[Paper Review] Little or no equalization is needed in energy-efficient sub-THz mobile access

Lorenzo Miretti, Thomas Kühne+6 more|arXiv (Cornell University)|Oct 11, 2022

Advanced MIMO Systems Optimization4 citations

TL;DR

This paper demonstrates that energy-efficient sub-THz mobile access can achieve high spectral efficiency with minimal or no channel equalization by leveraging highly directive beamforming, which suppresses multipath components and reduces inter-symbol interference. Using experimental 160 GHz channel measurements, the study shows that single-carrier QPSK modulation with simple linear equalization (7-tap MMSE) enables reliable communication at 1 bit/s/Hz with a 2.5–4 dB gap to the Shannon limit, outperforming complex multi-carrier OFDM designs in energy efficiency.

ABSTRACT

By trading coverage and hardware complexity for abundance of spectrum, sub-THz mobile access networks are expected to operate under highly directive and relatively spectrally inefficient transmission regimes, while still offering enormous capacity gains over current sub-6GHz alternatives. Building on this assumption, and supported by extensive indoor directional channel measurements at 160 GHz, this study advocates the use of very simple modulation and equalization techniques for sub-THz mobile access. Specifically, we demonstrate that, under the aforementioned transmission regimes, little or no equalization is needed for scoring significant capacity gain targets. In particular, we show that single-carrier or low-number-of-subcarriers modulations are very attractive competitors to the dramatically more complex and energy inefficient traditional multi-carrier designs.

Motivation & Objective

• To evaluate whether complex equalization is necessary in energy-efficient sub-THz mobile access systems.
• To assess the impact of highly directive beamforming on inter-symbol interference and channel equalization requirements.
• To compare the performance of single-carrier and multi-carrier modulation in terms of spectral efficiency and energy efficiency under realistic sub-THz channel conditions.
• To validate the feasibility of low-complexity modulation and equalization for reliable communication at 1 bit/s/Hz in indoor sub-THz environments.
• To provide a foundation for energy-efficient 6G sub-THz system design by integrating real-world channel measurements with information-theoretic analysis.

Proposed method

• Conducted extensive indoor directional channel measurements at 160 GHz to capture realistic impulse responses for sub-THz propagation.
• Used measured channel impulse responses to simulate system performance with single-carrier QPSK and multi-carrier OFDM waveforms.
• Applied linear minimum mean-square error (MMSE) equalization with up to 256 taps to evaluate equalization complexity trade-offs.
• Employed standard LDPC codes (rate 1/2, block length 1296) to assess coded bit error rate (BER) performance under inter-symbol interference.
• Treated inter-symbol interference as additive Gaussian noise in the LDPC decoder to simulate practical mitigation.
• Evaluated performance across all measured channel points, reporting median, worst-case, and best-case BER curves.

Experimental results

Research questions

• RQ1 Does highly directive beamforming in sub-THz bands significantly reduce inter-symbol interference, allowing for minimal or no equalization?
• RQ2 Can single-carrier modulation achieve reliable communication at 1 bit/s/Hz in sub-THz bands with low hardware complexity and energy efficiency?
• RQ3 How does the performance of single-carrier QPSK with simple equalization compare to multi-carrier OFDM in terms of spectral efficiency and robustness to channel variations?
• RQ4 What is the gap between practical system performance and the Shannon limit in realistic sub-THz indoor environments?
• RQ5 To what extent can inter-symbol interference be treated as additive noise in sub-THz systems with directive antennas?

Key findings

• Single-carrier QPSK with 7-tap linear MMSE equalization achieves reliable communication at 1 bit/s/Hz with a 2.5–4 dB gap to the Shannon limit across all measured channels.
• The worst-case BER performance at SNR ≈ 6 dB confirms that reliable transmission is feasible even under the most challenging channel conditions.
• Multi-carrier OFDM with K ≥ 16 subcarriers achieves better spectral efficiency than single-carrier modulation, but at the cost of higher peak-to-average power ratio (PAPR) and increased hardware complexity.
• Even with up to 256 equalization taps, the performance gain of multi-carrier systems over single-carrier is marginal in terms of spectral efficiency, especially in worst-case scenarios.
• The experimental channel measurements show that directive beamforming significantly attenuates multipath components, reducing delay spread and simplifying equalization.
• The results support the use of single-carrier modulation as a viable, energy-efficient alternative to complex OFDM-based designs in early-stage sub-THz 6G systems.