INTRODUCTION
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2-5
COM-00-21-04 OCTOBER 2021
Version No.: A.1
2.4.3 Intermediate and Radio Frequency Module (IRF)
Airlink Venus 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, USLF, and
the total gap), is identical across the entire system. Additionally, the beginning of the TDD frame
can be synchronized to a 1 pulse per second (PPS) generated by a GPS antenna. All Base
Stations 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, a Remote Station (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).
2.4.4 Filtering Architecture
The Airlink Venus radio is designed to operate over a wide range of frequencies and channel
configurations while minimizing interference. The Airlink Venus radio can operate from 70 MHz
to 6 GHz using channel sizes from 12.5 kHz to 10 MHz. With Quadrature Phase Shift Keying
(QPSK) using a 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.
