N-GEN Communications Web-based Seminars ©NGCS

Seminar - An Overview of Communications Technologies

Module 9 - Wireless Systems and Technologies                                                                                 Slide 13 of 80                                                                                                    ________________

    Direct Sequence Spread Spectrum (DSSS)

                                  Signal Power Spread over a Wider band of Frequencies

                                  Improved Signal-to-Noise Ratio

                                  DSSS Modulation Technique is Much Less Susceptible to
                                    Electrical Noise Than Conventional Radio Modulation Technique

                                  Chipping Code

                                                                                                   ________________

The DSSS modulation technique combines a data signal at the sending station with a higher data rate bit sequence. The resulting signal is resistant to interference. The higher data rate bit sequence is known as the chipping code.

For example:

Data Stream:    1011

Chipping Code:    1 = 00010011100
                        0 = 11101100011

Transmitted Signal:

                1                    0                    1                    1     (Data stream)

        00010011100    11101100011    00010011100    00010011100  (Transmitted signal)

DSSS technology is used for IEEE 802.11b standard, known as Wi-Fi specified operation in the 2.4 G Hz band ( ISM band) to achieve transmission speeds up to 11 M bps.