RFM22B/23B
8.9. Antenna Diversity
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 chip fully supports antenna diversity with an integrated antenna diversity control algorithm. The required
signals needed to control an external SPDT RF switch (such as PIN diode or GaAs switch) are available on the
GPIOx pins. The operation of these GPIO signals is programmable to allow for different antenna diversity
architectures and configurations. The antdiv[2:0] bits are found in register 08h “Operating & Function Control 2.”
The GPIO pins are capable of sourcing up to 5 mA of current, so it may be used directly to forward-bias a PIN
diode if desired.
The antenna diversity algorithm will automatically toggle back and forth between the antennas until the packet
starts to arrive. The recommended preamble length for optimal antenna selection is 8 bytes. A special antenna
diversity algorithm (antdiv[2:0] = 110 or 111) is included that allows for shorter preamble lengths for beacon mode in
TDMA-like systems where the arrival of the packet is synchronous to the receiver enable. The recommended
preamble length to obtain optimal antenna selection for synchronous mode is 4 bytes.
D7
D6
D5
D4
D3
D2
D1
D0
POR Def.
Add R/W Function/Description
08 R/W
Operating & Function
Control 2
antdiv[2] antdiv[1] antdiv[0] rxmpk autotx enldm ffclrrx
ffclrtx
00h
Table 17. Antenna Diversity Control
antdiv[2:0]
RX/TX State
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
Tel: +86-755-82973805 Fax: +86-755-82973550 E-mail: sales@hoperf.com http://www.hoperf.com
56
HOPERF [ HOPERF ]
