Spreading code

In spread spectrum method such as CDMA or DSSS , the message to be transmitted with the spreading code (engl. Spreading code ), which consists of a sequence of chips is, spread, that is, each individual user data bits of the message to be transmitted, with a plurality of chips of the spreading code multiplied. The useful bit rate is always lower than the chip rate . As a result, the user data signal is artificially expanded, creating a “spread” spectrum. The chip rate is specified in chips per second (cps) to distinguish it from the bit rate (bps) .

Several transmitters can send to one receiver at the same time by using spreading codes that are different and known to the receiver. Using the spreading codes known to it, the receiver can decode the signals and determine which user sent which signal. Such methods, which multiplex several digital channels, are called code division multiplex methods .

example

Chipping sequence: 1 1 0 0 0 1 1 1.

8 bits (i.e. one byte) have to be coded by 8 chips - this is done by an XOR operation .

The useful signal to be sent is the bit sequence " 1 0":

  Signal:    1    |   0
  Chip-S: 11000111|11000111
  XOR:    00111000|11000111

⇒ The result of the XOR operation would be transmitted with a data volume to be transmitted that is now increased by a factor of eight. This results in a lower usable data rate on the transmission path.

The receiver knows the chip sequence and is synchronized with the transmitter, which means that it can decode the original data again:

  Signal: 00111000|11000111
  Chip-S: 11000111|11000111
  XOR:    11111111|00000000

See also

• Orthogonal Variable Spreading Factor (OVSF)
• Walsh code
• Gold code
• Maximum length sequence

literature

• Alois MJ Goiser: manual of the spread spectrum technique . Springer, Vienna 1997, ISBN 3-211-83080-4 .