LE Coded PHY on EFR32xG13

by <a href="http://community.silabs.com/t5/Welcome-and-Announcements/Community-Ranking-System-and-Recognition-Program/m-p/140490#U140490"><font color="#000000"><font size="2">Legend Employee</font></font> </a> tmonte ‎06-15-2017 08:09 AM - edited ‎06-15-2017 10:06 PM

Introduction

Bluetooth 5.0 introduces a new long range PHY (LE Coded PHY) with 125kbps and 500kbps coding which gives range gains of 1.5-2x with improved sensitivity of 4 to 6dB.

 

The LE Coded PHY uses 1Mbit PHY but payload is coded at 125kbps or 500kbps. It also adds Forward Error Correction and Pattern Mapper.

 

Note: LE Coded PHY is only supported on EFR32xG13 devices.

 

Using the LE Coded PHY

Changing to LE Coded PHY can be requested from either slave or master device using the command gecko_cmd_le_connection_set_phy(connection, phy), where the phy value maps as follows:

  • 0x01: 1Mbit PHY
  • 0x02: 2Mbit PHY
  • 0x04: 125kbit Coded PHY (S=8)
  • 0x08: 500kbit Coded PHY (S=2)

For compatibility reasons the default PHY is always 1Mbit. This must the PHY used for advertising as well as establishing the connection. Once the connection is formed either side (master or slave) can request a new PHY to be taken into use. The PHY requests can be issued at any point during the connection and the stack takes care of negotiating the new PHY usage and notifying the application of whether the new PHY has been taken into use or not.

 

When a new PHY is requested it will take a minimum of 6 connection intervals for the device to switch to the new PHY. Once the PHY switch occurs the event le_connection_phy_status is raised by the stack to notify the application of the new PHY setting.

 

In the case of using 2 EFR32xG1x devices the event le_connection_phy_status will be raised on both devices in the following situations (if the new PHY request is accepted by the receiving device):

  • Switching to/from LE Coded PHY from/to another PHY (1Mbit/2Mbit)
  • Switching from 1Mbit to 2Mbit PHY and vice-versa

 

The event le_connection_phy_status will only be raised on the requesting device when:

  • The new PHY request is not accepted (e.g. the receiving device doesn’t support it)
  • Switching LE Coded PHY coding (from S=2 to S=8 and vice-versa) when LE Coded PHY is already in use
    • The reason is that changing the coding is not negotiated over-the-air, it’s done only locally on the requesting device and the receiving device continues using the same coding. At the RX stage the radio detects the coding based on the packet preamble and it can use a different coding for the TX stage.

 

An additional note is that the default coding for LE Coded PHY is S=8 (125kbit). This means that when an EFR32BG13 accepts a PHY change request to LE Coded PHY it will default to S=8. It is up for the application to change the coding if desired and the new coding will then be used if the PHY is again changed to 1Mbit/2Mbit and then back to LE Coded PHY.

 

Examples

Here are a few examples of changing between the PHYs using 2 EFR32xG13 devices in NCP mode and controlled with BGTool.

 

Changing to LE Coded PHY (S=8, 125kbit)

In this example the devices are connected using a 512ms connection interval. After the PHY change command is issued it takes ~5s to get the event which is raised on both devices.

switching to coded phy - Copy.png

 

Changing the Coding While in LE Coded PHY

As explained earlier in this article changing the coding is not negotiated over-the-air between the devices and for that reason the event le_connection_phy_status is only raised on the requesting side and very shortly after the command is sent. The receiving device will continue using the same coding.

switching coding - Copy.png

 

Attempted Change to LE Coded PHY

In the example below a new PHY request is attempted but the other device is an EFR32BG1 which only supports 1Mbit PHY so the event le_connection_phy_status is immediately raised on the requesting device to notify that the PHY has not been changed and no event is raised on the receiving device.

switching denied.png

 

Impact on Energy Consumption

Using LE Coded PHY will lead to higher current consumption due to the longer RX/TX periods. The next images show the current profile of a notification packet with 20 bytes of data payload on all the PHYs. The red blocks are highlighting the duration of the entire wake-up period and the average current consumption for said period.

 

These measurements were made from the slave device (RX precedes TX) with TX Power of 0dBm and the images are sorted from shortest to longest RX/TX periods: 2Mbit, 1Mbit, LE Coded S=2 (500kpbs) and LE Coded S=8 (125kpbs)

 

Note: When using LE Coded PHY the packet header is always coded with S=8 but the payload can be S=2 or S=8 coded and that is indicated by a bit in the header. This means that empty packets will not have a significant difference between S=8 and S=2 coding in terms of TX duration.

 

2Mbit PHY

2M_phy_notification_packet.png

 

1Mbit PHY

1M_phy_notification_packet.png

 

LE Coded PHY (S=2, 500kbps)

Coded_500_kbit_phy_notification_packet.png

 

LE Coded PHY (S=8, 125kbps)

Coded_125_kbit_notification_packet.png

 

Indoor Range Tests

The image below shows the result of range test in one of our offices. On the top right corner we placed an EFR32BG13 and then we walked around with a second EFR32BG13 kit to check the connection boundary.

 

Comparing between 1Mbit PHY and LE Coded PHY (125kbps) the range is roughly double due to the sensitivity improvements.

range_test.png