軟性多路徑干擾消除

比較(6.11)與(6.12)式的最後一行，我們可以發現兩系統在展頻後的資料

6.5 模擬結果與討論

調變方式(modulation) QPSK

載波頻率(carrier frequency) 2 GHz 頻寬(total bandwidth) 5 MHz 次載波個數(number of subcarriers) 256 有效符元長度(useful symbol time) 51.2 us

護衛間隔長度(guard interval) 12.8 us 完整符元時間(overall symbol time) 64 us

展頻因數(spreading factor) 256 車速(vehicle speed) 120 km/hr 都普勒頻率(Doppler frequency) 222 Hz

路徑個數(path number) 2

最大延遲擴散(max delay spread) 3.2 us 通道匹配方式(Channel matching)

MRC，EGC，ZFC，

PEC with =0.5 MMSEC, Optimum PEC

圖(6.3)與(6.4)顯示了在雙路徑衰減通道下，CDMA 系統接收端使 效應(Ping-pong effect)，常見於使用平行干擾消除(Parallel Interference Cancellation)的 CDMA 系統[20]，其成因與消除的方法屬於另一方面的 課題，值得我們在未來進行更深入的研究。

圖 6.3 在雙路徑衰減通道下使用 MRC 與 EGC 通道匹配方式與 硬/軟性多路徑干擾消除技術之系統效能

圖 6.4 在雙路徑衰減通道下使用 ZFC 與 PEC 通道匹配方式與 硬/軟性多路徑干擾消除技術之系統效能

圖 6.5 雙路徑衰減通道下使用最佳化 PEC 與 MMSEC 通道匹配方式 與軟性多路徑干擾消除技術之系統效能

第七章

由模擬的結果我們還可以發現，多路徑干擾消除技術經過多層級 的干擾消除，雖然可漸漸降低系統的位元錯誤率，但最大的改善都是 出現在第一次的干擾消除。而第一次的干擾消除是否理想也對其後的 干擾消除效果有著一定的影響。故為了提升第一次干擾消除的效能表 現，我們或許可以將系統加入錯誤更正碼(Error Control Coding)，以提 升第一級干擾重建的正確率，期能改善系統效能。甚至可以進一步求 出接收端解碼器輸出的位元軟資訊，回授給多路徑干擾消除器，形成 具有渦輪等化器(Turbo equalization) [21]能力的“渦輪多路徑干擾消除 技術＂。另外，通道估計的準確與否對系統效能亦有著深遠的影響，

是故如何設計一準確的通道估計方法，亦是未來值得我們努力的地方。

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