Research On Anti-interference Method Of Cognize Cdma Communication System Based On Waveform Designing

With the development of communication theory and technology,and the increasement of communication users,the spectrum resource becomes scarcer and scarcer.The Cognitive Users(CU)can use idle white spectrum to communicate with cognitive radio(CR)and the spectrum efficiency has been improved.The cognitive CDMA systems possess not only the advantages of CDMA technology,but also the advantage that the users possess the controllable spectrum,which can be used as the specific implementation technology in CR.A novel cognize wireless communication system can be constructed combining CDMA and CR technology.The anti-interference method of cognize CDMA communication system has been researched in the paper.The interference suppressing means three aspects.The first is that CU forms spectrum nulls to avoid the interference with AU.The second is that the multiple access interference(MAI)between cognize CDMA users and authorized CDMA users or among CDMA system itself will effectively resolved.The third is that the multipath interference(MPI)from fading channel in the practical communication systems.All of the three anti-interference methods are based on waveform designing.The specific work is shown as follows:(1)The classic analysis for bit error rate(BER)performance of users is carried out using statistical property of noise and signal in the time domain.However,the spectrum occupation of AU on basis of spectrum sensing is intuitionistic in CR systems.The spectrum of CU is non-ideal and overlap with AU,leading to BER performance deterioration of both of them.If the relation between the spectrum overlap between CU and AU and their BER performance can be obtained,the performance loss due to spectrum overlap can be intuitively presented.The BER of three classic modulation systems and spread-spectrum system with arbitrary chip waveform(CW)have been deduced in the paper.The spectrum overlap factor(SOF)is proposed and its relational expression with BER is given.The SOF is proved to be proportional to BER.The calculating method of SOF is given and turned out to be reasonable in the paper.(2)A novel spectrum nulling method of cognitive CDMA users with CW designing is proposed based on SOF criterion.The spectrum of CDMA users is proved to be determinedby both of CW and spread-spectrum sequence firstly.The spectrum of CDMA users can form spectrum nulls in the frequency band occupied by AU only by designing CW according to the spectrum template needed by CU.When CDMA users are taken as CU,both the BER performance of AU and CU can be improved.(3)The MAI exists in the practical CDMA communication environment and is the main factor to limit the system capacity and BER performance of users.With the increasment of users and the complexity of communication environment,novel methods to suppress MAI should be given.Two methods to suppress MAI are proposed in the paper.The first to suppress MAI is to use optimized multiple waveform optimizing(MWO)based on energy spectrum flatness criterion(ESFC).The ESFC is proposed through analyzing the principle to suppress MAI of double waveform optimizing(DWO).The calculation method of energy spectrum flatness is given.The optimizing waveform combinations can be selected using ESFC to suppress MAI and improve BER performance of CDMA users.The second is to suppress MAI using waveform sets(WS).The WS can be designed with only two or three conventional CWs for any numbers of users.Different users use only one CW of WS and can suppress MAI and improve BER performance.The principle to suppress MAI is analyzed and the solution of waveform sets is given.(4)The existing cognitive CDMA spectrum nulling method mainly uses composition sequence as spread-spectrum sequence and the spectrum of the users can form certain spectrum nulls.However,the position of spectrum nulls is fixed and the performance of users with composition sequence in fading channel is not analyzed.The autocorrelation of composition sequence is derived and is turned out to be worsen than classic m sequence,which can lower the anti-MPI ability of users.A method of cognitive recombination CDMA using modificatory Hermite function is given in the paper.The modificatory Hermite function possess better stypticity in the time domain,wonderful autocorrelation and cross-correlation performance,certain spectrum nulling.The autocorrelation performance of recombination spread-spectrum users can be compensated,the anti-MPI ablity can be improved and the MAI can be lower due to the wonderful cross-correlation performance when the modificatory Hermite function is used as CW for CDMA users.Moreover,the CDMA users with modificatory Hermite function of different orders can form different spectrum nulls,which is meaningful for CU to avoid AU.