The new features added in Rel7 for HSPA have made CS voice to be run over HSPA possible. It would have been possible to map CS voice over HSDPA and HSUPA in Rel6, but the resources at both the UE and the network would have been wasted, and it was not commercially viable.
The implementation till Rel 6 was to have a HSPA bearer for PS traffic and establish DCH bearer for CS voice. PS only traffic would be mapped to HSDPA in DL and HSUPA in UL and at the onset of CS call, everything would be moved to DCH. And when the CS call is disconnected, there would be radioBearerReconfiguration to move the PS to HSPA again.
The new features in UTRAN that have made this possible are:
Continuous Packet Connectivity: Allows the UE to be in CELL_DCH state for most of the time. UE can be configured with DRX cycles in this state and the battery resources conserved. HS-SCCH less operation on HS-DSCH channel saves DL codes. The HS-DSCH is switched to operation with HS-SCCH channel when the required data rate increases. With CPC, everything is mapped to HSPA, there is no DPCCH or DPDCH channel in operation in the DL.
enhanced Fractional-DPCH: This feature allows upto 7 UEs to be multiplexed on one DPCCH channel in the Downlink(for the calculation read enhanced F-DPCH). This F-DPCH channel carries TPC commands for the UE and is required along with HSDPA assignment in the downlink. Therefore, to have for example 35 active simultaneous voice calls, 5 such codes are required.
Non-Scheduled transmission on HSUPA: voice calls required guaranteed data rate(depending on codec, like 12.2 kbps). So, non scheduled grant will be used to transmit the voice packets in Uplink. Network allocates non-scheduled grant to the UE and makes provision in the UL resources (power transmitted by the UE) for the combined non scheduled grants for all the UEs in a cell. To read more about Non scheduled transmission go the MAC Scheduling paper.
Guaranteed QoS by the MAC-(e)hs Scheduler: MAC scheduler will have to be more robust in scheduling the voice packets on a priority basis and guarantee the codec rate in downlink. MAC Scheduling paper.