Wireless Notes

Multiple Access Techniques Comparison FDMA TDMA CDMA OFDMA SDMA

Complete comparison of all multiple access techniques FDMA TDMA CDMA OFDMA SDMA with generation mapping, performance metrics, selection criteria, evolution timeline, and capacity analysis for engineering students.

In wireless communication, multiple access techniques determine how many users will share a single spectrum. Each generation (1G to 5G) has adopted better techniques for higher capacity.

🌐 Overview {#overview}

│ Resource Dimensions	│
│ FDMA	Divide by FREQUENCY (each user gets a frequency band) │
│ TDMA	Divide by TIME (each user gets a time slot) │
│ CDMA	Divide by CODE (each user gets a unique code) │
│ OFDMA	Divide by FREQ + TIME (subcarriers × time slots) │
│ SDMA	Divide by SPACE (each user gets a spatial beam) │
│ Modern 5G uses ALL dimensions	│

📊 Master Comparison Table

Parameter	FDMA	TDMA	CDMA	OFDMA	SDMA
Divides by	Frequency	Time	Code	Freq+Time	Space
Signal type	Analog/Digital	Digital	Digital	Digital	Digital
Channel BW	Narrow (30 kHz)	Medium (200 kHz)	Wide (5 MHz)	Flexible (1.4-400 MHz)	Same as underlying
Bandwidth eff.	Low	Medium	Medium-High	Very High	Multiplier
Sync needed	No	Yes (strict)	Yes (codes)	Yes (time+freq)	Yes (CSI)
Guard	Freq bands	Time gaps	None (codes)	Cyclic prefix	Spatial nulls
Multipath	Not handled	Limited by guard	RAKE receiver	CP handles	Beamforming helps
Handoff	Hard	Hard	Soft ✅	Hard/Soft	Beam tracking
Complexity	Very Low	Low	Medium	Medium-High	High
Capacity	Low	Medium	High	Very High	Multiplies others
Generation	1G	2G	3G	4G/5G	4G/5G (with OFDMA)
Example	AMPS	GSM	WCDMA	LTE, 5G NR	Massive MIMO
Flexibility	Fixed	Semi	Moderate	Very High	High
Power control	Not critical	Not critical	Essential	Helpful	Needed
Freq planning	Required	Required	Not needed (reuse 1)	Not needed	Not needed

📱 Generation Mapping

┌─────────────────────────────────────────────────────────────────────┐ │ MOBILE GENERATION → ACCESS TECHNIQUE │ │ │ │ 1G (1980s) ──▶ FDMA │ │ • AMPS (30 kHz channels) │ │ • Analog voice only │ │ • ~400 users per cell │ │ │ │ 2G (1991) ──▶ TDMA + FDMA │ │ • GSM (8 slots × 200 kHz) │ │ • Digital voice + SMS │ │ • ~1000 users per cell │ │ │ │ 3G (2001) ──▶ CDMA │ │ • WCDMA (5 MHz), CDMA2000 (1.25 MHz) │ │ • Mobile data (2 Mbps) │ │ • ~60-80 voice users per carrier │ │ │ │ 4G (2009) ──▶ OFDMA + MIMO │ │ • LTE (up to 20 MHz, OFDMA DL, SC-FDMA UL) │ │ • Broadband data (100+ Mbps) │ │ • Dynamic scheduling │ │ │ │ 5G (2019) ──▶ OFDMA + Massive MIMO (SDMA) │ │ • NR (up to 400 MHz, flexible SCS) │ │ • Multi-Gbps, URLLC, mMTC │ │ • 64-256 antenna elements │ └─────────────────────────────────────────────────────────────────────┘

🎨 Visual Comparison

U1	U2	U3	U1U2U3	████████	U1	U2	U3	U1
U1	U2	U3	U1U2U3	████████	U2	U3	U1	U2
U1	U2	U3	U1U2U3	████████	U3	U1	U2	U3
U1	U2	U3	U1U2U3	████████	U1	U2	U3	U1

