LTE CRS

Cell Specific Reference Signal in LTE Release 8 :

To demodulate different downlink physical channels coherently, the UE requires complex valued channel estimates for each subcarrier. Known cell-specific reference symbols are inserted into the resource grid. The CRS is mapped to REs spread evenly in the resource grid, in an identical pattern in every RB. When transmitting with several antennas, each antenna must transmit a unique reference signal. When one antenna transmits its reference signal, the other? antenna must be silent. The mapping of the CRS on the resource grid therefore depends on the antenna configuration.

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Most of the channels (e.g, PDSCH, PDCCH, PBCH etc) are for carrying a special information (a sequence of bits) and they have some higher layer channel connected to them, but Reference Signal is a special signal that exists only at PHY layer. This is not for delivering any specific information. The purpose of this Reference Signal is to deliver the reference point for the downlink power.

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When UE try to figure out DL power (i.e, the power of the signal from an eNode B), it measure the power of this reference signal and take it as downlink cell power.

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These reference signal are carried by multiples of specific Resource Elements in each slots and the location of the resource elements are specifically determined by antenna configuration.

?From 3GPP TS 136 211

As you can see from the specifications, the mapping of the CSR grid is changing as per the PCI(Physical Cell Identity)

Also one important point in CSR mapping is higher the number of antennas higher the resource used for the CSR.

The orthogonality between the reference signals on different antenna ports enables the UE to so to say “see” each antenna port separately and estimate the total throughput on multiple layers.