PM16QAM

PM16QAM (Phase Modulated 16‑Quadrature Amplitude Modulation) is a digital modulation scheme that simultaneously encodes data in both the amplitude and phase of a carrier wave. It transmits four bits per symbol, yielding a spectral efficiency of 4 bits per hertz of symbol rate. The modulation uses a 16–point constellation that can be visualised as a 4×4 grid in the I–Q plane, with each point representing a unique 4‑bit pattern. Unlike pure phase modulation, PM16QAM exploits variations in both amplitude and phase, allowing more data to be sent in the same bandwidth compared with simpler schemes such as BPSK or QPSK.

The symbol set is typically arranged with uniformly spaced amplitude levels on both I and Q axes. An example mapping assigns the binary value 0000 to the lower‑left point (−3,−3), 0111 to the upper‑right point (+3,+3), and so forth, following Gray coding to minimise bit errors when adjacent symbols are confused. At the receiver, coherent demodulation requires an accurate carrier phase estimate; errors in the phase reference translate directly into symbol misclassification.

PM16QAM is widely used in modern wireless standards, including LTE, Wi‑Fi (802.11ac/ax), and 5G NR, where its higher data rates are valuable for downlink transmission. The main drawback is its sensitivity to channel impairments: phase noise, Doppler shifts, and non‑linearities in the amplifier can degrade performance, necessitating more complex equalisation and error‐correction coding. Nonetheless, by serving a balance between spectral efficiency and robustness, PM16QAM remains a key modulation format in contemporary broadband communication systems.