Adding Wi-Fi Coexistence support to short-range radio applications

This guide describes how to add Wi-Fi Coexistence (CX) support to your short-range radio application in nRF Connect SDK.

Short-range RF technologies (here referred to as SR), such as Bluetooth LE or 802.15.4, use different radios than Wi-Fi. However, if both SR and Wi-Fi attempt to transmit simultaneously, the radio frequency (RF) waves interfere with each other, causing decreased performance and unneeded higher power consumption. Also, in cases like receiving an acknowledgment (ACK), radios should not transmit to ensure correct reception.

These issues are defined as coexistence issues. To mitigate these issues and to improve performance, a Packet Traffic Arbiter (PTA) is used. When both SR and Wi-Fi request access to RF, the Packet Traffic Arbiter grants or denies that access.

Requirements

If your application uses SR protocols and requires coexistence with Wi-Fi, you must enable IEEE 802.15.4 External Radio Coexistence in the Multiprotocol Service Layer (MPSL) library.

Wi-Fi Coexistence and MPSL

The MPSL library provides an implementation of the Wi-Fi Coexistence for SR applications. To use this implementation, your application must use a protocol driver that enables the Wi-Fi Coexistence (CX) feature. The library provides multiprotocol support, but you can also use it in applications that require only one protocol.

Currently, the following protocols use the Wi-Fi Coexistence support provided by MPSL:

Enabling MPSL

The MPSL library provides an API to handle signals coming from the PTA and to control requests sent to PTA. See the Integration notes in the nrfxlib documentation for details.

Enable support for the MPSL implementation in nRF Connect SDK by setting the CONFIG_MPSL Kconfig option to y.

Note

When using the nRF5340, add the CONFIG_MPSL option only for the NET core.

Enabling Wi-Fi Coexistence

To enable Wi-Fi Coexistence, do as follows:

  1. Set the CONFIG_MPSL_CX Kconfig option to y.

  2. Select the specific Coexistence implementation. See Wi-Fi Coexistence implementations for details about the implementations currently supported.

  3. On the nRF5340, you must apply the settings to the Kconfig options mentioned in steps 1 and 2 also to the network core. See Multi-image builds.

Hardware description

The nRF Connect SDK provides a wrapper that configures Wi-Fi Coexistence based on devicetree (DTS) and Kconfig information.

To enable CX support for the currently supported CX implementation, you must add an nrf_radio_coex node in the devicetree source file. You can also provide the node using the devicetree source file of the target board or an overlay file. See Devicetree for more information about the DTS data structure, and Devicetree versus Kconfig for information about differences between DTS and Kconfig.

For the CX implementation currently supported, the nrf_radio_coex node has the compatible property set to generic-radio-coex-three-wire. Devicetree nodes compatible with generic-radio-coex-three-wire can be used when a three-wire interface to the PTA is provided (like the one shown in the following image). However, the role of each of the pins is dependent on the Wi-Fi Coexistence implementation used.

Note

  • When using one of the supported implementations, you must use the nrf_radio_coex name for the node. However, if you add a custom user implementation, you can also use a different name.

  • You can add a new device binding and use it as the compatible property for the node, if generic-radio-coex-three-wire is unsuitable.

PTA interface supported by the ``nrf_radio_coex`` node with an nRF52 Series SoC

To configure the hardware for a PTA using a 3-wire interface:

  1. Add the following node in the devicetree source file:

    / {
          nrf_radio_coex: radio_coex_three_wire {
             status = "okay";
             compatible = "generic-radio-coex-three-wire";
             req-gpios =     <&gpio0 24 (GPIO_ACTIVE_HIGH)>;
             pri-dir-gpios = <&gpio0 14 (GPIO_ACTIVE_HIGH)>;
             grant-gpios =   <&gpio0 25 (GPIO_ACTIVE_HIGH | GPIO_PULL_UP)>;
       };
    };
    
  2. Optionally replace the node name radio_coex_three_wire with a custom one.

  3. Replace the pin numbers provided for each of the required properties:

    • req-gpios - GPIO characteristic of the device that controls the REQUEST signal of the PTA.