📈 Performance Comparison

Spectral Efficiency:

Technique	Typical Spectral Eff.	Peak
FDMA (1G)	0.04 bps/Hz	Very low
TDMA (GSM)	0.2 bps/Hz	Low
CDMA (WCDMA)	1-2 bps/Hz	Medium
OFDMA (LTE)	3-5 bps/Hz	High
OFDMA+MIMO (LTE-A)	5-7 bps/Hz	Very High
OFDMA+mMIMO (5G NR)	10-15 bps/Hz	Highest

Capacity Scaling:

Factor	FDMA	TDMA	CDMA	OFDMA
More spectrum	Linear	Linear	Linear	Linear
Better modulation	N/A	Moderate	Moderate	High (adaptive)
More antennas	N/A	N/A	Limited	Excellent (MIMO)
Smaller cells	Linear	Linear	Linear	Linear
Scheduling gain	None	Minimal	Minimal	High

🎯 Selection Criteria

When each technique works best:

If you need...	Best Choice	Why
Simplest system	FDMA	No sync, basic
Digital voice (legacy)	TDMA	GSM proven
Soft handoff, security	CDMA	Code-based
High-speed data	OFDMA	Flexible, efficient
Maximum capacity	OFDMA + SDMA	5G approach
Low latency	OFDMA (mini-slot)	Flexible scheduling
Massive IoT	OFDMA (NB-IoT)	Narrowband subcarriers
Long range (rural)	OFDMA (low SCS)	Long CP for large cells

📅 Evolution Timeline

Year	Technology	Access	Peak Rate	Key Innovation
1983	AMPS (1G)	FDMA	– (analog)	Cellular concept
1991	GSM (2G)	TDMA	9.6 kbps	Digital, encryption
2000	GPRS (2.5G)	TDMA	56 kbps	Packet data
2001	WCDMA (3G)	CDMA	2 Mbps	Mobile broadband
2006	HSPA (3.5G)	CDMA	14 Mbps	Shared channel, AMC
2009	LTE (4G)	OFDMA	100 Mbps	OFDM, MIMO
2013	LTE-A	OFDMA	1 Gbps	Carrier agg, MU-MIMO
2019	5G NR	OFDMA+mMIMO	10 Gbps	Massive MIMO, mmWave
2025	5G-A	OFDMA+AI	20 Gbps	AI scheduling, ISAC
~2030	6G	TBD	1 Tbps?	THz, RIS, AI-native

📝 Summary

Quick Reference:

Generation	Access	Key Feature	Limitation
1G	FDMA	Simple analog	Low capacity, no security
2G	TDMA+FDMA	Digital, encrypted	Limited data
3G	CDMA	Soft handoff, security	Self-interference
4G	OFDMA+MIMO	Fast data, flexible	Complexity
5G	OFDMA+mMIMO	Ultra-fast, ultra-reliable	Cost, power

The Big Picture:

1G	2G: ~3× (TDMA time sharing)
2G	3G: ~5× (CDMA spread spectrum)
3G	4G: ~10× (OFDMA + MIMO)
4G	5G: ~10× (Massive MIMO + mmWave)
Total 1G	5G: ~1500× capacity improvement!
Achieved by	Better access + more spectrum + more antennas + smaller cells

❓ FAQ

Q: What access technique will be used in 6G? A: Research mein hai – likely OFDMA continue with enhancements: AI-based scheduling, Reconfigurable Intelligent Surfaces (RIS), THz communication, non-orthogonal multiple access (NOMA), and possibly semantic communication.

Q: What is NOMA? A: Non-Orthogonal Multiple Access – it separates users on the same resource in the power domain. The strong user decodes both its own and the weak user's signal (SIC). It is in the research phase for 5G.

Q: What access technique does WiFi use? A: WiFi 4/5: OFDM + CSMA/CA (contention-based, not scheduled). WiFi 6/7: OFDMA (scheduled, like cellular). WiFi's approach was different from cellular but is now converging.

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