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'[OT]:Spread Spectrum Efficiency for Continuous sig'
2002\04\12@133310 by Brandon Irwin

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     I've been reading about spread spectrum and I don't see how spread
spectrum techniques could be used for continuous transmissions.

     Everyone seems to want to convert everything to spread spectrum
because of its greater efficiency. I don't see how it could be any more
efficient when used on continuous transmissions such as a TV video signal.
The following assumes that they would keep the signal in the analog form.
If you have four available channels with four TV stations and you implement
the freq hopping method, then all four channels will always be full because
the TV stations would never have a pause to allow a fifth channel to
transmit. If you where to use a method similar to TDMA, then you would see
little gaps in the picture.
It seems to me that the only way the "more efficient" spread spectrum
methods could be used would be to convert the signal to a digital form so
it could be buffered on the TV side while the transmission ceased for
whatever amount of time.

     Is there something I'm missing? Maybe the whole concept?




-Brandon Irwin






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2002\04\12@135453 by Sean H. Breheny

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Hi Brandon,

I don't understand all of the aspects of this, but I'll take a stab at an
answer (engage mouth before brain ;-)

There are two types of efficiency involved: bandwidth and energy. You are
talking about bandwidth efficiency (i.e., how much information per time can
I send in a given bandwidth). Another possibility is to ask "How much
information per time can I send with a given average output power?"
Typically, I see the second type of efficiency emphasized more when it
comes to SS signals.

If you look at the Shannon-Hartley theorem (the information theorem that
says what is the absolute limit on the average bitrate you can get on an
additive Gaussian white noise channel with a given amount of noise,
bandwidth, and output power), you will see that the bitrate always
increases as bandwidth increases, but it levels off once you exceed a
certain bandwidth. Traditional types of communications systems typically
use much less than this threshold of bandwidth, which means that
inherently, they are using only a fraction of the bitrate they could
achieve with a given noise density and output power. By spreading their
signal out over a larger amount of spectrum, they could achieve a greater
power efficiency.

As for bandwidth efficiency, I don't know the details of this in regard to
SS. One of the issues is that SS signals, especially the ones that use DSSS
(Direct Sequence SS, using pseudonoise spreading sequences), essentially
just look like noise. So, if two of them are partially interfering, the
results might be less devastating than two signals with traditional
modulations types interfering. Also, you probably can arrange SS signals so
that they use all of their bandwidth equally, whereas some other modulation
types may require that there be no other signals within, say, 100kHz, but
they don't use all of that 100kHz very often or with an equal average power
(imagine an FM signal which only occasionally gets to full deviation), but
an SS signal using that 100kHz could use it more evenly, have the same
requirements as regards channel spacing, and yet send more bits per second.

Sean

At 10:29 AM 4/12/02 -0700, you wrote:
{Quote hidden}

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2002\04\12@142051 by Douglas Butler

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Spread spectrum is much more hype than real benefit.  Frequency hopping
is good to prevent eavesdropping.  Direct Sequence spreading helps you
hide in the noise.  Both are useful for military applications, but have
limited ligitimate civilian use.

If you had four channels of space with a varying number of users,
sometimes more than four, who couldn't agree on who should use what
space when, then spread spectrum helps to arbitrate the use of bandwith.
When too many users occupy the band everybody's signal to noise ratio
degrades, and everybody suffers equally.

I can stay on this soapbox for longer than anybody wants to read...

Sherpa Doug

> {Original Message removed}

2002\04\12@142706 by M. Adam Davis

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Spread spectrum doesn't necessarily mean frequency hopping.

A regular signal sends out a tall thin spike of energy at one frequency
with a small bandwidth.  A spread spectrum signal sends out a short fat
'lump' of energy at a frequency with a much larger bandwidth.

The key to spread spectrum, though, is the encoding.  By transmitting
data as a string of pseudo-random 1s and 0s then the signal appears as
noise to all regular transmitters and receivers.  Since the signal being
sent is low energy, and spread across a larger section of frequency then
the noise is acceptable.

