Chip (data transmission)

In digital information transmission, a chip is a single elementary modulation state in the context of Direct Sequence Spread Spectrum (DSSS) and the code division multiplexing (CDMA) based on it. The sequence of different chips is determined both by the spreading code and by the symbols transmitted, the chip rate of the spreading code typically being far higher than the symbol rate and the bit rate .

General

A sequence of chips is transmitted per symbol , whereby the sequence must have certain properties such as pseudo random noise for frequency spreading . For use in code division multiplexing , different sequences of chips are used for each subscriber who jointly use a transmission channel such as a frequency band for radio transmission. In addition to the frequency spread, these sequences must then have further properties such as the smallest possible periodic cross-correlation maximum with respect to one another.

In binary DSSS systems, a single chip is usually expressed as a square pulse with a duration t and an amplitude of +1 or −1, the sequence of the individual chips is lined up in time and modulated on a carrier frequency for transmission . To reduce intersymbol interference , the pulse shape of the individual chips is shaped with pulse shaping filters such as the raised cosine filter before modulation .

The chip rate, abbreviated cps for chips per second , is the number of chips per second that are sent or received. The chip rate is always greater than the symbol rate of the transmission system for the purpose of frequency spreading . The ratio between the chip rate and symbol rate is called spreading factor SF , English factor spreading , described and is defined as: ${\ displaystyle c_ {c}}$ ${\ displaystyle c_ {s}}$

{\ displaystyle {\ mbox {SF}} = {\ frac {c_ {c}} {c_ {s}}} \ gg 1}

The spreading factor varies depending on the application, typical values are over 1000. For example, the spreading factor in the civil Global Positioning System (GPS) is 20,460. This value results from the chip rate of 1.023 Mcps used in the civil C / A code and the useful data rate used in GPS, in this case equal to the symbol rate, of 50 bps.

The sequences used in code division multiplexing (CDMA) to generate the chips should have a relatively small periodic cross-correlation maximum with respect to one another, that is, they are almost orthogonal to one another. Codes that have this property and are particularly suitable for generating chips are the Kasami sequences and the Gold sequences . The latter are used, among other things, in civil GPS.

literature

Alois MJ Goiser: manual of the spread spectrum technique . Springer, Vienna 1997, ISBN 3-211-83080-4 .
Gérard Maral, Michel Bousquet: Satellite Communications Systems: Systems, Techniques and Technology . 4th edition. John Wiley & Sons, 2002, ISBN 978-0-471-49654-0 .