→ The link budget calculation enables the losses and gains to be seen,
and devising a link budget enables the apportionment of losses, gains
and power levels to be made if changes need to be made to enable the
radio communications system to meet its operational requirements.
→ Link budgets are used for Satellite systems. It is crucial that the
required signal levels are maintained to ensure that the received signal
levels are sufficiently high above the noise level to ensure that signal
to noise levels or bit error rates are within the required limits.
Larger antennas, high transmitter power levels that required add
considerably to the cost, so it is necessary to balance these to
minimize the cost of the system while still maintaining performance.
→ Also, link budget calculations are used for calculating the power
levels required for cellular communications systems, and for
investigating the base station coverage.
→ The link budget include the following items:
Transmitter power, Antenna gains (receiver and transmitter), Antenna
feeder losses (receiver and transmitter), Path losses, Signal
propagation losses (fading margin, polarization mismatch, losses
associated with medium through which signal is travelling, other
losses...)
→ Radio frequency (RF) circuit elements that are traditionally
considered to be linear frequently exhibit nonlinear properties that
affect the intended operation of many other RF systems. Devices such as
RF connectors, antennas, attenuators, resistors, and dissimilar metal
junctions generate nonlinear distortion that degrades primary RF system
performance. The high transmit power and tight channel spacing of the
communication channel makes communications very susceptible to nonlinear
distortion.
→ Nonlinearities in RF and Microwave systems are found in circuit
elements such as diodes, transistors, amplifiers, mixers, and others.
Also, nonlinearities have been found in passive intermodulation (PIM)
distortion, which occurs in antennas, cables, connectors, metal-to-metal
junctions, and various components.
→ PIM is observed when high-power signals interact with components that
are weakly nonlinear. In communication systems, the PIM produced can
fall close to the fundamental band and interfere with adjacent
communication channels. To combat this, need linearizing communication
systems.
→ Measuring weakly nonlinear RF circuit components requires specialized
RF hardware, which itself must be highly linear and devoid of any
self-generated nonlinearities. If the measurement system is not highly
linear, the measurements will reflect the distortions caused by the test
hardware in addition to the device under test (DUT).
→ The measurement system must create a highly linear probe signal and
must have the ability to measure very weak nonlinear signals in the
presence of the large fundamental probe signal.
→ The main task is the design of a PCB that eliminates antennas that can
radiate electromagnetic energy. Even if this can be achieved sometimes,
large loops of signal and corresponding ground-return lines that carry
high frequencies must be avoided. A careful positioning of the
integrated circuits is essential to achieve short interconnect lines.
→ Any time that multiple RF systems are co-located, there is an
opportunity for RF interference (RFI) to degrade or disrupt the
performance of receivers.
→ Most EMI-related crosstalk problems center around the crystal, when
the victim is located too close. No unrelated components should be
closer than 1 inch to the crystal. Every signal should be checked for
EMI issues. Routing near the crystal, and the crystal and tank circuit
itself, should be checked. Finally, the ground traces should be
gridded.
→ For Federal Communication Commission (FCC) limits, trace length
becomes important when it is greater than 1/10 of the wavelength. For
military standard limits, that number becomes 1/20 to 1/30 of the
wavelength. For automotive and consumer two-layer boards, 1/50 of the
wavelength begins to be critical, particularly in unshielded
applications. That says traces longer than 4 inches can be a problem for
FM-band noise.
→ If there is any impedance in the single ground wire, a portion of the
RF energy does not return to the microcomputer’s PCB via the ground
wire, but rather through a radiated path. The energy radiates off the
second board and couple back to the first, but, during the process, that
radiation also can add noise in other locations in the system, as well
as become direct radiation measured in the screen room.
→ Common-mode noise is a big problem in cables, but the fault does not
lie in the cable, it lies in the connections on the board that the
signals and returns tie to that form the common impedance.
→ Crosstalk in a cable is the same as in the PCB. Noise is coupled from
the source onto quiet victim signals. Therefore, run clocking or other
high-speed wires twisted with their own separate return. Crosstalk is a
problem in cables over 2-meters long, and can be a problem in cables as
short as 6 inches.
→ When an incident electric field traveling in the air hits a metal
surface, the metal causes the penetrating field strength to decrease.
