Chapter Summary

Chapter Summary

Key Points

• 1.

  Cell-free architecture eliminates cells. $L$ distributed APs jointly serve $K$ UEs with no predetermined cell assignment. Smooth macro-diversity replaces hard handovers. Cell-edge penalty disappears. The combination with OTFS addresses high- mobility cellular's deepest weakness.

• 2.

  Macro-diversity scales linearly: SINR $\propto L$. At $L = 50$: ~17 dB SINR vs 10 dB cellular. Combined with OTFS's DD- diversity $P$: aggregate effective diversity $L \cdot P$, BER at 20 dB SNR drops from $10^{-2}$ (cellular OFDM) to $10^{-13}$ (cell-free OTFS). 11 orders of magnitude.

• 3.

  Conjugate beamforming is simple and near-optimal. Each AP computes $\mathbf{v}^{(l, k)} = (\mathbf{H}^{(l, k)})^H / \|\mathbf{H}^{(l, k)}\|$ locally. No inter-AP coordination at symbol level. Asymptotic SINR scales linearly in $L$. Enables scalable deployment.

• 4.

  Embedded + superimposed pilots cut overhead to $< 1\%$. The CommIT embedded-pilot framework (Chapter 7) extends to cell-free with pilot reuse across spatially-separated UEs. For $K = 200$ UEs in 1 km² at 15 dB SNR: pilot overhead $< 1\%$ — vs $\sim 10\%$ for classical pilot-based.

• 5.

  CommIT contribution: 35% throughput gain. Mohammadi-Ngo- Matthaiou-Caire (IEEE TWC 2023) established the quantitative result: cell-free OTFS achieves $\sim 35\%$ improvement in 95%-likely per-user throughput vs cell-free OFDM at typical urban mobility (60-150 km/h). Scales as $L^{0.7} P^{0.3}$.

• 6.

  Coverage uniformity (R_95%/R_50%) improves from 0.05 (cellular) to 0.5-0.6 (cell-free OTFS). Operators can engineer for median users without cell-edge pathology. This is the operational benefit driving 6G cell-free adoption.

• 7.

  Deployment scales to $L = 100$-$1000$ APs. Per-AP fronthaul: ~77 kB/s with channel-estimate forwarding. CPU compute: $\sim 10^{8}$ ops/s at $L = 100$. Fits in 2024-era edge servers. Synchronization: PTP-1588v2 + GNSS-PPS at sub-microsecond accuracy.

• 8.

  6G standardization expected 2028+. 5G NR Rel. 18-19 includes cell-free support for select scenarios. Mass commercial deployment aligned with 6G rollout. OTFS cell-free is the primary deployment target for high-mobility, high-reliability wireless services.