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Spread Spectrum Communication Systems

2. Overview of Spread Spectrum Communications. Spread Spectrum is a means of transmission in which the data sequence occupies a bandwidth in excess of the minimum bandwidth necessary to send it.Effectively the signal is mapped to a higher dimension signal spaceThe signal component in each directio

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Spread Spectrum Communication Systems

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    1. 1 Spread Spectrum Communication Systems

    2. 2 Overview of Spread Spectrum Communications Spread Spectrum is a means of transmission in which the data sequence occupies a bandwidth in excess of the minimum bandwidth necessary to send it. Effectively the signal is mapped to a higher dimension signal space The signal component in each direction is sqrt(E/N) Signal spreading is done before transmission by using a spreading sequence. The same sequence is used at the receiver to retrieve the signal Spread Spectrum is most effective against interference (intentional or non-intentional) with fixed energy. Main commercial applications in wireless and GPS.

    3. 3 Spread Spectrum- Main Operation

    4. 4 Spread Spectrum: Main Operation

    5. 5 Generating the Spreading (Pseudo-Noise) Sequence The pseudo-noise (PN) sequence is a periodic binary sequence with a noise like waveform that is generated by means of a feedback shift register. The feedback shift register consists of m-stage shift registers and a logic circuit that perform modulo-2 (X-OR) arithmetic. A sequence with period 2m-1 is called Maximal-Length sequence

    6. 6 PN Sequence: Example

    7. 7 Properties of Maximal-Length Sequences In each period of a maximal-length sequence, the number of 1’s and the number of 0’s in the sequence always differ by 1. The autocorrelation function of a maximal-length sequence is periodic and binary valued.

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    9. 9 Direct Sequence Spread Spectrum with Coherent BPSK (DS/BPSK)

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    11. 11 Analysis The objective is to find the ratio of the SNR (SNRI) at the input of the receiver to the SNR (SNRO) at the output of the receiver

    12. 12 Analysis

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    14. 14 Probability of Error vcj is a random variable with zero mean and variance JTc/2 vcj can be viewed as a white noise with zero mean and variance N0/2 = JTc/2 Assuming Gaussian distribution for the jamming component, the probability of error due to jamming When noise and jamming exist the probability of error is

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    16. 16 Frequency-Hopped (FH) Spread Spectrum FH/SS is usually used with Binary FSK or M-ary FSK The carrier frequency is determined by the output sequence from a PN generator Slow hopping system has a hopping rate that is lower than the information rate (symbol rate) Several information symbols are transmitted by the same carrier frequency Fast hopping system has a hopping rate that is higher than the information rate One information symbol is transmitted by different carrier frequencies.

    17. 17 Slow Frequency Hopping Example

    18. 18 Fast Frequency Hopping Example

    19. 19 Code Division Multiple Access (CDMA) Multiple users can use the same channel as long as different users are assigned different PN sequence (code) Several users can transmit simultaneously on the same channel The transmissions from other users will look like interference. CDMA main application is wireless communication. CDMA is the wireless standard for North America Assuming all users have the same power, Ps, the number of user Nu that can be supported is given by:

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