CDRE - 利用網路編碼以降低封包錯誤率的機制

Abstract

無線訊號具有強烈的衰減和干擾效應，相對於有線訊號來說，顯得不穩定且不可靠。傳輸時的位元錯誤率會隨著訊噪比的下降而增加，過去實體層發展出來的各種頻道編碼技術，則是在訊噪比無法提升時用來降低位元錯誤率。而在 IEEE 802 家族中，位元錯誤率和封包長度完全左右了封包本身的錯誤率，也就是說，當位元錯誤率和封包長度固定時，封包錯誤率也隨之固定了。使用較長的封包在高速網路裡有更佳的效能，但是，較長的封包會讓封包錯誤率提高，為了不讓封包錯誤率太高，IEEE 標準裡制定了各種網路技術的封包長度上限，這限制了可用的封包長度。 此篇論文提出一個基於網路編碼的傳輸率增強機制，此機制可在位元錯誤率固定時再降低封包錯誤率，在連線品質惡劣時，可以用來提升傳輸率和產量，而且對於即時或非即時性的傳輸都有效。然而，這方法需要額外的運算和記憶體作為代價。透過模擬，顯示此機制在實體層沒有頻道編碼的情形下，傳輸時的訊噪比需求可降低約 2.4dB；搭配迴旋編碼的情況下則可以降低 1.8dB。

Wireless signals have strong fading and interference effects, therefore they are unstable and unreliable compared with wired signals. The bit error rate (BER) grows when the signal-to-noise ratio (SNR) gets low. Various channel coding technologies in physical layer were designed to reduce BER under the same SNR. In IEEE 802 families, the packet error rate (PER) of MAC layer is decided by BER and packet length. That means PER will be fixed if BER and packet length are both decided. Larger packet length brings better performance in high speed networks. However, the PER grows with the increasing of packet length. To against PER to be high, IEEE standards specified the max packet lengths of various network technologies. This restricts the raising of packet length. In this thesis, we proposed a flexible coding scheme for delivery rate enhancement (CDRE) based on network coding, which brings a lower PER than before with the same BER. This scheme can be used to improve the delivery rate and throughput under a poor link quality, and applied to both traffic type of realtime and best-effort. Of course, it needs some additional memory and computations in return. Through the simulation, we will show that this scheme can reduce the SNR requirement of about 2.4dB for transmitting without any channel coding in physical layer, or 1.8dB in case of convolutional coding.