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Wi-Fi Pro Lab › Modulation and waveforms

DSSS: direct-sequence spread spectrum

How a signal can be made deliberately wider so that noise and interference barely notice it. This is the radio of the original 802.11 standard and of 802.11b.

11 chipsper symbol (Barker code)
11 Mchip/schip rate
10.4 dBprocessing gain
1 and 2 Mbit/sthe Barker-coded rates

1. DefinitionWhat is direct-sequence spread spectrum?

DSSS multiplies every data symbol by a fast, fixed pattern of +1 and -1 values called a spreading code. Each value in the code is a chip, and the rate at which chips are sent is the chip rate. Because several chips are sent for every symbol, the signal needs far more bandwidth than the data alone would: that is the spreading. The number of chips per symbol is the spreading factor, and the benefit it brings is the processing gain.

Spreading is not modulation. Modulation (here DBPSK or DQPSK) decides how bits become phase. Spreading is applied afterwards, to the modulated symbols. They are separate steps, which is why a receiver has to undo both.

2. PurposeWhy 802.11 started here

  • Interference tolerance. The band is licence-exempt, so unknown interferers are certain. A narrowband interferer is spread out by the receiver’s despreading and mostly vanishes.
  • Low power density. The energy is spread over about 22 MHz, so at any single frequency the signal is weak, which suits rules written for spread-spectrum devices.
  • Robustness at low speed. 1 and 2 Mbit/s are slow but reach far, so they became the basic rates that every device could always fall back to.

3. MechanismStep by step

  1. Encode: at 1 Mbit/s each bit is encoded differentially (DBPSK): a 1 flips the phase of the next symbol, a 0 leaves it unchanged. At 2 Mbit/s two bits choose one of four phase changes (DQPSK).
  2. Spread: every symbol is multiplied by the 11-chip Barker code, +1 -1 +1 +1 -1 +1 +1 +1 -1 -1 -1, sent at 11 million chips a second. One 1-microsecond symbol becomes 11 chips of about 91 nanoseconds.
  3. Receive: multiply what arrives by the same code and add up the 11 chips. The wanted signal adds in step to +11 or -11; noise and interference do not.

4. VisualWhy the Barker code

A spreading code is only useful if it matches itself perfectly and nothing else. The 11-chip Barker code was chosen because of how it correlates with shifted copies of itself: one tall peak, and only tiny values everywhere else.

5. MathematicsProcessing gain and bandwidth

  • Processing gain = 10 log10(chips per symbol) = 10 log10(11) = 10.4 dB.
  • Chip rate = symbol rate x chips per symbol = 1 Msym/s x 11 = 11 Mchip/s.
  • Main-lobe bandwidth of the spread signal is about twice the chip rate, 22 MHz, against about 2 MHz for the same data unspread.
  • Data rate = symbol rate x bits per symbol: 1 x 1 = 1 Mbit/s (DBPSK) and 1 x 2 = 2 Mbit/s (DQPSK).
  • Why 10.4 dB: when the 11 chips are added in step the signal voltage grows 11 times, but noise, which adds randomly, grows only about 3.3 times (the square root of 11). The signal-to-noise power ratio improves by 11 times.

6. Real-world behaviourWhat DSSS looks like on a network

  • Slow rates, big airtime cost. A beacon of about 300 bytes at 1 Mbit/s takes roughly 2.6 ms; at 6 Mbit/s about 0.4 ms. With beacons every 102.4 ms per SSID, ten SSIDs at 1 Mbit/s can use about a quarter of the channel before any user traffic.
  • Compatibility overhead. When an 802.11b device is present, newer devices use protection mechanisms and long preambles so it can hear them. The ERP element in beacons has a Barker_Preamble_Mode bit for this.
  • Robust, but limited in speed. With the Barker code the rate stops at 2 Mbit/s, because each symbol carries at most 2 bits and symbols cannot be sent faster than a million a second.

Practical validationProve it in a capture

  • Add the data rate and PHY type of each frame as columns (wlan_radio.data_rate and wlan_radio.phy) and filter beacons with wlan.fc.type_subtype == 0x0008. Beacons at 1 Mbit/s are DSSS.
  • Count how many frames in a busy minute use the DSSS rates. Each one holds the channel far longer than an OFDM frame of the same size.
  • Question to answer: what happens to a network’s airtime if you disable the 1, 2, 5.5 and 11 Mbit/s rates, and which old devices would stop working?

Check your understandingQuick quiz

ReferencesSources and further reading

Facts checked against IEEE Std 802.11-2020: clause 16, 16.3.6.5 (the Barker sequence) and Tables 16-7 and 16-8 (DBPSK and DQPSK phase changes). The airtime example is a calculation from the stated frame size and rate.