F REQUENCY H OPPING (FH)? To Fit Legacy FH Modems into the Evolving - - PowerPoint PPT Presentation

AN EFFICIENT FREQUENCY HOPPING DEMODULATOR BASED ON POLYPHASE FILTER BANKS

fred harris, Xiaofei Chen, Elettra Venosa

SDR’11-WInnComm 29 Nov

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WHY SHOULD WE DO DIGITAL FREQUENCY HOPPING (FH)?

 To Fit Legacy FH Modems into the Evolving

Wireless Standards

 To Enjoy the Benefits Provided by Digital

Implementation

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THE IDEA OF FREQUENCY HOPPING

 MFSK Signal is Often Used as the Underlying

Modulation Signal

 The Center Frequency of the MFSK Signal is

Varied According to the PN Sequence

3

THE LEGACY FH DEMODULATOR

 A

Local Frequency Synthesizer Directed by the PN Sequence is Used to “De-hop” the Incoming FH signal.

 An M-FSK Demodulator Recovers the Transmitted Symbol. 4

MAJOR DRAWBACKS OF THE LEGACY FH MODEM

 Slow Acquisition Due to Serial Search  Performance Degradation Due to Imperfect Synchronization  Performance Degradation Due to Jamming

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THE IDEA OF CHANNELIZING FH SIGNAL

Why not implement a bank of digital band-pass filters ?

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M-PATH CHANNELIZER

This is a Bank of M Band-pass Filters! But, … At the cost of 1 filter plus an M point FFT !!!

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PROPOSED DIGITAL FREQUENCY HOPPING DEMODULATOR

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BENEFITS OF OUR PROPOSED DEMODULATOR

 Fast Acquisition Based on Parallel Search

The channelizer provides access to each hopping center, which allows us to obtain the hopping pattern

• instantaneously. Thus, parallel search of PN sequences

can be performed and, consequently, the expected acquisition time will be significantly reduced.

 Simultaneous and Diversity Hopping

• The proposed receiver structure can simultaneously

synchronize to and demodulate multiple FH signals.

• The same MFSK signals can be modulated onto

different PN sequences; and the receiver enjoys frequency diversity automatically.

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SIMULATION RESULTS: SIGNAL TO BE DEMODULATED

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

• 5

5 Frequency Hop Profile Hop Index Hop Frequency in MHz 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

• 1

1 Hop Time Sequence Time Index Amplitude

• 8
• 6
• 4
• 2

2 4 6 8 1000 2000 Spectrum of Frequency Hopping Signal Frequency in MHz Magnitude

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SIMULATION RESULTS: DE-HOPPED TIME SERIES

• 6.0-5.2-4.4-3.6-2.8-2.0-1.2-0.4 0.4 1.2 2.0 2.8 3.6 4.4 5.2 6.0

1000 2000 3000 4000 5000 6000 Hopping Frequency / MHz Time Index De-hopped Time Series

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SIMULATION RESULTS: ZOOM INTO 1.2 MHZ HOPPING CENTER

1800 1900 2000 2100 2200 2300 2400 2500 2600 2700

• 1

1 Output Time Series on 1.2 MHz Time Index Amplitude 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700

• 0.5

0.5 1 1.5 Signal Envelope Time Index Amplitude

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SIMULATION RESULTS: 2ND TIER CHANNELIZER OUTPUT

FOR 1.2 MHZ HOPPING CENTER

100 200 300 400 500 600 0.5 1 1.5 FSK Signal Envelope at del f = -200 kHz Amplitude 100 200 300 400 500 600 0.5 1 1.5 FSK Signal Envelope at del f = -40 kHz Time Index Amplitude 100 200 300 400 500 600 0.5 1 1.5 FSK Signal Envelope at del f = 120 kHz Time Index Amplitude

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CONCLUSION

Comparing with the legacy FH modem, the channelizer based FH modem has the following benefits:

 Reduced Acquisition Time  Enhanced System Flexibility and BER Performance

via Diversity Hopping and Simultaneous Hopping.

 Easy To Be Implemented On Any SDR Platform

THANKS  Open for Questions !

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