4G/5G & Wireless — Practice Q&A
Q: What actually changed structurally between LTE's EPC and 5G's Core, beyond "5G is faster"?
A: The 5G Core is built around network function virtualization and software-defined networking principles from the ground up, letting it dynamically instantiate and orchestrate logically isolated network slices — each with different throughput/latency/reliability guarantees — over shared physical infrastructure. LTE's EPC wasn't designed around this kind of dynamic virtualized orchestration, which is why network slicing is a genuinely 5G-native capability rather than something that could simply be added to 4G's core architecture.
Q: Explain OFDM's orthogonality property and why it matters practically.
A: OFDM divides spectrum into many narrow sub-carriers that are mathematically orthogonal to each other — meaning that even though they overlap in frequency, a receiver can extract each sub-carrier's data cleanly without interference from its neighbors, because their overlapping waveforms don't corrupt each other's information at the specific sampling points the receiver uses. Practically, this means sub-carriers can be packed much more densely than they could be with traditional non-overlapping frequency-division techniques, which need guard bands between channels — directly increasing spectral efficiency.
Q: Why does LTE use OFDMA for downlink but SC-FDMA for uplink?
A: It's a power-efficiency tradeoff specific to the device side of the link. OFDMA has a higher peak-to-average power ratio, which is fine for a base station with a large, grid-connected power supply, but costly for a battery-powered device's power amplifier. SC-FDMA achieves a lower peak-to-average power ratio, extending device battery life and allowing cheaper, less powerful device-side amplifiers — an acceptable tradeoff since the uplink typically carries less traffic than the downlink anyway.
Q: What's the practical difference between FR1 and FR2 deployment strategy, and why don't operators just deploy FR2 everywhere for the extra capacity?
A: FR1 (sub-6 GHz) offers the coverage-oriented range and obstacle penetration needed for wide-area, general-purpose connectivity, while FR2 (mmWave) offers dramatically higher capacity but at drastically shorter range and poor obstacle penetration, requiring dense small-cell deployment to cover any meaningful area. Deploying FR2 everywhere would require an economically impractical density of cell sites for the coverage most subscribers actually need most of the time, so operators use FR1 as the coverage backbone and deploy FR2 selectively where extreme localized capacity justifies the infrastructure cost — stadiums, transit hubs, dense downtown cores.
Q: How does massive MIMO increase capacity differently than earlier multiple-access techniques like OFDMA?
A: Earlier multiple-access techniques (TDMA, CDMA, OFDMA) all divide a fixed underlying resource — time, code space, or frequency sub-carriers — among users. Massive MIMO instead uses a large number of antenna elements to spatially separate multiple simultaneous data streams, effectively creating additional usable capacity through spatial multiplexing rather than dividing an existing fixed resource more finely. This is why 5G's realized capacity gains exceed what OFDMA's spectral-efficiency improvement over 4G's techniques alone would predict.
Q: Why is URLLC often paired with edge computing rather than solved through radio-link improvements alone?
A: Because URLLC's latency budget (often sub-5ms end-to-end) includes more than just radio transmission time — it includes propagation delay and routing through the core network to wherever the data is actually processed. Even a very fast radio link can't overcome the latency added by routing every packet back to a distant, centralized data center. Multi-access edge computing addresses this by moving processing physically closer to the radio access network, reducing the round-trip distance and the latency it contributes, which a radio-layer improvement alone cannot solve.