Theoretically, phase is a bit like an angle. An angle is
a way of stating the relationship between two lines; phase is a way
of stating the relationship between two SIGNALS. A signal can be
defined in terms of FREQUENCY, TIME, or SIGNAL SPACE and three
such definitions/descriptions will seem wildly different, even though
the underlying signal is the same. Much confusion comes
from interpreting a SIGNAL SPACE phase incorrectly. Also there
is a notion of STATIC phase and DYNAMIC phase and not paying attention to
the significant difference between them can be another source of
confusion.
I'm
not sure if this helps, but it's pretty much what phase is. I hope it isn't
too damn abstract. If you feel confused, it probably means that
you are SMART. Such a feeling of confusion suggests that you
have detected something that either you or the other person talking doesn't
yet altogether understand. This stuff is actually all very
intuitive. And is really a kind of abstract geometry: you just need
to learn and think about a few axioms. That don't mean it's
easy, at least for us mere mortals, but given enough time (and in my case
YEARS!) most of us can at least get some of
it.
> For QPSK, basically the same concept is
applied but the different phases are 0, 90, 180, 270.
This
is a phase comparison between two signals described in SIGNAL
SPACE. And this SIGNAL SPACE is based upon one static TIME phase and
two dynamic TIME phases. The dynamic TIME phase differences are
those of BPSK. (So in this sense the comment is correct.) The
static TIME phase difference is based on the concept of orthogonality,
which can be explained in simpler terms. (But in this sense the comment is a bit
cocky.) I'm not sure of a book without a lot of needless
calculus that describes orthogonality, but I'd guess that one
exists.
>BPSK sends two states, a "one" with one phase and "zero" with another
phase.
phase.
This is a phase comparison between two
signals described in SIGNAL SPACE. And this SIGNAL SPACE is based
upon one dynamic phase
difference.
Eric Caleb Black
| LA
Unplugged
Tel: +1(310)
600-0699 | Fax +1(310) 388-3239
http://www.LAunplugged.net | [EMAIL PROTECTED]
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Alexander Tascon
Sent: Thursday, December 18, 2003 7:20 AM
To: [EMAIL PROTECTED]
Subject: Re: [SOCALWUG] Difference between Frequency and Phase?The concept behind phase modulation is that there is a fundamental waveform and to generate different symbols the phase of that fundamental waveform is changed. For instance in BPSK there are two possible symbols one being the fundamental waveform (0 degrees phase) and the other one a shifted version of the fundamental at 180 degrees.For QPSK, basically the same concept is applied but the different phases are 0, 90, 180, 270.At 02:58 AM 12/18/2003, you wrote:
>Hello,
>
>Im having difficulties understanding exactly what Phase means, Im
reading
>802.11 Wi-Fi handbook
>(Neil Reid&Ron Seide) which explains that frequency is the complete
cycle
>of a wave.
>Phase is where the wave begins its cycle. Makes sense I guess.
>What becomes confusing is BFSK and BPSK, BFSK sends an on or off bit
with
>each frequency,
>BPSK sends two states, a "one" with one phase and "zero" with another
phase.
>If phase just determines at what point in time a frequency begins its
>cycle, how can there be a modulation scheme based on phase?
>
>Thanks
Sanford Weinberg <[EMAIL PROTECTED]> wrote:Great question... you are correct in that frequency is how long it takes to
complete one cycle (cycles per second) and phase is where it
starts...Disclaimer: this explanation is simplified for the concepts -
Remember this is a digital system. we don't look at the entire wave just
if we are at a peak or baseline at specific (sample) times. If we have a
frequency of one cycle per second (1 hertz) then to send one character
would take us typically 8 - 10 seconds (bit by bit plus overhead bits).
Remember my choice of frequency is probably limited by what I want to use
it for and the FCC
If I synchronize clocks on receiver and sender and I can process faster
then my frequency (say a tenth of a second in our example) then I can send
out theoretically 10 signals on the same frequency of 1 cycle per second at
different phases and get t! he character in one second instead of 8-10.
Let me know if you more detail is needed
At 02:58 AM 12/18/2003, you wrote:
>Hello,
>
>Im having difficulties understanding exactly what Phase means, Im reading
>802.11 Wi-Fi handbook
>(Neil Reid&Ron Seide) which explains that frequency is the complete cycle
>of a wave.
>Phase is where the wave begins its cycle. Makes sense I guess.
>What becomes confusing is BFSK and BPSK, BFSK sends an on or off bit with
>each frequency,
>BPSK sends two states, a "one" with one phase and "zero" with another phase.
>If phase just determines at what point in time a frequency begins its
>cycle, how can there be a modulation scheme based on phase?
>
>Thanks
>
>
>--------------------
>"Computer games don't affect kids, I mean if Pac Man affected us as kids,
>we'd all run around in a darkened room munching pills and listening to
>repetitive music." - Kristian Wilson, Nintendo, Inc, 1989.
>
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Alexander Tascsn
M.S. Electrical Engineering
The future is here. It's just not widely distributed yet - WIlliam Gibson
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