Multi-Access Radio Techniques — FDMA, TDMA & CDMA

The core challenge in wireless communication is how do multiple users share the same radio spectrum simultaneously? The answer lies in multiple access techniques — the backbone of every mobile generation from 1G to 3G.

The Problem: Shared Spectrum

Imagine a room full of people all trying to talk at once. Without a system, it's chaos. Mobile networks face the same problem: thousands of users need to share a limited radio spectrum. The solution is to divide the spectrum using time, frequency, or codes.

  Shared Radio Spectrum
  ┌─────────────────────────────────────────────┐
  │  User A  │  User B  │  User C  │  User D  │  → How do they share this?
  └─────────────────────────────────────────────┘
         ↓             ↓             ↓
       FDMA          TDMA          CDMA
  (By Frequency)  (By Time)    (By Code)

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1G — FDMA (Frequency Division Multiple Access)

Generation: 1G | Era: 1984 – 1996+
Technology: Analog Voice | Standard: AMPS, NMT, TACS

Concept: Each user gets a dedicated, unique frequency band for the entire call duration. No two users share the same frequency — they are simply separated by frequency.

How It Works

• The total spectrum is split into narrow frequency channels (e.g., 30 KHz each)
• Each call is assigned one channel exclusively
• The channel is held for the entire duration of the call
• Analogy: Separate lanes on a highway — each car has its own lane

Diagram — 1G FDMA

                   1G — Separate Frequencies
              FDMA — Frequency Division Multiple Access

 Frequency
    ↑
    │
    │  [30 KHz] ════════════════════════════════════════ User 8
    │  [30 KHz] ════════════════════════════════════════ User 7
    │  [30 KHz] ════════════════════════════════════════ User 6
    │  [30 KHz] ════════════════════════════════════════ User 5
    │  [30 KHz] ════════════════════════════════════════ User 4
    │  [30 KHz] ════════════════════════════════════════ User 3
    │  [30 KHz] ════════════════════════════════════════ User 2
    │  [30 KHz] ════════════════════════════════════════ User 1
    │
    └──────────────────────────────────────────────────→ Time
                (Channel is held for entire call)

Key Properties

Property	Value
Channel Width	30 KHz (AMPS)
Simultaneous Users	One per frequency band
Modulation	Analog FM
Voice Quality	Poor (analog, susceptible to noise)
Spectrum Efficiency	Low

Limitations

• Wasteful — frequency is reserved even during silence (pauses in speech)
• No digital data support
• Susceptible to noise, interference, and eavesdropping
• Low capacity — limited number of simultaneous calls

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2G — TDMA (Time Division Multiple Access)

Generation: 2G | Era: 1992 – 2000+
Technology: Digital Voice | Standard: GSM (Europe), D-AMPS (US)

Concept: Multiple users share the same frequency band but take turns using it in rapid time slots. Each user transmits in a brief burst during their assigned time slot, cycling rapidly so all users appear to have continuous service.

How It Works

• The spectrum is divided into 200 KHz channels (GSM)
• Each channel is further divided into 8 time slots per frame
• Each user is assigned one time slot per frame
• A TDMA frame = 8 timeslots × 0.577 ms = 4.615 ms per frame
• Analogy: A rotating speaking turn at a meeting — each person speaks briefly, then waits for their turn again

Diagram — 2G TDMA

                   2G — TDMA
           Time Division Multiple Access

 Frequency
    ↑
    │         ◄──── One TDMA frame (4.615 ms) ────►
    │         ┌──┬──┬──┬──┬──┬──┬──┬──┐ ┌──┬──┬──┬──┬──┬──┬──┬──┐
    │ 200 KHz │U1│U2│U3│U4│U5│U6│U7│U8│ │U1│U2│U3│U4│U5│U6│U7│U8│  ← 8 users
    │         │  │▓▓│  │  │  │  │  │  │ │  │▓▓│  │  │  │  │  │  │  ← User 2's slot
    │         ├──┴──┴──┴──┴──┴──┴──┴──┤ ├──┴──┴──┴──┴──┴──┴──┴──┤
    │ 200 KHz │U1│U2│U3│U4│U5│U6│U7│U8│ │U1│U2│U3│U4│U5│U6│U7│U8│
    │         ├──┴──┴──┴──┴──┴──┴──┴──┤ ├──┴──┴──┴──┴──┴──┴──┴──┤
    │ 200 KHz │U1│U2│U3│U4│U5│U6│U7│U8│ │U1│U2│U3│U4│U5│U6│U7│U8│
    │         ├──┴──┴──┴──┴──┴──┴──┴──┤ ├──┴──┴──┴──┴──┴──┴──┴──┤
    │ 200 KHz │U1│U2│U3│U4│U5│U6│U7│U8│ │U1│U2│U3│U4│U5│U6│U7│U8│
    │         └──┴──┴──┴──┴──┴──┴──┴──┘ └──┴──┴──┴──┴──┴──┴──┴──┘
    │
    └─────────────────────────────────────────────────────────────→ Time

