URLLC Configuration Report

OAI vs srsRAN Low Latency Analysis

Date: January 30, 2026
Author: Venu
Purpose: Configuration analysis for achieving URLLC (Ultra-Reliable Low-Latency Communication) with OAI gNB and B210 USRP

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Table of Contents

1. Executive Summary
2. TDD Configuration Comparison
3. OAI URLLC Parameters
4. srsRAN Low Latency Parameters
5. Parameter-by-Parameter Comparison
6. srsRAN-Only URLLC Features

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Executive Summary

This report compares URLLC configuration parameters between OpenAirInterface (OAI) and srsRAN Low Latency fork to identify optimal settings for achieving low-latency 5G NR operation with B210 USRP hardware.

Key Findings

• srsRAN Low Latency fork provides additional tunable parameters not available in OAI
• Both platforms support 2ms TDD periodicity for reduced latency
• srsRAN offers explicit K1/K2 timing control for HARQ optimization
• OAI requires sl_ahead=4 for B210 timing stability

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TDD Configuration Comparison

2ms TDD Pattern (DDDSU equivalent)

Parameter	OAI Value	srsRAN Value	Description
TDD Period	4 (2ms)	4 (2ms)	Switching periodicity
DL Slots	2	2	Downlink-only slots
DL Symbols	6	8	DL symbols in special slot
UL Slots	1	1	Uplink-only slots
UL Symbols	4	0	UL symbols in special slot

Visual TDD Pattern

2ms TDD Period (4 slots × 0.5ms each at 30kHz SCS)

Slot 0          Slot 1          Slot 2 (Special)     Slot 3
─────────       ─────────       ─────────────────    ─────────
│▼▼▼▼▼▼▼│       │▼▼▼▼▼▼▼│       │▼▼▼▼│░░░░│         │▲▲▲▲▲▲▲│
│  DL   │       │  DL   │       │ DL │ GP │         │  UL   │
│14 sym │       │14 sym │       │8sym│6sym│         │14 sym │
─────────       ─────────       ─────────────────    ─────────

▼ = Downlink (gNB → UE)    ▲ = Uplink (UE → gNB)    ░ = Guard Period

Config: nof_dl_slots=2, nof_dl_symbols=8, nof_ul_slots=1, nof_ul_symbols=0
Total: 36 DL symbols + 6 GP symbols + 14 UL symbols = 56 symbols

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OAI URLLC Parameters

TDD Configuration (gNB Config)

tdd_ul_dl_configurationCommon:
  referenceSubcarrierSpacing: 1  // 30 kHz SCS
  pattern1:
    dl_UL_TransmissionPeriodicity: 4        // 2ms period
    nrofDownlinkSlots: 2
    nrofDownlinkSymbols: 6
    nrofUplinkSlots: 1
    nrofUplinkSymbols: 4

MAC Scheduler Optimizations

# Force continuous UL scheduling
ulsch_max_frame_inactivity = 0;   // Never stop UL scheduling

# Minimum UL grant size
min_grant_prb = 20;               // Minimum PRBs for UL

# LDPC decoder optimization
max_ldpc_iterations = 4;          // Reduce decoding iterations

Critical Timing Parameters

# B210 USRP requirements
sl_ahead = 4;                     // 4 slots TX preparation time

# Thread configuration
L1s:
  num_cc: 1
  pusch_proc_threads: 8
  prach_dtx_threshold: 160

RACH Configuration

prach_ConfigurationIndex: 159     // Format B4, any slot
msg1_SubcarrierSpacing: 1         // 30 kHz SCS
ra_ResponseWindow: 7              // 20 slots window

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srsRAN Low Latency Parameters

TDD Configuration

tdd_ul_dl_cfg:
  dl_ul_tx_period: 4      # 2ms TDD period
  nof_dl_slots: 2
  nof_dl_symbols: 8
  nof_ul_slots: 1
  nof_ul_symbols: 0

Scheduling Optimizations

cell_cfg:
  slicing:
    - sst: 1
      sched_cfg:
        sr_free_access_enable: false
        min_ul_grant_size: 512      # Min UL grant in bytes

  mac_cell_group:
    bsr_cfg:
      periodic_bsr_timer: 1         # 1ms BSR reporting

  pusch:
    min_k2: 2                       # Min PUSCH scheduling delay

  pucch:
    sr_period_ms: 2                 # SR period matching TDD
    min_k1: 2                       # Min HARQ feedback delay

PHY Expert Parameters (Low Latency Fork Exclusive)

expert_phy:
  max_proc_delay: 1        # Max DL processing delay (M-slot)
  radio_heads_prep_time: 3 # Radio head prep time (H-slot)

Radio Configuration

ru_sdr:
  srate: 23.040       # 23.04 MSPS for 15 MHz BW
  otw_format: sc12    # 12-bit reduces USB3 bandwidth

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Parameter-by-Parameter Comparison

Feature	OAI Parameter	srsRAN Parameter	Purpose
TDD Period	dl_UL_TransmissionPeriodicity = 4	dl_ul_tx_period: 4	2ms switching
DL Slots	nrofDownlinkSlots = 2	nof_dl_slots: 2	DL-only slots
DL Symbols	nrofDownlinkSymbols = 6	nof_dl_symbols: 8	Special slot DL
UL Slots	nrofUplinkSlots = 1	nof_ul_slots: 1	UL-only slots
UL Symbols	nrofUplinkSymbols = 4	nof_ul_symbols: 0	Special slot UL
Slot Ahead	sl_ahead = 4	N/A (auto)	TX prep time
UL Scheduling	ulsch_max_frame_inactivity = 0	N/A (always)	Continuous UL
Min UL Grant	min_grant_prb = 20	min_ul_grant_size: 512	Min allocation
LDPC Iterations	max_ldpc_iterations = 4	N/A	Fast decoding
BSR Timer	N/A	periodic_bsr_timer: 1	Buffer reporting
SR Period	N/A	sr_period_ms: 2	SR frequency
K1 (HARQ)	N/A	min_k1: 2	HARQ feedback
K2 (PUSCH)	N/A	min_k2: 2	UL scheduling
Processing Delay	N/A	max_proc_delay: 1	DL processing
Radio Prep	N/A	radio_heads_prep_time: 3	Radio timing

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srsRAN-Only URLLC Features

These parameters are only available in srsRAN Low Latency fork:

Parameter	Value	Purpose
sr_free_access_enable	true/false	SR-free slice access
min_ul_grant_size	512 bytes	Guaranteed min UL allocation
max_proc_delay	1 slot	M-slot offset control
radio_heads_prep_time	3 slots	H-slot offset control
min_k1	2	Minimum HARQ-ACK timing
min_k2	2	Minimum PUSCH scheduling

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