Wireless and Mobile Communications
This domain covers radio propagation, cellular reuse, multiple access, antenna techniques, link budgets, mobility, and modern systems such as LTE and 5G NR. Questions often require converting between power, path loss, and interference ratios.
Core rules
- Free-space path loss grows with distance squared and frequency squared; in dB it is about 20 log10(d) + 20 log10(f) + constant.
- Large-scale shadowing is modelled log-normally; small-scale multipath fading is often Rayleigh or Rician. Doppler spread scales with velocity and carrier frequency.
- Cellular reuse: co-channel interference depends on the reuse factor N and path-loss exponent γ. Sectorization and antenna downtilt reduce interference.
- Multiple access: FDMA, TDMA, CDMA, OFDMA. OFDMA assigns subcarriers in time–frequency resource blocks; 5G NR uses scalable numerology with subcarrier spacing 15 × 2^μ kHz.
- MIMO provides diversity, beamforming, or spatial multiplexing; spatial multiplexing needs rich scattering and good SNR.
- Handover transfers a connection between cells; link budgets must include antenna gain, cable loss, and fade margin.
Worked example
For a cluster size N = 7 and γ = 4, the worst-case co-channel signal-to-interference ratio for an omnidirectional system is S/I = (1/6)(√(3N))^γ. Compute √21 ≈ 4.583; raise to the fourth power: 21² = 441. Divide by 6: 73.5, which is 10 log10(73.5) ≈ 18.7 dB. Increasing N improves S/I but reduces the number of channels per cell, so capacity and quality trade off.
Traps
- Adding dBm and dB values as if both were absolute powers.
- Using the reuse ratio without squaring or raising it to γ.
- Assuming Shannon capacity applies unchanged under fading without coding or diversity.
- Confusing beamforming gain with spatial multiplexing gain.
- Forgetting that higher carrier frequency increases path loss and Doppler for the same geometry.