Mobile and Wireless Communication (cellular systems, antennas, RF basics)
This section covers Mobile and Wireless Communication: cellular systems, antennas, and RF basics. The exam tests frequency reuse, handover, GSM/LTE architecture, and practical RF measurements.
Core rules and concepts
- Cellular planning divides coverage into cells and reuses frequencies in distant cells. Reuse factor N sets the cluster size; capacity rises as cells shrink and are sectorized.
- Handover transfers a call between cells; roaming lets a subscriber use another operator's network. GSM uses FDMA/TDMA with 200 kHz carriers and 8 timeslots; LTE uses OFDMA on the downlink; 5G NR adds wider bandwidths and beamforming.
- GSM architecture: MS/SIM, BTS, BSC, MSC, HLR, VLR, AUC, EIR. The HLR stores subscription data; the VLR holds visiting subscriber data.
- RF: frequency and wavelength are inversely related. Antenna gain (dBi) is directional focusing, not created power. EIRP = transmitter power + antenna gain - cable loss. VSWR measures impedance match; high VSWR reflects power back.
- Path loss rises with distance and frequency. Fresnel zone clearance, antenna height, tilt, and obstructions affect coverage.
Worked example
A network has 210 traffic channels and a reuse factor of 7. Channels per cell = 210/7 = 30. If each cell is split into 3 sectors, each sector gets about 30/3 = 10 channels. That shows the trade-off: smaller clusters increase capacity but also co-channel interference, so N cannot be reduced without careful RF design.
Traps that cost marks
Confusing antenna gain with amplification; mixing dBm (absolute) and dBi (relative); assuming handover and roaming are the same; ignoring cable loss in EIRP; and thinking a larger cell always gives more capacity.