WCDMA DL channel summation before D/A and carrier modulation

[Mctest]

Hello,experts!I have question:In many books after channelization and spreading there are gain block and then linear summation by all channels before D/A.How does signal look like after this process?I know that more channels - more power is required in order to maintain desired signal to noise,because more channels more non-orthogonality due to multipath and as consequence more interference.Am I right?I need more details on signal structure after summation!Thanks!

[dekili]

Signal is a pure some of voltages (signals). Those signals are uncorrelated but due to signal spreading power spectral density is relatively low (comparing to OFDM) thus not making too much peak-to-average ratio problems. Whole concept is in adding signals who are recognizable due to channalization codes (Walsh sequences) and after that pseudorandom sequence for spreading and despreading. Signal in the channel looks like additive white Gaussian noise when transmitted (from tranmistter to receiver) since multiple uncorrelated signals are sumarized. Also, signal is multiplied by the carrier or digitally up/downconverter at trnsmission and receiving side in order to position signal in appropriate radio channel. On the receiving side, RAKE receiver is used to achive better correlation with usually 4-8 fingers.

[Mctest]

Signal is a pure some of voltages (signals). Those signals are uncorrelated but due to signal spreading power spectral density is relatively low (comparing to OFDM) thus not making too much peak-to-average ratio problems. Whole concept is in adding signals who are recognizable due to channalization codes (Walsh sequences) and after that pseudorandom sequence for spreading and despreading. Signal in the channel looks like additive white Gaussian noise when transmitted (from tranmistter to receiver) since multiple uncorrelated signals are sumarized. Also, signal is multiplied by the carrier or digitally up/downconverter at trnsmission and receiving side in order to position signal in appropriate radio channel. On the receiving side, RAKE receiver is used to achive better correlation with usually 4-8 fingers.

Thanks for your explanation!You said that it is sum of voltages?How does the system add them digitally together and pass towards IQ modulator?

[dekili]

DSP (digital signal processing). You have bits or symbols who are shaped in baseband for transmission (Gaussian shape, Raised-Cosine shape etc...). These shpaes are represented by the samples taken in baseband (of course because complete system is digital) and you can look at them as bits. Every bit is having voltage value for the logical 1 or logical 0. In example, for unipolar signalling (1 and 0) you can have +1V for "1" and 0V for "0" or in bipolar signalling you can have +1V for "1" and -1V for "0". You can read in standard exact values. Don't forget that each bit in this case is the bit of Walsh sequence intended for channalization! That is why you can add all the voltages (samples in a digital world) and because codes are orthogonal, certain values could be achieved. Then you move to scrambling and upconversion to I/Q modulator.
On the receiving side you need to pass I/Q demodulator. This will also mean that you have achieved synchronization with the received signal and that you know bit/symbol period. By passing sugnal back to the descrambling and Walsh sequences multiplier.

[Mctest]

DSP (digital signal processing). You have bits or symbols who are shaped in baseband for transmission (Gaussian shape, Raised-Cosine shape etc...). These shpaes are represented by the samples taken in baseband (of course because complete system is digital) and you can look at them as bits. Every bit is having voltage value for the logical 1 or logical 0. In example, for unipolar signalling (1 and 0) you can have +1V for "1" and 0V for "0" or in bipolar signalling you can have +1V for "1" and -1V for "0". You can read in standard exact values. Don't forget that each bit in this case is the bit of Walsh sequence intended for channalization! That is why you can add all the voltages (samples in a digital world) and because codes are orthogonal, certain values could be achieved. Then you move to scrambling and upconversion to I/Q modulator.
On the receiving side you need to pass I/Q demodulator. This will also mean that you have achieved synchronization with the received signal and that you know bit/symbol period. By passing sugnal back to the descrambling and Walsh sequences multiplier.

Am I right that if I have +1V in 4 parallel channel it means that result is sample with +4V amplitude? And how does +4V sample upconvert to analog?Is it simple discretization and quantization?

[dekili]

Am I right that if I have +1V in 4 parallel channel it means that result is sample with +4V amplitude? And how does +4V sample upconvert to analog?Is it simple discretization and quantization?


Not directly +4V because you might use the code (in example in OFDM) to make lower peak to average value (otherwise you would saturate your aplifier) but yes. But, not to go to much in details - yes. Your samples are already discrete with values determined by the size of the bits f the regsiter used to present those values (resolution of the DSP system!). So you can just send them to I/Q mod/demod. A/D conversion is already made when you create baseband signal that you want to send.

[Mctest]

Not directly +4V because you might use the code (in example in OFDM) to make lower peak to average value (otherwise you would saturate your aplifier) but yes. But, not to go to much in details - yes. Your samples are already discrete with values determined by the size of the bits f the regsiter used to present those values (resolution of the DSP system!). So you can just send them to I/Q mod/demod. A/D conversion is already made when you create baseband signal that you want to send.

I like your explanations,but I need more and more details about resolution of dsp and D/A resolution and signal representation in all points:bits>baseband signal>channelization process > upconversion?Could you give me reference or give me example with 4 channel bit sequences?

[dekili]

I like your explanations,but I need more and more details about resolution of dsp and D/A resolution and signal representation in all points:bits>baseband signal>channelization process > upconversion?Could you give me reference or give me example with 4 channel bit sequences?


Hi,

you are asking model on the levl of the electronic circuit if I understand right? I was unable to find such reference, but it might be good to start investigating by reading:
H. Schulze, C. Luders: Theory & Design of OFDM &CDMA" from Wxley
and to check
hxxp://www.altera.com/end-markets/wireless/cellular/wcdma/wir-wcdma.html (http://www.altera.com/end-markets/wireless/cellular/wcdma/wir-wcdma.html)

hxxp://www.eetimes.com/electronics-news/4139933/W-CDMA-RAKE-Receiver-Comes-to-Life-in-DSP (http://www.eetimes.com/electronics-news/4139933/W-CDMA-RAKE-Receiver-Comes-to-Life-in-DSP)

(xx=tt)

Also, if you are member of Mathworks community you could get matlab code for what you are looking for. There are many codes which are even available to public...

[Mctest]

Yes,you are right!I'm really interested in such things!
Thank you for references!