Zaid Hayyeh Department of Electrical Engineering and Computer Science University of Kansas, Lawrence, Kansas

Transcription

1 Exploiting OFDM for Covert Communication Zaid Hayyeh Department of Electrical Engineering and Computer Science University of Kansas, Lawrence, Kansas 1

2 Covert Communication To hide, with a low probability of detection (LPD), the transmission of information A covert signal can be embedded within an existing non-covert communication Human scalp to embed a hidden message Hidden in the flow of data packets transmitted over the internet 2

3 What is OFDM? Orthogonal Frequency Division Multiplexing Lower rate narrow band as opposed to high rate wide band Nearly ideal response across each sub-channel W f= N 3

4 What is OFDM? 4

5 What is OFDM? Mitigate Intersymbol Interference (ISI) ISI caused by multi-path and the non-ideal response of channels Sub-carriers are orthogonal, do not interfere with one another T cos 2 f k t k cos 2 f j t j =0 0 k j 5

6 Cyclic Prefix Insertion Cyclic Prefix (CP) or Guard Time (TG) Last 6 or 7 samples for extended CP Prepended to the front of the OFDM symbol Based on channel's time dispersion Help to mitigate ISI 6

7 Bit & Power Allocation QPSK, 16-QAM, 64-QAM Divide power equally amongst sub-carriers SER below 10-4 eliminated Adjust bits per channel (B ) i Adjust power per channel (Pi) N 1 R b = Bi T i=1 N P total = P i i=1 7

8 Bit & Power Allocation 8

9 Benefits/Applications of OFDM More efficient use of spectrum Increase channel capacity Mitigate ISI LTE (Long Term Evolution) AT&T, Verizon WiMAX (802.16) Sprint 9

10 Hypothesis Utilize an unused sub-channel for covert communication Edge channel or middle channel Show effects of a covert communication signal embedded within an OFDM based wireless waveform Show the performance of the covert communications system in the presence of the non-covert OFDM signal 10

11 Simulation Methodology All simulations in MATLAB 10,000 OFDM symbols, approximately 10 minutes to run 100 bit errors per simulation for covert/noncovert system System performance will be evaluated in biterror-rate (BER) SNR will be given in Eb/No 11

12 Parameter Values Slotted structure of OFDM 5 MHz specification, actual BW = 7.68 MHz 512 point IFFT 15 KHz sub-channels Normal CP 301 out of 512 utilized 512 samples/symbols 518 samples/symbol with CP 12

13 Assumptions Known channel state information (CSI) Ideal phase/frequency recovery Fixed modulation Covert system has knowledge of utilized subchannels Channel spacing Poor performing sub-channels 13

14 Transmitter/Receiver Pairs & Channels 14

15 15

16 Received Signals N 1 r t = T2 C k=0 1 k A kc cos 2 f k t 1k N 1 T2 C i=0 Bic cos 2 f i t i 2 3 i 2 1 C k Aks sin 2 f k t 1 k T 2 3 C i Bis sin 2 f i t 2i T n t Avc & Avs=0 ; Bic & Bis =0 except for i=v N 1 ' r t = T2 C k=0 2 k Akc cos 2 f k t k3 N 1 T2 C i=0 Bic cos 2 f i t 4 i 4 i 2 2 C k Aks sin 2 f k t k3 T C i Bis sin 2 f i t i T n t Avc & Avs=0 ; Bic & Bis =0 except for i =v 16

17 Rb,covert = 7.40 kbps, Channel = -152, Ʈ = 128 samples/sym, Eb,covert/Eb,non-covert = db 17

18 Description of Covert Symbol/Bit rate for BPSK Rb Synchronous offset Ʈ Location/ Channel number 256 to +256 Eb,covert/Eb,non-covert Eb/No 18

19 Synchronous Offset Ʈ Symbol Sample (N samples/symbol) 19

20 Comparison of BER Curve With and Without Covert for Increasing Noise (Rb,covert = 7.40 kbps, Channel = -152 Eb,covert/Eb,non-covert = db, Ʈ = 128 samples/sym) 20

21 Effect of Increasing Covert Power on Non-Covert OFDM Signal (Rb,covert = 7.40 kbps, Channel = -152, Ʈ = 128 samples/sym) 21

22 Effect of Increasing Covert Power on Non-Covert OFDM Signal 22

23 Effect of Increasing Covert Power on Non-Covert OFDM Signal 23

24 Effect of Synchronous Offset on Non-Covert OFDM Signal Synchronous Offset ( Ʈ) Vs. Non-Covert BER (Rb,covert = 7.40 kbps, Channel = -152, Eb,covert/Eb,non-covert = db) 24

25 Effect of Synchronous Offset on Non-Covert OFDM Signal Synchronous Offset ( Ʈ) Vs. Non-Covert BER (Rb,covert = 7.40 kbps, Channel = -105, Eb,covert/Eb,non-covert = db) 25

26 Effect of Spectral Position on Covert Signal (Rb,covert = 7.40 kbps, Ʈ = 128 samples/sym) 26

27 Effect of Spectral Position on Covert Signal ( Ʈ = 128 samples/sym) 27

28 Effect of Synchronous Offset on Covert (Channel -154, Eb,covert/Eb,non-covert = db) 28

29 Effect of Spectral Synchronous Offset on Covert (Channel -105, Eb,covert/Eb,non-covert = db) 29

30 Received PSD at Non-Covert Receiver for Channel -105 (Eb,covert/Eb,non-covert = db, Rb =.925 kb/s) Covert Channel

31 Conclusion Demonstration of the feasibility of the concept Covert is hidden or difficult to detect Covert has little or no effect on non-covert system Location of covert is an unused sub-channel Synchronous offset has negligible effect Power of covert can achieve SER less than or equal to non-covert 31

32 Conclusion Effective covert system Covert synchronous offset can be ignored if... Covert power is set to achieve SER equal to or less than SER of non-covert system (10-4) This also aids in maintaining covert characteristic Bit rate several times below max allowed by sub-channel (935 bps) At least a few sub-channels away from utilized non-covert OFDM sub-channels BER of covert is too high for sub-channels next to those utilized by the non-covert OFDM system These conditions will allow covert communication to be successful 32

33 Future Work System for adjusting synchronous offset to achieve improved BER Study effect on most adjacent sub-carriers Study covert signal utilizing other symbol constellations Adaptive coding and modulation to enhance ability to hide covert 33

34 Thank You Questions? 34