What is ACPR (Adjacent Channel Power Ratio)

Adjacent Channel Power Ratio (ACPR) is a critical parameter in RF communication systems that assesses the interference potential of transmitted signals on adjacent frequency channels. Let’s dive into the technical details:

1. Definition:
   • ACPR represents the ratio between the power transmitted into adjacent frequency channels and the power transmitted into the main frequency channel.
   • It quantifies the amount of power that a communication system leaks into neighboring frequency bands.
2. Importance:
   • Ensures that a system does not interfere with other systems operating in nearby frequency bands.
   • Used in various communication systems, including wireless communication, radio broadcasting, and satellite communication.
3. Nonlinear Behavior of Power Amplifiers (PAs):
   • At times, PAs operate beyond their linear range for improved transmitter efficiency.
   • Due to PA nonlinearity, some power leaks into adjacent channels, primarily due to intermodulation distortion.
   • ACPR is also known as Adjacent Channel Leakage Ratio (ACLR).
4. Measurement and Calculation:
   • ACPR is expressed in decibels (dB).
   • Typically specified as a limit on the power transmitted into adjacent frequency channels.
   • The limit varies based on communication system type, frequency band, and regulatory body.
   • For example, in the United States, the FCC specifies ACPR limits for wireless communication systems.
   • Measurement standards (e.g., IS-95, CDMA, WCDMA, 802.11, Bluetooth) define ACPR as the ratio of average power in the main channel to adjacent channels.
   • Measurement offsets and bandwidths depend on the specific standard:
     • CDMA: Two offsets (885 kHz and 1.98 MHz) with a 30 kHz bandwidth.
     • WCDMA: Channels spaced 5 MHz apart with a 3.84 MHz carrier bandwidth.
5. Calculation Example:
   • To estimate ACPR for the outermost channel:
     • Compare total carrier power in a 3.84 MHz bandwidth centered around the main channel to the total power in a 3.84 MHz bandwidth offset by 5 MHz.
   • ACPR limit ensures minimal interference with neighboring systems.