[Paper Review] Spectrally-Precoded OFDM for 5G Wideband Operation in Fragmented sub-6GHz Spectrum

Renaud-Alexandre Pitaval, Branislav M. Popović+3 more|arXiv (Cornell University)|Jun 2, 2016

PAPR reduction in OFDM1 references3 citations

TL;DR

This paper proposes spectrally-precoded OFDM for 5G wideband transmission in fragmented sub-6GHz spectrum, using precoding of data symbols before OFDM modulation to achieve extremely low out-of-band emission and sharp spectral roll-off. The technique enables high spectral efficiency and flexible spectrum usage, particularly effective for wideband, short-duration transmissions in dense, fragmented spectrum bands.

ABSTRACT

We consider spectrally-precoded OFDM waveforms for 5G wideband transmission in sub-6GHz band. In this densely packed spectrum, a low out-of-band (OOB) waveform is a critical 5G component to achieve the promised high spectral efficiency. By precoding data symbols before OFDM modulation, it is possible to achieve extremely low out-of-band emission with very sharp spectrum transition enabling an efficient and flexible usage of frequency resources. Spectrally-precoded OFDM shows promising results for reaching 5G targets in high-data rate enhanced mobile broadband and ultra-reliable low-latency communications use cases. Spectral precoding is particularly efficient for wideband transmission enabling short-time transmission, which will often require flexible fragmented spectrum usage.

Motivation & Objective

• To address the challenge of spectral efficiency in 5G wideband transmission within fragmented sub-6GHz spectrum.
• To reduce out-of-band (OOB) emissions in dense spectrum environments to meet 5G spectral efficiency targets.
• To enable flexible, dynamic spectrum usage in wideband, short-duration transmissions.
• To develop a waveform that supports enhanced mobile broadband and ultra-reliable low-latency communications (URLLC) use cases in fragmented spectrum.

Proposed method

• Precoding of data symbols before OFDM modulation to shape the spectrum and suppress out-of-band emissions.
• Designing a precoding filter that enables sharp spectral roll-off with minimal spectral regrowth.
• Using a time-domain precoding approach to control the frequency-domain spectrum shape without modifying the OFDM structure.
• Applying a spectrally precoded OFDM waveform that maintains orthogonality while minimizing spectral regrowth.
• Optimizing the precoding filter coefficients to achieve minimal out-of-band power for a given spectral mask.
• Evaluating the performance in terms of spectral efficiency and OOB emission under fragmented spectrum conditions.

Experimental results

Research questions

• RQ1 How can out-of-band emissions be minimized in wideband 5G transmissions within fragmented sub-6GHz spectrum?
• RQ2 Can spectrally-precoded OFDM achieve sharp spectral roll-off while maintaining orthogonality and low peak-to-average power ratio?
• RQ3 What is the achievable spectral efficiency and OOB suppression performance of spectrally-precoded OFDM in fragmented spectrum scenarios?
• RQ4 How does spectrally-precoded OFDM support flexible, dynamic spectrum access in wideband 5G systems?
• RQ5 What is the impact of precoding on system latency and reliability in enhanced mobile broadband and URLLC use cases?

Key findings

• Spectrally-precoded OFDM achieves extremely low out-of-band emission, enabling efficient spectrum sharing in dense sub-6GHz bands.
• The waveform enables sharp spectral roll-off, allowing high spectral efficiency and minimal interference to adjacent channels.
• The technique is particularly effective for wideband, short-duration transmissions common in 5G use cases.
• Precoding significantly reduces spectral regrowth while preserving the orthogonality of the OFDM signal.
• The method supports flexible, dynamic spectrum usage in fragmented sub-6GHz bands, meeting 5G requirements for enhanced mobile broadband and URLLC.
• The approach maintains low peak-to-average power ratio, supporting energy-efficient transmission in practical systems.