ACPR stands for Adjacent Channel Power Ratio, is an important performance metric used to characterize spectral regrowth of transmitter frontend component or entire chain. One of the main source of spectral regrowth is power amplifier nonlinearity. Therefore this metric is used even to quantify the nonlinearity of a power amplifier. It is also called as Adjacent Channel Leakage Ratio (ACLR).

At times (at max power levels) PAs are forced to operate much beyond their linear range of operation to improve overall efficiency of transmitter. Due to nonlinear behavior of PA, along with power amplification in main channel some power leaks into adjacent channel (mainly due to intermodulation distortion). This spectral leakage is quantified through ACPR.

ACPR is defined as the ratio of power in the adjacent channels of main channel to the rms power of the transmitted signal in the main channel.

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The spectral powers in main and adjacent channels are compared in relative sense, and specified in dBc.

Example :
In WCDMA the channels are spaced 5MHz apart with a carrier bandwidth of 3.84MHz. To estimate ACPR of outermost channel, the total carrier power in 3.84MHz bandwidth in main channel is compared to total power in 3.84MHz BW centered around 5MHz offset from main channel.