NR Sound Reference Signal (NR-SRS)

5G, New Radio, NR, Tech Fundas

In NR there are two types of Reference Signal in UL which gives information about the channel quality.

  1. DMRS:- Demodulation Reference Signal
  2. SRS:- Sounding Reference Signal

With the help of above two RS, gNB takes smart decisions for resource allocation for uplink transmission, link adaptation and to decode transmitted data from UE. SRS is a UL reference signal which is transmitted by UE to Base station. SRS gives information about the combined effect of multipath fading, scattering, Doppler and power loss of transmitted signal.

Hence Base Station estimates the channel quality using this reference signal and manages further resource scheduling, Beam management, and power control of signal. So SRS provides information to gNB about the channel over full bandwidth and using this information gNB takes decision for resource allocation which has better channel quality comparing to other Bandwidth regions.

One reference signal(DMRS) is always associated with each channel (PUCCH/PUSCH), which provides information about the radio channel then question may arise that why  SRS  is required? and here is the answer.

  1. DMRS provides information about the frequency region which is being used by PUSCH/PUCCH specifically.
  2. If gNB assigns resources over full bandwidth region for a UE then there is no choice left for UE to select specific frequency region Hence SRS is optional. BUT DMRS will always be transmitted with PUSCH/PUCCH for coherent demodulation and channel estimation.

Difference Between Sounding Reference Signal (SRS) of 4G-LTE and 5G-NR.

S.No. 4G – LTE 5G – NR
1 Max SRS Symbols is one Nsrs sym = 1 Max SRS Symbols can be 1,2 or 4 N {1,2,4}
OR
In the Time Domain, SRS Resources Spans
Over 1,2 or 4 consecutive symbols.
(No. of symbols are more than LTE so NR
has better sounding capacity comparing to
LTE)
2 SRS is transmitted at the last
symbol of the slot
Symbol number for SRS transmission
depends on lo OR l offset , where lo is the
time domain starting position
OR SRS will always be transmitted in the last six
symbols of the slot.
3 LTE has two kinds of SRS

  • Common SRS / Cell-Specific SRS
  • Dedicated SRS / UE Specific SRS
Always configured as UE Specific only
4 SRS is transmitted always on alternate REs due to K TC =2  SRS transmission is flexible. It depends on
transmission comb K TC value [K TC = 2 or 4
5 Max number of cyclic shifts defined LTE SRS is 8. Hence, max 8 UEs can transmit SRS simultaneously using one antenna port 5G NR has additional flexibility of SRS transmission. For K TC = 4, then Max  number of cyclic shifts can be 12. Hence, max 12 UEs can transmit SRS simultaneously using one antenna port
6 Frequency Hopping Mechanism is same in LTE and 5G NR Frequency Hopping Mechanism is same in LTE and 5G NR

Article is submitted by Subhash Kumawat (Project Associate at 5G IIT-Hyderabad>)

About Subhash Kumawat  
Subhash is graduated from National Institute of Technology (NIT) NIT Kurukshetra with specialization in Electrical Engineering. He presently working with IIT Hyderabad as project associate on  5G NR Physical Layer Development.