Thus you can use essentially the same frequencies for two different
types of communication.

Both the transmitter and receiver know the sequence of pseudo random
numbers, and through some rather simple signal processing the signal can
be extracted very readily from a very noisy signal.

Modern spread spectrum signals are almost always digital.

The next nice thing is that the pseudo random sequences make it possible
for several spread spectrum devices to coexist in the same band without
causing problems, especially if the pseudo random sequences are
zero-balanced (ie, same number of ones as zeros, so the total signal
energy is zero)

-Adam

Brandon Irwin wrote:

{Quote hidden}

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2002\04\12@143532 by Sean H. Breheny

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Hi Doug,

I'd like to hear more. I have no practical experience with SS, but the
theoretical arguments that I've heard seem strong enough that I believed
that there was some benefit (especially with regard to more closely
approaching the theoretical channel capacity). Where does reality depart
significantly from this?

Thanks,

Sean

At 02:07 PM 4/12/02 -0400, you wrote:
{Quote hidden}

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2002\04\12@145951 by Jim

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The efficiencies are really 'gained' when using SS in
a multiple access/crowded/relatively un-coordinated
system, ala a cellular system (CDMA), un-licensed and
usually un-coordinated use of a swath of spectrum etc.

GPS is a GOOD example of the use of SS - all birds
transmit in the same 'channel' with different, but
known, 'spreading codes'.

'Know the code' of the bird you are interested in
and the 'codes' from all other birds appear as
noise and are 'un-correlated' (not recovered).

Any differences between that pre-set 'spreading code'
of the desired bird is an information bit-stream that
contains 'data' associated with the desired bird.

Jim



{Original Message removed}

2002\04\12@150417 by Jim

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  "Spread spectrum doesn't necessarily mean
   frequency hopping."

The secret, basically, is: "What is the IF of the system involved?"

A freq hopper can have a relatively narrow IF - roughly that
needed to accomodate the information bandwidth and the
radios 'jump (hop) around' seeking the next transmitted
'packet' -

- whereas a SS system must have an IF that will accomodate
the bandwidth of the 'spreading code' .. the 'information
bandwidth' of a SS signal is a fraction of the spreading
code.

Jim



{Original Message removed}

2002\04\12@152048 by Jim

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    "Spread spectrum is much more hype than real benefit.
     ...  Direct Sequence spreading helps you hide in the
     noise.  Both are useful for military applications, but
     have limited ligitimate civilian use."


Two civilain uses:

GPS and IS-95/CDMA 'PCS' (as employed by Verizon and, I believe, also
Sprint as pioneered by Qualcomm) -

Not to mention a variety of 'unlicenssed' apps (point to point T1 links)
in the ISM bands at 900 MHz and 2.4 GHz.

CDMA removes most of the nightamre that is known in the 'discrete
frequncy' world of telecom (all your flavors of TDMA like GSM,
IS-136) as 'system-channel planning'.

Instead of juggling just the same 416 channels (in a classical
cellular system) you get to also select different spreading
Walsh 'codes' from site to site ... that along with suitable
'offsets' in the stepping from sites with the same 'code'.

Jim


----- Original Message -----
From: "Douglas Butler" <spam_OUTdbutlerTakeThisOuTspamIMETRIX.COM>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Friday, April 12, 2002 1:07 PM
Subject: Re: [OT]:Spread Spectrum Efficiency for Continuous signals


{Quote hidden}

> > {Original Message removed}

2002\04\12@152843 by Douglas Butler

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I have not used Spread Spectrum, but I have studied it for a few
applications that didn't pan out.

As far as bandwidth efficiency, I can see some improvement, but a great
cost in complexity.  A digital bit stream has a SinX/X bandwidth.  We
allocate bandwidth by frequency so it comes in rectangular blocks.  A
SinX/X curve does not fit well in a rectangular block.  If you allocate
a block big enough to hold the SinX/X then there is significant
under-utilized bandwidth in the allocated block.  DSSS fills a
rectangular bandwidth block better.

