All About SRS (Sounding Reference Signal)

What is SRS?

- A Uplink Reference Signal.
- Not Associated with transmission of PUSCH or PUCCH.
- Use to measure Uplink Channel Quality over a Section of the Channel Bandwidth.
- Can be used by eNodeB to do Frequency Selective Scheduling  and Link Adaptation Decisions.
- eNodeB instructs UE to transmit SRS over a specific Section Of  The Channel Bandwidth
- eNodeB instructs UE to transmit SRS using a combination of  Common Information in SIB2 and UE  specific Dedicated  Information  in an RRC Connection Reconfiguration Message.
- SRS is always transmitted using the Last Symbol Of The Subframe.
- UE never instructed to send SRS over Entire Channel Bandwidth,  as it is NOT necessary to transmit  SRS within the RBs reserved  for PUCCH. PUCCH RBs are located at the Two Edges of the Channel  Bandwidth.
- SRS is used for Frequency Selective Scheduling of PUSCH, Not  PUCCH.

SRS Information In SIB2:

SRS Information In RRC Connection Reconfiguration Message: 

SRS-BandwidthConfig:(C-SRS)
- Broadcast on SIB2.
- Value from 0-7.
- Common to all UE within the Cell

SRS-Bandwidth:(B-SRS)
- Can be included in RRC Connection Reconfiguration Message.
- Can take values 0-3
- Can be UE specific.

According to 36.211: 

M-SRS : No of Resource Block over which the Sounding Reference Signal is Transmitted.

N0 - N3: One of parameter to decide the Starting position of the SRS in the Frequency Domain.

FreqDomainPosition: Received in RRC Connection Reconfiguration Message also has an impact on the Starting Position In The Frequency Domain.

SRS-SubframeConfig & SRS-ConfigIndex: The set of Subframes within which the SRS is transmitted is determined by Cell Specific SRS-SubframeConfig in SIB2 and UE specific SRS-ConfigIndex within in RRC Connection Reconfiguration Message.

SRS-SubframeConfig: 
- Takes a value between 0-14.
- Common within the Cell.
- Talks about in which subframe(s) SRS can be transmitted.

According to 36.211: 

SRS-ConfigIndex: (I-SRS)

According to 36.213: 

The SRS can transmitted in Subframes which satisfy:

(10 * nf +k-SRS -T-offset ) mod T-SRS =0

where,
  nf = SFN No (0-1023).
  k-SRS= SF No (0-9).

Duration: 
- Received in RRC Connection Reconfiguration Message.
- Takes a value TRUE or FALSE.
- TRUE - UE should Continue Transmitting SRS until instructed       otherwise.
- FALSE - UE should complete only a Single Transmission. 

TransmissionComb:
- Received in RRC Connection Reconfiguration Message.
- Allows 2 UE to Frequency Multiplex their SRS with in the Same     Resource Block

CyclicShift:(n-CS-SRS)
- Received in RRC Connection Reconfiguration Message.

SRS-HoppingBandwidth:(B-hop)

- Received in RRC Connection Reconfiguration Message.
- Allow SRS to move in the Frequency Domain Between Transmission.