Performance of an orthogonal frequency division multiplexing (OFDM) system is greatest when the exact channel state information (CSI) is used for transmitter rate control and power allocation. However, in real systems CSI can only be approximately known. Moreover, in video communication, it can be difficult to use any CSI for rate control of a video codec if the channel changes significantly during a group of pictures coded jointly, such as when the receiver is moving. We address this issue through a hybrid digital–analog (HDA) coding system where a standard video codec is used to generate a fixed-rate base layer upon which the analog quantization error is superimposed as a refinement layer. The system adapts to channel variations by proper transmit-power allocation between digital and analog components and across OFDM subcarriers, based on CSI. We present a power allocation scheme for this system which explicitly takes into account the imprecise nature of the available CSI. Experimental results obtained with simulated OFDM channel traces show that proposed scheme is able to achieve a much better quality-vs-reliability trade-off in video transmission, compared to the best known digital-only and analog-only alternatives.

当确切的信道状态信息（CSI）用于发射机速率控制和功率分配时，正交频分复用（OFDM）系统的性能最高。但是，在实际系统中，CSI只能近似得知。此外，在视频通信中，如果在联合编码的一组图片期间，例如接收器移动时，频道发生显着变化，则很难使用任何CSI来控制视频编解码器的速率。我们通过混合数模（HDA）编码系统解决了这个问题，在该系统中，标准视频编解码器用于生成固定速率的基础层，在该基础层上将模拟量化误差叠加为优化层。该系统通过基于CSI的数字和模拟组件之间以及OFDM子载波之间的适当发射功率分配来适应信道变化。我们为该系统提出了一种功率分配方案，该方案明确考虑了可用CSI的不精确性。通过模拟的OFDM信道迹线获得的实验结果表明，与最著名的纯数字和纯模拟替代方案相比，该方案能够在视频传输中实现更好的质量与可靠性的权衡。