Forums

why DFSK?

Started by zqchen November 23, 2007
DFSK (differential frequency shift keying) performs worse than FSK,
then why DFSK is also applied in practice?
what's the common demodulation method for DFSK, especially when
channel coding is also used?
thanks!
On Nov 23, 4:59 am, zqchen <zhiqun.c...@gmail.com> wrote:
> DFSK (differential frequency shift keying) performs worse than FSK, > then why DFSK is also applied in practice? > what's the common demodulation method for DFSK, especially when > channel coding is also used? > thanks!
I can't think of a generic reason why Differential Frequency Shift Keying would be used. Can you give an example? Maybe the practical reason is in the specifics. John
John <sampson164@gmail.com> writes:

> On Nov 23, 4:59 am, zqchen <zhiqun.c...@gmail.com> wrote: >> DFSK (differential frequency shift keying) performs worse than FSK, >> then why DFSK is also applied in practice? >> what's the common demodulation method for DFSK, especially when >> channel coding is also used? >> thanks! > > I can't think of a generic reason why Differential Frequency Shift > Keying would be used.
I'm with you, John - I can't think of a reason. Seems like a bad idea. The usual reason differential encoding is used is so that the receiver doesn't have to have phase lock in m-ary PSK, but phase lock isn't required for normal FSK anyway. -- % Randy Yates % "How's life on earth? %% Fuquay-Varina, NC % ... What is it worth?" %%% 919-577-9882 % 'Mission (A World Record)', %%%% <yates@ieee.org> % *A New World Record*, ELO http://www.digitalsignallabs.com

John wrote:


>>DFSK (differential frequency shift keying) performs worse than FSK, >>then why DFSK is also applied in practice? >>what's the common demodulation method for DFSK, especially when >>channel coding is also used? >>thanks! > > > I can't think of a generic reason why Differential Frequency Shift > Keying would be used. Can you give an example? Maybe the practical > reason is in the specifics.
Suppose you connect a simple modem to the analog input/output of FM radio. You don't know how many signal inversions are there in the modulator/demodulator path. The brain dead way to deal with that is the differential coding. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Vladimir Vassilevsky <antispam_bogus@hotmail.com> writes:

> John wrote: > > >>>DFSK (differential frequency shift keying) performs worse than FSK, >>>then why DFSK is also applied in practice? >>>what's the common demodulation method for DFSK, especially when >>>channel coding is also used? >>>thanks! >> >> >> I can't think of a generic reason why Differential Frequency Shift >> Keying would be used. Can you give an example? Maybe the practical >> reason is in the specifics. > > Suppose you connect a simple modem to the analog input/output of FM > radio. You don't know how many signal inversions are there in the > modulator/demodulator path. The brain dead way to deal with that is > the differential coding.
Yeah, it's a brain-dead method alright. Rather than build a simple frame structure with an identifiable synchronization sequence into the protocol that degrades the rate by a small fraction, you'd rather throw away a constant 3 dB of performance? -- % Randy Yates % "Though you ride on the wheels of tomorrow, %% Fuquay-Varina, NC % you still wander the fields of your %%% 919-577-9882 % sorrow." %%%% <yates@ieee.org> % '21st Century Man', *Time*, ELO http://www.digitalsignallabs.com
On Nov 23, 6:20 pm, Randy Yates <ya...@ieee.org> wrote:
> Vladimir Vassilevsky <antispam_bo...@hotmail.com> writes: > > John wrote: > > >>>DFSK (differential frequency shift keying) performs worse than FSK, > >>>then why DFSK is also applied in practice? > >>>what's the common demodulation method for DFSK, especially when > >>>channel coding is also used? > >>>thanks! > > >> I can't think of a generic reason why Differential Frequency Shift > >> Keying would be used. Can you give an example? Maybe the practical > >> reason is in the specifics. > > > Suppose you connect a simple modem to the analog input/output of FM > > radio. You don't know how many signal inversions are there in the > > modulator/demodulator path. The brain dead way to deal with that is > > the differential coding. > > Yeah, it's a brain-dead method alright. Rather than build a simple > frame structure with an identifiable synchronization sequence into > the protocol that degrades the rate by a small fraction, you'd > rather throw away a constant 3 dB of performance? > -- > % Randy Yates % "Though you ride on the wheels of tomorrow, > %% Fuquay-Varina, NC % you still wander the fields of your > %%% 919-577-9882 % sorrow." > %%%% <ya...@ieee.org> % '21st Century Man', *Time*, ELOhttp://www.digitalsignallabs.com
Randy, Why is the penalty 3 dB? John
John wrote:
> On Nov 23, 4:59 am, zqchen <zhiqun.c...@gmail.com> wrote: >> DFSK (differential frequency shift keying) performs worse than FSK, >> then why DFSK is also applied in practice? >> what's the common demodulation method for DFSK, especially when >> channel coding is also used? >> thanks! > > I can't think of a generic reason why Differential Frequency Shift > Keying would be used. Can you give an example? Maybe the practical > reason is in the specifics.
Its probably a homework question, rather than a practical one. They are probably looking for the standard answer to the standard drawback of differential coding - it doubles the BER. One screwed up symbol in the raw stream, results in 2 differentially screwed up symbols in the output stream. Steve
John <sampson164@gmail.com> writes:

> On Nov 23, 6:20 pm, Randy Yates <ya...@ieee.org> wrote: >> Vladimir Vassilevsky <antispam_bo...@hotmail.com> writes: >> > John wrote: >> >> >>>DFSK (differential frequency shift keying) performs worse than FSK, >> >>>then why DFSK is also applied in practice? >> >>>what's the common demodulation method for DFSK, especially when >> >>>channel coding is also used? >> >>>thanks! >> >> >> I can't think of a generic reason why Differential Frequency Shift >> >> Keying would be used. Can you give an example? Maybe the practical >> >> reason is in the specifics. >> >> > Suppose you connect a simple modem to the analog input/output of FM >> > radio. You don't know how many signal inversions are there in the >> > modulator/demodulator path. The brain dead way to deal with that is >> > the differential coding. >> >> Yeah, it's a brain-dead method alright. Rather than build a simple >> frame structure with an identifiable synchronization sequence into >> the protocol that degrades the rate by a small fraction, you'd >> rather throw away a constant 3 dB of performance? >> -- >> % Randy Yates % "Though you ride on the wheels of tomorrow, >> %% Fuquay-Varina, NC % you still wander the fields of your >> %%% 919-577-9882 % sorrow." >> %%%% <ya...@ieee.org> % '21st Century Man', *Time*, ELOhttp://www.digitalsignallabs.com > > Randy, > > Why is the penalty 3 dB? > > John
Because I was assuming it is the same as PSK, which is probably utterly wrong... -- % Randy Yates % "Maybe one day I'll feel her cold embrace, %% Fuquay-Varina, NC % and kiss her interface, %%% 919-577-9882 % til then, I'll leave her alone." %%%% <yates@ieee.org> % 'Yours Truly, 2095', *Time*, ELO http://www.digitalsignallabs.com
It's used in real world, like TETRA - terrestrial trunked radio.
zqchen wrote:
> It's used in real world, like TETRA - terrestrial trunked radio.
TETRA uses differential QPSK. Steve