Texas Instruments CC13 Series Manuel utilisateur

SWRA523B–September 2016–Revised March 2017

CC13xx Antenna Diversity

Application Report

SWRA523B–September 2016–Revised March 2017

CC13xx Antenna Diversity

ABSTRACT

This application report describes a preamble quality-based antenna-diversity patch for the CC13xx

SimpleLink™ Sub-1 GHz ultra-low power wireless microcontroller (MCU).

Recommended register settings discussed in this application report can be downloaded from

http://www.ti.com/lit/zip/SWRA523.

Contents

1 Introduction ................................................................................................................... 2

2 Antenna Diversity Patch .................................................................................................... 2

3 Building a Software Example............................................................................................... 6

4 Measurement Results....................................................................................................... 8

5 References.................................................................................................................. 15

List of Figures

1 Antenna Diversity Flow Chart.............................................................................................. 3

2 SmartRF™ Studio Setup for TX ........................................................................................... 7

3 PER vs Input Power Level (6-Byte Preamble, 6-dB Attenuator on One Antenna).................................. 8

4 PER vs Input Power Level (8-Byte Preamble, 6-dB Attenuator on One Antenna).................................. 9

5 PER vs Input Power Level (6-Byte Preamble, 6-dB Attenuator on One Antenna).................................. 9

6 PER vs Input Power Level (8-Byte Preamble, 6-dB Attenuator on One Antenna) ................................ 10

7 Block Diagram of Antenna Diversity Test Setup....................................................................... 11

8 Antenna Diversity Test Board............................................................................................. 12

List of Tables

1 Register Overrides to be Removed from SmartRF™ Studio Code Export........................................... 4

2 Register Override to be Added to the SmartRF™ Studio Code Export .............................................. 4

3 Configuration Registers..................................................................................................... 5

4 Status Registers ............................................................................................................. 5

5 Typical 1% BER Sensitivity (20% PER, 3-Byte Payload) .............................................................. 8

6 Test 1 PER for Boards Using ANT1, ANT2, or Diversity (DIV) ...................................................... 13

7 Test 2 PER for Boards using ANT1, ANT2, or Diversity (DIV)....................................................... 13

8 Test 3 PER for Boards Using ANT1, ANT2, or Diversity (DIV) ...................................................... 14

9 Test 4 PER for Boards Using ANT1, ANT2, or Diversity (DIV) ...................................................... 14

Trademarks

SimpleLink, SmartRF are trademarks of Texas Instruments.

ARM, Cortex are registered trademarks of ARM Limited (or its subsidiaries).

All other trademarks are the property of their respective owners.

Introduction

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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 [1].

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 [3] and recommended settings and overrides are

covered in Section 2.3,Section 2.4 and Section 2.5.

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).

Section 3 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. Figure 1 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 Section 2.5).

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. Section 2.5 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.

Toggle the switch

Measure RSSI1

Toggle the switch

Search for PQT1

Check

PQT2

Toggle the switch

Sync Search

RSSI2 > RSSI?

Measure RSSI2 Toggle the switch

PQT1 = 1

PreTimeout = 1 and PQT1 = 0

CsQual =1 and CS = 0

PQT2 = 1 PQT2 = 0

Yes

SyncTimeout = 1

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Antenna Diversity Patch

SWRA523B–September 2016–Revised March 2017

CC13xx Antenna Diversity

Figure 1. Antenna Diversity Flow Chart

Antenna Diversity Patch

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2.2 Automatic Gain Control (AGC)

The antenna diversity algorithm has a dedicated automatic gain control (AGC) (ANT_DIV AGC).

There is no gain adjustment during a preamble search. The AGC performs one front-end gain adjustment

when entering the Sync Search state (see Figure 1) and then uses the RSSI from the selected antenna to

calculate the optimal RF gain setting.

The user can change the signal level where the AGC starts to reduce the front-end gain by programming

the AGC RSSI reference level (AgcRssiRef) (see Table 3). The AGC uses the measured RSSI and the

programmed RSSI reference level to calculate the AGC_ERROR and optimum RF gain level, as shown in

Equation 1.

AGC_ERROR = RSSI – AgcRssiRef_dBm (1)

2.3 Recommended Operating Limits

The antenna diversity algorithm is flexible when it comes to data rate, but has a limitation related to the

minimum preamble length. Equation 2 shows the calculation for the minimum preamble length.

