- 學位論文
可實現零直流奈奎斯頻寬短碼直接序列分碼多重進接
Realizable Nyquist-bandwidth Short Code DS-CDMA Exhibiting Zero-DC
摘要
除了早期的第三代行動通訊，直接序列分碼多重進接目前在衛星導航與衛星定位系統中仍被廣泛應用。在現行的短碼直接序列分碼多重進接系統中，常使用方波來產生時域限制但頻域無限的展頻訊號，在先前的研究中，為了產生奈奎斯頻寬的短碼直接序列分碼多重進接信號，頻域方波則常被討論與使用，然而，頻域方波的不連續性使得系統無法實現。 另外，在衛星通訊中，直接轉換接收器為一接收器架構，直接轉換接收器嚴重地受到直流偏置影響，在本論文中，藉由對展頻序列提出一項新的限制，找出零直流成分且可實現的脈波成形，此脈波成形可幫助消除直流偏置影響，不會產生失真且可有效率地實現。為了找出適合使用此脈波成形的展頻序列，檢測了數個常用的正交碼以及偽隨機辨識碼並探討其多重存取干擾的影響。此外，兩個奇數碼長的正交碼也在本論文中提出。
並列摘要
Besides 3rd-Generation mobile telecommunications, direct-sequence code division multiple access (DS-CDMA) is still widely used in global navigation satellite system (GNSS) and global positioning system (GPS). In short code DS-CDMA systems, rectangular shaping pulse is widely applied to construct time-limited spreading waveforms, resulting in band-unlimited signals. To implement short code DS-CDMA systems occupying Nyquist bandwidth, frequency domain rectangular pulse is discussed in previous researches. However, the discontinuity of this pulse makes these systems unrealizale when implementing. Also, in satellite communications, direct conversion receiver (DCR) is one of the receiver structures. It is well-known that DCR suffers from DC offset severely. In thesis, by introducing a new constraint on codewords, a realizable zero-DC pulse shaping is proposed. This pulse shaping helps to eliminate DC offset without distortion and can be implemented efficiently. To find preferred codes for the new modulation scheme, several popular orthogonal codes and pseudo random (PN) codes are examined for system capacity and effect of multiple access interference (MAI). Two new orthogonal codes suitable for odd code lengths are proposed.