光纖有線電視網路之數位訊號傳輸研究

Abstract

我們探討在光纖有線電視網路中，一顆雷射二極體可傳送的最大 QAM 頻道容量。此一容量是由雷射截波所引起的非線性失真與雷射本身的相對 強度雜訊（RIN）所限制。我們的研究方法包含了非線性頻譜之理論分析 、傳輸系統之電腦模擬、與實驗驗證。在實驗部份，我們使用向量任意波 形產生器產生 10 到 70 個 QPSK 及 16-QAM 上行訊號，以調變三顆受測 的雷射，分別是一顆有隔離與溫控的 DFB、和二顆無隔離與溫控的 Fabry-Perot 雷射。研究結果顯示，對一個供上行所使用的雷射，即便其 RIN 高達 -115 dB/Hz，仍可傳送超過 1000 個 QPSK 或 170 個 16-QAM 頻道。然而，若是採用會產生碰撞的媒介接取控制（MAC），此一容量將 大幅降低。若考慮最壞的情況：除了一個使用中的頻道之外，其他頻道皆 有 8 個碰撞，則 QPSK 頻道容量將只剩 125 個。這個結果對採用頻帶堆 疊以及碰撞式 MAC 的上行系統設計，有重要的影響。另一方面，同樣的 分析也可以應用在下行系統，其結果顯示，一個 RIN 為 -135 dB/Hz的雷 射，最多可以傳送 600 個 64-QAM 或 128 個 256-QAM 頻道。 針對 混合 AM-VSB/QAM 傳輸系統，我們則提出「前置編碼」技術，以解決因 AM-VSB 訊號所引起的非線性失真，對 QAM 訊號所造成的干擾。我們以雷 射截波為非線性的來源，用電腦模擬證明，前置編碼技術可以完全去除誤 碼率高居不下的問題。這個首次提出的創見，預期對未來的有線電視系統 將有極大的貢獻。 We investigate the ultimate QAM channel capacity of a laser diode in a hybrid fiber/coax network which is limited by the laser-clipping induced nonlinear distortions (NLD's) and the relative intensity noise (RIN). Our study includes a spectral analysis, a complete system simulation, and an experiment. In the experiment, up to 70 channels of vector-arbitrary-waveform- synthesizer generated upstream QPSK or 16-QAM signals were used to modulate an isolated/cooled DFB laser and two unisolated/ uncooled Fabry-Perot lasers, respectively. Our analytical results show that for an upstream laser diode, over 1000 QPSK channels or 170 16-QAM channels can be delivered, even in the presence of a high RIN level of -115 dB/Hz. However, these high capacities are reduced significantly when we consider the effect of collision-based medium access control (MAC) protocols. We found that, in the worst case condition (collisions occur in all but one channels), the ultimate QPSK channel capacity of an upstream laser diode is dramatically reduced from over 1000 to 125 for 8 collisions/channel. These results have important implications to systems transporting frequency-stacked return- path bands with or without collision-based MAC channels. As regard to the ultimate capacity of a downstream laser diode with a RIN level of -135 dB/Hz, we found that as high as 600 and 128 channels of 64-QAM and 256-QAM signals can be transported, respectively. Through computer simulations, we also propose and verify the use of a precoding technique to eliminate the NLD's in a QAM channel which is transported along with multiple AM-VSB channels by a laser diode. We show that, for the first time, the precoding technique can completely remove the clipping-induced bit-error-rate floor at a cost of a 3 dB signal-to-noise ratio reduction.