SLM Selective Mapping

Selective Mapping (SLM) is a technique used in Orthogonal Frequency Division Multiplexing (OFDM) systems to reduce the effects of peak-to-average power ratio (PAPR) and improve system performance. OFDM is a widely used modulation scheme in modern wireless communication systems, such as Wi-Fi, 4G, and 5G.

In an OFDM system, data is transmitted over multiple subcarriers, which are closely spaced in the frequency domain. Each subcarrier carries a lower-rate data stream, and together they form a parallel transmission system. The parallel transmission allows for efficient use of the available frequency spectrum and provides resistance against multipath fading.

However, one of the challenges in OFDM is the high PAPR, which refers to the difference between the peak power and average power of the transmitted signal. High PAPR can lead to non-linear distortion in the power amplifiers, reducing the overall system efficiency and causing out-of-band radiation.

Selective Mapping (SLM) is a PAPR reduction technique that works by transmitting multiple versions of the same OFDM symbol, each with a different phase sequence. The different phase sequences are generated using a phase rotation algorithm, such as a random phase sequence or a predetermined set of phase sequences.

The SLM process can be summarized as follows:

1. Generate multiple phase sequences: Several different phase sequences are generated. The number of sequences depends on the desired PAPR reduction and the system requirements.
2. Apply phase rotation to the OFDM symbol: Each phase sequence is multiplied with the original OFDM symbol, resulting in multiple phase-rotated versions of the symbol.
3. Select the version with the lowest PAPR: The version of the symbol with the lowest PAPR is chosen for transmission. This selection is done based on a comparison of the PAPR values of the different versions.
4. Transmit the selected symbol: The selected symbol is transmitted over the wireless channel.
5. Notify the receiver: The transmitter sends information to the receiver about the selected phase sequence so that the receiver can reconstruct the transmitted symbol correctly.

At the receiver side, the SLM process is reversed to recover the original transmitted symbol. The receiver applies the same phase rotation as the transmitter using the information provided, and then performs the necessary demodulation and decoding processes.

The key advantage of SLM is its ability to reduce the PAPR without requiring any changes to the existing OFDM receiver structure. It provides flexibility in choosing the phase sequences and allows for efficient PAPR reduction with relatively low complexity. However, SLM does introduce some overhead in terms of additional signaling and computation requirements at both the transmitter and receiver sides.

Overall, Selective Mapping (SLM) is an effective technique to mitigate the high PAPR in OFDM systems, enabling improved performance and reduced distortion in power amplifiers.