A N E FFICIENT F REQUENCY H OPPING D EMODULATOR B ASED ON P OLYPHASE F ILTER B ANKS fred harris, Xiaofei Chen, Elettra Venosa SDR’11-WInnComm 29 Nov 1
W HY S HOULD WE DO D IGITAL F REQUENCY H OPPING (FH)? To Fit Legacy FH Modems into the Evolving Wireless Standards To Enjoy the Benefits Provided by Digital Implementation 2
T HE IDEA OF F REQUENCY H OPPING 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
T HE L EGACY FH D EMODULATOR 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
M AJOR D RAWBACKS OF THE L EGACY FH M ODEM Slow Acquisition Due to Serial Search Performance Degradation Due to Imperfect Synchronization Performance Degradation Due to Jamming 5
T HE I DEA OF C HANNELIZING FH S IGNAL Why not implement a bank of digital band-pass filters ? 6
M-P ATH C HANNELIZER This is a Bank of M Band-pass Filters! 7 But, … At the cost of 1 filter plus an M point FFT !!!
P ROPOSED D IGITAL F REQUENCY H OPPING D EMODULATOR 8
B ENEFITS OF O UR P ROPOSED D EMODULATOR 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 9 frequency diversity automatically.
S IMULATION R ESULTS : S IGNAL TO BE DEMODULATED Hop Frequency in MHz Frequency Hop Profile 5 0 -5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Hop Index Hop Time Sequence 1 Amplitude 0 -1 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Time Index Spectrum of Frequency Hopping Signal 2000 Magnitude 1000 0 -8 -6 -4 -2 0 2 4 6 8 10 Frequency in MHz
S IMULATION R ESULTS : D E - HOPPED T IME S ERIES De-hopped Time Series 6000 5000 4000 Time Index 3000 2000 1000 0 11 -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 Hopping Frequency / MHz
S IMULATION R ESULTS : Z OOM I NTO 1.2 MH Z H OPPING C ENTER Output Time Series on 1.2 MHz 1 Amplitude 0 -1 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 Time Index Signal Envelope 1.5 1 Amplitude 0.5 0 -0.5 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 Time Index 12
S IMULATION R ESULTS : 2 ND T IER C HANNELIZER O UTPUT FOR 1.2 MH Z H OPPING C ENTER FSK Signal Envelope at del f = -200 kHz 1.5 Amplitude 1 0.5 0 0 100 200 300 400 500 600 FSK Signal Envelope at del f = -40 kHz 1.5 Amplitude 1 0.5 0 0 100 200 300 400 500 600 Time Index FSK Signal Envelope at del f = 120 kHz 1.5 Amplitude 1 0.5 0 0 100 200 300 400 500 600 13 Time Index
C ONCLUSION 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 ! 14
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