Also when there are a few too many users for the available band a Spread
Spectrum system does not fail catastrophically by sitting a new user on
top of an existing user.  Instead all users begin to suffer a rising
noise level and service may not be good, but may be acceptable for the
short term overload.

GPS is a good use for SS.  The satellites don't have to keep track of
which of them is visible where to negotiate channel space.  And you
don't have to give each one its own channel.  If you have so many
satellites visible that they start to interfere with each others
reception then you must have lots of pseudorange data to feed into you
computation.  You will get a good position fix from not-so-good data, if
you have enough data.

{soapbox bold}

What really     raises my hackles is using SS for computer clocks, or clock
modulation to pass emissions tests.  The important point about clock
modulation that most people don't realize is that it does nothing to
reduce the actual interference that customers will see in the field.  It
is a clever scheme to circumvent government regulations and nothing
more.  If a milliwatt of energy at exactly 60MHz will cause a certain
amount of rectification in an audio pre-amp, smearing that same energy
from 59MHz to 61MHZ will cause almost exactly the same disruption.  Few
of the potential victims of EMI have any way to make such fine
distinctions in frequency.  The only real solution to the real problem
is to reduce the energy radiated, or shift it in frequency by a very
large amount, like at least a factor of two or three.

I work with EMI problems all the time in oceanographic equipment,
keeping digital noise out of sensitive sonar front ends.  For me the FCC
regulations are irrelevant.  There is nothing like an inch thick steel
pressure case and a thousand meters of seawater for shielding!  The only
EMI requirements I have to meet are Mother Natures, and she doesn't care
about peaks on a spectrum analyzer.  When a switching supply or a
microprocessor at one end of my instrument is radiating and causing
rectification in a sensitive amp at the other end modulating the
interfering source is not going to help at all.

The FCC regulations are not obstacles to be overcome.  They are tools to
help solve real problems.  When finding ways to comply with the rules,
you need to realize that rules are there to prevent problems
and simply complying to the exact letter of the rule may still leave the
underlying problem unsolved.  The FCC rules sometimes need some common
sense interpretation.  Unless we designers use our common sense, the FCC
will have to rewrite the rules to eliminate loopholes like clock
modulation, which will make the rules more complicated and restrictive
than they currently are.

{/bold /soapbox}

Sherpa Doug

> {Original Message removed}

2002\04\12@201908 by Barry Gershenfeld

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1. They would probably digitize the TV signal and push it through
the spread-spectrum system, and recover it on the other side.

2. In any of these schemes you have a bit rate for your signal, and
you have a clock rate for your "hopping".  If the clock is slower
than your bit rate, you have frequency hopping.  In this case you
might have a whole scan line come out on one frequency before
it hopped to the next.  Or at least some number of pixels.

Once you make the clock frequency *faster* than your bit rate
you now have spread spectum.   Now, a single pixel will
be spread across several "hops".  This will help you see how
interference might cause you to lose "some" of your pixel
but not all of it.

3. GPS uses spread spectrum, and the interesting thing is
that the signal level is so low (like -160dB) that it
would not be detectable using a conventional single-frequency
transmitter and conventional receiver.   The signal is
literally below the noise level.

4. One of the neat tricks CDMA uses is to actively control
the transmitter power level on *your* phone--it is reduced
to a level sufficient for a low-error signal.  This allows
more users with less interference.

Actually, systems that are really "analog" are a good application
here, because you can tolerate a certain amount of noise on
a TV picture or phone conversation, without degrading the
apparent quality.

Barry

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2002\04\12@234202 by dbengtson

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I suspect that Qualcomm and all the thousands of people who have made
millions from their stock would strongly disagree with you.



