New Millimeter, Terahertz, and Light-Wave Frequencies for Satellite Communications

Abstract

The last 50 years of satellite communications has followed a consistent trend to produce networks that can provide higher and higher rates of throughputs at lesser cost. Closely linked to this trend has been a parallel effort to seek more efficient use of the allocated frequencies. The first satellite systems were power limited, but as satellite engineers designed more powerful spacecraft, the challenge has been more and more to find ways to use frequencies more efficiently. In short, in a digital world, the objective has become to send more bits of information per available Hz of radio frequency. This has been primarily accomplished by using greater complexity in the coding and multiplexing systems. This has also been achieved by polarization isolation and higher-gain antennas (and thus narrower spot beams) that enable geographic isolation of the transmitted beams. If these narrow beams are spread sufficiently apart, this reduces interference and the radio frequencies can be reused over and over again. This process also minimizes the effective path loss of irradiated power by concentrating the beam to a tighter area.

This 50 years of satellite progress can be summarized by the ever-increasing power levels, frequency allocations, and system complexity to increases throughput efficiency. This “complexity” has allowed more throughput of information via the spectrum that is available. This is now typically measured in the metric of “digital bits” per hertz.

The fixed-satellite services (FSS), the mobile satellite services (MSS), and the broadcast satellite services (BSS) each in their own ways have applied this process to exploit the available frequency bands progressively over time. The lower-frequency bands have been used up first to meet initial demand in the earliest years. This is simply because these bands are easier to use. This is primarily because there is less rain attenuation in the lower frequencies and the radio transmission equipment and antennas are easier to design, manufacture, and use. In the case of the fixed-satellite services, the C band (at 6 and 4 GHz) was used first. Then the Ku bands (at 14 and 12 GHz) were utilized next and then they became largely saturated. Currently the greatest amount of expansion is in the so-called Ka band (this is the 30 and 20 GHz bands) that requires high power and encoding complexity to overcome rain attenuation issues. Despite the efficiency gains that come with the use of higher power, high-gain antennas, and coding complexity the current commercial satellite frequencies (in C, Ku, and Ka band) wiil eventually saturate. This is because of ever increasing demand for broadband video and data services and expanding access to users around the world. Further the satellite allocation for C-band was reduced as a fully protected service.

The next frontier thus seems to be the so-called Q/V bands of 47.2–50.2 GHz and 37.5–40.5 GHz, and beyond that, the expansion will be to even higher frequencies such as the W band, the terahertz (THz) band, and even the light-wave frequencies. The use of such high frequencies with ever-shrinking wavelengths is a challenge for satellite service, because atmospheric conditions make the use of such spectrum very difficult indeed. The one area where satellites have an advantage would be for transmissions that occur above the Earth’s atmosphere. The use of light waves or laser communications for intersatellite links (ISLs) or cross-links to connect satellites in orbit is not only possible but is starting to be used for this purpose. Laser cross-links for low Earth orbit constellations is easiest, but this is also possible for medium Earth orbit constellations or even GEO satellites.

There are many challenges represented by the higher radio frequency (RF) bands. These challenges include building radio equipment that can operate effectively and efficiently at these exceeding challenging frequencies. The great challenge is to utilize these microscopic wavelengths and to cope with the atmospheric interference that tends to block the signals at the Q/V and W bands and higher. Here it is a matter of not only rain scatter of the signal but also the oxygen absorption, scintillation, and other problems that weaken, distort, or otherwise interfere with satellite signals in these millimeter wave and even the THz frequency ranges. Light-wave transmissions from ground to space and back constitute an even greater difficulty.

Nevertheless, satellite communications systems of the future can be expected to operate in these challenging frequencies. In order to do so, however, new modulation and multiplexing equipment, signal regeneration, coding systems, antennas, and power systems will likely all be needed to deliver secure and reliable service in the future. In the meantime, better coding processes, higher power transmission, and improved and higher-gain antenna can extend and expand the efficiency of usage of the lower-frequency bands. It is possible that instead of such a heavy reliance on satellites in the GEO orbits, lower orbit satellite constellations and high-altitude platforms (HAPS) can also be deployed in the Ku band and Ka band to provide additional throughput capabilities. A third factor to consider in assessing future demand is the additional build-out of high-capacity fiber-optic networks. These extremely broadband systems could also serve to reduce the demand for future satellite services. Nevertheless, the demand for mobile, rural, and remote services plus broadcasting and multi-casting services should still sustain satellite growth for some time to come.

This chapter, in particular, focuses on the technical, operational, and practical issues associated with the development and future deployment of future satellites in the Q/V and W bands. It also briefly discusses the even higher terahertz frequencies and light-wave or laser transmission.