Physical Layer

1. Fourier Analysis

Fourier Analysis is a mathematical tool used to analyze and understand complex signals by breaking them down into simpler parts (specifically, sines and cosines).

Fourier proved that any "reasonably behaved" periodic function can be represented as the sum of sine and cosine waves with different frequencies. For example, if you have a periodic signal, Fourier analysis helps break it down to understand how much of each frequency is contributing to the overall signal.

2. Transmission Media

Transmission media are the pathways that signals travel through. These are classified into two main types: Guided Media and Unguided Media.

• Guided Media: These are physical conduits that guide the signal from one place to another. Think of it like a water pipe that channels water from one location to another.
  • Twisted Pair Cables: These consist of two copper wires twisted together, reducing interference from outside signals.
  • Coaxial Cables: These have a central conductor surrounded by insulation and an outer conductor, providing better protection from interference.
  • Fiber Optic Cables: These use light to transmit data through a glass or plastic core, which is protected by cladding.
    • Single
    • Multi
      • graded index
      • step index
• Unguided Media: These allow signals to travel without a physical conductor, like how sound waves move through the air.
  • Wireless communication falls into this category, using electromagnetic waves such as radio, microwaves, and infrared.

3. Wireless Communication

Wireless communication doesn't use a physical medium but instead relies on electromagnetic waves. Some common types include:

• Radio Transmission(无线电): Used for broadcasting audio signals.
• Microwave Transmission(微波): This is used for long-distance communication, like telephone and TV distribution.
• Infrared(红外): widely used for short-range communication(e.g., TV remote controller)
• Lightwave Transmission(光波)

Propagation Method:

• Line-of-Sight(>30MHz)
• Ground Propagation(2-30MHZ)
• Sky Propagation(<2MHZ)

Antenna:

• Omnidirectional Antenna(全向天线)
• Unidirectional Antennas(定向天线)

Communication Satellites

• Satellite likes a microwave repeater in the sky
• It listens and amplifies the incoming signal from somewhere on the earth, then rebroadcasts it at another frequency to another part on Earth