1. Physical Layer Security in Multiple Antenna Systems
Dr. Des McLernon
School of Electrical and Electronic Engineering. University of Leeds. UK
27 Nov 2011

2. School of Electronic and Electrical Engineering
Contents
A brief introduction to physical layer security.
Basic concepts and some techniques that enable physical layer security.
Outage Probability Based Power Distribution Between Data and Artificial Noise for Physical Layer Security.
Physical Layer Security of MIMO-OFDM Systems by Beamforming and Artificial Noise Generation
Conclusions.
Requirements to fullfill this characterisitcs
School of Electronic and Electrical Engineering
11/7/2018

3. Security on Wireless Networks
Wireless communications have changed personal communications and how people communicate each other allowing easy access to information everywhere, at anytime and with freedom of mobility.
Overuse of the radio channel
Cheating identities
Data alteration
Requirements to fullfill this characterisitcs
Eavesdropping
Jamming
Due to its broadcast nature, wireless communications have a great drawback: security. Many types of security vulnerabilities affecting wireless networks are jamming, data alteration, cheating identities, and eavesdropping.
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4. What is Physical Security Layer?
Physical layer security – an emerging new technique that improves secrecy by exploiting the characteristics of the wireless channel.
How is physical layer achieved?
By using techniques operating in physical layer such as multiple antennas schemes, modulation/coding techniques, diversity, multiple access schemes.
Why is more security necessary?
Existing security techniques are not completely secure, so security could be improved by opportunistically exploiting the secrecy possibilities that the wireless channel offers.
Which are the current used techniques?
Cryptographic algorithms working on presentation layer – public key encryption (e.g., RSA – no key transfer), DES (key transfer - broken in 1 day, 2008).
Which kind of attacks does Physical Layer Security prevent?
Mainly Eavesdropping and Jamming.
Where does it operate?
In the physical layer, but also could involved multi-layer approaches.
Requirements to fullfill this characterisitcs
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11/7/2018

5. Is it possible that Alice and Bob have a private conversation?
Eavesdropping
Requirements to fullfill this characterisitcs
Eavesdropping occurs when a non-authorized party hears a secret conversation between two nodes in the network and recover sensitive secret information broadcasted by the transmitter.
Is it possible that Alice and Bob have a private conversation?
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6. Two approaches against Eve.
Physical layer security vs. Cryptography.
Physical Layer
Cryptography
Advantages
Offers theoretical perfect secrecy.
Is secure enough for majority of applications.
Does not consider computational restrictions.
Offers compatibility and is already available.
Introduces Secrecy Capacity as performance metric.
Disadvantages
Only offers a probability of secrecy for passive eavesdropping cases.
Is not completely secure.
Is highly computationally complex.
Is not widely available.
Requires key interchange.
Does not offer a precise performance metric.
Requirements to fullfill this characterisitcs
Good News: both techniques can co-exist and improve the overall security of the system.
Bad News: The overall cost of deploying the system will increase.
School of Electronic and Electrical Engineering
11/7/2018

7. School of Electronic and Electrical Engineering
Contents
Brief introduction to physical layer security.
Basic concepts and some techniques that enable physical layer security.
Outage Probability Based Power Distribution Between Data and Artificial Noise for Physical Layer Security.
Physical Layer Security of MIMO-OFDM Systems by Beamforming and Artificial Noise Generation.
Conclusions.
Requirements to fullfill this characterisitcs
School of Electronic and Electrical Engineering
11/7/2018

8. Wiretap Channel and Secrecy
Non-degraded Gaussian Wiretap Channel
Spatial diversity in the system based on the different geographical location of each node, and acting as a virtual antenna array
element experiences unique channel characteristics, so spatial diversity can be achieved at the destination use of the different signals from nodes
Data rate decrease as result of reduction of transmission recoruses.
Secrecy is achieved if the main channel is “better” than the eavesdropper’s channel.
Secrecy capacity (CS) is given by the difference of both channels capacity, and it is the supreme secrecy data rate (RS) at which the eavesdropper is unable to decode any information.
School of Electronic and Electrical Engineering
11/7/2018

