Coexistence in multi-radio RF systems

Coexistence in IoT and smart wearables

The challenge of radio-frequency coexistence in IoT, home hub and smart wearable devices is evolving rapidly. Modern connected products increasingly integrate multiple radios within a single platform, combining broadband connectivity, low-power mesh protocols and highly sensitive long-range technologies. As the number of radios increases, so too does the potential for one wireless system to interfere with another. 

This is particularly challenging when radios with very different transmit powers, receiver sensitivities and bandwidths are integrated within the same compact device. Interference can reduce receiver sensitivity and communication range, increase packet loss and latency, and ultimately affect the reliability of the end product.

This Application Note describes the physical interference mechanisms behind RF coexistence, some of the limitations of established mitigation techniques, and how Forefront RF’s adaptive signal cancellation technology can provide an alternative approach to managing interference in increasingly complex multi-radio systems.

The changing nature of coexistence

The proliferation of connected devices has been accompanied by an equally rapid increase in the number of wireless protocols that need to operate alongside one another. What was once a relatively contained coexistence challenge — such as Wi-Fi and Bluetooth sharing the 2.4 GHz ISM (Industrial, Scientific and Medical) band — is evolving into a more complex environment involving multiple radios with very different operating characteristics.

Connected products can now combine Wi-Fi and Bluetooth with technologies such as Zigbee, Thread, LoRa, Wi-SUN FAN, Wi-Fi HaLow, Z-Wave, cellular and proprietary protocols. Some share the same spectrum, while others operate in adjacent bands but remain sufficiently close for a high-power transmission to interfere with a sensitive receiver.

Coexistence is no longer a case of sharing spectrum efficiently, but increasingly, it is also ensuring that one radio does not impair the operation of another.

When coexistence leads to signal interference

Wireless protocols operating at the same or nearby frequencies run the risk of interfering with each other. The problem is particularly acute when a relatively high-power transmitter is located close to a receiver designed to detect very weak signals. 

Broadly, two coexistence scenarios occur. The first is between technologies sharing unlicensed ISM or SRD (short range device) spectrum, such as Wi-Fi, Bluetooth and Zigbee at 2.4 GHz, or LoRa, Wi-SUN, Z-Wave, Wi-Fi HaLow in the sub-GHz spectrum.

The second occurs between licensed and unlicensed systems operating within the same product or in nearby spectrum. An example is a cellular transmitter operating close to a sensitive receiver. Even without direct channel overlap, transmitter leakage can cause receiver blocking or desensitisation.

Understanding the problem

The traditional coexistence problem can be illustrated by Wi-Fi and Zigbee. Both operate within the 2.4 GHz ISM band. Typically, non-overlapping channels are selected to avoid interference. However, when the two radio are in close physical proximity, interference can occur even when the channels do not directly overlap. The result can be collisions, packet loss, increased latency and reduced throughput.

Modern multi-radio products introduce an additional design challenge. A device may contain a relatively high-power, wideband radio alongside lower-power mesh connectivity and a highly sensitive long-range receiver. These radios may be separated by only a few centimetres and share the same PCB, power system and compact enclosure.

ISM band protocols

The ISM spectrum supports a wide variety of wireless connectivity technologies. Two particularly important areas for connected products are the sub-1 GHz spectrum and the 2.4 GHz band.

Sub-GHz frequencies provide favourable propagation characteristics and longer range, while 2.4 GHz offers greater available bandwidth and supports a large global ecosystem of wireless standards. In both cases, the concentration of multiple technologies within relatively limited spectrum creates the potential for interference.

Sub 1 GHz frequencies

The sub-GHz spectrum is used extensively for long-range and low-power connectivity. Common technologies include:

• LoRa/LoRaWAN - A low-power wide-area technology designed to provide long-range communication at relatively low data rates. Global regional frequency allocations differ.
• Wi-SUN FAN - Wireless Smart Utility Network Field Area Network, is designed for large-scale mesh networks.
• Wi-Fi HaLow - Extends Wi-Fi into sub-GHz spectrum, providing greater range than conventional Wi-Fi. Based on IEEE 802.11ah.
• Z-Wave - Primarily associated with low-power smart-home networks
• Numerous proprietary protocols.

Frequency Shift Keying (FSK) is a modulation technique in which digital information is represented by changes in carrier frequency. It is widely used by sub-GHz systems making the overall sub-GHz environment considerably broader than the above named wireless standards alone.

