What is ACPR?

Adjacent Channel Power Ratio (ACPR) is the ratio of the power that a communication system transmits into the adjacent frequency channels to the power that it transmits into the main frequency channel. It is a measure of the amount of power that a communication system transmits into the adjacent frequency channels. 

ACPR is an important parameter that is used to ensure that a communication system does not interfere with the other systems operating in the nearby frequency bands. It is used in a wide range of communication systems, including wireless communication systems, radio broadcasting, and satellite communication systems.

At times (at max power levels) power amplifiers are forced to operate much beyond their linear range of operation to improve the overall efficiency of the transmitter. Due to the nonlinear behavior of PA, along with power amplification in the main channel some power leaks into the adjacent channels (mainly due to intermodulation distortion). This is why ACPR is also called Adjacent Channel Leakage Ratio (ACLR). The spectral leakage is quantified through ACPR as the ratio of the power that a communication system transmits outside of its assigned frequency band to the power that it transmits within its assigned frequency band. It is expressed in decibels (dB), and it is typically specified as a limit on the amount of power that can be transmitted into the adjacent frequency channels.

The ACPR limit varies depending on the type of communication system, the frequency band, and the regulatory body that governs the system. For example, in the case of a wireless communication system, the ACPR limit is typically set by the regulatory body that governs the use of the frequency band. In the United States, the Federal Communications Commission (FCC) specifies the ACPR limit for wireless communication systems operating in the frequency bands that it governs. Many transmission standards, such as IS-95, CDMA, WCDMA, 802.11, and Bluetooth, contain a definition for ACPR measurements. Most standards define ACPR measurements as the ratio of the average power in the main channel and any adjacent channels. The offset frequencies and measurement bandwidths being used when obtaining ACPR measurements depend on the specific industry standard being used. For instance, measurements for CDMA amplifiers involve two offsets (from the carrier frequency) of 885 kHz and 1.98 MHz and a measurement bandwidth of 30 KHz. In WCDMA the channels are spaced 5 MHz apart with a carrier bandwidth of 3.84 MHz. To estimate the ACPR of the outermost channel, the total carrier power in 3.84 MHz bandwidth in the main channel is compared to the total power in 3.84 MHz bandwidth centered around 5 MHz offset from main channel.

The ACPR limit is important because if a communication system transmits too much power into the adjacent frequency channels, it can cause interference with the other systems operating in those frequency bands. This interference can degrade the performance of the other systems and make it difficult or impossible for them to operate properly. To ensure that a communication system meets the ACPR limit, it is important to design the system to have a well-defined frequency response. This can be achieved through careful design of the system's filters, amplifiers, and other components.

In addition to meeting the ACPR limit, it is also important to ensure that a communication system meets other performance requirements, such as signal-to-noise ratio (SNR), bit error rate (BER), and spectral efficiency. These requirements ensure that the system can transmit and receive data reliably and efficiently.

In conclusion, the Adjacent Channel Power Ratio (ACPR) is an important parameter that is used to ensure that a communication system does not interfere with the other systems operating in the nearby frequency bands. The ACPR limit is specified by regulatory bodies and varies depending on the type of communication system, the frequency band, and the regulatory body that governs the system. To ensure that a communication system meets the ACPR limit, it is important to design the system to have a well-defined frequency response.

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