DSSS — Direct-Sequence Spread-Spectrum
DSSS is one of the most popular modulation and transmission methods in wireless communications.
FHSS — Frequency Hopping Spread Spectrum
When you listen to your favorite radio station or watch your favorite TV channel, you tune-in to a specific channel…
Just like FHSS, DSSS also uses spread spectrum technology. The concept of spread spectrum — narrow band signals are intentionally spread over a much wider band.
FHSS vs DSSS
FHSS uses hopping to spread the data signals while DSSS on the other hand, combines data signals directly with a higher bit rate sequence.
Let’s look at an example to get a better idea.
- At the sending end, the original data is fed into a spreading modulator.
- Next, Pseudonoise (PN) Sequence is added and mixed in the spreading modulator. This PN sequence is of much higher bit-rate. It is a sequence of 1s and 0s.
NOTE: In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time.
- This PN code also referred to as chips, makes the original data signals identical to itself, close to noise-level signals .
NOTE: Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise.
- The spreading modulator distributes the signals over a wider frequency band.
- At the receiving end, the signal is demodulated with the PN code and the original message is restored.
— The ultimate satisfaction we get from this process is 2-fold —
- The signal is coded bit-by-bit.
- The signal is distributed over a wider band.
Let’s see how the PN code works on the original data bit-by-bit.
- Suppose, we need to send 2 bits of data namely, a 1 and a 0.
- In order to encode the data (makes sense), we add some noise (PN code/chips).
- Note that the PN code has higher bit-rate than original data.
- Now, to get the modulated and encoded signals to transmit, we use the XOR (Exclusive OR) truth table to calculate the transmitted signal.
PN code has a many radio pulses that are way shorter than the original signal, The shorter this duration, the larger the bandwidth of the resulting transmitted signal.
Why would spreading signals bypass the problems of interference, jamming and interception?
If you are driving on a narrow road during rush hour, you feel the resistance, more chances of accidents are there.
Similarly, in narrow-band communications, more power is consumed in overcoming the issues like jamming, and interference. DSSS uses wide band, noise-like signals, which make it difficult to detect and jam compared to narrowband signals. Since spread spectrum signals are wide, they transmit at a much lower spectral power density, making it less likely to interfere with narrowband communications.