RF Coexistence

[中文]

Overview

ESP32 has only one 2.4 GHz ISM band RF module, which is shared by Bluetooth (BT & BLE) and Wi-Fi, so Bluetooth can’t receive or transmit data while Wi-Fi is receiving or transmitting data and vice versa. Under such circumstances, ESP32 uses the time-division multiplexing method to receive and transmit packets.

Supported Coexistence Scenario for ESP32

Supported Features of Wi-Fi and BLE Coexistence

BLE

Scan

Advertising

Connecting

Connected

Wi-Fi

STA

Scan

Y

Y

Y

Y

Connecting

Y

Y

Y

Y

Connected

Y

Y

Y

Y

SOFTAP

TX Beacon

Y

Y

Y

Y

Connecting

C1

C1

C1

C1

Connected

C1

C1

C1

C1

Sniffer

RX

C1

C1

C1

C1

ESP-NOW

RX

S

S

S

S

TX

Y

Y

Y

Y
Supported Features of Wi-Fi and Classic Bluetooth (BT) Coexistence

BR/EDR

Inquiry

Inquiry scan

Page

Page scan

Connected

Wi-Fi

STA

Scan

Y

Y

Y

Y

Y

Connecting

Y

Y

Y

Y

Y

Connected

Y

Y

Y

Y

Y

SOFTAP

TX Beacon

Y

Y

Y

Y

Y

Connecting

C1

C1

C1

C1

C1

Connected

C1

C1

C1

C1

C1

Sniffer

RX

C1

C1

C1

C1

C1

ESP-NOW

RX

S

S

S

S

S

TX

Y

Y

Y

Y

Y

Note

Y: supported and performance is stable C1: supported but the performance is unstable X: not supported S: supported and performance is stable in STA mode, otherwise not supported

Coexistence Mechanism and Policy

Coexistence Mechanism

The RF resource allocation mechanism is based on priority. As shown below, both Bluetooth module and Wi-Fi module request RF resources from the coexistence module, and the coexistence module decides who will use the RF resource based on their priority.

Coexistence Mechanism

Coexistence Policy

Coexistence Period and Time Slice

Wi-Fi, BT, and BLE have their fixed time slice to use the RF. A coexistence period is divided into 3 time slices in the order of Wi-Fi, BT, and BLE. In the Wi-Fi slice, Wi-Fi’s request to the coexistence arbitration module will have higher priority. Similarly, BT/BLE can enjoy higher priority at their own time slices. The duration of the coexistence period and the proportion of each time slice are divided into four categories according to the Wi-Fi status:

  1. IDLE status: the coexistence of BT and BLE is controlled by Bluetooth module.

  2. CONNECTED status: the coexistence period starts at the Target Beacon Transmission Time (TBTT) and is more than 100 ms.

  3. SCAN status: Wi-Fi slice and coexistence period are longer than in the CONNECTED status. To ensure Bluetooth performance, the Bluetooth time slice will also be adjusted accordingly.

  4. CONNECTING status: Wi-Fi slice is longer than in the CONNECTED status. To ensure Bluetooth performance, the Bluetooth time slice will also be adjusted accordingly.

According to the coexistence logic, different coexistence periods and time slice strategies will be selected based on the Wi-Fi and Bluetooth usage scenarios. A Coexistence policy corresponding to a certain usage scenarios is called a “coexistence scheme”. For example, the scenario of Wi-Fi CONNECTED and BLE CONNECTED has a corresponding coexistence scheme. In this scheme, the time slices of Wi-Fi and BLE in a coexistence period each account for 50%. The time allocation is shown in the following figure:

Time Slice Under the Status of Wi-Fi CONNECTED and BLE CONNECTED

Time Slice Under the Status of Wi-Fi CONNECTED and BLE CONNECTED

Dynamic Priority

The coexistence module assigns different priorities to different status of Wi-Fi and Bluetooth. And the priority for each status is dynamic. For example, in every N BLE Advertising events, there is always one event with high priority. If a high-priority BLE Advertising event occurs within the Wi-Fi time slice, the right to use the RF may be preempted by BLE.

Wi-Fi Connectionless Modules Coexistence

To some extent, some combinations of connectionless power-saving parameters Window and Interval would lead to extra Wi-Fi priority request out of Wi-Fi time slice. It`s for obtaining RF resources at coexistence for customized parameters, while leading to impact on Bluetooth performance.

If connectionless power-saving parameters are configured with default values, the coexistence module would perform in stable mode and the behaviour above would not happen. So please configure Wi-Fi connectionless power-saving parameters to default values unless you have plenty of coexistence performance tests for customized parameters.

Please refer to connectionless module power save to get more detail.

How to Use the Coexistence Feature

Coexistence API

For most coexistence cases, ESP32 will switch the coexistence status automatically without calling API. However, ESP32 provides two APIs for the coexistence of BLE MESH and Wi-Fi. When the status of BLE MESH changes, call esp_coex_status_bit_clear to clear the previous status first and then call esp_coex_status_bit_set to set the current status.

BLE MESH Coexistence Status

As the firmware of Wi-Fi and Bluetooth are not aware of the current scenario of the upper layer application, some coexistence schemes require application code to call the coexistence API to take effect. The application layer needs to pass the working status of BLE MESH to the coexistence module for selecting the coexistence scheme.

  • ESP_COEX_BLE_ST_MESH_CONFIG: network is provisioning

  • ESP_COEX_BLE_ST_MESH_TRAFFIC: data is transmitting

  • ESP_COEX_BLE_ST_MESH_STANDBY: in idle status with no significant data interaction

Coexistence API Error Codes

All coexistence APIs have custom return values, i.e. error codes. These error codes can be categorized as:

  • No error. For example, the return value ESP_OK siginifies the API returned successfully.

  • Recoverable errors. For example, the return value ESP_ERR_INVALID_ARG signifies API parameter errors.

Setting Coexistence Compile-time Options

Note

Since the coexistence configuration option depends on the Bluetooth configuration option, please turn on the Bluetooth configuration option first before configuring the coexistence feature in the Wi-Fi configuration option.

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