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3.3.15. Blind Equalization
3.3.15.1.General
The equivalent channel of a satellite communication system is constituted of:
In order to get perfect recovery of transmitted pulses or symbols, the equivalent channel must verify the well-
known Nyquist criterion (for a symbol rate R = 2*fc where fc is the physical channel bandwidth, the impulse
response of the equivalent channel should be equal to 0 for each multiple of 1/R). This property is verified for a
chain made of a RRC emitter filter and a RRC receiver filter, as RRC are well-known Nyquist filters. However,
emitter filters at RF frequencies are very often cavity filter design to meet the regulatory rejection specifications,
they are able to work with the signal coming from the high-power amplifier and they have a non-linear phase
response. Moreover, changes of emitter filter characteristics (magnitude and phase) could not be avoided when
there are large variations of temperature like those which take place in space. So, the equivalent channel seldom
verifies the Nyquist criterion and this induces inter-symbol interferences (ISI) that are harmful for performances.
A second aspect that is harmful for performances is noise. Two types of noises are present: the thermal noise
and the interference noise, coming from close channels.
In order to reduce the ISI and the influence of noise, two types of receiver could be used:
As they are, these filters are not useful because the equivalent channel is changing. So, they are implemented
through conventional equalizers which are adaptive filters, but these filters need a training sequence which
causes a payload loss.
A better solution for equalizing the incoming signal is to use an automatic adaptive equalizer.
© Safran Data Systems
COMMAND RANGING & TELEMETRY UNIT CORTEX
CRT QUANTUM USER'S MANUAL
The emitter filter: for frequency division multiplexing (FDM), the pass-band emitter filter is
generally a cavity filter. Base-band filtering depends on the application.
The channel: the channel could be considered as an amplitude fading channel with additive
White Gaussian Noise. Amplitude fading is resolved by automatic gain control. One can
add the possibility of multi-paths created by the structure of the satellite that can be
modelled by a Finite Impulse Response filter.
The receiver filter: they are many possibilities, commonly used filters are Integral & Dump
and Root Raised Cosine (RRC) filters.
The Matched Filter
The Wiener Filter
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Safran Data Systems
.
Ref.
DTU 100042
Is.Rev.
5.17
Date:
Dec.
03, 2021Sept. 30, 2021
Page 103
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