Chapter Summary

Chapter Summary

Key Points

• 1.

  The cache-aided cloud-RAN architecture places caches of size $M$ at each of $N_\text{EN}$ edge nodes, connected to a central cloud via per-EN fronthaul capacity $C_F$. The aggregate caching gain is $t = N_\text{EN} M/N$.

• 2.

  Normalized delivery time (NDT) $\Delta$ is a dimensionless latency metric normalized to the infinite-fronthaul MU-MIMO baseline. NDT = 1 is the ideal; NDT > 1 reflects the architecture's bottleneck.

• 3.

  NDT formula. Achievable upper bound: $\Delta = \max(1, K(1-\mu)/(N_\text{EN} L_\text{EN}(1+t)) + K(1-\mu)/(N_\text{EN} C))$. Combines the downlink cooperative Lampiris-Caire DoF term with the fronthaul transfer term. Cut-set lower bound matches when $t = 0$.

• 4.

  Cache-fronthaul substitutability. Iso-NDT contours are hyperbolae in the $(\mu, C)$ plane. Doubling either resource reduces its contribution to NDT by half. Operators choose along the Pareto frontier based on cost structure.

• 5.

  The CommIT NDT framework (Sengupta-Tandon-Simeone, 2017, with Caire-adjacent follow-ups) is the unified language for cache- aided C-RAN analysis. It cleanly isolates the cache-fronthaul tradeoff from the underlying SNR.

• 6.

  Saturation behavior. $\mu \to 1$: NDT = 1 (full cache, no fronthaul needed). $C \to \infty$: NDT reduces to the Lampiris- Caire DoF formula for cooperative $N_\text{EN} L_\text{EN}$-antenna BC.

• 7.

  Deployment implications. 5G NR C-RAN with $N_\text{EN} = 2\text{-}8$ and $\mu \approx 0.1\text{-}0.3$ achieves NDT $\leq 3$ at typical fronthaul $C = 5\text{-}20$ files/use. Design tools benefit from the NDT framework when sizing fronthaul and cache budgets.