There are two kinds of reference signals in LTE. Downlink and Uplink
Downlink reference signal:-
To allow for coherent demodulation at the user equipment, reference symbols (or pilot symbols) are inserted in the OFDM time-frequency grid to allow for channel estimation. Downlink reference symbols are inserted within the first and third last OFDM symbol of each slot with a frequency domain spacing of six sub-carriers (this corresponds to the fifth and fourth OFDM symbols of the slot in case of normal and extended cyclic prefix, respectively) as shown in Figure 6 for an LTE system with one antenna in normal CP mode.
Furthermore, there is a frequency domain staggering of three sub-carriers between the first and second reference symbols. Therefore, there are four reference symbols within each Resource Block. The user equipment will interpolate over multiple reference symbols to estimate the channel. In case of two transmit antennas, reference signals are inserted from each antenna where the reference signals on the second antenna are offset in the frequency domain by three subcarriers. To allow the user equipment to accurately estimate the channel coefficients, nothing is transmitted on the other antenna at the same time-frequency location of reference signals.
The reference symbols have complex values which are determined according to the symbol position as well as of the cell. LTE specifications refer to this as a two-dimensional reference-signal sequence which indicates the LTE cell identity. There are 510 reference signal sequences corresponding to 510 different cell identities. The reference signals are derived from the product of a two-dimensional pseudo-random sequence and a two-dimensional orthogonal sequence. There are 170 different pseudo-random sequences corresponding to 170 cell-identity groups, and three orthogonal sequences each corresponding to a specific cell identity within the cell identity group.
UPLINK REFERENCE SIGNALS
There are two types of reference signals for uplink in LTE. The first is Demodulation Reference Signals (DM-RS) which are used to enable coherent signal demodulation at the eNodeB. These signals are time multiplexed with uplink data and are transmitted on the fourth or third SC-FDMA symbol of an uplink slot for normal or extended CP, respectively, using the same bandwidth as the data.
The second is Sounding Reference Signal (SRS) which is used to allow channel dependent (i.e. frequency selective) uplink scheduling as the DM-RS cannot be used for this purposes since they are assigned over the assigned bandwidth to a UE. The SRS is introduced as a wider band reference signal typically transmitted in the last SCFDMA symbol of a 1 ms subframe as shown in Figure 7. User data transmission is not allowed in this block which results in about 7% reduction in uplink capacity. The SRS is an optional feature and is highly configurable to control overhead – it can be turned off in a cell. Users with different transmission bandwidth share this sounding channel in the frequency domain.
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Webinar on LTE_DL/UL reference signals