Expert: Cezar L. Palconet - 12/28/2009

Question
QUESTION: Hi:

The following is not a homework question. It is a question of my genuine interest in electronics and telecommunications.

What is the maximum amount of bits-per-symbol of FSK possible using a telephone system [including the phone lines and any devices from start to finish of the phone's signal chain]? Let's assume a baud of only 1-symbol-per-second.


Thanks,

Green

ANSWER: Hi Green

The limiting factor is the line noise, a 64 kbps is possibly the maximum you can get from any  public switched telephone network, even with that, in real life it would be much less.

regards.

---------- FOLLOW-UP ----------

QUESTION: Thanks for your response but I was asking about the maximum amount of bits-per-symbol [not bits-per-second] possible using FSK devices on telephone systems.


Happy Holidays,

Green

ANSWER: hi Green

For an FSK, the bit rate is equal to the symbol rate, it is mainly determined by the amount of fixed set values for a specific bandwidth of frequency, the maximum rate you can achieve using this technique is basically determined by the degree at which the two states becomes incoherent, and how the receiver will decode the signal, with a given signal to noise ratio at the input of the receiver.

At this point you'll have so much inter-symbol interference that you will not be able to detect what you transmitted in the first place. So to this point, FSK is noise limited.

So my previous answer still hold valid for FSK.


Regards.

---------- FOLLOW-UP ----------

QUESTION: Thanks for your response

Sorry but I don't understand why FSK can't have more than one-bit-per-symbol. Why does FSK have this limitation when QAM and PSK don't?

PSK can have up to 3-bits-per-symbol and QAM can as much as 7-bits-per-symbol on a telephone line. So why the restriction for FSK?


Thanks again,

Green

Answer
Hi Green

For the same reason that you can only have two states in a BPSK or 2 PSK.

It uses two phases which are separated by 180° It is, however, only able to modulate at 1 bit/symbol and so is unsuitable for high data-rate applications when bandwidth is limited.

The only difference is the first one is a frequency dependent and the other is phase dependent, they both only have two conditions to key.

A good reference on this subject matter would be Hykins and Simon.

Haykin, Simon (1988). Digital Communications.
John Wiley %26 Sons
ISBN 0-471-62947-2  

Regards.