Department of Electrical & Computer Engineering Villanova University

Yimin Zhang, Villanova University 1

ECE 8708 Wireless Communications : Cooperative Diversity

Cooperative Diversity

Yimin Zhang, Ph.D.Department of Electrical & Computer Engineering

Villanova Universityhttp://yiminzhang.com/ECE8708



• Do you need cooperation?

• Fading and Diversity

• Cooperative Diversity Algorithms

• Cooperative Diversity Protocols

• Conclusion and Remarks

Outline



Do You Need Cooperation?

user 1 user 2

Base Station

Good channelBad channel

user 2

user 1

Good channel

User 1

User 2



Application Examples – Unmanned Vehicle Networks

Direct path

Relay path



Application Examples – Ad Hoc Networks



Application Examples – Sensor Networks

sink

Source








Outline



Propagation Fading in Wireless Communications

• Wireless Communication Environment– Noise– Multipath– Fading– Interference

• Demands– Multimedia applications ⇒ High rate– Data communication ⇒ Reliability



Propagation Fading in Wireless Communications



Diversity Techniques

Multiple antennas with a large separation receive signals with low correlation.

Multiple antennas with a large separation receive signals with low correlation.



Signal-to-noise ratio (SNR)

Bit e

rror

rate

(BER)

Flat fading channel,Rayleigh fading,

L = 1AWGN channel

(no fading)L = 2

L = 4 L = 3





We can also use multiple transmit antennas to achieve higher diversity gain.

In this case, space-time coding is usually applied to avoid interfering to each other.

We can also use multiple transmit antennas to achieve higher diversity gain.

In this case, space-time coding is usually applied to avoid interfering to each other.




A multiple-input-multiple-output (MIMO) system uses multipath to create parallel channels in the same band• Each transmit antenna transmits a unique data stream• Multipath scattering decorrelates the transmitted signals,

creates multiple, independent channels, which effectively increase the system capacity



Networked MIMO

For a MIMO system with m transmit antennas and n receiver antennas: • Diversity gain: mn• Data rate: min(m,n) log SNR








Outline



Cooperative Diversity

Scenario 1

The MIMO technique is good, but what happens if my terminal cannot equip multiple antennas?

….. Think about Cooperative Diversity!

Scenario 2

My terminal has several antennas, but all of them are experiencing deep shadowing fading.



Cooperating Systems

The user cooperates with other users and serves as a relay terminal for them.

Each transmit user receives an attenuated and noisy version of the partner’s transmitted signal and relays it



Cooperative Diversity Phases

Broadcast Phase: In this phase, the source users transmit information to relay terminals and the destination.

Relay Phase: In this phase, the users relay the information/ signals according to the specific cooperative scheme.



Cooperative Diversity Phases

Broadcast Phase: In this phase, the source users transmit information to relay terminals and the destination.

Relay Phase: In this phase, the users relay the information / signals according to the specific cooperative scheme.



Cooperative Diversity Algorithms – 1. Amplify-and-Forward

Tx2

Tx1 Rx

amplifier

Amplify and Forward



Cooperative Diversity Algorithms – 2. Decode-and-Forward

Tx2

Tx1 Rx

decode encode

Decode and Forward



Cooperative Diversity Algorithms – Comparison

Decode-and-ForwardRemove previous effect of noise and channel distortion. Higher complexity than amplify-and-forward. May raise security concerns.

Amplify-and-Forward

Preferred for simplicity.

The additive noise and channel distortion in the relay processes is accumulated and amplified.








Outline



Non-Cooperative Scheme

without cooperative diversity

Data rate = 1, diversity gain =1

Tx 1

Tx 2

Rx User 1 transmits

time

Ch. 1

Ch. 2 User 2 transmits



Repetition-Based Cooperative Diversity

After the source broadcasts its information to the destination and the relays, all the relays repeat this information in a sequential order.

Suitable for both the amplify-and-forward algorithm and the decode-and-forward algorithm.

Data rate = 1/M, diversity gain = M



Space-Time Cooperation Protocol – Scheme 1

Provides effective bandwidth use, particularly when the number of users is large.

Protocol requires decode-and-forward algorithm.

Not always implementable.

Data rate = ½ , diversity gain = M



Space-Time Cooperative Protocol – Scheme 2

The i-th user generates an MxM space-time codeword Xifrom the information.

At the M-th time block (last time block) all the users cooperate in transmitting the whole codeword Xi in the i-th user channel.

Data rate = flexible, diversity gain = M



Proposed Space-Time Cooperative Protocol

No information can be transmitted by the relay at this time because it doesn’t have the information.

Space-Time codeword(for single user with two Tx antennas)

Space-Time cooperative codeword (for two users with

one Tx antenna each)

time

antenna

time

user



Simulation Results

Two Rayleigh channels with variances [1, 0.01]

User 1

User 2





• Cooperative Diversity

• Proposed Technologies


Outline



Conclusion and Remarks - 1

• We have demonstrated the effectiveness of using cooperative diversity techniques in various wireless network applications.

• The significance of the diversity gain is particularly evident when part of the communication links is less reliable.

• The study on cooperative diversity is relatively new and many issues remain open for future research. For example, – Synchronization– Effective relaying in large scale sensor network– Location and energy distribution over relaying sensors



References

[1] A. Sendonaris, E. Erkip, and B. Aazhang, “User cooperative diversity – Part I and Part II,” IEEE Trans. Commun., vol. 51, no. 11, pp. 1927–1948, Nov. 2003.

[2] J. N. Laneman, D. N. C. Tse, and G. W.Wornell, “Cooperative diversity in wireless networks: effective protocols and outrage behavior,” IEEE Trans. Inform. Theory, vol. 50, no. 12, pp. 3062 – 3080, Dec. 2004.

[3] J. N. Laneman and G. W. Wornell, “Distributed Space-Time Coded Protocols for Exploiting Cooperative Diversity in Wireless Networks”, IEEE Trans. Inform. Theory, vol. 49, no. 10, pp. 2415–2425, Oct. 2003.

[4] G. Wang, Y. Zhang, and M. G. Amin, “Space-time cooperation diversity using high-rate codes,” International Symposium on Antennas and Propagation, Sendai, Japan, Aug. 2004.

Department of Electrical & Computer Engineering Villanova University

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