Direct sequence spread spectrum
Direct sequence spread spectrum, also known as direct sequence code division multiple access (DS-CDMA), is one of two approaches to spread spectrum modulation for digital signal transmission over the airwaves. In direct sequence spread spectrum, the stream of information to be transmitted is divided into small pieces, each of which is allocated across to a frequency channel across the spectrum. A data signal at the point of transmission is combined with a higher data-rate bit sequence (also known as a chipping code) that divides the data according to a spreading ratio. The redundant chipping code helps the signal resist interference and also enables the original data to be recovered if data bits are damaged during transmission.
Spreading with DSSS:
- Direct sequence spread spectrum (DSSS) systems take a user bit stream and perform an (XOR) with a so-called chipping sequence
- The example shows that the result is either the sequence 0110101 (if the user bit equals 0) or its complement 1001010 (if the user bit equals 1).
- Each user bit has a duration tb, the chipping sequence consists of smaller pulses, calledchips, with a duration tc.
- If the chipping sequence is generated properly it appears as random noise: this sequence is also sometimes called pseudo-noise sequence.
- Spreading factor s = tb/tc; determines the bandwidth of the resulting signal. If the original signal needs a bandwidth w, the resulting signal needs s·w after spreading.
- The first step in a DSSS transmitter is the spreading of the user data with the chipping sequence
- The spread signal is then modulated with a radio carrier
- The radio carrier then shifts this signal to the carrier frequency
- This signal is then transmitted
- The receiver only performs the inverse functions of the two transmitter modulation steps.This is achieved using the same carrier as the transmitter reversing the modulation and results in a signal with approximately the same bandwidth as the original spread spectrum signal.
- In the next step the receiver has to know the original chipping sequence, i.e., the receiver basically generates the same pseudo random sequence as the transmitter.
- Sequences at the sender and receiver have to be precisely synchronized because the receiver calculates the product of a chip with the incoming signal. This comprises another XOR operation together with a medium access mechanism that relies on this scheme.
- During a bit period, which also has to be derived via synchronization, an integrator adds all these products.
- Finally, in each bit period a decision unit samples the sums generated by the integrator and decides if this sum represents a binary 1 or a 0.