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Optics and Channel Coding

Started by Ted June 2, 2004
Have been looking at some reports on optical transmission. Looks like
channel coding concepts used in wireless, wired comms can be applied
to optics as well. It appears to have convolutional encoding. viterbi
decoding etc etc.

How prevelant is this channel coding in optics ?  What are the major
differences in channel coding between wireless and optics ?
ed_ted_ed@yahoo.com (Ted) wrote in message news:<39c154be.0406020848.6da7793@posting.google.com>...
> Have been looking at some reports on optical transmission. Looks like > channel coding concepts used in wireless, wired comms can be applied > to optics as well. It appears to have convolutional encoding. viterbi > decoding etc etc. > > How prevelant is this channel coding in optics ? What are the major > differences in channel coding between wireless and optics ?
Optics use Reed-Solomon codes a lot.
On 2 Jun 2004 09:48:06 -0700, ed_ted_ed@yahoo.com (Ted) wrote:

>Have been looking at some reports on optical transmission. Looks like >channel coding concepts used in wireless, wired comms can be applied >to optics as well. It appears to have convolutional encoding. viterbi >decoding etc etc. > >How prevelant is this channel coding in optics ? What are the major >differences in channel coding between wireless and optics ?
Think about Shannon's channel capacity theorem and *why* we use channel coding at all. There is a tradeoff between bandwidth and SNR and implementation complexity. Wired and Wireless systems are (usually) bandwidth limited. The carrier frequencies are in the range ~kHz through 0.1 THz or so. They use multi-level signalling and coherent detection. They employ means to limit the transmitted bandwidth to avoid interfering with other channels. They must use some sort of channel coding to improve throughput. Optical systems have a huge bandwidth available. The carrier frequencies are usually in the range 190 - 360 THz. (850nm, 1310nm and 1550nm are the common wavelengths.) Optical systems usually use two level signalling (OOK), with non-coherent detection (e.g. PiN diode). "Wireless"-style channel coding is not used at all in modern systems (AFAIK); there is no point in doing so, since we can run several Tbps through a fibre and not even come close to the shannon bound. So far, commercial systems transmit up to 40Gbps per wavelength. Many wavelengths can be transmitted per fibre. (The ITU-T specify both 50GHz and 100GHz channel spacings for WDM (Wavelength Division Multiplexing).) Note: long haul optical systems often use FEC. This is usually something simple like Reed Solomon coding as another poster pointed out. Regards, Allan.
"Ted" <ed_ted_ed@yahoo.com> ha scritto nel messaggio
news:39c154be.0406020848.6da7793@posting.google.com...

> How prevelant is this channel coding in optics ? What are the major > differences in channel coding between wireless and optics ?
I have read about Turbo Product Codes (www.aha.com: they have a few technical documents). This kind of channel codes, with iterative (turbo) decoding, allow you to reach very low BERs using long codes. In AWGN channels the gain over Reed-Solom+Viterbi is at least 3dB. Don't know much about optical channels, but I guess turbo product codes are a very good choice for high data rates due to their highly parallelizable architecture. Regards, Emanuele