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Sec. 8–6 Link Budget Analysis 597
the satellites appear overhead so data is received from both simultaneously. This provides for
redundancy (i.e., path diversity reception). Frequency diversity is also provided by transmitting
the data redundantly on two different downlink bands as shown in Fig. 8–15a. Time
diversity is provided by a data delay of 4 seconds on one of the satellites. Using these diversity
techniques, outage effects and data errors are minimized.
XM uses two geostationary (GEO) satellites located at 85° W and 115° W longitude
above the equator. In the United States, signals can be received simultaneously from both
satellites. Path, frequency and time diversity are also used to minimize outage effects and
data errors.
The SDARS systems have downlink frequencies in the 2.3 Ghz band (see Table 8–3) and
each SDARS system is assigned a channel with 12.5 MHZ bandwidth as shown in Fig. 8–15.
The bit streams of the multiple audio channels are time-division-multiplexed (TDM) to provide
a composite data rate of about 4 Mbs. In addition, concatenated forward-error-correction coding
(FEC) is added this TDM data stream. This coded data stream is QPSK modulated on the
downlink signal provided to the subscriber. Unfortunately, these received satellite
TDMQPSK signals will be intermittently blocked (i.e., attenuated) by large buildings, tunnels,
and subways. There may also be multipath effects due to reflections from the buildings
and other objects. These effects occur mainly in cities and not in rural areas. To counteract
these effects, terrestrial repeaters are placed in and around urban and city areas. The parameters
of these repeaters are shown in Table 8–3. They use OFDM and have frequency assignments as
shown in Fig. 8–15. The data on the repeaters are delayed about 4 seconds with respect to the
data received from the satellites. These repeaters provide additional space, time and frequency
diversity reception for additional system reliability.
For more information about communication satellite systems, the reader is referred to
an excellent book [Pratt et al, 2003].
8–6 LINK BUDGET ANALYSIS
In this section, formulas will be developed for the signal-to-noise ratio at the detector input as
a function of the transmitted effective isotropic radiated power (EIRP), the free-space loss,
the receiver antenna gain, and the receiver system noise figure. These results will allow us to
evaluate the quality of the receiver output (as measured by the probability of bit error for
digital systems and output signal-to-noise ratio for analog systems), provided that the noise
characteristics of the receiver detector circuit are known.
Signal Power Received
In communication systems, the received signal power (as opposed to the voltage or current
level) is the important quantity. The power of the received signal is of prime importance when
trying to minimize the effect of noise sources that feed into the system and are amplified. The
voltage gain, current gain, and impedance levels have to be such that the required power gain
is achieved. In FET circuits, this accomplished by using relatively large voltage levels and
small currents (high-impedance circuitry). With bipolar transistors, power gain is achieved
with relatively small voltages and large currents (low-impedance circuitry).