Sounding Reference Signal in LTE

By LTE Expert | June 8, 2014

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There are two types of uplink reference signals in LTE, reference signals for channel estimation to support coherent uplink
transmission and so-called sounding reference signal in LTE.

Uplink Channel-Sounding Reference Signal

SRS are used for:-

• Uplink Channel Estimation 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 subframes 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.

–Sounding reference signals are not limited to the bandwidth that each UE is scheduled on PUSCH, but can be transmitted on order from eNodeB on almost any part of the system bandwidth. Therfore the SRS is able to provide channel quality information beyond scheduled for PUSCH. This way SRS is an important element for the frequency selective scheduling.

Sounding reference signal transmissions are configured with

• a certain bandwidth, indicating how many resource blocks (in Freq domain) the SRS signal will cover
• a certain period, indicating the distance, in time, between consecutive SRS transmission(1,2,5,10 Subframes)
• a certain duration, indicating how many times the sounding reference signal will be transmitted

The SRS can be allocated per cell and per cell QCI up to every 5ms. The uplink Frequency-Selective scheduling feature enables the operator to select is an SRS should be allocated oe not per QCI. Also It depends on the configuration set by the signaling message (SIB2, RRC Connection Setup, RRC Connection Reconfiguration etc), but UE can transmit it every two subframes at the most and every 32 frame (320 subframe) at the least (10 bit signaling parameter srs-ConfigIndex tells UE of the periodicity of SRS transmission and the period can be 2,5,10,20,40,80,160,320 ms). Actually there is an option in which UE does not transmit SRS at all.

SRS configuration options can be seen as:

1. Cell specific configuration

• SRS allocated in every 5th subframe
• TDD- SRS in allocated only on UpPTS

2. UE specific Configuration

• enable/disable per QCI
• Sounding bandwidth and periodicity selected based on system bandwidth