In the early days of radio communication, sophisticated data protocols were impossible due to the lack of semiconductors and microcontrollers. As a result, simple OOK and AM modulations dominated.
This changed with the advent of transistors, specialized chips, and microcontrollers, making signal processing more affordable. Today, almost all professional radio transmissions use FM.
Aviation radio is one of the last domains where amplitude modulation (AM) is still in use, primarily for historical reasons. Even public broadcasting has long since switched from AM to FM, moving away from shortwave frequencies (SW/MW) to modern VHF frequencies.
Whenever you need to reliably transmit large amounts of data at high speeds, frequency modulation (FM) is the preferred choice.
Modern FM breakout boards are only slightly more expensive than simple OOK transmitters and receivers and are available for various frequency bands.
Overview
Simple OOK is the most basic way to transmit radio signals: the transmitter is turned on and off at intervals to send signals—that’s it. With encodings like EV1527, this is sufficient for transmitting small chunks of data, such as the 24-bit IDs used in remote controls.
However, when higher data rates, longer transmission distances, and lower power consumption are required, more advanced modulations like FM (Frequency Modulation) become necessary.
Specialized radio chips such as CC1101, SI4463, nRF24L01, and SX1231 handle much of the complexity introduced by FM modulation.
Transceivers
Most FM chips on breakout boards are transceivers, meaning they can both transmit and receive. Unlike OOK modules, which require separate sender and receiver boards, FM transceivers allow two-way communication.
Two-way communication is crucial for robust data transfer:
- A sender can transmit data and then switch to receiver mode to listen for an acknowledgment from the receiver, ensuring the data was received correctly.
- The receiver can switch to sender mode to report back whether the data was received correctly or if a retransmission is needed.
Most modern communication protocols rely on this continuous switching between sending and receiving to ensure data integrity.
Microcontroller Required
Most FM breakout boards require a microcontroller to operate. While these modules handle signal processing, they require an external microcontroller to program them, setting the desired modulation, speed, RF power, and data encoding. Standard microcontroller interfaces are used, such as:
Occasionally, FM radio chips are integrated into specialized boards designed for very specific use cases, in which case no external microcontroller or programming is required.
A common example is wireless serial modules, which automatically transmit serial data wirelessly:
- Connect your serial RX and TX pins to the corresponding pins on the module.
- Connect RX and TX of another (paired) module elsewhere.
Once paired, the modules transmit serial data wirelessly at a predefined baud rate—eliminating the need for expensive cables or custom programming.
Frequency Modulation (FM)
FM encodes information by subtly shifting the frequency. This process, known as frequency modulation, allows for more reliable signal transmission compared to amplitude-based methods:
- Improved noise immunity
- Btter signal clarity
- Resistance to amplitude-based interference.
Multipath Fading
To better understand why FM transmits information more reliably and with less power, let’s examine a phenomenon called multipath fading, which weakens OOK and AM signals.
Multipath fading occurs when a transmitted signal takes multiple paths to reach the receiver due to reflections, diffractions, and scattering caused by obstacles like walls, buildings, mountains, and even vehicles. Nearly all radio communication is affected by multipath in some way.
Radio wave behavior is similar to the ripples you see when you throw a stone into a lake. When reflected waves intersect, their amplitudes either strengthen or cancel out, depending on their phase shift (timing).
Any modulation that relies on amplitude variations—such as OOK and AM—is therefore highly susceptible to multipath fading.
In contrast, FM encodes information using frequency shifts rather than amplitude changes, making it inherently more resistant to signal degradation.
Additionally, in FM, the strongest received signal dominates, reducing the impact of weaker multipath components. As a result, FM signals are far less affected by natural radio noise and interference from other sources.
Speed and Power Efficiency
But FM has even more advantages: it requires less bandwidth to transmit the same amount of information, allowing for faster data transfer.
Faster (and therefore shorter) transmissions consume less power, which is a significant benefit for battery- or solar-powered projects.
