CDMA MODEM

Multiple access using CDMA for 2 users in an AWGN channel.

Description

Description of program

1. Convert input bits to bipolar bits.. 1 to 1 and 0 to -1 for user1 and user2
2. Take 100 samples per bit for both user1 and user2 and then plot base band signal which is in bipolar NRZ format.
3. Then BPSK modulate the signal. Take care that sampling rate of sinusoidal carrier matches the sampling rate per bit. Here it is 100 samples per carrier and then plot the BPSK signal
4.
This segment of code:

               for i=1:length_user1
    for j=1:10 %chip rate is 10 times the bit rate
        pn1=[pn1 seed1(4)];
        if seed1 (4)==seed1(3) temp=-1;
          else temp=1;
          end
              seed1(4)=seed1(3);
              seed1(3)=seed1(2);
              seed1(2)=seed1(1);
              seed1(1)=temp;
    end
end
 
performs the function of exclusive or gate and shift registers. There are 4 shift registers XOR the contents of register3 and register4 and put the result in register1 , move the original contents of shift register1 to register2 and register2 to register3, register3 to register 4 and the content of register4 is the chip of PN code at each clock pulse

5. Multiply the BPSK modulated signal with the PN code. Here again the care should be taken to match the sampling rate. i.e no. of chip per bit* no of samples per chip = no of samples per bit of BPSK modulated signal.
6. Same procedure is carried out for user2 bits.
7. The channel is AWGN channel with SNR 5 dbs. In channel the signal from user1 is added to signal from user2 and white Guassian noise is added.
8. At receiver end , first received signal is multiples with PN then BPSK demodulated by multiplying with the carrier(coherent demod)
9. then the samples over 1 bit interval is summed. And if the sum is greater than 0 than the received bit is 1 else rxbit is 0. Summation is used in place of integration because it is a discrete time system
10. Same procedure is repeated for user2.

Conclusion: If SNR is of more than some reasonable value there are no bit errors.

• MATLAB release: MATLAB 7 (R14)

[Altamash Ahmed]

good shot!

[ALI ALNEHMY]

 

[le tuan]

 

[maha ali]

im new in this site.i need ur email id mr waqas mansoor

[Karim George]

 

[mahmood abbasi]

This is an excellent technique.

[abdalbaset ali]

simulation of pn offset plannin g in cdma

[Shahriar Ferdous]

 

[fengky wijaya]

thank you very much for your help...^_^

[Muhammad Rizwan]

Hi, I want to know something if you can help.
I have spread the bits with walsh code then with PN code and added awgn noise to it. On receiving side when I multiplied this received signal with conjugate of PN code it was removed from bit .Now how can I remove the walsh function to gain actual original bits.

[sharique mohd]

sir, iI want to know something if u help.
Sir i want to do work on spreading sequence that will also use phase combination i.e, phase combination [0 pi/2 3*pi/2]
i want to use as[0 1 3].
is it possible in cdma

[abwd]

thx