Peak-to-Average Power Ratio (PAPR) is a critical metric in signal transmission, especially in communication systems, where it measures the ratio of the maximum instantaneous power of a signal to its average power. High PAPR values can lead to distortion, reduced efficiency, and increased cost in amplification systems. Therefore, PAPR reduction techniques aim to mitigate these issues by minimizing the difference between the peak and average power levels of a signal. Here are some principles of PAPR reduction techniques in AC (alternating current) signal transmission:
Clipping and Filtering: This technique involves clipping the peaks of the AC signal to reduce its amplitude, followed by filtering to remove the resulting distortion. While this method can lower the PAPR, it introduces non-linear distortion, which may impact signal quality and require more complex filtering.
Peak Windowing: In this method, the signal is divided into smaller blocks, and the peak values within each block are reduced by applying a window function. This process reduces the chances of high peaks occurring in the overall signal, thus lowering the PAPR. However, it might introduce spectral spreading and adjacent channel interference.
Selected Mapping (SLM): SLM is a probabilistic technique where multiple versions of the transmitted signal are generated, and the one with the lowest PAPR is chosen for transmission. The receiver is provided with information to reverse the chosen mapping, ensuring correct signal recovery. While effective, this method requires additional overhead for transmitting the mapping information.
Partial Transmit Sequences (PTS): PTS divides the signal into disjoint subblocks, and different phase sequences are applied to each subblock. This reduces the chances of high peaks occurring across the entire signal. The receiver uses side information to determine the correct phase sequences and reconstruct the original signal.
Tone Reservation: This technique sets aside specific tones (frequencies) in the signal to account for potential high peaks. By intentionally allocating some power to these reserved tones, the PAPR of the remaining signal is reduced. However, this method requires careful frequency planning and coordination.
Active Constellation Extension (ACE): ACE extends the signal's constellation points beyond their nominal values to create additional "virtual" points. This expansion allows for more flexibility in representing the signal and reduces the probability of large peaks, thus lowering the PAPR.
Precoding Techniques: Precoding involves applying a linear transformation to the original signal before transmission. The transformation is designed to reduce the PAPR while maintaining the integrity of the transmitted information. Examples include the Tomlinson-Harashima Precoding (THP) and the Clipping Precoding technique.
Probabilistic Amplitude Clipping (PAC): PAC is a statistical approach that applies amplitude clipping to the signal with a certain probability distribution. This reduces the likelihood of high peaks occurring and offers a trade-off between PAPR reduction and distortion.
It's important to note that each PAPR reduction technique comes with its advantages and drawbacks, and the choice of technique depends on the specific application, signal characteristics, and the desired trade-offs between PAPR reduction, signal quality, complexity, and implementation cost.