The 3G LTE uses 3 Quadrature Amplitude Modulations (QAMs) depending on the radio quality.
QAM uses both the amplitude and the phase.
LTE devices use QPSK, 16QAM and 64QAM to modulate data and control information. The eNodeB supports all of these modulation techniques for the downlink direction. However, 64QAM is optional in the uplink direction. A modulation technique is selected based on the measured signal-to-noise ratio (SNR).
The LTE supports in DL and UL the following modulations:
- QPSK, the most robust but the less efficient
- 16-QAM
- 64-QAM, the less robust but the most efficient
QAM is a modulation method modifying the phase and the amplitude of the carrier signal.
With good signal conditions, 64QAM is used in the LTE downlink direction. QAM stands for Quadrature Amplitude Modulation and is a modulation technique that encodes several bits per transmission step in the amplitude of a sine wave signal and additionally in a phase shift compared to a reference signal.
QAM symbols are represented by the carrier signal being transmitted with specific phase / amplitude (dictated by the message), for finite periods of time.
One symbol is identified by a Q and an I value.
Transmission channels with a limited bandwidth limit the amount of symbols per second (Baud rate) that canbe transmitted.
To increase the bit per sec (bps) capacity of a channel, while keeping the Baud rate at the low values imposed by the channel bandwidth, the symbols carry (represent) more than one single bit.
Symbols will represent a number of n bits, increasing the channel capacity by a factor of n.
The price paid is the presence of multiple symbols in the channel, increasing the probability of incorrect symbol identification at the receiver.