3GPP LTE standard TS36.211 specifies the down link reference signals (DL RSs) in section 6.10. The specification defines the DL RSs in three types: Cell-specific reference signals, UE-specific reference signals, and MBSFN reference signals. DL RSs are needed to carry out coherent demodulation at UE through DL channel estimation.
The cell-specific RSs are available to all UEs in a cell and span the entire cell bandwidth. The structure and characteristics of the cell-specific RSs are as follows:
1. The OFDM symbols form a two-dimensional sequence (corresponds to one of 504 difference cell identities, NID_cell) and are arranged in a 'diamond shape' lattice structure to achieve the minimum interpolation errors for the channel estimation.
2. Considering the case of high channel frequency selectivity and high UE velocity, there are four reference symbols per RB (Resource Block) for optimal channel estimation.
3. There are six frequency shifts of the reference symbols corresponds to 84 different cell identities (the reference symbols are inserted within the first and the third from the last OFDM symbol for a given slot of RBs and staggered by three subcarriers relative to each other).
4. For the case of four antenna ports, the reference symbols of the second antenna port are shifted by three subcarriers with respect to the reference symbols of the first antenna port in each OFDM symbols. The reference symbols for the third and fourth antenna ports are located within the second OFDM symbols of each slot (two reference symbols per RB) to reduce the overhead in multiple channel estimations.
5. DL RSs are generated using QPSK modulation for low PAPR (Peak-to Average Power Ratio).
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