Little or no equalization is needed in energy-efficient sub-THz mobile access

TL;DR

This study advocates the use of very simple modulation and equalization techniques for sub-THz mobile access, and shows that single-carrier or low-number-of-subcarriers modulations are very attractive competitors to the dramatically more complex and energy inefficient traditional multi-carriers designs.

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.

Figures (6)

• Figure 1: Pictorial representation of the radio channel comprising of a multipath propagation channel and (a) low directivity antennas; (b) high directivity antennas. In contrast to case (a), using highly directive antennas as in case (b) significantly attenuates most multipath components of the propagation channel. As shown experimentally in the remainder of this study, this effect reduces inter-symbol interference, and hence simplifies equalization.
• Figure 2: Schematic visualization of the measurement scenario. The upper drawing is a top-down illustration of the atrium's dimension and composition. The green star represents the TX's first position and the blue star it's second. The dashed lines indicate both the TX's as well the RX's orientation during the measurement campaign's execution. The lower drawing illustrates the TX's and RX's height, as well as the measured distances.
• Figure 3: Power delay profile of three distinguishing measurements (red, green and blue) with enlarged range around the first peak (upper right corner). The time axis is given in channel taps as integers (one tap corresponds to 0.25ns).
• Figure 4: Achievable spectral efficiency for single-carrier modulation with $N$-taps linear equalization, in the considered indoor scenario assuming $\mathrm{SNR}=6$ dB. The line plots are the median value over all measurement points, and the matched filter upper bound. The variations across measurement points are captured by the empirical cumulative density function, illustrated through a color map, and truncated within the range of observed spectral efficiencies. Due to the specific structure of the measured channels, single-carrier modulation with very few equalization taps appears sufficient to achieve spectral efficiencies of about $1.4-2.3$ bit/s/Hz, and, hence, score the capacity gains promised by sub-THz networks in an energy efficient manner.
• Figure 5: Achievable spectral efficiency for multi-carrier modulation with $K$ subcarriers and no prefix, in the considered indoor scenario assuming $\mathrm{SNR}=6$ dB. The line plots are the median value over all measurement points, and the matched filter upper bound. The variations across measurement points are captured by the empirical cumulative density function, illustrated through a color map, and truncated within the range of observed spectral efficiencies. Using a low number of subcarriers and no prefix is sufficient to score the target capacity goals, with a much better PAPR than conventional OFDM.