The downlink reference signals consist of so-called reference symbols which are known symbols inserted within the OFDM time/frequency grid. These reference signals can be used by the UE for downlink channel estimation to enable coherent detection.
For LTE Downlink, three types of reference signals are defined:Up to four cell-specific reference signals, each corresponding to one downlink antenna port.MBSFN reference signals within MBSFN sub-framesUE-specific reference signals, sometimes also referred to as dedicated reference signals.
In case of a single antenna port (a single transmit antenna), a single cell-specific reference signal is inserted into the time/frequency grid according to Figure below.
As observed, the reference symbols are inserted within the first and the third last OFDM symbol of each slot and with a frequency-domain spacing of six sub-carriers. Furthermore, there is a frequency-domain shifting of three sub-carriers between the first and second reference symbols. Within each resource block there is a total of four reference symbols.
In case of downlink multi-antenna transmission (transmit diversity and spatial multiplexing) the UE must be able to identify and estimate the channel corresponding of each transmit antenna port. In order to enable this, there is one downlink reference signal transmitted from each antenna port, as illustrated in Figure below.
Concept of Holes
As can also be seen from Figure above, in a resource element carrying a reference symbol for a certain antenna port, nothing is being transmitted at the other antenna ports. The blank resource elements on which nothing is transmitted as a result of reference signals on other antenna ports are called holes (unused resource element). Thus there is no interference to reference symbols from other antenna ports within the cell.
Note: In new 3GPP releases, additional downlink reference signals have been defined.