Anti-interference performance is always a top concern among RC enthusiasts, and it also poses a major challenge for manufacturers. Traditionally, RC transmitters operate on frequency bands including 27MHz, 35MHz, 40MHz and 72MHz, each with a narrow bandwidth of only 1MHz. Moreover, frequency regulations vary across countries.
The 2.4GHz band is a license-free global public band with a total bandwidth of 83.5MHz and abundant supporting chips. For these reasons, nearly all modern RC transmitters adopt the 2.4GHz frequency band.
Since the 2.4GHz band is open to all, it is crowded with Wi-Fi, Bluetooth, cordless phones, Zigbee and numerous other wireless devices. A quick Wi-Fi scan on a mobile phone outdoors will reveal dense signals across the entire band. In such a congested and harsh wireless environment, reliable control and signal stability are critical, making anti-interference capability the core indicator for evaluating RC transmitters. Currently, spread spectrum communication is the mainstream technology for anti-interference solutions.
Before introducing spread spectrum technology, let us first review standard wireless communication principles. Wireless transmission works by loading information (such as audio and video) onto a high-frequency carrier wave for over-the-air delivery. Direct transmission of raw low-frequency signals is impractical: audio signals are below 20kHz, and lower frequencies correspond to longer wavelengths. For a 20kHz signal, the wavelength reaches 15,000 meters, which would require an impractically large transmitting antenna.
The process of loading baseband signals onto a high-frequency carrier is called modulation. Common analog modulation types are as follows:
Digital systems convert continuous analog signals into binary codes (0s and 1s) for digital processing. Early 72MHz RC transmitters adopted PCM digital modulation. Corresponding digital modulation formats for AM, FM and PM are ASK, FSK and PSK respectively.
An unmodulated carrier appears as a single frequency point on the frequency spectrum. After modulation, the signal occupies a continuous frequency range, known as bandwidth, which is determined by the baseband signal and modulation scheme.
(Modulation block diagrams omitted)
Literally, spread spectrum technology expands the signal over a much wider frequency range. The two most widely used types are Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS). DSSS suppresses co-channel interference, while FHSS evades interference. Both are digital modulation technologies.
DSSS adds an extra spread spectrum modulation stage to conventional communication. The baseband signal is multiplied with a high-rate pseudo-random code to raise its bit rate, before modulating the carrier. Phase Shift Keying (PSK) is the optimal modulation for spectrum spreading.
For example:
Take TI CC2533 as an example: it modulates the phase of the 2.4GHz carrier. The original baseband rate is 250 kbps. After spreading via pseudo-random codes, the rate increases to 2 Mbps, corresponding to an actual signal bandwidth of approximately 4MHz. After spectrum spreading at the transmitter, signal energy is distributed across a broad frequency range.
Its anti-interference advantages are reflected in three aspects:
Strong narrowband interference concentrated on a single frequency is spread out, which drastically reduces its power spectral density and turns severe interference into weak noise.
The transmitter and receiver use identical pseudo-random codes. Interfering signals from other devices are further spread and weakened during reception. Even if part of the local signal spectrum is heavily disturbed, the receiver can still recover valid data via signal correlation, enabling reliable reception in noisy environments.
Received signals include both direct transmission and reflected signals from the ground, walls and other obstacles, which normally cause multipath interference. DSSS is immune to this issue and can even utilize reflected signals to enhance reception.
Additionally, DSSS features low power spectral density, so it causes minimal interference to other wireless devices. Multiple devices can operate on the same frequency simultaneously, allowing more users to fly or operate RC models in the same area.
(DSSS baseband spreading & despreading diagram, DSSS spectrum diagram omitted)
FHSS expands the spectrum by continuously switching carrier frequencies across the entire band. A qualified FHSS system requires no fewer than 15 hopping channels, and the hopping sequence must be pseudo-random (similar to noise). Fixed repetitive frequency switching cannot be defined as FHSS.
DSSS relies on dedicated RF chips, while FHSS is mainly implemented via communication algorithms. Its anti-interference capabilities are listed below:
Interference usually only occupies part of the frequency band. With frequencies hopping across the full spectrum, the system can always find clear channels to avoid data loss.
Reflected signals arrive at the receiver with a time delay. By then the channel has already switched, so overlapping multipath signals are effectively avoided.
Diversified hopping sequences reduce channel collisions between different transmitters. Since RC remote control is a one-way communication system with no feedback from receivers, adaptive channel switching is not available. Well-designed low-collision hopping sequences effectively cut down signal conflicts.
When the hopping sequence features true randomness, FHSS delivers longer transmission range compared with fixed-frequency or regular hopping systems.
The hopping sequence is the core of FHSS design, and it must meet the following criteria:
FHSS sequence design is based on discrete mathematics, related to channel quantity, sequence cycle, sequence quantity, mutual correlation, link recovery time and maximum concurrent users. Advanced mathematical algorithms are essential for optimal design.
(FHSS hopping sequence diagram omitted)
DSSS excels at suppressing interference but may fail to demodulate signals under extreme narrowband high-power interference. FHSS avoids interference rather than suppressing it. Combining DSSS and FHSS creates a hybrid spread spectrum system with comprehensive anti-interference performance.
Technology keeps evolving. 4G mobile communication adopts OFDM (Orthogonal Frequency Division Multiplexing), which supports larger data throughput within the same bandwidth. As OFDM chip costs decrease, this technology will eventually be applied to next-generation RC transmitters.