Chirp Spread Spectrum
The term Chirp Spread Spectrum (CSS ) or chirp spread spectrum modulation refers to a technique that uses the spread spectrum ( Spread Spectrum English ) chirp pulses ( = chirping ). This modulation method is used for wireless data transmission over short distances, such as Wireless Personal Area Network and is in the IEEE 802.15.4a, an enhancement of IEEE 802.15.4, normalized.
A chirp pulse is a sinusoidal waveform is, which increases continuously over time in the frequency or drop. This waveform is used as part of the CSS as a fundamental transmit pulse which represents a symbol. Data transmission is done by a temporal succession of a series of on - and descending chirp pulses which are referred to in the standard as "sub- chirp ".
Like other spread spectrum such as Direct Sequence Spread Spectrum (DSSS) also uses CSS a wide range, which is caused directly by the chirp pulse. In contrast to DSSS, the method is not based on pseudo-random. As an essential feature of this method is robust against interference, according to the Doppler effect, because only the change in frequency over time of a chirp pulse is of importance, and the absolute frequency is not important, within certain limits.
The under IEEE 802.15.4a CSS to transmitted user data to be implemented in the modulation first with a differential quadrature phase shift keying ( DQPSK ) into a complex signal, which is then constant amplitude changes the phase of the sub -chip pulse which emitted in a row will. The transmit signal thus formed is provided in the standard referred to as DQCSK for Differential Quadrature Shift Keying chirp.
Channel selection different, fixed -defined sub- chirp sequences, as in the illustration, there are illustrated by the example of two sub- chirp sequences. The frequency ω is mapped over time, which is constantly varied: a rising straight line means an increasing in frequency sub- chirp or a falling Just a descending sub- chirp. The abrupt transitions between two chirp pulses are smoothed by a Gaussian filter.
The receiver, which is tuned to a certain channel, analyzes by correlating only the selected sub - chirp sequence for further data processing. CSS thus represents a kind of code division multiple dar.