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 SC-FDMA symbol of a 1 ms subframe as shown in Figure below. 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.
