[Paper Review] Millimeter Wave and Terahertz Urban Microcell Propagation Measurements and Models

Yunchou Xing, Theodore S. Rappaport|arXiv (Cornell University)|Sep 28, 2021

Millimeter-Wave Propagation and Modeling30 references104 citations

TL;DR

This paper presents extensive outdoor millimeter wave and terahertz propagation measurements in urban microcell environments across 28–142 GHz, comparing path loss, delay spread, and angular spread. It finds remarkably similar path loss exponents across frequencies when referenced to 1 m free-space loss, indicating that base station spacing need not change as carrier frequencies increase toward THz bands.

ABSTRACT

Comparisons of outdoor Urban Microcell (UMi) large-scale path loss models, root mean square (RMS) delay spreads (DS), angular spreads (AS), and the number of spatial beams for extensive measurements performed at 28, 38, 73, and 142 GHz are presented in this letter. Measurement campaigns were conducted from 2011-2020 in downtown Austin, Texas, Manhattan (New York City), and Brooklyn, New York with communication ranges up to 930 m. Key similarities and differences in outdoor wireless channels are observed when comparing the channel statistics across a wide range of frequencies from millimeter-wave to sub-THz bands. Path loss exponents (PLEs) are remarkably similar over all measured frequencies, when referenced to the first meter free space path loss, and the RMS DS and AS decrease as frequency increases. The similar PLEs from millimeter-wave to THz frequencies imply that spacing between cellular base stations will not have to change as carrier frequencies increase towards THz, since wider bandwidth channels at sub-THz or THz carrier frequencies will cover similar distances because antenna gains increase quadratically with increasing frequency when the physical antenna area remain constant.

Motivation & Objective

• To characterize outdoor urban microcell (UMi) propagation at millimeter wave and sub-THz frequencies from 28 to 142 GHz.
• To compare large-scale path loss models, RMS delay spread, and angular spread across multiple frequency bands.
• To evaluate the impact of frequency on channel sparsity and beamforming requirements for future 6G systems.
• To provide empirical data for designing multi-band, high-capacity wireless systems at THz frequencies.

Proposed method

• Conducted four large-scale outdoor measurement campaigns in Austin, Manhattan, and Brooklyn from 2011 to 2020, covering ranges up to 930 m.
• Used wideband sliding correlation-based channel sounding systems with rotatable horn antennas at each frequency band.
• Applied the close-in (CI) free-space reference distance model with d₀ = 1 m to standardize path loss comparisons across frequencies.
• Calculated path loss exponents, shadow fading, RMS delay spread, and angular spread (AOA/AOD) using a 30 dB power threshold to detect multipath components.
• Analyzed both omnidirectional and directional channel models, including CIF (close-in free-space) path loss models.
• Used consistent antenna beamwidths (7–10°) across bands to ensure comparability of angular statistics.

Experimental results

Research questions

• RQ1 How do path loss exponents vary across 28, 38, 73, and 142 GHz in outdoor UMi environments?
• RQ2 How does RMS delay spread change with increasing frequency in LOS and NLOS scenarios?
• RQ3 How do angular spreads (AOA and AOD) and the number of spatial paths evolve with frequency?
• RQ4 To what extent do channel characteristics at sub-THz bands resemble those at mmWave bands when referenced to 1 m free-space path loss?
• RQ5 What implications do these findings have for base station spacing and beamforming design in 6G systems?

Key findings

• Path loss exponents (PLEs) are remarkably similar across 28–142 GHz when referenced to 1 m free-space path loss: 2.3 at 28 GHz, 1.9 at 38 GHz, 2.0 at 73 GHz, and 2.1 at 142 GHz.
• RMS delay spread decreases with increasing frequency: 26 ns at 28 GHz, 11 ns at 38 GHz, 23 ns at 73 GHz, and 9 ns at 142 GHz.
• RMS angular spread (AOA and AOD) decreases with frequency, indicating sparser channels at higher bands: mean AOA spread drops from 30.8° at 28 GHz to 10.1° at 142 GHz in LOS.
• The average number of AOA directions decreases from 3.6 at 28 GHz to 1.9 at 142 GHz in LOS, and from 4.7 to 1.6 in NLOS, confirming increased channel sparsity at higher frequencies.
• Shadow fading standard deviation decreases with frequency: 4.3 dB at 28 GHz, 3.5 dB at 38 GHz, 1.9 dB at 73 GHz, and 2.8 dB at 142 GHz.
• The similarity in PLEs across mmWave and sub-THz bands implies that base station spacing does not need to be reduced as carrier frequency increases, provided antenna aperture remains constant.