Wireless and Mobile Communication Systems
This domain tests cellular radio and short-range wireless: frequency reuse, handover, GSM/LTE/5G basics, antennas and Wi-Fi channel planning. Numerical questions often involve capacity per cell or path-loss change.
Core rules
- Cellular systems divide an area into cells and reuse the same frequencies in cells separated by a reuse distance. Cluster size N sets capacity per cell = total channels ÷ N.
- Handover transfers an active call between cells; roaming uses another operator's network.
- GSM uses a SIM to identify the subscriber (IMSI); the handset carries an IMEI. Indian GSM operates in 900 and 1800 MHz bands.
- LTE uses OFDMA on the downlink and SC-FDMA on the uplink; 5G NR adds wider bandwidth, massive MIMO and lower latency.
- Antenna gain is in dBi; sectorization commonly uses three 120° sectors; downtilt controls coverage.
- Path loss rises with distance and frequency. Wi-Fi 2.4 GHz uses non-overlapping channels 1, 6 and 11; 5 GHz has more channels but shorter range. Bluetooth forms a piconet at 2.4 GHz.
Worked example
A cluster of 7 cells shares 210 voice channels: 210 ÷ 7 = 30 channels per cell. If each cell is split into 3 sectors, each sector carries about 10 channels, and trunking efficiency must be recalculated rather than assuming 30 per sector. Doubling the distance from the antenna raises free-space loss by 20 log10 2 ≈ 6 dB.
Traps
Using dBm for antenna gain; thinking 5 GHz penetrates walls better; selecting overlapping 2.4 GHz channels; confusing handover with roaming; assuming higher frequency gives longer range; ignoring sector count in capacity sums.