On Fri, 12 Apr 2002 14:07:32 -0400, you wrote:

{Quote hidden}

>> {Original Message removed}

2002\04\12@234905 by dbengtson

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While I agree (somewhat) with your assessment of spread spectrum
clocks, I disagree that they are only a means of skirting an FCC
Emissions regulation. In your example, if I have a strong 60 MHz clock
that is rectifying in a junction (Essentially generating harmonics and
self mixing) then if I spread out the energy across 2 MHz, the problem
will go away. While the energy is still there, the rectification that
does take place will mean that the products are under the thermal
noise floor. Keep in mind that IP2 products go up 2 dB for each 1 dB
in input power increase. IP3 Products go up 3 dB for each 1 dB. I I
can drop the peak power by 10 dB, then my IP3 products go down by 30
dB. That's enough to make a big difference in noise problems.

Dave





On Fri, 12 Apr 2002 15:20:56 -0400, you wrote:

{Quote hidden}

>> {Original Message removed}

2002\04\12@235047 by dbengtson

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You can use Spread spectrum for continuous transmissions, but you must
digitize the signal. Analog Spread spectrum was used in WW2 for crypto
applications, but it's necessary to digitize to get the multiple user
stuff done effeciently.

Dave



On Fri, 12 Apr 2002 10:29:43 -0700, you wrote:

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2002\04\13@010937 by Dale Botkin

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>Spread spectrum is much more hype than real benefit.  Frequency hopping
>is good to prevent eavesdropping.  Direct Sequence spreading helps you
>hide in the noise.  Both are useful for military applications, but have
>limited ligitimate civilian use.

Dunno -- I won't even look at non-SS cordless phones.  Yes, I know SS
phones *can* be monitored by someone with enough time, money and desire.
It's not those folks I'm worried about, I just don't want the neighbor's
kid with a scanner to hear every call.

Dale

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2002\04\13@055747 by Scott Stephens
2002\04\13@115239 by Jim

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  "if I have a strong 60 MHz clock that
   is rectifying in a junction"

THAT takes a lot of 'leakage' from a 'box' to do that.

I rather doubt this condition is ever experienced in real
life ...

My cell phone and 440 MHz HT (in the 1/2 Watt setting) can
both cause 'clicks' in my active/amplified PC speakers from
a foot (12 inches/30 cm) away -

The cell phone will cause a 'click' in the speakers on
origination (origination PL (power level) probably set for
PL = 2 or 3 (.6/.250 Watts) for origination (the power level
set for default before power level is set by the MTSO based
on received signal strength)) and the TDMA rep rate (a fuzzy
'buzzing' sound) can then be heard with the phone right next
to the speaker with the amplifier during a call (power level,
PL, now probably 5 or 6 (sub 100 mW range) since I'm line of
sight to ATTWS cell site #197 on a water tower to my north).

A prior example cited 1 mW of leakage power as causing a
junction to rectify? Not likely (unless the coupling to the
affected circuit is REALLY, REALLY tight) - and it's also
not likely that as much energy as what constitutes 1 mW
is likely yo leak from any bos (unless, of course, that
box is a transmitter).

Jim



----- Original Message -----
From: "David Bengtson" <dbengtsonspamKILLspampobox.com>
To: <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
Sent: Friday, April 12, 2002 10:46 PM
Subject: Re: [OT]:Spread Spectrum Efficiency for Continuous signals


{Quote hidden}

clock
{Quote hidden}

> >> {Original Message removed}

2002\04\15@013857 by ISO-8859-1?Q?Ruben_J=F6nsson?=

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I haven't seen anybody mention the advantage of eliminating multi path
fading problems with SS.

In an industrial environment with transmitters moving around you could have
several paths from the transmitter to the receiver due to the signal
bouncing on the surrounding structures inside a building. When two
different paths differs in length with a multiple of one half wavelength,
the signal could effectively be blanked out at the receiver. By
continuously changing the channel this problem could be limited to just
some channels.

