# Capacity vs bandwidth

1. Aug 3, 2007

### znb

hi,
1.please tel me in detail why do we say that for higher capacity we require more bandwidth?
higher capacity means more data rate, meaning more bits per seconds(and i know BW is bps) but tell me in terms of frequencies availble in a band..coz we transmit our signal on 1 freq then what it has to do with the other freq in that band, when we transmit our signal after modulating it on some higher freq carrier then how does it actually travel in the air..what my understanding is that only sinusiods (with single frequency)travel then where is the data? and also what is the use of the bandwidth, let say if i modulate my signal on 2Ghz and it has a BW of say 500MHz then my antenna generates the signal of 2GHz sinusiod..so where is the data coz its just a plain sinusiod of 2G and also what are the other freq in that band doing?..does this antenna generate 2.1G, 2.2G also and propagates like a signal with variable frequencies to extract the data in terms of variation in freq??Plz explain..um so confused and i nkow um very bad with basics.

2. also why we say that more bits per seconds means more variations in the signal(in terms of time domain) and does more variations in the signal refer to more frequency componets present in that signal?

2. Aug 3, 2007

### chroot

Staff Emeritus
You cannot transmit any meaningful information on a single frequency.

A signal of exactly one frequency is nothing more than a sine wave. You can represent ones and zeros by the presence or absence of this signal, but that's it, and it's really unreliable.

When people say "I'm transmitting my data at 2 GHz," they do not mean that they're using only one specific frequency. What they mean is they're transmitting over some band of frequencies centered around 2 GHz.

If you imagine modulating this pure sine wave in any way, you will introduce components of other frequencies. The two most familiar modulation schemes are instructive. Amplitude modulation (AM) varies the amplitude of a carrier frequency. This actually smears the signal out over a nearby range of frequencies, called a channel. In frequency modulation (FM), the carrier frequency itself is varied to encode the transmitted information. This obviously involves a range of frequencies.

Your intuition is corect: you cannot change a signal in the time domain without also changing its spectrum in the frequency domain.

- Warren

Last edited: Aug 3, 2007
3. Aug 4, 2007

### rcgldr

A 2 ghz FM signal with 500mhz bandwidth could be implemented as a signal with a rage from 2ghz-250mhz to 2ghz+250mhz. With AM signals, it's not quite as obvious, the rate of change in amplitude uses up bandwidth. Morse code transmitters modulate the ramp on and off time to take several milliseconds to reduce bandwidth.

4. Aug 4, 2007

### znb

>>When people say "I'm transmitting my data at 2 GHz," they do not mean that they're using only one specific frequency. What they mean is they're transmitting over some band of frequencies centered around 2 GHz.

I got this thing sir but how does it transmit in the air. does your antenna transmit all these frequencies in that band at the same time? and if lets suppose u are using FM then how your anteena transmit the signal.
eg. for 101 it transmit one freq then for 010 another freq from the band and so on...in a consecutive fashion..is it right??

and how does it go with AM? coz here only the amplitude of the waveform changes and this waveform will be having only one frequency..then what the other frequencies of the band be doing??

5. Aug 4, 2007

### znb

also tell me why we do need more BW for higher cap data.

higher cap>>more bits per sec>>more variations in the data>>more freq components present coz of variations>>therefore more BW?

is this right?? please correct me.

BUT variations can me bw 2-3 frequencies?right then how this whole thing goes??
please HELP

6. Aug 4, 2007

### chroot

Staff Emeritus
Of course it does. If it didn't, the receiver would not be able to recreate the signal.

No, the FM transmitter simultaneously transmits on all the frequencies in the band.

Think about your voice. Your voice contains frequencies from about 100 Hz all the way up to about 5 kHz. When you speak, your voice uses frequencies all throughout this band, simultaneously. Your listener will only be able to understand you if his/her ears receive all of thse frequencies simultaneously.

If you continuously vary the amplitude of a pure sine wave, you no longer have a signal of just one frequency.

Think about this: one way to vary the amplitude of a sine wave is to add other sinusoids of different frequencies to it. Try it on a graphing calculator. So, a sine wave with a varying amplitude is not a pure sine wave: it contains all kinds of different frequency components.

