PDCP SDU PDCP SDU Header PDCP SDU Header leade PDCP PDUs (Header Compression and Ciphering) RLC SDU RLC SDU RLC SDU RLC Sublayer Header Concatenated RLC SDU Header Segmented RLC SDU Header Segmented RLC SDU RLC PDUs (Segmentation and Concatenation) (Part MAC Sublayer Header Multiplexed MAC SDUs MAC SDU Header MAC PDUs (Multiplexing) Physical Layer Transport Block Transport Block FIGURE 7-26 Packet processing in 3GPP LTE [9, ,12] 7.4 3GPP LTE radio link control and medium access control sub-layers always scheduled through PDCCH. A synchronous HARQ scheme is supported in the uplink. The maximum number of re-transmissions can be configured per UE basis, as opposed to per radio bearer. The downlink ACK/NACK signaling in response to uplink (re)transmissions is sent on PHICH. The ARQ functionality within the RLC sub-layer is responsible for re-transmission of RLC PDUs or RLC PDU segments. The ARQ re-transmissions are based on RLC status reports and optionally based on HARQ/ARQ interactions. The polling for an RLC status report is used when needed by RLC and status reports can be triggered by upper layers [9]. When a PDCCH for the UE is correctly received, the UE follows the instructions by PDCCH, i.e., to perform a transmission or a re-transmission regardless of the content of the HARQ feedback (ACK or NACK). When no PDCCH addressed to the C-RNTI of the UE is detected, the HARQ feedback determines how the UE should perform the re-transmissions. The UE performs a non-adaptive re-transmission, i.e., a re-transmission on the same uplink resource as previously used by the same process. The UE does not perform any UL (re)transmission and maintains the data in the HARQ buffer. A PDCCH is then required to perform a re-transmission, i.e., a non- adaptive re-transmission cannot follow. Measurement gaps are of higher priority than HARQ re-transmissions; whenever a HARQ re-transmission collides with a measurement gap, the latter prevails. The ARQ function within the RLC sub-layer re-transmits RLC PDUs or RLC PDU segments based on