9. Wiretap Channel and Secrecy
Let’s get an example
CM=5
Spatial diversity in the system based on the different geographical location of each node, and acting as a virtual antenna array
element experiences unique channel characteristics, so spatial diversity can be achieved at the destination use of the different signals from nodes
Data rate decrease as result of reduction of transmission recoruses.
CW=3
Transmission rate R = 5. Secrecy Rs=Cs=2 (Max Rs)
Transmission rate R = 4. Secrecy Rs=1.
Transmission rate R = 3. No Secrecy.
All capacities are given in bps/Hz.
School of Electronic and Electrical Engineering
11/7/2018

10. Passive Eavesdropping?
Spatial diversity in the system based on the different geographical location of each node, and acting as a virtual antenna array
element experiences unique channel characteristics, so spatial diversity can be achieved at the destination use of the different signals from nodes
Data rate decrease as result of reduction of transmission recoruses.
Eavesdropper hears “silently” the conversation between Alice and Bob. No information regarding Eve is available at Alice. ?
It is not possible to calculate the secrecy capacity, so perfect secrecy cannot be guaranteed.
For passive eavesdropping other approaches are used to define secrecy:
Outage formulation of secrecy (Probability of Secrecy)
Secrecy based in guaranteeing QoS constraints. (Target SNR at Bob and Eve)
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11. Some enabling techniques
Beamforming is a multiple antenna technique that adjusts the strength of the transmitted/received signal. It is the optimum strategy in MISO systems for maximizing the secrecy capacity.
Spatial diversity in the system based on the different geographical location of each node, and acting as a virtual antenna array
element experiences unique channel characteristics, so spatial diversity can be achieved at the destination use of the different signals from nodes
Data rate decrease as result of reduction of transmission recoruses.
Artificial Noise Generation improves the probability of secrecy in passive eavesdropping scenarios by broadcasting isotropically artificial noise over the null space of the channel between Alice-to-Bob.
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12. Some enabling techniques
OFDM – Orthogonal Frequency Division Multiplexing. Data is passed through a serial-to-parallel convertor that splits the signal into a number of parallel channels to then be modulated by the IDFT. N N
N+CP
Adding Cyclic Prefix
Symbol
Mapping
Serial to Parallel
Parallel to Serial
IDFT
DAC
RF Mod
Spatial diversity in the system based on the different geographical location of each node, and acting as a virtual antenna array
element experiences unique channel characteristics, so spatial diversity can be achieved at the destination use of the different signals from nodes
Data rate decrease as result of reduction of transmission recoruses.
Elegant way to deal with the effects of a time dispersive channel (multipath).
Frequency selective channels (L taps) moved to m parallel flat fading channels.
Efficient implementation by FFT.
It allows adaptive modulation/power allocation per sub carrier.
Increase security by transmitting over N flat fading channels.
It is robust to interference.
School of Electronic and Electrical Engineering
11/7/2018

13. School of Electronic and Electrical Engineering
Contents
Brief introduction to physical layer security.
Basic concepts and some techniques that enable physical layer security.
Outage Probability Based Power Distribution Between Data and Artificial Noise for Physical Layer Security.
Physical Layer Security of MIMO-OFDM Systems by Beamforming and Artificial Noise Generation.
Conclusions.
Requirements to fullfill this characterisitcs
School of Electronic and Electrical Engineering
11/7/2018

