheme for both the Downlink and Uplink is always controlled and selected by the eNodeB.