---
title: "TDMA, CDMA and SDMA"
subject: "Wireless Communication"
chapter: 7
key: "semester_7/wireless_communication:tdma-cdma-sdma"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/tdma-cdma-sdma
site: BCT Engineering
---

# TDMA, CDMA and SDMA

Wireless Communication · Chapter 7: Multiple Access in Wireless Communication

- Asked in: 2071 Bhadra, 2+6 marks
- Video: [FDMA, TDMA, CDMA and SDMA in one video 🔥 | Wireless Communication](https://www.youtube.com/watch?v=sArkTtUMDgg) — Er Sahil ka Gyan
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/tdma-cdma-sdma
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-7

## Question

What is a multiple access technique? Explain TDMA, CDMA and SDMA.

## Short answer

Multiple access is the technique for sharing limited radio spectrum among many users via frequency, time, code, or space divisions. TDMA allocates time slots, allowing burst transmission on one carrier, saving battery and simplifying handoff. CDMA spreads each user's data with a unique code over the same frequency, providing soft capacity but requiring power control. SDMA separates users by spatial position using directional or adaptive antennas, increasing capacity and reducing interference.

## Answer

#### Multiple access technique
Multiple access is the technique by which **many users share a common, limited radio spectrum at the same time** without causing serious interference to each other. The shared resource is divided in frequency (FDMA), time (TDMA), code (CDMA) or space (SDMA).

#### TDMA (Time Division Multiple Access)
TDMA divides one radio carrier into **time slots**; each user transmits only in its own slot, in bursts, using the full carrier bandwidth.

```text
 |<-------------- one TDMA frame -------------->|
 |pre| slot1 | slot2 | slot3 | ... | slotN |
        U1      U2      U3            UN
 slot: |tail|data|training|data|tail|guard|
```
- Each frame has preamble (sync/address), slots and guard times.
- Transmission is discontinuous → battery saving and mobile-assisted handoff in idle slots.
- No duplexer needed; high burst rate needs equalization; tight synchronisation needed.
- Example: GSM (8 slots per 200 kHz carrier), IS-136 (3 users per 30 kHz).

#### CDMA (Code Division Multiple Access)
All users transmit **on the same frequency at the same time**; each user's data is multiplied by a unique PN spreading code with chip rate much higher than the bit rate.

```text
 d1->(x)c1 --\
 d2->(x)c2 ---(+)--> channel --> (x)c1 --> d1
 d3->(x)c3 --/
```
- Receiver correlates with the wanted code; others remain as noise (reduced by the processing gain).
- Features: frequency reuse factor 1, soft capacity, soft handoff, RAKE receiver for multipath.
- Problems: near-far effect (needs power control), self-jamming.
- Example: IS-95 (1.25 MHz, 1.2288 Mcps), WCDMA.

#### SDMA (Space Division Multiple Access)
SDMA separates users by their **spatial position**. The base station uses directional or smart (adaptive) antennas that form a beam toward each user, so users in different directions can reuse the same frequency/time/code in the same cell.

```text
           /--beam1--> user A
 [BS ant]--+--beam2--> user B
           \--beam3--> user C
```
- Advantages: higher capacity, less co-channel interference, lower transmit power, wider coverage.
- Simplest form: sectored antennas (120°); ideal form: adaptive arrays tracking each user.

## Check yourself

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

1. How does TDMA share a carrier?
   A. Each user transmits continuously on its own carrier
   B. Each user transmits in bursts in its own time slot using the full carrier bandwidth
   C. Users are separated by codes
   D. Users are separated by beams
   Answer: B. TDMA divides a carrier into time slots; transmission is discontinuous. (Key point: TDMA principle)

2. How many slots does GSM have per 200 kHz carrier?
   A. 3
   B. 16
   C. 4
   D. 8
   Answer: D. GSM has 8 slots per 200 kHz carrier (IS-136 has 3 users per 30 kHz). (Key point: TDMA example)

3. How does a CDMA receiver extract a user?
   A. It listens in the user's time slot
   B. It correlates with the wanted code, leaving other users as noise reduced by processing gain
   C. It tunes to the user's carrier
   D. It points a beam at the user
   Answer: B. All users share frequency and time; the unique PN code separates them. (Key point: CDMA operation)

4. Which are the problems of CDMA?
   A. Guard bands and duplexer
   B. Slot synchronisation only
   C. Beam tracking
   D. Near-far effect and self-jamming
   Answer: D. Near-far needs power control; imperfect orthogonality causes self-jamming. (Key point: CDMA problems)

5. How does SDMA allow reuse of the same frequency in a cell?
   A. Smart antennas form beams toward users in different directions
   B. Users hop frequencies
   C. Users get different codes
   D. Users get different slots
   Answer: A. Spatially separated beams let users reuse frequency/time/code. (Key point: SDMA principle)

## Related in Wireless Communication

- [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
- [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; FDMA vs CDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-ma-fdma-cdma) — Wireless Communication, chapter 7
- [Advantages and disadvantages of FDMA, TDMA, CDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Afdma-tdma-cdma-pros-cons) — Wireless Communication, chapter 7
- [Multiple access and near-far effect mitigation](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amultiple-access-near-far) — Wireless Communication, chapter 7
- [Multiple access, TCDMA and TDFH](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Atcdma-tdfh) — Wireless Communication, chapter 7

## Related in other subjects

- [Multiple tube fermentation (MPN) technique](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Amtf-mpn-procedure) — Water Supply Engineering, chapter 4, asked 5×
- [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
- [Multiplexing vs multiple access](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Amux-vs-multiple-access) — Data Communication, chapter 6