Sharan123 <99077@DSPRelated> wrote:

>Dear Steve, Glen, Eric,
>
>I am opening this thread to seek some more clarifications ... I am keeping
>questions generic as sometimes standard specific terminologies tend to
>mask the underlying explanations (to me).

Okay, I am skipping your question (1) as Eric has answered it.

>2) On the RF side, are there multiple ways to carry baseband signal over
>the air? Is one better than the other?

One could define there between two majorly different ways of doing this: 
carrying the signal in the near-field or in the far-field.  But in many 
(most) circumstances the designer doesn't really have this as a choice.

Much more frequently one is faced with designing and/or specifying
the antenna design.  Antennas are crucial to system performance, can be 
quite complex, and there is a vast number of different antenna types.  
Additionally this is one place where combining may be used (beamforming 
or MIMO).  

>So, sometimes baseband bandwidth of X does not always translated to X
>bandwidth at the carrier frequency. Is this correct?

That is correct, in that selectivity can be obtained either within the 
baseband processor or at an IF or RF stage, or quite commonly both; and 
this applies separately to the transmitter and the receiver.

>BTW, I am also interested to know if converting baseband signal into RF in
>mobile communication is called modulation (in the above response, it was
>mentioned as I/Q modulation)? If so, is this the best way to carry I/Q or
>baseband data over air?

As Eric stated, this is often called modulation, but it could be
more precisely be called mixing.  How exactly this is approached is
related to, or a part of, the frequency plan for your system.
Having a good frequency plan is important.

Steve

On Tue, 29 Dec 2015 09:54:05 -0600, "Sharan123" <99077@DSPRelated>
wrote:

>Dear Steve, Glen, Eric,
>
>I am opening this thread to seek some more clarifications ... I am keeping
>questions generic as sometimes standard specific terminologies tend to
>mask the underlying explanations (to me).
>
>1) If I have to design a baseband transmission system and bandwidth is
>allocated or known to me then what is the lever I have in hand to limit
>the baseband modulated signal to baseband bandwidth. Is it the rate at
>which data arrives for baseband modulation?

I don't think there's a single way to answer this, and there are a lot
of nuanced tradeoffs and different ways to do things.

In general, though, the bandwidth will be approximated by the data
rate (bits/sec) divided by the modulation density (bits/Hz).
Increasing modulation density usually costs range or reliability.

If you're starting with a particular bandwidth, for a single-carrier
system the bandwidth is essentially the symbol rate, and the
modulation density can be increased by increasing the modulation order
(e.g., moving up from QPSK -> 16QAM -> 64QAM, etc., etc.).    For a
multi-carrier system you can pick how many subcarriers, how wide they
are, etc., etc., to fill up your usable bandwidth.

>2) On the RF side, are there multiple ways to carry baseband signal over
>the air? Is one better than the other?

Generally the RF transmission system and the baseband modulation are
matched for compatibility.   e.g., it makes no sense to send an OFDM
baseband waveform into either an RF phase modulator or a class-C power
amplifier.

Beyond that I'm not sure what you're asking.

>In the above responses, it was said that AM tends to occupy smaller RF
>BW.
>So, sometimes baseband bandwidth of X does not always translated to X
>bandwidth at the carrier frequency. Is this correct?

FM doesn't inherently occupy more bandwidth than AM.   Generally
analog FM systems adjust the modulation index in order to get an
improvement in SNR in the receiver.   So, yes, in analog FM the
modulation index can be adjusted so that the signal bandwidth and the
RF bandwidth differ by controllable amounts.

This is similar to changing the modulation density or modulation order
in a digital system.

>BTW, I am also interested to know if converting baseband signal into RF in
>mobile communication is called modulation (in the above response, it was
>mentioned as I/Q modulation)? If so, is this the best way to carry I/Q or
>baseband data over air?

Yes, the word "modulation" is overloaded and means both the type of
modulation that is used as well as the conversion process from
baseband to IF/RF.

>Once again, thanks
>
>---------------------------------------
>Posted through http://www.DSPRelated.com

Eric Jacobsen
Anchor Hill Communications
http://www.anchorhill.com

Dear Steve, Glen, Eric,

I am opening this thread to seek some more clarifications ... I am keeping
questions generic as sometimes standard specific terminologies tend to
mask the underlying explanations (to me).

1) If I have to design a baseband transmission system and bandwidth is
allocated or known to me then what is the lever I have in hand to limit
the baseband modulated signal to baseband bandwidth. Is it the rate at
which data arrives for baseband modulation?

2) On the RF side, are there multiple ways to carry baseband signal over
the air? Is one better than the other?

In the above responses, it was said that AM tends to occupy smaller RF
BW.
So, sometimes baseband bandwidth of X does not always translated to X
bandwidth at the carrier frequency. Is this correct?

BTW, I am also interested to know if converting baseband signal into RF in
mobile communication is called modulation (in the above response, it was
mentioned as I/Q modulation)? If so, is this the best way to carry I/Q or
baseband data over air?

Once again, thanks

---------------------------------------
Posted through http://www.DSPRelated.com

glen herrmannsfeldt  <gah@ugcs.caltech.edu> wrote:

>Eric Jacobsen <eric.jacobsen@ieee.org> wrote:

>(snip on ethernet, baseband, broadband, and such)

>(Steve wrote)

>>>An older definition of "baseband" is a digital signal that is
>>>intended to be directly transmitted over a line.  This corresponds,
>>>I think, to the terminology used in Ethernet.