- Warren

7. Aug 4, 2007

### znb

ok if transmitter transmits all the frequencies in that band simultaneously then we would have different sinusoids in the air as an output of our antenna..lets say 900Mhz, 900.1 Mhz and so on rite?? but they r pure sinusoids where is our data?there must be some thing that indicates..e.g.
usually in telecom our digitized voice is modulated on some higher frequency..
for bits 00 we assign some freq1 and for 10 another freq2 and so on ...if our speech Sx is 00 10 00 10 10...so freq1 and freq2 ll be trasmitted?? BUT 00 is transmitted only 2 times where as 10 is 3 times then what is done at the transmitter that tells this thing..PLZ EXPLAIN

Last edited: Aug 4, 2007
8. Aug 4, 2007

### chroot

Staff Emeritus
Not just discrete frequencies like 900 MHz and 900.1 MHz. The information can be transmitted at every frequency simultaneously, in a continuum.

As an example, one of the simplest sorts of digital information transmission is called BPSK, or "binary phase-shift keying." This modulation uses a single carrier frequency, but can broadcast the carrier at two different phases, 180 degrees apart.

Each bit will occupy a specific amount of time. Let's say the transmitter transmits 0 degrees phase for one microsecond, then switches to 180 degrees phase for the next microsecond. That might be an encoding for a zero, and a one.

It may sound like this information occupies only a single frequency, but it does not. The transitions from one phase to the next involve abrupt changes in the waveform, and these discontinuities involve energy in other frequencies.

- Warren

9. Aug 4, 2007

### znb

ok that means in air we transmits all these frequencies that helps in constructing a waveform that changes its phase from 0 to 180 and to get this, all the frequencies are sent at different power levels..
MY main concern is that how we transmit them in air??i mean all these freq ll be in sinusoid form,it is the power with which they r transmitted creates the difference?...

10. Aug 4, 2007

### chroot

Staff Emeritus
You can think about a transmitted waveform in either the time domain or in the frequency domain. It's not particularly helpful to imagine a transmitter designed to simultaneously transmit multiple perfect sinusoids on different frequencies, with different powers.

Instead, just understand that a transmitter which transmits an arbitrary, complex waveform is doing fundamentally the same thing.

- Warren

11. Aug 4, 2007

### znb

but complex waveform doesnt travel in air?

12. Aug 4, 2007

### chroot

Staff Emeritus
Of course complex waveforms can travel through air (in the form of electromagnetic radiation)! Complex waveforms are nothing but a sum of simple sinusoids, and simple sinusoids travel through air just fine.

- Warren

13. Aug 4, 2007

### znb

you tell me how it goes in this case..

-yu have binary data 1 0 and you are required to do frequency modulation.
-if ur data is 1000001
-then for 1 you slect one freq and for 0 another
-but 1 only appeares twice in the message where as 0 appears 5 times
-you transmit freq A and B for 0 1 respectively
-BUT how you manage to tell that 1 appears only 2 times where as 0 appears for 5 times?????

14. Aug 4, 2007

### chroot

Staff Emeritus
What you're describing is binary frequency-shift keying (BFSK).

You break the transmission down into units of time. For the first microsecond, you transmit frequency A. For the second, third, fourth, fifth, and sixth microseconds, you transmit frequency B. For the seventh microsecond, you transmit frequency A again.

- Warren

15. Aug 4, 2007

### znb

but you said earlier
and if lets suppose u are using FM then how your anteena transmit the signal.
eg. for 101 it transmit one freq then for 010 another freq from the band and so on...in a consecutive fashion..is it right??

No, the FM transmitter simultaneously transmits on all the frequencies in the band.

16. Aug 4, 2007

### chroot

Staff Emeritus
I never said anything like that. FM is an analog modulation scheme not, a digital one.

- Warren

17. Aug 4, 2007

### znb

ME:
and if lets suppose u are using FM then how your anteena transmit the signal.
eg. for 101 it transmit one freq then for 010 another freq from the band and so on...in a consecutive fashion..is it right??

YOU:
No, the FM transmitter simultaneously transmits on all the frequencies in the band.

SCROLL UP

18. Aug 4, 2007

### znb

why dont you come to yahoo sir??

19. Aug 4, 2007

### chroot

Staff Emeritus
Again, FM is an analog (continuous) form of modulation, in which an analog signal (like a voice) is used to modulate a carrier.

BFSK is a digital (discrete) form of modulation, in which only two specific frequencies are used.

Your questions are so vague that I really do not understand where you are stuck. Simply telling me "SCROLL UP" doesn't help.

I prefer not to use Yahoo.

- Warren

20. Aug 4, 2007

### znb

but there i can make you understand more clearly what i exactly want to know

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