IoT gateways and hubs benefit from enabling multiple protocols such as Wi-Fi, ZigBee, Thread, and Bluetooth to support a diverse end node landscape. Wireless coexistence studies and mitigation technologies for unlicensed 2.4 GHz frequency bands have been around for at least 20 years.
The issue is that different 2.4 GHz wireless technologies meet different needs for the same devices, and therefore must operate simultaneously without noticeable performance degradation.
The Importance of Wi-Fi Coexistence
Figure 1: 2.4GHz protocols frequency overlap
Wi-Fi coexistence allows multiple 2.4 GHz technologies including Wi-Fi, ZigBee, Thread, and Bluetooth to operate without signals from one radio interfering with adjacent radios. Figure 1 shows the 2.4GHz protocol frequency overlap.
Due to the shared nature of the channel, these technologies cause interference to each other when they operate in the same time-frequency-space region. Depending on the strength of the interfering channels and the power of transmission, such interference can cause considerable performance degradation. Interference degrades wireless performance through message failures, resulting in more message retries. These issues can also lead to reduced device responsiveness and increased power consumption.
For example, when a Wi-Fi network is on the same channel as a ZigBee network, the Wi-Fi network will usually interfere with the ZigBee network. Figure 2 shows the situation of that Wi-Fi sideband and ZigBee frequency interference.
The Wi-Fi spectrum has two components: the 20 MHz “square” section that contains the data subcarriers and sideband lobes on each side, which are normal side effects. Sideband lobes might not carry Wi-Fi data, but they are fully capable of drowning out ZigBee transmissions.
This is especially evident when your ZigBee access point and Wi-Fi access point are in very close proximity to each other. So designing Wi-Fi coexistence is a big issue when multiple wireless sources are present in the same environment.
（a）WIFI 20MHz spectrum
(b) WIFI sideband and ZigBee frequency overlap
Figure 2: WIFI sideband and ZigBee frequency interference
The potential for interference issues and the need for co-existence solutions were realized early on during the development of these technologies. The IEEE 802.15.2 standard addresses the issue of coexistence between WLAN and WPAN networks. According to the standards and the protocols, coexistence without direct interaction between ZigBee/Thread, BLE, and Wi-Fi radios can be improved in the following ways:
1. Implement Frequency Separation
Co-channel operation of 802.15.4 with 100% duty cycle Wi-Fi blocks most of the 802.15.4 messages and must be avoided. It is better to use different frequency channels.
For Wi-Fi networks, the Access Point (AP) establishes the initial channel and, in auto channel configuration, is free to move the network to another channel using the Channel Switch Announcement (CSA), introduced in 802.11h, to schedule the channel change.
For Thread networks, frequency separation implementation depends on the application layer. For Zigbee networks, the Coordinator establishes the initial channel. However, when implemented by the product designer, a Network Manager function, which can be on the Coordinator or on a Router, can solicit energy scans from the mesh network nodes and initiate a network channel change as necessary to a quieter channel.
Bluetooth is using frequency-hopping spread-spectrum technology (FHSS). The frequency hop technique divides the whole frequency spectrum into several hop channels with a nominal bandwidth of 1 MHz for each channel.
During a connection, Bluetooth transceivers use all 79 channels and rapidly hop from one channel to another in a random manner across all the channels. Every channel is divided into time segments, slots with 11 durations of 625 µsec.
In this approach, the collocated radios avoid the common frequencies for their operation. For example, if the WLAN radio is operating on channel 1, the Bluetooth radio avoids channels 0-21 or the ZigBee radio avoids channels 11-14.
2. Operate Wi-Fi with 20MHz Bandwidth
Since Wi-Fi 802.11n uses OFDM (Orthogonal Frequency-Division Multiplexing) sub-carriers, third-order distortion products from these sub-carriers extend one bandwidth on each side of the Wi-Fi channel. 802.11n can operate in 20MHz or 40MHz modes. If operated in 40MHz mode, 40MHz of the 80MHz ISM band is consumed by the Wi-Fi channel. However, an additional 40MHz on each side can be affected by third-order distortion products. These third-order products can block the 802.15.4 receiver and is the primary reason adjacent channel performance is up to 20dB worse than “far-away” channel performance.
3. Increase Antenna Isolation
Minimizing the Wi-Fi energy can improve the 802.15.4 or BLE receive range. Antenna isolation between the Wi-Fi transmitter and 802.15.4 RF /BLE input is required to improve receive range. Increased antenna isolation can be achieved by:
（1）Increasing the distance between antennas. In open-space, far-field, power received is proportional to 1/R2, where R is the distance between antennas.