Minimum Preamble Length = 4 × PreTimeout + 8 [symbols] (2)

The worst-case scenario occurs when a signal is received at the sensitivity level. For example, if the first

antenna misses the desired preamble and the signal level is too low for the second antenna, two preamble

time-outs have occurred without receiving a preamble hit. Then the receiver gets a preamble hit and must

check the other antenna. This is the theory behind the worst-case scenario of 4-preamble time-outs. In

addition, it is necessary to add the internal hardware delay.

Thus, for the default setting with PreTimeout = 10 symbols, the minimum preamble length is 6 bytes.

Because there is no AGC during antenna switching, the algorithm might not select the antenna with

highest RSSI if both antenna signals are above approximately –50 dBm.

2.4 Register Overrides

SmartRF™ Studio [2] should be used to generate the register settings. However, three of the radio

diversity patch and one must be added (see Table 2).

Table 1. Register Overrides to be Removed from SmartRF™ Studio Code Export

Override Comment

HW_REG_OVERRIDE(0x6084,0xXXXX) Remove from SmartRF Studio code export

HW_REG_OVERRIDE(0x6088,0xXXXX) Remove from SmartRF Studio code export

HW_REG_OVERRIDE(0x608C,0xXXXX) Remove from SmartRF Studio code export

Table 2 lists the radio register settings from the SmartRF Studio that must be changed to use the antenna

diversity patch.

Table 2. Register Override to be Added to the SmartRF™ Studio Code Export

Override Comment

MCE_RFE_OVERRIDE(1, 0, 0, 1, 0, 0) Run the MCE and RFE patches

[4] contains a smartrf_settings.c file that has the complete override list to be used with the patch.

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Antenna Diversity Patch

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CC13xx Antenna Diversity

2.5 Configuration Registers

Table 3 lists the configuration registers available in the antenna diversity patch.

Table 3. Configuration Registers

0x4004 52B0 15–8 PreTimeout PreTimeout_bits = PreTimeout + 8 0x02

7–0 SyncTimeout SyncTimeout_bits = SyncBits + PreamBits + SyncTimeout – 31 0x18

0x4004 52B4

15 NA NA 0x00

14 CsQual Carrier sense qualified preamble 0x01

13–12 NA NA 0x00

11–8 PreLen Number of preamble bits for correlation

PreLen_bits = PreLen + 1 0x07

7–0 PreThr Preamble correlation threshold 0x0B

0x4004 6088 15–0 AgcRssiRef AGC RSSI reference level for gain adjustment

AgcRssiRef_dBm = AgcRssiRef – 216 [dBm] 0xFFA6

0x4004 6090 15–8 Reserved 0x0A

7–0 CS_THR Carrier sense threshold

CS_THR_dBm = CS_THR – 256 [dBm] 0x91

This register can only be changed by static overrides of the following format:

HW_REG_OVERRIDE(0xXXXX,0xYYYY),

where:

• XXXX is the 2 least significant bytes of the register address (52B0, 52B4, 6088, or 6090)

• YYYY is the value to write

As an example, the following register write sets the carrier sense threshold to –111 dBm:

HW_REG_OVERRIDE(0x6090,0x0A91),

Increasing the carrier threshold to –107 dBm (for example) yields the following:

HW_REG_OVERRIDE(0x6090,0x0A95),

2.6 Status Registers

Table 4 lists the status registers available in the antenna diversity patch.

Table 4. Status Registers

0x4004 5178 15–0 AntSelect Antenna selection TX and RX

0: Antenna0

1: Antenna1 0x00

0x4004 517C 15–8 RSSI1 RSSI antenna 1 0x00

7–0 RSSI0 RSSI antenna 0 0x00

0x4004 51C8 11–0 AntSelect0Cnt Antenna select 0 counter 0x00

0x4004 51CC 11–0 AntSelect1Cnt Antenna select 1 counter 0x00

In TX, the AntSelect selects the antenna that is used for the transmission. The configuration registers do

not have retention, so if the RF Core is power cycled, the application must then update the AntSelect

and then the processor must write the register value again before entering TX.

The registers can be read using the CMD_READ_RFREG API command but can be written only as an

override (when doing a CMD_PROP_RADIO_DIV_SETUP).

Building a Software Example

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3 Building a Software Example

Using the patch is transparent to the application as long as the correct settings are used and the

RFC_SMI_CL_OUT signal is output on the DIO used to control the switch.

