Section: 6.2.4 Resource-element groups (REGs)
Content: Resource-element groups are used for defining the mapping of control channels to resource elements. A resource-element group is represented by the index pair of the resource element with the lowest index in the group with all resource elements in the group having the same value of . The set of resource elements in a resource-element group depends on the number of cell-specific reference signals configured as described below with , . - In the first OFDM symbol of the first slot in a subframe the two resource-element groups in physical resource block consist of resource elements with and , respectively. - In the second OFDM symbol of the first slot in a subframe in case of one or two cell-specific reference signals configured, the three resource-element groups in physical resource block consist of resource elements with , and , respectively. - In the second OFDM symbol of the first slot in a subframe in case of four cell-specific reference signals configured, the two resource-element groups in physical resource block consist of resource elements with and , respectively. - In the third OFDM symbol of the first slot in a subframe, the three resource-element groups in physical resource block consist of resource elements with , and , respectively. - In the fourth OFDM symbol of the first slot in a subframe in case of normal cyclic prefix, the three resource-element groups in physical resource block consist of resource elements with , and , respectively. - In the fourth OFDM symbol of the first slot in a subframe in case of extended cyclic prefix, the two resource-element groups in physical resource block consist of resource elements with and , respectively. Mapping of a symbol-quadruplet onto a resource-element group represented by resource-element is defined such that elements are mapped to resource elements of the resource-element group not used for cell-specific reference signals in increasing order of and . In case a single cell-specific reference signal is configured, cell-specific reference signals shall be assumed to be present on antenna ports 0 and 1 for the purpose of mapping a symbol-quadruplet to a resource-element group, otherwise the number of cell-specific reference signals shall be assumed equal to the actual number of antenna ports used for cell-specific reference signals. The UE shall not make any assumptions about resource elements assumed to be reserved for reference signals but not used for transmission of a reference signal. For frame structure type 3, if the higher layer parameter subframeStartPosition indicates 's07' and the downlink transmission starts in the second slot of a subframe, the above definition applies to the second slot of that subframe instead of the first slot.