Coding, Modulation & the relationship between frequency and data rate

[Turksen]

Hi,

I'm a bit confused by the relationship between frequency and data rate and wondered if someone could help clarify please.

Say I want to achieve a data rate of 10Mbit/s - 10 millions bits transmitted across a link every second - and am using on-off modulation (say +5V = '1' bit and 0V = '0' bit), which by default is NRZ (non return to zero). What is the required frequency: 10 MHz, or could we say that each cycle can transmit 2 bits - so a frequency of 5 MHz would do?

Also, to then improve this could I use a different modulation scheme or form of coding? I've read that QPSK or QAM are more 'spectrally efficient' and can transmit more bits per symbol, but isn't the frequency determined by the BIT rate not SYMBOL rate?

Thanks for any tips!

 

[hexreader]

QAM is used by Modems for transmitting data across analogue circuits. I do not think it is normally appropriate for use with simple square wave digital transmission.

The (analogue) frequency of a perfect square wave is infinite, and contains all frequencies. But what you are talking about, when you say "frequency" is actually bit-rate, not really frequency, hence the difference in terms. Sorry if this adds further confusion!

If you really want to learn about QAM, then you need to google lots, and read up on a lot of background information. Data transmission over analogue lines is a HUGE subject, and a dying technology for telephone circuits (probably expanding technology somewhere else I don't know about).

Maybe a simple theoretical idea will help:

Imagine that instead of sending 1 as +5V and 0 as 0v..... you send (binary) 00 as 0v, 01 as 1 v, 10 as 2v, 11 as 3v. This gives you two bits of information in the space of one bit. This would be a simple (but ridiculous) version of something similar to a QAM type of idea.

Don't know if this helps, but it is what happens to spill out of my brain today.

EDIT: By the way.. Bit rate and symbol rate (and Baud rate) are the same in 99.9% of cases that I know about. The hypothetical case I gave above is the other 0.01%. I would forget about the difference unless you want to do some serious research into the subject.

I saw your thread on the other forum, and I can see why you are now confused.

 

[Turksen]

Ok, thanks for that.

So are we saying that bit rate and frequency are synonymous? i.e. use a 10MHz clock to transmit 10 Mbit/s data.

Or should we consider some of the harmonic make-up of a square wave?

Perhaps I'm actually overcomplicating this and trying to read too deep into it.

 

[hexreader]

So are we saying that bit rate and frequency are synonymous? i.e. use a 10MHz clock to transmit 10 Mbit/s data.

In your application, then probably yes.

Or should we consider some of the harmonic make-up of a square wave?

No, I do not think that there is any need, unless you are building your own Modem

Perhaps I'm actually overcomplicating this and trying to read too deep into it

Yes, I think this is true.

 

[Turksen]

Quote:
Perhaps I'm actually overcomplicating this and trying to read too deep into it

Yes, I think this is true.

Ha. I'll leave it there then. Thanks for your help!