# CDMA

#### Nabla1

Feb 20, 2012
9
Hi,

TDMA (Time Division Multiple Access) and FDMA (Frequency Division Multiple Access) both work because each 'user' of the communication medium is transmitting at either a different frequency or at a different time.

I don't understand how a receiver can physically distinguish between users in a CDMA scheme when each is allowed to transmit with the same carrier frequency and at the same time. Even if the codes are different, how can the receiver distinguish one users code from the other if they are at the same carrier frequency at the same time?

Wouldn't it just 'hear' a nonsense merging of the two codes as one code?

I think I've missed something important here.

Thanks

#### davenn

Moderator
Sep 5, 2009
14,150
from Wikipedia....

One of the concepts in data communication is the idea of allowing several transmitters to send information simultaneously over a single communication channel. This allows several users to share a band of frequencies (see bandwidth). This concept is called multiple access. CDMA employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code) to allow multiple users to be multiplexed over the same physical channel. By contrast, time division multiple access (TDMA) divides access by time, while frequency-division multiple access (FDMA) divides it by frequency. CDMA is a form of spread-spectrum signalling, since the modulated coded signal has a much higher data bandwidth than the data being communicated.

An analogy to the problem of multiple access is a room (channel) in which people wish to talk to each other simultaneously. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different languages (code division). CDMA is analogous to the last example where people speaking the same language can understand each other, but other languages are perceived as noise and rejected. Similarly, in radio CDMA, each group of users is given a shared code. Many codes occupy the same channel, but only users associated with a particular code can communicate.

hope that helps

cheers
Dave

#### jackorocko

Apr 4, 2010
1,284
Wouldn't it just 'hear' a nonsense merging of the two codes as one code?

I think his question is still very valid. The code is analogous to the different languages, but how does the receiver know to distinguish between the codes? More or less if I was in a room with 10 different people speaking 10 different languages all at the same time. I would find it hard to distinguish them apart. Specially since I wouldn't know the languages until they where presented to me. Does that make sense?

Knowing verizon, this may well be a highly regarded secret, but I see where the OP struggles to understand this very important part.

Last edited:

#### peter.mitchell

Feb 17, 2012
14
Spreaddspectrum technology implies that it is not the exact same frequeny. Probably have their transmission coded in such a way that it can be on a VERY close but not the same freq?

#### Nabla1

Feb 20, 2012
9
I think his question is still very valid. The code is analogous to the different languages, but how does the receiver know to distinguish between the codes? More or less if I was in a room with 10 different people speaking 10 different languages all at the same time. I would find it hard to distinguish them apart. Specially since I wouldn't know the languages until they where presented to me. Does that make sense?

Knowing verizon, this may well be a highly regarded secret, but I see where the OP struggles to understand this very important part.

Thanks, glad you've understood where I'm coming from. I have heard the language <-> code analogy, but I can still see a problem:

Many things I've read say CDMA allows multiple transmitters to share a channel at the SAME time, and the SAME frequency, as opposed to both TDMA and FDMA. A visual representation of this idea, I have seen used a few times is the set of graphs in the first figure of the below:

http://www.umtsworld.com/technology/cdmabasics.htm

The problem is lets say a receiver receives signals from user A and user B at the same time, which by the above explanation may have the same RF carrier frequency. From what I've read, the data signal is modulated ('multiplied' - can be a logical XOR) with the unique 'code', and then modulated by the RF carrier at the transmitter. The reverse process happens at the receiver - the signal is received at the antenna, demodulated from the RF carrier, and THEN demodulated using the 'code' to retrieve the data.

Now if user A and B's signals arrive at the receiver antenna at the same instant in time, with the same RF carrier and user A signalling a logical 1, while user B is signalling a logical 0, then surely what the receiver sees is just a logical 1 (At least in the case of AM RF modulation), which is an interference of the two signals, perceived as a single signal.

Surely the codes used to demodulate the data from this signal in the following receiver stage are redundant now, since neither of the users codes will receive any useful data (i.e. not nonsensical) from the perceived signal.

Maybe another way to ask the question would be: how does the receiver distinguish between the logical '1' of user A, and the logical '1' of user B, when they arrive at the antenna at the same time, with the same carrier frequency, and same RF carrier modulation scheme? Surely the 2 different codes are redundant now?

#### davenn

Moderator
Sep 5, 2009
14,150
Thanks, glad you've understood where I'm coming from. I have heard the language <-> code analogy, but I can still see a problem:

Many things I've read say CDMA allows multiple transmitters to share a channel at the SAME time, and the SAME frequency, as opposed to both TDMA and FDMA. A visual representation of this idea, I have seen used a few times is the set of graphs in the first figure of the

Its not really an issue. I think you are making it seem more complicated than it really is... even using basic CTCSS tone encoding/decoding you can have multiple transmitters and receivers on the identical freq. Your receiver will only respond to the correct CTCSS code. its been used in the commercial and amateur radio world for many years and thats in the analog world. It gets even easier when everything is all digital.

Dave

#### Laplace

Apr 4, 2010
1,252
When you take a low frequency signal and multiply it by a high data rate pseudo-random code, what you get is a high data rate noise signal or a spread spectrum signal that can be transmitted over the air. At the receiver you can take the spread spectrum signal and multiply it by the same high data rate pseudo random code, what you get is the original low frequency signal but all the power that was spread over a wide bandwidth is now concentrated in the low frequency signal. However, if there were also a second spread spectrum signal received that had been generated by a different pseudo-random code, when this is multiplied by the non-matching code signal what you get is still a high data rate noise signal or spread spectrum signal -- there is no concentration of signal power into the low frequency signal as was seen with a matching code. So with CDMA you just need to consider the signal to noise degradation caused by additional spread spectrum signals versus the signal power concentration for the original low frequency signal. And since a spread spectrum signal is indistinguishable from noise, it does not matter whether an interfering signal is another non-matching spread spectrum signal or just random noise. If just a small component of a received noise signal is a spread spectrum signal, correlating that whole noise signal with the matching code will leave the noise signal power spread over the whole bandwidth but will also concentrate the matched signal power into its narrow frequency band where it can be filtered and recovered.

#### jackorocko

Apr 4, 2010
1,284
ok, So you said the receiver knows the same pseudo-random code. How does this work, does the receiver tell the transmitter to use a certain pseudo-random code, like during the initial handshake between the receiver and transmitter?

#### Laplace

Apr 4, 2010
1,252
The details of the code synchronization are implementation specific for any system design. But it is more than just using the same code. The receiver needs to maintain phase sync with the transmitted code to within a fraction of a microsecond for high spreading factors or else the correlation will be lost. The CDMA Basics website posted by Nabla1 has a lot of good information, but it assumes the reader has some level of prior knowledge about the process of signal spreading and correlation.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
It's a bit like comedy.

It relies on a shared experience and really good timing.

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