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INTRODUCTION
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2.3.10 GPS Antenna
The Mars GPS antenna is used to provide the GPS signal to the CBB. The antenna should be
an active device to support the 5 VDC provided by the CBB.
2.4
Functional Theory of Operation
Airlink can use Time Division Duplex (TDD), meaning the downlink and uplink communication
uses the same frequency, but at different times or Half Duplex FDD where the Downlink and the
uplink use different frequencies (still at different times - simultaneous transmission is not
supported currently). Time separation between the downlink and uplink messages is done with
TDD framing. A TDD radio is constantly toggling between transmit and receive states. The base
station transmits during the downlink subframe (DLSF) and receives during the uplink subframe
(ULSF) while the remote station transmits during the ULSF and receives during the DLSF.
The Airlink TDD radio uses a fast switching power amplifier (PA) and transmit/receive (TX/RX)
switch. The PA gate voltage is used to switch the PA in less than 5 µs between transmit and
receive states. The TX/RX switch switches the antenna to the PA during the transmit phase and
to the low noise amplifier (LNA) during the receive phase. The PA does not transmit during the
receive state and the PA is maintained in the off state during the transmit phase if transmission
is not needed to minimize power consumption.
To avoid self-interference, the TDD frame structure (i.e. the duration of the DLSF, ULSF, and
the total gap), is identical across the entire system. Additionally, the beginning of the TDD frame
is synchronized to a 1 pulse per second (PPS) generated by a GPS receiver. All BSs in the
system transmit at the same time. A BS and RS never transmit at the same time, so they never
interfere with each other.
The DLSF and ULSF may be further divided into zones that are identical across the entire
system. Zones may be able to make the network more efficient during certain conditions. For
example, an RS in the middle of the sector serving area is less susceptible to interference from
other sectors and generates less interference to the remotes of other sectors. Therefore, these
remotes may use a more aggressive frequency reuse scheme than remotes in the sector that
are located closer to the sector boundaries. The DLSF and ULSF can be partitioned into two
zones. One zone will be used for remotes at the boundaries of the sector with a less aggressive
frequency reuse scheme and the other zone will be used for remotes near the center of the
sector with a more aggressive frequency reuse scheme. This is known as fractional frequency
reuse (FFR).
2.4.1 Filtering Architecture
The Mars radio is designed to operate over a wide range of frequencies and channel
configurations while minimizing interference. The radio can operate from 70 MHz to 6 GHz
using channel sizes from 12.5 kHz to 10 MHz. At QPSK using convolution turbo coding (CTC)
rate of ½, the receiver sensitivity ranges from -127 dBm for a 12.5 kHz channel to -98 dBm for a
10 MHz channel.
2-7
COM-00-21-05
OCTOBER 2021
Version No.: A
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