Wireless and Mobile Communication (including 4G/5G basics)
This domain tests cellular architecture, frequency reuse, handover, multiple access, and the evolution from GSM through LTE to 5G NR. Reuse-distance and capacity reasoning are common numericals.
Core rules
- A cellular system divides coverage into cells and reuses frequencies in cells separated by the reuse distance D = R√(3N), where N is the cluster size.
- Reuse factor is 1/N; smaller N increases capacity but raises co-channel interference.
- Handoff transfers a call between cells; GSM uses FDMA/TDMA, LTE uses OFDMA downlink and SC-FDMA uplink.
- LTE core is EPC with MME, SGW, PGW; 5G core is service-based with AMF, SMF, UPF.
- 5G NR operates in sub-6 GHz and mmWave bands, uses flexible numerology and massive MIMO beamforming.
- SA (standalone) 5G uses a 5G core; NSA anchors on LTE.
Worked example
A cluster size N = 7 with cell radius R = 2 km. Reuse distance D = 2 × √21 ≈ 2 × 4.58 = 9.17 km. Co-channel cells are therefore about 9.17 km apart. If N were reduced to 3, D = 2√9 = 6 km, increasing capacity but worsening interference-an engineering trade-off.
Traps that cost marks
- Confusing reuse factor 1/N with cluster size N.
- Using D = R√N instead of R√(3N).
- Mixing GSM TDMA with LTE OFDMA.
- Claiming 5G mmWave gives better coverage than sub-6 GHz; it gives capacity but poorer penetration.
- Confusing NSA and SA architectures.
Exam approach
Write the reuse formula, compute D, then comment on interference or capacity. For architecture questions, name the core nodes and their functions rather than listing generations only.