---
tags: Communication System Design, ss, ncu
author: N0-Ball
title: Introduction
GA: UA-208228992-1
---

[ToC]

# Preknowledge

## Baseband Signals

- Analog baseband signals re message signals that are generated or sensed by hummans, such as speech, music, video images...
- Contains frequencies close to 0

## Radio Frequency Signals

- RF signals carry messages, but the message cannot be sensed directly by humans

## Radio Spectrum


UHF
: Ultra High Frequency
VHF
: Very High Frequency

### Band
EHF, SHF
- L-band
- S-band
- X-band

## Geostationary-orbit (GEO)

- Always available wthin its service area
- Appears stationary in sky

### Geostationary satelllite


- Links can be established quickly
- from anywhere to anywhere
- Boardcasting
- Direct to home TV
- Signals received at the satellite are always week
- 10^-12^ W
- Very **low noise** receivers are needed
- Operate in C, Ku, Ka band
- Covers **no man's land**
- no internet connection at certain area
- e.g. Antartica
- Limited bandwidth for Space
- 

## Dynamic Range

- the ratio of the largest to the smallest signal - usually in dBs
- Communication systems ofthen cannot carry the full dynamic range of signals
- Compression may be applied to the signal before transmission
- Expansion in receiver ensures linearity
- The process together is known as Companding
- Humans have sensing systems with wide dynamic range
- Eyes (24 f-stops, + brains)
- Cameras (14.4 f-stops)
- Ears (~140dB)
- CDs (96dBs)


:::info
While camera is either left or middle, human eye can be left due to dynamic range.
:::

## Decibel

dB
: A **relative** unit of measurement
: A logarithmic way of describing a ratio of a power or root-meansquare quantity
: Used in electronics, signals, and communication
: $10 \times log_{10}(\frac{P_2}{P_1})$

### Rule of thumb

$\pm 10$ dB difference
: An order of magnitude difference

## OSI model


- Physical layer is where message signals are transmitted and received
- Some aspects are grouped in the data link layer
- error control
- type definition

## Frequency Bands


[中華民國頻率分配表](https://freqdbo.ncc.gov.tw/upload/FILESAVE/080724081215.pdf)

- avoid interference
- radios that allocates at same location means that they can live together

## Modulations

:::info
makes carrier able to carry a message and transmit for distant users
:::

- It is used to shift the spectral content of a message signal so that it lies inside the
operating frequency band
- It provides a mechanism for putting the information content into a form that may
be less vulnerable to noise or interference
- It permits the use of multiple-access techniques

Demodulation
: the opposite process

Modulating signal
: the message signal

Modulated signal
: the output of the modulation process

Modulator
: the device used for modulation

Demodulator
: the device used for demodulation

# Communication Systems

- Message Signal
- Generate a **message signal**
- Symbols
- Represeent the message signal as a sequence of **symbols**
- Encode
- **Encode** the symbols
- Transmit
- **Transmit** the encoded symbol sequence
- Receive
- **Receive** the encoded symbol sequence
- Decode
- **Decode** the received signal and recover original symbols
- Original Message
- Output the **original message**


## Morse Code (Sample of Communication Systems)



- Most Commonly use to Least Commonly use

Origin Message -> MorseCode -> Origin Message

> History: Fast data transmittion

## Objective
- The message signal should not be degraded by the transmission process

## Reality
- The message will suffer some degradation
- better controlled in a digital transmission system than an analog system
- The performance of the communication system defines how much degradation may occur

## Type

Point to Point
: One to one communicaion

Broadcast (point to multi-point)
: One transmitter, many receivers

# Performance

:::info
All new communication system are digital
:::

## Potential Impairments
- Noise
- Random Occurance
- Thermal or random noise
- Interference
- Certain Occurance
- Possible occurance
- Time
- Frequency
- Sometimes treat as noise
- CDMA spread spectrum system
- Unwanted signals

## Performance Checks
- Limited resources
- Channel bandwidth
- Transmitter power
- Links may be bandwidth limited or power limited
- Performance measure is carrier power to noise power ratio (C/N) at input of receiver
- C/N sets S/N at baseband (analog signal) or bit error rate (BER) (digital signal)

