bandwidth: Figure 10.15 IC scheme for small cell. © 2009 by Taylor & Francis Group, LLC Radio Access Network VolP Optimization different sectors use different sub-bands, implying that they do not interfere with each other. On the other hand, the interference among the three sectors within one cell seems to be a bit larger. Of course, some kind of combined scheme by utilizing the advantages of the two IC schemes can be designed to further enhance the performance as well. 10.6.2.6 TTI Bundling for LTE UL A natural solution to improve the coverage in a noise limited scenario would be to use RLC layer segmentation of VoIP packets. The drawback of this method is mainly the significantly increased overhead. To overcome this problem, the TTI bundling method is adopted in 3GPP [28,29], in which a few consecutive TTIs are bundled together and a single transport block is first coded and then trans- mitted by using this bundled set of consecutive TTIs. With TTI bundling only one L1/L2 grant is needed to schedule the transmission and only one HARQ feedback signal is sent from the eNodeB for the bundled subframes. Moreover, the same hybrid ARQ process number is used in each of the bundled subframes. The operation is like an autonomous retransmission by the UE in consecutive HARQ processes without waiting for ACK/NAK feedback. The redundancy version on each autonomous retransmission in consecutive subframes changes in a prede- termined manner. There is a possibility that the code rate might be larger than one for the first subframe, but decoding becomes possible at the eNodeB as soon as the code rate becomes less than or equal to one. TTI bundling is a technology that improves the data rate, reducing overheads and leading to a packet-optimized transmission. Network Performance of VolP 10.7.1 10.7.1.1 Simulation Methodology and Assumptions The quasi-static system level simulators for HSUPA and HSDPA, where all neces- sary RRM algorithms as well as their interactions are modeled, are used to investi- gate the performance of VoIP