14. School of Electronic and Electrical Engineering
The general idea
In this section we address physical layer security in MISO communications in the presence of passive eavesdroppers.
Spatial beamforming and artificial noise broadcasting are chosen as the strategy for secure transmission.
An optimum power allocation strategy between transmitted information and artificial noise is proposed.
The aim is to guarantee a given probability of secrecy, defined by quality of service constraints at the receiver and at the eavesdroppers.
NDMA – Multiplicity - multilayer protocol because it explores MAC functionality by taking advantage of retransmissions
ALLIANCES – ALLow Improved Access in the Network via Cooperation and Energy Savings
Low rate transmission Cooperative Transmission Epoch – CTE
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15. Transmit Strategy System Model Transmitted Signal
Information power
Single Antenna
Artificial Noise power
Single Antenna
SNR at Bob and the kth Eve
Multiple Antenna
NDMA – Multiplicity - multilayer protocol because it explores MAC functionality by taking advantage of retransmissions
ALLIANCES – ALLow Improved Access in the Network via Cooperation and Energy Savings
Low rate transmission Cooperative Transmission Epoch – CTE
Beamformer and artificial noise
principal eigenvector of
Single Antenna
Passive eavesdropping:
Perfect CSI available at Alice.
Assuming Eve’s channel distribution and statistics.
Artificial Noise Vector
Artificial Noise Cov. Matrix
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16. Power Allocation Problem and Secrecy Constraints.
To find the optimal power allocation strategy to guarantee a given probability of secrecy β є [0; 1) satisfying the quality constraints γb ,γe.
QoS Constraint at Bob
Secrecy Achieved whether:
SNR at Bob is above the target QoS
AND
SNR at Eve is below the given QoS bound.
QoS Constraint at kth Eve
Given Probability of Secrecy
NDMA – Multiplicity - multilayer protocol because it explores MAC functionality by taking advantage of retransmissions
ALLIANCES – ALLow Improved Access in the Network via Cooperation and Energy Savings
Low rate transmission Cooperative Transmission Epoch – CTE
After re-writing the probability of secrecy constraint in terms of a random Hermitian quadratic form and its cumulative distribution function (CDF), the original problem and the solution respectively are
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17. Results
Trade off of guaranteeing high probability of secrecy is a reduction in data throughput
Probability of Secrecy can be guaranteed
NDMA – Multiplicity - multilayer protocol because it explores MAC functionality by taking advantage of retransmissions
ALLIANCES – ALLow Improved Access in the Network via Cooperation and Energy Savings
Low rate transmission Cooperative Transmission Epoch – CTE
Achieved probability of secrecy (β) when K=3 and Nt=5 for both unconstrained and constrained (Pmax=6) transmit power.
Normalized secrecy throughput in attempting to achieve a target probability of secrecy (β) when K=3 and Nt=5 for both unconstrained and constrained (Pmax = 6) transmit power systems.
School of Electronic and Electrical Engineering
11/7/2018

18. Results
Higher probability of secrecy request more power at the transmitter.
NDMA – Multiplicity - multilayer protocol because it explores MAC functionality by taking advantage of retransmissions
ALLIANCES – ALLow Improved Access in the Network via Cooperation and Energy Savings
Low rate transmission Cooperative Transmission Epoch – CTE
Throughput can be improved providing more power at the transmitter.
Normalized secrecy throughput for different maximum power (Pmax) available at transmitter when K=3, Nt=5, and target probability of secrecy β=0.8 and 0.9 for both unconstrained and constrained transmit power systems.
Power distribution between information and artificial noise for achieving a target probability of secrecy (β) in an unconstrained transmit power system when K=3 and Nt=5.
School of Electronic and Electrical Engineering
11/7/2018

19. It is worth to pointing out…
The introduced effective power distribution guarantees a given probability of secrecy defined by the QoS constraints both at the intended receiver and at the eavesdroppers.
The power allocation is given by simple closed-form expressions.
Simulation results show that this strategy is energy consumption efficient.
The proposed outage probability-based approach compares favourably to an existing second-order-statistic-based technique.
For the constrained power case, there is a trade off between secrecy throughput and achieving a high probability of secrecy that can be improved by augmenting the total amount of power available at the transmitter.
NDMA – Multiplicity - multilayer protocol because it explores MAC functionality by taking advantage of retransmissions
ALLIANCES – ALLow Improved Access in the Network via Cooperation and Energy Savings
Low rate transmission Cooperative Transmission Epoch – CTE
School of Electronic and Electrical Engineering
11/7/2018

