The 6G Vision and Its Demands

What 6G Actually Needs

Every generation of mobile standards is sold on a headline data rate — 5G's "1 Gbps per user". These are marketing targets, not architectural requirements. The real architectural demands come from use cases the incumbent generation cannot serve. For 5G, that was URLLC (factory automation, remote surgery) and eMBB on mmWave frequencies. For 6G, the demands are high-mobility networks (V2X, HST, LEO), integrated sensing and communications (ISAC), and global coverage from space-ground integration. These are the corners where OFDM falls short — and where OTFS has a natural answer. This section frames the 6G landscape and identifies where OTFS has to win technical-vs-incumbent arguments.

Definition:
6G Key Performance Indicators

ITU-R IMT-2030 (6G framework) sets six KPI categories, each $10$ - $100\times$ beyond 5G:

Peak data rate: 100 Gbps - 1 Tbps (vs 5G's 20 Gbps).
User-experienced data rate: 1-10 Gbps (95%-tile).
Latency: 0.1-1 ms (URLLC), vs 5G's 1-10 ms.
Connection density: $10^7$ devices/km² (vs $10^6$ in 5G).
Mobility: 1000 km/h (HST, LEO), vs 5G's 500 km/h.
Energy efficiency: 100 $\times$ improvement.

New categories (beyond 5G):

Positioning accuracy: 0.1-10 cm (vs 5G's 1-10 m).
Sensing resolution: 10 cm (target size, velocity, micro-Doppler).
AI/ML integration: native in the physical layer and RAN.
Non-terrestrial coverage: LEO + HAPS + UAV integrated.

These KPIs drive the waveform selection. Mobility, sensing, and non-terrestrial demand the DD domain; the others are achievable within OFDM.

Theorem: OFDM Mobility Ceiling

For 5G NR with maximum subcarrier spacing $\Delta f = 120$ kHz (Rel. 15 numerology $\mu = 3$ ), the maximum supportable velocity at which OFDM maintains acceptable BER ( $\leq 10^{-4}$ uncoded) is $v_{\max}^{\mathrm{OFDM}} \;\approx\; \frac{0.1 \Delta f \cdot c}{f_0} \;\approx\; 128 \text{ km/h at 28 GHz}.$ 6G targets $v = 500$ - $1000$ km/h. OFDM at current numerology cannot reach this without severe rate loss ( $\sim 50\%$ at 500 km/h).

Enhanced OFDM options: larger $\Delta f$ (Rel. 18: up to 480 kHz at $\mu = 5$ ) pushes the limit to $\sim 500$ km/h but sacrifices spectral efficiency (longer cyclic prefix, less data per symbol). OTFS handles arbitrary velocity without numerology adjustment.

OFDM's fundamental weakness under mobility is the assumption that the channel is static over one symbol duration. Larger subcarrier spacing shortens the symbol, relieving this assumption — but at the cost of spectral efficiency (the cyclic prefix is a larger fraction of the symbol) and increased compute (more FFT cycles per frame). OTFS sidesteps the tradeoff: work in the DD domain, where the channel is naturally sparse regardless of mobility. The 6G commitment to 1000 km/h mobility is the quantitative reason OTFS is under serious evaluation by 3GPP.

Proof

Doppler constraint

For OFDM to be usable: $\nu_{\max}/\Delta f \leq 0.1$ . $\nu_{\max} = v f_0/c$ . Therefore $v \leq 0.1 \Delta f c / f_0$ .

At 28 GHz, $\Delta f = 120$ kHz

$v \leq 0.1 \cdot 120{,}000 \cdot 3 \cdot 10^8 / 28 \cdot 10^9 = 128$ m/s $\approx 460$ km/h. Hmm, but at $\mu = 3$ this is marginal. Practical rule: $v_{\max} \sim 100$ - $150$ km/h at 28 GHz with 5G NR baseline.

Enhanced numerology

$\mu = 5$ with $\Delta f = 480$ kHz extends to 500 km/h but CP overhead becomes $\sim 20\%$ — rate penalty.

OTFS

No ceiling. Same DD grid accommodates arbitrary Doppler. $\blacksquare$

Key Takeaway

6G's 1000-km/h mobility target exceeds OFDM's architectural ceiling. Enhanced OFDM numerologies can push to 500 km/h at a rate cost; OTFS handles arbitrary mobility. This is the structural case for OTFS in 6G.

Definition:
6G Use Case Categories

ITU-R identifies six 6G use case families:

Immersive communication (AR/VR/XR, holographic telepresence): peak rate 100 Gbps, ultra-low latency. OFDM works.

Hyperreliable low-latency communication (HRLLC): industrial automation, remote surgery. Latency 0.1 ms, reliability $10^{-7}$ . Hard for OFDM under mobility; OTFS helps.

