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I am working in acoustic communication, I want to test CDMA network capabilities in acoustic environment. I had an acoustic modem that has serial interface, a WHOI micromodem.

How can i serialized coded data from various node in the same channel?

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  • $\begingroup$ Without knowing what your modem does exactly, it's impossible to even tell whether it's possible to do CDMA with your modem. $\endgroup$
    – Marcus Müller
    Jun 23 '20 at 14:26
  • $\begingroup$ Generally, what indication do you have that this might be possible? Even if your modem uses a linear modulation, which would be necessary for the superposition of different users, it'll do more than that, including channel estimation, equalization, frequency synchronization; if it's not meant for CDMA, it can't do that in the presence of multiple transmissions, so I honestly don't see how this would work, unless the modem is designed to do CDMA from the start (in which case this isn't a signal processing question, but a "which control command do I need to send to the modem?" question.) $\endgroup$
    – Marcus Müller
    Jun 23 '20 at 14:28
  • $\begingroup$ @MarcusMüller - Its WHOI Micromodem. I had worked with SDR in air-scenario where I used ADLAM Pluto SDR to configure two different Pluto SDR at Tx and Rx end. It did worked in practical, I am not very sure about this modems as this is entire different than our in-air conventional modems. What if I connect a process on-board which does all the channel estimation, equalization, frequency synchronization(not sure this works in underwater environment which is frequency dependent in nature.) $\endgroup$
    – Jay Patel
    Jun 23 '20 at 14:50
  • $\begingroup$ Please edit your question to include that info!! $\endgroup$
    – Marcus Müller
    Jun 23 '20 at 14:50
  • $\begingroup$ I'm a bit confused why you'd need a dedicated Blackfin to play modem at these very low symbol rates? You'd do the same what you do in SDR: instead of using a fixed-functionality frontend like your modem, you'd simply use a sound card and software running on your PC? As soon as that works on your PC, you can still port it to your modem's DSP. But honestly, I don't see that ever becoming necessary. $\endgroup$
    – Marcus Müller
    Jun 23 '20 at 14:52
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If underwater communications is as complicated as I expect it to be, you get reflections from your own vessel, the ground, larger swarms of fish, and doppler due to water currents, and interesting pressure-dependent nonlinearties. An interesting field, indeed!

Now, that puts your modem in the position to have to compensate a lot of interesting¹ effects:

  • High attenuation
  • Doppler due to currents
  • Reflection on you vessel, ground, fish, submarines, cables, density strata, and hence a multipath channel
  • Frequency drift
  • potentially strong nonlinearity due to transducers and density effects in greater depths
  • …
  • Temporal variation of all the above effects

Thus, your modem will have to not only add error correction to the data bits you give to it, map these bits to some sort of symbol, and modulate that, and reverse that at the receiver. It will also have to estimate the channel and revert it; that's what frequency, phase and timing synchronization are all about, and what equalization does to revert a multipath channel as far as possible.

But the channel estimation would fall flat the moment multiple modems are transmitting.

Hence, you can't just add CDMA as extra layer to this problem that you can address on the data-providing side of the modem. You need to redesign the modem to be able to work as CDMA modem.

How to Tx CDMA signal physically using an existing point-to-point modem?

Redesign the modem.

Since you say in the comments that you've worked with SDRs: exactly like that, you'd develop your whole system in SDR software, simulate it, evaluate performance and improve it before you start working with "real" hardware transducers. One of such might be your current modem, but with a pretty different firmware that would be a port of your prototype to the specific Blackfin platform.

Maybe this talk (slides, paper) is a nice starting point for you; they do long reach, low speed communications using SDR approaches.

¹ and "interesting" here is the engineering definition of the word: "your rocket explodes whenever someone installs a printer in the office building? *Interesting!*"
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  • $\begingroup$ Thanks for your valuable inputs. I'll keep updating this post progressively. $\endgroup$
    – Jay Patel
    Jun 23 '20 at 15:48
  • $\begingroup$ Hm, I don't think you should mark your post as answered if you plan on updating it. That sounds more like you have new questions? If that's the case, ask them in separate questions, i.e. as new posts. $\endgroup$
    – Marcus Müller
    Jun 23 '20 at 15:56

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