There are two types of uplink reference signals in LTE:
Reference signals for channel estimation to support coherent uplink transmission and Sounding reference signals.
Reference signals for channel estimation
As illustrated in Figure below, the uplink reference signals used for channel estimation are transmitted within the fourth block of each uplink slot and with an instantaneous bandwidth equal to the bandwidth of the data transmission. Note that in the general case, uplink frequency hopping may be applied on different, and perhaps substantially separated frequencies.
Reference signals for channel sounding also known as Sounding Reference Signal
Sounding reference signals (SRS) are transmitted on the LTE uplink. The intention with the sounding reference signals is for the network to be able to estimate the channel quality of the uplink channel for different UEs in order to be able to apply uplink channel-dependent scheduling. The sounding reference signals can also be used to estimate the timing of UE transmissions and to derive timing-control commands for uplink time alignment.
Sounding reference signals are transmitted independently of the transmission of any uplink data, i.e., a UE may transmit a sounding reference signal also in sub-frames where the UE does not have any data transmission. Furthermore, the sounding reference-signal bandwidth can very well be, and typically is different from that of any simultaneous data transmission from the same UE.