Enabling the IoT, part 4: Collaborative coexistence
July 14, 2017
IEEE 802.15.2 has defined several collaborative and non-collaborative methods to implement coexistence. Here are two recommended collaborative approaches.
In previous parts of this article series, we discussed the basics of the Bluetooth and Wi-Fi interfaces and the types of coexistence between them. Now we will discuss collaborative coexistence in detail.
IEEE 802.15.2 has defined several collaborative and non-collaborative methods to implement coexistence. Two recommended collaborative approaches are:
- Alternating wireless medium access (AWMA)
- Packet traffic arbitration (PTA)
Alternating wireless medium access implements time-division multiple access with WLAN and Bluetooth radios taking turns accessing the spectrum. The primary disadvantage of this approach is that it requires collaboration with the wireless access point and may not be possible for many systems. In addition, Bluetooth synchronous connection-oriented links – critical for voice-related applications – are not possible when using this method.
Packet traffic arbitration employs an arbiter that decides whether WLAN or Bluetooth gets access to the radio. If Bluetooth needs to access the medium, it requests the arbiter for access and the arbiter makes the decision. Performance depends on the way the arbiter is implemented and how much information is available to it from both Bluetooth and Wi-Fi devices. IEEE 802.15.2 recommends a three-wire PTA architecture.
Three-wire collaborative coexistence
[Figure 1 | Coexistence mechanism for three-wire PTA]
Bluetooth and Wi-Fi use three signals to implement coexistence (Figure 1), hence the name, three-wire coexistence interface. PTA resides inside Wi-Fi device and three signals are exposed on the chip interface to connect to a Bluetooth device. The three interface pins/signals are:
The Bluetooth link manager is responsible for providing status information to the PTA. The Bluetooth link manager drives the BT_PRIORITY_AND_STATUS signal to indicate the priority and RX/TX slot. BT_ACTIVITY is asserted by the Bluetooth device to request medium access when a Bluetooth transaction needs to be done. Every time the Bluetooth device wants to access the medium, it asserts this signal. Following a request, the PTA uses the WLAN_ACTIVITY signal to indicate whether the Bluetooth device can proceed with the transmission.
Both BT_ACTIVITY and WLAN_ACTIVITY are exchanged before every packet transmission attempt (Figure 2). Timing requirements may vary from device to device and vendor to vendor.
[Figure 2 | A sample signal timing diagram for three-wire coexistence]
Serial enhanced coexistence interface (SECI)
While the above method allows both Bluetooth and Wi-Fi to coexist, performance is greatly impacted by the limited information that is shared between the devices and PTA. To address this limitation and enable more sophisticated and efficient coexistence, developers can use the two-wire serial enhanced coexistence interface (SECI) (Figure 3).
[Figure 3 | A high-level representation of SECI]
SECI supports UART communication between Wi-Fi and Bluetooth devices. With SECI, 48- or 64-bit coexistence data can be exchanged to help the PTA manage Wi-Fi and Bluetooth traffic efficiently. Information conveyed using this interface includes packet type, duration, hop frequency, RSSI, etc. SECI is a proprietary implementation so to take advantage of SECI or any other proprietary interface, it is important to select both Wi-Fi and Bluetooth components from the same vendor.
Coexistence in combo devices
All methods discussed above are primarily used between discrete Wi-Fi and Bluetooth devices. The most efficient solution for coexistence between Wi-Fi and Bluetooth is to use combo devices that have both technologies integrated onto a single die. When both Wi-Fi and Bluetooth are on the same die, they can share a parallel interface with the PTA to allow faster communication and improve overall performance. The integrated nature of combo devices allows for an enhanced coexistence interface (ECI) for Bluetooth and Wi-Fi combo devices that helps improve the quality of simultaneous voice, video, and data communication, such as video-over-WLAN and high-fidelity Bluetooth stereo. Implementations vary from vendor to vendor, so coexistence performance varies as well.
Hardware plays a major role in implementing a reliable coexistence between collocated Bluetooth and Wi-Fi. Some of the common antenna configurations for collocated devices are:
In this configuration, both Bluetooth and Wi-Fi share the antenna/bandwidth and cannot operate simultaneously. This limits Wi-Fi throughput. Such a configuration is suitable for applications requiring limited throughput, so it may work well for most IoT applications. A single low noise amplifier (LNA) may be used to receive both Bluetooth and Wi-Fi.
In this configuration, both Bluetooth and Wi-Fi have their own dedicated antenna. With dedicated antennas, both radios can operate simultaneously, provided there is sufficient isolation between the antennas. Isolation needs to be enough to ensure transmission that the use of one antenna does not saturate the antenna of another device or technology. Lack of enough isolation will limit operation to time division multiplexing, the same method as the one used between shared antenna. Typically, 35 dB or more isolation is required between Bluetooth and Wi-Fi antennas to support simultaneous transmission.
A hybrid implementation allows 25 dBm isolation where Bluetooth Tx and Wi-Fi Rx can happen at the same time, given that transmit power for Bluetooth is much less than for Wi-Fi. For example, Wi-Fi can transmit an ACK while Bluetooth is receiving. However, Wi-Fi range and throughput will be limited or determined by the isolation and high Bluetooth Low Energy (BLE) power at the Wi-Fi antenna can cause packet loss due to receiver saturation.
Real simultaneous dual band
If high availability is a requirement of Wi-Fi, the best approach is to use the 5 GHz band since Bluetooth operates at 2.4 GHz. IEEE 802.11ac allows both the 2.4 GHz and 5 GHz bands to be used for Wi-Fi. Some combo devices support real simultaneous dual band (RSDB) operation that allows devices to transmit at 2.4 GHz and 5 GHz simultaneously. Hence, Wi-Fi throughput is not compromised and offers the best coexistence with collocated Bluetooth devices and other wireless devices operating in the vicinity. RSDB is especially needed in automotive applications where Wi-Fi is used for high definition video streaming.
IoT applications that need collocated Bluetooth and Wi-Fi require special attention while selecting right device and designing hardware. The coexistence interface offered by the device must be examined carefully to ensure it achieves the required Wi-Fi throughput while still providing reliable Bluetooth communication. Combo devices offer a compact approach to meeting some of the challenges in coexistence.