How do I select between TDMA, FDMA, and CDMA for a given communication system design?
Multiple Access Scheme Selection
The choice of multiple access scheme is one of the most fundamental decisions in communication system design, affecting frequency planning, terminal hardware, network capacity, and interference management.
| Parameter | Free Space | Urban | Indoor |
|---|---|---|---|
| Path Loss Model | Friis (1/r²) | Okumura-Hata | IEEE 802.11 |
| Fading Margin | 0 dB | 10-30 dB | 5-15 dB |
| Multipath | None | Severe | Moderate-severe |
| Typical Range | Line of sight | 1-30 km | 10-100 m |
| Shadow Fading (σ) | 0 dB | 6-12 dB | 3-8 dB |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Frequently Asked Questions
Why did cellular evolve from FDMA to TDMA to CDMA to OFDMA?
1G (FDMA, analog FM): simple but low capacity and no data capability. 2G (GSM/TDMA, IS-136): digital voice, higher capacity than analog, SMS capability. 2G (IS-95/CDMA): even higher capacity (3-5×), better soft handoff, improved building penetration. 3G (WCDMA/CDMA2000): wideband CDMA with higher data rates (up to 42 Mbps with HSPA+). 4G (LTE/OFDMA): OFDM handles multipath elegantly, MIMO integration is natural with OFDM, very high spectral efficiency (up to 30 bps/Hz with 8×8 MIMO). 5G (NR/OFDMA): extends LTE OFDMA to mmWave with flexible numerology and massive MIMO. Each generation addressed the key limitation of the previous one.
Can I combine multiple access schemes?
Yes, many systems are hybrids: (1) GSM: FDMA (200 kHz channels) + TDMA (8 time slots per channel). (2) IS-95/CDMA: FDMA (1.25 MHz channels) + CDMA (users within each channel separated by Walsh codes). (3) LTE: OFDMA (downlink) + SC-FDMA (uplink), with the OFDMA itself combining FDMA (frequency-domain) and TDMA (time-domain) resource allocation. (4) Satellite systems: FDMA between transponders + TDMA within each transponder for multiple earth stations. Hybrid approaches exploit the strengths of each scheme at different levels of the protocol stack.
What about SDMA (Space Division Multiple Access)?
SDMA uses spatially separated antenna beams to serve multiple users on the same time-frequency resource. This is implemented through: (1) Sectorization: dividing a cell into 3-6 sectors with directional antennas. Each sector reuses the same frequencies. Capacity improvement: proportional to the number of sectors. (2) Beamforming: forming narrow beams toward individual users. Users in different spatial directions can share the same channel. (3) Massive MIMO: the ultimate form of SDMA. With 64-256 antenna elements, the base station forms independent beams to 8-16+ users simultaneously on the same time-frequency resource. Capacity improvement: proportional to the number of spatial streams. 5G massive MIMO is essentially OFDMA + SDMA.