Zahoor Ahmed J.P.Cances V.Meghdadi Cryptographic Spread Spectrum Relay Communication NGMAST 2008

Break up Spread Spectrum Cryptography Relay Network (Orthogonal) Conclusions

NGMAST 2008 Introduction Spread Spectrum Cryptography to hide the Signal below noise level Message Encryption Doubly secured information Transmission through Relay Channel

NGMAST 2008 Spread Spectrum Spread spectrum is an RF communications system in which the baseband signal bandwidth is intentionally spread over a larger bandwidth by injecting a higher frequency signal Reason ? secure communications Makes jamming and interception harder prevent detection.

NGMAST t t t Data Signal PN Signal Data x PN T b =8T c TbTb TcTc ,-1,- 1,1,-1,1,1,-1

NGMAST 2008 Example

NGMAST 2008 The data transmitted with bit duration T b d(t)= d n  { 1,-1} PN Sequence N=T b /T c transmitted with chip duration T c The Data in channel Spread Spectrum

NGMAST 2008 Cryptography Message Secret key Cryptogram To recover the original message, the receiver computes

NGMAST 2008 SS and Cryptography To encrypt the message data or PN sequence Suppose we want to transmit the Message with SF N=T b /T c PN Sequence to synchronize bit and chip time, add S in M where is N time repetition of M

NGMAST 2008 Synchronization Problem Cryptogram Since the energy of SS is lower than ambient noise, so it is not possible to recover the message without the knowledge of S Problem ? Good cryptographic PN Sequences confronts the problem of synchronization.

NGMAST 2008 Suggestion To counter the problem of synchronization, we use a technique very like that of steganography We can spread N over a more larger bandwidth to mask the message inside ambient noise. We mask the secret SS signal with a classical dummy communication. This method is more robust than classical stegano.

NGMAST 2008 SS using BPSK Channel Let Be the non-secret message Then the transmitted signal will be let Be the hidden message Then the secret communication is

NGMAST 2008 SS using BPSK Channel Then the transmitted signal is The receiver uses non-secret communication to synchronization, recover the signal The secret message is smaller than the noise, so is not detectable if the PN sequence is not known to compute correlation

NGMAST 2008 SS and Orthogonal transmission If an error occurs on symbol M, this error induces very high noise on secret message m To avoid this problem we use a relay channel in which two signals are sent orthogonally, so now it is unlikely that that the non secret message can cause an error in the secret message. So now the transmitted signal will be Where

NGMAST 2008 Relay Network DestinationSource Relay

NGMAST 2008 Relay Network(Orthogonal)

NGMAST 2008 Relay network Capacity Achievability is proved by cut-set bound

NGMAST 2008 Conclusion SS Communications are difficult to detect. Encrypted PN sequences makes the communication more secure. The selection of good key make the communication absolutely secure Synchronization problem is addressed by Splitting the signal into phase and quadrature.

NGMAST 2008