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Enhanced Cell FACH

 

As the name stands for, Enhanced Cell FACH is an improved version of the conventional Cell FACH (If you are not familiar with the concept of conventional Cell FACH, refer to RRC State Change page). Assuming that you have at least a rough understanding of the conventional Cell FACH, let me as one thing. What would be the problems in the conventional Cell FACH ?

In my understanding and experience, the problems in conventional Cell FACH would be

  • Transistion Time from Cell FACH to Cell DCH is too long (due to signaling TTI used in R99)
  • Data rate on the Cell FACH is too low (just around 38 Kbps maximum)

Enhanced Cell FACH is a new technology to solve the problem mentioned above. Then, what is the logic behind the technology. The most important idea is to map Cell FACH onto HSPA channel. Since HSPA has much shorter TTI (usually 2 ms) comparing to R99 and data rate is much higher than R99 data rate, you can easily guess that Cell FACH mapped onto HSPA can improve the problems mentioned above. Enhanced Cell FACH was introduced in Release 7.

 

Detailed understanding on how Enhanced Cell FACH requires detailed understanding of channel mapping for the technology. Overall channel mapping of Enhanced Cell FACH is as follows. (Compare this channel mapping to the conventional R99 Cell FACH : Channel Mapping - Cell FACH : R99/R5/R6)

 

 

Following is RRC Connection Setup Complete message showing UE capability for Enhanced Cell FACH

 

 

Following is SIB5 showing the mapping for CCCH and some additional information labeled in LCID = SIB5 in the above illustration.

 

 

* Additional information showing the mapping of other channels will be updated later.