**4. Conclusions**

442 Wireless Communications and Networks – Recent Advances

Fig. 7. Application layer throughput (in bit/sec.) comparison among various schemes under

 2 10 , <sup>4</sup> 2 

illustrate the effective throughput results for various schemes under different Nakagami-*m*, *m*=1/2, 1, 3, slow-fading channels, and summarize the results in Table 2. From Table 2, it can be found that both analytical results based on (8)-(9) and simulation results of the optimized fast HARQ scheme also always have better throughput results than those of other schemes as in Table1. The results of the non-CD HARQ scheme for both Table 1 and Table 2 are the same since its performance only depends on channel-1's BER. We also notice that

In summary, based on Figs. 6 and 7 and Tables 1 and 2, we can thus conclude that the fast HARQ scheme is an excellent approach for transporting delay-constrained streaming-type or real-time multimedia flows in CD environments even when there is an error probability on the cooperative path. It is for the reasons that the retransmission strategy can be adaptively adjusted according to the channel condition and that the decoding procedure

10 , and <sup>3</sup>

3 

> 2

10 , as an example, to

10 in the sender-to-

Nakagami-3 slow-fading channels when channel-2 is not error-free.

1 

throughput improvement of our scheme is significant even with <sup>4</sup>

involving MRC and the Viterbi decoding are appropriately designed.

Last, for completeness, we take <sup>3</sup>

partner channel.

same as those in Fig. 6 due to the fact that MRC is much powerful.

3 10 , the performance results of the first 2 good schemes are almost the

that when <sup>3</sup> 1 

> A fast HARQ packet retransmission scheme has been successfully proposed to transport delay-sensitive flows in a general CD environment. The presented scheme incorporates 2 retransmission policies, and these 2 policies can be selected adaptively by the channel SNRs and the APDU sizes. In ideal conditions, the best retransmission policy can always be selected to achieve optimized performance.

> In this case study, our cooperative fast retransmission scheme has been shown to be an excellent approach for improving the effective throughput in transporting delay-sensitive flows in CD environments. Numerical results verified via simulations, show that when optimized, the proposed scheme can achieve effective throughput much better than that of other ARQ schemes (such as [26]) and non-CD HARQ schemes, especially when the senderto-partner channel condition is good. The performance improvement is still significant even when there is an error probability (e.g. BER < 10-4) in the sender-to-partner channel. Moreover, in the aspect of the battery saving, the presented scheme should save much more power than that of other schemes due to the one-time retransmission design. It is thus concluded that the proposed fast HARQ retransmission scheme is an excellent ARQ candidate for the multimedia or real-time transport in CD environments, when the timeconstraint is imposed.
