MSG1 – PRACH in 5G NR


What is MSG1 in 5G?

        MSG-1 is the first message in the Random Acccess Procedure of 5G (NR). It is transmitted by the User Equipment (UE) to the gNodeB (gNB) over the Physical Random Access Channel (PRACH).

MSG1 contains a Random Access Preamble, which is a special signal used by the UE to:

  • Request initial access to the network
  • Re-establish connection after radio link failure
  • Perform handover
  • Synchronize uplink timing

Why is MSG1 Required?

MSG1 is essential because:

  • The UE doesn’t yet have uplink timing aligned with the gNB.
  • It allows the gNB to detect the UE, measure timing offset, and allocate resources.
  • It initiates communication when the UE is in RRC_IDLE, RRC_INACTIVE, or during beam failure recovery.

MSG1 Structure (PRACH Preamble)

MSG1 is not a regular/normal message with headers and payload. It’s a waveform generated using Zadoff-Chu sequences. It includes:

FieldExplanation
Preamble IndexIdentifies which preamble UE is using (used for contention resolution).
Sequence FormatLong (839) or Short (139) depending on cell size and deployment scenario.
Subcarrier Spacingit is not constant varies by frequency range (like FR1: 15/30 kHz, FR2: 60/120 kHz).
PRACH Configuration IndexDetermines time/frequency resources for PRACH transmission.
RA-RNTIit is used to identify the UE. it is being used during Random Access Procedure only.
it stands for  "Random Access Radio Network Temporary Identifier"
How MSG1 is Transmitted
  1. UE selects a preamble index and PRACH resource based on configuration from SIB1 or RRC.
  2. UE transmits the PRACH waveform using selected format and power.
  3. The transmission is blind—UE doesn’t know if gNB received it.


What Happens at gNB After Receiving MSG1?

Once gNB receives MSG1:

  1. It detects the preamble and estimates timing offset.
  2. It sends MSG2 (Random Access Response) via PDCCH and PDSCH.
  3. MSG2 includes:
    • Timing Advance
    • Temporary C-RNTI
    • Uplink grant for MSG3

If multiple UEs send the same preamble (contention-based access), gNB resolves it in later steps (MSG4).


MSG1 in the Full Random Access Procedure


UE                                                                    gNB
│                                                                          │
├── MSG1: PRACH Preamble   ─────▶│  (Initial access)
│◀── MSG2: RAR (Timing, Grant)    ── ┤
├── MSG3: RRC Setup Request  ────▶│
│◀── MSG4: Contention Resolution  ──┤
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NR Interview Questions:

 LTE/NR Interview Questions 

 ———————1st———————————

1. What is 5G and why do we need it over existing LTE? - OK

 2. SA and NSA mode operation- OK

 3. Sub 6[FR1] and mmWave [FR2] - OK

 4. 5G Numerology [SCS/Subcarrier spacing details], RE, RB- 

 5. 5G supported Bandwidth, MCS, MAS, FR1, and FR2 supported bands as per 3GPP Rel 15/16

 6. 5G service-based architecture

 7. 5G Deployment options [option 2/3/3a/3x/5/6/7/7a/7x]

 8. 5G supported main features- eMBB, Network slicing, URLCC, mMTC, Massive MIMO, Beamforming, etc

 9. 5G NSA and 5G SA power on call flow with each message/IE

 10. 5G protocol stack, channels, Frame structure, and physical layer parameters

———————2nd———————————

1. Difference between 3, 3a and 3x deployment options?

2. 5G gNodeB architecture?

3. Functionality of 5G MAC, RLC and PDCP compared to LTE?

4. Explain about 5G NR new protocol SDAP?

5. 5G SA and NSA registration call flow and VoNR call flow?

6. 5G throughput calculation formula and main parameters?

7. CORESET in 5G and how does it different from LTE corset?

8. 5G core major interfaces?

9. Network slicing, S-NSSI, SST, and SD?

10. Explain about SSB?

———————3rd———————————

1. 5G NR BWP Types and BWP Operations?

2. Explain DCNR and MRDC ?

3. Explain MU MIMO and Massive MIMO?

4. 5G TDD and FDD frame structure?

5. 5G NR PDCP ROHC modes and profiles supported?

6. Signaling radio bearers and importance of SRB 3?

7. 5G NR UE and Network identifiers?

8. 5G NR Modulation and Coding Scheme (MCS) Characteristics?

9. 5G NR PSS and LTE PSS comparison?

10. 5G NR SSS and LTE SSS comparison?

———————4th———————————

1. 5G UE category types?

2. 5G NR Transport block size[TBS] calculation?

3. 5G NR RACH procedure and RACH types?

4. 5G NR CBRA and CFRA RACH?

5. 5G NR SCG failure, Beamforming failure, and RLF log analysis and debug?

6. 5G NR Measurements: RSRP, RSSI, RSRQ, and SINR?

7. Handovers in 5G?

8. Explain QOS in 5G?

9. 5G NR Logical, Transport, and Physical Channels Mapping?

10. 5G NR Radio Network Temporary Identifier (RNTI) and RNTI types?

