1 Introduction
Fractional DPCH in Rel 6 made the DL DPCH channel more optimised. Rel 5 had introduced the robust and shared HS-DSCH transport channel which was a favorite for the data bearers because of high speed, low latency and sharing of resources. The use of DL DPCH was only limited to the DL TPC signalling for correction of Uplink Transmission power. This is called the A-DPCH or Associated DPCH.
With one 256 SF code for every UE, this was using up as many codes as was the number of UEs being served on the HS-DSCH channel. With the limited channelisation codes, it was wiser to make the DL DPCH shared too among several UEs, so that the remaining channelisation codes could be used for HS-DPDCH. This was done with Fractional DPCH in Rel 6. It allowed for slots to be assigned to the UEs on DL DPCH channel to listen for their TPC commands while HS-DSCH was assigned to them. This resulted in higher cell capacity and reduced interference on the DPCH channel becasue of the DTX periods.
But there was a limitation on which slot should be assigned to the UE during SHO. Rel 7 does away with this limitation and extends the number of UEs that can share the DL DPCH channel with Enhanced Fractional DPCH.
The UE sleeps for the rest of the time on that channelisation code and wakes up at the assigned instance to get its TPC command.
The limitation in Rel 6 was that during a SHO, the offsets of all the Radio Links in the active set had to be same. There was only one slot format defined for F-DPCH. As shown below, the position of the TPC bits(2 bits) was fixed in the slot.
This is a big problem for the network as it has to assign the same slot # to the UE on all the Radio Links in the ative set. This reduced the effective number of UEs on one F-DPCH channel to 3.
Note that slot format #2 corresponds to the rel 6 slot format, providing backward compatibility.
This arrangement of 10 slot formats allows the UE to have different offsets assigned on different Radio Links in the active set.
Since about 30-40% of UEs are in SHO at a given moment, the effective number of UEs supported with enhanced F-DPCH becomes 7 (still more than R6 F-DPCH)
With one 256 SF code for every UE, this was using up as many codes as was the number of UEs being served on the HS-DSCH channel. With the limited channelisation codes, it was wiser to make the DL DPCH shared too among several UEs, so that the remaining channelisation codes could be used for HS-DPDCH. This was done with Fractional DPCH in Rel 6. It allowed for slots to be assigned to the UEs on DL DPCH channel to listen for their TPC commands while HS-DSCH was assigned to them. This resulted in higher cell capacity and reduced interference on the DPCH channel becasue of the DTX periods.
But there was a limitation on which slot should be assigned to the UE during SHO. Rel 7 does away with this limitation and extends the number of UEs that can share the DL DPCH channel with Enhanced Fractional DPCH.
2 Rel 6 Limitation
Fractional DPCH was implemented by giving each participating UE an offset on the DL DPCH channel where it could listen for its TPC command.The UE sleeps for the rest of the time on that channelisation code and wakes up at the assigned instance to get its TPC command.
The limitation in Rel 6 was that during a SHO, the offsets of all the Radio Links in the active set had to be same. There was only one slot format defined for F-DPCH. As shown below, the position of the TPC bits(2 bits) was fixed in the slot.
This is a big problem for the network as it has to assign the same slot # to the UE on all the Radio Links in the ative set. This reduced the effective number of UEs on one F-DPCH channel to 3.
3 Rel 7 Solution
In rel 7 there are 10 slot formats defined where the position of the TPC bits and Noff1 and Noff2 bits changes as shown below.Note that slot format #2 corresponds to the rel 6 slot format, providing backward compatibility.
This arrangement of 10 slot formats allows the UE to have different offsets assigned on different Radio Links in the active set.
Since about 30-40% of UEs are in SHO at a given moment, the effective number of UEs supported with enhanced F-DPCH becomes 7 (still more than R6 F-DPCH)