20. School of Electronic and Electrical Engineering
Contents
Brief introduction to physical layer security.
Basic concepts and some techniques that enable physical layer security.
Outage Probability Based Power Distribution Between Data and Artificial Noise for Physical Layer Security.
Physical Layer Security of MIMO-OFDM Systems by Beamforming and Artificial Noise Generation.
Conclusions.
Requirements to fullfill this characterisitcs
School of Electronic and Electrical Engineering
11/7/2018

21. School of Electronic and Electrical Engineering
The general idea
In this section we address physical layer security in MIMO-OFDM frequency selective wireless channels in the presence of a passive eavesdropper.
Spatial beamforming and artificial noise generation are chosen as the strategy for secure transmission.
The contribution of channel frequency selectivity to improve secrecy is studied by performance and probabilistic analysis.
It is investigated the capability of the eavesdropper to jeopardize the security of the system based on the knowledge that it might have regarding the transmission strategy.
NDMA – Multiplicity - multilayer protocol because it explores MAC functionality by taking advantage of retransmissions
ALLIANCES – ALLow Improved Access in the Network via Cooperation and Energy Savings
Low rate transmission Cooperative Transmission Epoch – CTE
School of Electronic and Electrical Engineering
11/7/2018

22. Artificial Noise Covariance
Transmit Strategy
Multiple Antenna
Multiple Antenna
Passive eavesdropping:
Perfect CSI available at Alice.
No information regarding Eve’s CSI.
NDMA – Multiplicity - multilayer protocol because it explores MAC functionality by taking advantage of retransmissions
ALLIANCES – ALLow Improved Access in the Network via Cooperation and Energy Savings
Low rate transmission Cooperative Transmission Epoch – CTE
Multiple Antenna
Transmitted Signal
Power per Subcarrier
Fraction of power for AN
Beamformer and artificial noise
principal eigenvector corresponding to the largest eigenvalue of
Artificial Noise
Artificial Noise Covariance
Linear combination of Nt-1 remaining eigen vectors
Power equally distributed among the remaining eigen vectors
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23. Transmit Strategy – Block Diagram
Block diagram at the transmitter. Nt Nt Nt
NDMA – Multiplicity - multilayer protocol because it explores MAC functionality by taking advantage of retransmissions
ALLIANCES – ALLow Improved Access in the Network via Cooperation and Energy Savings
Low rate transmission Cooperative Transmission Epoch – CTE Nt Nt Nt
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11/7/2018

