RADIO REGULATION AND SPECTRUM MANAGEMENT
FOR SATELLITE COMMUNICATIONS
Part 1
Les Barclay
1INTRODUCTION TO THE RADIO SPECTRUM AND UTILISATION
1.1Introduction to the needs for spectrum management
For line or optical-fibre telecommunication systems, where the system characteristics, standards and protocols to be used do not generally affect unconnected systems, it is in principle only necessary to reach agreement between those concerned with making and using the systems. In contrast, radiocommunication systems and services cannot be considered in isolation. Radiation into the atmosphere or space will spread and overshoot the receiving antenna, dependent on the propagation characteristics, and may then adversely affect other radio systems. Management of use of the radio spectrum and radio regulation is thus essential, to control the chaos which might well ensue.
The radio frequency spectrum is limited in extent and the most useful parts of it are crowded. The growth in demand - for communications; for increased data rates with higher quality; and particularly for mobility - increases the crowding and necessitates the extensive application of co-channel frequency sharing. It is easy to see that two radio systems occupying the same frequency band have the potential for causing harmful interference to each other. The extent of the interference depends on system characteristics and the robustness of the modulation, on the acceptable quality and availability for the service, and also on the propagation characteristics of both the wanted and interfering paths.
As an example of this, in some frequency bands, e.g. below say 30 MHz, ionospheric refraction may permit signals to be propagated world-wide, so that full international coordination between users who wish to use the same or adjacent frequencies is necessary. At higher frequencies satellite communication systems also need world-wide coordination, both for the radio frequencies and system parameters and also for the satellite orbit positions. Terrestrial services at VHF and higher frequencies may have a more limited propagation extent, but even so interference may well be caused far beyond national boundaries. In such cases regional agreements and regional coordination is necessary.
To permit maximum spectrum occupancy, it is necessary to design modulation methods which can transfer the information with minimum impact - either by using the minimum bandwidth (the traditional approach) or by minimising the product of bandwidth, power density, time for transmission, etc. in some other way.
Although not the main need as far as regulation is concerned, the advantages to industry and users of standardisation of equipments and systems within Europe and across the world are also very significant. ETSI has been established to provide standard specifications for equipment and there are now procedures for the type approval of manufactured equipment as a prerequisite for licensing.
In addition the all pervasive use of radio systems for a wide variety of purposes in domestic, business and industrial environments, means that unwanted susceptibilty of non-radio electronic equipment to radio emissions and also the unwanted and unnecessary radiation from transmitters and receivers must be controlled and regulated if the maximum use of radio is to continue. These electromagnetic compatibility aspects are dealt with within the European Union by the Radio and Telecommunication Terminal Equipment Directive, and also by the specification of relevant radio equipment parameters by the CEPT and by CENELEC.
Despite good system design and implementation, electromagnetic radiation cannot be strictly confined to the ray path between the transmitter and the receiver. Antenna beams have a finite beamwidth, allowing the spreading of energy and perhaps exciting unforeseen propagation modes. Transmitter powers are designed to ensure an adequate received signal during periods of signal fading due to propagation conditions, so that in most circumstances more signal is radiated than is strictly necessary. Moreover, signal levels capable of causing unacceptable interference to other co- or adjacent-channel systems will be at a substantially lower level than that needed for a high quality wanted signal; thus the transmitted energy will overshoot the receiving antenna and cause interference at longer ranges.
If these effects could be confined within national boundaries, then the planning and control of radio emissions would be a wholly national matter. However this is often not the case; radio signals extend across national boundaries and there is a consequent need to find ways to achieve regional and international coordination and agreement over the use of radio. In addition there are similar arguments concerning the use of the radio transmissions from satellites, particularly for satellites in or near the important geostationary orbit. In fact the extensive use of radio means that the available radio frequency spectrum, limited by propagation characteristics and the state of economic equipment development, is under stress. Thus there is a need to ensure good spectrum utilisation if room is to be found for new radio systems and services as they emerge.
Furthermore, there are other benefits to be had from agreements on system specification and modulation methods. Some standardisation here will give economic and convenience benefits to the user and manufacturer of equipments by increasing market size and assuring some stability in product types.
Thus there is a need for radio regulation at several levels: nationally, regionally and internationally.
