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5/26/2018 Ky Thu t Tra i Ph Trc Ti p

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K thut tri phtrc tip

Hthng DS-SS tc tri phbng cch nhn tn hiu ngun vi tn hiu gi

ngu nhin. hthng DS-SS nhiu thu bao cng dung chung mtbngtn v phttn hiu ca hng thi. My thu sdng tn hiu gingu nhin chnh xc lytn hiu mong mun bng cch gii tri ph. y lhthngc bit n nhiu nhttrong cc h thng tri phv c cu tongin, khng yu cu tnh ninhk nhanhhoc tctng hp tn scao.

Figure 1: Spread spectrum systems

ChuiPN: chui mang tnh ngu nhin v phnm trong vng nhiu tp mtrng nhngli c tnh tun hon v hon ton xc nhupht v u thu


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Figure 2:s triphtrc tip

Vic tri ph c thc hin bngcch iu ch trc tip nhbiu chtn hiusong mang. ng thi, d liu bnghp c nhn vi tn hiu m gi ngu nhin PN

Bng rng,

Figure 3:DS-SS BPSK digital communication system block diagram

Truyn mt chui dliuc di 103 trn h thng DS-SS BPSK. Chui d liu sc to ra hon ton ngu nhin bng cc s0 v 1vi xc sutnh nhau.

Gi stc ddng bit d liu l fb =1Hz. Vi mi bit sdng chui PN vi chiu diNc = 20 tri ph tn hiu.

Tn hiu sng mang c iu ch c tn sfc = 4* fb * Nc

Mc ch m phng:

M phng tuyn truyn dn DS-SS BPSK


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Quan st tn hiu tri phtrc tip Quan st tn hiu sau khi tri ph M phng tuyn truyn v nhn tn hiu ca 1 hthng DS-SS BPSK Quan st v so snh tn hiu cuicng thu c v tn hiu gcpht i

To d liu gi i

M phng tuyn truyn dn

Trong tuyn truyn dn,u tin phi khi to mt chui sngu nhin

Sau , cn to mt chui PN vidi thch hp, thc hin tri phbng cch nhntn hiu gcbng cbn vi chui PN va to.

%% phia phat

base_band_signal = zeros(1, length(t));for jj = 1:Nif data(jj)base_band_signal((1:M)+M*(jj-1)) = 1;end

endbase_band_signal = base_band_signal * 2 - 1;% khoi tao chuoi PNPN = round(rand(1, N*Nc));% Chuyen chuoi PN thanh tin hieu lay mau.PN_signal = zeros(1, length(t));for jj = 1:N*Ncif PN(jj)

PN_signal((1:M/Nc)+M/Nc*(jj-1)) = 1;end

endPN_signal = PN_signal * 2 - 1;%Thuc hien trai pho truc tiepspreaded_signal = base_band_signal .* PN_signal;%Dieu che BPSK cho tin hieu trai phomodulated_signal = spreaded_signal .* (sqrt(2)/sqrt(Tb)).* sin(2*pi*fc.*t);

Tn hiu gcbngc bn, chui PN v tn hiu tri ph

figuresubplot(3,1,1)plot(t(1:10*M), base_band_signal(1:10*M))axis([t(1), t(10*M), -1.5,1.5])title('Base Band Signal');xlabel('Time');subplot(3,1,2)plot(t(1:10*M), PN_signal(1:10*M))axis([t(1), t(10*M), -1.5,1.5])title('PN Sequence');xlabel('Time');subplot(3,1,3)plot(t(1:10*M), spreaded_signal(1:10*M))

axis([t(1), t(10*M), -1.5,1.5])title('Spectrum Spread Signal');xlabel('Time');


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Figure 4: Tn hiugcbngc bn,chuiPN v tn hiusau triph


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V phca tn hiu gcbngcbn v tn hiu tri ph

n = 2^(nextpow2(length(base_band_signal)));BBS = fft(base_band_signal, n)/fs;SSS = fft(spreaded_signal, n)/fs;df = fs/n;

f = (0:df:df*(n-1)) -fs/2;figuresubplot(2,1,1)semilogy(f, fftshift(abs(BBS)))axis([-5, 5, 1e-2,1e2])title('Base Band Digital Signal');xlabel('Frequency');subplot(2,1,2)semilogy(f, fftshift(abs(SSS)))axis([-5, 5, 1e-2,1e2])title('Spread Spectrum Signal');xlabel('Frequency');

M phng knh truyn

Figure 5: tn hiugc bngcbnv tn hiutri ph

Knh truynngin nht cho vic m phngl knh truyn c nhiu AWGN.

Cn phi tnh ton nhiu thc t tha mn gi tr Eb/N0

%% Tren kenh truyen% tinh toan nhieu duoc cong vao tren kenh truyenEbNo_dB = 20;% nhieu trn gauss voi gia tri 0dBnoise = 1/sqrt(2)*(randn(1,length(t)) +1i*rand(1, length(t)));% nhieu thuc te thoa man gia tri EbNosnr = 10.^(EbNo_dB/10)*(1/Tb)/fs;actual_noise = noise./sqrt(snr);

received_signal = modulated_signal + actual_noise;


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Sdng PSK giiiu chtn hiu nhn c. u tin, giiiu chBPSK tnhiu bng cch nhn vi sng mang. Tip theo l gii tri ph tn hiu va giiiuch.

%% tai phia thu% Giai dieu che BPSK tin hieu nhan duocdemodulated_signal = received_signal .* (sqrt(2)/sqrt(Tb)).* sin(2*pi*fc.*t);% Giai trai pho tin hieu vua giai dieu chedeSpreaded_signal = demodulated_signal .* PN_signal;% dua tin hieu giai trai pho qua bo locwindowSize = 15;deSpreaded_signal_filtered = filter(ones(1,windowSize)/windowSize,1,deSpreaded_signal);% thu lai du lieufor ii = 1:Nif deSpreaded_signal_filtered(ii*M - M/2) > 0demodulated_data(ii) = 1;elsedemodulated_data(ii) = 0;endend% tinh toan BER

Err = demodulated_data - data;nErr = length(find(Err ~= 0));Err_rate = nErr / N;

figuresubplot(2,1,1)plot(t(1:10*M), base_band_signal(1:10*M))axis([t(1),t(10*M),-1.5,1.5])title('Base Band Signal');xlabel('Time');subplot(2,1,2)plot(t(1:10*M), real(deSpreaded_signal_filtered(1:10*M)))axis([t(1),t(10*M),-3,3])title('Recovered Signal');xlabel('Time');


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