FEC over Wireless Channels using Cryptographic Check Values as Coding Redundancy

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Universität Siegen

Institut für Digitale Kommunikationssysteme

Univ.
-
Prof. Dr. Christoph Ruland

Hölderlinstraße 3

D
-
57076 Siegen

http://www.dcs.uni
-
siegen.de

FEC over Wireless Channels using
Cryptographic Check Values as Coding
Redundancy


Belgrade, April 2, 2009

Nataša Živić

Wireless Communication Systems


Wireless communication systems have found heavy
deployments over recent years


Many popular applications have emerged for wireless
networks including, voice calls, SMS, MMS, web browsing
and video conferencing, to name a few.


These services not only needs to be provided but rather
provided in a secure and reliable manner


Our work focus on reliablity of data transfer, from source to
destination, using security mechanisms as a tool

18.04.2008

ITG
-
Fachgruppensitzung

2

Channel Coding & Cryptography


We consider the following two basic limitations of a wireless
channel,

1.
Wireless channel is inherently prone to transmission errors


Forward Error Correction (FEC) is used as a solution in those
scenarios where the feedback link is missing and thus Automatic
Repeat Request (ARQ) can not be used

2.
Wireless channel is prone to the problem of data security
too


Cryptography and digital signatures are used as a solution to these
problems


Cryptography solves the problems of eves
-
dropping & digital
signatures is a solution to repudiation by the sender

18.04.2008

ITG
-
Fachgruppensitzung

3

Joint Channel Coding & Cryptography


So far, data security and the correction of errors induced by
the transmission channel have been dealt separately


In this work we present a novel technique which combines
these two different areas together to give a joint solution to
both the problems discussed before


We use cryptography (in the form of cryptographic check
functions) to improve the BER over the channel


Moreover we use the channel coding to improve the results
of cryptography as well


We call this technique as Joint Channel Coding &
Cryptography

18.04.2008

ITG
-
Fachgruppensitzung

4

SOURCE

SOURCE

ENCODER

CHANNEL

ENCODER

MODULATOR

DEMODULATOR

CHANNEL

DECODER

SINK

Data Communication (Building Blocks)

SOURCE

DECODER

L
-
values

NOISY
CHANNEL

Data (Text, Audio, video)

Received Data

(probably with errors)

Objective

MODULATOR

CHANNEL

ENCODER

SOURCE

ENCODER

SOURCE

ENCRYPTOR

DECRYPTOR

DEMODULATOR

CHANNEL

DECODER

SOURCE

DECODER

SINK

L
-
values

L
-
values

Data (Text, Audio, video)

Received Data

(probably with errors)

NOISY
CHANNEL

CHANNEL DECODER

(SISO)

SOFT INPUT DECRYPTOR

SID block

L (1.bit) L (2.bit) L (3.bit)…

Soft

Input

L = L
-
Values

(Obtained from probability distribution)

Soft Input Decryption

Soft Input Decryption Algorithm

Channel

SISO Channel Decoder

Number of
trials exceeded
the max?

No

Source Decoder

No

Bit flipping

No

Successfully
verified data

Verification
successful?

Unsuccessfully
verified data
(contains errors)

Yes

L Values of
bits

Ordering of bits into an
increasing array of their |L|
-
values

Decryptor
(Verification)

Soft Input Decryption Process

Feedback information


about correct

decoded bits


CHANNEL DECODER

(SISO)

SOFT INPUT DECRYPTOR

SID block

L (1.bit) L (2.bit) L (3.bit)…

Soft

Input

Soft Input Decryption with Feedback

ma || na

block a

mb || nb

block b

a

b



b

a

b



b

a





a

b



b

u



V:



ma

mb

na

nb

CCF

CCF

key

key

CCF


Cryptographic Check
Function

Assembling message u for Transmission

Parallel Soft Input Decryption Algorithm

Demodulation

Channel

SID of Block a‘

SID of Block b‘


SISO Channel Decoding

Block a‘

Block b‘

BER
1,SID

BER
1,SID

SISO Channel Decoding

BER
feedback

SID of Block a‘

SID of Block b‘


BER
2,SID

S
t
e
p
1

S
t
e
p
1

Step 2

Step 3

BER
2,SID

BER
cd1

SISO Channel Decoding

Segmentation of u‘

(into blocks a‘ & b‘)

BER
feedback


Encoder:


-

Convolutional encoder (1/2)


BPSK modulation


AWGN channel


Decoder:


-

Convolutional MAP decoder (1/2 and 1/3)


Length of

SID Blocks: 192 bits (128 bit message and 64 bit cryptographic
check value)


8 (16) bits with the minimal absolute L
-
values are tested


E
b
/N
0

increased in steps of 0,5 dB


For each result (point of curves) 50 000 tests are done





Simulation Parameters

Simulation Results


Joint Soft Input Decryption and Source Channel Coding



Improvement of arithmetic efficiency of Soft Input Decryption method



Development of new strategies for Soft Input Decryption



Extension of Soft Input Decryption with Feedback to Turbo principle



Analysis of the impact of the channel encoder to Soft Input Decryption



SISO decryption




Summary




Thank you very much

for your attention!