Cooperative Diversity in Wireless Systems Elzbieta Beres June 24 th, 2005

2 Outline  Nature of the Wireless Channel  Diversity – what is it and how does it help ?  MIMO Systems Traditional multi-antenna systems Cooperative diversity and relaying Challenges

3 Attenuation in a Wireless Channel  Large-scale Distance ~ (deterministic) Shadowing - Large obstructions (random)  Small-scale fading Many signal paths due to reflections Paths may add in phase or out of phase (Random)

4 Wireless Channel

5 Spatial Diversity  Independent paths to the destination  Increases probability that at least one path is not in a low fade  Significantly improves performance

6 Spatial Diversity – Performance

7 Traditional MIMO Systems  Multiple antennas at receiver and transmitter  Not sure that multiple antennas can be implemented on mobiles  Cannot overcome shadowing

8 Cooperative Diversity and Relaying  Independent paths can be created using other terminals  Can also overcome shadowing!

9 Half-duplex constraint  The relay must be assigned an additional orthogonal channel c2c2 c1c1 c1c1

10 When does relaying help?

11 Main challenges  How should relays send information? Amplify and forward? Decode and forward? Decode and re-encode?  How can multiple relays be incorporated into the system? How will they interact?

12 Questions ?