This requires that the system allows that data that is lost at some hops
could be retransmitted in another hop, just like collisions in a CSMA
scheme, which implies that it isn't suited for continuous transmission.

I don't really know if this works in practice but it is one of the
advantages that is mentioned in favor of using SS methods.

Ruben


Date sent:              Fri, 12 Apr 2002 10:29:43 -0700
Send reply to:          pic microcontroller discussion list
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From:                   Brandon Irwin <BrandonIspamspam_OUTLEAKED.INFO>
Subject:                [OT]:Spread Spectrum Efficiency for Continuous signals
To:                     @spam@PICLISTKILLspamspamMITVMA.MIT.EDU

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2002\04\15@101112 by Douglas Butler

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>
>    "if I have a strong 60 MHz clock that
>     is rectifying in a junction"
>
> THAT takes a lot of 'leakage' from a 'box' to do that.
>
> I rather doubt this condition is ever experienced in real
> life ...
>

Actually that is quite common.  It is the principal of the Bose
detector.

http://www.tuc.nrao.edu/~demerson/bose/bose.html

With modern high performance op amps it is becomming more of a problem.
At the operating frequency (audio or sonar) the op amp can close the
loop so it operates with a tiny input and in its linear range.  But if
an RF signal can apply 1mV of differential signal to the input, and it
is too fast for the feedback loop to handle, but the op amp front end is
fast enough to amplify it, then the middle and output sections of the op
amp will go non-linear and cause distortion of legitimate signals the
are being amplified at the same time.

Sherpa Doug

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2002\04\15@102409 by Douglas Butler

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The point is that the amplitude of each cycle is the same whether it is
at the 60MHz nominal frequency, or if this particular cycle is shifted
to 61MHz.  It is only when the spectrum is averaged over time that the
signal amplitude appears lower.  If you do a spectrum analysis of any
single cycle of a SS modulated clock, it amplitude will be almost
exactly the same as a cycle of unmodulated clock.  When the signal self
mixes it is mixing with itself.  It is not mixing with an average cycle.
So the amplitude of the individual cycles is what matters.  This mixing
produces a difference signal which is DC which disrupts low frequency
legitimate signals the amplifier is supposed to handle.

Sherpa Doug

> {Original Message removed}

2002\04\15@102425 by Jon Baker

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----- Original Message -----
From: "Douglas Butler" <TakeThisOuTdbutlerEraseMEspamspam_OUTIMETRIX.COM>

> Actually that is quite common.  It is the principal of the Bose
> detector.
>
> http://www.tuc.nrao.edu/~demerson/bose/bose.html

Thats quite an interesting paper. I never realised people were working at
those wavelengths so soon after the discovery of EM radiation. And to
predict the existence of p and n type semiconductors over 100 years ago.. my
hat goes off the the man.

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Jon Baker

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2002\04\15@143451 by Jim

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  "Actually that is quite common."

I have never seen this in practice.

  "But if an RF signal can apply 1mV of
   differential signal to the input"

Some thing called *Good Design Practice* can
minimize these effects to sensitive circuitry.

Your previous example also cited a power level
or 1 mW as being capable of this sort of effect.

I suspect that those circuits didn't function under
simple fluorescent lighting either (Hi-E fields
if you know what I mean).

I would still like a practical cite of a case where
a 1 mW emission from a 'box' affects another device
to the degree that it affects operation. Such devices,
I suspect, were 'one of' laboratory prototypes and
not production.

I *have* had 'leakage' from RF power amplifiers that
then affected instrumentation (thermocouples) during
test - but nothing to the degree you seem to indicate.

I build radio systems with sub-microvolt sensitivity
receivers that share a common device called a
duplexer, A -120 dBm capable receiver intimately
working with a +50 dBm output transmitter into a
common antemma without desense, noise or blocking.