    One timeslot  = 0.577 ms
    One TDMA frame = 8 timeslots = 4.615 ms
    Each user transmits in burst → appears continuous to the ear

Key Properties

Property	Value
Channel Width	200 KHz (GSM)
Timeslots per Frame	8
Timeslot Duration	0.577 ms
Frame Duration	4.615 ms
Capacity vs FDMA	~3–8× more users per MHz
Voice Quality	Good (digital)

Advantages over FDMA

• Digital — better voice quality, encryption possible
• More efficient — up to 8 users share one 200 KHz channel
• Supports SMS and basic data services
• Silent periods detected → timeslot can be freed

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3G — CDMA (Code Division Multiple Access)

Generation: 3G | Era: 2001 – 2010+
Technology: Multimedia / Packet Data | Standard: UMTS (W-CDMA), CDMA2000

Concept: All users transmit on the same frequency at the same time, but each user's signal is spread across the spectrum using a unique mathematical code (spreading code). The receiver uses the same code to extract only the intended signal.

How It Works

• Every user gets a unique orthogonal code (e.g., Walsh codes)
• All users transmit simultaneously across the entire bandwidth
• At the receiver: multiply received signal × user's code → extract the signal
• All other users' signals appear as background noise (spread out)
• Analogy: Multiple conversations in different languages in the same room — you understand only your language, others sound like noise

Spreading Process (Simplified)

  User Data Bit:    1    (low rate: e.g., 9.6 Kbps)
        ×
  Spreading Code:  1011010110...  (high rate: e.g., 1.2288 Mcps)
        =
  Spread Signal:   1011010110...  (occupies full wideband spectrum)

  At Receiver:
  Full Wideband Signal × Same Spreading Code = Original Data Bit
  (Other users' spread signals → average out to noise floor)

Diagram — CDMA

                   3G — CDMA
           Code Division Multiple Access

 Frequency
    ↑
    │  ┌─────────────────────────────────────────────┐
    │  │ User 1 (Code A):  ▓▓▒▒▓▒▓▓▒▓▒▒▓▓▓▒▓▒▒▒▓▓▒▓ │
    │  │ User 2 (Code B):  ▒▓▓▒▒▓▒▒▓▒▓▓▒▒▒▓▒▓▓▓▒▒▓▒ │ ← All users on
    │  │ User 3 (Code C):  ▓▒▒▓▓▒▓▒▒▓▓▒▓▒▓▒▒▓▒▓▒▓▒▓ │    SAME frequency
    │  │ User 4 (Code D):  ▒▒▓▓▒▒▒▓▓▒▒▓▒▓▓▓▒▒▓▒▒▓▓▒ │    SAME time
    │  │ ─────────────── All transmitted together ─── │
    │  │ Wideband (~5 MHz)                            │
    │  └─────────────────────────────────────────────┘
    │
    └──────────────────────────────────────────────→ Time

  Each user's code is orthogonal (mathematically independent)
  Receiver uses matching code to isolate signal from noise

Key Properties

Property	Value
Channel Width	5 MHz (W-CDMA / UMTS)
Multiple Access	Spreading codes (Walsh / PN codes)
Chip Rate	3.84 Mcps (W-CDMA)
Capacity	Soft — degrades gracefully with more users
Voice Quality	Excellent
Data Speed	Up to 2 Mbps (UMTS), 14 Mbps (HSDPA)

