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Learn FDMA with working principle, frequency channel allocation, guard bands, advantages disadvantages, AMPS 1G system, and comparison with TDMA CDMA OFDMA explained for engineering students.
FDMA is the oldest multiple access technique in which the available spectrum is divided into separate frequency bands (channels). Each user gets a dedicated frequency channel.
🎯 What is FDMA?
In FDMA, the total available bandwidth is divided into smaller frequency channels. Each user gets a unique frequency channel – they occupy it for as long as the call lasts.
| ┌─────────────────────────────────┐ User 5 | |
|---|---|
| ├─────────────────────────────────┤ Guard Band | |
| ┌─────────────────────────────────┐ User 4 | |
| ├─────────────────────────────────┤ Guard Band | |
| ┌─────────────────────────────────┐ User 3 | |
| ├─────────────────────────────────┤ Guard Band | |
| ┌─────────────────────────────────┐ User 2 | |
| ├─────────────────────────────────┤ Guard Band | |
| ┌─────────────────────────────────┐ User 1 | |
| └─────────────────────────────────┘ |
⚙️ Working Principle
The basic idea of FDMA is simple – divide the spectrum and give each user a slice.
Steps:
- Total available bandwidth determined (e.g., 25 MHz)
- Divide into narrow channels (e.g., 30 kHz each)
- When user requests call → assign an available channel
- User occupies that frequency for entire call
- When call ends → channel released for others
Key Characteristics:
- One channel per user at any time
- Continuous transmission on assigned frequency
- No time-sharing – exclusive frequency use
- Narrowband channels (10-30 kHz typically)
- Analog or Digital signals can be used
📊 Channel Allocation
┌──────────────────────────────────────────────────────┐
│ │
│ Number of Channels = (Total BW - 2×Guard) / Channel BW│
│ │
│ Example (AMPS): │
│ Total BW = 12.5 MHz (one direction) │
│ Channel BW = 30 kHz │
│ Channels = 12,500 / 30 = 416 channels │
│ │
│ But some channels reserved for control: │
│ Traffic channels ≈ 395 │
│ Control channels ≈ 21 │
│ │
└──────────────────────────────────────────────────────┘
Duplex in FDMA:
| Duplex Method | How | Example |
|---|---|---|
| FDD (Frequency Division Duplex) | Uplink & downlink on different frequencies | AMPS: 824-849 MHz (up), 869-894 MHz (down) |
| Separation | Guard band between UL and DL | 20 MHz separation in AMPS |
🛡️ Guard Bands
Guard bands are small frequency gaps between adjacent channels that prevent interference.
│ Ch 1 │ Guard │ Ch 2 │ Guard │ Ch 3 │
│30 kHz│ ~10kHz│30 kHz│ ~10kHz│30 kHz│
Without guard bands: Adjacent channels would overlap
and interfere with each other (adjacent channel interference)
Guard band = wasted spectrum (no data transmitted) but necessary for practical filters.
✅ Advantages & Disadvantages
Advantages:
| Advantage | Hindi |
|---|---|
| Simple concept & implementation | Easy to understand and build |
| No synchronization needed | Users do not need to sync |
| Works well with analog signals | Analog systems ke liye perfect |
| Continuous channel (good for voice) | Voice ke liye achha (no gaps) |
| Well-established technology | Purani proven technology |
| Low delay | Kam delay (dedicated channel) |
Disadvantages:
| Disadvantage | Hindi |
|---|---|
| Inefficient for bursty data | Data traffic ke liye wasteful |
| Guard bands waste spectrum | Guard bands mein spectrum waste |
| Limited number of users | Users ki sankhya limited |
| Intermodulation interference | Non-linear mixing products |
| Expensive narrowband filters | Precise filters costly |
| Channel unused if user silent | Channel blocked even during silence |
| No flexibility (fixed allocation) | Fixed, not dynamic |
The biggest problem with FDMA is that when a user is silent (listening in a call), the channel still remains occupied – spectrum is wasted.
📱 FDMA in 1G Systems
AMPS (Advanced Mobile Phone System):
| Parameter | Value |
|---|---|
| Frequency (Uplink) | 824-849 MHz |
| Frequency (Downlink) | 869-894 MHz |
| Channel BW | 30 kHz |
| Total Channels | 832 (416 per operator) |
| Duplex | FDD (45 MHz separation) |
| Modulation | FM (analog voice) |
| Region | North America (1983) |
Other 1G FDMA Systems:
- NMT (Nordic Mobile Telephone) – 12.5/25 kHz channels
- TACS (Total Access Communication System) – UK, 25 kHz
- C-Netz – Germany
⚔️ FDMA vs Other Techniques
| Parameter | FDMA | TDMA | CDMA |
|---|---|---|---|
| Division by | Frequency | Time | Code |
| Channel BW | Narrow (30 kHz) | Wider (200 kHz) | Wide (1.25 MHz) |
| Users share | Nothing (dedicated) | Frequency (time slots) | Frequency + Time |
| Sync needed | No | Yes (strict) | Yes (codes) |
| Guard | Frequency gaps | Time gaps | None (codes orthogonal) |
| Efficiency | Low | Medium | High |
| Data traffic | Poor | Better | Good |
| Generation | 1G | 2G (GSM) | 3G (CDMA2000) |
| Flexibility | Fixed | Semi-flexible | Flexible |
📝 Summary
| Concept | Key Point |
|---|---|
| Principle | Divide spectrum into frequency channels |
| User gets | One dedicated frequency for entire duration |
| Guard bands | Small gaps to prevent adjacent interference |
| Efficiency | Low (channel wasted during silence) |
| Complexity | Simple (no synchronization) |
| Best for | Continuous analog voice (1G) |
| Used in | AMPS (1G), some satellite systems |
| Replaced by | TDMA (2G), CDMA (3G), OFDMA (4G/5G) |
❓ FAQ
Q: Where is FDMA still used today? A: Pure FDMA is rarely used now. But the FDMA concept is still used in FDD (Frequency Division Duplex) – in 4G/5G, uplink and downlink are on different frequencies.
Q: Why isn't FDMA good for data? A: Data is bursty (sometimes sending, sometimes idle). In FDMA, the channel remains blocked even during idle time = waste. In TDMA/OFDMA, idle slots can be given to others.
Q: How many users can FDMA support? A: Total BW / Channel BW = max users. This is limited. In CDMA/OFDMA, more users can fit in the same spectrum.
Exam Focus
Revise definitions, diagrams, examples, and short-answer points for FDMA Frequency Division Multiple Access.
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Prepare one clear explanation, one practical example, and one common mistake for this Wireless Communications topic.
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