persubcarrier

Per-subcarrier, or persubcarrier, refers to the concept of applying modulation order, coding rate, and sometimes transmit power on an individual subcarrier within a multicarrier system such as orthogonal frequency-division multiplexing (OFDM). In OFDM-based systems data are carried on many subcarriers, and each subcarrier can experience a different channel condition due to frequency-selective fading. Per-subcarrier adaptation exploits this diversity by assigning stronger modulation and coding to subcarriers with good channel quality and lighter or no modulation to poorer ones, potentially with tailored power allocations.

Implementation commonly involves bit loading and adaptive modulation and coding (AMC) at the transmitter. The receiver estimates the channel gain per subcarrier and, if feedback is available, conveys this information to the transmitter. The transmitter then selects the modulation order (for example, QPSK, 16-QAM, 64-QAM) and coding rate for each subcarrier. Power allocation across subcarriers may follow strategies such as water-filling or other schemes that satisfy a total power constraint. Signaling overhead for per-subcarrier schemes can be significant, so many systems group subcarriers into larger units called resource blocks or code blocks to balance performance and signaling bandwidth.

Applications and practicality vary by system. In many OFDM-based standards, adaptation is performed at the level of resource blocks, which reduces signaling needs compared with true per-subcarrier granularity. Per-subcarrier adaptation remains an area of study and is found in research and some advanced or configurable systems to maximize spectral efficiency in highly frequency-selective channels.

Related concepts include OFDM, adaptive modulation and coding, and advanced power loading techniques such as water-filling.