Cooperative diversity is a cooperative multiple antenna technique for improving or maximising total network channel capacities for any given set of bandwidths which exploits user diversity by decoding the combined signal of the relayed signal and the direct signal in wireless multihop networks.
That is, cooperative diversity decodes the information from the combination of two signals.
The simplest cooperative relaying network consists of three nodes, namely source, destination, and a third node supporting the direct communication between source and destination denoted as relay.
If the direct transmission of a message from source to destination is not (fully) successful, the overheard information from the source is forwarded by the relay to reach the destination via a different path.
[3] The relaying strategies can be further distinguished by the amplify-and-forward, decode-and-forward, and compress-and-forward strategies:[4] Serial relay transmission is used for long distance communication and range-extension in shadowy regions.
For outdoors and non-line-of-sight propagation, signal wavelength may be large and installation of multiple antennas are not possible.
To increase the robustness against multi-path fading, parallel relay transmission can be used.
It is assumed that the channel is in a half-duplex, orthogonal and amplify-and-forward relaying mode.
Differently to the conventional direct transmission system, we exploit a time division relaying function where this system can deliver information with two temporal phases.
The received signal at the destination and the relay nodes are respectively written as: where
On the second phase, the relay can transmit its received signal to the destination node except the direct transmission mode.
In the direct scheme, the destination decodes the data using the signal received from the source node on the first phase where the second phase transmission is omitted so that the relay node is not involved in transmission.
The decoding signal received from the source node is written as: While the advantage of the direct scheme is its simplicity in terms of the decoding processing, the received signal power can be severely low if the distance between the source node and the destination node is large.
In the non-cooperative scheme, the destination decodes the data using the signal received from the relay on the second phase, which results in the signal power boosting gain.
The reliability of decoding can be low since the degree of freedom is not increased by signal relaying.
When we can take advantage of such a signal and increase in diversity order results.
The whole received signal vector at the destination node can be modeled as: where
As a linear decoding technique, the destination combines elements of the received signal vector as follows: where
is the linear combining weight which can be obtained to maximize signal-to-noise ratio (SNR) of the combined signals subject to given the complexity level of the weight calculation.
Adaptive scheme selects one of the three modes described above which are the direct, the non-cooperative, and the cooperative schemes relying on the network channel state information and other network parameters.
It is noteworthy that cooperative diversity can increase the diversity gain at the cost of losing the wireless resource such as frequency, time and power resources for the relaying phase.
In June 2005, A. Høst-Madsen published a paper in-depth analyzing the channel capacity of the cooperative relay network.
Using the max-flow min-cut theorem yields the upper bound of full duplex relaying where
Note that the max-flow min-cut theorem states that the maximum amount of flow is equal to the capacity of a minimum cut, i.e., dictated by its bottleneck.
Thus, the upper bound is rewritten as Using a relay which decodes and forwards its captured signal yields the achievable rate as follows: where the broadcast channel is reduced to the point-to-point channel because of decoding at the relay node, i.e.,
The capacity of the reduced broadcast channel is Thus, the achievable rate is rewritten as The capacity of the TD relay channel is upper-bounded by with In a cognitive radio system, unlicensed secondary users can use the resources which is licensed for primary users.
It is very challenging to sense the activity of spatially distributed primary users in wireless channel.
Spatially distributed nodes can improve the channel sensing reliability by sharing the information and reduce the probability of false alarming.
Such networks have been successfully deployed for military communication and have lot of potential for civilian applications, to include commercial and educational use, disaster management, road vehicle network etc.
Careful incorporation of relay cooperation into routing process can selects better communication links and precious battery power can be saved.