Digital FM Modes
Digital FM Modes
FM was originally designed for analog signals. While it’s possible to transmit digital data using analog FM, the process is highly inefficient and slow—as anyone who used an acoustic coupler in the early days of the Internet can attest.
To address this, FM radio boards use specialized FM modulations optimized for digital data transmission, similar to ASK (Amplitude Shift Keying) on AM.
While digital FM modulations can no longer transfer analog signals directly, you can still build a walkie-talkie: use an ADC (analog-to-digital converter) to digitize the microphone’s amplitudes. On the receiver side, run the bits through a DAC (digital-to-analog converter) to drive a speaker. Alternatively, you can use a digital microphone and digital amplifiers directly (e.g., using I2S).
FSK (Frequency Shift Keying)
In FSK, binary data (0 and 1) is represented by two distinct frequency shifts.
Different variations of FSK exist to meet additional requirements:
-
GFSK (Gaussian Frequency Shift Keying)
Bluetooth operates with high RF emissions. To meet regulatory standards and minimize spurious emissions, it uses GFSK.GFSK applies a Gaussian low-pass filter to smooth out frequency transitions, reducing high-frequency components that cause spectral spreading and interference.
However, this smoothing makes demodulation more complex. The receiver must precisely detect frequency deviations, as noise, Doppler shifts, or oscillator drift can introduce bit errors—especially at low signal-to-noise ratios (SNR).
GFSK is widely used in Bluetooth Classic, DECT (cordless phones), and many radio breakout boards.
-
MSK (Minimum Shift Keying)
The Gaussian filter in GFSK spreads frequency shifts over time, potentially causing overlapping transitions between bits. At higher data rates, this leads to inter-symbol interference (ISI).MSK reduces ISI while maintaining spectral efficiency, improving data integrity.
-
GMSK (Gaussian Minimum Shift Keying)
A filtered version of MSK that further enhances spectral efficiency. It is commonly used in GSM mobile networks.
Each modulation has distinct advantages and disadvantages and are not just evolutionary steps: the more sophisticated GFSK is not automatically superior to basic FSK:
| Modulation | Key Advantages |
|---|---|
| FSK | Simpler implementation, lower energy consumption |
| GFSK | Reduced spurious emissions at high RF power, more bandwidth-efficient |
| MSK | High speed, narrow bandwidth, constant envelope (useful in non-linear amplifiers) |
| GMSK | Reduced spurious emissions, bandwidth-efficient, better suited for regulatory environments |
FM boards often support some or all of these modulations. Depending on your project and use case, pick the modulation that suits you best.
Beyond FM
The evolution of digital radio transmission does not stop with FM. As industry and consumers demand higher speeds, available frequency bands become increasingly scarce and overcrowded.
New transmission techniques aim to address three key objectives:
- Speed & Bandwidth Efficiency
Reduce bandwidth requirements to enable either higher data transfer rates or more simultaneous communication channels. - Minimizing RF Power
Reduce the energy required to transmit a signal over a given distance, lowering power consumption and interference. - Security & Resilience
Make it harder for adversaries to detect, eavesdrop, or jam radio communication.
Modern approaches tackle these objectives using phase shift modulation and spread spectrum techniques, both of which became feasible only recently due to affordable signal processing and fast-switching transceivers. These methods are now widely used in Wi-Fi, Bluetooth, smartphones, and DIY radio transceiver boards.
Phase Shift
While AM encodes information in amplitude, and FM in frequency shifts, phase modulation (PM) encodes information in timing shifts or in changes relative to previously transmitted signals (differential approaches).
This method is highly complex and was not technologically feasible until recently. Today, however, powerful and inexpensive chips handle this modulation in devices such as modern smartphones and Bluetooth transceivers.
PSK (Phase Shift Keying)
PSK uses phase shifts instead of frequency shifts, making it more bandwidth-efficient than FSK, though it requires more complex receivers.
