Communication Systems
Every scheme in this subject is a different answer to the same question: how do you get a signal across a channel that distorts it, shares it and adds noise to it? These topics animate the spectrum, not just the waveform.
Start from the beginning →9 topics you can watch now, 18 still to come.
Foundations
The block diagram, and why the hard part is in the middle.
- Elements of a communication system
- Baseband vs passband
- Bandwidth and the channel
- Noise, SNR and the noise figure
Amplitude modulation
Put the message in the size of the carrier.
- Amplitude ModulationTwo thirds of a broadcast transmitter's power carries no information at all — and the reason that was the right decision is in the receiver, not the transmitter.
- The Superheterodyne ReceiverA 1918 architecture still in your phone. Don't tune the filter — move the signal to it, and pay for it with the image frequency.
- DSB-SC, SSB and VSB in detail
- Coherent detection and carrier recovery
Angle modulation
Put the message in the frequency instead, and buy noise immunity.
- Phase modulation and its relation to FM
- FM demodulator circuits
- Stereo multiplex
Pulse and digital modulation
Sample it, number it, and send the numbers.
- Sampling and the Nyquist TheoremTwo different sine waves passing through identical samples. That picture is why the factor of two is not arbitrary, and why the anti-aliasing filter cannot be digital.
- PCM, Quantisation and CompandingSampling can be lossless; quantisation never is. Then a ladder with uneven rungs buys four bits for free, which is why a phone channel is 64 kbit/s.
- Digital ModulationOnce symbols are points in a plane, bit rate and noise immunity are both geometry — and 256-QAM's need for a clean channel stops being a separate fact.
- PAM, PWM and PPM
- Matched filter derivation
- Equalisation and intersymbol interference
Multiplexing and access
One channel, many users, without them destroying each other.
- OFDM in detail
- Random access and ALOHA
- Cellular frequency reuse planning
Information theory
The limits no clever engineering can get past.
- Huffman and Shannon-Fano coding worked through
- Error-detecting and correcting codes
- LDPC and turbo codes
About Communication Systems
Communication systems answers a question with a surprisingly physical answer: why can you not simply put a voice signal onto an antenna? Because an efficient antenna is sized to the wavelength, and audio wavelengths are kilometres long. Modulation exists to move information up to a frequency that can actually be radiated.
Once that is clear, the subject organises itself. Amplitude and frequency modulation are two ways of carrying information on a carrier, with different noise behaviour and different bandwidth costs. Digital modulation trades continuous values for discrete symbols and buys regeneration in return. Multiplexing shares one channel between many conversations by dividing frequency, time or code.
Every one of these is a relationship between a time waveform and a spectrum, changing together — which is why they are shown as animations with both views side by side rather than as separate figures.
What to know first
- Signals and Systems, especially frequency-domain thinking
- Basic probability for the noise material
Where it gets used
- Understanding why Wi-Fi, LTE and broadcast radio occupy the bands they do
- Reading a link budget and seeing where the margin went
- Explaining why a weak digital signal fails abruptly rather than gradually