Project based on OCDMA

[Irfan Morshed]

Hi,

This is basically a 3 user OCDMA system, done using spectral amplitude coding. So I have used walsh code for encoding and decoding, using complementary/balanced detection, by using FBGs. However, the BER (0.5) and eye diagrams are bad. This is for a 16 QAM system. Could anyone please help me locate the problem?

Thanks in advance 🙂

[Irfan Morshed]

My project is basically about a 3 user OCDMA system for 16 QAM, where I have done spectral amplitude coding (SAC) using FBGs. I have used BER test set and the BER I am getting is 0.5 which is bad, and the eye diagram is also horrible. I have used optical amplifier of gain 10, FBGs encoded and decoded using Walsh code (you can check the wavelength and bandwidth values, I have done it by using complementary/balanced detection), laser linewidth of 300 GHZ ( since wavelength = 2.4 nm), and sequence length = 2048 bits. I also changed sequence length to 65,536 bits to support BER test set, still no change in results. All other design parameters have ideal values. However, the M-ary threshold detector values are kept as the default values for 64 QAM. The constellation and eye diagrams are both horrible.

Could anyone please be kind enough to help me locate and solve the problem? Your one tiny help can make things much easier for me here. My thesis work is getting badly hampered due to this. I have been brainstorming since weeks, still no outcome.

Please anyone help me out! Its a sincere request.

Please.

[Irfan Morshed]

Here’s a brief on how Spectral Amplitude Coding (SAC) was done using Walsh Code :

We have 4 wavelength values –

lamda 1 = 1550 nm
lamda 2 = 1550.8 nm
lamda 3 = 1551.6 nm
lamda 4 = 1552.4 nm

Walsh code for Encoding :

User 1 (1010) : 2 FBGs at encoder coded with lamba 1 and lamda 3 (lamda 2,4 are cut off)
User 2 (1100) : 2 FBGs at encoder coded with lamba 1 and lamda 2
User 3 (1001) : 2 FBGs at encoder coded with lamba 1 and lamda 4

Walsh code for decoding using complementary/balanced detection :

User 1 (1010) : 2 top FBGs at decoder coded with lamba 1 and lamda 3, 2 bottom FBGs at decoder coded with lamba 2 and lamda 4
User 2 (1100) : 2 top FBGs at decoder coded with lamba 1 and lamda 2, 2 bottom FBGs at decoder coded with lamba 3 and lamda 4
User 3 (1001) : 2 top FBGs at decoder coded with lamba 1 and lamda 4, 2 bottom FBGs at decoder coded with lamba 2 and lamda 3

[Irfan Morshed]

Also at the transmitter side, upconversion from baseband to passband is done, from 2.5 Gigs to 60 Gigs. And at receiver side, 60 gigs RF passband signal is converted back to 2.5 Gigs baseband signal.

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