Modulation and Signal Quality at LTE

Each symbol represents bits depending on the modulation technique, Each data point represents a different bit pattern.  LTE devices use QPSK, 16QAM and 64QAM 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.

A modulation technique is selected based on the measured Signal to Noise Ratio (SNR). Subscribers located away from the eNodeB must use more robust modulation schemes (lower throughput), or they will experience unacceptable data loss rates. In addition, subscribers close to a sector boundary may experience inter-sector interference.

Each modulation scheme has a threshold SNR. For example, let’s assume

• QPSK is associated with SNR 9 dB.
• 16QAM with 15 dB.
• 64QAM with 25 dB. 

If the SNR drops below 15 dB, the eNodeB will instruct the UE to use QPSK modulation.

Measuring Signal Quality

? Signal quality measured using Reference Signals

? eNodeB initially measures UE signal quality during network entry.

? eNodeB controls the modulation selected for both DL and UL.

Both the eNodeB and the UE measure signal quality using Reference Signals.

The Reference Signals carry a known (pseudo-noise) bit pattern at a boosted power level.

The receiver performs channel equalization based on the received Reference Signals.

The eNodeB calculates the UE-transmitted signal quality by measuring UE Reference Signals, and by requesting Channel Quality Indication (CQI) feedback from the UE.

The eNodeB always controls and selects the modulation and coding scheme for both the Downlink and Uplink.

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