ND SatCom SKYWAN IDU 7000, Руководство пользователя

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2010-10-26 Network Design and Engineering Guide 77
Outdoor Unit and Satellite Link Design
Fundamentals of Link Budget Calculation
Path Loss
Due to the very long distance earth station – satellite (> 36000 km) only a small fraction of the
radiated power will be picked up by the receive antenna of the earth station or the satellite. This
signal attenuation is called the free space path loss a
Path
and depends on both the distance
and the radiation frequency. In addition to this constant loss at ’clear sky’ conditions, additional
atmospheric loss happens temporarily under rain conditions (rain fade). This will be discussed
in more detail in a later section.
Saturation Flux Density (SFD)
The satellite transponder’s saturation flux density defines the power density which is needed
to generate a signal on the downlink with the maximum downlink EIRP
Sat
. A lower SFD means
a higher satellite transponder gain.
Noise, Figure of Merit G/T and Signal-to-Noise Ratio Eb/No
Besides signal power losses and gains there is also an accumulation of noise along a satellite
link. The most important noise sources are thermal noise picked up by the receiving antennas
and noise produced by the Low Noise Amplifiers (LNA) in the earth station and satellite recep-
tion systems. The noise power N is typically represented by an effective noise temperature
which is defined by the formula
where k
B
is Boltzmann’s constant and B the signal bandwidth. The noise production by both
the satellite and the earth station is usually combined with the antenna gain to the system’s fig-
ure of merit G/T.
The resulting signal received at the earth station’s demodulator will have both a signal carrier
power C and a noise power N. The signal quality will be determined by the signal-to-noise ratio
C/N. Instead of using the bandwidth dependent quantities C and N, the signal-to-noise ratio is
expressed by Eb/No. Here No is the spectral noise power per 1 Hz bandwidth and Eb the en-
ergy per information bit which represents the carrier power divided by the carrier’s modem data
rate. For the definition of modem data rate please refer to chapter 2.6.
Satellite Link Quality Dependencies
The quality of a satellite link is defined by the average ratio of bit errors which is generated on
the link. Typical requirements for a satellite link quality is that the bit error rate must be lower
than 10
-8
…10
-7
. The required Eb/No level for that link quality depends on the modulation and
error correction of the carrier. Lower FEC rates i.e. higher information redundancy on the car-
rier reduces the required Eb/No, 8PSK modulation increases the required Eb/No compared to
QPSK. There is also a slight dependency on the TDMA container size: For gross container siz-
es smaller than 200 byte a higher Eb/No is needed due to a lower performance of the
Turbo-Phi coding for short code words.
N = k
B
x T x B