Baseband Bandwidth

Hello,

With respect to wireless communication, I have a question I would like to
clarify.

I have some trouble conceptually appreciating the bandwidth on the
baseband side.
For example, if the baseband bandwidths available are 5 & 10 MHz then does
it mean that one can transmit with double rate on 10 MHz signal compared
to 5 MHz signal? 

In other words, is the user data carried out at the rate of 10 MHz signal
versus a 5 MHz in the above cases?

Thanks ...

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On Sat, 21 Nov 2015 00:08:02 -0600, "Sharan123" <99077@DSPRelated>
wrote:

>Hello,
>
>With respect to wireless communication, I have a question I would like to
>clarify.
>
>I have some trouble conceptually appreciating the bandwidth on the
>baseband side.
>For example, if the baseband bandwidths available are 5 & 10 MHz then does
>it mean that one can transmit with double rate on 10 MHz signal compared
>to 5 MHz signal? 
>
>In other words, is the user data carried out at the rate of 10 MHz signal
>versus a 5 MHz in the above cases?
>
>Thanks ...

If the modulation type and code rate are not changed, then increasing
the signal bandwidth from 5MHz to 10MHz would essentially mean that
the symbol rate has doubled, so, yes, the information rate doubles.


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

On Saturday, November 21, 2015 at 1:08:06 AM UTC-5, Sharan123 wrote:
> Hello,
> 
> With respect to wireless communication, I have a question I would like to
> clarify.
> 
> I have some trouble conceptually appreciating the bandwidth on the
> baseband side.
> For example, if the baseband bandwidths available are 5 & 10 MHz then does
> it mean that one can transmit with double rate on 10 MHz signal compared
> to 5 MHz signal? 
> 
> In other words, is the user data carried out at the rate of 10 MHz signal
> versus a 5 MHz in the above cases?
> 
> Thanks ...
> 
> ---------------------------------------
> Posted through http://www.DSPRelated.com

The information rate is not a function of the signal's location in frequency. The information rate is linearly proportional to the available bandwidth that you have, baseband or passband.

Your confusion might be in I/Q modulation though. If you use only the in-phase part of the carrier (i.e., real signals in baseband), it means you are not utilizing the half of the available bandwidth for information transfer. The signal will be symmetric in frequency domain. But if you use both in-phase and quadrature part simultaneously, you can fully use total bandwidth for information transfer. 

Hi All,

I thought through this and here is the source of my confusion.

In case of wired communication, when we say 1 GHz clock then it generally
means that we can transfer at Gbps+. In wired communication, there is no
such concept as 800 MHz - 1 GHz or 200 MHz bandwidth.

So, when we say 800 MHz frequency to carry data in wireless, I can
understand, but when you say 20 MHz bandwidth, it confuses me. Or, in
simple terms, if someone can explain how this 20 MHz bandwidth is used, it
would help a lot ...
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"Sharan123" <99077@DSPRelated> writes:

> Hello,
>
> With respect to wireless communication, I have a question I would like to
> clarify.
>
> I have some trouble conceptually appreciating the bandwidth on the
> baseband side.
> For example, if the baseband bandwidths available are 5 & 10 MHz then does
> it mean that one can transmit with double rate on 10 MHz signal compared
> to 5 MHz signal? 
>
> In other words, is the user data carried out at the rate of 10 MHz signal
> versus a 5 MHz in the above cases?

Do not confuse the terms "data rate" and "bandwidth". While they are
related, they are definitely not the same thing.

Take a look at the block diagram in Figure 1 here:
http://www.digitalsignallabs.com/tutorial.pdf. The "data rate" is the
rate at the output of the data source. However, the bandwidth is the
bandwidth of the signal m(t) (at the output of the modulator).

The bandwidth of the signal depends on several things other than the
data rate, e.g., modulation scheme, type and amount of coding, access
type (e.g., CDMA spreads the bandwidth very wide).

Finally, it doesn't matter whether the signal is baseand or carrier:
the same bandwidth and data rate applies either way.
-- 
Randy Yates
Digital Signal Labs
http://www.digitalsignallabs.com

You are it seems confusing CARRIER frequency with CLOCK
Frequency and BANDWIDTH.

These are 3 different things.

Mark

>Do not confuse the terms "data rate" and "bandwidth". While they are
>related, they are definitely not the same thing.
>
>Take a look at the block diagram in Figure 1 here:
>http://www.digitalsignallabs.com/tutorial.pdf. The "data rate" is the
>rate at the output of the data source. However, the bandwidth is the
>bandwidth of the signal m(t) (at the output of the modulator).

>The bandwidth of the signal depends on several things other than the
>data rate, e.g., modulation scheme, type and amount of coding, access
>type (e.g., CDMA spreads the bandwidth very wide).
>
>Finally, it doesn't matter whether the signal is baseand or carrier:
>the same bandwidth and data rate applies either way.

Dear Randy,

If I may ask, in case of modern communications (like mobile), we have data
source, baseband bandwidth and then RF carrier. 
I understand that data source sources data limited by max. data rate.
I can understand that RF carrier occupies certain frequency range as the
RF carrier frequency is modulated with user data.

Where I get a little confused is how the baseband bandwidth is used.
Is there a frequency modulation due to which frequency variations happen?

Also, I have one more conceptual question - in mobile communication,
spectrum is scarce and every operator is given a frequency band to
operate. My question is, why not use some sort of amplitude modulation, in
which case you need just one frequency per operator and not frequency
band?

That also brings to one more question - what type of modulation is applied
at the RF frequency in mobile communication?

PS: sorry for some basic questions but I don't have much background into
wireless communication ...


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Sharan123 <99077@dsprelated> wrote:

(snip)

> In case of wired communication, when we say 1 GHz clock then it generally
> means that we can transfer at Gbps+. In wired communication, there is no
> such concept as 800 MHz - 1 GHz or 200 MHz bandwidth.

