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What is QAM (quadrature amplitude modulation)?

A digital modulation scheme that represents data by changing both the amplitude and the phase of the carrier. Each symbol sends one of a grid of signal points formed by combinations of in-phase (I) and quadrature (Q) components, and M-QAM with M points sends log₂ M bits per symbol: 4 bits for 16QAM, 6 for 64QAM and 8 for 256QAM. More levels mean higher speed, but the points are closer together, so a higher S/N is needed.

Japanese name: QAM(直交振幅変調)

QAM (quadrature amplitude modulation) appears in 6 practice questions across 2 topics, including Wireless Communication Basics, Silicon photonics.

Field: Wireless Communication Basics

Try it: questions about QAM (quadrature amplitude modulation)

In 64QAM, which is the number of bits that can be sent per symbol?

  1. A6 bits
  2. B8 bits
  3. C64 bits
Show answer and explanation

Answer: A. 6 bits

In M-ary modulation, each symbol sends one of M possible signal points, so the number of bits it carries is log₂ M. 64QAM has 64 signal points, so it carries log₂ 64 = 6 bits. The 64 points form a grid with 8 levels on each of the in-phase (I) and quadrature (Q) axes. Similarly, 16QAM carries 4 bits and 256QAM carries 8 bits.

With 16QAM at a symbol rate of 5 M symbols per second (5 Mbaud), which is the bit rate?

  1. A1.25 Mbit/s
  2. B5 Mbit/s
  3. C20 Mbit/s
Show answer and explanation

Answer: C. 20 Mbit/s

16QAM sends log₂ 16 = 4 bits per symbol. The bit rate is "symbol rate × bits per symbol", so it is 5 M × 4 = 20 Mbit/s. The symbol rate (baud rate) is the number of symbols sent per second, and with multilevel modulation it does not equal the bit rate. Because the occupied bandwidth is set mainly by the symbol rate, using more levels raises the bit rate without widening the bandwidth.

QAM (quadrature amplitude modulation) in the explanations

Questions that mention QAM (quadrature amplitude modulation) (6)

  1. In 64QAM, which is the number of bits that can be sent per symbol?
  2. With 16QAM at a symbol rate of 5 M symbols per second (5 Mbaud), which is the bit rate?
  3. Which is a general property when the modulation is changed from 16QAM to 256QAM at the same symbol rate?
  4. In a transmitter sending OFDM or multilevel QAM signals, which is the main reason for operating the power amplifier (PA) at a point below where its output saturates (with back-off)?
  5. Which correctly describes the coherent scheme, used for links of several tens of km or more such as between data centers, compared with IM-DD?
  6. Which digital modulation scheme represents data by changing the phase while keeping the carrier's amplitude and frequency constant?

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Related terms

Baud rate OFDM (orthogonal frequency-division multiplexing) Chromatic dispersion Beamforming Bluetooth Low Energy (BLE) Direct-conversion receiver Fading Free-space path loss (FSPL) Hidden node problem Link budget

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