题目内容
（请给出正确答案）
The word perceives in the passage is closest in meaning to
A．imitates.
B．discharges.
C．notices.
D．carries.
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Write MATLAB codes to implement DSSS system sinmulation, The system is a 2-user system with a 2-path frequency selective fadingchannel for each user, The power for each path is half the total transmit power for each user. A RAKR receiver with ideal channel estimation should be used in the receiver for each user. The relative delay between two paths is 1 chip. BPSK modulation is assumed. The BER-versus SNR curve should be drawn for both users. The SNR is defined by
, and is assumed to change from 0 dB to 10 dB with step 1 dB. The simulation results should be compared with the theoretical BER versus SNR curve in flat Rayleigh fading channel.
A、N_Trials=300; N_number=100; N_snr=10; N_users=2; N_path=2; Delay=1; Q=16; N_samples=N_number*Q; E_M=0; for trials=1:N_Trials trials noise=randn(N_users.*N_path,Q*N_number)+j.*randn(N_users*N_path,Q*N_number); s10=round(rand(N_users,N_number)); ss_user=s10*2-1; % without consideration of delayed path pn01=round(rand(N_users,Q)); pn=(pn01.*2-1)./sqrt(Q); S_spread=[]; for user=1:N_users s=kron(ss_user(user,:), pn(user,:)); S_spread=[S_spread; s]; end S_multipath=[]; for user=1:N_users s_user=S_spread(user,:); Suser_delayed=[zeros(1,Delay), s_user(1,1:(N_samples-Delay))]; S_multipath=[S_multipath; [s_user;Suser_delayed]]; end %___________the following is to generate Rayleigh fading coefficients N=N_users.*N_path; fad_c=fading(8,0.005,N_samples, N); fad=fad_c.'; % only use the amplitude; %phase is not used, which is equivalent to have an ideal phase %compensation by using channel equalization S_spread=S_multipath.*fad; %S_spread is the signal matrix of size users x N_number sgma=1; Error_M=[ ]; for snr_db=0:1:N_snr snr=10.^(snr_db./10)/2; %Evaluate the SNR from SNR in dB N0=2*sgma.^2; Eb=snr.*N0; yy=sqrt(Eb./2)*S_spread+noise; %received spread signals in the baseband Error_v_user=[]; for user=1:N_users y_path1=yy((user-1).*N_path+1, :); y_path_temp=yy((user-1).*N_path+2, :); y_path2=[y_path_temp(1,Delay+1:N_samples), zeros(1,Delay)]; Y_M_path1=[ ]; Y_M_path2=[]; for k=1:N_number ym1=y_path1(1,(k-1)*Q+1:k*Q); ym2=y_path2(1,(k-1)*Q+1:k*Q); Y_M_path1=[Y_M_path1;ym1]; Y_M_path2=[Y_M_path2;ym2]; end % Y_M is a matrix of size N_number x Q, each row correspinding to a % BPSK symbol ys=Y_M_path1*pn(user,:).'+Y_M_path2*pn(user,:).'; %despreading for a user y=ys.'; y_real=real(y); s_e=sign(y_real); s_e10=(s_e+1)./2; Error_snr=sum(abs(s10(user,:)-s_e10)); Error_v_user=[Error_v_user;Error_snr./N_number]; %A BER collumn vector for all the users and for each snr end %for user Error_M=[Error_M,Error_v_user]; %BER matrix for all users and for all the SNRs end % end for snr E_M=E_M+Error_M ; end % end for trials BER=E_M./N_Trials; BER_T=[ ]; for snr_db=0:1:N_snr snr=10.^(snr_db./10); temp=sqrt(snr./(1+snr)); BER_THEROY=(1-temp)/2; BER_T=[BER_T,BER_THEROY]; end i=0:1:N_snr; semilogy(i,BER(1,:),'-r',i,BER(2,:),'ob',i,BER_T ,'*g'); xlabel('E_b/N_0(dB)') ylabel('BER') legend('User1 RAKE','User2 RAKE', 'Theoretical flat fading');
B、no need
C、no need
D、no need
A．served.
B．affected.
C．raised.
D．designed.
A．The lack of variety and short life of film reels prevented the industry from experiencing real success.
B．In order to attract larger audiences to their showings, exhibitors needed to expand their stocks of films.
C．Due to difficulties transporting the films long distances, traveling cinemas were not commercially successful.
D．The public soon began demanding higher-quality films, but most exhibitors were unable to obtain them.
A．Square Ａ.
B．Square Ｂ.
C．Square Ｃ.
D．Square Ｄ.
A．It requires divers to undergo strenuous training.
B．The algae's roots are usually too strong to pull out.
C．It is necessary to remove every last piece of algae.
D．The algae grow back faster than they can be removed.
A．discussing ancient Greek and Roman history.
B．relating the accomplishments of Plato and Cicero.
C．providing a definition for the name itself.
D．describing Florence, Italy, in the fourteenth century.
A．discussing ancient Greek and Roman history.
B．relating the accomplishments of Plato and Cicero.
C．providing a definition for the name itself.
D．describing Florence, Italy, in the fourteenth century.
A．remained with.
B．held off.
C．brought forth.
D．stood for.
A．At the time, the U.S. was struggling against many other nations to gain control of western lands.
B．The American government encouraged all people, regardless of their background, to settle in western regions.
C．Mexico, England, and the Native Americans all fought with America for the rights to the western territories.
D．U.S. authorities supported westward migration as it kept other peoples from taking over the land there.
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