us (re)transmission using the same resources. The benefit of synchronous nonadaptive HARQ is that the UL control signaling can be minimized since only a NAK needs to be signaled back to start a HARQ retransmission. However, due to orthogonality and single carrier requirements in © 2009 by Taylor & Francis Group, LLC Radio Access Network VoIP Optimization LTE UL, there are some obvious problems for nonadaptive HARQ retransmissions combined with semi-persistent allocation: Resource fragmentation: When several UEs are scheduled in one TTI and some users require retransmissions whereas others do not, the UL resources can be fragmented and the scheduling of new users becomes more difficult or the required resources cannot be scheduled to a user due to single carrier requirement [36]. Low resource efficiency: For semi-persistent allocation, part of the resources are allocated persistently and part are scheduled dynamically. A nonadaptive HARQ sets unnecessary restrictions and leads to poor resource utilization. Since with synchronous nonadaptive HARQ the possible retransmission would overlap with the persistent allocation, a persistent allocation in a given time-frequency resource implies that nothing can be scheduled on the same frequency resource at a HARQ RTT earlier (or a few HARQ RTTs earlier). Separate ACK/NAK channel in DL: With synchronous nonadaptive HARQ, a separate ACK/NAK has to be sent strongly coded (repetition) and with high power in order to guarantee low error probability. Synchronous adaptive HARQ can then naturally be used to solve the problems: when resource fragmentation happens, an adaptive HARQ allows moving of the retransmissions to the edges of the band, thus avoiding the fragmentation; when the collision happens between initial persistent transmissions and retransmissions, by allowing adaptive HARQ, the possible retransmission can be sent on different frequency resources, thus avoiding the problem; adaptive HARQ implies that all the retransmissions are scheduled. This has the advantage that no