DIRECT SEQUENCE SPREAD SPECTRUM EBOOK

DIRECT SEQUENCE SPREAD SPECTRUM EBOOK

DIRECT SEQUENCE SPREAD SPECTRUM EBOOK!

Direct Sequence Spread Spectrum (DSSS) is a spread spectrum technique whereby the original data signal is multiplied with a pseudo random noise spreading. Spectrum access. Direct Sequence Spread Spectrum. (DSSS). ○ Each bit in original signal is represented by multiple bits (chips) in the transmitted signal. Spread spectrum signals may be divided into two main groups - direct sequence spread spectrum. (DSSS), and frequency hopping spread spectrum (FHSS).


DIRECT SEQUENCE SPREAD SPECTRUM EBOOK

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DIRECT SEQUENCE SPREAD SPECTRUM EBOOK


Direct sequence spread spectrum, also known as direct sequence code division multiple access DS-CDMAis one of two approaches to spread spectrum modulation for digital signal transmission over the airwaves.

When transmitting a DSSS spread spectrum signal, the required data signal is multiplied with what is known as a spreading or chip code data stream.

The resulting data stream has a higher data rate than the data itself. Each bit in the spreading sequence is called a chip, and this is much shorter than each information bit. Purely digital spread-spectrum despreading chips can contain more than several million equivalent 2-input NAND gates, switching at several tens of megahertz.

Different Modulation Spreading Techniques for Spread Spectrum Different spread-spectrum techniques are distinguished according to the point in the system at which a PRN is inserted in the communication channel.

This is very basically illustrated in the RF front-end schematic in Direct sequence spread spectrum Several spreading techniques are applied at different stages of the transmit chain. In practice, the pseudo-random direct sequence spread spectrum is mixed or multiplied with the information signal, giving an impression that the original data flow was "hashed" by the PRN.

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There is also the "chirp" technique, which linearly sweeps the carrier frequency in time. The modulator, therefore, sees a much larger bit rate, which corresponds to the chip rate of the PRN sequence.

The main lobe of this spectrum null to null has a bandwidth twice direct sequence spread spectrum clock rate of the modulating code, and the side lobes have null-to-null bandwidths equal to the code's clock rate.

An Introduction to Spread-Spectrum Communications

Illustrated in Figure 13 is the most common type of direct-sequence-modulated spread-spectrum signal. Direct-sequence spectra vary somewhat in spectral shape, depending on the actual carrier and data modulation used. Below is direct sequence spread spectrum binary phase shift keyed BPSK signal, which is the most common modulation type used in direct-sequence systems.

Spectrum-analyzer photo of a DSSS signal. Note the original signal nonspread would only occupy direct sequence spread spectrum of the central lobe. The speed at which the hops are executed depends on the data rate of the original information. The latter method, the most common, allows several consecutive data bits to modulate the same frequency.

DIRECT SEQUENCE SPREAD SPECTRUM EBOOK

FFHSS is characterized by several hops within each data bit. The transmitted spectrum of a frequency-hopping signal is quite different from that of a direct-sequence system.

Understanding Spread Spectrum for Communications - National Instruments

The bandwidth of a frequency-hopping signal is simply N times the number of frequency slots direct sequence spread spectrum, where N is the bandwidth of each hop channel. Spectrum-analyzer photo of a FHSS signal. Figure 15 illustrates THSS, a method not well developed today.

Implementations and Conclusions A complete spread-spectrum communication link requires various advanced and up-to-date technologies and disciplines: Though designers and manufacturers compete, they are also joining in their effort to implement spread-spectrum systems.

Recuperate original signal for two different users. User 1 has the correct sequence. User 2 has different sequence and therefore the output message will have many errors no information M-sequences are the basics of PN sequences and they are used in real systems GPS but these are not the only PN sequences.

While for useful process gain the transmitted DSSS signal must occupy much wider bandwidth than simple amplitude modulation of the original signal alone would require, its frequency spectrum can be somewhat restricted for spectrum economy by a conventional analog bandpass filter to give a roughly bell-shaped envelope centered on the carrier frequency.

In contrast, frequency-hopping spread spectrum which pseudo-randomly re-tunes the carrier, instead of adding pseudo-random noise to the data, requires a uniform frequency response since any bandwidth shaping would cause amplitude modulation of the signal by the hopping code.

If an undesired transmitter transmits on the same channel but with a different PN sequence or no sequence at allthe de-spreading process has reduced processing gain for that signal.



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