Wireless Communication Basics
Which describes direct-sequence spread spectrum (DSSS)?
- AA scheme that lowers the data rate to make the occupied bandwidth as narrow as possible
- BA scheme that multiplies the data by a spreading code that changes faster than the data, sends it spread over a wide band, and restores it at the receiver by multiplying by the same code
- CA scheme that sends while switching the transmit frequency at short intervals in a set order
Show answer and explanation
Answer: B. A scheme that multiplies the data by a spreading code that changes faster than the data, sends it spread over a wide band, and restores it at the receiver by multiplying by the same code
Direct-sequence spreading multiplies the data by a spreading code that changes faster than the data bits (each element of the code is called a chip), so the signal is spread over a wide band. When the receiver multiplies by the same code, the desired signal returns to its original narrow band, while narrowband interference picked up along the way is instead spread over a wide band, reducing its effect. The spreading ratio is called the processing gain (spreading gain). Using different codes lets several users share the same frequency, which is the basis of CDMA.
- A. Spread spectrum does the opposite: it widens the bandwidth used.
- B. Correct. The signal is spread by the code and restored at the receiver by multiplying by the same code.
- C. This describes frequency-hopping spread spectrum (FHSS), used in Bluetooth and elsewhere.