---
title: "Spread spectrum multiple access; FDMA vs CDMA"
subject: "Wireless Communication"
chapter: 7
key: "semester_7/wireless_communication:spread-spectrum-ma-fdma-cdma"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/spread-spectrum-ma-fdma-cdma
site: BCT Engineering
---

# Spread spectrum multiple access; FDMA vs CDMA

Wireless Communication · Chapter 7: Multiple Access in Wireless Communication

- Asked in: 2072 Magh, 6+2 marks
- Video: [Spread Spectrum Multiple Access - Mobile Radio Propagation - Mobile Communication System](https://www.youtube.com/watch?v=6r7pOoogNBo) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/spread-spectrum-ma-fdma-cdma
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-7

## Question

Explain the different types of spread spectrum multiple access techniques. Compare FDMA with CDMA.

## Short answer

Spread spectrum multiple access includes FHMA, where carrier frequencies hop per a PN code, and CDMA (DS-SS), where data is spread by a high-rate PN code over a shared band. FDMA assigns separate frequencies to users, has hard capacity, and requires frequency planning, while CDMA lets all users share the same band with soft capacity, needs power control, and supports soft handoff and RAKE reception for multipath.

## Answer

#### Types of spread spectrum multiple access
Spread spectrum multiple access (SSMA) spreads each user's narrowband signal over a band much wider than needed, using a PN code. All users share the wide band; the code separates them. Processing gain G_p = (spread bandwidth)/(data rate) gives resistance to interference and multipath.

**1. Frequency Hopped Multiple Access (FHMA)**
- The carrier of each user hops pseudo-randomly over many narrow channels within the wide band.
- Slow FH: several symbols per hop; fast FH: several hops per symbol.
- Collisions (two users on the same frequency) are handled by error-correcting codes.
- Examples: Bluetooth, GSM slow hopping.
```text
 data->[FSK]->(x)->Tx      synth driven by PN code
               ^           selects hop frequency
            [synth]<-[PN]
```

**2. Direct Sequence Multiple Access (DS-SS / CDMA)**
- Data is multiplied by a high-rate PN code (chip rate >> bit rate), spreading it over the full band. All users transmit together on the same carrier.
- The receiver correlates with the wanted code; others remain as wideband noise.
- Needs power control (near-far problem). Examples: IS-95, WCDMA.
```text
 d(t)->(x)->(x)cos wct -> channel
        ^
      c(t) PN code
```

**3. Hybrid spread spectrum techniques**
- **FCDMA**: band split into sub-bands, each running DS-CDMA.
- **DS/FHMA**: DS signal whose centre frequency hops; avoids near-far effect.
- **TCDMA**: different code per cell, time slots per user inside a cell.
- **TDFH**: new hop frequency each TDMA frame (GSM), gives fading diversity.

#### FDMA vs CDMA

| Point | FDMA | CDMA |
|---|---|---|
| Separation | Frequency | Unique code |
| Bandwidth per user | Narrow channel | Whole wide band |
| Frequency reuse | N = 4–12, needs planning | N = 1, no planning |
| Capacity | Hard limit (fixed channels) | Soft limit |
| Handoff | Hard | Soft |
| Power control | Not critical | Essential (near-far) |
| Multipath | Fading problem | RAKE receiver exploits it |
| Example | AMPS | IS-95 |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Processing gain G_p is defined as:
   A. data rate / spread bandwidth
   B. spread bandwidth / data rate
   C. chip rate x bit rate
   D. transmit power / noise power
   Answer: B. G_p = spread bandwidth / data rate. (Key point: Processing gain formula)

2. Which statement matches DS multiple access?
   A. Carrier hops among narrow channels
   B. Each cell has a time slot
   C. Data is multiplied by a high-rate PN code and all users transmit together on the same carrier
   D. Users are separated by beams
   Answer: C. DS spreads data with chips over the full band; the receiver correlates with the wanted code. (Key point: FHMA vs DS)

3. How does frequency reuse compare for FDMA and CDMA?
   A. FDMA N=1, CDMA N=4-12
   B. FDMA N=4-12 needing planning, CDMA N=1
   C. Both N=1
   D. Both N=4-12
   Answer: B. FDMA needs cluster planning; CDMA reuses the same frequency everywhere. (Key point: Frequency reuse comparison)

4. CDMA compared with FDMA has:
   A. Hard capacity and hard handoff
   B. Soft capacity and hard handoff
   C. Hard capacity and soft handoff
   D. Soft capacity and soft handoff
   Answer: D. CDMA is interference-limited (soft capacity) and uses soft handoff. (Key point: Capacity and handoff)

5. What is true of power control and multipath in CDMA vs FDMA?
   A. Power control essential in CDMA; multipath exploited by RAKE
   B. Power control essential in FDMA; multipath causes fading in CDMA
   C. Power control not needed in either
   D. RAKE is used in FDMA
   Answer: A. CDMA needs power control for near-far and uses RAKE to exploit multipath, while FDMA suffers fading. (Key point: Power control and multipath)

## Related in Wireless Communication

- [Multiple access; FDMA vs CDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amultiple-access-fdma-vs-cdma) — Wireless Communication, chapter 7
- [Spread spectrum multiple access variants](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Assma-variants-applications) — Wireless Communication, chapter 7
- [CDMA vs TDMA; FHSS multiple access](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Acdma-advantages-fhss-types) — Wireless Communication, chapter 7
- [CDMA features and hybrid spread spectrum](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Acdma-features-hybrid-ss) — Wireless Communication, chapter 7
- [Spread spectrum disadvantages; CDMA implementation](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-disadvantages-cdma) — Wireless Communication, chapter 7
- [Hybrid multiple access techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Ahybrid-multiple-access) — Wireless Communication, chapter 7

## Related in other subjects

- [Digital vs analog spread spectrum](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aspread-spectrum-digital-vs-analog) — Data Communication, chapter 6
- [Multiplexing vs multiple access](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Amux-vs-multiple-access) — Data Communication, chapter 6
- [Multiple access protocols](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Amultiple-access-protocols) — Computer Networks and Security, chapter 3, asked 2×
- [Multiple access in IEEE 802.5](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aieee-802-5-access) — Computer Networks and Security, chapter 3