Advantages over TDMA

• Soft capacity — no hard limit; performance degrades gracefully
• Frequency reuse factor of 1 — every cell uses same frequency
• Rake receiver handles multipath signals (improves quality)
• Supports high-speed packet data (internet, video)
• Better handling of handoff between cells (soft handoff)

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Side-by-Side Comparison

        Multi-Access Radio Techniques — 3D View

     FDMA                TDMA                 CDMA
 ┌──────────┐        ┌──────────┐         ┌──────────┐
 │▓▓▓▓▓▓▓▓▓▓│ freq   │ │ │ │ │ │  freq   │░░░░░░░░░░│ freq
 │░░░░░░░░░░│   ↑    │ │ │ │ │ │    ↑    │░░░░░░░░░░│   ↑
 │▓▓▓▓▓▓▓▓▓▓│   │    │ │ │ │ │ │    │    │░░░░░░░░░░│   │
 │░░░░░░░░░░│   │    │ │ │ │ │ │    │    │░░░░░░░░░░│   │
 │▓▓▓▓▓▓▓▓▓▓│   │    │ │ │ │ │ │    │    │░░░░░░░░░░│   │
 │░░░░░░░░░░│   │    │ │ │ │ │ │    │    │░░░░░░░░░░│   │
 └──────────┘   │    └──────────┘    │    └──────────┘   │
  ──────────→time     ──────────→time      ──────────→time

 Users separated      Users separated       Users separated
  by FREQUENCY          by TIME              by CODE
  (different            (same freq,          (same freq,
   channels)             diff slots)          same time,
                                              diff codes)

Feature	FDMA (1G)	TDMA (2G)	CDMA (3G)
Multiple Access	Frequency	Time	Code
Channel Width	30 KHz	200 KHz	5 MHz
Users/Channel	1	8	Soft (10–35+)
Signal Type	Analog	Digital	Digital (spread)
Spectrum Efficiency	Low	Medium	High
Data Support	None	Basic (9.6 Kbps)	Up to 2+ Mbps
Voice Quality	Poor	Good	Excellent
Generation	1G	2G	3G
Example Standard	AMPS, NMT	GSM, D-AMPS	UMTS, CDMA2000

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Generation Migration — 1G to 3G

Key Milestones

Generation	Period	Technology	Peak Speed	Key Feature
1G	1984–1996	FDMA (Analog)	—	Voice only, analog
2G	1992–2000	TDMA / CDMA (Digital)	9.6–14.4 Kbps	Digital voice, SMS
2.5G	2001+	GPRS	115 Kbps	Packet data (always-on)
2.75G	2003+	EDGE	384 Kbps	Mobile internet
3G	2003–2004	W-CDMA / CDMA2000	2+ Mbps	Video calls, mobile broadband

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Quick Mental Model

Think of radio spectrum like a block of bandwidth:

• FDMA = Slice the block horizontally (by frequency) — each user gets a thin strip
• TDMA = Slice the block vertically (by time) — each user gets a brief moment to use the whole strip
• CDMA = Don't slice at all — everyone uses the whole block, but with different secret codes

   FDMA           TDMA           CDMA
 ┌────────┐     ┌────────┐     ┌────────┐
 │ User 4 │     │U│U│U│U│     │  All   │ ← Code A
 │────────│     │1│2│3│4│     │ users  │ ← Code B
 │ User 3 │     │ │ │ │ │     │  on    │ ← Code C
 │────────│     │ │ │ │ │     │  same  │ ← Code D
 │ User 2 │     │ │ │ │ │     │  freq  │
 │────────│     │ │ │ │ │     │   +    │
 │ User 1 │     │ │ │ │ │     │  time  │
 └────────┘     └────────┘     └────────┘
  Frequency       Time           Codes
   division      division       division

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Summary

	FDMA	TDMA	CDMA
Generation	1G	2G	3G
Divide by	Frequency	Time	Code
Analog/Digital	Analog	Digital	Digital
Spectrum use	Inefficient	Moderate	Efficient
Data	❌	Basic	✅ Broadband

These three techniques form the foundation of all modern mobile networks. Even 4G LTE and 5G NR build upon these concepts — using OFDMA (Orthogonal Frequency Division Multiple Access), which is an advanced evolution of FDMA combined with digital modulation.