UMTS/3G Definitions

By Zahid Ghadialy (zahidtg@yahoo.com)

Last Updated: 16/09/2007

What is URA and URA_PCH state?

URA or UTRAN Registration Area is a colection of cells that are used for fast moving UE's in Connected mode when they are not transferring any data. In this case the UE is in CELL_PCH state.

Everytime a fast moving UE in CELL_PCH state changes the cell, a CELL UPDATE needs to be performed to let the UTRAN know of the new position of the UE. This is done because in the connected mode (CELL_PCH), UE is known at cell level rather than UTRAN level as in IDLE state. If too many CELL UPDATES are performed, it defeats the purpose of UE being in CELL_PCH. Hence in this case the UE is put in URA_PCH state. Now the UE will perform CELL UPDATE only when the URA is changed for a UE. The drawback is that when UE needs to be paged the paging area is now extended to many cells belonging to the URA.

Also Note that the CELL_PCH state is actually a subset of the URA_PCH state. It is possible to define overlapping URAs to be used in the URA_PCH state. Thus, the UTRAN operator could define that each cell is a separate URA in addition to other larger URAs. Then the operator could assign small one-cell URAs for slow-moving mobiles, and larger URAs for mobiles with greater mobility. The small URAs could nicely perform the task of the CELL_PCH state. However, it has been decided to keep these states separate.

The URAs can be overlapping or even hierarchical. The same cell may belong to several different URAs, and the UEs in that cell may have been registered to different URAs. SIB 2 contains a list of URA identities indicating which URAs this cell belongs to. This arrangement is done to further reduce the amount of location update signaling because now the UEs moving back and forth in the boundary area of two URAs do not have to update their URA location information if the boundary cells do belong to both URAs.

Every one knows about DRX, but what is DTX, does it has any relationship with downlink rate matching?
Contributed by Senthil Kumar

DRX is quite often used and is frequently discussed. But DTX
Rate matching is used to match the amount of data to be transmitted to the available capacity of the different physical channels. It can be done either through puncturing the bits(applicable for both uplink and downlink) if there are too many data for the capacity of the physical channel or through repeating the bits(applicable for uplink) if there is less number of bits when compared to the physical channel capacity.

But for the downlink rate matching, if the number of bits to be transmitted is lower than the maximum allowed bits then DTX indication bits are used to fill up the radio frame. DTX indication bits only indicate when the transmission should be turned off, they are not transmitted.

Unlike uplink where the data rate can be changed every TTI, the downlink data rate is fixed unless changed via higher layer scheduling or through the use of compress mode patterns.

Downlink rate matching is classified on the basis of two approaches.

  1. Fixed position for transport channel.
    Here the decision to puncture or to use DTX bits is specific to a particular transport channel i.e., each transport channel is allocated a specific fixed transmission resource.
  2. Flexible position for transport channel.
    Here the decision on whether to puncture or to use DTX bits is only made once all the data to be transmitted with in a radio frame are assembled. The advantage with this approach is that any spare capacity for one transport channel can be shared with transport channel that require additional capacity.




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