 problems. This is caused by poor UE power utilization from the small 1 ms TTI duration. To overcome this, a TTI bundling technique [28,29] was adopted as a part of LTE 3GPP standard. The TTI bundling technique is described in Section 10.6.2.6. 10.6 VolP Solutions to Avoid System Related Limitations 10.6.1 Solutions in HSPA 10.6.1.1 Uplink Gating Associated with Packet Bundling To address the practical constraints for VoIP over HSPA, several solutions have been proposed to improve VoIP performance. In 3GPP releases up to Release 6, the DPCCH channel is transmitted con- tinuously regardless of whether there is actual user data to be transmitted or not, thereby highly loading the cell (see Section 10.5.1.1). The idea of gating, intro- duced in 3GPP Release 7 and presented in [30], is to stop the transmission of DPCCH when there is no data to be sent on E-DCH and no L1 feedback signaling on HS-DPCCH (see Figure 10.7). This would reduce interference to other users and increase the UL capacity. Besides, it can increase UE standby time efficiently because of less power consumption. DPCCH gating might pose a difficulty for the network to distinguish an inac- tive period from a lost connection. Hence it is desirable that DPCCH transmission is not totally silent but transmitted periodically following a predefined pattern. Another issue to consider is transmission power. After a gating period without track- ing the received SIR on DPCCH, the channel response variations could degrade the transmission and increase the interference between the UEs. Thus, in the 3GPP specifications for DPCCH gating, the optional power control preambles are defined to be sent prior to the data channel reactivation. As a further improvement, E-DCH DPCCH (continuous) E-DCH DPCCH (gated) Figure 10.7 Illustration of VolP transmission on E-DCH assuming 10-ms TTI and bundling of two VolP packets into one TTI with continuous and gated DPCCH transmission. © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology 