Wednesday, 10 July 2013

Difference between SRS and DMRS


There are two types of reference signals used in LTE uplink, to estimate uplink channel quality. Which allow eNB to take smart decisions for resource allocation for uplink transmission, link adaptation and to decode transmitted data from UE .

So to take first smart decision by eNB Sounding Reference Signal  (SRS) is being used. SRS is being transmitted by UE on the last symbol of subframe (in which subframe will come to know later). This SRS report the channel quality of over all bandwidth and using this information eNB assign the resource (to UE for uplink transmission )has better channel quality comparing to  other bandwidth  region.

So is SRS optional in LTE? Yes. SRS is configurable and infact we do not need SRS at all in case eNB assign all resource block or full bandwidth or have no choice.

Now on the basis of configuration and node wise there are two types of SRS (refer 36.211), cell specific (Common SRS) and UE specific (Dedicated SRS).  eNB notify UE about the configuration of SRS parameter by RRC messages. 

There are two types of SRS on the basis of periodicity. Periodic and Aperiodic (In Rel. 10 LTE Advance). The minimum periodicity of SRS is 2ms (1ms=1subframe) and the maximum is 320ms (it is even more than 320ms which is reserved according to specs 36.213).

Now you might be thinking what if all UEs transmit the SRS with same interval and periodicity or in other words how eNB distinguish the UE specific SRS in case of overlapped SRS transmission. Well in that case using transmission_comb and cyclic shift parameters present in RRC Connection setup and RRC Connection Reconfiguration, eNB distinguish and decode different UE specific SRS.

Demodulation reference signal (DMRS) in uplink transmission is used for channel estimation and for coherent demodulation which comes along with PUSCH and PUCCH. If DMRS is bad or by some reason not decoded properly by base station , PUSCH or PUCCH will be not decoded as well. Hence DMRS is not optional like SRS.

DMRS only state channel quality of frequency region in which PUSCH or PUCCH is being transmitted. So what about positioning of DMRS in resource grid, is this fixed ? Answer is Yes and No both. So, when DMRS sent by UE with PUCCH, position of reference signal vary according to PUCCH format indicator. But in case of PUSCH it is always the center symbol of a slot (3rd symbol of slot0 and 10th symbol of slot1).

To support a large number of UEs (User terminal), a large number of DMRS sequences needed and it is achieved by cyclic shifts of a base sequence. As we know in LTE -Advance we will have concept of MIMO in uplink as well, hence DMRS have to enhance for MIMO transmission and each UE will use different DMRS sequences.

DMRS is always mapped to PUSCH in multiple of 12 sub-carriers , however  DMRS mapped to PUCCH  is always in terms of 12 sub-carriers only.

The only similarity in between SRS and DMRS is both uses Constant Amplitude Zero Autocorrelation (CAZAC) sequences.

You may observe less throughput in case of SRS enabled data transmission , because to report SRS during  uplink data transmission , eNB schedule some RBs to UE which could have been used for actual data. 

Guys you may have multiple questions roaming in your mind , so please post those question here.  We will try to learn and explore more.

> What if SRS and CQI coincide on the same subframe?
 Well in that case a UE shall not transmit SRS whenever SRS and PUCCH format 2/2a/2b transmissions (CQI, CQI with 1 or 2 bit HARQ ACK/NACK) happen to coincide in the same subframe [3GPP 36.213 Section 8.2]. Having said that, does it mean L2 scheduler will not schedule SRS and CQI on the same subframe ?

Questions are welcome.


17 comments:

  1. IS aperiodic SRS available only from Release 10?

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    1. Hi Buddy,

      Yes Aperiodic SRS is introduced in LTE-A (Rel 10). you may refer below links for further details:

      http://books.google.co.in/books?id=tT4sWfCcyEUC&pg=SA7-PA5&lpg=SA7-PA5&dq=is+aperiodic+SRS+in+release+9&source=bl&ots=L6io4JBnNH&sig=dFqy8GVYaz25od5KvDpNfusR0j8&hl=en&sa=X&ei=Dy3gUYGCCImzrAeVv4DwAw&ved=0CCsQ6AEwAA#v=onepage&q=is%20aperiodic%20SRS%20in%20release%209&f=false

      or this patent as well:

      http://www.google.com/patents/WO2011155711A2?cl=en

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  2. SRSconfigindex (RRC Setup etc) plays a role in UE specific periodicity and subframe index (to derive the subframe where to send SRS). So I think, tranmission comb and cyclic shift are not the only parameters to separate UE SRS's.

