My son got a quadcopter for his birthday, a "NiGHT LiONS TECH® X9 Flying Car 4 Channel 2.4Ghz RC Quadcopte Drone".

On the box, it says that it communicates at 2.4 GHz on the "100-200 band"; on the website it says:

2..4 Ghz spread spectrum transmitter uses multiple frequencies across 100-200 band

What is the 100-200 band? Does this refer to 2.4 GHz + an extra 100-200 Hz?


I smell a translation error. "Spread spectrum" is a term for technologies that take a narrowband signal (e.g. the very low-rate signal needed to communicate a few button/joystick levels) over a large bandwidth. You can do that in a number of ways – including FHSS (Frequency-hopping spread spectrum), that is, taking a single transmit symbol (e.g. the value "4") and quickly jumping to multiple frequencies ("hop frequencies", or "channels"), and sending e.g. the same symbol there.

Now, "channel" is a term very narrowly related to "band". I could imagine that for a non-native speaker, a "100-channel frequency hopping spread-spectrum system" becomes a "multiple frequencies across 100-200 band"; notice that in any case, the grammar is slightly off in that sentence.

As a technological comment: it certainly doesn't mean +100-200 Hz, because if you have an oscillator at 2.4 GHz, even generating a tone that is a couple hundred Hertz close to its specified frequency is very nontrivial. Consider the relative oscillator accuracy needed to hit exactly within 100 Hz of a wanted 2.4 GHz tone:

$$\frac{10^2}{2.4\cdot10^9}\approx 40\,\text{ppb .}$$

That is parts per billion. I did a quick search on Mouser for oscillators that have a better accuracy than 50 ppb – and the cheapest one starts at roughly 38€ (without taxes), so about the same price as your complete drone.
For this very reason, and because things can be moving relative to each other (Doppler!), frequency synchronization is a crucial part of every wireless communication device; the oscillators within a cheap 2G telephone certainly aren't great enough to "hit" the center of the 200 kHz GSM channels, but with a couple of tricks, the oscillators can be "trained" to run at much more exactly the same rate as the base station's oscillators, at least for a short duration of time. For 30-years-old GSM that was already a necessity – now imagine things like 3G (which, at least in Europe, uses a really offset-sensitive modulation) where symbol rates are much higher, and a smaller frequency offset has even greater effect, or 4G, which has 1200 subcarriers in a 20 MHz uplink, meaning that you need to know very exactly what frequency you are at – otherwise, you'd be receiving the wrong things.

  • \$\begingroup\$ Marcus, I actually bought a 2.4 GHz transceiver from SparkFun.com in hopes that I could program an Arduino to control something like the quad copter. You think that an off-the-shelf 2.4 GHz transceiver would be able to talk to something like this? \$\endgroup\$ – mj_ Jan 6 '17 at 3:51
  • \$\begingroup\$ Probably not. The band (2.4GHz) in which something operates doesn't imply the same transmission format - otherwise, your Bluetooth could talk to your WiFi, and your cordless phone, and your cordless mouse and your microwave oven. \$\endgroup\$ – Marcus Müller Jan 6 '17 at 8:59
  • \$\begingroup\$ @mj_ also see this answer electronics.stackexchange.com/a/273585 \$\endgroup\$ – Marcus Müller Jan 6 '17 at 9:15
  •  Since each bit is sent as multiple chips, you need more bps bandwidth to send the signal.
    • 11Mbps uses 8bits/Baud
    • In the 2.4 GHz or “S-band,” the Commission allocated the 2495-2500 MHz bandwidth or 505 bands of 1MHz
    • many parallel channels out of 100~200 band choices are used simultaneously for redundancy and lower Fading tracking loss from phase cancellation in flight.
    • Used frequency hopping, 96 channels of 1 MHz using 2GFSK
    • 802.11 Standard defined 26 orthogonal hop sequences - Can support multiple simultaneous transmissions – use different hop sequences E.g. up to 10 co-located APs with their clients
    • Number of chips per bit is called the spreading ratio
    • This is the spreading part of spread spectrum
  •  Given the Nyquist and Shannon results, you need more spectral bandwidth to do this. » Spreading the signal over the spectrum
  •  Advantage is that is transmission is more resilient. » DSSS signal will look like noise in a narrow band » Can lose some chips in a word and recover easily
  •  Multiple users can share bandwidth (easily).
    • Follows directly from Shannon (capacity is there)

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