random access preambles in special subframes) offers the shortest handover interruption time. 12.10.6 Estimation of the L1/L2 Overhead The downlink L1/L2 overhead in the 3GPP LTE-Advanced includes different types of reference signals, i.e., Cell-Specific Reference Signals (CRS) or common pilots transmitted within each resource block, UE-specific Reference Signals (URS) or dedicated pilots, and the reference signals that are specifically utilized in the estimation of channel state information (CSI-RS). The overhead channels further include control signaling channels that are transmitted in the first L OFDM symbols (L = 1, 2, or 3) of each subframe (L = 4 in the case of 1.4 MHz bandwidth) and the synchronization signals, as well as the physical broadcast control channel. The PDU headers corresponding to Layer 2 sub-layers (MAC/RLC/PDCP) are also included in the self-evaluation results. The overhead due to CRS and L1/L2 control signaling depends on the number of cell-specific antenna ports and the number of OFDM symbols used for the L1/L2 control signaling. The number of overhead resource elements per resource-block pair (168 resource elements in total) and the corresponding relative overhead are shown in Table 12-26 for non-MBSFN and MBSFN subframes. The overhead (due to L1/L2 control signaling and cell-specific reference signals) in Table 12-26 is shown in terms of the number of resource elements per resource-block pair in percentages. In the case of MBSFN subframes, the overhead is reduced. All the subframes in a radio frame are assumed to be either regular or MBSFN type in overhead estimation. The overall overhead depends on the fraction of MBSFN subframes. In the self-evaluation results, a fraction of 0% and 60% has been assumed. The overhead due to UE-specific reference signals depends on the number of UE-specific 12.10 3GPP LTE-advanced link-level and system-level performance TABLE 12-26 3GPP LTE-Advanced L1/L2 Overhead [3] FDD Duplex Scheme TDD Duplex Scheme Number of Cell-Specific Antenna Control- Number of