Sunday, April 15, 2007

Cooperative Diversity for Wireless Networks

Cooperative diversity can emmulate an antenna array and then exploit spatial diversity in wireless networks with fading channels. It exploits the broadcast nature of the wireless medium and allows radios to jointly transmit information through relaying.

Traditionally, in ad hoc networks, packets are forwarded from the source to the destination through multi hops serially. The well-known capacity of wireless networks obtained by Gupta and Kumar in 2000 is with this strategy. By using Cooperative diversity, the source and the relay nodes may cooperate together for data transmission. The capacity of this kind of network has been investigated by Xie and Kumar.

The use of cooperative diversity is manifold:
1) Increase capacity (enlarge capacity region)
2) Improve reliability in terms of diversity gain
3) Diversity-multiplexing tradeoff

Different algorithms can be applied for cooperative communication, some examples are:

1) Amplify-and-Forward
In this case, relays just simply amplify the received signal with a linear transformation.
2) Decode-and-Forward
Relays first detect and decode the received signals and then re-encode them. This decoding and re-encoding procedure can be considered as a non-linear transformation.
3) Selection and dynamic relaying
In this case, cooperative terminals adapt their transmission format according to the measured channel state information.
4) Incremental relaying
This scheme exploits limited feedback from the destination terminal for relaying.

Incorporating the cooperative diversity into the network is still an open problem.

[1] J. Nicholas Laneman, "Cooperative diversity: models, algorithms and architectures," Cooperation in wireless networks, Springer, 2006

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