:::warning
**BER** is probability of bit error not a rate
:::

## Objective

- **C/N ratio** must typicaly **exceed 10 ( or 10 dB )** at input of receiver
- **S/N** ratio in baseband of analog signal should be **above 50 dB**
- **BER** shold be lower than **10^-6^**
- Performance may be permitted to degrade for short periods of time

## In practice
- Thermal noise, cannot be removed, is usually the limitation on radio links
- Noise cannot be removed (easily) from an analog signal
- Error can be removed from a (well-designed) digital signal

# Transport

- Earth is curve
- repeater are needed for microwave (VHF, UHF) communication systems to get around curved Earth
- Reflect radio waves from the ionosphere
- Narrowband communicationL 0-30 Mhz RF

## Satellite
- Geostationary (3800km) satellite can see 1/3 Earth


## Fiber
- Optical fiber communication can be considered as a form of RF (at 300 GHz)
communications
- The carrier is an infrared light wave with wavelength of 1.55 μm
- Frequency of carrier is 2 x 1014 Hz = 200 THz
- Modulation is ASK (or OOK, On-Off Keying)
- Bandwidth is huge: 1013 Hz theoretically
- Wavelength division multiplexing allows 100 Gbps per fiber
- Optical fibers go from point to point
- Fiber has very low loss (0.1 dB/km). Amplifier can be >100 km apart.
- Good for interconnecting cities
- Not so good for broadcasting
- Fibers are never laid singly – a fiber cable has many individual fibers

# Signal Characeristics

## Analog

Bandwidth
- Speech: 100 Hz - 10k Hz
- Telephone sstem bandwidth
- US: 300 - 3100 Hz
- ITU-T: 300 - 3400 Hz
- Music: 50 Hz - 20 kHz (human hearing range)
- NTSC video: 25 Hz - 4.2 MHz
- US, Japan, Taiwan ...
- Taiwan stopped using NTSC in 2012
- PAL video: 25 Hz - 5.5 MHz
- Europe, Australia ...

## Digital

Bit rates
- PCM Speech: 64kbps
- CD replay: 3.4 Mbps
- Raw digitized video: 80 Mbps
- MPEG-2 videos: 5-80Mbps
- MPEG-4 videos: 60kps - 8 Mbps

## Mobile Network in Taiwan

- 2G
- GSM 900
- GSM 1800
- 3G
- UMTS 2100
- CDMA 800
- 4G
- LTE 700
- LTE 900
- LTE 1800
- 5G
- 5G 3500 (78)
- 28 (257)

# Analog Sound Recording and Replay


## Cassette
1. Change in air pressure on microphone (sound wave) creates voltage waveform
2. Amplified and applied to recording head
3. Iron oxide particles on tape are magnetized in patterns created by voltage waveform
4. Magnetic fields passing replay head create voltage waveform
5. Power amplifier drives current into speaker coil and creates sound wave in air
6. Ideally, output into air by speaker is identical to input into microphone

## CD


1. Analog signals are filtered
2. converted to digital signals by an Analog-to-Digital Converter (ADC)
3. Digitized signals are multiplxed, modulated, or synchronized.
4. Laser beam records such signals as pits
5. bumps pits on the CD surface.

# Designing Satellite Downlinks

1. Calculate the availability of the link
2. LNAs are important for ground receivers
3. weather effects sky noise
4. link margin allows C/N to fall in rain
- Find C/N in clear air and C/N threshold
- Calculate path attenuation and sky temperature(T~a~')
- check percentage of outage -> availability

# History

## First communication satellite

Sputnik 1
: Radio transmitter onboard launched in 1957

## 1960 oversea telephone

- 550 overseas telephone lines
- US $1.00 per min
- Average wage US $1.50 per hour
- 40 min of work per call minute

## Passive relay

- Moon in 1955
- Reflection

## 2004 oversea telephone

- US $0.09 per min
- Average wage US $5.20 per hour
- ~1 min of work per call minute

## 2021 oversea telephone

- Too cheap that we stop care

## 2021 satellite telephone
* [Reference](https://satellitephonestore.com/iridium-services)