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density ratio for communication at the
desired average BER.
is the carrier-tenoise density ratio achieved
[']ACHIEVED by the end-to-end link.
The required carrier-bnoise density ratio is determined by the
particular signalling waveform used and by the noise and
propagation characteristics of the channel. Statistical methods
can be used to determine the minimum required carrier-tm
noise ratio needed to assure operation at an average BER.
Statistical methods can also be used to include the effects of
propagation environment and other random losses in the form
of a required margin.
Hence, for operation at a desired average BER the following
relationship must hold:
where: M, is the link margin required for the propagation
environment and various random RF parameter variations.
The carrier-tenoise performance measures must be allocated
to various portions of the RF link, which are discussed below.
Al.1 To-aircraft link analysis
The achieved carrier-tc-noise density ratio on the forward link
is determined by a number of noise sources in the RF link.
With simple, frequency translating transponders, the achieved
signal (carrier)-to-noise power ratio can be computed from the
expression:
where:
NvF = the thermal noise power density of the uplink
feeder link.
ND = the t h e d noise power density of the L-band
downlink.
I, = the intermodulation power density on the L-band
downlink due to the satellite transponder.
I, = the intrasystem interference power density.
roll = the downlink Lband intersystem interference
power density at the receiver.
IavF = the intersystem interference power density on the
feeder link uplink.
An important assumption is inherent in equation [A.3]. It is
assumed that in an individual channel bandwidth all the noise
sources can be considered to be "white Gaussian" in nature.
A1.2 From-nrircraR link analysis
In the same manner as the forward link, the achieved cmierbnoise
density ratio is determined by a number of noise
sources in the return link. The achieved carrier-to-noise power
density ratio can be obtained from the expression:
ANNEX 10 - VOLUME III 293 9/11/95
Annex 10 - Aeronautical T~hcommunicatbns Volume IIZ
where:
NDF = the thermal noise power density of the downlink
feeder link.
N, = the thefmd noise power density of the L-band
uplink.
I, , = the minimum operable intermodulation power
density expected on the L-band uplink from the
multi-carrier operation of the AES high power
amplifiers.
I = the intrasystem interference power density.
I,, = the intersystem interference power density at L
band expected on the uplink.
I,,, = the intersystem interference power density on the
feeder link downlink.
A1.3 Propagation anomalies and
required margins
An idealized RF link can be adversely affected by a number
of factors which can be divided into two basic classes:
deterministic and nondeterministic. Deterministic factors
influencing RF link margin requirements depend on the propagation
path established by the relative locations of the aircraft,
satellite and earth in a particular situation. Other deterministic
factors are fixed by the system design, such as, information bit
rates, modulation type, interleaver depths, coding schemes, etc.
The nondeterministic factors that influence the RF link requirements
are system design and operational elements specified by
the service provider, degradation due to interference and other
propag ation-related random losses.
Many factors that influence the RF link requirements may be
viewed as losses that reduce the available carrier power and
degrade link performance. Detailed discussions of several of
these factors are included in,the following sections.
A 1.3.1 MULTIPATHFA DING
The term "multipath" refers to a condition in which energy
reaches the receiver of a telecommunications system by m e
than one path. Multipath propagation may result from reflection
from land and water surfaces and man-made structures.
Multipath operation is generally undesirable, because signals
arriving over the different paths arrive with variable relative
phase, with the result that they alternatively add constructively
m destructively in space. Hence, the total received signal will
be characterized by fading, involving repeated minima which
may fall below the signal level required for acceptable communications
performance. Fading is also significantly higher
over water as opposed to land. Furthermore, the signals
arriving over the different paths also have different time
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