Expert: Cezar L. Palconet - 12/30/2009

Question
QUESTION: Hi:

This is a follow-up to the answer you gave about FSK in http://www.allexperts.com/user.cgi?m=6&catID=2447&expID=70388&qID=4890406

You wrote:

"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"


Thanks for the book. However, from what I've read in wikipedia, PSK can have a maximum-order of 8, not 2. In other words, 8PSK is the max possible. 8PSK equates to a PSK with 3-bits-per-symbol because 2-to-the-power-3 is 8.

I hope my persistence doesn't annoy you but this topic really interests me. Is FSK really limited to just 1-bit-per-symbol? 1-bit-per-symbol for FSK equates to BFSK [or 2FSK] because 1 bit only allows two states. 3-bits allows for 8 states. 2 bits allows for 4 states.


Thanks for your understanding,

Green

ANSWER: Hi, Green

Let us see the original question.

Questions;
•   What is the maximum amount of bits-per-symbol of FSK possible
•   Why FSK can't have more than one-bit-per-symbol?


My answer to you was based primarily with FSK and not in any variant form of FSK.

The example describes a BPSK having the same bits/Symbol as an FSK, due to its two conditions.

4PSK,8PSK or any variants of PSK, cannot be compared with an FSK, you would have to compare it with its equivalent FSK variant like 4FSK, 8FSK, 16FSK etc.

FSK can only have one bit per symbol.


Regards.


Anything beyond FSK is similar to Spread spectrum.


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

QUESTION: Do 4FSK, 8FSK, and 16FSK use more than 1-bit-per-symbol? I guess the answer is yes but I could be wrong.

What about MFSK [Multiple Frequency Shift Keying]? What is the maximum amount of bits-per-symbol possible using MFSK on telephone devices?

In MFSK, more amount of different frequencies used [per symbol] translates into more bits-per-symbol. Right?

Once again, I hope my persistence on this subject doesn't frustrate you.


Thanks,

Green

Answer
Hi Green,

You are a consistent guest, and you are always welcome.

MFSK means Multi - Frequency Shift Keying, and should not be confused with MSK (Minimum Shift Keying). There are a number of different techniques, using concurrent and sequential frequencies, or a combinations of frequencies.

MFSK uses relatively narrow frequency spacing, so remarkable data rates are achieved for a given bandwidth - 64 bps in a signal bandwidth of 316 Hz is typical.

In MFSK systems, each symbol carries information according to how many distinct frequencies there are - three bits of information for 8 distinct frequencies, four bits for 16 distinct frequencies and so on, each MFSK frequency burst is one symbol. The symbol rate is always measured in baud (symbols/second), the reciprocal of the duration of the symbol.

MFSK16 signal (16 carriers) with a spacing of 15.625 Hz and operating at 15.625 baud are typical. The transmission operates at 62.5 bps and occupies about 316 Hz bandwidth. MFSK16 is always operated with FEC, so the text throughput is actually lower at only about 42 WPM (31.25 bps).
At this point it is very clear that only the bandwidth is the major factor of concern for MFSK, as you increase the amount of distinct frequencies.

However increasing bandwidth also increases noise due to intermodulation and cross talk, this limits the usable amount of spectrum and efficiency of FSK.

regards.


Regards.