Three Customers, One Radio
Every generation before it was designed for one service, so it could freeze its parameters — LTE fixed 15 kHz spacing for every band and every speed (M10-L2). NR was specified against three services whose demands contradict each other.
They Pull Opposite Ways
- eMBB — 20 Gbit/s peak. Wants the widest channel and 256-QAM
- URLLC — 1 ms and 99.999%. Wants the shortest transmission and spare margin
- mMTC — 10⁶ devices/km² = one per square metre. Wants a narrow channel and sleep
- 99.999% is 1 in 10⁵: at 1000 packets/s, one failure per 100 s
- LTE’s HARQ round trip is 8 ms — eight budgets, for one retry (M6-L4)
No fixed parameter set is best at all three. So NR was made adjustable instead.
Double the Spacing, Halve the Slot
15, 30, 60, 120 kHz → slots of 1, 0.5, 0.25, 0.125 ms. The resource block stays 12 subcarriers (M10-L2), so at 120 kHz it is 12 × 120 = 1.44 MHz wide.
Latency Costs Prefix Reach
- Overhead never moves: 4.69/71.36 = 6.6% at every spacing (M10-L2)
- But the prefix duration halves: 4.69 → 0.59 µs
- At 300 m per µs, reach falls 1407 m → 176 m
- Doppler at 28 GHz, 100 km/h: 27.78/0.0107 = 2593 Hz
- That is 17.3% of 15 kHz — but 2.2% of 120 kHz ✓
28/3.5 = 120/15 = 8, so scaling spacing with frequency holds the Doppler fraction constant. Short slots suit small cells and high bands, and nothing else.
Bandwidth Beats SNR
1384/666 = 2.08× the capacity on 4× the bandwidth at 10 dB worse SNR. Matching 1.38 Gbit/s inside 100 MHz would need 41.7 dB — which no mobile link has.
Pick a Numerology
The Penalty Is the Cure
λ = 10.7 mm, so 8 × 8 at λ/2 spans 7 × 5.36 = 37.5 mm. It does not cure blockage: a body still costs 20–40 dB (M8-L2).
The Radio Is Not the Budget
gNB splits into CU, DU and RU; the 5GC is services — AMF control, SMF sessions, UPF your packets. A gateway 150 km away burns 1.5 ms of fibre alone; an edge UPF 10 km away costs 100 µs.
Standalone or Not
- Slicing — separate logical networks, each with its own SLA, on one infrastructure
- Honest limit: it is orchestration, not spectrum — one cell, one scheduler still
- NSA anchors the control plane on an LTE base station and the LTE core
- So early phones showed 5G while signalling over 4G — the throughput was real
- SA is gNB plus 5GC end to end — and slicing and URLLC need it
Every non-radio feature in this lesson requires standalone. That migration matters more than any peak rate.
What you learned
- Three corners contradict, so Δf = 15·2k kHz and slots are 1 → 0.125 ms
- CP overhead stays 6.6%; CP reach falls 1407 m → 176 m
- 100 MHz at 20 dB = 666 Mbit/s; 400 MHz at 10 dB = 1.38 Gbit/s
- +18.1 dB of path loss, −18.1 dB from 64 elements — the same 20 log₁₀8
- 20 Gbit/s needs 800 MHz, 4 layers, 256-QAM and line of sight at once