Combining multiple radios in a single solution

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

Early IOT solutions typically involved creating a simple link between a device and some other system. However, as solutions become more sophisticated, they increasingly include multiple radios. This article looks at why this can be required, and what the technical challenges can be in implementing such solutions.

Read the article on Electronics Weekly

Sharing wireless spectrum

Any wireless solution has to transmit on some part of the wireless spectrum – and the thing about spectrum is that they aren’t making any more of it! This means any spectrum must be shared amongst different users and 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 – with typically range, throughput and power consumption being 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 of the solution necessary to make the radio function effectively.

Since it is very hard to achieve a solution that is optimal in all of the three main performance metrics, more sophisticated IoT solutions will now 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 solution

One of the most typical dual radio solutions is to combine WiFi and Bluetooth. Both these radios operate on the same 2.4 GHz band although WiFi also operates on the 5GHz frequency. WiFi offers higher bandwidth, whereas Bluetooth, especially in its Low Energy variant, offers extremely low power consumption and much easier ad-hoc connectivity. So, a solution might have a BLE “Radio- Active” to detect a connection, perhaps have some initial exchanges or for some interactive connections (for example with a user with a smartphone), and then switch on WiFi when bulk data transfer isrequired. This approach would optimize both power consumption and throughput by intelligent management of the radios.

Technical issues

What are the technical issues in implementing such a solution? 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 2.4 GHz band, WiFi transmits on 22 MHz wide bandwidth channels, whereas Bluetooth uses a frequency hopping technique across all or some of 79 1 MHz or fewer 2MHz channels. In practice this means that around 1/3 of the time you can expect collisions and the solution would not work. Bluetooth’s frequency hoping technique means it is reasonably resilient to interference, a key design aim, but the WiFi connection would have a high chance to fail.

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. So each will simply 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 WiFi to stop 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, or for latency sensitivity data like 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 & WiFi transmission

It is possible to arrange for Bluetooth and WiFi to transmit simultaneously by implementing “Adaptative 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 solution may suffer from the higher power WiFi 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 WiFi Bluetooth co-existence, although which channels are occupied may change in time, so it will not be an entirely robust solution. In such a scheme, the WiFi and Bluetooth antennas will need to be different to avoid direct interference between the two radios, and ideally with a certain degree of physical separation also.

Two radios, two bands

What about the case where you have two radios operating in different bands? Such demands are relatively common to benefit from the different characteristics of radios operating in different areas of spectrum. This would be the case for a Bluetooth short range device working with a lower data rate sub-Giga long range device like 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. If you have the two radios close to each on the same application board, you still need to make sure that subtle resonances in the board don’t degrade the radio performance. But if you have two largely independent systems, then the task is relatively simple.

Single antenna for two radios

If you wish to design a very small device, you might want to use a single antenna for the two radios. This poses two main technical challenges – firstly designing a dual band antenna, and secondly, how to isolate the two radios suitably whilst providing the antenna connection.

Designing a dual band antenna is not simple and there isn’t space to discuss the issues here. Realistically, one is unlikely to be able to totally optimise the antenna for both frequencies, particularly if space is at a premium. So typically one would have to make a choice of which one to prioritise. In the example of the LoRa/BLE solution above, one would expect that it would often make sense to prioritise the long range 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 wouldn’t be required.

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

A more appropriate solution 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 only transmit 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.