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What is spread spectrum? • A transmission technique in which a pseudo-noise code, independant of the information data, is employed as a modulation waveform to “spread” the signal energy over a bandwidth much greater than the signal information bandwidth. At the receiver the signal is “despread” using a synchronized replica of the pseudo-noise code.
Principle of Spread spectrum • The spread-spectrum concept has developed from the principle of Shannon theorem. • Shannon equation: C = B· log2(1 + SNR) where C=channel capacity B=Bandwidth of signal SNR=Signal to noise ratio
Principle of SS. • If data is transmitted at a rate of Rb over a channel occupying a bandwidth much greater than Rb, Shannon theorem indicates that reliable communications can be achieved at a reduced SNR. • However, if the transmitted power is kept fixed, even though the power density is substantially reduced, a surplus in the SNR is generated and can be used to combat interference and jamming. • This surplus in SNR is called processing gain.
Advantages of SS. • Signal hiding (lower power density, noise-like), non-interference with conventional systems and other SS systems • Secure communication (privacy) • Code Division Multiple Access CDMA (multi-user) • Mitigation (rejection) of multipath, hold only the direct path • Protection to intentional interference (Jamming) • Rejection of unintentional interference (narrowband) • Low probability of detection and interception (LPI)
Disadvantages of SS • No improve in performance in the presence of Gaussian noise. • Increased bandwidth (frequency usage, wideband receiver). • Increased complexity and computational load.
Criteria for spread spectrum. To qualify as a spread spectrum signal, two criteria should be met:- • The transmitted signal bandwidth is much greater than the information bandwidth. • Some function other than information being transmitted is employed to determine the resultant transmitted bandwidth. E.g PN & Walsh codes.
Applications of Spread Spectrum:- • Wireless local area network (WLAN). • Space systems. • Global positioning system (GPS). • Cordless phone
DSSS (continued…) • A block diagram of the modulator that generates DS-SS signals is shown in Figure. • The binary data m(t) is first multiplied by the high rate code sequence to acquire the energy spreading. • The baseband signal Sn(t) is filtered to confine energy within the bandwidth defined by the code rate. • The carrier modulation commonly used in spread spectrum is phase shift keying. • Sn(t) = m(t) · C(t)
DSSS (continued…) • The baseband signal Sn(t) is convoluted with the impulse response of the spectrum-shaping filter to yield y(t) y(t) = Sn(t) ∗ h(t) where * denotesconvolution • The bandpass signal SS(t) = [Sn(t) ∗ h(t)] · cosωCt
Types of Spread spectrum • DSSS (Direct sequence spread spectrum) • FHSS (Frequency hopped spread spectrum)
DSSS receiver:- • A basic block diagram of the matched filter receiver is shown in Figure • The received bandpass signal SS(t) is converted to an equivalent complex lowpass signal A(t) by mixing with a locally generated coherent carrier. • The lowpass spread spectrumis caused to collapseby multiplying by a locally generated in-phase copy of the transmitted code sequence.
DSSS receiver:- • The de-spread signal B is matched filtered and sampled. • The complex lowpass signal A(t) = SS(t) · cosωCt • The de-spread signal B(t) = A(t) · [C(t) ∗ h(t)]
DSSS receiver:- • The receiver decodes the data according to the following rule: D(T) > 0 decode binary ‘1’ otherwise decode binary ‘0’.
DSSS:- • Each bit represented by multiple bits using spreading code • Spreading code spreads signal across wider frequency band • In proportion to number of bits used • 10 bit spreading code spreads signal across 10 times bandwidth of 1 bit code • One method: • Combine input with spreading code using XOR • Input bit 1 inverts spreading code bit • Input zero bit doesn’t alter spreading code bit • Data rate equal to original spreading code
DSSS generation:-example Low-Bandwidth Signal: High-Bandwidth Spreading Code: Mix is a simple multiply … and transmit.
DSSS signal recovery:-MA example To Decode / Receive, take the signal: Multiply by the same Spreading Code: … to get ...
Frequency Hopping Spread Spectrum (FHSS) • Signal broadcast over seemingly random series of frequencies • Receiver hops between frequencies in sync with transmitter • Eavesdroppers hear unintelligible blips • Jamming on one frequency affects only a few bits
FHSS:- • Above figure shows an example of a frequency hopping signal. • Typically there are 2k carrier frequencies forming 2k channles. • The spacing between carrier frequencies and hence the width of each channel usually corresponds to the bandwidth of the input signal. • The transmitter operates in one channel at a time for a fixed interval. • During this interval some number of bits is transmitted using some encoding scheme. • Both transmitter & receiver use the same code to tune into a sequence of channels in synchronization.
Frequency Hopping Spread Spectrum System (Transmitter) • For transmission ,binary data are fed into a modulator using some digital to analog encoding scheme, such as FSK or BPSK. • The resulting signal is centered on some base frequency. • A PN number source serves as an index in to a table of frequencies. • Each k bits of the PN source specifies one of the 2k carrier frequencies. • At each successive interval,a new carrier frequency c(t) is selected.
Frequency Hopping Spread Spectrum System (Transmitter) • This frequency is then modulated by the signal produced from the initial modulator to produce a new signal s(t) with the same shape but now centered on the selected carrier frequency. • On reception the spread spectrum signal is demodulated using the same sequence of PN derived frequencies and then
Processing Gain:- • In each of these processing methods, certain characteristics of the input signal are being modified or amplified. The effectiveness of the processor is measured with a factor called the processing gain Gp defined as:
Processing Gain(Cont….) • In spread-spectrum systems, the parameter of interest is the signal spectrum at the input (Bb) and the spectrum of the output (Bs). Thus: • Thus the processing gain (Gp) expresses the bandwidth expansion factor. For a DS-SS system:
Slow and Fast FHSS • Frequency shifted every Tc seconds • Duration of signal element is Ts seconds • Slow FHSS has Tc Ts • Fast FHSS has Tc < Ts • Generally fast FHSS gives improved performance in noise (or jamming)
Spreading codes • PN codes • Walsh codes
PN (pseudo random) code • The PN code sequence is a Pseudo-Noise or Pseudo-Random sequence of 1’s and 0’s, but not a real random sequence (because periodic). • Not random, but it looks randomly for the user who doesn’t know the code. • Deterministic, periodical signal that is known to both the transmitter and the receiver. The longer the period of the PN spreading code, the closer will the transmitted signal be a truly random binary wave, and the harder it is to detect.
Properties of PN Sequences • Balance Property:- • In each period of the sequence the number of binary ones differs from the number of binary zeros by at most one digit (for Nc odd). • Independence:- • No one value in the sequence can be inferred from others. • Correlation property:- • If a period of the sequence is compared term by term with any cycle shift of itself, the number of terms that are the same differs from those that are different by at most 1.
