 important practical limitations in radio interface optimization. Existing solutions (if any) to avoid these limitations are covered in Section 10.6. 10.5.1 Control Channel Limitations With VoIP traffic the number of supported users may increase drastically. To sup- port VoIP transmission of DL and UL transport channels, there is a need for certain associated DL and UL control signaling. Therefore, the overhead from associated control signaling in UL and DL may become a bottleneck for the VoIP system per- formance. Sections 10.5.1.1 and 10.5.1.2 describe the main contributors for control channel overhead for HSPA and LTE radio systems. Methods to reduce control channel overhead are described in Sections 10.6.1 and 10.6.2. 10.5.1.1 For VoIP traffic, the packet size is considerably small, corresponding to a low data rate transmission on the data channel, for example, 32 kbps with 10 ms TTI and 160 kbps with 2 ms TTI in HSUPA. In other words, it implies that the overhead from control channels would consume relatively more power for the transmission of VoIP traffic than the other packet data traffic transmitted with the high data rate. On the other hand, the control channels cannot be subject to HARQ retrans- missions; that is, the information carried by the control channels cannot be soft combined. Thus, more retransmissions may lead to the higher overhead from the control channels associated with data transmission. It makes sense to study the effect of the control channel overhead to performance and investigate how the over- head could be reduced. In HSPA, there are mainly three types of dedicated physical control channels in the UL and one in the DL: the UL Dedicated Physical Control Channel (UL DPCCH), the E-DCH Dedicated Physical Control Channel (UL E-DPCCH), the UL Dedicated Physical Control Channel associated with HS-DSCH trans- mission (UL HS-DPCCH), and the DL Shared Control Channel for HS-DSCH © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology (HS-SCCH). Their 