Forums

DPSK's relation with OFDM and Demodulation

Started by karthikbalaguru January 6, 2008
Hi,
I understand that simplified equalization is one of the good reasons
for going with OFDM. But, how do they manage it ?
That is,
The OFDM may be viewed as using many slowly-modulated narrowband
signals rather than one rapidly-modulated wideband signal.
So, The Differential modulation such as DPSK / DQPSK if applied to
each sub-carrier, equalization can be completely omitted. ( Reason -
These schemes are insensitive to slowly changing amplitude and phase
distortion )
But, it produces more erroneous demodulations. :(:(

So, how do they overcome this erroneous demodulation scenarios in
WiMAX ? What is the demodulation technique
normally used in overcoming erroneous demodulations w.r.t
DPSK ?

Thx in advans,
Karthik Balaguru
On Jan 6, 8:30 pm, karthikbalaguru <karthikbalagur...@gmail.com>
wrote:
> Hi, > I understand that simplified equalization is one of the good reasons > for going with OFDM. But, how do they manage it ? > That is, > The OFDM may be viewed as using many slowly-modulated narrowband > signals rather than one rapidly-modulated wideband signal. > So, The Differential modulation such as DPSK / DQPSK if applied to > each sub-carrier, equalization can be completely omitted. ( Reason - > These schemes are insensitive to slowly changing amplitude and phase > distortion ) > But, it produces more erroneous demodulations. :(:( > > So, how do they overcome this erroneous demodulation scenarios in > WiMAX ? What is the demodulation technique > normally used in overcoming erroneous demodulations w.r.t > DPSK ?
By not using DPSK! Instead, QAM is used, along with interleaving and error-correction coding. -- Oli
On Jan 7, 1:30 am, karthikbalaguru <karthikbalagur...@gmail.com>
wrote:
> Hi, > I understand that simplified equalization is one of the good reasons > for going with OFDM. But, how do they manage it ?
assuming no inter symbol interference, the transmit symbol X gets multplied by channel H to produce Y (and N is the noise) Y = HX + N so, dividing Y by H gives an estimate of X, aka single tap equalizer.
> That is, > The OFDM may be viewed as using many slowly-modulated narrowband > signals rather than one rapidly-modulated wideband signal. > So, The Differential modulation such as DPSK / DQPSK if applied to > each sub-carrier, equalization can be completely omitted. ( Reason - > These schemes are insensitive to slowly changing amplitude and phase > distortion )
hmm... I do not follow your train of thoughts. Phase okay, but how do you extent to amplitude?
> But, it produces more erroneous demodulations. :(:(
:)
> > So, how do they overcome this erroneous demodulation scenarios in > WiMAX ? What is the demodulation technique > normally used in overcoming erroneous demodulations w.r.t > DPSK ?
Nice write up on DBPSK http://tinyurl.com/2l9ukh ~ Krishna http://www.dsplog.com