Direct Sequence Spread Spectrum: the basics

Direct Sequence Spread Spectrum, DSSS Includes:
DSSS basics    

See also:     CDMA basics    

DSSS, direct sequence spread spectrum is a form of spread spectrum transmission which uses spreading codes to spread the signal out over a wider bandwidth then would normally be required.

The technique behind direct sequence spread spectrum, DSSS is at first sight counter-intuitive, but DSSS is used in a number of areas where it enables considerable benefits to be gained.

Direct sequence spread spectrum basics

Direct sequence spread spectrum is a form of transmission that looks very similar to white noise over the bandwidth of the transmission. However once received and processed with the correct descrambling codes, it is possible to extract the required data.

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. Often the data is multiplied using the XOR (exclusive OR) function.

Each bit in the spreading sequence is called a chip, and this is much shorter than each information bit. The spreading sequence or chip sequence has the same data rate as the final output from the spreading multiplier. The rate is called the chip rate, and this is often measured in terms of a number of M chips / sec.

The baseband data stream is then modulated onto a carrier and in this way the overall the overall signal is spread over a much wider bandwidth than if the data had been simply modulated onto the carrier. This is because, signals with high data rates occupy wider signal bandwidths than those with low data rates.

To decode the signal and receive the original data, the CDMA signal is first demodulated from the carrier to reconstitute the high speed data stream. This is multiplied with the spreading code to regenerate the original data. When this is done, then only the data with that was generated with the same spreading code is regenerated, all the other data that is generated from different spreading code streams is ignored.

The use of direct sequence spread spectrum is a powerful principle and has many advantages.

DSSS direct sequence spread spectrum encode / decode process

In order to visualise how the direct sequence spread spectrum process operates, the easiest method is to show an example of how the system actually operates in terms of data bits, and how the data is recovered from the DSSS, direct sequence spread spectrum signal.

The first part of the process is to generate the DSSS signal. Take as an example that the data to be transmitted is 1001, and the chip or spreading code is 0010. For each data bit, the complete spreading code is used to multiple the data, and in this way, for each data bits, the spread or expanded signal consists of four bits.

With the signal obtained and transmitted, it needs to be decoded within the remote receiver:

NB: 1 x 1 = 0     1 x 0 = 1

In this way it can be seen that the original data is recovered exactly by using the same spreading or chip code. Had another code been used to regenerate the CDMA spread spectrum signal, then it would have resulted in a random sequence after de-spreading. This would have appeared as noise in the system.

The spreading code used in this example was only four bits long. This enabled the process to be visualised more easily. Commonly spreading codes may be 64 bits, or even 128 bits long to provide the required performance.

DSSS spreading gain

The bandwidth of the spread spectrum signal will be much wider than the original data stream. To quantify the increase in bandwidth, a term known as the spreading gain is used. If the bandwidth of the DSSS, direct sequence spread spectrum signal is W and the input data bit length or period 1/R then the DSSS spreading gain can be defined:

It is found that the larger the spreading gain of the direct sequence spread spectrum signal, the more effective the performance of the system is. This is because the wanted signal becomes larger. In the example shown above, the spreading gain is four, as seen by the fact that four "1"s are generated for each required data bit. Data produced by other dispreading codes would appear as noise and can be discarded as it would be lower in value.

Direct sequence spread spectrum applications

DSSS is used in a number of areas where its properties have enabled it to provide some unique advantages over other techniques.

• Covert communications:   DSSS was first used to provide secure and covert communications. The signals were initially difficult to detect as they sounded like broadband noise and often would have been mistaken for that. Also to access the data it is necessary to know the code used to generate the signal
• CDMA cellphone technology:   The DSSS technique was used to provide a multiple access scheme that was used for 3G cellophane technology. Each mobile used a different access code or spreading code and this enabled multiple users to access the base station on the same frequency.
  Read more about . . . . CDMA cellphone multiple access technology.
• GNSS:   Satellite based navigation systems use DSSS as this gives a signal gain by spreading the signal out over a wide bandwidth. It also enables different satellites to use the same channel without mutual interference.

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