直接序列分碼多重進接無線通訊系統之分析及設計

Abstract

直接序列分碼多重進接(DS-CDMA)技術已被廣泛的應用於第二代行動通訊系統並成為下一代通訊系統的主要技術之一。本論文的主要內容在深入地分析傳統技術之效能，並提出一個可增進其效能的片碼間插(chip-interleaving)技術。

首先，我們針對傳統系統在經迴旋碼(convolutional codes)之通道編碼後的檢測結果進行理論上的分析。基於隨機展頻碼(random-spreading codes)的假設，我們提出一個新穎且簡單的分析結果。我們也分析使用短展頻碼(short-spreading codes)和長展頻碼(long-spreading codes)的差別，分析結果顯示雖然在未經通道編解碼時，展頻碼的週期長度並未對檢測結果造成影響，但是由於長展頻碼有較佳的干擾隨機性(randomness)，其在經通道編碼的系統中效能明顯優於短展頻碼；而此差異在淡化通道下，相對較不明顯。此外，我們也發現直接序列分碼系統雖然能經耙狀接收器(rake receiver)及位元間插器(bit interleaver)分別得到路徑分集(path diversity)及時間分集(time diversity)，進而在淡化(fading)通道中有較好的表現，但是其效果仍較未經淡道化通道時來得差。因此，仍需進一步的分集技術。

多重用戶干擾(multiple access interference)是影響分碼多重進接系統容量的重要因素。因此，我們接著討論應用平行式干擾消除(parallel interference cancellation)檢測器以處理多重用戶干擾問題的系統。針對硬式(hard-limiting)干擾消除器，我們研究其在同步(synchronous)系統中的效能極限。經由分析我們發現，此檢測器在第兩級後的效能即未有明顯的差異，而當僅有兩用戶時，只要一級的干擾消除就夠；我們也驗證此效能的限制是來自於用戶間相互的干擾影響。

在接下來的部份，我們研究片碼技術。我們首先提出一個使用片碼間插技術的直接序列分碼系統(CIDS-CDMA)。此系統由於能額外提供內符碼分集(intra-bit diversity)，在淡化通道下有很穩健的效能。當合併使用位元間插技術時，此系統最大的分集度是展頻係數(spreading factor)和迴旋碼之自由距離(free distance)的乘積。我們從理論上分析此系統在未經和經過通道編碼的效能。分析顯示，檢測對象的移動速度及其片碼間插深度幾可完全決定其檢測效果的好壞，其他干擾用戶的情形並未對其有明顯的影響。我們也發現在此系統中，展頻碼的週期長度在完美片碼間插情形下，並未如同傳統系統一樣影響檢測結果。

在第四個部份，我們基於3GPP的寬頻分碼多重進接系統(WCDMA)設計了片碼間插寬頻分碼系統(CI-WCDMA)。片碼間插技術應用於實數分支(I-branch)以傳遞用戶資料。虛數分支(Q-branch)則用以傳送前導序列(pilot sequence)而在接收端利用傳統非片碼間插技術進行通道估測。在此系統中，我們亦應用了數種不同的非線性平行式干擾消除器。經由模擬驗證，此系統在淡化通道下的確有極為優異的效能表現。

最後，我們研究應用多展頻碼(multicode)的片碼間插分碼系統於慢速淡化(slow fading)通道中。經由適當的零填補(zero-padding)，此系統在非同步(asynchronous)及多重路徑(multipath)反射的情形下仍能維持展頻碼間的同步相關性(synchronous correlation)，進而減少干擾。我們提出一個基於高登序列(gold sequences)的設計範例，其能在檢測品質及系統容量中有較好的效能取捨。和最近所提出的片碼間插區塊展頻系統(chip-interleaved block-spread CDMA)相較，我們所提出的系統在通道編碼的系統中有較佳的效能。

Direct-sequence code division multiple access (DS-CDMA) has been widely implemented in second generation cellular communications systems and become one of the most promising techniques in next generation system. In this thesis, we study the performance of conventional DS-CDMA systems and propose a chip-interleaved system which can greatly improve the performance.

First, we concentrate on the analysis of conventional DS-CDMA and begin the discussion with convolutional coded systems. Random-spreading codes are assumed. A novel and relatively simple theoretical result is presented. The performance difference between short- and long-code spreading is also studied and the results show that due to the randomness of interference over symbols, the system with long-code spreading performs much better than with short-code spreading although they have similar performance when channel coding is not applied. The results also show that although DS-CDMA can gain diversity from rake receiving (path diversity) and bit interleaver (time diversity), it still degrades a lot in fading channels and hence the need for other diversity sources.

Secondly, we consider the use of Parallel Interference Cancellation (PIC) in combating multiple access interference (MAI), which is the limiting factor to the capacity of DS-CDMA. The performance of hard-limiting PIC in synchronous systems is investigated. We show that hard-limiting PIC in such case has performance limit at the second stage. In addition, for two-user, one stage is enough. And the performance limit comes from the mutual influence between desired and interfering users.

The use of chip-interleaving in DS-CDMA is studied. We present a chip-interleaved DS-CDMA (CIDS-CDMA) which is resistant to channel fading by leveraging intra-bit diversity. When combined with bit-interleaving, this system can, in the limit, provide a diversity order equal to the product of spreading factor and the convolutional code’s free distance. Thus, CIDS-CDMA is much resistant to channel fading. Both channel coded and uncoded systems are theoretically analyzed. The analysis shows that the fading rate and chip-interleaving depth of the desired user itself almost single-handedly determines the resulting performance and the interfering users’ fading speeds do not affect the performance significantly. It also reveals that in the case of perfect chip-interleaving, the spreading code period has no influence on the performance.

Fourthly, we proposed a chip-interleaved WCDMA (CI-WCDMA) based on several 3GPP WCDMA system features. In this system, data symbols are transmitted through I-branch while Q-branch is used for the transmission of pilot sequence. Channel estimation is done in Q-branch before chip de-interleaving as conventional systems. Several nonlinear PIC detectors are considered as the receiving structures. The result shows that the proposed system has a great improvement over conventional DS-CDMA system in fading channels.

Lastly, we demonstrate a multicode CIDS-CDMA which can improve the performance of DS-CDMA in asynchronous and multipath channels by effecting the synchronous correlations among the spreading codes in such conditions. Slow fading channels are considered. An example using gold sequences is presented and shows a better trade-off between performance and capacity. Compared to the recently proposed chip-interleaved block-spread CDMA (CIBS-CDMA), the presented scheme can attain better performance in channel-coded systems.