311 CDMA Transmitter/Receiver :: Quicker, easier and cheaper to make your own chip!

How it works

This is a very simple circuit, it consists in two LSFR register lineary connecated in an specific way in order to generate two m-sequences, in order to generate this pair of m-sequences, we input an initial vaule called seed, this is because we cannot generate a PN signal if the LSFR registers have an initial value of 0s. For this design we used two LSFR with 5 D Flip-Flops. With this we can generate a PN signal with a length of(2^5)-1. With this PN signal and modulus 2 adding the signal we want to tranmit, we generate a CDMA signal, which we are going to study simulating a channel with an OPAM in order to add noise to the CDMA and feed it back to the designed circuit to see the bit-error rate of this device. With this we hope to study and put to test.

CDMA
Gold Sequences.
The effect of the noise in the CDMA.
Reception process with a simulated channel.
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How to test

As we used a hardware description language (Verilog), we created an specific testbench for the cdma.v, this testbench simply initializes the input signals to 0 in order to generate the adequate signal such as the clock, a test signal to send that lasts 31 clock cycles (Spread-spectrum), a seed value to load both LSFR The stimulus simple will assign a value to set_i and deactivate it to load the seed, after that, the LSFR have a linear feedback and will contantly generate the Gold signal each clock cycle. With the test signal_i we the system will generate the CDMA signal and assign it to CDMA_o, the Gold signal is assigned to Gold_o, this is the tranmition process.

For the reception process, we simply assign the value of CDMA_o to receptor_i and the output will be observerd at receptor_o, we can observe that we recovered signal_i as it shows the same time diagram as receptor_o.

As for the LED_o it works as a simple indicator that the inputed seed is valid for transmission.

The template will include the verilog file with its testbench.

External hardware

OPAM, Testboard, LED

#	Input	Output
0	signal_i	cdma_o
1	seed_i[0]	gold_o
2	seed_i[1]	receptor_o
3	seed_i[2]	led_o
4	seed_i[3]
5	seed_i[4]
6	receptor_i
7	load_i

311 CDMA Transmitter/Receiver