Terahertz channels in atmospheric conditions: Propagation characteristics and security performance

Abstract

With the growing demand for higher wireless data rates, the interest in extending the carrier frequency of wireless links to the terahertz (THz) range has significantly increased. For long-distance outdoor wireless communications, THz channels may suffer substantial power loss and security issues due to atmospheric weather effects. It is crucial to assess the impact of weather on high-capacity data transmission to evaluate wireless system link budgets and performance accurately. In this article, we provide an insight into the propagation characteristics of THz channels under atmospheric conditions and the security aspects of THz communication systems in future applications. We conduct a comprehensive survey of our recent research and experimental findings on THz channel transmission and physical layer security, synthesizing and categorizing the state-of-the-art research in this domain. Our analysis encompasses various atmospheric phenomena, including molecular absorption, scattering effects, and turbulence, elucidating their intricate interactions with THz waves and the resultant implications for channel modeling and system design. Furthermore, we investigate the unique security challenges posed by THz communications, examining potential vulnerabilities and proposing novel countermeasures to enhance the resilience of these high-frequency systems against eavesdropping and other security threats. Finally, we discuss the challenges and limitations of such high-frequency wireless communications and provide insights into future research prospects for realizing the 6G vision, emphasizing the need for innovative solutions to overcome the atmospheric hurdles and security concerns in THz communications.

Keywords: Atmospheric conditions; Atmospheric turbulence; Channel propagation characteristic; Physical layer security; Rain; Snow; Terahertz wireless channel.

Fig. 1

6G enabled by THz communications.

Fig. 2

Impact of atmospheric effect on THz communications.

Fig. 3

Channel path loss in free space. (T = 0 °C, P = 1013 hPa and RH = 30%, 60% and 90%).

Fig. 4

Rayleigh and Mie scattering.

Fig. 5

Atmospheric turbulence - induced phase front distortion and power fluctuation at receiver.

Fig. 6

Schematic measurement setup. Source: Reprinted from [56] with the permission of Elsevier.

Fig. 7

Attenuation by rain under a rainfall rate of 12 mm/hr (heavy rain) when the Marshall-Palmer distribution employed. (T = 25 °C, P = 1013 hPa and RH = 97%).

Fig. 8

(a) Receive spectra of broadband THz pulses propagating either through free space (red) or rain (blue). (b) Phase spectra of transmitted THz pulses with the same legend as Fig. 8a. The black curve corresponds to the difference between both after using an unwrapping algorithm.

Fig. 9

Degradation of a THz channel in rain as a function of transmitted power from the antenna at the transmitter side. (a) Schematic of measurement setup. (b) Measured real-time BER performance of the THz link as a function of the received power at a data rate of 5 Gbps. Source: Reprinted from [56] with the permission of Elsevier.

Fig. 10

Power attenuation on wireless channelsoperating at (a) 140 GHz, (b) 220 GHz, (c) 340 GHz and (d) 675 GHz caused by rain at rain rates between 50 mm/hr and 450 mm/hr over a channel distance of 4m. Source: Reprinted from [56] with the permission of Elsevier.

Fig. 11

Schematic representation of the outdoor setup designed to gauge rainfall attenuation experienced by a 140 GHz channel.

Fig. 12

Power loss due to (a) dry snow at −1 °C and (b) wet snow (water content 25%) at 0 °C, under the G-M distributions with a snowfall rate (equivalent rainfall rate) of 10 mm/hr. (P = 1013hPa, RH=97%). (b) keeps the same legend with (a).

Fig. 13

Predicted average BER performance of the channels operating at (a) 140 GHz and (b) 220 GHz. The upper and lower bounds of the predicting area correspond to the predictions by ITU model (dry) and Scott model, respectively. Channel distance 1 km, relative humidity RH 50%, temperature 0 °C, transmitted power 20 dBm, noise level of receiver −60 dBm; the gain at the transmitter and receiver side are identical and equals to 40 dB (combination of antenna and lens).

Fig. 14

(a) THz wireless link measurement setup; (b) Measurement with (blue) and without snow (black), and the calculated result (red). Source: Reprinted from [160] with the permission of Springer Nature.

Fig. 15

THz channel measurement setup implemented in the campus of Beijing Institute of Technology (BIT). (a) Outdoor channel on the rooftop of Building 4 at BIT with both transmitter (Tx) and receiver (Rx) safeguarded by waterproof coverings; CDF profile for received SNR with and without snowfall with operating frequencies at (b) 220 GHz, alongside the fitted CDF to the measured data at (c) 220 GHz in snowfall conditions.

