Investigation Of Orthogonal Frequency Division Multiplexing And Multiple Access Techniques For Wireless Communication Systems

In wireless communication,multiplexing and multiple access(MA)refers to the sharing of a communication resource,and are the focus of wireless communication researches.By orthogonal or non-orthogonal resource allocations among system users,many popular schemes,e.g.,the former as time division(TD),and frequence division(FD),the latter as code division(CD),can achieve good performance of wireless signal transmission.However,to some extent,fading channels in wireless communication badly influence the communication performance which can be achieved theoretically.There are many methods to resist the effects.Generally the theory of designs includes two ways.One is designing new orthogonal system with equalization suitable for fading channels,such as vector orthogonal frequence division multiplexing(V-OFDM).Another is using non-orthogonal multiple access with more complex iterative receiver,such as turbo code decoder,e.g.,developing the interleave-division multiple access(IDMA)scheme.In this thesis,some beneficial results and improvement schemes are obtained by investigating on the above techniques from the points of view of improving the system performance,mitigating the degradation causing by speed mobile,and increasing the system capacity.Firstly,to improve system performance,V-OFDM characters are analyzed.As a result,by introducing Fourier transform and scrambler,this thesis proposes a new DFT scrambling vector OFDM(DFT-SV-OFDM)system based on Fourier expansion and gives the system performance analysis theoretically.Both theoretical analyses and computer simulations show that the new multiplexing scheme lowers V-OFDM peak-to-average power ratio(PAPR),reduces the receiver complexity,and has the advantages of defending against fading channels.Secondly,based on DFT-SV-OFDM system characters,combining differential encoding,a frequency-domain scrambling differential detection and equalization for DFT-SV-OFDM system is presented for eliminating the effect of the channel estimation errors in flat fading channels.Furthermore,when handsets are moving quickly,i.e.,signals are traveling under fast fading channels,the thesis proposes a joint time and frequency domain differential detection and equalization for DFT-SV-OFDM system to mitigate errors from equalization by fast fading channels.Theoretical analyses and computer simulations are given for the two schemes.It is shown that these new schemes are both better than V-OFDM and DFT-SV-OFDM with channel estimation errors.Then,two multiple access methods are provided based on DFT-SV-OFDM.In one method,sharing of the communication resource is accomplished by allocating sub-carriers among users,named DFT scrambling vector orthogonal frequence division multiple access(DFT-SV-OFDMA).Another is merging CDMA technique,called adaptive multi-rate DFT scrambling vector multi-carrier CDMA(AMR-DFT-VMC-CDMA),in which signals are spread by different codes before divided to different sub-carriers,and the parameters of the system could be adaptively adjusted to ensure frame length long enough.The simulation results show that DFT-SV-OFDMA is better than DFT-S-OFDMA,compared with the traditional MC-DS-CDMA,AMR-DFT-VMC-CDMA could achieve multi-rate and multi-user information transmission flexibly with better performance and better spectrum efficiency in fading channels.Moreover,it can satisfy the requirement of frame length under certain channel utilization by adjusting the parameters.Finally,uncoded IDMA is discussed.By investigating the construction of transmitted signals and the decode process in the iterative detector of IDMA from the point of spread-spectrum communication view,the user capacity bound on given signal-noise-ratio(SNR)in AWGN channel is deduced.According to the results obtained,the relationship between the average SNR,user number,and spreading length is revealed.The derivation also shows that the behaviors of IDMA are conformed to the basic theories of spread-spectrum communication using spreading code though it uses random interleavers to separate users.Hence,a hybrid high capacity orthogonal and random multiple access scheme is presented,which is a combination of the conventional orthogonal CDMA scheme and the synchronous IDMA scheme.Since the two systems both follow the same spread-spectrum communication theory,it is easy to construct two sets of user signals which distinguished by decoder and chip-level iterative interleaver respectively.By the different receivers,the new system performs much better than the conventional CDMA schemes with large users,and lower the complexity of receiver compared with synchronous IDMA scheme.Therefore,the system is suitable for high load communication,and the simulation results also support the conclusion.