Mobile and Wireless Communication
Mobile and Wireless Communication
This domain tests cellular concepts, radio propagation and mobile access technologies. Expect questions on frequency reuse, handover, multiple access, antennas, coverage and basic link calculations.
Core rules
- A cellular network divides coverage into cells and reuses frequencies at a distance that keeps co-channel interference acceptable.
- Cluster size N determines reuse distance: D = R√(3N), where R is cell radius. Larger N means less interference but fewer channels per area.
- Handover transfers an active call between cells; roaming lets a subscriber use another operator's network.
- Multiple access methods include FDMA, TDMA, CDMA and OFDMA. GSM uses TDMA/FDMA; LTE uses OFDMA on the downlink and SC-FDMA on the uplink.
- Radio signals suffer path loss, shadowing and multipath fading. Antenna gain, height and tilt affect coverage.
Worked example
For a cluster size N = 7 and cell radius R = 1 km, reuse distance D = 1 × √(3×7) = √21 ≈ 4.58 km. If N is increased to 12, D = √36 = 6 km. The larger distance reduces co-channel interference, but each cluster needs more frequency groups, so fewer channels are available per cell. The reasoning links a coverage number to a capacity trade-off.
Traps that cost marks
Using diameter instead of radius; treating handover and roaming as the same; mixing uplink and downlink; and adding dBm values as if they were ordinary powers. Also remember that a higher-frequency signal generally has greater path loss for the same antenna conditions.