Difference in PCI & Scrambling Codes

[T_ADD]

This question is for all guys working in WCDMA, CDMA and LTE.

What is the real difference between scrambling codes (WCDMA) or PN codes (CDMA) and Physical Cell Identity (which is unique) in LTE?

To my knowledge, scrambling or PN codes scramble a signal in code domain to maintain uniqueness among sectors. This also mitigates interference.

But in LTE is the PCI just a unique number in every cell and therefore LTE doesn't have any sort of protection in the code domain or the PCI is used to scramble data just like in WCDMA or CDMA? If so, can someone point out where in the 3GPP specs is this information. I do understand that OFDM subcarriers are orthogonal but then the PCI still exists.

Any thoughts?

Sent from my ADR6300

[dekili]

Since OFDM subcarriers are orthogonal, LTE doesn't need Walsh sequences like in WCDMA, to provide orthogonality between the user signals. PN sequence serves as sequence to spread signal in spectrum and to be unique identifier. PCI keeps the same role of unique identifying...

[T_ADD]

Yeah, but is PCI used to scramble data like PN is used to?

Sent from my ADR6300

[Shakeel_15]

Hello,

In very easy words you can say that PCI in LTE = PSC in WCDMA. But it is not as simple.

The minimum unit of capacity in WCDMA is code while on the other hand it is RB (resource block) in LTE. So there is no more code in LTE now is RB which is a combination of subcarieers in frequency domain and symbol periods in time domain hence the concept of OFDMA. i can say that it is the enhancement in FDMA from basic. PCI (Physical cell id) is a combination of Cell group id and cell id and its range is 0-503.

When you plsn PSC in WCDMA you have enough codes and it is much easier than PCI planning even though you have big range in PCI too. The limitation is that during PCI planning you have to avoid same MOD (like MOD3) values PCI in same facing sectors otherwise the situation will worst. This MOD is actually the reminder which inreal is the difference between two reference signals and its mapping is different in different MIMO stratagies.

In short you have to plan PCI in such a way to avoid collisions in RS. Anyways you have to read in details to understand all concepts.

BR


This question is for all guys working in WCDMA, CDMA and LTE.

What is the real difference between scrambling codes (WCDMA) or PN codes (CDMA) and Physical Cell Identity (which is unique) in LTE?

To my knowledge, scrambling or PN codes scramble a signal in code domain to maintain uniqueness among sectors. This also mitigates interference.

But in LTE is the PCI just a unique number in every cell and therefore LTE doesn't have any sort of protection in the code domain or the PCI is used to scramble data just like in WCDMA or CDMA? If so, can someone point out where in the 3GPP specs is this information. I do understand that OFDM subcarriers are orthogonal but then the PCI still exists.

Any thoughts?

Sent from my ADR6300

[T_ADD]

Hi, all this ok. But it hasn't been clear from your post that if PCI is used to scramble data. When you said there's no more code in LTE but RBs, did you mean that the PCI is not used to scramble data?

Sent from my ADR6300

[s52d]

Hi, all this ok. But it hasn't been clear from your post that if PCI is used to scramble data. When you said there's no more code in LTE but RBs, did you mean that the PCI is not used to scramble data?

Sent from my ADR6300

Hi!
No, PCI is not used to scramble data.

Simmilarity between PCI (LTE) and primary scrambing code (WCDMA) and BCC(GSM) is:
- we use them to distinguish cells on same carrier (ARFCN, UARFCN, EUARFCN) for measurements
- we have to plan then simmilary based on distance (distance in hops, kilometers, whatever)
- in fact, I use the same code to plan BCCH, BCC, SCR (just a bit modified).

Physically they are different.
In LTE, PCI indicates how reference signal is distributed. Inwithin site,
5 TXes are silent when 6th transmits, so there is less interference in order to get good cell detection.
If sites are in sync (time sync, not just freq sync) we can do it between sites as well.

BR
s52d

[T_ADD]

So does that mean LTE doesn't have any data scrambling scheme like CDMA/wcdma?



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[s52d]

depends what means "like CDMA".

If like means: wow, nobody can listen, it is scrambled: cyphering is elsewhere, not in scrambling.

If it means: we need ortogonality to lower interference, then LTE has completely different mechanism.
You just allocate different frequencies/ resource blocks.

If it means: we need something to make signal more "random like" and no DC component, then again, LTE is different.
And this is not done by scrambling codes in WCDMA, ch. codes does it.

Quickly: PCI is simmilar to promary scrmabling code like it is simmilar to BCC in GSM.
Just a way to distinguish different cells on the same carrier.

BR
s52d

[mobilink999]

In terms of functionality PCI serve the same purpose as Scrambling code in WCDMA. However intricacies are much more different.

[T_ADD]

So are you guys implying that PCI is just an arbitrary value designed to uniquely identify cells (and is not used to scramble any data) unlike scrambling/pn code which is an important ingredient to scramble data?

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[s52d]

So are you guys implying that PCI is just an arbitrary value designed to uniquely identify cells (and is not used to scramble any data) unlike scrambling/pn code which is an important ingredient to scramble data?

Sent from my ADR6300

There is no scrambling in W-CDMA sense in LTE. So, PCI is not used for it.
It is not arbitrary: there is a story behind, but direct sequence spread spectrum scrambling is not part of it.

BR

[T_ADD]

I had thought so. I'll have to find out what's the story behind the PCI.

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