A connected device that once carried a single radio now carries four or more. Cellular, GNSS, Wi-Fi, and Bluetooth Low Energy (BLE) share one board and one enclosure, and that enclosure keeps shrinking. These radios do not take turns by default. A transmitter sitting centimetres from a receiver can raise the noise floor enough to leave that receiver deaf to the signal it is trying to hear.

Fitting several radios onto one board is the easy part; making them transmit at the same time without degrading each other is an engineering problem that has to be solved during layout, not after.

What Is Radio Coexistence?

Radio coexistence is the discipline of getting several radios to operate simultaneously inside one device without degrading each other's performance. It sits at the centre of modern RF engineering, and it differs from the interference most people picture. Two separate products interfering across a room is an environmental problem, solved by emission limits and distance. Coexistence is the self-interference problem inside a single enclosure, where the aggressor and the victim share the same supply rail, the same ground, and the same few square centimetres of board.

Proximity is what makes it severe. A signal from a satellite or a distant base station arrives faint, often below a microwatt, while a transmitter on the same board hits a neighbouring receiver with raw radio-wave energy billions of times stronger. The two radios may occupy different frequency bands, but that near energy still spills into the victim's front end. Wireless coexistence, then, is less about keeping signals apart in theory than about managing real energy in a confined space, one of the harder challenges surrounding RF design in dense, multi-radio products.

Why Radios on the Same Board Deafen Each Other

The dominant failure mode has a name: receiver desensitisation, or RF desense. A nearby transmitter raises the noise floor across a receiver's band until the wanted signal can no longer be resolved above it, and the receiver goes deaf. When radios on one board fail to coexist, RF desense is usually the reason, and it reaches the field as dropped connections, lost GNSS fixes, and throughput that collapses whenever another radio transmits.

Three mechanisms account for most of it:

• Out-of-band emissions: no transmitter confines all its energy to its assigned channel. Skirts of emission spill into adjacent bands, and a neighbouring receiver tuned there absorbs them as noise.
• Harmonics: a transmitter also radiates at integer multiples of its fundamental frequency, and those harmonics can land squarely on another radio. The uplink of LTE Band 13 (777 to 787 MHz) has a second harmonic at 1554 to 1574 MHz, right against the GPS L1 frequency of 1575.42 MHz, so a cellular transmission can blind a GNSS receiver a few millimetres away.
• Broadband blocking: a strong out-of-band signal overwhelms a receiver's low-noise amplifier, compressing it so its gain collapses across the whole band, not only at the interfering frequency.

How RF Engineers Design Multiple Radios to Coexist

Making radios coexist comes down to decisions across three domains: frequency, space, and time. Strong multi-radio coexistence is the product of working all three deliberately rather than trusting any one of them to carry the design.

• Frequency planning and filtering: choose band combinations that minimise harmonic and adjacent-channel overlap, then add front-end filters so each radio sees only its own spectrum. Surface acoustic wave (SAW) filters and notch filters attenuate the out-of-band energy a given layout has to reject.
• Antenna isolation and placement: separate antennas physically, orient them for opposing polarisation, and use ground planes between them so radiated energy from one does not couple into another. Antenna isolation is often the largest single lever on desense, and it costs nothing in the bill of materials when planned early.
• RF shielding and grounding: shielding cans over sensitive stages and disciplined grounding stop radiated and conducted energy from crossing the board between RF domains.
• Time-domain coordination: where radios share a band, such as Wi-Fi and Bluetooth in the 2.4 GHz ISM (industrial, scientific, and medical) band, on-chip and firmware arbitration schedules them to transmit in turns instead of on top of each other.

Every one of these choices spends something: board area, insertion loss, component cost, or throughput surrendered to a time-sharing scheme. The engineering skill lies in knowing how each part behaves and interacts, then spending that budget where it buys the most isolation for the least compromise.

What Affects Radio Coexistence on the Hardware Layer

Firmware arbitration and adaptive power control help, but they operate within a ceiling the board itself sets. In RF board design, the physical layout decides how much interference exists before a single line of wireless coexistence firmware runs, and no amount of scheduling recovers margin the layout gave away. Four hardware factors set that ceiling: 

• Component placement and layout: keep aggressor and victim radios, and their feed traces, physically apart, and treat antenna position as a first-class layout constraint, not a late compromise around mechanical parts.
• Filter and front-end selection: specify filtering for the exact band combination the product uses. A generic front end tuned for no one in particular leaves measurable desense on the table.
• Power and ground integrity: design clean supply and ground so noise from one radio's power draw does not couple into another through shared rails, a path that bypasses every antenna precaution above it.
• Validation under real conditions: exercise each receiver while every other radio transmits at full power, so desense surfaces on the bench instead of in a returned unit.

The same layout discipline separates reliable RF-enabled IoT devices from ones that pass a single-radio bench test and then fail in the field.

Designing and Manufacturing the Hardware Behind Coexistence-Ready Devices

Coexistence is engineered, and then it has to be built without eroding the margin the design created. This is the work PCI does as an electronics manufacturing services (EMS) partner across smart home, industrial, and transportation programmes, taking a multi-radio product from schematic to certified hardware:

• RF hardware design: coexistence planned into the layout, the antenna plan, and the front-end filtering from the first board revision, not retrofitted after desense appears.
• Multi-radio antenna integration: multiband, multi-antenna designs placed and isolated to hold performance when every radio is active.
• Shielding and grounding discipline: shielding cans, ground-plane strategy, and controlled impedance carried through into the manufactured board.
• Certification and validation: PTCRB certification for putting a cellular device onto live mobile networks, alongside the coexistence testing and quality control that proves each radio survives the others.

The discipline that starts at layout is only worth as much as the manufacturing that preserves it.

Conclusion

A multi-radio device is only as reliable as the wireless coexistence engineered into its board, and that engineering has to happen at the design stage, when isolation is still free to design in, and desense is still cheap to remove. Left to firmware and field updates, it becomes a permanent tax on performance. To design and build coexistence-ready hardware that holds up in the field, talk to PCI about its Radio Wave Frequency Engineering services.

Frequently Asked Questions

What Is RF Desense?

RF desense, short for receiver desensitisation, is the loss of a receiver's sensitivity when a nearby transmitter raises the noise floor across its band. The wanted signal, already faint, can no longer be distinguished from the noise, so range, data rate, and reliability fall together. On a multi-radio board, it is the main reason two radios that work alone fail when run together, and it is corrected mainly through filtering, antenna isolation, and shielding rather than software.

How Do You Stop Wi-Fi and Bluetooth Interfering on the Same Device?

Wi-Fi and Bluetooth share the 2.4 GHz band, so they are separated in time rather than in frequency. A coexistence arbiter, on-chip or in firmware, schedules the two radios to transmit in turns, while board-level antenna isolation and filtering keep each one from desensitising the other when both are active.