24. Power Allocation
Power Constrained system:
Power requested for Cyclic Prefix is considered.
NDMA – Multiplicity - multilayer protocol because it explores MAC functionality by taking advantage of retransmissions
ALLIANCES – ALLow Improved Access in the Network via Cooperation and Energy Savings
Low rate transmission Cooperative Transmission Epoch – CTE
Power allocation per subcarrier given by the water-filling algorithm
Total Power considering CP
Subcarriers for transmission
mth channel power to noise ratio.
Power allocation for artificial noise:
Guaranteeing a target SNR at Bob:
Target SNR at Bob
AWGN power
Equally distribution of the power
(Maximizes in average the SC)
Manual allocation
Channel Power (Principal Eigen Value)
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25. Receive Strategy Beamformer at Bob given by MRC
Signal received at Bob
Eve using MMSE:
Eve somehow is aware of power allocation, transmitting beamforming vector, artificial noise covariance matrix.
Bob signal after beamforming
Signal received at Eve
NDMA – Multiplicity - multilayer protocol because it explores MAC functionality by taking advantage of retransmissions
ALLIANCES – ALLow Improved Access in the Network via Cooperation and Energy Savings
Low rate transmission Cooperative Transmission Epoch – CTE
Eve signal after beamforming
Beamformer at Bob given by MRC
Bob using MRC:
Due to the artificial noise is cancelled, MRC is similar to MMSE. The difference is only the AWGN term.
Beamformer at Eve given by MMSE
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26. Artificial Noise Mitigation by ZF
Eve using ZF:
Eve somehow is aware of transmitting beamforming vector.
Mitigating Artificial Noise Effect
Even though that MMSE is the best strategy for maximizing the SNR, Eve can mitigate the effect of the artificial noise by using ZF and then obtain a better performance compromising the security of the system.
Signal received at Eve
NDMA – Multiplicity - multilayer protocol because it explores MAC functionality by taking advantage of retransmissions
ALLIANCES – ALLow Improved Access in the Network via Cooperation and Energy Savings
Low rate transmission Cooperative Transmission Epoch – CTE
Eve signal after beamforming
Beamformer at Eve given by ZF
If Ne ≥ Nt
Yields to an identity matrix
AN orthogonal to beamformer
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27. Artificial Noise Mitigation by ZF
Ne < Nt
The resulting channel is a fat matrix. The product of the channel pseudo inverse and the channel does not yield to an identity matrix. Thus the artificial noise cannot be completed cancelled. However, this method allows to mitigate the effect of the artificial noise.
Eve using ZF:
Eve somehow is aware of transmitting beamforming vector.
Signal received at Eve
NDMA – Multiplicity - multilayer protocol because it explores MAC functionality by taking advantage of retransmissions
ALLIANCES – ALLow Improved Access in the Network via Cooperation and Energy Savings
Low rate transmission Cooperative Transmission Epoch – CTE
Eve signal after beamforming
Beamformer at Eve given by ZF
If Ne < Nt
Does not yields to an identity matrix
AN is mitigated but is not completely cancelled
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28. Probability of Secrecy
Passive eavesdropping
Alice is not aware of Eve’s CSI.
NDMA – Multiplicity - multilayer protocol because it explores MAC functionality by taking advantage of retransmissions
ALLIANCES – ALLow Improved Access in the Network via Cooperation and Energy Savings
Low rate transmission Cooperative Transmission Epoch – CTE
For passive eavesdropping scenario Alice cannot determine the system’s secrecy capacity, thus perfect secrecy cannot be guaranteed.
An alternative way to define secrecy is by introducing the concept of outage probability.
Probability of achieving secrecy between Alice and Bob is the likelihood that information on the main link can be transmitted secretly at a certain rate C.
Given data rate that defines secrecy
Bob’s channel Capacity
Bob’s channel Capacity
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29. Results – System Performance
Gap between Bob’s and Eve’s SNR increases
Increasing the number of OFDM subcarriers increase system secrecy.
Gap between Bob’s and Eve’s SNR smaller by ZF
NDMA – Multiplicity - multilayer protocol because it explores MAC functionality by taking advantage of retransmissions
ALLIANCES – ALLow Improved Access in the Network via Cooperation and Energy Savings
Low rate transmission Cooperative Transmission Epoch – CTE
ZF outperforms MMSE due the artificial noise cancelation
Flat fading Case
Average SNR at Bob and Eve vs. number of OFDM subcarriers (N) when Eve uses MMSE and ZF, є(m)=0.5, Nt=Nr=Ne=5 and L=4.
Average SNR at Bob and Eve vs. target SNR for different number of OFDM subcarriers, N=8, 32, 128, when Eve uses MMSE, Nt=Nr=Ne=5 and L=4.
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30. Results – Probability of Secrecy
Maximum allowable data rate given by SNR
Lower data rate supported with the same probability of secrecy
Probability of secrecy increase with N
Higher data rate is supported with the same probability of secrecy.
NDMA – Multiplicity - multilayer protocol because it explores MAC functionality by taking advantage of retransmissions
ALLIANCES – ALLow Improved Access in the Network via Cooperation and Energy Savings
Low rate transmission Cooperative Transmission Epoch – CTE
Increasing the number of antennas at Eve compromise secrecy.
Achieved probability of secure communications with data greater than C vs. target data rate C when Eve uses MMSE, target SNR at Bob is SNR=7,14,21, Nt=Nr=Ne=5, N=8, 32, 64, 128 and L=4.
Achieved probability of secure communications with data greater than C vs. target data rate C for different number of antennas at Eve Ne=2,5,10,25,50 when Eve uses MMSE, target SNR at Bob is SNR=10, Nt=Nr=5, N=8 and L=4.
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31. Effect of Artificial Noise Cancellation
High artificial noise power
No power for artificial noise
Even when it is not possible to complete cancel the artificial noise, ZF outperforms MMSE.
NDMA – Multiplicity - multilayer protocol because it explores MAC functionality by taking advantage of retransmissions
ALLIANCES – ALLow Improved Access in the Network via Cooperation and Energy Savings
Low rate transmission Cooperative Transmission Epoch – CTE
Ne=3
ZF outperforms MMSE due to the artificial noise cancellation
Average SNR at Bob and Eve vs. fraction of power for AN generation (є(m)) for different number of OFDM subcarriers (N=8, 16) when Eve uses MMSE and ZF, Nt=Nr=Ne=5 and L=4.
Achieved probability of secure communications with data greater than C vs. target data rate C for different number of antennas at Eve (Ne=3,5,8 ) when Eve uses MMSE and ZF, Nt=Nr=Ne=5 , N=16, and L=4.
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11/7/2018