I have also worlked on design teams that designed
portable two-way radios ('handy - talkies') who
didn't experience the kind of effects you describe
when designing microphone amplifier and limiting
circuitry - *as long as* proper design techniques
were observed and the means attention paid to
decoupling AND circuit layout ...

Jim

{Original Message removed}

2002\04\15@144449 by Jim

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The 'leakage' you're addressing most likely *isn't*
capable of directly affecting other devices, but, rather
would cause, say, a nasty herring bone on a CH 2 TV
signal. Direct rectification of signal at junction
requires that the jucntion see a voltage at least
sufficient to shift the Q point and either turn off
(bipolar device) or turn on and that requires in the
neighbor hood of .65V. Even for a milivolt sensitive
op-amp - it still requires that the applied errant
RF signal affect the cirtcuit's BIAS point (assuming
the RF signal is much above the upper bandwidth of
the op-amp) by rectification of the applied hi-frequency
AC (RF) signal).


By spreading this 'tone' (as we in the RF field are
prone to term them) over a much wider range a barely
perceptable effect might be the result. This is a
far cry better effect than a noticible 'herring
bone' appearing on a TV set. Bear in mind that a
signal (tone) 40 dB down from a TV station's carrier
can be evident on a TV screen ...

Jim


{Original Message removed}

2002\04\15@220820 by Barry Gershenfeld

picon face
>I haven't seen anybody mention the advantage of eliminating multi path
>fading problems with SS.

I didn't want to get that deep into it :)

>In an industrial environment with transmitters moving around you could have
>several paths from the transmitter to the receiver due to the signal
>bouncing on the surrounding structures inside a building. When two
>different paths differs in length with a multiple of one half wavelength,
>the signal could effectively be blanked out at the receiver. By
>continuously changing the channel this problem could be limited to just
>some channels.

This is called "diversity reception" and might be applicable while
hopping, but SS is different enough that you can't just ignore
some channels.

>This requires that the system allows that data that is lost at some hops
>could be retransmitted in another hop, just like collisions in a CSMA
>scheme, which implies that it isn't suited for continuous transmission.

As I said before, in SS each bit is spread over several hops.  And it's
worse (or better) than that.  The hopping is so fast that the each
"hop", as we would think of it, doesn't look like a discrete signal
on a certain frequency.  It is a wideband signal that covers a large
range.

>I don't really know if this works in practice but it is one of the
>advantages that is mentioned in favor of using SS methods.

What they actually are able to do is see the echoed signal being
received at a later time (remember SS clocks are _fast_) and
so they can recognize and filter them out.

When I heard this last item, I said, "If they can see them they ought
to be able delay them and add them back into the real signal to
recover even more of the signal."   I don't know if that's been
done but it seems obvious so I expect they have.

>>
>> -Brandon Irwin
>>
>> -------------------------------------
>> This message was written using the Dvorak keyboard layout.

lgid ks mddk tsfw mo h.sipv
or is it
bcj. yr m..y frgw mp ekrpat

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2002\04\15@225047 by Jim

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This kind of 'trick' can also be/is employed on what
I call 'discrete frequency' systems like the TDMA
systems using a 'channel equalizer' that works with
the delays contribited by the different propagation
'paths' that otherwise can intoduce ISI (Inter-Symbol
Interference) due to this 'Time Diversity' effect.

By 'equalizing' each uniquely identified delay path (and
tracking same as the delay will change as a subscriber
moves about) and it's associated 'channel' these
'channels' may be 'equalized' before being numerically
summed to represent the true digital waveform that was
transmitted - and is now being 'demodulated'/decoded.

So, this approach isn't unique to SS/CDMA systems only.

Jim


{Original Message removed}

2002\04\15@233036 by Jim

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   "As I said before, in SS each bit is spread
    over several hops.  And it's worse (or better)
    than that.  The hopping is so fast that the each
    "hop","

I don't know why you guys refer to 'hops' when discussing
a spread spectrum system.