While Bluetooth Classic originally used GFSK, it later adopted π/4-DQPSK and 8-DPSK in its Enhanced Data Rate (EDR) modes, tripling speed from 1 Mbps to up to 3 Mbps.
-
π/4-DQPSK (π/4-Differential Quadrature Phase Shift Keying)
A type of QPSK (Quadrature Phase Shift Keying) where the phase shifts by π/4 (45°) or -3π/4 (-135°) for each symbol. This reduces large phase jumps, improving noise resistance.Since DQPSK encodes information in phase changes rather than absolute values, it is less sensitive to phase noise.
-
8-DPSK (8-level Differential Phase Shift Keying)
Uses 8 different phase shifts instead of 4 (QPSK), allowing each symbol to carry 3 bits. This increases data rates (up to 3 Mbps in Bluetooth EDR), but also makes it more susceptible to noise.
| Modulation | Bits per Symbol | Data Rate (Bluetooth) | Noise Sensitivity | Complexity |
|---|---|---|---|---|
| GFSK | 1 | 1 Mbps | Low (resistant to amplitude noise) | Low (simple demodulation) |
| π/4-DQPSK | 2 | 2 Mbps | Moderate (phase noise is a factor) | Medium |
| 8-DPSK | 3 | 3 Mbps | High (phase noise affects decoding) | High |
Spread Spectrum
Spread Spectrum is a different approach that distributes a signal over a wider frequency range to improve resistance against interference, jamming, and multipath fading.
Since spread spectrum requires rapid frequency changes within microseconds, it relies on modern transceivers capable of fast frequency switching.
- DSSS (Direct Sequence Spread Spectrum)
Spreads data over a wide bandwidth using a pseudo-random noise (PN) sequence. This makes it highly resistant to narrowband interference (e.g., a single-tone jammer).- Used in older Wi-Fi (802.11b), Zigbee, and GPS.
- FHSS (Frequency Hopping Spread Spectrum)
Rapidly switches between frequencies in a pseudo-random pattern, reducing susceptibility to interference and eavesdropping.- Lower complexity than DSSS, but also lower data rates.
- Used in Bluetooth Classic and older cordless phones.
- OFDM (Orthogonal Frequency Division Multiplexing)
Divides the signal into multiple subcarriers, each modulated with QPSK, 16-QAM, or 64-QAM.- Highly resistant to multipath fading and interference, making it ideal for urban environments.
- Supports extremely high data rates but requires precise synchronization.
- Used in Wi-Fi (802.11a/g/n/ac), LTE, and Bluetooth LE Audio.
- CSS (Chirp Spread Spectrum)
Optimized for long range and low power, sacrificing speed.- Uses “chirps” that gradually sweep up or down in frequency, increasing resistance to noise and jamming.
- Allows signals to be received even below the noise floor, making it ideal for noisy environments.
- Enables communication over tens of kilometers at ultra-low power, allowing devices to run for years on a coin cell battery.
- Used in LoRa, IoT sensors, metering, and tracking.
| Feature | GFSK | π/4-DQPSK | 8-DPSK | DSSS | FHSS | OFDM | LoRa |
|---|---|---|---|---|---|---|---|
| Modulation Type | Frequency | Phase | Phase | Spread Spectrum | Spread Spectrum | Multi-carrier | Chirp Spread Spectrum |
| Bandwidth Efficiency | Low | Medium | High | Low | Low | Very High | Very Low |
| Noise Immunity | High | Moderate | Low | Very High | High | Very High | Extremely High |
| Interference Resistance | Medium | Medium | Low | High | Very High | Very High | Extremely High |
| Range | Short | Short | Short | Medium | Medium | Medium | Very Long (10+ km) |
| Power Consumption | Low | Medium | High | Medium | Medium | High | Very Low |
| Data Rate | ~1 Mbps | ~2 Mbps | ~3 Mbps | ~11 Mbps | ~1 Mbps | ~600 Mbps | <50 kbps |
| Use Cases | Bluetooth 1.x | Bluetooth EDR | Bluetooth EDR | Wi-Fi 802.11b, Zigbee | Bluetooth Classic | Wi-Fi 802.11a/n/ac, LTE | LoRaWAN, IoT, Smart Cities |
Programming
As mentioned, most FM boards require an external microcontroller to operate.