>> In other words, it hasn't been mixed up to some IF or RF or other
>> channel frequency.    If you look at it that way, it's consistent
>> across a broader scope.

>I might have thought that you could create a non-baseband signal
>directly, without a mixer, but the process of doing that seems
>similar enough to a mixer that it might as well be called one.

>You could, for example, do all the modulation in the digital
>domain, then through a DAC, just a little filtering, and out to
>a wire or transmitter. No analog mixer. 

>But the main time when the distinction is imporant is when you
>need another signal to fit below it. For example, DSL allows
>for a baseband analog telephone signal on the same line.

The older definition I had in mind above is specifically meant to
convey that digital communications is occuring, but with a digital
logic signal being simply buffered and sent on the line, without
there being a modem per se -- just digital line drivers/receivers.

So in that sense, things like NRZ, Manchester, RZ, RLL, RS-232, were all 
thought of as baseband signals, whereas V.22 was not a baseband signal.

The more modern treatment might be that this distinction is too
easily blurred -- e.g. a Manchester encoded signal might nonetheless
involve preambles, equalization, shaping, on other features commonly
thought of as components of a modem.

More recently, "the baseband" refers to that part of a PHY layer
that is not at IF or RF, regardless of modulation method, as Eric
notes above.

Steve

Eric Jacobsen <eric.jacobsen@ieee.org> wrote:

(snip on ethernet, baseband, broadband, and such)
(Steve wrote)
>>An older definition of "baseband" is a digital signal that is
>>intended to be directly transmitted over a line.  This corresponds,
>>I think, to the terminology used in Ethernet.

> In other words, it hasn't been mixed up to some IF or RF or other
> channel frequency.    If you look at it that way, it's consistent
> across a broader scope.

I might have thought that you could create a non-baseband signal
directly, without a mixer, but the process of doing that seems
similar enough to a mixer that it might as well be called one.

You could, for example, do all the modulation in the digital
domain, then through a DAC, just a little filtering, and out to
a wire or transmitter. No analog mixer. 

But the main time when the distinction is imporant is when you
need another signal to fit below it. For example, DSL allows
for a baseband analog telephone signal on the same line.

-- glen

On Tue, 24 Nov 2015 00:14:33 +0000 (UTC), spope33@speedymail.org
(Steve Pope) wrote:

>glen herrmannsfeldt  <gah@ugcs.caltech.edu> wrote:
>
>>Eric Jacobsen <eric.jacobsen@ieee.org> wrote:
>
>>> "Baseband" generally means the signal is at it's lowest frequency for
>>> simplest processing.   For OFDM and most digital signals this means
>>> that the signal spectrum is centered at 0 and the signal spans from
>>> -BW/2 to +BW/2.    For things like AM this means the signal may run
>>> from 0 to BW  (or BW/2 for a double-sideband signal).
>
>>In the case of ethernet, baseband is the alternative to broadband.
>
>An older definition of "baseband" is a digital signal that is
>intended to be directly transmitted over a line.  This corresponds,
>I think, to the terminology used in Ethernet.
>
>Steve

In other words, it hasn't been mixed up to some IF or RF or other
channel frequency.    If you look at it that way, it's consistent
across a broader scope.


Eric Jacobsen
Anchor Hill Communications
http://www.anchorhill.com

glen herrmannsfeldt  <gah@ugcs.caltech.edu> wrote:

>Eric Jacobsen <eric.jacobsen@ieee.org> wrote:

>> "Baseband" generally means the signal is at it's lowest frequency for
>> simplest processing.   For OFDM and most digital signals this means
>> that the signal spectrum is centered at 0 and the signal spans from
>> -BW/2 to +BW/2.    For things like AM this means the signal may run
>> from 0 to BW  (or BW/2 for a double-sideband signal).

>In the case of ethernet, baseband is the alternative to broadband.

An older definition of "baseband" is a digital signal that is
intended to be directly transmitted over a line.  This corresponds,
I think, to the terminology used in Ethernet.

Steve

Eric Jacobsen <eric.jacobsen@ieee.org> wrote:

(snip)

> "Baseband" generally means the signal is at it's lowest frequency for
> simplest processing.   For OFDM and most digital signals this means
> that the signal spectrum is centered at 0 and the signal spans from
> -BW/2 to +BW/2.    For things like AM this means the signal may run
> from 0 to BW  (or BW/2 for a double-sideband signal).

In the case of ethernet, baseband is the alternative to broadband.

As well as I know it, the 6MHz wide channels used for TV signals
qualify as broad, relative to the carrier frequency (at VHF anyway).

The 10broad36 ethernet form never got very popular, but fits the
signal into a TV channel, such that it could be implemented on
cable TV systems. Current cable modems are the successors, again
using one (or more) TV channels to get the signal through.

You can normally only have one baseband signal on a cable, but
usually more than one non-baseband using different channels.

> Once the signal is mixed (i.e., translated) up to the frequency at
> which it is sent over the air, it is at RF.    It may go through a
> two-step or multiple-step conversion to get to the RF frequency, and
> the intermediate steps are called IF (Intermediate Frequency).

> Generally the signal will have the same bandwidth at baseband, IF and
> RF.   Things like double-sideband AM are an exception, where the
> modulated bandwidth is really half of the RF signal bandwidth.

If you include the negative frequencies, then you can consider the
AM bandwidth equal to the modulator input. 

-- glen