COOPERATIVE DIVERSITY

Cooperative diversity enables to achieve the antenna diversity gain by the use of the cooperation of distributed antennas belonging to each node.

Contents

Technology

System model

Decoding at the destination

Trade-off

See also

Systems

Technologies

Technology

System model

We consider a wireless relay system that consists of source, relay and destination nodes. 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, this system delivers information from the source to the destination node with two phase transmission.
On the first phase, the source node broadcasts information $x\_\{s\}$ toward both the destination and the relay nodes. The received signal at the destination and the relay nodes are respectively written as:
:$$
r_{d,s} = h_{d,s} x_{s} + n_{d,s}, quad
r_{r,s} = h_{r,s} x_{s} + n_{r,s}

where $h\_\{d,s\}$ is the channel from the source to the destination nodes, $h\_\{r,s\}$ is the channel from the source to the relay node, $n\_\{r,s\}$ is the noise signal added to $h\_\{r,s\}$ and $n\_\{d,s\}$ is the noise signal added to $h\_\{d,s\}$.
On the second phase, the relay transmits its received signal to the destination node.

Decoding at the destination

There is two fundamental ways to decode the signal at the destination which are stand-alone decoding and cooperative decoding.
For stand-alone decoding, the destination decodes the data just based on the received signal from the relay on the second phase, which results in the signal power boosting gain. The signal received from the relay node which retransmits the signal received from the source node is written as:
:$$
r_{d,r} = h_{d,r} r_{r,s} + n_{d,r}
= h_{d,r} h_{r,s} x_{r,s} + h_{d,r} n_{r,s} + n_{d,r}

where $h\_\{d,r\}$ is the channel from the relay to the destination nodes and $n\_\{r,s\}$ is the noise signal added to $h\_\{d,r\}$.
For cooperative decoding, the destination node combines two signals received from the source and the relay nodes which results in the diversity advantage. The whole received signal vector at the destination node can be modeled as:
:$$
mathbf{r} = [r_{d,s}, r_{d,r}]^T
= [h_{d,s}, h_{d,r} h_{r,s}]^T x_{s} + [1, sqrt{|h_{d,r}|^2+1}]^T n_{d}
= mathbf{h} x_{s} + mathbf{q} n_{d}

where $x\_\{d,s\}$ and $x\_\{d,r\}$ are the received signals from the source and relay nodes to the destination node, respectively and $n\_\{d,s\}$ and $n\_\{d,r\}$ are the noise signals added to the link paths from the source and relay modes to the destination node, respectively. As a linear decoding technique, the destination combines elements of the received signal vector as follows:
:$$
y = mathbf{w}^H mathbf{y}

where $mathbf\{w\}$ 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.

Trade-off

It is noteworthy that cooperative diversity can increase the diversity gain at the cost of lossing the wireless resource such as frequency, time and power resources for the relaying phase. Wireless resources are wasted since the relay node must use wireless resources to relay the signal between the source and destination nodes. Hence, it is important to remark that there is trade-off between the diversity gain and the waste of the spectrum resource in cooperative diversity.

See also

Systems

★ Cooperative wireless communications

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★ Wireless ad-hoc network

★
★ Mobile ad-hoc network

★
★ Wireless mesh network

★
★
★ Mesh network

★ Wireless community network

Technologies

★ Diversity schemes

★ Space–time code

★ Multiple-input multiple-output communications (MIMO)

★ Diversity combining

★ Transmit diversity

★ Diversity gain