Constellation in LTE

Constellation in LTE

In LTE (Long-Term Evolution), the term “constellation” typically refers to the modulation scheme used for transmitting data over the air interface. LTE uses Quadrature Amplitude Modulation (QAM) to encode multiple bits into a single symbol. The constellation diagram visually represents how the amplitude and phase of the modulated signals correspond to specific bit patterns.

Key points about constellations in LTE:

  1. QAM Modulation:
    • LTE commonly uses QAM modulation, such as 16-QAM or 64-QAM. The number in the QAM notation represents the number of states (points) in the constellation diagram.
  2. Constellation Diagram:
    • The constellation diagram is a graphical representation of the modulation scheme. It shows the positions of the signal points in the complex plane.
  3. 16-QAM and 64-QAM:
    • In LTE, 16-QAM uses a constellation with 16 points, allowing each symbol to represent 4 bits. Similarly, 64-QAM has 64 points, allowing each symbol to represent 6 bits. The use of higher-order QAM increases the data rate but makes the system more susceptible to noise and interference.
  4. Adaptive Modulation and Coding (AMC):
    • LTE employs Adaptive Modulation and Coding to dynamically adjust the modulation scheme and coding rate based on the channel conditions. In good conditions, higher-order modulations like 64-QAM can be used for increased data rates, while in poor conditions, a lower-order modulation like QPSK (Quadrature Phase Shift Keying) may be chosen for better reliability.
  5. Error Performance:
    • The choice of constellation affects the error performance of the system. Higher-order modulations provide higher data rates but are more susceptible to errors in noisy environments.

In summary, the constellation in LTE refers to the modulation scheme, typically represented by QAM, and the constellation diagram visually depicts the positions of the signal points used for encoding and decoding data over the LTE air interface. The choice of constellation is a crucial aspect of LTE design to balance data rate and error performance based on varying channel conditions.

The QPSK is the most robust modulation. It can be represented by a constellation:The radius, R, represents the amplitude.The angle, ?, represents the phase.

There are 1 amplitude but 4 phases to 4 different states.2 bits can be coded with 1 QPSK symbol. The 16-QAM can modulate 4 bits per symbol.

The 64-QAM can modulate 6 bits per symbol.
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