Abstract
本論文研究展頻多重接近(spread-spectrum multiple-access)之系統設計有關效能及 容量方面、以及提出減少多重接近干擾之新觀念。在系統效能方面，本文分析在室內多路徑衰褪通道之Direct-sequence(DS)以RAKE為接收器及slow frequency hopped(SFH)以多樣技術(diversity)為接收器的錯誤率。DS系統是用BPSK調變，等增益(equal-gain)及選擇性selective 的結合(combining)多樣技術。 SFH系統是用BFSK調變、等增益及選擇性的結合多樣技術。我們使用一群一群(cluster)到達的路徑 (path)表示多路徑衰褪通道以符合實際室內環境。 從結果觀察得知在低位元傳輸速率時SFH與DS差不多。而在高位元傳輸速率時，DS較SFH有較好效能。此外編碼與多樣技術確能改進效能。在系統容量方面(定義為相對於錯誤率之同時使用頻道用戶數)，本文分析低速如語音與高 速如數據於固定頻寬傳輸時應如何使用頻寬，是使用整個頻寬(wideband)或是分割成數個頻帶(hybrid FDMA)再使用呢？我們考慮了DS-SSMA 與SFH-SSMA系統於室內環境情形。 結果顯示hybrid FDMA/DS-SSMA於高速傳輸速率時較wideband DS-SSMA有較高系統容量，於低速傳輸速率時，兩者系統容量之優劣難以分出因其與衰褪通道特性有關。 我們探討另一展頻系統fastfrequency hopped(FFH)於室內多重路徑衰褪通道之系統容量，考慮等增益與自標準化(self-normalization)結合、Reed-Solomon錯誤更正碼、關連性 (correlation) 接收，結果發現FFH-SSMA於關連性接收考量下效能明顯變差。由上述分析知系統容量受多重接近干擾與窄頻干擾嚴重，本文提出跳頻多使用者接收器以解決多重接近干擾問題，提出頻道化及BFSK調變之系統， 利用已知的用戶跳頻形式(qopping pattern)與估測到之使用者振幅，從偵測理論之最大相似性(maximum likelihood)推導得最佳偵測器。我們提出一次佳且簡單之 新的多使用者接收器。結合多樣技術後其效能更為改進，我們發現慢跳頻用此接收器對多重接近干擾有很好抵抗能力，快跳頻亦有優於常用M-ary FSK之容量。為降低窄頻干擾 ，本文提出將蜂巢式narrowbandcode-division multiple-access(N-CDMA)安排於相鄰於類比式蜂巢電話之頻道上，因高負載之兩系統安置在相同頻道會造成容量下降。System design issues and concepts for spread-spectrum multiple-access (SSMA) communications are studied in this research.Design issues have two important criteria includingperformance and capacity ( bit error probability versussimultaneous users ). For performance criterion, we considerthe best policy of applying SSMA systems in indoor channels. Forcapacity criterion, we consider the policy of maximum systemcapacity and evaluate new application. Two concepts of systemdesign including multiuser detection of frequency-hopped systemand coexisting two systems in adjacent spectrum are proposedhere to improve capacity criterion. The first concerned issuefor system designer is the best policy of applying direct-sequence (DS) and slow frequency hopped (SFH) systems in indoorchannels. We compare performance of single-user DS and SFHsystems to answer this question. Bit error probabilities ofDS systems with RAKE receivers and SFH systems with diversityreceptions in indoor multipath fading channels are analyzed.Equal-gain and selective combinings for DS systems with binaryphase-shift keying (BPSK) modulation and SFH systems with binaryfrequency-shift keying (BFSK) modulation are considered. BCHcodes with single or double error-correcting capability arechosen to further improve the performance of DS and SFHsystems. Multipath fading channel models with clusters ofarriving rays are used to meet the practical indoorenvironments. From the numerical results, we observe that theSFH system is competitive under moderate data rates. For higherdata rates, the DS system has better performance than that ofthe SFH system. We demonstrate that coding and diversitytechniques can improve the performance of DS and SFH systems.The policy of maximum system capacity is to spread the signalinto the entire available bandwidth or to divide the entirebandwidth into several frequency channels and spread signal ateach band. To answer this question, we compare totalcapacities of hybrid frequency division multiple access (FDMA)/SSMA (DS and SFH) systems with those of wideband SSMA systems.Using an indoor multipath fading channel model with clusters ofarriving rays, we investigate multi-user DS systems with RAKEand diversity reception by selection combining (SC), multi-user SFH systems with equal-gain (EG) diversity reception.Reed-Solomon codes are considered to further improve the systemperformance. Given a fixed available bandwidth with narrow bandinterference (NBI), capacities and packet error rates aredetermined under various system configurations. For high datarate communications, hybrid FDMA/DS-SSMA systems have largercapacities than wideband DS-SSMA systems. For low data ratecommunications, a capacity comparison between wideband DS-SSMAand hybrid FDMA/DS-SSMA systems depends on fading statistics.Hybrid FDMA/SFH-SSMA systems have larger capacities thanwideband DS-SSMA systems. Since fast frequency hopped (FFH)systems not applied in indoor channels, we evaluate systemcapacity of multi-user FFH systems with correlated EG and self-normalization (SN) combining techniques. Results show thatFFH-SSMA systems could not provide satisfactory performancedue to correlation among hopping bands. System evaluations ofDS-SSMA, SFH-SSMA and FFH-SSMA demonstrate that multiple-access interference degrade performance and capacitysignificantly. One concept of multiple-access interferenceresistant is proposed in this study based on multiuserdetection. New multiuser detectors of frequency-hopped spreadspectrum multiple access (FH-SSMA) based on binary frequencyshift keying (BFSK) modulation and channelized frequencyhopping are proposed. Being knowledge of hopping sequences andenvelope of active users, the proposed scheme is a sub-optimaldetector under maximum likelihood test. Diversity combining isemployed and considered as technique of anti-multiple-accessinterference which improves the performance significantly. InSFH systems, we demonstrate that this multiuser detectorcombined with diversity is robust to multiple-accessinterference. In FFH systems with heavy load of multiple-access, this detector employs more simultaneous users than themultiuser detector with M-ary frequency shift keying (MFSK)modulation. Results also show that narrowband interferenceimpairs the performance and capacity. Another concept ofreducing narrowband interference is proposed. We proposecoexisting analog frequency modulation (FM) based on frequencydivision multiple access (FDMA) and narrowband code divisionmultiple access (N-CDMA) in adjacent spectrum. The allocationon different spectrum can avoid the serious performancedegradation as transitioning from fully loaded analog FM/FDMAto N-CDMA. The guard-band needed between the adjacent spectrumto maintain acceptable performance degradation is analyticallyderived and evaluated. Numerical results indicate that atleast 255 kHz bandwidth to separate the two system is necessary.The worst case of the interference from FDMA will reduce theperformance of N-CDMA, even under proper guard-band separation.This study will demonstrate the capacity reduction of N-CDMA ascoexisting analog FM/FDMA in adjacent spectrum.