Why bandwidth is limited?

Hi,

I have read many times that bandwidth is limited. Why is that? What is the bandwidth of a wireless channel anyway? I mean how to know that the channel bandwidth is X MHz? and what does that mean?

Thanks

Are you talking about channel capacity or analog signal bandwitdh? The latter simply means the frequency band occupied by the radio frequency signal.

FvM said:

Are you talking about channel capacity or analog signal bandwitdh? The latter simply means the frequency band occupied by the radio frequency signal.

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I mean the channel bandwidth. For example, they say that there are two primary resources of any communication system:

1- the channel bandwidth and
2- transmit power

For transmit power I can understand that, and for the bandwidth of a wire-line cable. But why the bandwidth of a wireless channel is limited, how, and what does it mean?

Thanks

There several factors restricting bandwidth in wireless transmission, e.g. regulations, transmitter technology, antennas.

FvM said:

There several factors restricting bandwidth in wireless transmission, e.g. regulations, transmitter technology, antennas.

Click to expand...

It is still not clear. Now, an operator company when get a licensed bandwidth, what they actually get?

It is still not clear.

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Neither the question is...

because the more the bandwidth the more the noise it has..

noise is any unwanted signal that is not helpful

I think I understand your question because this is what I wanted to know before. However I am still not clear if I understand it right now.

But what I understand is...

The wireless channel is limited because there is a limit "range of the frequency" that the signal can travel efficiently. If it's out of bandwidth, the signal can still be travel through this channel but not very well because there will be noise or whatever interruption. This causes the receiver not to be able to determine what signal was transmitted. So there is a limit bandwidth.

Anyone can correct me if I am wrong.

I think it has to do with regularizing the different wireless systems to not to interfere with each others. For example, if the FCC or whatever agency gives a mobile phone company to operate with range of frequencies from 800MHz to 2GHz, then the bandwidth is limited to 1.2 GHz. Am I right?

No mobile phone has a bandwidth of 1.2 GHz. Individual GSM bands have bandwitdhs in a 10 MHz range, GSM channels about 100-200 kHz for up- and downlink each. See: GSM frequency bands - Wikipedia, the free encyclopedia

The requirement to use rather small bands is obviously related to "not to interfere with each others". But your question seems to assume, that unlimited bandwitdh would be preferable for wireless transmission without this interference problem. Why do you think so?

P.S.: mazdaspring has added an interesting point. For a given transmitter power and noise level, increasing the bandwidth implies reducing the signal-to-noise ratio. Although a coherent wide-band signal can be recovered with a power density even below the noise level, as we do in spread spectrum and some UWB applications, it's at least involving some effort at the receiver side and won't work with arbitrary high bandwidths.

1.2GHz is just a hypothetical assumption.

I implied that more bandwidth is better, because more bandwidth means more data rates. Right?

I implied that more bandwidth is better, because more bandwidth means more data rates. Right?

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Depends on. For low signal-to-noise ratios, increasing the bandwidth will bring only a small improvement in theoretical channel capacity. With excess SNR, the higher bandwidth translates directly to channel capacity. See Shannon-Hartley theorem

The practical limitations set by RF technology and available frequency bands mostly dominate the bandwidth decision in wireless transmission, however.

Bandwidth is a scarce resource in wireless systems because:

All technologies are sharing the transmission medium (the air), to be able to multiplex and demultiplex the signal we have to send every technology's signal on a different carrier frequency and with a limited BW to avoid interference between technologies.

Bandwidth is also limited in cabled connections because:

The transmission medium here is a material (copper, fiber,..etc) you have no other users but the medium itself is a low pass filter, if your signal is very wide band it will be distorted by the medium's transfer function.

A signal on its own is never bandlimited!!! Because:

In order to be bandlimited, a signal has to be of infinite duration (starts before universe creation and ends when the universe vanishes), no signal can be non causal and infinite. On the channel the unlimited signal spectrum is either limited by a limited filter (in wireless) or by the medium itself (dispersion).

... the medium itself is a low pass filter ...

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but as I know, the medium is a bandpass filter. right? it has a low pass equivalent representation, but it is in itself is bandpass.

David83 said:

but as I know, the medium is a bandpass filter. right? it has a low pass equivalent representation, but it is in itself is bandpass.