32. It is worth to pointing out…
Frequency selectivity contributes positively to the secrecy of the MIMO-OFDM system through frequency diversity and opportunistic power distribution.
If the eavesdropper has:
a large number of antennas,
knows the main channel CSI (principal eigen vector),
uses zero forcing
then the secrecy of the system can be put at risk.
NDMA – Multiplicity - multilayer protocol because it explores MAC functionality by taking advantage of retransmissions
ALLIANCES – ALLow Improved Access in the Network via Cooperation and Energy Savings
Low rate transmission Cooperative Transmission Epoch – CTE
School of Electronic and Electrical Engineering
11/7/2018

33. School of Electronic and Electrical Engineering
Contents
Brief introduction to physical layer security.
Basic concepts and some techniques that enable physical layer security.
Physical Layer Security of MIMO-OFDM Systems by Beamforming and Artificial Noise Generation.
Outage Probability Based Power Distribution Between Data and Artificial Noise for Physical Layer Security.
Conclusion.
Requirements to fullfill this characterisitcs
School of Electronic and Electrical Engineering
11/7/2018

34. School of Electronic and Electrical Engineering
Conclusions
Physical layer security is an emerging technique that augment the security of wireless networks by exploiting the characteristics of the wireless channels.
Beamforming and artificial noise generation is an efficient transmit technique to improve the security for passive eavesdropping scenarios.
Frequency selectivity contributes positively to the secrecy of the system, however, the technical capabilities available at the eavesdropper might put at risk the security of the system.
The novel outage based power allocation technique guarantees a given probability of secrecy defined by quality of services constraints at both the intended receiver and the eavesdropper.
DF extra processing capabilities. AF and EF are simpler introduce an important gain to the system compared with a non cooperative scheme.
After considering the theoretical background,
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11/7/2018

35. Further information available on:
N. Romero-Zurita, M. Ghogho, and D. McLernon, “Physical Layer Security of MIMO Frequency Selective Channels by Beamforming and Noise Generation,” in EUSIPCO 2011 (19th European Signal Processing Conference), Barcelona, Spain, Aug
N. Romero-Zurita, M. Ghogho, and D. McLernon, “Physical Layer Security of MIMO-OFDM Systems by Beamforming and Artificial Noise Generation”, Physical Communication, 2011, doi: /j.phycom
N. Romero-Zurita, M. Ghogho, and D. McLernon, “Outage Probability Based Power Distribution Between Data and Artificial Noise for Physical Layer Security," IEEE Signal Processing Letters, 2011.
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36. School of Electronic and Electrical Engineering
Thank You
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