Lecture 12: Final Lecture on Multiplexing

Recap of Previous Lectures

• Lecture 11: Digital band pass modulation (Amplitude Modulation, Frequency Modulation, Phase Modulation)
  • Generation, detection, frequency domain representation
  • Variants of AM, FM, PM with digital signals

Class Test Information

• Date: Wednesday, 12th May at 10:00 AM
• Worth: 15%
• Content Covered: Lectures 7-12
• Format: Two parts, A and B
  • Part A: 10 questions, 7.5 marks each
  • Part B: 5 questions, 5 marks each
  • Duration: 2 hours total
  • Instructions: Part B involves handwritten solutions, demo available online

Introduction to Multiplexing

• Definition: Several communication channels sharing the same channel
• *Types:
  • Time Division Multiplexing (TDM): Digital signals, time slots for each user
  • Frequency Division Multiplexing (FDM): Analog signals, different frequency bands for each user
  • Spread Spectrum: Combination of TDM and FDM, used in Bluetooth, Wi-Fi

Digital System Performance

• Bit Error Rate (BER):
  • Measure of errors in received bits
  • BER = Number of bits received in error / Total number of bits transmitted or received
  • Ideal BER = 0 (never happens in reality)
  • BER vs. Signal-to-Noise Ratio (SNR) Curve:
    • Low SNR: high noise, high BER
    • High SNR: low noise, low BER
  • Comparison of communication systems using BER curves
    • E.g., Different levels of QAM (2-level, 16-level, 64-level)
    • Higher levels of QAM: higher BER but faster communication
• *Analog & Digital Signal Comparison:
  • High-level QAM more affected by noise
  • Trade-off: speed vs. bit error rate

Multiplexing in Detail

• Multiplexing (MUX): Combining multiple signals
• Demultiplexing (DMUX): Recovering original signals
• FDM (Frequency Division Multiplexing):
  • Multiple users share the channel at the same time, different frequency bands
  • Used in Radio, TV, telephone lines
  • Requires modulation to avoid overlap
  • Example: Radio stations broadcasting simultaneously with different frequencies
• TDM (Time Division Multiplexing):
  • Multiple users share the same frequency at different times
  • Used in fiber optic, telephone communications
  • Requires synchronization between multiplexer and demultiplexer
• Spread Spectrum:
  • Combination of FDM and TDM
  • Users occupy different frequencies at different times
  • Secure communication (e.g., Bluetooth, Wi-Fi)
  • Frequency hopping: signals change frequency over time
  • Difficult to intercept without knowing the code

Multiplexing Techniques

• Simplex, Duplex:
  • Simplex: One-way communication (e.g., radio)
  • Half Duplex: Two-way, one party at a time (e.g., walkie-talkie)
  • Full Duplex: Two-way, both parties can communicate simultaneously (e.g., telephone)
• Comparison of ASK and DPSK: Using BER vs SNR graph
  • ASK more error-prone than DPSK
  • Example: ASK 100x more likely to have an error than DPSK at 9 dB SNR

Exam Preparation

• Date: 19th May
• Instructions: Unique set of questions based on student ID
• Question Format: Design, explain, compare, sketch
• Content Focus: Not directly from lecture notes
• Preparation Tips: Review last year's exam, mock exam on Canvas

Conclusion

• Course Wrap-Up: Enjoyed teaching, future courses (ELEC 377, ELEC 477)
• Exam Tips: Be creative, use imagination, different correct answers possible
• Good Luck: Revision, class test, final exams
  • Next year's course: ELEC 352

Good luck and stay safe!