Telecom Fundamentals — Practice Q&A
Q: Why does higher-frequency spectrum offer more capacity but worse coverage than lower-frequency spectrum?
A: It's a fundamental physical property of radio waves — higher frequencies carry more information per unit time (more capacity) but attenuate faster over distance and are blocked more easily by obstacles like buildings, meaning a higher-frequency signal needs more, closely-spaced transmission points to cover the same area a lower-frequency signal could cover with fewer, more widely-spaced ones. This is why 5G's high-capacity mmWave bands require dense urban small-cell deployment, while lower-frequency bands remain important for wide-area rural coverage even in a 5G-era network.
Q: What's the actual difference between GSM and CDMA, beyond "both are 2G standards"?
A: They use fundamentally different multiple-access techniques to let multiple users share spectrum — GSM uses TDMA (each user gets a distinct time slot on a shared frequency), while CDMA uses code-division (each user's signal is spread with a unique code, allowing simultaneous use of the same frequency and time). This isn't just an implementation detail — it produces genuinely different network-planning characteristics: GSM requires careful frequency-reuse planning across cells to avoid interference, while CDMA allows universal frequency reuse but manages capacity through interference/noise management instead.
Q: Why did 4G LTE need a specific solution like VoLTE for something as basic as voice calling?
A: Because LTE was designed from the start as an all-IP, packet-switched-only network with no native circuit-switched domain — unlike 2G/3G, which had dedicated circuit-switched infrastructure specifically built for voice. Voice calling had no built-in mechanism on pure LTE, so the industry needed either CSFB (falling back to older circuit-switched 2G/3G networks for the call) or VoLTE (carrying voice as packetized IP data over LTE itself, using IMS architecture for call signaling). VoLTE became the long-term solution because it doesn't require dropping back to older, slower network technology just to make a call.
Q: Explain the HLR's role in a GSM network, and why it matters for mobility.
A: The Home Location Register is a central database tracking each subscriber's identity, service permissions, and current location area — it's how a network knows where to route an incoming call to a device that could physically be anywhere within coverage. Combined with the VLR (a temporary, local copy of relevant data for the subscriber's current area), this HLR/VLR split lets a call be routed correctly without every single call needing to query one central database from anywhere in the network, distributing that lookup load geographically instead.
Q: What's the difference between hard handover and soft handover, and which technologies use which?
A: Hard handover ("break-before-make," used by GSM) briefly ends the connection to the old cell before establishing the new one during a mobile device's transition between coverage areas. Soft handover ("make-before-break," enabled by CDMA's code-division nature) maintains connections to both the old and new cell briefly before dropping the old one, generally producing a smoother transition with less risk of a dropped call at the handover point.
Q: Why is spectral efficiency a more accurate way to describe generational telecom progress than "more bandwidth"?
A: Because a meaningful share of each generation's capacity gain comes from extracting more data throughput from a comparable amount of spectrum, not just from being allocated more raw spectrum — OFDMA's flexible sub-carrier allocation achieves higher spectral efficiency than TDMA's fixed time-slot structure, for instance. Techniques like MIMO and beamforming extend this further, squeezing additional capacity from the same underlying spectrum through multiple-antenna and directional-transmission techniques rather than requiring more spectrum allocation at all.