Most of existing cooperative communications are designed based on the assumption that the receivers have the full knowledge of the Channel State Information (CSI), which inevitably increases the cost of frequent retraining of the receivers.However, the channel estimates may be unavailable in a fast-fading environment. At this time, practical schemes should be developed to solve this problem, so the differential modulations for decode and forward cooperation communications are considered. Although the CSI isn't needed in differential modulations, the performances of systems are not ideal. When combined with convolutional coding technology, the performances of differential modulations become better in a certain extent.%现存的协作分集通信系统都是在假设接收端完全掌握了信道状态信息(CSI)的情况下建立起来的,这就不可避免地增加了接收端对CSI的重传.然而,在某些实际情况下,由于衰落信道条件变化太快难以进行有效的信道估计或为了降低移动终端的复杂度和造价而免去信道估计,这时就必须研究其他方案来弥补这个不足.协作分集的差分调制模型可以解决这个问题.差分调制模型虽然不需要知道CSI,但是系统性能不是很理想.将卷积编码与差分调制相结合,就可以在很大程度上提高系统的性能.
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