RFM31 Datasheet, PDF (53/137 Pages) –

RFM31
8.9. Antenna-Diversity For RF31 IC
To mitigate the problem of frequency-selective fading due to multi-path propagation, some radio systems use a
scheme known as Antenna Diversity. In this scheme, two antennas are used. Each time the radio 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.
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.
Add
08
R/W
R/W
Function/Des
cription
Operating &
Function Control 2
D7
antdiv[2]
D6
D5
D4
antdiv[1] antdiv[0] rxmpk
D3
Reserv
ed
D2
enldm
D1
ffclrrx
D0
Reserv
ed
POR
Def.
00h
Table 23. Antenna Diversity Control
antdiv[2:0]
000
001
010
011
100
101
110
111
RX State
GPIO Ant1
GPIO Ant2
0
1
1
0
0
1
1
0
Antenna Diversity Algorithm
Antenna Diversity Algorithm
Antenna Diversity Algorithm in Beacon Mode
Antenna Diversity Algorithm in Beacon Mode
Non RX State
GPIO Ant1
GPIO Ant2
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
53
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