Combining multiple radios in a single solution

There are technical challenges to implementing multiple radios in the IoT, explain Chris Barratt and Dr Nick Wood of Insight SiP.

The IoT, or Internet of Things, is something of a catch-all term, but it is all about providing connectivity between electronic devices and the internet. In a large proportion of cases that involves wireless connectivity, because frequently the connectivity is being added to existing devices. Adding a wireless connection is an order of magnitude simpler than adding a wired infrastructure.

Early IoT connectivity typically involved creating a simple link between a device and some other system. Increasing sophistication led to including multiple radios.

Sharing wireless spectrum

Any wireless solution has to transmit on some part of the wireless spectrum. The thing about a spectrum is that there is no more being produced! It must therefore be shared among different users and be subject to rules to ensure fair use. In addition, each part of the spectrum has different underlying physics, which leads to different performance characteristics. The result is that most radio protocols are, to some extent, a compromise or trade-off between different performance characteristics – typically range, throughput and power consumption are the key metrics. Within those broad categories there are finer details, such as the sensitivity of the protocol to environment, obstructions and interference from other devices, and the physical size and design necessary to make the radio function effectively.



Since it is very hard to achieve a design that is optimal in all of the three main performance metrics, more sophisticated IoT designs will often incorporate two or more radios. This, however, introduces additional technical complexities in the design as radio signals, by their nature being pervasive, are prone to interfere with one another.

Typical dual radio design

One of the most typical dual radio solutions is to combine Wi-Fi and Bluetooth. Both operate on the same 2.4GHz band, although Wi-Fi also operates on the 5GHz frequency. Wi-Fi offers higher bandwidth, whereas Bluetooth, especially in its Low Energy (BLE) variant, offers low power consumption and much easier ad hoc connectivity.

A BLE ‘radio-active’ may be used to detect a connection, perhaps have some initial exchanges or for some interactive connections (for example with a user with a smartphone). It will then switch on Wi-Fi when bulk data transfer is required. This approach would optimise both power consumption and throughput by intelligent management of the radios.

Since both radios operate in the same frequency bands, in general, it is not possible to operate both at the same time. If left to just simplistically operate, then the two radios will often cause collisions when the two radios try and transmit on the same channels.

Within the overall 2.4GHz band, Wi-Fi transmits on 22MHz wide bandwidth channels, whereas Bluetooth uses a frequency-hopping technique across all or some of 79 1.0MHz or fewer 2.0MHz channels. In practice, this means that around one-third of the time you can expect collisions and the connection would not work. Bluetooth’s frequency- hopping technique means it is reasonably resilient to interference – a key design aim – but the Wi-Fi connection would have a high chance of failure.

Time-division multiplexing

The most common solution to this is to operate a time-based contention mechanism. In such a scheme, each radio has a digital radio-active pin connected to the other; each will wait until the other has stopped before starting transmission.

A more sophisticated approach is to add a third line for ‘Bluetooth priority’, which means the Bluetooth can request the Wi-Fi stops mid-transmission so it can transmit. This might be useful because Bluetooth is more likely to have a time-sensitive connection, as the protocol is based on timing. It might also be useful for latency sensitivity data, such as streamed audio.

In such a scenario, the two radios can use the same antenna, as only one will be transmitting at a time. This of course will require an RF switch to choose which radio is transmitting at a given time with consequent loss in sensitivity and signal strength.

Simultaneous Bluetooth/Wi-Fi transmission

It is possible to arrange for Bluetooth and Wi-Fi to transmit simultaneously by implementing adaptive frequency hopping within Bluetooth. Under this scheme, the Bluetooth device scans the full 2.4GHz band to determine which channels are occupied, and then notifies the other devices to avoid these channels in the frequency- hopping scheme. This scheme may suffer from the higher power Wi-Fi signals saturating the Bluetooth receiver and causing a significant loss in sensitivity, and hence range, for the Bluetooth connection.

Such a system can allow for Wi-Fi Bluetooth co-existence, although which channels are occupied may change in time, so it will not be entirely robust. In such a scheme, the Wi-Fi and Bluetooth antennas will need to be different to avoid direct interference between the two radios, and ideally, also include a certain degree of physical separation.

Two radios, two bands

What about when there are two radios operating in different bands? Such demands are relatively common to benefit from the different characteristics of radios operating in different areas of the spectrum. This would be the case for a Bluetooth short-range device working with a lower data rate sub-G long-range device such as a LoRa radio.

In some ways this is simpler, as there is less concern about the radios directly interfering with each other over the air. Where there are two radios close to each other on the same application board, it is important to ensure that subtle resonances in the board do not degrade the radio performance; where there are two largely independent systems, then the task is relatively simple.

Single antenna for two radios

To design a very small device, it might be preferable to use a single antenna for the two radios. This poses two main technical challenges. The first is designing a dual-band antenna and the second is how to isolate the two radios suitably while providing the antenna connection.

Realistically, one is unlikely to be able to totally optimise a dual-band antenna for both frequencies, particularly if space is at a premium. Typically, one would have to make a choice of which one to prioritise. In the above LoRa/BLE example, one would expect that it would often make sense to prioritise the long range of the LoRa radio, whereas the BLE radio would be more likely to be used to configure the device by someone in close proximity, so maximum range would not be required.

If the two radios are both attached to the same antenna, then there is a risk that energy transmitted by one gets sunk in the receiver of the other. There are two basic options. First, to put an RF switch to select only one radio at a time. This deals with the problem effectively but creates a new one – how to manage the control between the two radios. This is not obvious, especially if they may be receiving data asynchronously.

A more appropriate approach is to use a frequency diplexer that uses a low pass-high pass filter combination between the antenna and the two radios. This prevents the low frequency radio transmitting into the high frequency radio and vice versa, such that both radios transmit only their signals to the antenna. This allows the radios to operate simultaneously without interfering with one another.

In either case, one has to be aware that putting anything between the analogue radio output of the radio chip and the antenna will cause some level of loss, and badly designed circuits or cheap components could make these losses significant.

It is clear that RF circuits are complex and combining more than one can more than double the challenges. Building a complex radio from the ground up is a specialist RF task while less experienced designers can make the most use of pre-integrated radios to avoid an endless trial-and-error design cycle or sub-optimal designs.

About The Author

Chris Barratt is chief technical officer and Dr Nick Wood is director of sales and marketing at Insight SiP


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