2.4 GHz

The 2.4 GHz ISM band supports Wi-Fi, Bluetooth, Zigbee, Thread and numerous proprietary wireless protocols. Its global availability has made it one of the most widely used frequency ranges for short-range wireless connectivity.

As a result, several radios can be present in a single product while operating within the same spectrum. Smart-home hubs, wearables, smart glasses, security products and other connected devices can require concurrent operation of Wi-Fi and Bluetooth, with Zigbee or Thread connectivity added in some applications.

This density creates both co-channel and adjacent-channel interference challenges, particularly within compact products where the number of antennas or physical separation between antennas is limited.

Product applications for ISM band protocols

ISM-band connectivity is used across a broad range of consumer, industrial and infrastructure products. Sub-GHz technologies are particularly common in smart meters, asset trackers, alarm and security systems, locks, alarms, heating controls,  industrial and environmental sensors, agricultural monitoring, smart-grid, smart-city infrastructure, smart buildings and network gateways.

In the home, multi-radio connectivity is increasingly used in hubs, security cameras, smart lighting, sensors, doorbells and other connected products. Amazon Sidewalk is one example of this trend, using sub-GHz connectivity alongside other wireless technologies to extend connectivity beyond the normal range of a home Wi-Fi network.

Wearable devices and smart glasses create another demanding environment. Multiple antennas and radios must be integrated into a very small physical space while maintaining reliable simultaneous connectivity.

In many applications these ISM-band radios may also operate alongside LTE-M, NB-IoT, Cat-1 bis or other cellular connectivity, further increasing the number of RF systems that must coexist within a single platform.

The coexistence challenges

Coexistence problems vary depending on the protocols, frequency bands and product architecture involved. However, the underlying challenge is consistent: a strong interfering signal can impair a receiver attempting to detect a much weaker wanted signal.

The customer-visible effects can include shorter communication range, intermittent connections, reduced throughput, packet loss and increased latency. At the product-design level, achieving adequate isolation can require additional filtering, shielding, antenna separation and careful placement of RF components.

Sub-GHz frequency challenges

LoRa is particularly sensitive to the coexistence problem because long-range operation depends upon its ability to receive signals at very low power levels, yet it operates at similar frequencies to other short-range systems and cellular networks. A reduction in effective receiver sensitivity can directly translate into reduced range, packet loss or an unreliable connection. LoRa, Wi-SUN, Z-Wave and proprietary radios can operate within the same regional frequency allocations, increasing the potential for collisions and cross-technology interference.

Amazon Sidewalk provides a recognisable example in the North American 900 MHz spectrum. Sidewalk extends connectivity for devices such as smart-home sensors, trackers and compatible Echo and Ring products using sub-GHz communications. Interference that reduces receiver sensitivity can therefore compromise one of the fundamental benefits of the technology: reliable connectivity over distances beyond conventional home Wi-Fi.

Concurrent Wi-Fi and Bluetooth in small form factor smart devices

At 2.4 GHz, Wi-Fi and Bluetooth coexistence is already a well-established RF design challenge. Concurrent functioning becomes increasingly difficult as products become smaller. Smart glasses and other compact wearables may integrate Wi-Fi, Bluetooth and additional radios within a very limited physical area. Smart-home hubs face a related challenge as designers attempt to accommodate Wi-Fi, Bluetooth, Zigbee or Thread and other connectivity within a single enclosure.

A conventional response is to provide greater physical separation between antennas and RF paths. However, antenna separation consumes valuable space and can impose constraints on PCB layout, enclosure design and product dimensions. As more radios are added, achieving sufficient isolation through physical separation alone becomes increasingly difficult.

Technical considerations

Dynamic Range as a Fundamental Constraint

The difference between transmitter power and receiver sensitivity illustrates the scale of the coexistence challenge. A wireless transmitter can operate at tens of dBm while a long-range receiver may be designed to detect signals below −100 dBm and, in some cases, approaching −130 dBm.

The resulting dynamic range can exceed 100 dB within a single product. Even a small proportion of transmitter energy coupling into the receive path can therefore be sufficient to degrade receiver performance.

As products become smaller and the number of integrated radios increases, achieving the necessary isolation becomes progressively more difficult using physical separation alone.

Desensitisation and Receiver Blocking

In multi-radio systems, interference does not need to occupy exactly the same channel as the wanted signal. A sufficiently strong nearby transmission can overload parts of the receiver chain or raise its effective noise floor, reducing its ability to detect weak signals.

