Modulation in LTE

The 3G LTE uses 3 Quadrature Amplitude Modulations (QAMs) depending on the radio quality.
QAM uses both the amplitude and the phase.
The LTE supports in DL and UL the following modulations:
  1. QPSK, the most robust but the less efficient
  2. 16-QAM
  3. 64-QAM, the less robust but the most efficient
QAM is a modulation method modifying the phase and the amplitude of the carrier 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 can
be 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.