Communication Systems: Introduction

I’m using my website as platform for taking and storing notes from my classes. This is part of my collection of notes for 525.416 Communication Systems Engineering at JHU. Content is derived from JHU EP and the course instructor, Rob Nichols. These notes primarily follow the format and topics of the course.

Introduction

Examples of Communication Systems

Example Associated Channel
Cable Based Technologies Fiber Optics, Coaxial Cable, Twisted Pair
Mobile Cellular Troposphere Wave Propogation
Microwave Links Troposphere Wave Propogation, Traversing Mountainous Terrain
Satellite Systems One or two way propogation through atmospheric layers

Signals vs. Systems

Signals - A signal is a set of information or data.
Systems - Signals are processed by systems, which can modify them or extract additional information from them.

Analog vs. Digital

Analog Data is characterized by values that vary over a continuous over a time period that also has a continuous range. Digital Data is a set of finite symbols that are ordered to form a code word.

Power and Energy

A repeating signal that goes on forever would have infinite energy, so signalstrength is measured in energy over time, or power. Instantaneous Power is defined as the square of the amplitude of the signal. Total Power is the sum of the square of the signal over a given period of time.

Signal Power

Where $s^2(t)$ is the signal instantaneous power.

Noise Power

Where $n^2(t)$ is the noise instantaneous power.

Modulation

Amplitude Modulation (AM)

• Information is encoded on the amplitude of the carrier signal
• Because of the limitations of 20th century electronics, signals could only achieve the the kHz range.
• AM uses 580 - 1090 kHz
• Because of frequencies, AM needed large towers (~500 ft)

Frequency Modulation (FM)

• Enhanced noise rejection. More robust than AM.
• FM is a subclass of Phase Modulation (PM)
• When FM was introduced in the 1950s, electronics could achieve the MHz range.
• FM uses 87 - 108 MHz
• Antenna size reduced to 5 to 10 ft.

Analog-to-Digital Conversion

Analog to digital conversion occurs in two steps:

Sampling captures the waveform values at discrete time intervals. Digitization converts numbers into bits.

Encoding

Pulse Code Modulation

Pulse Code Modulation assigns a +1V to a 1 bit and -1V to a 0 bit.

Example

Source: 1

In this example bits are transitted with the least significant bit (LSB) first. Alternatively, messages could be transmitted most significant bit first.

Pulse Amplitude Modulation

• Makes use of modulating the amplitude level in a signal
• Limited by noise

Pulse Width Modulation

• Varies the width of the signal pulse
• Classic example: rotary dial phone

Communication Systems

Source: 1

Some sort of device (e.g. a microphone) transduces the signal into an electrical waveform. The output is the baseband signal.

The signal then enters the transmitter where it is amplified in power and released through channel, where noise and distortion are introduced.

The receiver then captures the signal and recreates the baseband signal.

Then the output transducer converts the signal back into it’s original form.

Channel Capacity

Channel Capacity describes how much information can be carried.

The Data Rate (sometimes synonymous with baud rate) is the number of bits per second traversing a digital channel.

Where B is the bandwidth of the signal and noise and SNR is the signal-to-noise ratio. C is the number of bits per second that can cross the channel without errors occuring.

Cable / Fiber Channel Technologies

Twisted (Copper) Pair

• Original technology for traditional telephone systems
• Cables were twisted together to reduce the impact of the Earth’s magnetic field.

Coaxial Cable

• For short distances
• Used to route TV signals within a house

Fiber Optic Cable

• Handles wide bandwidths

Cellular Mobile Telephone

• First cell phone developed by Bell Labs in 1979
• First mobile phone was analog, nowadays they are digital.
• Cellular networks use dedicated towers that monitor a patch of ground called a cell.
• Towers communicate with handsets and route calls.
• Used to connect large population centers to save on cost of laying wires
• Classic example: Malaysia - Islands conencted by microwave radio tower.

Satellite Systems

• Used for radio and TV
• Orbits:
• Geosynchronous: high up, wide view
• Low Earth Orbit: Constantly moving. Useful when power is a concern.

AM radio broadcasting - 1920’s Used towers on the order of 500 ft. FM radio improved noise rejection.

1941 NTSC (National Television System Committee) defined first black and white TV standard. 1953 NTSC Color Standard used modulation to maintain backwards compatibility with previous standard.

AM Example

Input transducer is a microphone. The transmitter uses AM1480 (which is 1480 kHz) and sends the signal through the atmosphere.

In the atmosphere is is subject to noise (mainly from lightning strikes, sunspots, solar flares)

The Receiver must filter out the band of the frequecies that is 5 kHz on either side of 1480. Then reconstructs the signal back into audio.

References

[1] Nichols, R., Module 01. 525.416 Communication Systems Engineering. Summer 2017.