Capacity or Data Rate?

maximum data rate of a channel

Are the capacity of the channel different when we use FDMA, TDMA, CDMA or OFDMA? Does the capacity of a channel depends on modulation or multiplexing scheme, or is it the data rate?

-how to calculate capacity and data rate?

Hi,

pmonon said:

Are the capacity of the channel different when we use FDMA, TDMA, CDMA or OFDMA? Does the capacity of a channel depends on modulation or multiplexing scheme, or is it the data rate?

Click to expand...

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Definitely capacity is different when we are using one of the abobe mentioned multiple access techniques. As for as capacity issue concerns, it depends upon many things which includes the modulation, spatial multiplexing, knowledge of channel at transmitter/receiver etc. The channel cpacity and its data rate may be different, but usually we assume that whatever the channel capacity is, we are getting the same data rate.
For further discussion on capacity issue, there are many good references and for which i will suggest the Book and work of Prof. Andrea Goldsmith. If you search her name on google u will find interesitng results on capacity from her research.

BR,

MAK

channel capacity and data rates

Hi
I think answer of mkhan is not true.
Capacity is the same in all of the mentioned method. Capacity is just depend on SNR and BW.
Capacity of the channel is the famous work of Shannon.
Capacity is not depent on modulation but it is different when transmitter have some knowledge about channel.

channel capacity and data rate

i think m_lla is correct. because it obvoius the capacity depends only on the bandwidth and SNR(signal to noise ratio).
and hows data rate is related is this that the data rate will be different for each technique.
FDMA depends upon FDM.in which the channel is shared between the users with different frequencies.
in TDMA,the channel is given to a user for a pecified time and data rate should be more for this.
and in CDMA, a code carries the users data.and this technique should have a better data rate then the others discussed

maximum data rate of a channel

Hi,

The channel capacity is the limit or the maximum rate that can be achieved over any channel and it depends on the signal set used in transmission in addition to the power and the bandwidth of the channel.

For more details, check this link:



I think, this thesis will clarify this problem.

Regards,
Ahmed

Capacity is same for a channel for any modulation scheme used to transmit data through that channel .

but datarate is the rate of sending data per second , which may vary .

thus they are different parameters for a channel .

Capacity is the maximum data rate that can be achieved with error free transmision and is a purely theoretical concept. The maximum data-rate is determined by the system design, and is associated with a given error rate.

Hi Folks,

I am sorry that i was away for couple of days and could not respond actively.

Thank you for the discussion (m_lla) and all others.

I replied that capacity may be different than data rate and i still believe in this. The argument is as follows.

Lets consider that we have a water pipe which can allow the flow of maximum 2Liter/sec through it. But the water that is flowing through it is 1L/sec. So what is the capacity of the pipe and what rate it is offering right now.

Same is the analogy with channel capacity. To be very humble, i know the Shannon formulation of capacity and work of Foschini and Teletar on the capacity of single antenna (SISO) and scaling laws for the MIMO, and to be very specific i am talking about point to point link, bcz capacity for the Broad cast channel may differe from point to point link calculations due to many factors. So lets assume that the capacity of my wireless channel is C, but the data that I want to send on that channel is with rate R where R ≤ C. Now data rate (R) can be equal or less than the capacity of the channel. According to my understanding C is the channel charactersitc that is just like pipe available to some one, and data rate (R) is the actual data that some one sending/receiveing on that particular channel. Now that data rate can be equal or less to the capacity according to the application.

To give more specific example, consider the ISDN telephone line which normally offer (128Kbps). Now if you are only using this ISDN line for just single telephone and not for any data purpose, you might be using only 64Kbps (while ur capacity is still 128Kbps i.e. your pipe is still capable of carrying 128 Kbps) but you are using only 64Kbps. So in this sense capacity and data rate may differe from each other and can be treated as different terms, but usually we consider R as same as C.

I hope I cleared my point of view.

BR,

MAK

Added after 6 minutes:

cancer06 said:

Capacity is the maximum data rate that can be achieved with error free transmision and is a purely theoretical concept. The maximum data-rate is determined by the system design, and is associated with a given error rate.

Click to expand...

Further to my previous discussion on the topic, i fully agree with subahik and cancer (sorry friends if i m not spelling ur nicks correctly). Their explanation that the capacity can be equal to the maximum error free data rate that can be achived is very much correct.

BR,

MAK