UL SC Planning in WCDMA

[baig_kashan@yahoo.com]

Why there is no planning of UL scrambling codes in WCDMA. if it use different SC for UL and downlink then like DL SC UL SC should also be planned accordingly.

[wolverine]

In the DL you only have 512 scrambling codes, so you have to be careful how you assign them as two neighbour cells using the same scrambling code will interfere with each other. In the uplink you have a few million codes available so the system can easily assign different ones to each user in a cell. Having so many also means that the probability of two users in neighbour cells having the same UL scrambling code assigned is very limited so the possibility of UL interference is not an issue.

Any book on basic WCDMA theory will elaborate on the above.

[zloy_qwer]

No need to plan UL SC. Fact.

[baig_kashan@yahoo.com]

In the DL you only have 512 scrambling codes, so you have to be careful how you assign them as two neighbour cells using the same scrambling code will interfere with each other. In the uplink you have a few million codes available so the system can easily assign different ones to each user in a cell. Having so many also means that the probability of two users in neighbour cells having the same UL scrambling code assigned is very limited so the possibility of UL interference is not an issue.

Any book on basic WCDMA theory will elaborate on the above.

Thanks very much. let me know one more thing that in DL one code can be spread upto 512 chips while in UL it is only possible to spread code up to 256 chips. why is that?

[boring]

In the DL the spreading can go up to 512, if there is a very-low data rate service requirement. Actually, this spreading for DL is not implemented by any of the UTRAN vendors, and in fact the highest spreading that we have in the DL is 256.

From a standard's point of view, the equivalent UL spreading for this very-low data rate is 256 (also it is not implemented by any vendor).

now the difference comes from the fact that all information bits are transmitted in the DL using QPSK (2 bits/symbol) whereas in the uplink information data rate is transmitted over only one of the I/Q branches (so we have BPSK modulation = 1 bits symbol), which roughly means that for the same service rate, in UL we need half of the spreading that we use in the DL

hope this helps

[firstmaxim]

Why there is no planning of UL scrambling codes in WCDMA. if it use different SC for UL and downlink then like DL SC UL SC should also be planned accordingly.

On the uplink, we have several million scrambling codes available, and allocation will be managed by the RNC. The uplink scrambling code is assigned to UE at the time of call set up. Also, remember that there are instances, when a UE assigned a scrambling code X , enters a handover to another cell with the same scrambling code X already assigned to another UE. In this case the RNCs communicate on the application part of the Iur interface, and a softest handoff (change of scrambling code is executed prior to handoff.

[baig_kashan@yahoo.com]

In the DL the spreading can go up to 512, if there is a very-low data rate service requirement. Actually, this spreading for DL is not implemented by any of the UTRAN vendors, and in fact the highest spreading that we have in the DL is 256.

From a standard's point of view, the equivalent UL spreading for this very-low data rate is 256 (also it is not implemented by any vendor).

now the difference comes from the fact that all information bits are transmitted in the DL using QPSK (2 bits/symbol) whereas in the uplink information data rate is transmitted over only one of the I/Q branches (so we have BPSK modulation = 1 bits symbol), which roughly means that for the same service rate, in UL we need half of the spreading that we use in the DL

hope this helps

Thanks a lot boring.

can you tell me one more thing there are two extra modes in 3G which are nor dedicated neither Idle, URA PCH and Cell_FACH.

What is exactly the purpose of these extra states and why don't we categorize them as idle or dedicated state. For these states extra channel type transport channel has been defined.

and how do mobile distinguish between types of different channel

I mean different type of scrambling code define different types of physical channel, different burst structure define different types of logical channel but how exactly transport channel get identify.

[boring]

in 3G we have two RRC_Connection modes: idle and connected
idle: no RRC connection with RNC
connected: there is an RRC connection with RNC

connected mode has different RRC states: CELL_DCH, CELL_FACH, CELL_PCH and URA_PCH
CELL_DCH: dedicated transport connection between UE and RNC (i.e. DCH, HSDPA, HSUPA)
CELL_FACH: UE communicates only through common transport (i.e.FACH)
CELL_PCH: no transport connection. however the RNC knows the cell the UE camps upon and cell update happens whenever the UE moves to new cell
URA_PCH: no transport connection. however the RNC knows the URA (like LA) the UE camps upon and URA update happens whenever the UE moves to new URA (improved battery consumption)

hope this helps (reputation greatrly appreciated!)