# How to modulate a WCDMA Signal?

#### PersianEmpire

Joined Oct 29, 2017
1
Hello all,

I have got a basic question which I appreciate if someone can help me on it. Given the attached scenario, how are we going to modulate the output signal using QPSK? More specifically, how we can send for instance +2 and -2 using the QPSK?

Many thanks

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#### Motanache

Joined Mar 2, 2015
481

You should start from quadrature modulation(I/Q modulation).
There are many IC who use that

You have two sinusoid at 90 degrees phase shift that summed up.
The first sine wave is modulating in amplitude with I, and the second is amplitude modulated by Q.

After summarizing, we have a wave that is modulated in both amplitude and phase.

Depending on the choice of I and Q values, it can be modulated only in the phase.

By I and Q the information is coded.
E.g.
Micro controller=>D/A converter for I and Q => I/Q transceiver =>RF amplifier..
...................RF reception amplification=> I/Q transceiver=> I and Q in A/D converter => microcontroller

#### Motanache

Joined Mar 2, 2015
481
Obviously CDMA also involves coding. (Code-division multiple access)
Off-topic but interesting:
https://en.wikipedia.org/wiki/Code-division_multiple_access

"The technology of code-division multiple access channels has long been known. In the (U SSR), the first work devoted to this subject was published in 1935"

http://www.telecomhall.com/what-does-orthogonal-means-in-wireless-networks.aspx

The problem is that for CDMA will not find much documentation.
For example, for 2G there is Open BTS. Those who like programming can use their mobile phone on their own network. (Open BTS).

Mobile phone producers do not want this and defend themselves by making things harder to understand in 3G and 4G.

The TDMA 2G for phone is assigned by BTS a frequency channel. The same in CDMA 3G

#### Motanache

Joined Mar 2, 2015
481
In TDMA 2G on a single channel, several phones emit one at a time in the allocated time slot. Like more people would talk in turn. Here the time slot has a fixed size,
If all time slots are occupied and all frequency channels are busy the next user can not dial the phone.
It is what happens sometimes the new year or the holidays when everyone calls and you can not call.
At CDMA is just the same but the time slot does not have a fixed size.

If more users appear, each phone decreases how long it emit (shortens the time slot).
Even if voice quality is lost at the reception (in case too many users) but everyone can call.

Third, there is a coding.
Among the transmitted data and a code is XOR.
This makes it much more difficult to be received by unwanted third parties for a 3G conversation.
E.g.

In the post #1 figure we had:
- the data we want to be transported above
- a code that we know and the reception at the bottom

Last edited:

#### Motanache

Joined Mar 2, 2015
481
From left to right in the figure:
- upper
data -1 code 1 => Z=data*code=-1
- lower
data 1 code 1 => Z=data*code=1
after the adding sign we will have 0

- upper
data -1 code 1 => Z=data*code=-1
- lower
data 1 code -1 => Z=data*code=-1
after the adding sign we will have -2
It's the first -2 after the blue line on the left.

And so on...........
As you can see there is a logic part that run on the micro controller(DSP in the case of phones) And a part of the radio transmission I described above.