The metal causes the field to be replaced with conduction currents that
flow in the metal close to the surface. A very small (exponential decay)
amount of the field does pass through, but for emissions, this is never
a problem. The metal chassis serves as a shield. The fields from all the
radiating surfaces inside it are blocked and kept inside the box, with
the only noise coming from the cables or wires that enter or exit the
box and from holes or slots made in the box.
→ The shield should be thought of as an RF conducting plane, with the
least number of breaks and impedance’s between the source of the RF
currents and the ground reference point.
→ Real capacitors have limited effective frequency ranges due to their
self-resonant frequency (SRF). The SRF is available from the
manufacturer, but sometimes must be characterized by direct measurement.
Above the SRF, the capacitor is inductive, and therefore will not
perform the decoupling or bypass function.
→ Autorouters for PCBs do not take any noise reduction actions;
therefore, care should be taken in their use. Power and ground routing,
as well as signals that impact susceptibility, should be laid out by
hand.
→ Experience and careful work by the design engineer are much more
effective than sophisticated computer-aided design tools.
→ Satellites provide a variety of mobile and fixed communications
services such as Government & Defence, ATM Communication, Aeronautical
Broadband, Emergency & Safety, Media Distribution, Media
Broadcast(fixed), IP Backhaul Backup VSAT, Intelligent Transport
Systems, Media Broadcast(mobile), Media & IP, Media Broadcast & IP
Connectivity, Satellite News Gathering, Backhaul & Media Provisioning,
Disaster Relief & Humanitarian Aid, Automatic Identification Maritime &
M2M, Digital Oil Field M2M/SCADA Smart Grids.
→ A satellite system consists of a space and a ground segment. The space
segment is composed of satellites, which classified into geostationary
earth orbit (GEO) altitude above the Earth surface 35,786 km (22,366
miles), middle earth orbit (MEO) between 2,000 and 24,000 km (1,250 -
15,000 miles) and low earth orbit (LEO) between 160 and 2,000 km (100 -
1,250 miles).
GPS satellites orbit
at a height of 20,200 km (12,550 miles). Global mobile communication,
like disaster relief or maritime operations, LEO is the technology of
choice.
→ Since MEO/LEO satellites are closer to the Earth’s surface, the
necessary antenna size and transmission power level are much smaller.
However, the MEO’s/LEO’s footprint is also much smaller than GEO’s. A
constellation of a large number of MEO/LEO satellites is necessary for
global coverage, while three GEO satellites are sufficient (Transmitted
signal from one onboard antenna cover 42% of Earth’s surface). The lower
the orbit altitude, the greater the number of satellites required for
MEO (10 - 15 satellites), LEO (>32 satellites).
→ Important - Increased exposure to Van Allen Belt
radiation creates degradation/hazards to onboard electronic systems. The
Van Allen radiation belts are two torus-shaped regions of energetic
ions, protons and electrons trapped in the Earth’s magnetic field, which
can harm electronic circuits onboard a satellite. The Van Allen
Radiation Belts locations is Inner Belt - 1,500 to 5,000 km (940 - 3,150
miles) and Outer Belt - 13,000 to 20,000km (8,150 - 12,500 miles). Also,
solar weather can very adversely affect communications satellites,
sometimes to the extent that a flare will actually render a
communications satellite inoperable.
→ Propagation Concerns for Satellite Communications Systems:
Attenuation, sky noise (Rain, atmospheric gases, cloud), Signal
depolarization (Rain, ice crystals), Atmospheric multipath (Atmospheric
gases). Sky noise is a combination of galactic and atmospheric effects.
An optimal window between 1 GHz and 10 GHz for satellite transmissions
due to its fairly low noise temperature. There is greater attenuation of
the signal by the atmosphere as we get closer to the frequencies that
are absorbed by water and free Oxygen. Losses occur in the earth
atmosphere as a result of energy absorption by the atmospheric gases.
These losses are treated quite separately from those which result from
adverse weather conditions, which of course are also atmospheric losses.
