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 spread over a wider bandwidth before being Communication transmitted. i.e. the Navigation bandwidth employed for Test systems transmission is much larger ……. than the minimum bandwidth required to transmit the information SDR'11 2
Low probability of detection (energy density reduction) Interference suppression Fine time resolution Communication resource sharing (multiple access transmission techniques) SDR'11 3
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 SDR'11 4
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 ANALOG OG ANALOG OG or or DIGITAL AL SDR'11 5
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 output 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 The M-FSK Th SK data modulate ted d signal is then mixed with the synth thesi sizer output t patte tern to produce the frequency y hopped signal SDR'11 6
7 SDR'11
fs M fs Digital up h (n) x(n,0) 0 conversion to higher Nyquist x(n,1) h (n) 1 zones by the IFFT x(n,2) h (n) Spectral 2 shaping and M-P NT x(n,3) h (n) filtering by the M- 3 IFFT ..... FDM ..... path partitioned P olyphase . . . . . filter weights P artition h (n) Sample rate x(n,M-2) M-2 change by the h (n) x(n,M-1) M-port output M-1 commutator h (n)= h(r+ nM) r SDR'11 8
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 SDR'11 9
Prototype Low-Pass Filter (2nd Channelizer) - Impulse Response Prototype Low-Pass Filter (1st Channelizer) - Impulse Response 1 1 0.8 Amplitude Amplitude 0.6 0.5 0.4 0.2 0 0 -0.2 -100 -50 0 50 100 -400 -300 -200 -100 0 100 200 300 400 Time Samples Time Samples Prototype Low-Pass Filter (2nd Channelizer) - Frequency Response Prototype Low-Pass Filter (1st Channelizer) - Frequency Response 0 0 Log Mag (dB) -20 Log Mag (dB) -50 -40 -60 -100 -80 -8 -6 -4 -2 0 2 4 6 8 -80 -60 -40 -20 0 20 40 60 80 Normalized Frequency Normalized Frequency First Tier Channelizer Second Tier Channelizer -Impulse and Frequency -Impulse and Frequency Response- Response- SDR'11 10
Spectrum - FSK Modulated Signal Spectrum - FSK Modulated Signal Spectrum - Hopped Signal Spectrum - Hopped Signal Magnitude 0 Magnitude 0 0 0 Magnitude Magnitude -50 -50 -50 -50 -5 0 5 -5 0 5 -50 0 50 -50 0 50 Frequency Frequency Frequency Frequency Spectrum - FSK Modulated Signal Spectrum - FSK Modulated Signal Spectrum - Hopped Signal Spectrum - Hopped Signal 0 0 Magnitude Magnitude Magnitude 0 Magnitude 0 -50 -50 -50 -50 -5 0 5 -5 0 5 -50 0 50 -50 0 50 Frequency Frequency Frequency Frequency Spectrum - FSK Modulated Signal Spectrum - FSK Modulated Signal Spectrum - Hopped Signal Spectrum - Hopped Signal 0 0 0 0 Magnitude Magnitude Magnitude Magnitude -50 -50 -50 -50 -5 0 5 -5 0 5 -50 0 50 -50 0 50 Frequency Frequency Frequency Frequency Spectrum - FSK Modulated Signal Spectrum - FSK Modulated Signal Spectrum - Hopped Signal Spectrum - Hopped Signal Magnitude 0 Magnitude 0 0 0 Magnitude Magnitude -50 -50 -50 -50 -5 0 5 -5 0 5 -50 0 50 -50 0 50 Frequency Frequency Frequency Frequency Spectrum - FSK Modulated Signal Spectrum - FSK Modulated Signal Spectrum - Hopped Signal Spectrum - Hopped Signal Magnitude 0 Magnitude 0 Magnitude 0 Magnitude 0 -50 -50 -50 -50 -5 0 5 -5 0 5 -50 0 50 -50 0 50 Frequency Frequency Frequency Frequency First Tier Channelizer Outputs Second Tier Channelizer Outputs SDR'11 11
10 9 8 7 Frequency 6 5 4 3 2 1 200 400 600 800 1000 1200 1400 1600 1800 2000 Time Contour plot, for ten symbols, of the hopped 8-FSK signals SDR'11 12
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 SDR'11 13
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. SDR'11 14
….Thanks for your attention SDR'11 15
Recommend
More recommend
