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SC Selection Combining

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Selection Combining (SC) is a technique used in wireless communication systems to improve the reliability and quality of received signals in the presence of fading and interference. It is commonly employed in scenarios where multiple copies of the same signal are received through different transmission paths, such as in a multipath fading environment.

The basic idea behind SC is to select the best version of the received signal from multiple replicas and combine them to obtain a more reliable estimate of the transmitted signal. This selection process is based on the received signal's signal-to-noise ratio (SNR) or some other metric that reflects the quality of the received signal.

Here's a step-by-step explanation of the SC process:

  1. Reception of multiple replicas: In a wireless communication system, the transmitted signal may reach the receiver through multiple paths due to reflections, scattering, and diffraction. These multiple copies of the transmitted signal are subject to fading, where the signal strength fluctuates over time due to constructive and destructive interference. The receiver captures these replicas simultaneously.
  2. Calculation of the quality metric: Each received replica is evaluated based on a quality metric, typically the SNR. The SNR represents the power of the received signal relative to the power of the noise present in the channel. Other metrics like the received signal strength indicator (RSSI) or channel state information (CSI) may also be used to assess the quality of the received signal.
  3. Selection of the best replica: The replica with the highest quality metric is selected as the best version of the transmitted signal. This selection is made based on the assumption that the replica with the highest quality metric experienced the least amount of fading and interference.
  4. Combining the selected replicas: The selected replica is combined with the replicas received over time to mitigate the effects of fading and interference. The combining process can be as simple as taking the average of the selected replicas or using more advanced techniques like maximal ratio combining (MRC) or equal gain combining (EGC). These techniques aim to amplify the desired signal and suppress the effects of fading and interference.
  5. Demodulation and decoding: The combined signal is then demodulated and decoded to recover the original transmitted information. The demodulation process depends on the modulation scheme used, while the decoding process is typically based on error-correcting codes such as forward error correction (FEC) or convolutional codes.
  6. Signal quality assessment: After demodulation and decoding, the receiver assesses the quality of the received signal by checking for errors or comparing the received information with the known transmitted information. This assessment provides feedback to the transmitter or higher-level protocols for possible retransmission or other actions to improve the overall communication performance.

Selection Combining offers several advantages in wireless communication systems. By selecting the best replica and combining them, it improves the overall reliability and quality of the received signal. It helps combat the adverse effects of fading, interference, and other impairments, resulting in enhanced system performance and better user experience.