Spread spectrum (SS) techniques were, at the beginning, investigated - - PowerPoint PPT Presentation

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Spread spectrum (SS) techniques were, at the beginning, investigated - - PowerPoint PPT Presentation

Sep 12, 2022 42 likes •193 views

Spread spectrum (SS) techniques were, at the beginning, investigated for military applications because of their characteristic of being highly jamming resistant The name, spread spectrum, derives from the fact that the modulated signal is

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  • The name, spread

spectrum, derives from the fact that the modulated signal is spread over a wider bandwidth before being

  • transmitted. i.e. the

bandwidth employed for transmission is much larger than the minimum bandwidth required to transmit the information Spread spectrum (SS) techniques were, at the beginning, investigated for military applications because of their characteristic of being highly jamming resistant Communication Navigation Test systems …….

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 Low probability of detection (energy density reduction)  Interference suppression  Fine time resolution  Communication resource sharing (multiple access transmission techniques)

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Frequency hopping is one of the most common spread spectrum techniques in which the carrier frequency of the signal is periodically changed before transmission In frequency hopping spread spectrum (FHSS) transmissions, a frequency band, called hopping band, that includes M channels, is accessed by a controlled pseudorandom sequence, called frequency hopping pattern, that shifts it to a different center frequency which is selected from N possible center frequencies The receiver knows the pseudorandom sequence

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In the frequency hopping transmitters, the modulation process occurs in two steps:

  • 1. The input signal is baseband

modulated (generally by using an analog or a digital M-FSK modulator)

  • 2. The complete hopping band

is hopped over one of the N possible hopping frequencies by a second tier up converter

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ANALOG OG

  • r
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DIGITAL AL ANALOG OG

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Th The frequency y synthe thesiz sizer produces s frequency y hopping patterns s determined d by by the time-varyi ying g multil tilevel sequence specified d by by the

  • utput

t bits s of the code generator tor At each hop time me the pseudorandom m code generator tor fe feeds a frequency y synthe thesiz sizer a frequency y wo word which dictate tes s one of the possi sible center frequencies s from the hopset Th The M-FSK SK data modulate ted d signal is then mixed with the synth thesi sizer

  • utput

t patte tern to produce the frequency y hopped signal

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M fs fs h (n) h (n)

2

h (n)= h(r+ nM)

r

P

  • lyphase

P artition h (n)

M-2

h (n)

1

h (n)

3

h (n)

M-1

FDM

M-P NT IFFT

. . . . .

.....

.....

x(n,0) x(n,1) x(n,2) x(n,3) x(n,M-1) x(n,M-2)  Digital up conversion to higher Nyquist zones by the IFFT  Spectral shaping and filtering by the M- path partitioned filter weights  Sample rate change by the M-port output commutator

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First Tier Channelizer Second Tier Channelizer

The channel selector, controlled by a pseudo noise sequence generator, and placed between the two engines, delivers the input signals to the proper input port of the second up converter channelizer for performing the desired hops

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0.2 0.4 0.6 0.8 1 Prototype Low-Pass Filter (1st Channelizer) - Impulse Response Time Samples Amplitude

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Prototype Low-Pass Filter (1st Channelizer) - Frequency Response Normalized Frequency Log Mag (dB)

First Tier Channelizer

  • Impulse and Frequency

Response- Second Tier Channelizer

  • Impulse and Frequency

Response-

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100 200 300 400 0.5 1 Prototype Low-Pass Filter (2nd Channelizer) - Impulse Response Time Samples Amplitude

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Prototype Low-Pass Filter (2nd Channelizer) - Frequency Response Normalized Frequency Log Mag (dB)

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First Tier Channelizer Outputs Second Tier Channelizer Outputs

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Spectrum - FSK Modulated Signal Frequency Magnitude

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Spectrum - FSK Modulated Signal Frequency Magnitude

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Spectrum - FSK Modulated Signal Frequency Magnitude

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Spectrum - FSK Modulated Signal Frequency Magnitude

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Spectrum - FSK Modulated Signal Frequency Magnitude

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Spectrum - FSK Modulated Signal Frequency Magnitude

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Spectrum - FSK Modulated Signal Frequency Magnitude

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Spectrum - FSK Modulated Signal Frequency Magnitude

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Spectrum - FSK Modulated Signal Frequency Magnitude

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Spectrum - FSK Modulated Signal Frequency Magnitude

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Spectrum - Hopped Signal Frequency Magnitude

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Spectrum - Hopped Signal Frequency Magnitude

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Spectrum - Hopped Signal Frequency Magnitude

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Spectrum - Hopped Signal Frequency Magnitude

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Spectrum - Hopped Signal Frequency Magnitude

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Spectrum - Hopped Signal Frequency Magnitude

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Spectrum - Hopped Signal Frequency Magnitude

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Spectrum - Hopped Signal Frequency Magnitude

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Spectrum - Hopped Signal Frequency Magnitude

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Spectrum - Hopped Signal Frequency Magnitude

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Contour plot, for ten symbols, of the hopped 8-FSK signals

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Time Frequency 200 400 600 800 1000 1200 1400 1600 1800 2000 1 2 3 4 5 6 7 8 9 10

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The up converter channelizer provides the digital frequency hopping modulator with the unique capability of performing multiple simultaneous hopping without adding complexity to the design (Multiple simultaneous hops can be performed in the analog FH modulator at the cost of including multiple up converters in the design) Dual and multiple code hopping sequences can be easily performed Increase the frequency diversity capability of the FH transmitter

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We proposed a fully digital frequency hopping modulator architecture. Two polyphase up converter channelizers, in cascade, compose its core. The first channelizer performs the M-FSK modulation of the baseband spectrum while the second one hops the modulated spectra over N possible center frequencies. The novel architecture inherits the flexibility of these engines that allows us to select the levels of the M-FSK modulation, as well as the dimensionality of the hopset and the hopping bandwidth while, due to the efficiency of the IFFT algorithm, the total workload of the structure is kept low which makes feasible the realization of the proposed fully digital frequency hopping modulator.

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….Thanks for your attention

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