1. Telecommunications Richard Patient Suffolk New College
Multiplexing
Telecommunications
Richard Patient
Suffolk New College

2. Topic Aims Level Method Equipment Duration
Multiplexing – Code Division Multiple Access (CDMA)
Aims
To teach Code Division Multiple Access
Link to History of Communications
Link to modern mobile phone technology
Level
Level 3
Method
PowerPoint slides, animated. Teacher/Lecturer to run through PowerPoint slides with Students and discuss each element. Can also be ran off as Hand-Outs
Equipment
Laptop/Projector
Pens/Pencils
Notepads
Duration
>30 Minutes

3. Objectives
Be able to describe more complex types of multiplexing.

4. What is the link to Multiplexing?
The story of the Actress, the Pianist and the Torpedo!
What is the link to Multiplexing?

5. Hedy Lamarr Actress Born in Austria 1914
Star of controversial 1933 film “Ecstasy”
Married Friedrich Mandl, German Arms dealer
1937 she left her husband and fled to Hollywood

6. Hedy Lamarr Inventor
Neighbour of George Antheil, composer and pianist.
Discussed the problem of jamming the radio signals used to control torpedoes.
Invented Frequency Hopping.

7. “Player Piano” and Piano Roll

8. Frequency Hopping Very hard to Jam the signal Can share the spectrum
Moves around almost “randomly”
Can share the spectrum
Different users have different hopping sequences
Bluetooth uses this to avoid interference

9. Code Division Multiple Access
All users transmit on all of the frequencies, all of the time!
Users have unique codes that allow them to only receive the correct signal.

10. Cocktail Party Analogy

11. Does it Really Work 1 Bit 13 Chips
Each bit of data is coded with a “code word” (chips)
The “chip rate” is greater than the “bit rate”
1 Bit
13 Chips

12. Transmitting “1” or “0”
A zero “inverts” the chips
= “1”
= “0”

13. Decoding a Signal
The worksheet has a signal created from 4 different users
Each user has a unique code
Codes are chosen to ensure decoding works reliably
Difficult maths!
Each signal is coded with the users code
All the signals are added together
The user combines their code with the received signal
They make decision on what their signal must have been
The other signals “disappear” in the decoding

14. Example Decode
Received Signal -4 2 -2
Your Code -1 1
Signal x Code 4

15. Decision
If the total for the signal is more than 0 then the signal sent was “1”
If the total for the signal was less than 0 then the signal sent was a “-1”

16. Signals Code 1 Signal  1 1 -1 -1 Code 2 Signal  -1 1 -1 1

18. TDMA We have looked at “Synchronous” TDMA
Each channel always uses the same time slot
Timeslot 1  Channel 1
Timeslot 2  Channel 2
Etc
Simple, and ideal if each channel always has data

19. Asynchronous TDM Channel divided into equal timeslots, but..
Also known as “Statistical Multiplexing”
Channel divided into equal timeslots, but..
Timeslots filled with data from any channel that has data ready to transmit.
Channel identification is added so that the data can be sent to the correct receiver.

20. Wasted Capacity

21. Dense Wavelength Division Multiplexing
Coarse WDM  Approximately 8 wavelengths
Dense WDM  32 or 64 wavelengths used.