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In this paper, a modified access mechanism named Constrained-send DCF (CDCF) is proposed to improve the performance of IEEE 802.11 DCF protocol. It is found that, in DCF, the transmission probability is higher than the reasonable value when the node number is greater than 4 under basic access scheme or than 17 under RTS/CTS scheme, and it results in serious collision. To avoid collision of high access loading, a constrained-send probability is introduced at the end of each back off procedure for the station transmitting. The performance of this mechanism is analyzed based on a 2-Dimension Markov analytical model, after that the optimum constrained-send probability is derived. Numerical results show that the CDCF mechanism has much better performance than DCF with respect to both system throughput and average packet delay. And under RTS/CTS scheme, although CDCF slightly improve the throughput performance (due to the natural good throughput performance of RTS/CTS-scheme DCF), it leads to a much better average packet delay performance compared to DCF. The CDCF keeps all the features of the IEEE 802.11 DCF protocol and is quite easy to implement. 相似文献
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与目前普遍应用的IB…BP预测结构相比,在相同参考帧数目前提下,层次B帧预测结构对大部分序列,在相同码率情况下,其图像客观质量PSNR平均提高了1 dB左右。但是,对不同运动特性的图像序列,选择不同图像组大小的层次B帧预测结构编码,编码性能差异较大。基于此,提出了基于层次B帧预测结构的自适应图像组大小编码技术,并给出了具体的图像组大小判断方法和编码过程。实验结果表明,相对于固定图像组大小的层次B帧预测编码技术,对所测序列,本文提出的方法编码性能提高了0.60 dB左右。 相似文献
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