Nationally, the administration has the right to authorise any transmissions. In addition, for the effective use of the spectrum and to ensure a good level of performance for the user it is necessary to plan and control the use of radio, typically by the use of licensing and the associated charges. In the UK, the national authority is OFCOM, an Agency which combines the scope of several bodies concerned with various aspects of the regulation of telecommunications and broadcasting. Until the end of 2003 the radio regulatory function was undertaken by the Radiocommunications Agency.
Note that a radio station on a satellite is not within the national territory of any country, so there is a particular problem in regulating satellite systems. This is covered by international agreements.
Regionally, particularly in Europe where countries are small and where there are common objectives through the European Union, there is a need to coordinate the use of radio so as to avoid interference, and also a need to employ common equipment specifications and regulations for usage so as to provide a European-wide market and to permit cross-border use, e.g. of cellular telephones, etc. These objectives are achieved through:
The Electronic Communication Committee (ECC), of the Conference of European Post and Telecommunication administrations (CEPT), which deals with spectrum utilisation and frequency planning matters, and
The European Telecommunications Standards Institute (ETSI) which deals with equipment specifications and performance. These notes will not address such aspects, although they will be of increasing importance as specifications are prepared for a wide variety of equipments, and such standards may in some cases have the authority of European Directives.
Internationally, the wider goals are very similar. There is a need to plan for good spectrum utilisation and for coordination between administrations, and to ensure, as far as political and commercial constraints allow, that the parameters of equipment specifications which affect spectrum utilisation have world wide agreement. Particularly for space systems, where transmissions from a geostationary satellite may illuminate nearly half of the earth’s surface, and a satellite in a lower orbit may over-fly every country in the world, the need for world-wide agreement is obvious. This function is undertaken by the International Telecommunication Union (ITU), based in Geneva; most of these lectures will concentrate on the agreements reached in the ITU, since this provides the overall framework within which Regional and National provisions are set.
1.2The Radio Spectrum
Radio waves are electromagnetic waves, subject to the conditions contained in the Maxwell equations. Electromagnetic waves may exist with an extremely wide range of wavelengths, and with corresponding frequencies, where the product of frequency in Hertz and wavelength in metres is the velocity of em waves in free space, about 3 x 108 m/s.
It is convenient to consider that the radio spectrum lies in the range of frequencies between say 3 kHz and 3 THz, although significant use is within the range 10 kHz to say 50 GHz; the range covered in detail by the ITU Radio Regulations extends from 9 kHz to 275 GHz. The conventional nomenclature for the spectrum is summarised in Table 1 below:
Table 1
Frequency Bands defined by the ITU
Band No / Symbols / Frequency Range / Wavelength / Corresponding metricsub-division / Metric abbreviations for the bands
ELF / below 3 kHz / Greater than 100 km / (unofficial designation)
4 / VLF / 3 kHz - 30 kHz / 100 km - 10 km / Myriametric waves / B.Mam
5 / LF / 30 kHz - 300 kHz / 10 km - 1 km / Kilometric waves / B.km
6 / MF / 300 kHz - 3 MHz / 1 km - 100 m / Hectometric waves / B.hm
7 / HF / 3 MHz - 30 MHz / 100 m - 10 m / Decametric waves / B.dam
8 / VHF / 30 MHz - 300 MHz / 10 m - 1 m / Metric waves / B.m
9 / UHF / 300 MHz - 3 GHz / 1 m - 100 mm / Decimetric waves / B.dm
10 / SHF / 3 GHz - 30 GHz / 100 mm - 10 mm / Centimetric waves / B.cm
11 / EHF / 30 GHz - 300 GHz / 10 mm - 1 mm / Millimetric waves / B.mm
12 / 300 GHz - 3 THz / 1 mm - 100 m / Decimillimetric waves
Note 1: “Band number N” (N = band number) extends from ) 0.3x10N to 3x10N Hz
Note 2: prefix: k = kilo (103); M = mega (106); G = giga (109); T = tera (1012); m = milli (10-3); = micro (10-6)
In some cases letter designations are used for some frequency bands. These are not recommended as the precise limits are not universally recognised. Some of these letter designations, as listed by the ITU, are given in Table 2.