-
UART/Serial:
Some modules use built-in microcontrollers and communicate via UART/serial and simple commands. Such commands can be easily sent via Serial and do not require specialized external libraries (e.g., the HC-12 board using SI4463 or the less performant SI4438).Here is an extensive tutorial on using the HC-12 board (in German, use Google Translate).
-
SPI:
More cost-effective radio boards do not have built-in microcontrollers. Instead, they implement interfaces such as SPI and provide direct access to the radio chip (e.g., most boards using the CC1101). This requires a specialized library for such modules.
| Radio Chip | Arduino Library | platformio.ini |
|---|---|---|
| CC1101 | SmartRC-CC1101-Driver-Lib | lsatan/SmartRC-CC1101-Driver-Lib@^2.5.7 |
| SI4438/SI4463 | UART | UART |
| nRF24L01 | NRFLite | dparson55/NRFLite@^3.0.5 |
Universal Libraries
In an effort to abstract commands from particular radio hardware, a few projects/libraries have evolved that target a wide range of radio boards, including those with UART interfaces:
-
RadioLib:
An actively maintained library that integrates many different wireless communication modules, protocols, and digital modes into a single consistent system. It supports modules based on CC1101, LLCC68, LR11x0, RF69, RFM2x, RFM9x, Si443x, STM32WL, SX126x, SX127x, Sx128x, and Sx123x. -
RadioHead:
An older and still popular library, though it does not seem to be actively maintained anymore. The last significant updates appear to date back to 2018. Here is a version on GitHub polished by Adafruit.ESPHome
ESPHome does not natively support FM radio.
ESPHome has built-in components like remote_receiver and remote_transmitter, but these are designed for simple OOK/EV1527 communications, typically used in the context of remote controls.
Conclusion
FSK (based on FM) offers a reliable and efficient way of transmitting data at high speeds. Affordable breakout boards are available but require a microcontroller for operation. There is good community support for the most commonly used radio chips.
To transfer digital data, FSK (Frequency Shift Keying) or one of its derivatives is commonly used:
| Modulation | Use Case |
|---|---|
| FSK | Fast and reliable data transfer |
| GFSK | Reduces unwanted spurious emissions with higher RF power |
| MSK | Supports even higher data transfer speeds at the expense of increased complexity. Typically not supported by simple breakout boards |
| GMSK | Reduces spurious emissions with higher RF power. Also typically not supported by simple breakout boards |
Slow Website?
This website is very fast, and pages should appear instantly. If this site is slow for you, then your routing may be messed up, and this issue does not only affect done.land, but potentially a few other websites and downloads as well. Here are simple steps to speed up your Internet experience and fix issues with slow websites and downloads..
Comments
Please do leave comments below. I am using utteran.ce, an open-source and ad-free light-weight commenting system.
Here is how your comments are stored
Whenever you leave a comment, a new github issue is created on your behalf.
-
All comments become trackable issues in the Github Issues section, and I (and you) can follow up on them.
-
There is no third-party provider, no disrupting ads, and everything remains transparent inside github.
Github Users Yes, Spammers No
To keep spammers out and comments attributable, all you do is log in using your (free) github account and grant utteranc.es the permission to submit issues on your behalf.
If you don’t have a github account yet, go get yourself one - it’s free and simple.
If for any reason you do not feel comfortable with letting the commenting system submit issues for you, then visit Github Issues directly, i.e. by clicking the red button Submit Issue at the bottom of each page, and submit your issue manually. You control everything.
Discussions
For chit-chat and quick questions, feel free to visit and participate in Discussions. They work much like classic forums or bulletin boards. Just keep in mind: your valued input isn’t equally well trackable there.
(content created Mar 21, 2025 - last updated Mar 22, 2025)