 Indicator Channel) (PHICH): It carries Hybrid ARQ ACK/NAKs (ac- knowledged/not acknowledged) in response to uplink transmissions. 3.5.3.3 Physical Signals A downlink signal corresponds to a set of resource elements used by the physical layer but does not carry information originating from higher layers. There are two physical signals (reference signals and synchronization signals) which are defined as follows [13]. 3.5.3.3.1 Reference Signals Three types of downlink reference signals are defined and there is one reference signal transmitted per downlink antenna port: Cell Specific Reference Signals: These signals are transmitted in all downlink subframes in a cell supporting unicast transmission and are associated with non-MBSFN transmission. In case the sub- frame is used for transmission with MBSFN, only the first two OFDM symbols in a subframe can be used for transmission of cell-specific reference symbols. MBSFN Reference Signals: MBSFN reference signals can only be transmitted in subframes allocated for MBSFN transmissions. MBSFN reference signals are transmitted on antenna port 4 and are defined for extended cyclic prefix only. UE Specific Reference Signals: UE-specific reference signals are supported for single-antenna-port transmission of PDSCH and are transmitted on antenna port 5. The UE is informed by higher layers whether the UE-specific reference signal is present and is a valid reference for PDSCH demodulation or not. UE-specific reference signals are transmitted only on the resource blocks upon which the corresponding PDSCH is mapped. 3.5.3.3.2 Synchronization Signals There are 504 unique physical layer cell identities. The physical layer cell identities are grouped into 168 unique physical layer cell-identity groups, each group containing three unique identities. The grouping Next Generation Mobile Communications Ecosystem code words layers antenna ports Modulation Resource Scrambling OFDM signal element mapper generation mapper Layer Precoding mapper Resource Modulation Scrambling element OFDM s