---
title: "Spread spectrum multiple access variants"
subject: "Wireless Communication"
chapter: 7
key: "semester_7/wireless_communication:ssma-variants-applications"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/ssma-variants-applications
site: BCT Engineering
---

# Spread spectrum multiple access variants

Wireless Communication · Chapter 7: Multiple Access in Wireless Communication

- Asked in: 2074 Magh, 8 marks
- Video: [Tutorial: CDMA](https://www.youtube.com/watch?v=ZK3gokdlJrU) — Teracom Training Institute
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/ssma-variants-applications
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-7

## Question

Describe spread spectrum multiple access, SSMA variants and application with suitable diagram.

## Short answer

SSMA spreads a narrowband message over a much wider bandwidth using PN sequences; processing gain gives immunity to interference and jamming. Variants include FHMA (Bluetooth, GSM), DS-CDMA (IS-95, WCDMA, GPS), and hybrids such as FCDMA, DS/FHMA, TCDMA, and TDFH. Applications cover cellular, WLAN 802.11b, Bluetooth, and satellite navigation.

## Answer

#### Spread spectrum multiple access (SSMA)
SSMA uses signals whose transmission bandwidth is **several orders of magnitude larger** than the minimum required bandwidth. A pseudo-noise (PN) sequence spreads a narrowband message into a wideband, noise-like signal. Many users share the same wide band at the same time; each is separated by its own PN code or hopping pattern. The ratio of spread bandwidth to data rate is the **processing gain**, which gives immunity to interference, jamming and multipath.

There are two main variants: **FHMA** and **DS-SS (CDMA)**, plus hybrid forms.

#### 1. Frequency Hopped Multiple Access (FHMA)
- Carrier frequency of each user hops pseudo-randomly over the wide band; at any time the signal is narrowband.
- Slow FH: several symbols per hop; fast FH: several hops per symbol.
```text
 data->[FSK mod]->(x)->[BPF]-> channel
                   ^
          [synth]<-[PN generator]
```
- Applications: Bluetooth, GSM slow frequency hopping, military radios.

#### 2. Direct Sequence CDMA (DS-CDMA)
- Data bits are multiplied directly by a high-rate PN code (chips); all users use the same carrier at the same time.
- The receiver correlates with the wanted user's code; other users appear as noise.
```text
 d(t)->(x)->(x)-> channel -> (x)->(x)->[∫]-> d
        ^    ^               ^    ^
      c(t) carrier        carrier c(t)
```
- Needs power control (near-far problem); uses RAKE receiver.
- Applications: IS-95 (cdmaOne), CDMA2000, WCDMA (3G UMTS), GPS.

#### 3. Hybrid spread spectrum techniques
| Hybrid | Idea | Benefit |
|---|---|---|
| FCDMA (FDMA/CDMA) | Band split into sub-bands, each a DS-CDMA system | Uses non-contiguous spectrum |
| DS/FHMA | DS signal whose centre frequency hops | Avoids near-far effect |
| TCDMA | Codes per cell, time slots per user | Only one user per cell at a time |
| TDFH | Hop to new frequency each TDMA frame | Diversity against fading (GSM) |

#### Advantages and applications of SSMA
- No frequency planning; universal reuse (CDMA).
- Soft capacity and soft handoff.
- Multipath diversity via RAKE receiver.
- Security and anti-jam capability (origin in military use).
- Used in cellular (IS-95, 3G), WLAN 802.11b (DSSS), Bluetooth (FHSS), satellite navigation (GPS).

## Check yourself

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

1. What defines spread spectrum multiple access?
   A. Transmission bandwidth is the same as the message bandwidth
   B. Each user gets a dedicated narrow channel
   C. Transmission bandwidth is far larger than needed, spread by a PN sequence, with users separated by their code or hop pattern
   D. Users are separated by time slots only
   Answer: C. A PN sequence spreads a narrowband message over a wide band shared by all users. (Key point: SSMA definition)

2. What is processing gain?
   A. Ratio of spread bandwidth to data rate
   B. Ratio of transmit power to noise power
   C. Number of users per cell
   D. Number of hops per second
   Answer: A. Processing gain = spread bandwidth / data rate; it gives immunity to interference, jamming and multipath. (Key point: Processing gain)

3. How does DS-CDMA spread the data?
   A. By changing the carrier frequency pseudo-randomly
   B. By assigning time slots
   C. By transmitting from narrow beams
   D. By multiplying data bits by a high-rate PN code of chips
   Answer: D. Data is multiplied directly by a chip-rate PN code; the receiver correlates with the wanted code. (Key point: DS-CDMA operation)

4. What is TDFH and its benefit?
   A. A new hop frequency each TDMA frame, giving diversity against fading (GSM)
   B. Codes per cell with time slots per user
   C. Sub-bands each running DS-CDMA
   D. A DS signal with a hopping centre frequency to avoid near-far
   Answer: A. TDFH combines TDMA with slow hopping per frame; the others describe TCDMA, FCDMA and DS/FHMA. (Key point: Hybrid TDFH)

## Related in Wireless Communication

- [Spread spectrum multiple access; FDMA vs CDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-ma-fdma-cdma) — 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
- [Hybrid spread spectrum and FHMA types](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Ahybrid-ss-fhma-types) — Wireless Communication, chapter 7
- [Direct sequence spread spectrum (DSSS)](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adsss-block-diagram) — Wireless Communication, chapter 4
- [Spread spectrum and FHSS](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-fhss) — Wireless Communication, chapter 4
- [Frequency hopped multiple access (FHMA)](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Afhma-operation) — Wireless Communication, chapter 7, asked 5×

## Related in other subjects

- [Spread spectrum frequency hopping in Bluetooth](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Afreq-hopping-bt) — Instrumentation II, chapter 5, asked 2×
- [Spread spectrum: FHSS and DSSS](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aspread-spectrum-fhss-dsss) — Data Communication, chapter 6, asked 4×
- [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