Coexistence
Introducion
Bluetooth (BT) and Wi-Fi both operate in the 2.4GHz frequency band. Therefore, it is necessary to carefully manage mutual interference when they coexist and operate simultaneously.
This chapter describes several BT and Wi-Fi coexistence solutions for reducing the mutual interference. Both BT and Wi-Fi can achieve appropriate performance with these solutions.
There is no mutual interference between BT and Wi-Fi when Wi-Fi working at 5GHz, so this chapter does not cover this case.
Features
The Wi-Fi of chip complies with the IEEE 802.11n specification, and operates at both 2.4GHz and 5GHz. It has three working modes: station mode, soft AP mode and concurrent mode.
The Bluetooth of chip only supports LE controller and complies with v5.0 specification. The related BT features can be classified into several categories including BLE, BLE mesh, BLE audio and power saving. The details of supported features are listed in following table.
Items |
Wi-Fi |
BT |
|---|---|---|
Specification |
IEEE 802.11n |
BT core specification v5.0 |
Working frequency |
2.4GHz/5GHz |
2.4GHz |
Working modes |
|
|
Power saving |
|
|
Radio Frequency Path
There is only one radio path in chip, which is shared between Wi-Fi and BT module. The path is described below.
Application-Oriented Path Selection
The RF path can be configured for adapting to different applications as listed below.
Case |
Shared path |
Shared path |
Antenna number |
Need coexistence? |
|---|---|---|---|---|
Case |
BT |
Wi-Fi |
Antenna number |
Need coexistence? |
#1 (BT-only application) |
√ |
X |
One |
No |
#2 (Wi-Fi-only application) |
X |
√ |
One |
No |
#3 (Wi-Fi and BT application) |
√ |
√ |
One |
Yes |
Coexistence in Shared RF Path
Introduction
For guaranteeing appropriate performance of Wi-Fi and BT under the shared RF path, the coexistence mechanism composed of Packet Traffic Arbitration (PTA) and Time Division Multiple Access (TDMA) must be applied.
The following figure is a diagram of Wi-Fi and BT coexistence architecture using shared RF path. The key components are the PTA circuit and TDMA scheme.
PTA readily arbitrates the simultaneous transmitting or receiving request from Wi-Fi and BT to decide who can use the shared RF path. For example, we can configure the PTA logic to BT traffic priority > Wi-Fi traffic priority to ensure that BT traffic can always preempt Wi-Fi traffic.
The coexistence algorithm collects proper information from Wi-Fi driver and BT core stack, and modify the PTA logic due to different coexistence applications. When Wi-Fi or BT status change, relative information should be updated in time so that the coexistence algorithm can adjust the coexistence strategy rapidly.
TDMA is responsible for divide the RF usage into Wi-Fi time slot and BT time slot. The PTA logic can be different in Wi-Fi or BT slot, which provides more transmit privileges during their respective slot for Wi-Fi and BT. For instance, we can set the PTA logic of Wi-Fi slot to BT high-pri Tx > Wi-Fi > BT others, while BT slot to BT > Wi-Fi. In such case, BLE ADV traffic which is assigned with high priority can be transmitted during both Wi-Fi slot and BT slot. BLE SCAN traffic which is assigned with low priority is forbidden to transmit during Wi-Fi slot, but only permitted during BT slot.
PTA Mode
As described in the following figure, PTA gathers the traffic status information (priority, traffic request, …) from both sides to decide which side can use the shared RF path. Then PTA do arbitration depending on the compare logic set by coexistence algorithm to send out the output status confirm and the grant signal. The traffic status confirms are sent to Wi-Fi and BT as the feedbacks to control traffic transmission. The grant signal controls the shared RF circuits switching to the right side.
Although PTA is an effective coexistence mechanism for Wi-Fi and BT, its inherent limitations in heavy traffic or noisy environment, which causes excessively frequent shared RF circuits switching between Wi-Fi and BT, can break down the traffic unexpectedly. So PTA cannot be solely relied on to solve coexistence issues all the time.
TDMA Mode
TDMA is an enhancement to PTA. TDMA divide Wi-Fi TBTT (Target Beacon Transmission Time) interval to Wi-Fi time slot and BT time slot. Wi-Fi time slot starts at several milliseconds earlier than TBTT for receiving 802.11 beacon frames. The remaining time of the TBTT interval is BT time slot.
In most cases, we use 2-slot and 4-slot TDMA to share the RF usage. Figure TDMA type with 2-slot and TDMA type with 4-slot demonstrate TDMA with 2-slot and 4-slot mode separately.
TDMA should be used combined with PTA instead of using TDMA separately.
Considering the difficulty to understand some hardware characteristics of the on-chip RF system, we do not recommend customers to change TDMA parameters by themselves.
TDMA type with 2-slot
TDMA type with 4-slot
Working Scenarios
The Wi-Fi operation modes include STA mode and soft AP mode. The BLE contains peripheral and central roles according to generic access profile (GAP). Besides, the state of power saving is a key topic in coexistence, which should be treated carefully.
Based on these working modes and roles, there are many Wi-Fi and BT coexistence scenarios with different parameter configuration.
Here, we take a scenario as an example to illustrate the parameter configuration under shared RF path coexistence. In this example, Wi-Fi is in connected state while BT in LE scan state. In Wi-Fi time slot, Wi-Fi packets has higher priority than LE scan. In BT time slot, LE scan has higher priority than Wi-Fi packets. The details is described below.