As an example, the following steps must be completed to implement antenna diversity on the rfPacketRX

example. The rfPacketRX example is available when downloading the SimpleLink™ CC13x0 Software

Development Kit [3]. See the Proprietary RF Quick Start Guide for more information. A link to this guide

can be found in the documentation_overview_simplelink_cc13x0_sdk.html found here:

C:\ti\simplelink_cc13x0_sdk_x_xx_xx_xx\docs (assuming that installation has been to default location)

1. The smartrf_settings.c file must be replaced with the one included with this document [4].

2. The following defines must be added:

#define PINCC26XX_MUX_RFC_SMI_CL_OUT 0x37

#define ANTENNA_SELECT IOID_X

#define ANTENNA_SELECT_INV IOID_Y // Added if the switch requires two control signals

3. The ANTENNA_SELECT and optional ATENNA_SELECT_INV signals must be added to the pinTable:

PIN_Config pinTable[] =

{Board_LED1 | PIN_GPIO_OUTPUT_EN | PIN_GPIO_LOW | PIN_PUSHPULL | PIN_DRVSTR_MAX,

ANTENNA_SELECT | PIN_GPIO_OUTPUT_EN | PIN_GPIO_LOW | PIN_PUSHPULL | PIN_DRVSTR_MAX,

PIN_INV_INOUT,

PIN_TERMINATE

};

4. Add the following line or lines to the int main(void) function before starting the BIOS. Adding these lines

is to connect the RFC_SMI_CL_OUT signal to wanted DIOs (ANTENNA_SELECT and

ANTENNA_SELECT_INV).

PINCC26XX_setMux(pinHandle, PIN_ID(ANTENNA_SELECT), PINCC26XX_MUX_RFC_SMI_CL_OUT);

PINCC26XX_setMux(pinHandle, PIN_ID(ANTENNA_SELECT_INV), PINCC26XX_MUX_RFC_SMI_CL_OUT);

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Building a Software Example

SWRA523B–September 2016–Revised March 2017

CC13xx Antenna Diversity

For the transmitter, use the SmartRF Studio and the 50-kbps setting with the preamble count set to

6 bytes (see Figure 2).

Figure 2. SmartRF™ Studio Setup for TX

Input Power Level (dBm)

Packet Error Rate

-115 -105 -95 -85 -75 -65 -55 -45 -35

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

D001

Measurement Results

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4 Measurement Results

4.1 System Parameters

The system parameters used when testing the antenna diversity patch are as follows:

• 2-GFSK modulation

• 50-kbps data rate, ±25 kHz deviation

• 868 MHz

• 6- or 8-byte preamble

• 4-byte sync word (0x930B 51DE)

• Carrier sense (CS) threshold set to –111 dBm

4.2 Conducted Measurements

4.2.1 Sensitivity

Table 5 lists the sensitivity measurement results.

Table 5. Typical 1% BER Sensitivity (20% PER, 3-Byte Payload)

Preamble Length Typical Sensitivity [dBm]

6 bytes –109

8 bytes –109

4.2.2 Packet Error Rate (PER) versus Input Power Level

A 20% PER corresponds to a 1% bit error rate (BER).

4.2.2.1 6-Byte Preamble

Figure 3 shows the PER versus the input power level with a 6-byte preamble and a 6-dB attenuator on

one antenna.

Figure 3. PER vs Input Power Level

(6-Byte Preamble, 6-dB Attenuator on One Antenna)

−110

−100

−90

−80

−70

−60

−50

−40

−30

−20

−50 0 50

Offset Frequency (ppm)

Level (dBm)

100

Input Power Level (dBm)

Packet Error Rate

-115 -105 -95 -85 -75 -65 -55 -45 -35

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

D002

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Measurement Results

SWRA523B–September 2016–Revised March 2017

CC13xx Antenna Diversity

4.2.2.2 8-Byte Preamble

Figure 4 shows the PER versus the input power level with an 8-byte preamble and a 6-dB attenuator on

one antenna.

Figure 4. PER vs Input Power Level

(8-Byte Preamble, 6-dB Attenuator on One Antenna)

4.2.3 PER versus Frequency Offset

4.2.3.1 6-Byte Preamble

Figure 5 shows the PER versus the input power level with a 6-byte preamble and a 6-dB attenuator on

one antenna.

Figure 5. PER vs Input Power Level

(6-Byte Preamble, 6-dB Attenuator on One Antenna)

−110

−100

−90

−80

−70

−60

−50

−40

−30

−20

−50 0 50

Offset Frequency (ppm)

Level (dBm)

100

Measurement Results

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4.2.3.2 8-Byte Preamble

Figure 6 shows the PER versus the input power level with an 8-byte preamble and a 6-dB attenuator on

one antenna.

Figure 6. PER vs Input Power Level

(8-Byte Preamble, 6-dB Attenuator on One Antenna)

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