Click to expand...

In most mediums, like copper, you can send a CW (DC) signal down them, as well as a signal with some frequency content. However, as the frequency of operation increases, the loss of the medium increases, so the insertion loss rolls off quickly. This form a (primarily) low-pass filter type of response.

---------- Post added at 21:45 ---------- Previous post was at 21:31 ----------

David83 said:

I think it has to do with regularizing the different wireless systems to not to interfere with each others. For example, if the FCC or whatever agency gives a mobile phone company to operate with range of frequencies from 800MHz to 2GHz, then the bandwidth is limited to 1.2 GHz. Am I right?

Click to expand...

You are heading down the right path with this line of thinking. We have access to an infinite spectrum of frequencies, however, only some are usable (under a few hundred GHz). Since these usable frequency ranges are somewhat limited, regulatory agencies were set up to develop some rules that governs how those limited resources are allocated, and used. Within a range of frequencies, say 88 to 108 MHz (broadcast FM radio in the USA), many stations need to share the same, finite range of frequencies. The FCC decided to divy up the range into channels spaced 0.2 MHz apart. A station's signal bandwidth is around 20kHz-30 kHz. The rest of the 200 kHz bandwidth is allocated for +/- 75 kHz of carrier frequency deviation, and a few kHz for guard bands, so one transmitter's noise floor doesn't drown out a transmitter on an adjacent channel.

A similar process has happened for 802.11 wireless standards, GSM, CDMA and other cellphone standards. The frequency range is divided up into a number of channels, and the bandwidth is the range divided by # of channels. The agencies that collaborated to form the standard did trade-offs to find a reasonably optimal trade off between bandwidth (which correlates to possible data rates), and potential number of users within that frequency range.

Bandwidth is required to pass information.

the smallest information that can be passed is on/off of a single frequency for wireless - this is morse code.
Theoretically someone could send on/off on say 1.000.000MHz and someone else on 1.000.001MHz.
but this is impossible in practice, since a receiver cannot tune to receive exactly one frequency and no other side
frequencies, so there is a "channel" spacing needed, at least 200Hz for morse code so the receiver only hears one
of them.

Now if everyone would be happily stuck in the 1910's and use morse code for information that would be dandy or not :twisted:

so with voice information, you know this is sound, with intelligible speech containing frequencies between 300Hz-3000Hz in the minimum.
This voice information needs to be modulated on top of the frequency that is to carry this information, this is the basis of the
bandwidth.
(you could in theory fit 15 different operators talking in morse code in the same bandwidth of the above example voice bandwidth of a
single operator)

better sounding speech uses 300-12000Hz, so this improved quality needs more bandwidth - more information.

the same can be extrapolated when its video information.

Thus I hope I have shown how a bandwidth is "limited" as your 1st Question asked.

The "limit" put in by authoritative regulation is allow to planning for n number of users over a given part of the spectrum.
The n increases if each user is allowed a smaller piece of the pie only.
The piece of the pie size is determined by the amount of information each user is going to transmit.
And also assuming all the users are using simultaneously.





To learn further about this search "Shannon Theory" or "Information Theory"

The best to think about bandwidth is Water flowing through the Pipe. If pipe's diameter is small then at at any given time from one end to another there is less water flowing through it. Even if you have larger bucket of water to pass through the pipe you can only pass at the full capacity of pipe. If you do so you will spill water around and hence water will be wasted.

In order to pass greater quantity of water you need to get bigger pipe (>diameter), it means you have increase the capacity of the system. Now you can pass more water/sec.

In the same way if we want error free transmission then the rate of the transfer R from one end to another based on the Capability of the channel to allow the transfer of information. This capability is the bandwidth of the system.
You can either transmit at R < this Bandwidth or equal to this Bandwidth. When the R is equal to B then this is the max information you can transmit and it is known as Capacity of the system C.

