Introduction
2
SWRA523B – September 2016 – Revised March 2017
Copyright © 2016–2017, Texas Instruments Incorporated
CC13xx Antenna Diversity
1
Introduction
Multipath is a phenomenon that occurs when electromagnetic waves bounce off of surfaces (such as
floors, ceilings, walls, trees, people walking by, and so on) and arrive at the receiver at different times.
When these signals arrive at the receiver out-of-phase, they may result in a form of cancellation called
fading
.
Antenna diversity is a technique that can be used to improve radio communication and maximize the
chance of a packet getting through at a given time and in a given position between a receiver and
transmitter in a nonstatic environment.
This application report presents a brief description of the patch, relevant status registers, configuration
registers, and required overrides. Performance figures (when using the patch with CC1310) are provided
with real-world measurement results that show the benefit of antenna diversity.
All measurements were performed on the CC13xxEM-7793_4L reference design
.
2
Antenna Diversity Patch
Antenna diversity is not available in ROM code, and a patch is required to support this feature.
The antenna diversity feature can be used only with the proprietary radio commands. The patch is part of
the SimpleLink™ CC13x0 Software Development Kit
and recommended settings and overrides are
covered in
,
and
2.1
Preamble-Based Antenna Diversity With External Switch
The antenna diversity algorithm controls an external antenna using an internal signal from the RF Core
(RFC_SMI_CL_OUT). The RFC_SMI_CL_OUT signal can be routed to any available digital I/O (DIO).
describes how this signal is routed out on a DIO.
The antenna diversity algorithm is based on Preamble Quality Threshold (PQT), meaning that it searches
for a preamble. If there is no preamble the algorithm toggles the external switch and starts over again. If a
preamble is detected, the algorithm toggles the switch and checks if a preamble is also present at the
other antenna. If a preamble is found on both antennas, the algorithm selects the antenna with highest
RSSI and then starts searching for a sync word. If a preamble is found on the first antenna only, it toggles
the switch back again and starts searching for a sync word on the first antenna.
shows the state
machine diagram.
To improve the robustness of preamble detection, an additional carrier-sense qualifier check is added
before the preamble is accepted. This feature can be turned on or turned off (see
).
The carrier sense (CS) threshold is set to –111 dBm. In an environment with a noise floor above
–111 dBm, it is advantageous to increase the CS threshold to reduce the likelihood of processing a false
packet. If the CC13xx device is preoccupied by processing a false packet, there is a finite possibility that a
true packet will be missed.
explains how to change the CS threshold.
If the RF Core is not power cycled, the last antenna used by the radio will be used the next time it enters
the TX state. This condition ensures that an acknowledgment is sent with the same antenna that received
the last packet.
