• Preknowledge
  • Baseband Signals
  • Radio Frequency Signals
  • Radio Spectrum
    • Band
  • Geostationary-orbit (GEO)
    • Geostationary satelllite
  • Dynamic Range
  • Decibel
    • Rule of thumb
  • OSI model
  • Frequency Bands
  • Modulations
• Communication Systems
  • Morse Code (Sample of Communication Systems)
  • Objective
  • Reality
  • Type
• Performance
  • Potential Impairments
  • Performance Checks
  • Objective
  • In practice
• Transport
  • Satellite
  • Fiber
• Signal Characeristics
  • Analog
  • Digital
  • Mobile Network in Taiwan
• Analog Sound Recording and Replay
  • Cassette
  • CD
• Designing Satellite Downlinks
• History
  • First communication satellite
  • 1960 oversea telephone
  • Passive relay
  • 2004 oversea telephone
  • 2021 oversea telephone
  • 2021 satellite telephone

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)

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

$10\times lo{g}_{10}(\frac{P_2}{P_1})$

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

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

Modulations

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

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)

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