What is SSS Subcarrier Spectrum Sidelobe

Subcarrier Spectrum Sidelobe (SSS) Explained Technically

In Orthogonal Frequency-Division Multiplexing (OFDM) based communication systems, Subcarrier Spectrum Sidelobes (SSS) are unwanted spectral components associated with each subcarrier. While the primary signal power is concentrated on the designated subcarriers, these sidelobes extend beyond the intended bandwidth, potentially causing interference to adjacent channels.

Here's a breakdown of the technical details of SSS:

Origin of SSS:

• OFDM divides the available channel bandwidth into multiple narrowband subcarriers. Ideally, each subcarrier would occupy a perfect rectangular spectrum with no leakage outside its allocated band.
• However, due to the finite nature of the transmitted signal (i.e., it has a defined start and end point in time), the abrupt transitions between on and off states create spectral roll-off. This roll-off manifests as sidelobes flanking the main lobe of the subcarrier spectrum.

Impact of SSS:

• The level of SSS depends on the specific windowing function used to shape the subcarrier waveforms. Common windowing functions like rectangular, raised cosine, and Hann offer varying trade-offs between spectral containment (lower sidelobe levels) and signal bandwidth efficiency.
• High levels of SSS can lead to:
  • Adjacent Channel Interference (ACI): When the sidelobes of a subcarrier bleed into the bandwidth of neighboring channels, they can interfere with the signals transmitted on those channels, degrading communication quality.
  • Out-of-Band Emissions: Excessive sidelobes can violate regulatory limits on out-of-band emissions, potentially causing interference to other systems operating in nearby frequency bands.

Techniques to Mitigate SSS:

• Windowing Functions: As mentioned earlier, careful selection of the windowing function used to shape the subcarrier waveforms can significantly reduce sidelobe levels. Functions like raised cosine and Hann offer better sidelobe suppression compared to a rectangular window.
• Cyclic Prefix (CP): In OFDM systems, a Cyclic Prefix (CP) is a cyclic extension of the symbol appended to the beginning of each symbol. This CP allows for channel equalization and also helps mitigate the effects of inter-symbol interference (ISI) caused by multipath propagation. Additionally, the CP acts as a guard interval between symbols, preventing overlap between the main lobe and the sidelobes of adjacent symbols.

Understanding SSS is crucial for engineers working on:

• OFDM system design and optimization.
• Spectral efficiency improvement in communication systems.
• Minimizing interference in congested frequency bands.

Additional Considerations:

• The trade-off between spectral efficiency (narrower subcarriers) and sidelobe levels is a key factor in OFDM system design.
• Advanced techniques like filtering and precoding can be employed in specific scenarios to further suppress sidelobes.

By understanding and mitigating SSS, engineers can design efficient and reliable OFDM communication systems that minimize interference and optimize spectrum utilization.