Modulation Techniques in LTE
- LTE devices use several modulation techniques to modulate data and control information. These modulation techniques include: QPSK (2 bits per symbol), 16QAM (4 bits per symbol), and 64QAM (6 bits per symbol). All of these modulation techniques are supported in the downlink direction; and all but 64QAM, which is optional, are supported in the uplink direction.
- A modulation technique is selected based on the measured signal to interference plus noise ratio (SINR). Each modulation scheme has a threshold SINR. Subscribers located farther from the eNodeB (i.e. with lower SINR values) use a more robust modulation scheme (lower throughput), while subscribers closer to the eNodeB (i.e. with higher SINR values) can use less robust modulation schemes (higher throughput).
- Both the eNodeB and the UE measure signal quality using the Reference Signals. The Reference Signals carry a known (pseudo-noise) bit pattern at a boosted power level.
- The eNodeB always controls and selects the modulation and coding scheme for both
the downlink and uplink.