Constant Average Power

Constant Average Power (CAP) is a property of some modulation schemes that ensures that the transmitted signal has a constant average power over time. In other words, the power of the transmitted signal is constant on average, even though the instantaneous power may vary.

CAP is important in communication systems because it helps to reduce the peak-to-average power ratio (PAPR) of the transmitted signal, which can cause distortion and interference in the signal. A high PAPR can also reduce the efficiency of the power amplifier used to amplify the signal.

Some modulation schemes, such as amplitude modulation (AM), have a variable average power and therefore do not have the CAP property. However, other modulation schemes, such as frequency modulation (FM) and phase modulation (PM), can be designed to have the CAP property.

One common method for achieving CAP in FM and PM modulation is to use pre-emphasis and de-emphasis circuits in the transmitter and receiver, respectively. These circuits adjust the amplitude of the modulating signal before and after modulation, such that the average power of the transmitted signal is constant.

Overall, the CAP property is an important consideration in designing modulation schemes for communication systems, particularly those that require high efficiency and low distortion.