Why Capacity Planning Matters in 5G NR

Coverage planning answers where signals reach; capacity planning answers how much traffic each cell can carry. In 5G NR, the fundamental capacity unit is the Physical Resource Block (PRB). Every user session, every QoS flow, and every network slice consumes PRBs. When PRB utilization consistently exceeds 70--80%, users experience throughput degradation, increased latency, and higher call drop rates.

3GPP defines PRB utilization measurements in TS 28.552 clause 5.1.1.3 (DL.TotalPRBUsage.Avg and UL.TotalPRBUsage.Avg). These counters are the bedrock of capacity planning -- they tell you exactly how loaded each cell is at every measurement interval.

T-Mobile US reported at their 2024 Analyst Day that their 5G network averages 42% DL PRB utilization nationally, with urban dense cells reaching 78% during busy hour. They target a 70% PRB utilization threshold for capacity expansion triggers. Reliance Jio shared similar metrics, noting their 3.5 GHz TDD cells in Mumbai average 65% PRB utilization during evening busy hour, driving a densification program adding 12,000 small cells in 2025.

5G NR Resource Grid Fundamentals

The NR resource grid dimensions depend on numerology (subcarrier spacing), channel bandwidth, and TDD pattern. The following table lists PRB counts for common 5G configurations per TS 38.101-1 Table 5.3.2-1.

For a typical macro deployment on n78 (3.5 GHz) with 100 MHz bandwidth and 30 kHz SCS, each cell has 273 PRBs per slot. With a TDD pattern of DDDSU (3GPP configuration per TS 38.213), approximately 70% of slots are available for downlink, giving an effective DL capacity of ~191 PRBs per slot on average.

Traffic Modelling for 5G

Busy Hour Traffic Estimation

Capacity planning revolves around the Busy Hour (BH) -- the contiguous 60-minute period with the highest traffic volume. The key metric is Busy Hour Traffic (BHT) measured in Gbps per cell.

The general formula is:

`

BHT_per_cell = (Subscribers_per_cell × BH_activity_factor × Avg_session_throughput × Sessions_per_BH × Avg_session_duration) / 3600

`

Traffic Profile by Service Type

Worked Example 1: Urban Macro Cell Dimensioning

Given:

• Band: n78 (3.5 GHz), 100 MHz BW, 30 kHz SCS → 273 PRBs
• TDD pattern: DDDSU → 70% DL ratio
• MIMO: 4x4 MIMO, average rank 2.5
• MCS: Average MCS index 20 (64-QAM, code rate 0.65) → spectral efficiency ~4.5 bps/Hz per layer
• Subscribers per cell: 800
• BH activity factor: 30% (240 active users during BH)
• Target PRB utilization: ≤70%

Step 1: Calculate peak cell DL throughput `

PRBs_available_DL = 273 × 0.70 (TDD) = 191 PRBs per slot

Subcarriers_per_PRB = 12

Symbols_per_slot = 14

SCS = 30 kHz → slot duration = 0.5 ms → 2000 slots/s

Bits_per_PRB_per_slot = 12 subcarriers × 14 symbols × 4.5 bps/Hz × 2.5 MIMO layers

= 12 × 14 × 4.5 × 2.5 = 1,890 bits

Peak_DL_throughput = 191 PRBs × 1,890 bits × 2,000 slots/s = 722 Mbps

` Step 2: Calculate usable capacity at 70% PRB load `

Usable_capacity = 722 × 0.70 = 505 Mbps per cell

` Step 3: Estimate BH traffic demand `

Avg_user_DL_throughput = 10 Mbps (weighted mix from traffic profile table)

Active_users_in_BH = 240

Simultaneous_users (activity factor within BH) = 240 × 0.25 = 60

Traffic_demand = 60 × 10 Mbps = 600 Mbps

` Step 4: Capacity assessment `

Demand (600 Mbps) > Usable capacity (505 Mbps) → Cell is OVER-CAPACITY

Utilization at demand = 600 / 722 = 83% → exceeds 70% threshold

` Recommendation: Split cell into 2 sectors or add a small cell layer. Alternatively, upgrade to 8T8R massive MIMO to increase spectral efficiency by ~40%.

Worked Example 2: PRB Utilization Trending and Forecast

Given: Historical PRB utilization data for Cell-ID 47832 (n78, urban) Analysis: `

Monthly DL PRB growth rate = (72 - 52) / 5 months = 4% per month

At current trend:

- April 2026: 76% (threshold breached)

- May 2026: 80% (degraded user experience)

- June 2026: 84% (critical)

Time to 70% threshold: Already exceeded in March 2026

Time to 80% critical: ~2 months (May 2026)

` Capacity intervention plan:

1. Immediate (April): Enable carrier aggregation with n1 (2100 MHz, 20 MHz) to offload 15--20% traffic
2. Short-term (May): Activate 256-QAM DL (requires SW upgrade) for ~15% spectral efficiency gain
3. Medium-term (Q3): Deploy small cell at nearby high-traffic POI to absorb 30% of macro traffic

PRB Utilization Thresholds and Actions

Dimensioning Formula Summary

The core dimensioning formula links subscriber count to required PRBs:

`

Required_PRBs = (N_users × Activity_factor × Avg_PRBs_per_user) / PRB_efficiency_factor

`

Where:

• N_users = subscriber count on the cell
• Activity_factor = fraction of users active in BH (typically 0.20--0.35)
• Avg_PRBs_per_user = average PRBs consumed per active user per slot (depends on QoS mix)
• PRB_efficiency_factor = scheduler efficiency, typically 0.85--0.92

3GPP TS 28.552 defines the relevant performance counters: DL.TotalPRBUsage.Avg, DL.UEThpDl.Avg, RRC.ConnMax, and DRB.SessionTime.Avg. These feed directly into the traffic model.

Operator Capacity Planning Practices

T-Mobile US uses a three-tier capacity planning framework:

1. Cell-level: Weekly automated PRB utilization checks against 70% threshold across 85,000 5G cells
2. Cluster-level: Monthly traffic growth forecasting per market cluster (20--50 cells)
3. Market-level: Quarterly spectrum deployment and site acquisition planning

Bharti Airtel reported at India Mobile Congress 2024 that their capacity planning engine processes 12 billion PRB utilization samples daily across 350,000 cells, using linear regression to forecast 90-day utilization trajectories. Cells predicted to exceed 75% within 90 days are automatically flagged for intervention.

3GPP References

• TS 28.552: Performance measurements for 5G NR -- defines all PRB utilization, throughput, and session counters
• TS 38.101-1: NR User Equipment radio transmission and reception, Part 1: Range 1 -- defines bandwidth and PRB configurations
• TS 38.214: Physical layer procedures for data -- defines MCS tables, CQI mapping, and scheduling parameters used in throughput calculations
• TS 38.213: Physical layer procedures for control -- defines TDD slot configurations that determine DL/UL capacity split

Conclusion

5G capacity planning is fundamentally a PRB management discipline. By combining resource grid calculations (TS 38.101-1), traffic demand modelling, and PRB utilization trending (TS 28.552), engineers can predict congestion months in advance and plan interventions -- carrier aggregation, MIMO upgrades, densification -- before users experience degradation. The target is to maintain busy-hour PRB utilization below 70%, with automated forecasting to trigger actions at least 90 days before threshold breach.