This receiver desensitisation can arise from out-of-band emissions, harmonics, antenna coupling, PCB coupling and other leakage paths within the product.

The practical result is a reduction in effective receiver sensitivity, which can manifest itself as shorter range, reduced throughput or intermittent connectivity.

Hopping and time scheduling

Many existing coexistence techniques seek to avoid interference rather than remove it. Frequency hopping moves transmissions between channels, while Listen Before Talk (LBT) and other time-domain techniques allow radios to share available spectrum by avoiding simultaneous transmission.

These approaches can be highly effective where radios are able to coordinate their activity. However, they become more difficult when a receiver needs to remain continuously available or when the source of interference cannot be coordinated. Long-range IoT radios, for example, may need to listen for transmissions that cannot be predicted or synchronised with other radio activity within the product.

Temporarily disabling a receiver while another radio transmits can therefore create gaps in connectivity, while restricting simultaneous operation can affect throughput and latency. This creates a growing requirement for coexistence techniques that can support concurrent radio operation rather than relying solely on frequency or time-domain avoidance.

Why Existing Solutions Fall Short

Traditional coexistence techniques are predominantly based on avoidance: avoiding frequency overlap through channel planning, avoiding simultaneous transmissions through time arbitration, or avoiding RF coupling through filtering, shielding and physical isolation.

Each remains an important part of RF system design. However, their effectiveness becomes constrained as products become smaller, radios become more numerous and simultaneous operation becomes more important.

Channel planning cannot eliminate all transmitter leakage or coupling. Scheduling can restrict simultaneous operation and may not be suitable for continuously listening receivers such as smart glasses. Physical isolation requires space, while increasingly selective filtering can add components, insertion loss and design complexity.

These techniques therefore manage coexistence principally by reducing exposure to interference rather than addressing the interfering signal itself.

Adaptive signal cancellation

Forefront RF's approach to this problem builds upon technology originally developed to address transmitter-to-receiver isolation in cellular RF front-ends. 

Central to Forefront RF's Foretune™ technology is adaptive signal cancellation. Rather than relying solely on passive filtering, physical separation or scheduling, cancellation uses knowledge of the interfering signal to suppress it at the receiver.

Conceptually, the signal arriving at a receiver contains both the wanted signal and an unwanted interfering component:

• Received signal = Desired signal + Interference

By measuring the interfering signal, generating a matched inverse replica, and subtracting it from the receive path, the system can effectively suppress interference:

• Result ≈ Desired signal preserved

This principle is already fundamental to Foretune™, where Adaptive Passive Cancellation complements the Electrical Balance n-plexer to maintain isolation between transmit and receive paths across changing frequency and impedance conditions. Applied to multi-radio coexistence, the same underlying approach provides a means of addressing interference directly at RF.

Strategic shift: From avoidance to cancellation

As multi-radio products become more complex, coexistence design can evolve from relying exclusively on interference avoidance towards actively managing interference when it occurs.

Active cancellation can support concurrent operation because the interfering radio does not necessarily need to stop transmitting while the other radio receives. It can also reduce reliance on physical antenna separation and does not require the two wireless protocols to coordinate their operation.

For applications combining high-power transmitters with sensitive receivers, implementing interference cancellation changes the design objective. Rather than ensuring that the interfering signal is never present, the RF system can be designed to suppress that signal sufficiently to preserve receiver performance

Most importantly, suppressing the interfering signal before it impairs the receiver can help preserve the sensitivity on which long-range and low-power connectivity depends.

Foretune™ provides Forefront RF with a technology platform through which these principles can be applied beyond their original cellular duplexing application to coexistence challenges in multi-radio connected products.

Conclusion

RF coexistence is becoming an increasingly important consideration in IoT, wearable and connected-device design. The integration of multiple wireless technologies within compact products places radios with very different transmit powers, bandwidths and receiver sensitivities in close physical proximity. 

The effects of interference are both technical and practical. Receiver desensitisation can reduce range and reliability, while conventional mitigation can require additional filtering, antenna separation, shielding or restrictions on simultaneous operation. These measures can, in turn, influence PCB layout, product size and overall RF architecture.

Established coexistence techniques based on frequency, time and physical separation remain important, but increasingly complex multi-radio systems create circumstances where avoidance alone may not be sufficient.

Adaptive signal cancellation provides another approach. By suppressing an interfering signal at RF rather than simply attempting to avoid it, Forefront RF's Foretune™ technology can preserve receiver sensitivity while supporting concurrent operation and greater flexibility in multi-radio product design