CDMA with out carrier modulation

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Hectoryx

Joined: 27 Aug 2009
Posts: 14
Location: Beijing

27 Aug 2009 3:28 greek carrier cdma

Generally speaking, the two major points of CDMA transceiver are as following,
1, pseudo-random code sequences, like M-sequences.
2, carrier modulation and demodulation, like PSK, QPSK etc.
Our problem is that we face a baseband transmission system, which does not contain carrier. Now, we need to implement Multiple Access. Of course TDMA can be used for that, however for some reasons; we do not plan to use this method.
Instead, we plan to “spread the spectrum” by only using the pseudo-random code sequences, without the carrier modulation.
Does it sound possible?
Could this method be also named CDMA?
Please give you advice. Thanks in advance.

Joatt

Joined: 22 Apr 2009
Posts: 18
Helped: 4

27 Aug 2009 8:30 modulation without carrier

In baseband transmission, you don't use carrier modulation/demodulation. Instead, you use line coding such as polar NRZ, differential RZ, Manchester code, etc (read more about line coding if that is a new topic to you). After the line coding, you can use CDMA or any technique you prefer for multiplexing.

Regards

Hectoryx

Joined: 27 Aug 2009
Posts: 14
Location: Beijing

27 Aug 2009 9:32 coding new line on cdma

Thanks for your reply.
The point to point transmission using line coding has been already implemented. And our task is to find a multiplexing method.
We want to use CDMA. The question is the original communication system is a baseband transmission system without carrier. However, common CDMA system contains a step which is carrier modulation. Our question is that if we do not fulfill this step and only multiply by the pseudo-random code sequences directly, does such a “CDMA” system work?
Best Regards.

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Joatt

Joined: 22 Apr 2009
Posts: 18
Helped: 4

27 Aug 2009 14:05 Re: CDMA with out carrier modulation

CDMA is usually used in wireless systems, that are passband channels. That's why common CDMA systems contain a step which is carrier modulation. However, you are using a basedban transmission channel, so you can go without this step since the line coding is already implemented as you mentioned.

Regards