Terahertz (THz) wireless systems for space applications

NASA has been leading the Terahertz (THz) technology development for the sensors and instruments in astronomy in the past 20 years. THz technologies are expanding into much broader applications in recent years. Due to the vast available multiple gigahertz (GHz) broad bandwidths, THz radios offer the possibility for wireless transmission of high data rates. Multi-Gigabits per second (MGbps) broadband wireless access based on THz waves are closer to reality. The THz signal high atmosphere attenuation could significantly decrease the communication ranges and transmittable data rates for the ground systems. Contrary to the THz applications on the ground, the space applications in the atmosphere free environment do not suffer the atmosphere attenuation. The manufacturing technologies for the THz electronic components are advancing and maturing. There is great potential for the NASA future high data wireless applications in environments with difficult cabling and size/weight constraints. In this study, the THz wireless systems for potential space applications were investigated. The applicability of THz systems for space applications was analyzed. The link analysis indicates that MGbps data rates are achievable with compact sized high gain antennas.


INTRODUCTION
Previously, Terahertz (THz) technology has been driven by applications in astronomy. In the past 20 years, NASA has successfully launched and deployed scientific satellites with THz instruments and sensors for applications in astronomy [1,2]. The recent research and development activities in THz technologies are expanding into much broader applications such as security screening, medical imaging, and wireless sensors and communications [3][4][5][6][7][8].
There is no limit on the demand for the data rates and capacity of wireless communications for today's applications. Thus, new technologies for spectral efficient modulations and the reduction of interference were developed to achieve the growth of data rates in recent years. To meet the demand, new technologies are needed to offer data capacity and to reduce energy consumption requirements in the future wireless networks. One possibility is the exploitation of new frequency spectrum for the radio systems. In the THz range of the frequency spectrum (from 300 GHz to 3000 GHz), multiple gigahertz channel bandwidths are available, shown in Figure 1. This provides the possibility to transmit multi-gigabits per second (MGbps) data rates with less power consumption and higher channel capacity of the network.
The millimeter wave (MMW) technologies have been successful demonstrated at 220 GHz carrier frequency with 25 Gbps data rates [9]. The semiconductor devices and photodiodes for the THz frequency band between 300 and 500 GHz have been demonstrated [10]. The manufacturing technologies for the THz electronic components are advancing and maturing. THz-band wireless systems have some desired advantages over currently available wireless systems. Much higher bandwidths are available than the conventional microwave and millimeter wave systems. The THz system efficiency is higher than laser systems. The THz-band signals have smaller attenuations than the optical/laser signals. The capability of transmission and reflection off the dielectric materials could be useful in many non line of sight indoor applications for the THz waves.  An important advantage of the THz wireless system is the potential low complexity system design with the simplest modulation schemes coupled with the multiple GHz channel bandwidth to achieve the multi-gigabit throughput performance.  achievable dat sized high ga feasibility stu stem capability ould also be te uiring precisio a beamwidth is a beamwidth d Thus, the anten nts will be hi half-power beamwi ngth of the TH , a high gain an aperture, whic ations. Recent na system wa a reflector ante lly fed by a b ts show a 60-6 1 and 2 THz w anned confoc reported [12] B gain at 0.67 lector antenna diam the required re achieve a 60 orted in [11] gain. The THz radio system has an advantage for compact sized high gain antenna design. However, technical challenges still exist in the practical THz antenna system design and fabrication.
Recently, a breakthrough on nano phased array (NPA) design was reported [13]. It demonstrated a large-scale twodimensional array antenna in which 64 × 64 (4,096) elements are densely integrated on a silicon chip with all of the nanoantennas precisely balanced in power and aligned in phase to generate a designed, sophisticated radiation pattern in the far field. A phased array antenna could electronically steer the antenna beam at the target with high precision, which is an advantage over the conventional reflector antenna. This technology breakthrough could greatly increase the THz system applications in the future for non point to point fixed wireless network.

II. SPACECRAFT LINK ANALYSIS
Wireless communication is an enabling technology for both manned and unmanned spacecraft; it enables untethered mobility of crew and instruments, increasing safety and science return, and decreasing mass and maintenance costs by eliminating cabling.
Terahertz wireless system links for potential space applications are theoretically analyzed in this section. The wireless link is assumed to have an additive white Gaussian noise (AWGN) channel. In such an AWGN channel the theoretical maximum data rate is defined by its capacity which can be calculated with the Shannon formula C=B log 2 (1+SNR) (1) where SNR is the signal-to-noise ratio and B is the available bandwidth in the channel. The noise power can be calculated from the thermal motion of the charges in the receiver. The additional noise due to the non ideal receiver is defined by the noise figure F. Hence, the signal-to-noiseratio is SNR=P r /(FkTB) (2) where P r is the received power, k is the Boltzmann constant, and T is the ambient temperature. According to the Farri free-space path-loss model, the received power is where P t is the transmitted power, G t and G r are antenna gain for transmitter and receiver, and L s is the space loss. Therefore, with (2) and (3) in (1) we can calculate the maximum achievable data rate for the proposed wireless THz links. Note that this maximum data rate is the theoretical upper limit. The following parameters were assumed in the following data rate calculations. The frequency is 0.5 THz (500 GHz); the bandwidth is 10 GHz or 50 GHz; the transmit power is 10 mW or 1 W; the noise figure is 10 dB; the ambient temperature is 300 K.

A. Interior WLAN
Due to the atmospheric attenuation of THz signals, the practical THz indoor communication distances are limited to 50 meters. Since the diffracted fields or creeping wave at THz frequency are insignificant compared to microwave signals, THz signals could not overcome the structure blockage. THz systems would require line of sight operations between transmitter and receiver. Crews moving in the module could block and disrupt the communication links. A THz wireless system could provide Gbps high data rate WLAN services to the crew modules of the Space Station, as shown in Figure 6 [14]. A 10 dB additional path loss due to the atmospheric attenuation is assumed for the 0.5 THz signals traveling a 10 m distance.  Figure 7 shows the achievable data rate versus required antenna gain for 10 m range interior WLAN applications. A 30 dB gain antenna is needed to compensate for the free space path loss and atmospheric attenuation. The data rate increases with the allocated bandwidth as well as the noise. From (1) the receiver power and the maximum achievable data rate can be calculated. A maximum data rate of 55 Gbps is achieved with a moderate 30 dB gain antenna and 10 mW transmit power, as shown in Figure 7. The rate could be higher with higher channel bandwidth and higher gain antenna, as shown in Figure 8.   ground is typ m monopole or n loss and atmo gher than the m h gain antenna k budget. As fo have a time de th. The data ra terference (ISI propagation pa odulation schem n. This sheet ve indoor THz pagation paths f maximum achieva 0 and 50 GHz chan face Applicatio rate wireless sy ublic outreach maging, radar, mum achievable planet surface s. 10 and 11. equired to have e path loss to na aperture size wn in figure 12 ommunication netw missions [16].  ial interior munication ted that the Gbps per 1 required to overcome the high space loss at THz band. High gain antenna could be compact size at THz band. It's a technical challenge for long range applications at THz band. In addition to high gain antenna, high power transmitters would be required for long range communications to be feasible.