The 'modulator' we used in out GPS test sets consisted
simply of an XOR gate - this effectively controls the
inversion the phase of the 'carrier' sent through the
XOR gate under the control of the spreading code (and
information bit stream), and, indeed, this is the
technique (effectively) used to modulate a carrier
with 'GPS information'.

Where 'hopping' comes in when 'phase' modulating a signal
with using an XOR gate - I can't grasp and just doesn't ring
true ...

This XORing (phase modulation technique actually) instead
serves to 'spread' the signal over a frequency range as
determined by the rate at which the XOR gate is controlled
by a 'spreading code'. The 'spreading code' is normally
modified as to sequence by a much slower and normally
integrally related 'information code' train. The two result
in a w-i-d-e bandwidth digital signal that has the actual
information spread over a wide (well, whatever the
spreading code set it to be) bandwidth.

Demodulation/recovery of the information bit stream calls for
phase-locking to the *known* 'spreading code' and looking
for any differences - those differences being the applied
(and much slower and integrally related) 'information code
train'. This process is called 'correlation' - any differences
from what would be *perfect* correlation is due to either
un-expected noise (in the real world) or the intended
modulation bit stream ...

Prior to demodulation is where a 'channel equalizer' (RAKE
receiver) would be inserted to correct for/make use of the
time diversity aspect of delay spread due to multi-path
in a cluttered/urban environment ...

Jim




{Original Message removed}

2002\04\16@022558 by ISO-8859-1?Q?Ruben_J=F6nsson?=

flavicon
face
I was talking about frequency hopping spread spectrum only, not
direct sequence.

Sorry if I confused anyone...

Ruben

Date sent:              Mon, 15 Apr 2002 18:56:16 -0700
Send reply to:          pic microcontroller discussion list
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From:                   Barry Gershenfeld <RemoveMEbarryEraseMEspamEraseMEZMICRO.COM>
Subject:                Re: [OT]:Spread Spectrum Efficiency for
Continuous signals
To:                     RemoveMEPICLISTspam_OUTspamKILLspamMITVMA.MIT.EDU

{Quote hidden}

==============================
Ruben Jvnsson
AB Liros Electronic
Box 9124, 200 39 Malmv, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
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2002\04\17@040802 by Peter L. Peres

picon face
>I haven't seen anybody mention the advantage of eliminating multi path
>fading problems with SS.

Afaik no commercial band permitting SS is wide enough to allow the SS to
'overcome' standing wave 'holes' easily. Space diversity works much better
(at least two antennas at least at the base station). The SS can be
equated to space diversity antennas at a given distance (expressed in
lambdas). Since the band is narrow, this distance will be small wrt. what
can be achieved by simply putting the second antenna N/4 lambdas (or some
other well chosen number) away from the first. To achieve the same effect
with SS you would need a bandwidth of 25%.  Only the military can afford
that sort of thing or so I think.

Peter

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2002\04\17@070150 by ISO-8859-1?Q?Ruben_J=F6nsson?=

flavicon
face
We manufacture a radio modem (RS232/422 to RF and back) which
works in the 2.4GHz ISM band. The following is quoted from the
the integration guide for the radio part.

Begin quote

1.1. Why spread spectrum?

The radio transmission channel is very hostile, corrupted by
noise, path loss and interfering transmissions from other
radios. Even in a pure interference-free environment, radio
performance faces serious degradation through a phenomenon
known as multipath fading, a problem particularly prevalent for
indoor installations, which results when two or more reflected
rays of the transmitted signal arrive at the receiving antenna
with opposing phase, thereby partially or completely canceling
the desired signal. In the frequency domain, a multipath fade
can be described as a frequency-selective notch that shifts in
location and intensity over time as reflections change due to
motion of the radio or of objects within its range. At any
given time, multipath fades will typically occupy perhaps 5% of
the band, which means that from a probabilistic viewpoint, a
conventional radio system faces a 5% chance of signal
impairment at any given time due to multipath.

