We present a multiuser cooperative transmission scheme with opportunistic network coding (CTONC), which can improve system performance. In contrast to direct transmission and conventional cooperative transmission, the CTONC allows the relay node to decide whether or not to help do cooperation and employ network coding based on the limited feedback from the destinations. It will not help a transmission pair unless its direct transmission fails. This enables CTONC to make efficient use of the degrees of freedom of the channels. We derive and analyze the diversity-multiplexing tradeoff for the CTONC over Rayleigh fading channels at finite signal-to-noise ratios (SNRs). Theoretical analysis and numerical results show that the proposed scheme achieves better performance gain in terms of average mutual information, outage probability, and finite-SNR diversity-multiplexing tradeoff.

Cooperative transmission has recently received significant attention, both from academia and industry, as a new approach to achieve spatial diversity gains, increase coverage [

Recently, there have been some studies incorporating network coding [

In this paper, we consider a cooperative network with two sources, one relay and two destinations, and study the relay cooperation assuming DF cooperation as it offers the design flexibility for implementing network coding. In such system, each node is equipped with one antenna, and the finite-SNR diversity-multiplexing tradeoff is achieved across multiple nodes via the cooperation with opportunistic network coding. “Opportunistic” means that the relay decides whether or not to help do cooperation for the

Multiuser cooperative transmission scenario.

The rest of this paper is organized as follows. Section

We consider a cooperative network with two sources, one relay and two destinations (see Figure

The source node

For simplicity, we assume all the transmit nodes have a common transmit power, which is denoted as

In this section, we describe the CTONC scheme that exploits limited feedbacks from the destinations, for example, a single bit indicating the success or failure of the direct transmission, which can remarkably improve spectral efficiency. The proposed scheme is described in the wireless cooperative network in details as follow.

During the first and second time slots, the node

The relay takes different actions based on different status of the direct transmissions of the two

If the direct transmission of only one of the two pairs succeeds, during the next coming time slot, the relay will decode, reencode, and forward what it receives from the source of the pair whose direct transmission fails towards the corresponding destination. For cooperative diversity transmission, after the source

If the direct transmissions of both pairs fail, the relay will perform network coding (XOR operation) on the data it receives from both

In this section, we investigate the diversity-multiplexing tradeoff of the CTONC scheme over Rayleigh fading channels. While the conventional definitions of diversity and multiplexing gains of a system refer to asymptotic quantities as the SNR approaches infinity [

We denote

If only the direct transmission of

In the case, two pairs of direct transmissions fail, the relay will perform cooperative transmission with network coding. The transmissions, from the source to the destination and from the cooperative relay to the destination, can be viewed as parallel channels [

To obtain diversity-multiplexing gain tradeoff for CTONC at finite SNR, we need to compute the probability of outage and its derivative. We consider

Substituting (

Using efficient nonlinear programming techniques, the computational time of

In this section, we demonstrate the performance of the CTONC scheme. For the simplicity, we assume that the pairs

Figure

Mutual information comparison of the three schemes.

Outage probability comparison of the three schemes.

In Figure

Finite-SNR diversity-multiplexing comparison of the three schemes.

The outage probability versus SNR for the three schemes is illustrated at different multiplexing gains in Figure

Outage probability comparison of the three schemes with different multiplexing gains.

In this paper, we propose the opportunistic network coding for relay cooperative transmissions to provide a feasible method for performance improvement and study mutual information, outage probability, and the finite-SNR diversity-multiplexing tradeoff. On the whole, the proposed CTONC scheme outperforms other schemes in terms of mutual information and outage probability. For low multiplexing gains, the CTONC scheme achieves larger diversity gains than other schemes at finite SNRs. As a future work, we will extend the proposed scheme to cooperative MIMO in the context of cellular mobile systems, consider fractional Gaussian noise and further investigate performance gains.

Obviously,

Similarly, the probability of outage

This work has been supported by National S&T Major Program (no. 2010ZX03003-003-01), the National Natural Science Foundation of China (no. 61070204), and the Fundamental Research Funds for the Central Universities (BUPT2010PTB0503), the Open-ended fund program of Guangxi Key Lab of Wireless Wideband Communication & Signal Processing (no. 21106).