Scheduling Request (SR) & Random Access Fallback

← MAC Overview ← Scheduling Master SPS & CG → DL Assignment → BSR → LCP → RACH Details → PHR → TA → DRX → ☆ NTN MAC

1. The Problem — UE Has Data But No Grant

The core issue: UE has uplink data to send but no valid UL grant from gNB. Three paths to get one: (1) Configured Grant — pre-allocated, (2) Scheduling Request — fast PUCCH signal, (3) Random Access — slower but always works.

Path 1: Configured Grant

Pre-allocated periodic resources. Zero latency for grant acquisition. Best for URLLC/VoNR.

Path 2: Scheduling Request

1-bit signal on PUCCH. "I need a grant!" — 1–4 ms latency. Best for bursty traffic.

Path 3: Random Access

4-step or 2-step RACH. Always works. 5–20 ms latency. Fallback when SR fails.

2. SR — The Fast Path to a Grant

SR is a 1-bit signal on PUCCH (Format 0 or Format 1). It tells the gNB "I have data" without saying how much — that's the BSR's job, sent later in the grant.

Parameter	PUCCH Format 0	PUCCH Format 1
Duration	1–2 symbols (short)	4–14 symbols (long)
Capacity	1–2 bits	1–2 bits
Latency	Lower	Higher
Coverage	Limited	Better (longer)
Use case	Cell center, low latency	Cell edge, coverage limited

🎛️ SR State Machine — Interactive

Watch the SR state machine in action. Trigger data arrival, see SR transmissions with counter, and observe success or failure → RA fallback.

sr-TransMax: sr-Prohibit (slots):

3. SR State Machine — MAC Internal State

State Variable	Initial Value	Purpose
SR_COUNTER	0	Number of SR transmissions so far
SR_PENDING	false	Whether SR is currently triggered
sr-ProhibitTimer	stopped	Min time between consecutive SRs
sr-TransMax	RRC configured	Max SR attempts before RA fallback

SR Cancellation: A pending SR is cancelled when: (1) UL grant received that can carry the BSR, (2) BSR is included in UL transmission, (3) MAC reset (handover), or (4) SR failure (counter exceeds max).

4. SR Failure → Random Access Fallback

When SR_COUNTER > sr-TransMax, MAC notifies RRC to release PUCCH/SRS resources, then initiates contention-based Random Access. Msg3 carries the BSR — the same information that would have been sent if SR had succeeded.

⚡ SR Failure → RA Fallback Timeline

Animated timeline showing SR attempts, failure detection, and RA fallback (4-step). Adjust sr-TransMax to see how many retries before fallback.

sr-TransMax: 4 Speed:

Why RA Works as Fallback

PRACH uses broadcast preamble pool — always available. No PUCCH config needed. Works even after radio link failure.

Trade-off

RA is slower (10–20 ms vs 1–4 ms for SR), uses more resources (4 messages), and has collision risk. But it ALWAYS works.

What RRC Does

Releases PUCCH SR resources and SRS resources. UE can't transmit on UL until RA completes.

5. 2-Step RACH — Faster Fallback (Rel-16)

Aspect	4-Step RACH	2-Step RACH
Messages	Msg1 + Msg2 + Msg3 + Msg4	MsgA + MsgB (combined)
MsgA/Msg1	PRACH preamble only	PRACH preamble + PUSCH payload (BSR + ID)
Latency	12–20 ms	5–10 ms
Collision handling	Msg4 resolves	MsgB resolves (fallback to 4-step if fail)
Signal requirement	Any RSRP	RSRP > threshold (strong signal)

🔄 4-Step vs 2-Step RACH Comparison

Side-by-side animated message flow. Watch the latency difference — 2-step completes in roughly half the time.

RA Type:

6. SR Timing — Periodicity vs Latency

Periodicity	Avg Wait	SR→Grant	Total Latency	Use Case
sl1 (1 slot)	0.5 ms	~1 ms	~1.5 ms	URLLC
sl2 (2 slots)	1 ms	~1 ms	~2 ms	Voice (VoNR)
sl5 (5 slots)	2.5 ms	~1 ms	~3.5 ms	Video
sl20 (20 slots)	10 ms	~1 ms	~11 ms	Web
sl80 (80 slots)	40 ms	~1 ms	~41 ms	Background

⏱️ SR Periodicity vs Latency

Adjust the SR periodicity slider to see how average wait time changes. Shorter periodicity = lower latency but more PUCCH overhead.

SR Periodicity (slots): 5 Avg wait: 2.5 ms Overhead: high

7. SR vs Configured Grants

SR (Dynamic)

Bursty, unpredictable traffic. 2–5 ms latency. PDCCH overhead per grant. gNB decides per-TTI. Best for eMBB, web, video.

CG (Grant-Free)

Periodic, predictable traffic. 0.5–2 ms latency. No PDCCH overhead. Fixed pre-allocated resources. Best for VoNR, URLLC, V2X.

Hybrid (Common)

CG for periodic VoNR packets (every 20 ms). SR for bursty web/video data. RA fallback if SR fails. Best of both worlds.

8. Practical Field Considerations

Scenario	sr-Periodicity	sr-TransMax	sr-Prohibit
VoNR (voice)	sl2 (2 slots)	n8	ms2
Video streaming	sl5 (5 slots)	n16	ms5
Web browsing	sl20 (20 slots)	n32	ms10
IoT background	sl80 (80 slots)	n64	ms40
URLLC	sl1 (1 slot)	n4	ms1

Common pitfall: sr-TransMax too low → SR fails quickly → RA fallback → 10–20 ms penalty → noticeable voice quality degradation. Always configure appropriately per traffic type.

9. 6G Evolution — Predictive Grants & SR-Free Access

Predictive Grant

AI at gNB predicts when UE will have data → pre-sends grant before data arrives → zero SR delay. Targets 60–80% SR reduction.

Grant on Demand

1-symbol "demand signal" → gNB responds within same slot. No pre-configured SR resources. Total latency < 1 slot.

Unified RA

Single RA procedure handles all access types. AI selects 2-step vs 4-step, CBRA vs CFRA. Adaptive to channel, urgency, load.

🚀 6G Evolution — SR Latency Across Generations

Compare SR/grant-acquisition latency from 4G through 6G with animated timeline.

Generation:

Dimension	4G LTE	5G NR	6G (Target)
SR format	PUCCH F1 only	Format 0 + Format 1	+ Demand signal
Min SR latency	1 ms	0.125 ms	< 0.1 ms
SR configs per UE	1	8	AI-adaptive
RA fallback	4-step only	4-step + 2-step	Unified RA
Predictive grant	None	None	AI predicts