Much of the terminology is reused, where the meaning isn't the
original meaning anymore.

Baseband, more or less, means that the spectrum goes, more or less,
down to zero. More realistically, if the low end of the spectrum
is lower than the spectral bandwidth it should probably be baseband.

That is, there isn't room for another signal with that bandwidth
below the one in question.
 
> So, when we say 800 MHz frequency to carry data in wireless, I can
> understand, but when you say 20 MHz bandwidth, it confuses me. Or, in
> simple terms, if someone can explain how this 20 MHz bandwidth is used, it
> would help a lot ...

In analog AM radio, double sideband modulation, the bandwidth is
twice the highest modulation frequency. A digital signal, with 
appropriate filtering, has its highest frequency component half the
highest transition rate. (This is actually what Nyquist found.)
The combination of those two is that the bandwidth of AM and
digital modulation is bandwidth equal to the highest transition
rate, which is often the clock rate.

So, there is one example where the data rate matches the bandwidth.

The term, though, is often used even when other modulation methods
are used. First, note that AM is wasteful, using twice the bandwidth
needed, as there are both upper and lower sidebands. The simplicity
of modulation is wasteful by a factor of two.

Next note that one has to extract a clock from the signal to get
the bits out again. Manchester coding is an old favorite for
simplifying the extraction of a clock from the data, while doubling
the bandwidth.

So far, the spectral bandwidth is within a factor of two of the
data rate. More or less, that is close enough to equate data rate
and bandwidth.

Better modulation methods reduce the bandwidth required, but
make demodulation harder.

-- glen

Sharan123 <99077@dsprelated> wrote:

(snip)

> If I may ask, in case of modern communications (like mobile), we have data
> source, baseband bandwidth and then RF carrier. 
> I understand that data source sources data limited by max. data rate.
> I can understand that RF carrier occupies certain frequency range as the
> RF carrier frequency is modulated with user data.

Consider a popular baseband data transmission method, ethernet.

Usual 10 megabit/s ethernet is manchester coded, such that it
needs 10MHz of bandwidth. If you send a stream of all 0's or
all 1's, the frequency on the wire is 10MHz. A stream of 
alternating 1's and 0's is 5MHz, and other bit patterns are
in between. Note that it doesn't go down to 0Hz, and since
it is transformer coupled can't go down to 0Hz, but is still
considered baseband.  If you add another signal onto the cable,
the ethernet decoder likely won't track the data anymore.

Broadcast radio and TV signals are modulated such that one can
send and receive more than one at a time on the same cable or
over the air. An AM radio station has about 10kHz bandwidth,
that is, 5kHz above and below the frequency on the dial.

FM broadcasts fit in 200kHz, with some guard band. (Theoretically,
an FM signal has infinite bandwidth, but practically it works.)
Broadcast TV in the US uses 6MHz wide channels, either for analog
or digital.
 
> Where I get a little confused is how the baseband bandwidth is used.
> Is there a frequency modulation due to which frequency 
> variations happen?

For AM/DSB, about all I can explain here, the bandwidth is twice
the highest modulating frequency. AM radios are supposed to fit
within a 10kHz band, so the highest modulating frequency should
be 5MHz. While audio equipment commonly claims 20Hz to 20kHz,
there is little that is actually that far up. Stations are
assigned such that adjacent channels are not broadcast from too
near each other.
 
> Also, I have one more conceptual question - in mobile communication,
> spectrum is scarce and every operator is given a frequency band to
> operate. My question is, why not use some sort of amplitude modulation, in
> which case you need just one frequency per operator and not frequency
> band?

As noted above, AM is not one frequency per operator. In the beginning
of radio, it took a little while to understand how modulation and
bandwidth worked.
 
> That also brings to one more question - what type of modulation is applied
> at the RF frequency in mobile communication?

Which mobile communication? AM radio uses AM/DSB (double side band).

FM uses frequency modulation with 75kHz deviation. That is, if
you follow the instantaneous frequency, it should stay within 75kHz
of the stated carrier. (Note that FM carriers are always odd 
multiples of 100kHz, such that they fit within +/- 100kHz evenly
spaced channels. The actual bandwidth with 75kHz deviation is
somewhat wider, but small enough to avoid problems with nearby
signals. Again, they space the stations out to avoid adjacent
channels being too close physically.

> PS: sorry for some basic questions but I don't have much background into
> wireless communication ...

You might try the ARRL handbook, which a library might have, for
a lot of information on modulation methods and bandwidth.

-- glen

Dear Glen,

>Consider a popular baseband data transmission method, ethernet.
>
>Usual 10 megabit/s ethernet is manchester coded, such that it
>needs 10MHz of bandwidth. If you send a stream of all 0's or
>all 1's, the frequency on the wire is 10MHz. A stream of 
>alternating 1's and 0's is 5MHz, and other bit patterns are
>in between. Note that it doesn't go down to 0Hz, and since
>it is transformer coupled can't go down to 0Hz, but is still
>considered baseband.  

Thanks. The above explanation helps a lot.

>An AM radio station has about 10kHz bandwidth,
>that is, 5kHz above and below the frequency on the dial.

Are you saying that the frequency of a specific AM signal that is
transmitted can vary by +-5KHz. But being AM modulation, I assumed that
carrier frequency should not vary (in theory at least).

>> That also brings to one more question - what type of modulation is
>applied
>> at the RF frequency in mobile communication?
>
>Which mobile communication? AM radio uses AM/DSB (double side band).

I was mainly interested to know the modulation used in case of cell phone
communication.

>You might try the ARRL handbook, which a library might have, for
>a lot of information on modulation methods and bandwidth.

Sure. Thanks ...

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