تقييم اداء نظام الاتصالات المتعدد المستخدمين المرتكز على متجهات فوضوية متعامدة == Performance Evaluation Of Multiuser Communication System Based On Orthogonal Chaotic Vectors
Author name:
بسمة نزار ناظم
Supervisor name:
ماهر خضير محمود
General topic:
Electrical, Electronic and Communications Engineering
Specific topic:
Electronics and Communications Engineering
Degree:
Master
University:
Mustansiriyah University - College Of Engineering - Department Of Electrical Engineering
Language:
English
University location:
Baghdad
First pages:
34T522 - p.pdf
Abstract:
In DS - CDMA systems, the use of orthogonal spreading codes with low cross - correlation values is needed to suppress the effect of multiple access interference (MAI) which increases as the number of active users in system is increased.Currently, the conventional PN sequences, such as (Gold codes, Kasami codes …etc.), generated by linear feedback shift registers (LFSRs) and orthogonal variable spreading factor (OVSF) were used as spreading codes in DS - CDMA system. Using these sequences, it is not possible to increase the number of active users in the system due to the fact that the possible sequences generated in conventional PN - system are limited in number and offer mutual interference as the number of active users is increased due to its non - zero cross - correlation values. The chaotic system can be used to generate very large number of quasi - orthogonal sequences compared with the number of PN sequences due to its sensitivity to initial conditions. Another type of chaotic sequences with zero values of cross - correlation called Orthogonal Chaotic Vectors (OCVs) is generated using chaotic systems and the basic idea of Gram - Schmidt ortho - normalization process.In this work, the use of OCVs in synchronous DS - CDMA system for transmission of data over (AWGN, Flat fading and Frequency selective channels) is introduced. The simulation results show that the DS - CDMA based on OCVs gives better BER performance when spread factor ? is greater than the number of active users due to the reduction of MAI between active users.Results show also that, at BER = - in AWGN channel and for 5 users, there are gain of (0.3, 1.2, 1.75 and 3.75) dB between system based on OCVs and systems based on small set Kasami sequence, Gold codes, large set of Kasami sequence and Non - OCVs respectively. Under the effect of flat fading channel, it can be noted that at BER = - and for 30 user, there is 1.7 dB gain between system based on OCVs and system based on small set of Kasami sequences, and this gain increases to 2 dB for 50 user.Under time invariant frequency selective fading channel and for 8 users at BER= - there is about 6dB gain between the system based on OCVs and system based on Non - OCVs.