Radio spectrum usage for communications and navigation purposes began at the lower frequencies, and moved to higher frequencies as the demand for bandwidth grew and as necessary technologies were developed. However the effect of the Earth’s atmosphere and of terrain and other obstacles varies with frequency. The combination of propagation characteristics and bandwidth has led to different uses of the various parts of the spectrum. These uses may not in fact be ideal for the purpose since an established use with large investment in equipment restricts the practical possibilities for moving that use to another part of the spectrum. Spectrum management involves a combination of technical considerations, the constraints of established uses, the possibility and practicality of using equipment with non-ideal emission and susceptibility specifications, and the economics of equipment design, construction, implementation and of the use of improved technology and system design.
Table 2
Letter symbols used to denote some frequency bands
(not recommended for use)
Letter symbol / Radar (GHz) / Space RadiocommunicationsSpectrum region
GHz / Examples
GHz / Nominal designations / Examples
(GHz)
L / 1 – 2 / 1.215 - 1.4 / 1.5GHz band / 1.525 - 1.710
S / 2 – 4 / 2.3 - 2.5
2.7 - 3.4 / 2.5GHz band / 2.5 - 2.690
C / 4 – 8 / 5.25 - 5.85 / 4/6 GHz band / 3.4 - 4.2
4.5 - 4.8
5.85 - 7.075
X / 8 – 12 / 8.5 - 10.5
Ku / 12 – 18 / 13.4 - 14.0
15.3 - 17.3 / 11/14 GHz band
12/14 GHz band / 10.7 - 13.25
14.0 - 14.5
K (1) / 18 – 27 / 24.05 - 24.25 / 20 GHz band / 17.7 - 20.2
Ka(1) / 27 – 40 / 33.4 - 36.0 / 30 GHz band / 27.5 - 30.0
V / 40 GHz bands / 37.5 - 42.5
47.2 - 50.2
(1)For space radiocommunications K and Ka bands are often designated by the single symbol Ka.
1.3. Spectrum use
It is useful to review the way in which each decade of the spectrum is used. Although the conventional way of describing the spectrum in decade frequency bands does not match the applications or the propagation characteristics very well, it is used here as a convenient short-hand. None of the frequency boundaries indicated are clear and precise in terms of differing usage.
1.3.1 ELF (below 3 kHz) and VLF (3-30 kHz)
Typical services: world-wide telegraphy to ships and submarine communications; navigational aids (e.g. Omega – now closed); time standards; worldwide communication, mine and subterranean communication;
System considerations: even the largest antennas are only a small fraction of a wavelength with low radiation resistance; difficult to make transmitter antennas directional; bandwidth very limited, only low or very low data rates; high external atmospheric noise so that inefficient receiving antennas are satisfactory
Propagation:In the Earth-ionosphere waveguide, relatively stable propagation; affected by thick ice masses (e.g. Greenland); asymmetric propagation east/west and west/east. Propagation through sea water which has significant skin depth for these wavelengths.
Comment: There are no international frequency allocations below 9 kHz. Limited use of frequencies below 9 kHz for military purposes.
1.3.2. LF (30-300 kHz)
Typical services:long-distance shore-to-ship communication; fixed services over
continental distances; broadcasting; time signals
System considerations: vertical polarisation (for ground wave propagation, and for antenna efficiency); large efficient antennas possible; directional antennas very large; high atmospheric noise; limited bandwidth.
Propagation:up to several thousand km; ground wave, strong sky wave at night, slow fading
1.3.3. MF (300 kHz -3 MHz)
Typical services: broadcasting; radionavigation; maritime mobile communications;
System considerations: half wavelength vertical antenna at 1 MHz is 150 m high; directional antennas possible, magnetic receiving antennas;
Propagation:ground wave more pronounced over sea; strong sky wave absorption
during the day, but little absorption at night; high atmospheric noise levels
1.3.4. HF ( 3-30 MHz)
Typical services: international broadcasting, national broadcasting in tropical regions;
long-distance point-to-point communications; aeronautical and maritime mobile communications;
System considerations: arrays of horizontal dipoles; log-periodic antennas (vertical or horizontal), vertical whip antennas, frequency agility essential; crowded spectrum needing good intermodulation performance; external noise environment varies with time and location. Bandwidths up to about 6 kHz
Propagation:propagation up to world wide distances by ionospheric sky-wave, very variable in time. Propagation window between MUF and LUF (maximum and lowest usable frequencies) varies from a few MHz to about 20 MHz
Comment:necessary to change the operating frequency several times during 24 hours. Broadcasting uses schedule of frequencies. Fixed and some mobile services use intelligent frequency adaptive systems. Continues to provide the main intercontinental air traffic control system. Most modulation bandwidths may exceed the correlation bandwidth. New digital modulation methods (DRM) now being introduced for broadcasting.