Spread spectrum reduces the vulnerability of a radio system to
both interference from jammers and multipath fading by
distributing the transmitted signal over a larger region of the
frequency band than would otherwise be necessary to send the
information. This allows the signal to be reconstructed even
though part of it may be lost or corrupted in transit.

End quote

This device uses frequency hopping spread spectrum. Not direct
sequence.


Ruben

{Quote hidden}

==============================
Ruben Jvnsson
AB Liros Electronic
Box 9124, 200 39 Malmv, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
RemoveMErubenKILLspamspampp.sbbs.se
==============================

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2002\04\17@082732 by Alan B. Pearce

face picon face
>In the frequency domain, a multipath fade can be
>described as a frequency-selective notch that shifts
>in location and intensity over time as reflections
>change due to motion of the radio or of objects
>within its range.

Well put.

Just do not have the mid point of your range over a tidal water course :)
Strange fades happen every 12 and a bit hours if both the transmitter and
receiver can see this point. I believe this is a well known problem for
telecommunications engineers dealing with microwave links for telephones and
broadcasting distribution.

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2002\04\17@092131 by Douglas Butler

flavicon
face
> >In the frequency domain, a multipath fade can be
> >described as a frequency-selective notch that shifts
> >in location and intensity over time as reflections
> >change due to motion of the radio or of objects
> >within its range.
>

It is frequency selective, but at a fairly low Q.  So I agree with Peter
that no commercial system has a wide enough bandwidth see much benefit
from this effect.

{Quote hidden}

I see this effect all the time with sonar.  I never thought about it
with radio!

Sherpa Doug

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2002\04\17@112932 by Graeme Zimmer

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face
Hi,

> It is frequency selective, but at a fairly low Q.  So I agree with Peter
> that no commercial system has a wide enough bandwidth see much benefit
> from this effect.

In Australia we now have CDMA mobile phones. CDMA of course uses DS spread
spectrum.

Where I live we have terrible multipath reception, TV is an unwatchable mess
and conventional mobile phones are useless. Even a car-phone is unusable.

My new CDMA phone can't understand what the fuss is. It works solid with no
drop-outs or distortion. There isn't a single dead spot.

............................. Zim

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2002\04\17@141110 by Tom Messenger

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> TV is an unwatchable mess

Same here.

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2002\04\17@144847 by Dale Botkin

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face
On Wed, 17 Apr 2002, Tom Messenger wrote:

> > TV is an unwatchable mess
>
> Same here.

Me too, especially since I got cable.

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2002\04\17@162415 by Bob Ammerman

picon face
You don't need any interference or multipath for TV to be an unwatchable
mess. It is generally such 24/7 everywhere you might be.

Bob Ammerman
RAm Systems

{Original Message removed}

2002\04\17@212209 by John Ferrell

flavicon
face
Y'all are flogging a dead horse. 50 years ago (1951 or 52?) the head of the
US FCC described Television as "Bubblegum for the mind."
Quantity has increased, but quality may have gone down hill.

{Original Message removed}

2002\04\18@164533 by Peter L. Peres

picon face
> 2.4GHz spread spectrum

Ruben, I believe that the quotation about your product is true, but I also
believe (strongly)  that the 2.4GHz band is at most 10% wide. If the
standing wave pattern will not have narrow deep notches (as it usually
hasn't - they are nice |sin| ones) then by not using space diversity you
have a very good chance to be in the 'dark' 50% of the time.  This
experience I have is mostly from 2.4GHz video links which do this,
especially near full range with lots of metal around. It is common
practice to bend a piece of piano wire and put it as 'reflector' behind
the rubber ducky receiver antenna. With some tweaking you get rid of the
'hole' (and have some gain). Paddle antennas are 'fixed' using metal trays
(aluminium) of comparable size behind their back ... with a SD transmitter
these problems do not exist.

Peter

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