    • pri-dir-gpios - GPIO characteristic of the device that controls the PRIORITY signal of the PTA.

    • grant-gpios - GPIO characteristic of the device that controls the GRANT signal of the PTA (RF medium access granted). Note that GPIO_PULL_UP is added to avoid a floating input pin and is required on some boards only. If the target board is designed to avoid this signal being left floating, you can remove GPIO_PULL_UP to save power.

    The phandle-array type is used, as it is commonly used in Zephyr’s devicetree to describe GPIO signals. The first element &gpio0 indicates the GPIO port (port 0 has been selected in the example shown). The second element is the pin number on that port.

  4. On the nRF5340, you must also apply the same devicetree node mentioned in step 2 to the network core. To do so, apply the overlay to the correct network-core child image by creating an overlay file named child_image/*childImageName*.overlay in your application directory, for example child_image/multiprotocol_rpmsg.overlay.

    The *childImageName* string must assume one of the following values:

    • multiprotocol_rpmsg for multiprotocol applications having support for both 802.15.4 and Bluetooth.

    • 802154_rpmsg for applications having support for 802.15.4, but not for Bluetooth.

    • hci_rpmsg for application having support for Bluetooth, but not for 802.15.4.

Wi-Fi Coexistence implementations

The following CX implementations are available:

Each implementation is a plugin composed of one or more C source files interfacing with the MPSL API, which, in turn, communicates with the SR protocol drivers.

When one of the CX variants is enabled, the driver requests access to the RF medium from the PTA and informs it about the operation it is about to perform. It also reacts properly to the information from the PTA that access to the RF medium was either granted or denied.

Generic 3-pin interface implementation

This implementation uses the following pins to communicate with the PTA:

  • REQUEST pin - it is the output controlled by the protocol driver. It is asserted to request RF access from the PTA. It can be configured by setting the req-gpios property of the nrf_radio_coex devicetree node.

  • PRIORITY pin - it is the output controlled by the protocol driver. It contains information about the type of operation (either RX or TX) to perform. It can be configured by setting the pri-dir-gpios property of the nrf_radio_coex devicetree node.

  • GRANT pin - it is the input of the SoC controlled by the PTA. It asserts when PTA grants access to the RF to the 802.15.4 and deasserts when it denies the access. It can be configured by setting the grant-gpios property of the nrf_radio_coex devicetree node.

The support for this interface is provided in a single-file plugin located in the sdk-nrf repo, in the subsys/mpsl/cx directory.

Adding support for the CX generic 3-pin interface

The generic 3-pin interface is supported out of the box. To use it, complete the following steps:

  1. Set the CONFIG_MPSL_CX_GENERIC_3PIN Kconfig option to y.

  2. Add the devicetree nrf_radio_coex node as described in Hardware description.

Thread implementation

This implementation uses the following pins to communicate with the PTA:

  • REQUEST pin - it is the output controlled by the protocol driver. It is asserted to request RF access from the PTA. It can be configured by setting the req-gpios property of the nrf_radio_coex devicetree node.

  • PRIORITY pin - it is the output controlled by the protocol driver. It is asserted if the operation priority is high and deasserted if it is low. It can be configured by setting the pri-dir-gpios property of the nrf_radio_coex devicetree node.

  • GRANT pin - it is the input of the SoC controlled by the PTA. It asserts when PTA grants access to the RF to the 802.15.4 and deasserts when it denies the access. It can be configured by setting the grant-gpios property of the nrf_radio_coex devicetree node.

The support for this interface is provided in a single-file plugin located in the sdk-nrf repo, in the subsys/mpsl/cx directory.

Adding support for the CX Thread interface

To use the Thread interface PTA, complete the following steps:

  1. Set the CONFIG_MPSL_CX_THREAD Kconfig option to y.

  2. Add the devicetree nrf_radio_coex node as described in Hardware description.

Custom user implementations

Implementing a custom user CX implementation is described in IEEE 802.15.4 External Radio Coexistence. If the nrf_radio_coex devicetree node is not sufficient for the implementation, you must add a custom devicetree node.