I am a relative beginner regarding wireless communications protocols and I am looking into the protocols used by toy helicopters, in particular the Hubsan X4. According to more experienced people who have done similar work the X4 uses "an A7105 transceiver chip".

Now according to the A7105 specification (pages 1 and 5), the A7105 runs on the 2.4GHz band, and is suitable for "2400 ~ 2483.5MHz ISM Systems".

Returning to my original source, he describes the binding mechanism of the helicopter:

First the Tx scans the RSSI on the following channels and picks the best one:

14 1e 28 32 3c 46 50 5a 64 6e 78 82 [HEX]

I was under the impression that the 2.4GHz band had only 14 bands (Wiki), so how can there be 130(+) channels for the helicopter to search through?

  • 2
    \$\begingroup\$ The 2.4GHz ISM band is divided into a number of channels (13-16 depending on your country I believe) for Wifi purposes. The helicopter (or more likely the RF link) manufacturer has chosen to divide the same spectrum more finely for their purposes, because they're not using Wifi for their control channel, they're just using the same radio spectrum as Wifi. The reason there are only a few Wifi channels is because each Wifi channel is very wide, but lower data rate radio links can use narrower (and therefore more) channels. \$\endgroup\$
    – markt
    Commented Jan 9, 2015 at 1:35

2 Answers 2


Every protocol defines channels as they see fit, on the continuum of frequencies.

WiFi defines 14 channels with wide spacing, because typical transmissions use a large amount of bandwidth, and communication in adjacent channels should not interfere. Your remote control uses only little bandwidth, so the channels could be packed tighter together.


De required bandwidth (BW) of a channel depends on the amount of information that needs to go through that channel. This is true for analog as well as digital data.

  • Intelligible speech requires 2-3kHz analog BW
  • For music with a certain fidelity one needs 15-20kHz analog BW

This is simply because music contains more detail, more information so to speak. If you would have a hypothetical frequency spectrum available from 100 to 300kHz you could put roughly (300.000 - 100.000)/ 3000 = 66 channels on that BW, for music only 10. assuming no spacing between channels would be needed.

Unfortunately, noise has a detrimental effect of how much information can pass through a certain BW. For digital information this reflected in Shannon's Law.

  • 2
    \$\begingroup\$ Speech doesn't need 2-3 kHz. That's the bandwidth of the baseband signal, sure. But to transmit that we only need a sufficient bitrate, and we can manipulate any of the terms in Shannon's law to achieve that (for example, we can increase the signal power), some of those solutions resulting in a signal bandwidth narrower than the baseband signal. \$\endgroup\$
    – Phil Frost
    Commented Jan 9, 2015 at 3:13
  • \$\begingroup\$ Bandwidth limitation also applies to analog signals. In this digital world many have often forgotten that. Also easier to comprehend than digital BW IMO. \$\endgroup\$
    – Guest
    Commented Jan 9, 2015 at 4:49
  • \$\begingroup\$ I don't think it does. If you are using a simple modulation like SSB, sure. However, there are any number of techniques, even analog ones, that could reduce the RF bandwidth required. For example, I could pass the baseband signal through a function that halves all of the frequency components, so the 3 kHz baseband signal now fits in 1.5 kHz. On demodulation, double all the frequency components. The only reason this isn't done is because it would be complex to implement in analog electronics, which is why digital communications are the state of the art. \$\endgroup\$
    – Phil Frost
    Commented Jan 9, 2015 at 12:49

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