應用於高記錄密度光碟機上之非線性MLSD技術

Abstract

在高記錄密度磁光光碟機(Magneto-Optical Disk Drive)的設計過程中， 對於其讀寫通道(read/write channel)的了解是非常重要的．唯有準確的 讀寫通道模式，才有可能設計出最佳之接收器．在較低記錄密度時常用的 線性讀寫通道模式，並不適用於高記錄密度的磁光光碟機上．由於高記錄 密度所引起的非線性現象可能嚴重地影響接收器的效能，因此必須加以考 慮．本論文的第一部份處理磁光光碟機的通道辨識(identification)，我 們使用了線性及非線性的方法．在三種線性方法上，其結果並不相同．其 中第二及第三種結果顯示高記錄密度磁光光碟機的讀寫通道實際上是一種 非線性的通道．所以，我們使用非線性的方法重新再做一次通道辨識．從 均方值誤差(Mean-Squared Error)來分析，在本實驗中的非線性方法較線 性方法的均方值誤差明顯地小了約20倍．從數值結果來看，非線性通道辨 識方法可以提供較為準確的通道模式．論文的第二部份集中在最佳接收器 的設計，從非線性通道辨識所導出的非線性通道模式可以作為最佳接收器 的設計基礎．除了NRZ記錄方式外，另一種常用的(2,7)RLL碼也使用在實 驗上．這兩種非線性的Viterbi detectors在5*10^5 bits測試中，兩者都 沒有錯誤發生，但是以儲存設備的規格而言，錯誤率應該要到達10^-9． 這樣的要求不太可能藉由直接量測而獲知．因此我們使用了SAM( Sequenced Amplitude Margin)方法來評估上述兩種Viterbi detectors的 錯誤率．SAM分析的結果顯示，NRZ編碼的Viterbi detector大約可以到 達10^-6的錯誤率而(2,7)RLL編碼的Viterbi detector幾乎可以到達10^-9 的錯誤率．亦即，(2,7)RLL編碼以及本論文所提出非線性Viterbi detector的組合可以提供較原系統高出約15%的儲存容量提昇． To implement a high recording density MO (magneto-optical) disk drive, the priorknowledge about the MO read/write channel is needed. An accurate channel model will make an optimal system design possible. Linear MO channel models which are often used in low density recording systems are not suitable for describing the channel effect in a high density recording system. Nonlinear channel characteristic which is caused mainly by the interaction between the two closely allocated recorded domains must be taken into account. It is known that this nonlinear effect will seriously degrade theperformance of a linear receiver no matter how complicated the associated equalizer is. he first part of this thesis deals with the MO channel identification problem. Both linear and nonlinear methods are used. We find that the linear method using PN sequence as the input is not stable. Channel responses using three linear algorithms are not consistent. There are indications that strong nonlinearity does exit in the MO channel. We then use a Volterra-decomposed nonlinear model to identify MO channels. Numerical results show that this nonlinear model can charaterize the MO channel to a very high degree of accuracy. In fact, as far as mean-squared identification error (MSIE) is concerned, the nonlinear method yields MSIE 20 times smaller than that resulted from any linear model. The second part concentrates on the detector design, assuming a nonlinear channel characteristic. We derive a maximum-likelihood sequence detector and evaluate the resulting performance. Based upon the nonlinear channel model, we derive two nonlinear Viterbi detectors, one for an NRZ-coded (non- return zero) channel and the other for a (2,7) RLL-coded nonlinear MODD channel. Using the SAM (sequenced amplitude margin) scheme we are able to predict the error rate performance up to 10^-9 with only about 10^5 ~ 10^6 sample bits. Due to the practical limitation of hardware resolution for both the transimitter (pattern generator) and front-end digitizer (digital oscilloscope), there are only finite number of clock rate from which we can choose. Experiment results indicate that the error rate performance of the NRZ-coded nonlinear Viterbi detector is around 10^-6 while the (2,7) RLL-coded nonlinear Viterbi detector can be almost 10^-9. The selected recording density is at least 1.15 times higher than the original specification; in other words, there are almost 15% capacity enhancement for the (2,7) RLL-coded nonlinear Viterbi detector over the original linear detector.