1.3.5. VHF ( 30-300 MHz)
Typical services: land mobile for civil, military and emergency purposes, maritime and
aeronautical mobile; sound (FM and DAB) and (outside UK) television broadcasting (to about 100 km); aeronautical radionavigation and landing systems; cordless telephones; paging; very limited little LEO satellite systems. The lowest range of the frequencies which may be used for satellite services (ionospheric effects prevent satellite use at lower frequencies, and the ionosphere will cause significant effects in the VHF range)
System considerations: multi-element dipole (Yagi) antennas, rod antennas suitable for vehicle
mounting, atmospheric noise small but man-made noise significant. Some use for meteor burst communications
Propagation:usually by refraction in troposphere; reflections may cause multipath on line-of sight paths; screening by major hills, but diffraction losses generally small; some anomalous propagation due to refractivity; unwanted ionospheric modes due to sporadic E and meteor scatter. Substantial Faraday rotation and ionospheric scintillation on Earth-space paths
1.3.6. UHF (300 MHz - 3 GHz)
Typical services: television broadcasting; cellular and personal communications; TETRA; satellite mobile; GPS; important radio astronomy bands; surveillance radars; terrestrial point-to-point service; radio fixed access; telemetry; cordless telephones (DECT); tropospheric scatter links.
System considerations: small rod antennas; multi-element dipole (Yagi) antennas; parabolic dishes for higher frequencies; wide bandwidths available
Propagation: : line-of sight and very slightly beyond; tropospheric scatter for transhorizon paths, screening by hills, buildings and trees; refraction effects; ducting possible; ionospheric scintillation
1.3.7 SHF ( 3-30 GHz)
Typical services: fixed (terrestrial point-to-point up to 155 Mb/s); fixed satellite; radar;
satellite television; GSO and NGSO fixed satellite services; remote sensing from satellites; wireless fixed access systems
System considerations: high-gain parabolic dishes and horns; waveguides; major inter-service frequency sharing; wide bandwidths
Propagation:severe screening; refraction and ducting; scintillation; rain attenuation and scatter increasing above about 10 GHz; atmospheric attenuation above about 15 GHz, trans-ionospheric effects becoming small.
1.3.8. EHF (30-300 GHz)
Typical services: line-of sight communications, future satellite applications; remote sensing from satellites; WFA; fixed service using stratospheric platforms
System considerations: small highly directional antennas; equipment costs increase with frequency; little use at present above 60 GHz; very wide bandwidths; short range
Propagation:severe difficulties: screening; atmospheric absorption; rain; fog; scintillation
1.4. Measures of Spectrum Usage
1.4.1. Spectrum utilisation
In an ideal world with perfect transmitters, receivers and antennas, with a constant propagation environment and with a steady information flow, it would be possible to pack emissions into the spectrum and use this limited resource to a maximum extent. Even if possible, this theoretical approach would be undsesirable since the spectrum usage would then be inflexible and could nto accommodate growth in demand. In practice the real situation is very different and measures of how near spectrum usage approaches a theoretical maximum are difficult to determine.
One significant reason for this is the economic and competitive environment. There will always be some pressure to use equipments with imperfect characteristics since these may be cheaper, smaller or lighter, and may be able to be manufactured and marketed faster. The radio regulator aims to ensure an agreed minimum standard for equipment specification, on the one hand to ensure equitable access to a competitive environment, and on the other to provide a basis from which spectrum planning may undertaken.
Propagation characteristics vary markedly with frequency, location and time. For mobile and transportable systems the location of one or both ends of a circuit will also vary. For a reliable service, suitable propagation will be needed for nearly all of the time (say 95 to 99.99% of the time, dependent on the needs of the service). Predictions for such extreme conditions will inevitable be less well specified than for median conditions and an allowance for a statistical confidence level may be large.