Most modern communication techniques use waves
Modulation
Both types of modulation use a "Carrier Wave"
A Carrier wave is a high-frequency Electromagnetic wave modulated in either frequency or amplitude to transfer a signal
Amplitude Modulation
In amplitude modulation the frequency of the modulated wave is constant, and the amplitude of the modulated wave varies proportionally to and in phase with the signal
Carrier Wave
+ Signal wave
= Amplitude modulated wave
Frequency Modulation
In frequency modulation (FM), the amplitude of the carrier wave remains constant. The frequency of the carrier wave is made to vary in sync with the displacement of this information signal
Carrier Wave
+ Signal wave
= Frequency modulated wave
Sidebands and bandwidth
TODO
TODO: Advantages and disadvantages to AM and FM
| Communication Type | Frequency Range | Wavelength in air | Frequency Band |
|---|---|---|---|
| LW Radio | 30 KHz - 300 Khz | 10km - 1km | Low frequency (LF) |
| MW Radio | 300KHz - 3 MHz | 1km - 100m | Medium frequency (MF) |
| Short-Wave (SW) radio | 3 MHz - 30MHz | 100m - 1m | High frequency (HF) |
| FM Radio | 30 MHz - 300 MHz | 10m - 1m | Very high frequency (VHF) |
| TV Broadcasting and Phones | 300MHz - 3GHz | 1m - 10cm | Ultra-high frequency (UHF) |
| Microwave link | 3 GHz - 30 GHz | 10cm - 1cm | Super-high frequency (SHF) |
| Satellite link | 30 GHz - 300 GHz | 1cm - 1mm | Extra-high frequency (EHF) |
Analog Signals
Signal has the same variation as the data and is continuously variable. When an analog signal is transmitted over a long distance it will be attenuated and pick up the background noise.
For the signal to continue to travel, it must be periodically amplified by a repeater, however, this also amplifies any distortion/noise, negatively affecting the signal
Digital Signals
Digital Signals consist of highs and lows (1's and 0's - discrete) with no intermediate value. Because of this, even if the signal becomes noisy during transit, amplifiers in the middle are able to recreate the exact original waveform and remove the data
Advantages of digital signals
Binary number representation
Binary is a base-2 scale
| Denary (Base 10) | Binary (Base 2) | Hexadecimal (Base 16) |
|---|---|---|
| 1 | 0001 | 01 |
| 2 | 0010 | 02 |
| 3 | 0011 | 03 |
| 4 | 0100 | 04 |
Converting Analogue to digital
In digital transmission, the analog signal (e.g voice) is converted to digital using an ADC (Analog to digital converter). When received, the signal can be regenerated into its analog form using a DAC (Digital to analog converter)
e.g. if the sampling rate is 10 Hz, then 10 samples per second are taken and the interval between samples is 0.1s
The maximum sampling rate required is only twice the highest frequency present in the signal.
The human ear can detect 20Hz - 20KHz. Max sampling required for ADC (Analog to digital convert) is 40KHz ($20,000 * 2 = 40,000$)
These samples (instantaneous voltages) are converted into a binary number representing their value. How accurate this depends on the number of bits.
Parallel to serial converter can be used to transmit bits one after another through a single wire, rather than having 8 wires to transmit 8 bits.
When received, a serial to parallel converter can be used to convert the signal back to its original form.
Improving reproduction of the input signal
Increase sampling frequency to reduce the width of each step (space between samples)
Increase the number of bits per sample to allow for a more precise binary representation of the analog signal's initial voltage
Channels of communications
Wire pairs are normally used for phone lines, the potential difference between the wires is the signal. Each wire acts as an aerial and picks up unwanted noise. attenuation is high since energy is lost as heat due to the high resistance.
I need to do this later lol
Attenuation
The gradual decrease in the power of signal over distance
Measured in decibels, value is negative as it is a loss of power
Signal-to-noise ratio (SNR)
Repeaters will amplify both signal and noise, and thus will not affect SNR. Attenuation WILL lower SNR, as the signal is getting weaker relative to noise.
Repeaters repeating a digital signal can remove noise and amplify the signal, improving SNR.
Satellites
For any satellite in constant orbital motion:
as
and
and if $F_c = F_g$, then
Satellite communications
Carrier wave is transmitted from earth to the satellite, the signal is attenuated a lot and received. The signal is amplified and transmitted back to earth at a different carrier frequency, which will avoid "Swapping" of the highly attenuated signal by the high power transmission.
The Electromagnetic frequency used is very high (10GHz-30GHz) to prevent ionosphere reflections and to have a large information-carrying capacity.
| Polar Satellite | Geostationary Satellite |
|---|---|
| Travels from pole to pole with a shorter period | Geostationary. Travels from west to east with the same orbital period as the earth (24 hours) |
| Able to cover the surface of the earth | Can only cover $\frac13$ of the earth |
| Lower altitude 700km-800km (Cheaper to launch into space) | Higher altitude 36000km |
| Lower latency (as closer to Earth), however, you will need to calculate or track its position to communicate with it | Higher latency (takes approximately 0.5s for 1 signal to travel to and back again) |
| As lower, it is able to resolve smaller objects while imaging. | As it remains in a fixed position above a point on the equator it can be used for continuous communication and weather |