———————5th———————————

1. Handover call flow in detail (conurer questions like: path switch, SN status transfer ) 

2. 256 qam related link adaptation (DL UL link adaptation , alt cqi table)

3. TTI bundling

4. Sps (its an volte feature)

5. Call flow in detail (rach, Ue id acquisition, location update related counter questions )

6. Emergency call flow

7. Establishment cause (like in what case what establishment cause will come)

8.DL UL throughput debugging

9. SON (ANR, MRO)

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SRS(Sounding reference signal) in NR:

 SRS(Sounding reference signal) in NR:

       In the world of 5G New Radio (NR), efficient and accurate uplink channel estimation is crucial for maintaining high data rates and reliable connectivity. One of the key tools used for this purpose is the Sounding Reference Signal (SRS).


What is SRS?

          SRS is a type of uplink reference signal transmitted by the User Equipment (UE) to help the gNodeB (5G base station) assess the quality of the uplink channel. Unlike other reference signals that are tied to data transmission, SRS is often sent independently of data, purely for the purpose of channel sounding.



Why is SRS Important in 5G?

5G networks operate across a wide range of frequencies, including millimeter wave bands, where channel conditions can vary rapidly. SRS helps the network:

  • Estimate uplink channel quality across different frequency resources.
  • Support beamforming and massive MIMO by providing spatial channel information.
  • Enable frequency-selective scheduling, allowing the network to assign resources based on real-time channel conditions.
  • Assist in mobility management, especially in scenarios involving handovers or dual connectivity.

How Does SRS Work?

The gNodeB configures the UE to transmit SRS periodically or on-demand. These signals are sent over specific resource blocks and are designed to be orthogonal to other uplink transmissions to avoid interference.

Key aspects of SRS configuration include:

  • Bandwidth and frequency hopping: SRS can span wide bandwidths and hop across frequencies to provide a comprehensive channel view.
  • Time-domain configuration: SRS can be scheduled at regular intervals or triggered dynamically.
  • Antenna port mapping: In MIMO setups, SRS can be transmitted from multiple antenna ports to help the gNodeB understand spatial characteristics.

1. SRS Triggering Mechanisms

SRS transmission can be initiated in two main ways:

  • Periodic Triggering: The UE sends SRS at regular intervals based on a predefined schedule.
  • Aperiodic Triggering: The gNodeB can request SRS on-demand via Downlink Control Information (DCI), allowing dynamic channel sounding when needed.

This flexibility helps balance overhead and responsiveness.

2. Time-Domain Configuration

SRS can be configured to occur in specific time slots or symbols. Key parameters include:

  • SRS Periodicity: Defines how often the UE should transmit SRS (e.g., every 20 ms, 40 ms).
  • Offset: Determines the starting point of the periodic transmission within a frame.
  • Duration: Specifies how many symbols are used for SRS in a slot.

This allows operators to optimize SRS timing based on traffic load and mobility.

3. Frequency-Domain Configuration

SRS can be transmitted over a wide or narrow frequency range. Important aspects include:

  • Bandwidth Configuration: SRS can span multiple resource blocks (RBs), enabling wideband channel estimation.
  • Frequency Hopping: SRS can hop across different frequency locations to provide a broader view of the channel.
  • Comb Size: Determines the spacing between SRS tones, affecting resolution and overhead.

These settings help the gNodeB assess frequency-selective fading and optimize resource allocation.

4. Spatial Configuration

In MIMO systems, SRS can be transmitted from multiple antenna ports. This supports:

  • Uplink Beamforming: By analyzing SRS from different spatial directions, the gNodeB can select optimal beams.
  • Channel Reciprocity: In TDD systems, uplink SRS can help infer downlink channel conditions.

This is especially useful in massive MIMO deployments.

5. Group and Sequence Configuration

SRS uses specific sequences and cyclic shifts to maintain orthogonality between UEs:

  • Sequence Group and ID: Defines the base sequence used for SRS.
  • Cyclic Shift: Allows multiple UEs to transmit SRS simultaneously without interference.

This ensures scalability and efficient multi-user support.

6. SRS Resource Configuration

The gNodeB defines SRS resources using RRC signaling. Each resource includes:

  • Resource ID
  • Time and frequency allocation
  • Antenna port mapping
  • Transmission comb and sequence parameters

These configurations are managed via the SRS-Config structure in the RRC protocol.


SRS Configuration Summary Table

ParameterDescription
Trigger TypePeriodic or Aperiodic (on-demand via DCI)
PeriodicityDefines how often SRS is transmitted (e.g., every 20 ms, 40 ms)
OffsetTime offset within the frame for periodic SRS
DurationNumber of OFDM symbols used for SRS in a slot
Bandwidth ConfigurationNumber of resource blocks (RBs) allocated for SRS
Frequency HoppingEnables SRS transmission across different frequency locations
Comb SizeSpacing between SRS tones (e.g., 2, 4, 8)
Antenna PortsNumber of antenna ports used for SRS (supports MIMO and beamforming)
Sequence Group & IDDefines the base sequence used for SRS
Cyclic ShiftAllows multiple UEs to transmit SRS simultaneously without interference
SRS Resource IDUnique identifier for each configured SRS resource
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