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    Replies
    1. In below explanation, my last question might help you to explore more about overlapped SRS reporting.

      Using SRSconfigindex parameter we define the periodicity of SRS reporting and the subframe offset value. Like for SRSConfigindex value 17, Table 8.2-1 of 36.213 define periodicity as 20 with subframe offset value 0. That is if for UE1 if you have configured SRSconfigindex as "17", then this UE1 will always send SRS value on subframe 0 for each 20ms (or on alternate system frame). But What if we configure the same SRSconfigindex value "17" for UE2 as well? How eNB will handle this overlapped SRS reporting?

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    2. I hope you might have found the answer of "Overlapped SRS Reporting and decoding". If not then see below: (I am assuming all UE have reported SRS on the last symbol of same subframe).
      In case of overlapped SRS reporting, "transmission comb and cyclic shift" shared by eNB to UEs during RRC messaging will be used for differentiating the UE specific SRS. The Value of transmission comb parameter (0-1) and cyclic shift (0-7) is UE specific and eNB knows this value corresponding to each UE.

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  3. I really, liked the simple explanation bypassing difficult to understand math formulae!

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  4. What is the relation between Cell SRS and UE SRS, can we say that UE specific SRS is a subset of the Cell SRS subrames ?
    Is there any relation between srs_subframeconfig and srs_configindex ? And how they work? It looks no ralations between ue subframe and common srs subframe from 3GPP .

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  5. UE transmit SRS using a combination of Common Information in SIB2 and UE specific dedicated Information in an RRC Connection Reconfiguration Message. All terminals within a cell are aware of the set of subframes within which SRS may be transmitted by any terminal within the cell. Information about the set of subframes in which SRS may be transmitted within a cell is provided as part of the cell system information. So yes you can assume that "UE specific SRS is a subset of the Cell SRS subrames".

    The set of Subframes within which the SRS is transmitted is determined by Cell Specific SRS-SubframeConfig in SIB2 (According to 36.211, Table 5.5.3.3-1) and UE specific SRS-ConfigIndex within in RRC Connection Reconfiguration Message (According to 36.213, Table 8.2-1).

    You can do experiment and verify with these two parameter srs_subframeconfig and srs_configindex. There is relation between these two parameters. From (According to 36.213, Table 8.2-1) following formula must be satisfiable and must belong to subframe set of 36.211, Table 5.5.3.3-1:

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

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

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  6. Can you please explin cyclic shift.
    Thank You

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  7. Hi,
    Very useful blog.
    (3rd symbol of slot0 and 10th symbol of slot1)
    we have only 6 or 7 symbol per slot right?

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    Replies
    1. Here it is considered as total 1 subframe(slot 0 and 1) and each slot having 7 symbols then total 14 symbols available.
      So DMRS is transmitted in 3rd and 10th symbol of the subframe

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  8. Can link adaptation be carried out by the eNB with only DMRS present and with SRS NOT configured?

    If so, is this a standard provided capability, or just that the eNB does have the capability and can optionally use it?

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  9. So do we need SRS if we can have CQI as well?......

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    1. SRS is optional, but you need SRS for uplink channel estimation and CQI for downlink channel estimation. So based on SRS uplink MCS can vary and based on CQI downlink MCS vary.

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  10. In case of tdd, we have channel reciprocity. So, cqi report should suffice to estimate uplonk channel. Why SRS is still provided. Is it only for TA ?

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  11. Hi Can you please share any log where I can check the DMRS slot number, which message in Uu interface will carry this info?
    I searched for the same in 0xB139 message in QXDM but it's giving the value = 2, for both slot 0 and slot 1.
    Thanks,
    Br.
    Anupam

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  12. Can you please elaborate what is exactly use of DMRS and SRS ?
    Is DMRS is used for both PUCCH and PUSCH ?

    We do not need SRS at all in case eNB assign all resource block or full bandwidth or have no choice.
    - If whole bandwidth is used then SRS is not useful. What is that means ?
    - I mean if single UE has been alloted all RBs (like 100 RB in case of 20 MHz) for PUSCH then SRS is not useful.


    - DMRS is transmitted with PUCCH/PUSCH whereas SRS transmission is useful for further allocation of resources for PUSCH. Am i correct ?

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