（2）Taking advantage of antenna directionality. A monopole antenna provides a null along the axis of the antenna, which can be directed toward the Wi-Fi antenna(s).
4. Use Zigbee/Thread/Bluetooth Retry Mechanisms
The IEEE 802.15.4 specification requires retries at the MAC layer. To further improve message delivery robustness, the hardware MAC layer implements network (NWK) retries, wrapping the MAC retries. The user application can also take advantage of APS retries, wrapping the NWK retries. Bluetooth (point-to-point) messages require responses. If a response is not received within programmable time, the application can resend the message up to a programmable limit.
Managed Coexistence takes advantage of communication between the co-located Wi-Fi and BLE/802.15.4 radios to coordinate each radio’s access to the 2.4GHz ISM band for transmission and receive. The IEEE 802.15.2 standard describes collaborative solutions including packet traffic arbitration(PTA) which is an effective managed coexistence solution. A separate PTA block authorizes all transmissions from the different interfaces using the same channel. The PTA block coordinates the sharing of the medium depending on traffic load and priority.
Figure 3: Four kinds of PTA
There are four kinds of PTA implementations as shown in Figure 3. They are 1-Wire PTA，2-Wire PTA，3-Wire PTA， and 4-Wire PTA. In 1-Wire PTA, the Wi-Fi/PTA device asserts a GRANT signal when Wi-Fi is not busy transmitting or receiving.
When GRANT is asserted, the Bluetooth/ZigBee radio is allowed to transmit or receive. In 2-Wire PTA, the REQUEST signal is added, allowing the Bluetooth/ZigBee radio to request the 2.4GHz ISM band.
The Wi-Fi/PTA device internally controls the prioritization between Bluetooth/ZigBee and Wi-Fi and, in a conflict, the PTA can choose to either GRANT Bluetooth/ZigBee or Wi-Fi. In 3-Wire PTA, the PRIORITY signal is added, allowing the Bluetooth/ZigBee radio to signify a high- or low-priority message is either being received or transmitted.
The Wi-Fi/PTA device compares this external priority request against the internal Wi-Fi priority, which may be high/low or high/mid/low, and can choose to either GRANT Bluetooth/ZigBee or Wi-Fi.
In 4-Wire PTA, the FREQ signal is added, allowing the Bluetooth radio to signify an “in-band” or “out-of-band” message is either being received or transmitted. Figure 1 shows the four kinds of PTA. 1-Wire PTA does not allow external radio to request the 2.4GHz ISM and is not recommended.
The 802.15.2 recommendation only addresses a single 802.11b radio connected to a single 802.15.1 radio. However, market trends are requiring multiple co-located 2.4GHz ISM radios (ZigBee, Thread, and Bluetooth low energy) to operate with a Wi-Fi/PTA device only designed for one external radio. Many chip manufacturers have addressed this requirement by enhancing the REQUEST signal with the “shared” REQUEST feature.
Figure 4: The popular 3-wire PTA
Figure 5: PTA & PTA disabled comparison
The Dusun IoT gateway uses IoT wireless chips which have enhanced PTA features. Figure 4 shows the popular 3-wire PTA realized in the wireless chips Dusun used. The number of ZigBee retries per message and ZigBee message failure rate under PTA conditions and PTA disabled conditions are shown in figure 5(adapted from the test results of the wireless chip used by Dusun).
It can be seen that PTA can improve the ZigBee communication performance. It is reported that the BLE communication performance can also be improved by PTA. Hence PTA is an effective coexistence strategy.
As the Internet of Things (IoT) expands and evolves, an increasing number of Wi-Fi-enabled gateways will add Bluetooth, ZigBee, Thread, and other wireless protocols to enable communication with connected devices in homes and buildings. Co-located, strong Wi-Fi can have a substantial impact on IEEE 802.15.4/BLE performance. IEEE 802.15.4/BLE performance with co-located Wi-Fi can be improved through unmanaged and managed coexistence techniques. Unmanaged coexistence recommendations include:
- Implement frequency separation
- Operate Wi-Fi with 20 MHz bandwidth
- Increase antenna isolation
- Use ZigBee/Thread/BLE retry mechanisms
Wi-Fi/IEEE 802.15.4/Bluetooth coexistence test results show substantial 802.15.4/BLE performance improvements when PTA is utilized. Unmanaged coexistence remains important; as best managed coexistence performance is under good unmanaged coexistence conditions.
Dusun’s smart gateways utilized wireless chips which have good PTA function and good frequency separation function. Antenna isolation is considered in the PCB gateways’ design. The application software uses the ZigBee/Thread/BLE retry mechanisms. The Dusun gateways thus have increased throughput and may integrate up to four 2.4 GHz radios.
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