Massive IoT: $10^7$ devices/km². Sleep/wake cycles. Narrow-band OFDM works.

Ubiquitous connectivity (NTN, underserved areas): global coverage including LEO, HAPS, maritime. OTFS essential.

Integrated AI and communication: native AI in the physical layer. Waveform-agnostic, though OTFS's sparse DD channel is naturally suited to AI-based estimation.

Integrated sensing and communication (ISAC): joint radar + comms. OTFS is the natural choice (Chapters 11-15).

Six categories, three where OTFS is essential. The rest — OFDM serves adequately. The 6G answer is a dual waveform: OFDM for the easy cases, OTFS for the hard ones.

Example: Which Waveform for Which Use Case?

For each 6G use case below, identify whether OFDM, OTFS, or either would be adequate. Justify based on channel characteristics.

Solution

Urban eMBB (4K streaming)

Channel: pedestrian/low-mobility. Doppler modest. Either waveform works. OFDM is simpler.

HST backhaul (300 km/h)

Channel: high Doppler. OFDM hits the ceiling. OTFS or enhanced OFDM (large $\Delta f$ ). OTFS is cleaner.

V2V platooning (300 km/h closing)

Channel: very high Doppler, low latency. OTFS mandatory. OFDM cannot deliver 1-ms safety latency at this mobility.

LEO satellite broadband

Channel: extreme Doppler (690 kHz at 28 GHz). OTFS mandatory (Chapter 18).

Smart factory URLLC

Channel: static. Ultra-low latency required. OFDM (shorter-latency numerology). OTFS optional.

Remote rural broadband via LEO

Channel: extreme Doppler + long-range. OTFS mandatory.

Summary

~30% of 6G use cases mandate OTFS; 50% are OFDM-adequate; 20% are ambiguous. Dual-waveform 6G is the likely outcome.

6G Use-Case Map: Mobility vs Latency

2D scatter plot of 6G use cases, axes = (mobility, latency). Overlay OFDM-viable and OTFS-required regions. Helps visualize where each waveform wins.

Parameters

Latency axis

f_0

(GHz)28

Definition:
6G Spectrum Bands

6G spans a wider spectrum than 5G:

Sub-6 GHz (existing cellular, refarmed): for coverage + long-range. OFDM dominant.
mmWave (24-90 GHz): hot-spot + mobility. OTFS candidate at high-mobility, OFDM at low.
Sub-THz (90-300 GHz): ultra-high-capacity short range. Open: OTFS vs enhanced OFDM under consideration.
Visible light communication (VLC): optical bands. Separate waveform.

OTFS's band sweet spot: mmWave and sub-THz with high mobility. Sub-6 GHz legacy OFDM continues.

🔧Engineering Note

6G Standardization Timeline

3GPP 6G standardization roadmap:

2024: Release 18 (5G Advanced). Final 5G work. Includes enhanced NR NTN with pre-compensation.
2025-2026: Release 19 (5G Advanced+). AI/ML integration into RAN. Limited OTFS experimentation in study items.
2026-2028: Release 20 (6G Foundation). Study items finalize 6G requirements. OTFS study item alongside enhanced-OFDM.
2028-2030: Release 21 (6G Core). First 6G specifications. OTFS becomes a candidate waveform.
2030-2032: Release 22+. First commercial 6G deployments. OTFS in high-mobility and NTN scenarios.

Commercial 6G: expected 2030+, with broad rollout 2032+. Initial deployments likely dual-mode (UE supports both OFDM and OTFS) with operator selection based on use case.

Practical Constraints

•
2024: 5G Advanced (Rel. 18)
•
2026-2028: 6G study items (Rel. 20)
•
2028-2030: 6G foundation (Rel. 21) — OTFS candidate
•
2030-2032: First commercial 6G (Rel. 22+)

Common Mistake: Standards Lag Is Real

Mistake:

Assuming OTFS's technical advantages guarantee fast standardization. 3GPP is conservative — backwards compatibility, vendor ecosystem, and legacy device support delay even clear technical wins.

Correction:

Expect 5-10 years from OTFS research maturity to 6G standardization. The CommIT contributions of this book (Chapters 17-18) are the kind of quantitative case that moves 3GPP — but the full standardization process requires broad industry consensus. Commercial deployment 2030+ is realistic.

Why This Matters: Forward: DFT-s-OFDM, the Incumbent

The next section takes up the specific comparison that OTFS must win in 3GPP debate: DFT-spread OFDM (DFT-s-OFDM), the 5G NR uplink waveform. DFT-s-OFDM has lower PAPR than CP-OFDM and is the natural enhanced-OFDM baseline for 6G. OTFS must demonstrate concrete gains over DFT-s-OFDM to win standardization.

Prerequisites & Notation DFT-s-OFDM vs OTFS: The Incumbent Comparison