Fig. 16

Power loss relative to the LWE precipitation rate for the 120 GHz and 220 GHz channels, over distances of 21 m and 11 m, respectively. (b) keeps an identical legend and marks as (a).

Fig. 17

Scintillation attenuation for THz wave under different turbulence strengths. Source: Reprinted from [50] with the permission of Springer Nature.

Fig. 18

Schematics of a Mach-Zehnder interferometer setup to characterize turbulences with visible light. Inset: Position of photodectors D1, D2 relative to interference fringe intensity. (a) Attenuation of THz channel power as the function of time, for warm air at high speed (b) corresponding Log(BER) of THz channel as the function of time.

Fig. 19

Atmospheric attenuation spectrum for different humidity levels.

Fig. 20

Geographic of the point-to-point THz channel with a security attacker (Eve) in rain or snow, together with the coordinate system of the legitimate (LoS) and eavesdropping (NLoS) channels.

Fig. 21

(a) LoS (solid) and NLoS (dashed) link gain with respect to rainfall rate with carriers at 140 GHz (black), 220 GHz (blue) with Eve at a position of (200 m, 10 m) and Bob located at (1 km, 0 m). (T = 25 °C, P = 1013 hPa and RH = 97%. Secrecy capacity distribution for 2-D positions of Eve when (b) 140 GHz channel, (c) 220 GHz channel, and (d) 340 GHz channel with rainfall rate Rr = 15 mm/hr and Bob located at (1 km, 0 m). (T = 25 °C, P = 1013 hPa and RH = 97%. The color bar denotes the safe transmission rate in Gbps).

Fig. 22

(a) LoS (solid) and NLoS (dashed) channel gain with respect to snowfall rate (equivalent rainfall rate) with carriers at 140 GHz (black), 220 GHz (blue), 340 GHz (red) with Eve at a position of (200 m, 10 m) and Bob located at (1 km, 0 m). (T = −1 °C, P = 1013 hPa and RH = 97%). Secrecy capacity distribution for 2-D positions of Eve when (b) 140 GHz channel, (c) 220 GHz channel and (d) 340 GHz channel with snowfall rate (equivalent rainfall rate) Rr = 15 mm/hr and Bob located at (1 km, 0 m). (T = 25 °C, P = 1013 hPa and RH = 97%. The color bar denotes the safe transmission rates in Gbps).

Fig. 23

LoS (solid) and NLoS (dashed) channel gain with respect to snowfall rate (equivalent rainfall rate) with carriers at 140 GHz (black), 220 GHz (blue) and 340 GHz (red) with Eve positioned at (200 m, 10 m) and Bob located at (1 km, 0 m). (T = 0 °C, P = 1013 hPa and RH = 97%). Secrecy capacity distribution for 2-D positions of Eve when (b) 140 GHz channel, (c) 220 GHz channel and (d) 340 GHz channel with snowfall rate (equivalent rainfall rate) Rr = 15 mm/hr and Bob located at (1 km, 0 m). (T = 25 °C, P = 1013 hPa and RH = 97%. The color bar denotes the safe transmission rates in Gbps).

Fig. 24

Attenuation due to atmospheric turbulence with different strengths. (pressure P = 1013 hPa, humidity RH = 20%, channel distance d = 1km).

Fig. 25

(a) Evolution of channel gain received by Bob (LoS) and Eve (NLoS) versus turbulence strength when Eve is located at (500 m, 50 m); (b) Evolution of channel secrecy capacity distribution versus x-position of Eve (y = 50 m); (c) Evolution of channel secrecy capacity distribution versus y-position of Eve (x = 500 m); (d) Secrecy capacity distribution for 2-D positions of Eve with a unit of Gbps in the color bar.

Fig. 26

Variation of secrecy capacity with respect to the y-position of Eve under different (a) carrier frequencies, (b) turbulence strengths, (c) divergence angles, and (d) receiver sensitivities and FOV angle for Eve. Inset of (a): secrecy capacity distribution for 2-D positions of Eve with a unit of Gbps in the color bar.

Fig. 27

Variation of outage probability with respect to the y-position of Eve for different (a) carrier frequencies, (b) turbulence strengths, (c) divergence angles, and (d) receiver sensitivities and FOV angle of Eve.

Fig. 28

Outline of future research directions.