See Figure attached


1st Part of Figure * R≤C can be error free flow or transmission ideally, but other factors are also considered like effect of impairment in the channel. Noise (in communication systems)
2nd Part of Figure * Keeping B constant we can only achieve C capacity even if we increase the R’ bits/sec, and by doing so we get errors in Transmission
3rd Part of Figure * Way to achieve R’ is to increase B to B’. Hence the channel capacity is increase by increasing Bandwidth B (directly proportional)

It seems that we can achieve error free transmission even at higher rate if we increase the Bandwidth of the system
Theoretically we can get Infinite capacity or by Increasing B we can achieve infinite Transmission Rate R
But Unfortunately there is a limiting factor in communication system which is Noise
As bandwidth increases the Noise power also increases
For N = ηB , η/2 is the Noise power spectral density
R=C and at Bandwidth B,
Re-arranging Shannon- Hartley and the plotted curve is shown below.




Some point about figure are as follows:

* No error Free Transmission at -1.59dB regardless of Information rate
* Keeping B Constant (i.e, Numerator of y-axis) and Increasing the R (bits/sec) we can follow this Error free transmission but we have to increase the signal power (energy) if we want to achieve more Data Rate (Spectral Efficiency using various modulation scheme BPSK, QPSK, QAM etc
* Now, Keeping R constant (bits/sec) we can increase/decrease the Bandwidth to move along red curve as desired. It means that for a given rate we can achieve error free transmission at higher Power if B is small. Or we can achieve error free transmission at higher bandwidth and keeping Signal power small
* Hence there is a Trade-off between spectral efficiency and Power

Bandwidth Limited system - Use spectrally efficient modulation scheme (e.g. higher order QAM) & the Transmit power is not a major issue
Power Limited systems - save power at the expense of bandwidth

Hope that might help.

Rgds
Kalim

in8214 said:

The best to think about bandwidth is Water flowing through the Pipe. If pipe's diameter is small then at at any given time from one end to another there is less water flowing through it. Even if you have larger bucket of water to pass through the pipe you can only pass at the full capacity of pipe. If you do so you will spill water around and hence water will be wasted.

In order to pass greater quantity of water you need to get bigger pipe (>diameter), it means you have increase the capacity of the system. Now you can pass more water/sec.

In the same way if we want error free transmission then the rate of the transfer R from one end to another based on the Capability of the channel to allow the transfer of information. This capability is the bandwidth of the system.
You can either transmit at R < this Bandwidth or equal to this Bandwidth. When the R is equal to B then this is the max information you can transmit and it is known as Capacity of the system C.

See Figure attached

1st Part of Figure * R≤C can be error free flow or transmission ideally, but other factors are also considered like effect of impairment in the channel. Noise (in communication systems)
2nd Part of Figure * Keeping B constant we can only achieve C capacity even if we increase the R’ bits/sec, and by doing so we get errors in Transmission
3rd Part of Figure * Way to achieve R’ is to increase B to B’. Hence the channel capacity is increase by increasing Bandwidth B (directly proportional)

It seems that we can achieve error free transmission even at higher rate if we increase the Bandwidth of the system
Theoretically we can get Infinite capacity or by Increasing B we can achieve infinite Transmission Rate R
But Unfortunately there is a limiting factor in communication system which is Noise
As bandwidth increases the Noise power also increases
For N = ηB , η/2 is the Noise power spectral density
R=C and at Bandwidth B,
Re-arranging Shannon- Hartley and the plotted curve is shown below.




Some point about figure are as follows:

* No error Free Transmission at -1.59dB regardless of Information rate
* Keeping B Constant (i.e, Numerator of y-axis) and Increasing the R (bits/sec) we can follow this Error free transmission but we have to increase the signal power (energy) if we want to achieve more Data Rate (Spectral Efficiency using various modulation scheme BPSK, QPSK, QAM etc
* Now, Keeping R constant (bits/sec) we can increase/decrease the Bandwidth to move along red curve as desired. It means that for a given rate we can achieve error free transmission at higher Power if B is small. Or we can achieve error free transmission at higher bandwidth and keeping Signal power small
* Hence there is a Trade-off between spectral efficiency and Power

Bandwidth Limited system - Use spectrally efficient modulation scheme (e.g. higher order QAM) & the Transmit power is not a major issue
Power Limited systems - save power at the expense of bandwidth

Hope that might help.

Rgds
Kalim

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i want to know to what physical thing in badwidth is the diameter of the pipe analogous to?and how to increase the channel capacity?