Satellite Spectrum Issues

 
Satellite Spectrum Issues
When you’re walking down the street, or shooting hoops in the nearby park, or sailing
on the lake, you are oblivious to the invisible waves of electromagnetic energy
that bring us TV, radio, and cellular phone calls. The waves aren’t relevant until one
furnishes the right antenna and receiver to tune in their signals.34
Radio waves can be as long as a football field and as short as the size of molecules,
having in common that they both travel at light speed and obey the laws of
physics. Wireless, in this case satellite links, are transmissions through air rather
than plasticor fibers.35 The relationship between the wavelength in fiber and that of
satellite RF links is key to end-to-end broadband for the mobile user.
The frequency of any communication signal is the number of cycles per second
at which the radio wave vibrates or cycles. The distance a wave travels during a single
cycle is called its wavelength. There is an inverse relationship between frequency
and wavelength: the higher the former, the shorter the latter. A cycle of a very low
frequency signal is measured in hertz (Hz), and a frequency of one thousand cycles
per second is known as a kilohertz. One million cycles per second is a megahertz
(MHz) and one billion is a gigahertz (GHz). We refer generally to frequencies from
3 KHz to 300 GHz as the electromagnetic spectrum, although the spectrum continues
into lightwave frequencies and beyond (see Table 4-1).
When commercial communication satellites were launched in the 1960s, they
did not need long radio waves that could bounce around the ionosphere for thousands
of miles. They needed a direct line-of-sight RF link that would travel in a
directed path from the Earth station antenna to the satellite’s antenna. The
International Telecommunication Union, an agency of the United Nations, allocated
frequencies in the Super High Frequency (SHF) range from 2.5 to 22 GHz for satellite
communications. These very short frequencies are called microwaves, with the
same characteristics of visible light.36 Although most communication satellites operate
in the SHF range, military and navigation satellites operate at lower frequencies
that yield a larger footprint for signal reception and require less precision for
acquiring the uplink.
In the late 1970s and early 1980s, the first commercial Ku-band satellites were
launched. Because few terrestrial microwave systems used these frequencies, the
Ku-band satellites could use higher power transmitters without causing interference.
In addition to the higher power, the beam of a Ku-band signal is significantly
Satellite Communications 145
narrower than a comparable C-band parabolic dish antenna,38 correcting the tendency
of one satellite’s uplink signals to interfere with another’s as the geostationary
band crowded up. In orbit locations for North America, satellites are normally separated
by 2 degrees so narrower beamwidths significantly reduces interference.39
One drawback with using frequencies above 10 GHz is that the wavelength is so
short that rain and snow can reduce the strength of the signal.40 Larger antennas are
used to overcome the loss to rain. As frequencies expand into the Ka-band from 17
to 31 GHz, the rain fade issue will become more pronounced. Ka-band satellites,
however, will play a major role in the future. Ku-band satellites are major players
today, demonstrating their potential with increased bandwidth, on-board processing,
and multiple-spot beams. These attributes equate to more throughput, which
provides the critical link to end-to-end solutions.