RFM23
8.9. Antenna-Diversity For RF23 IC
To mitigate the problem of frequency-selective fading due to multi-path propagation, some transceiver systems use
a scheme known as Antenna Diversity. In this scheme, two antennas are used. Each time the transceiver enters
RX mode the receive signal strength from each antenna is evaluated. This evaluation process takes place during
the preamble portion of the packet. The antenna with the strongest received signal is then used for the remainder
of that RX packet. The same antenna will also be used for the next corresponding TX packet.
This module fully supports Antenna Diversity with an integrated Antenna Diversity Control Algorithm. By setting
GPIOx[4:0] = 10111 and 11000, the required signal needed to control an external SPDT RF switch (such as PIN
diode or GaAs switch) is made available on the GPIOx pins. The operation of these switches is programmable to
allow for different Antenna Diversity architectures and configurations. The antdiv[2:0] register is found in register
08h. The GPIO pin is capable of sourcing up to 5 mA of current, so it may be used directly to forward-bias a PIN
diode if desired.
When the arrival of the packet is unknown by the receiver the antenna diversity algorithm (antdiv[2:0] = 100 or 101)
will detect both packet arrival and selects the antenna with the strongest signal. The recommended preamble
length to obtain good antenna selection is 8 bytes. A special antenna diversity algorithm (antdiv[2:0] = 110 or 111)
is included that allows for shorter preamble for TDMA like systems where the arrival of the packet is synchronized
to the receiver enable. The recommended preamble length to obtain good antenna selection for synchronized
mode is 4 bytes.
POR
Def.
Function/Des
cription
D7
D6
D5
D4
D3
D2
D1
D0
Add R/W
Operating &
R/W
antdiv[2]
antdiv[1] antdiv[0] rxmpk
autotx
enldm
ffclrrx
ffclrtx
00h
08
Function Control 2
Table 24. Antenna Diversity Control
RX/TX State
antdiv[2:0]
Non RX/TX State
GPIO Ant1 GPIO Ant2
GPIO Ant1
GPIO Ant2
000
001
010
011
100
101
110
111
0
1
0
1
1
0
1
0
0
0
1
1
0
1
0
1
0
0
1
1
0
1
0
1
Antenna Diversity Algorithm
Antenna Diversity Algorithm
Antenna Diversity Algorithm in Beacon Mode
Antenna Diversity Algorithm in Beacon Mode
63
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