What is Direct Sequence Spread Spectrum

In telecommunications, direct-sequence spread spectrum (DSSS) is a spread spectrum modulation technique used to reduce overall signal interference. The spreading of this signal makes the resulting wideband channel more noisy, allowing for greater resistance to unintentional and intentional interference.

Spread  spectrum signals may be divided into two main groups - direct sequence spread spectrum (DSSS), and frequency hopping spread spectrum (FHSS). This tutorial is concerned with demonstrating some of the principles of the first.

With DSSS, the data is divided and simultaneously transmitted on as many frequencies as possible within a particular frequency band (the channel). DSSS adds redundant bits of data known as chips to the data to represent binary 0s or 1s. The ratio of chips to data is known as the spreading ratio:The higher the ratio, the more immune to interference the signal is, because if part of the transmission is corrupted, the data can still be recovered from the remaining part of the chipping code.

This method provides greater rates of transmission than FHSS, which uses a limited number of frequencies, but fewer channels in a given frequency range.And, DSSS also protects against data loss through the redundant, simultaneous transmission of data. However, because DSSS floods the channel it is using, it is also more vulnerable to interference from Electromagnetic (EM) devices operating in the same range.

The number of center-channel frequencies used by 802.11 DSSS devices depends on the country. For example, North America allows 11 channels operating in the 2.4 to 2.4835 GHz range, Europe allows 13, and Japan allows 1. Because each channel is 22-MHz-wide, they may overlap each other. Of the 11 available channels in North America, only a maximum of three (1, 6, and 11) may be used concurrently without the use of overlapping frequencies.


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