Telecommunications Richard Patient Suffolk New College Multiplexing Telecommunications Richard Patient Suffolk New College

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

Objectives Be able to describe more complex types of multiplexing.

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

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

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.

“Player Piano” and Piano Roll

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

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.

Cocktail Party Analogy

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

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

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

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

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”

Signals Code 1 Signal  1 1 -1 -1 Code 2 Signal  -1 1 -1 1

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

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.

Wasted Capacity

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