Chirp Spread Spectrum
The term Chirp Spread Spectrum (CSS) or chirp spread spectrum refers to a modulation technique which the frequency spread ( English Spread Spectrum ) Chirp uses pulses (= chirping). This modulation method is used for wireless data transmission over mostly short distances such as the wireless personal area network and is standardized in this application area in the standard IEEE 802.15.4a , an extension of IEEE 802.15.4 .
General
A chirp pulse represents a sinusoidal signal curve, which continuously increases or decreases in frequency over time . This signal course is used in CSS as an elementary transmission pulse, which represents a symbol. The data transmission takes place through a chronological sequence of ascending and descending chirp pulses, which are referred to as "sub-chirp" in the standard.
Analogous to other frequency spreading methods such as Direct Sequence Spread Spectrum (DSSS), CSS also uses a large bandwidth , which is directly caused by the chirp pulse. In contrast to DSSS, the process is not based on pseudo randomness . As an essential property, this method is robust against disturbances due to the Doppler effect , since only the frequency change over the time of a chirp pulse is important, and the absolute frequency plays no role within certain limits.
Procedure
Modulation of the user data
The user data to be transmitted with CSS as part of IEEE 802.15.4a is first converted into a complex signal during modulation using differential quadrature phase shift keying (DQPSK) , which then changes the phase position of the sub-chirp pulse with constant amplitude , which is then transmitted becomes. The transmission signal formed in this way is also referred to as DQCSK for Differential Quadrature Chirp Shift Keying within the framework of the standard .
Channel selection
There are various, permanently defined sub-chirp sequences for channel selection, as shown in the figure on the right using the example of two sub-chirp sequences. The frequency ω is mapped over time, which is constantly varied: a rising straight line means a sub-chirp that rises in frequency and a falling straight line means a descending sub-chirp. The sudden transitions between two chirp pulses are smoothed by a Gaussian filter .
The receiver, which is set to a specific channel, uses correlation to evaluate only the selected sub-chirp sequence for further data processing. CSS thus represents a type of code division multiplexing procedure.