To distinguish between these, the weather-related losses are referred to
as atmospheric attenuation. The main scatterer particles in troposphere
are hydrometeors like raindrops, hail, ice, fog or clouds, and these
particles represent a problem for frequencies higher than 10 GHz. Rain
Introduces attenuation by absorption and scattering, increases noise
through absorption, at either 20 or 30 GHz, attenuations of more than 30
dB may occur 0.1% of the time. Techniques to counter rain fade include
dropping bit rate and changing to undesirable effects may transform it
into an elliptical different error-correction codes.
→ Satellite communications use linear and circular polarization, but
undesirable effects may transform it into an elliptical polarization.
→ Type of Satellite Services: FSS (Fixed Satellite Service), MSS (Mobile
Satellite Service), BSS (Broadcast Satellite Service), PSS (Personal
Satellite Service), Navigational Satellite Services, Meteorological
Satellite Services.
→ Band and frequency assignments authorized by
FCC (US domestic) and
ITU
(International Telecommunications Union; non-US). Satellite
Communication Frequency Bands: VHF, UHF, L, S, C, X, Ku, Ka, Q, V, W.
Earth Stations transmit their uplink signals to the satellite at the
higher operating frequency of the communication link, while satellites
transmit the lower operating frequency, in order to avoid the
interference between the transmitted and received signals.
→ Communications satellites divide bandwidth between transponders. There
are two types of transponders: Frequency Translation Transponder (Bent
pipe or Repeater transponder) and On-board Processing Transponder
(Regenerative repeater demodulation/remodulation, decoding/recoding,
transponder). A transponder is the hardware needed to take signals in on
one set of frequencies and re-send to the ground on other frequencies. A
transponder is capable of sending and receiving about 45 Mb/sec. These
are analog signal processors, so the modulation technique will determine
the bandwidth in C-band transponders 36 MHz wide, Ku-band 54 MHz wide,
Ka-band 100 MHz wide. Frequency reuse is obtained by use of beams. They
separate the direction of the transmissions and reception by pointing
the antenna at the ground in different directions. Additional bandwidth
is obtained by use of polarization that doubles usable bandwidth.
→ Latency is the time it takes a bit of information to traverse a
network from its originating point to its final destination. latency in
a satellite network will be described differently and broken down into
two discrete components: propagation delay and processing delay.
Propagation delay is the finite time it takes a radio wave travelling at
the speed of light to cover the distance from the Earth’s surface to a
satellite and back to the Earth’s surface. Processing delay, on the
other hand, is the cumulative delayed caused by the hardware and
software in every device the signal passes through. The significant
problem for GEO satellite links is the large propagation delay, value of
round-trip delay is 250- 280ms, for a MEO’s is 110-130ms. The round-trip
delay for a LEO satellite system is 20-25ms, which is comparable to that
of a terrestrial necessary antenna size and transmission power level are
much smaller link. Minimization latency in satellite network - reduce
signal processing delay or moving from a geosynchronous satellite to one
in low-Earth or medium-Earth orbit.
→ Satellite carrier bit rates 2400 b/s – 100 Mb/s and up. Bit rate is a
function of amount of information to be transmitted. BER - fraction of
received bits in error, which is excellent(1e-10), very good(1e-8),
good(1e-6), marginal(1e-4).
→ Common technique modulation - Phase Shift Keyed (PSK) modulation: 2
phase or bi-phase shift keying (BPSK), 4 phase (QPSK - 2bits/RE), 8
phase PSK, 16 phase PSK, and Quadrature Amplitude Modulation (QAM): 16
QAM (4 bits/RE), 64 QAM (6 bits/RE) and 256 QAM (8 bits/RE). Mostly used
modulations in satellite communication are Quadrature Phase Shift Keying
(QPSK), 8-Phase Shift Keying (8-PSK), 16-Quadrature Amplitude Modulation
(16-QAM). These are used for carrying more information in less
bandwidth.
→ Bandwidth required to support a particular bit rate is a function of
information rate and modulation technique. Bandwidth increases with bit
rate. For an information rate of 1000 bits/sec using BPSK (bandwidth =
1000 Hz), QPSK (bandwidth = 500 Hz), using 8PSK (bandwidth = 250 Hz).
→ Dish alignment with Dishpointer -
Dishpointer
→ Orbits and satellite positions -
SatelliteXplorer
→ Space Weather Prediction Center (Satellite Community Dashboard) -
Space Weather Conditions