Physical Layer

Dr. Alexios Louridas

Physical Layer

Analogue

Fourier Series

Bandwidth

Data Rate

Fourier Analysis

Fourier Series

A continuous repeated time varying signal can be represented by an infinite sum of sines and cosines.

Fourier Series of a function f(x)

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Square Wave

To produce a square wave you would need to add odd harmonics only to the Fourier series. The more harmonics that are added the better a square wave is represented.

Β 

𝑓(π‘₯)=𝑒+\sum_{n=1}^\infty π‘Ž_𝑛 cos⁑(𝑛π‘₯) + \sum_{n=1}^\infty 𝑏_𝑛 sin⁑(𝑛π‘₯)

The Nyquist theorem specifies that a sinusoidal function in time or distance can be regenerated with no loss of information as long as it is sampled at a frequency greater than or equal to twice per cycle.

Nyquist and Shannon’s Theorem

π΅π‘–π‘‘π‘…π‘Žπ‘‘π‘’_{π‘šπ‘Žπ‘₯}=2π΅π‘™π‘œπ‘”_2 𝑀 𝑏𝑖𝑑𝑠/π‘ π‘’π‘π‘œπ‘›π‘‘ \\ \text{, where B is the bandwidth (in Hz) and M is the number of signal levels. }

The Shannon capacity theorem defines the maximum amount of information, or data capacity, which can be sent over any channel or medium

π΅π‘–π‘‘π‘…π‘Žπ‘‘π‘’_{π‘šπ‘Žπ‘₯}=π΅π‘™π‘œπ‘”_2 (1+\frac{𝑆}{𝑁})𝑏𝑖𝑑𝑠/π‘ π‘’π‘π‘œπ‘›π‘‘ \\ \text{, where B is the bandwidth (in Hz) and} \\ \frac{𝑆}{𝑁} \text {is the signal to noise ratio in linear power or voltage ratio.}
SNR=10\log{\frac{S}{N}}dB\\ \text{For Power Ratio}
SNR=20\log{\frac{S}{N}}dB\\ \text{For Voltage Ratio}

γ€ž

Prove why we digitise audio at more than 40kHz and voice at 8kHz

- Use Nyquist theorem to prove it

Guided Transmission Media

There are a variety of media which can be categorised in 4 types. Each type has a variety of bandwidths and bitrates.Β 

Electromagnetic Spectrum and Telecommunications

Frequency Allocation

Apple Newton (1992)

Nokia 3110 (1995)

Psion 5 (1997)

iPhone 3G (2009)

Compaq iPad(200)

Blackberry Pearl 9105 (2010)

Looking back create a timeline of features concerning connectivityΒ  you though were cool

Generation Icon Technology Max (Mbit/s) Typical (Mbit/s)
2G G GPRS 0.1 <0.1
E EDGE 0.3 0.1
3G 3G 3G 0.3 0.1
H HSPA 7.2 1.5
H+ HSPA+ 21 4
H+ DC-HSPA+ 42 8
4G 4G LTE Cat 4 150 12-15
4G+ LTE Cat 6 300 24-30
4G+ LTE Cat 9 450 60
5G and 5G+ 5G NR sub-6 GHz 10000 200-400
5G+ NR mmWave

Mobile Networks (Download Speed)

Generation Upload (Mbps) Latency(ms) Packet Loss (%)
5G 10000 1 (10 typical) 0.1
4G 10 21 0.5
3G 2.2 90 1.20

Mobile Networks (other comparatives)

Cell Shape
Highest Coverage Area


Re-usage of Frequencies
Adjacent cells different frequency


MS should share the same cell
Multiple Access Methods

  • FDMA
  • TDMA
  • CDMA
  • SDMA

Mobile Cells

Re-use of resources

  • Frequency
  • Time
  • Code
  • Space

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Handover

Cell change


Cell Selection
Overlapping

Mobile Communication Aspects

FDMA

A different frequency for each user

Supports both analog and digital communication
Streaming
Handover difficult
CrossTalk
1G and 2G (GSM)
Β 

  • Channel divided in multiple frequencies

  • Each subscriber gets his own frequency

  • A guard band is required to reduce interference

TDMA

Same Frequency
Time slot division

  • Fixed Length
  • Time Slot Re-Use

Simpler handover
Streaming not possible
2G GSM

  • Combined with FDMA

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  • Each subscriber gets his own time to use all the frequencies of the channel

  • Guard times are required between different user times

CDMA

Single frequency
Different codes between users
No interference
Significant complexity
Handover

  • Code Re-use

2G
WCDMA in 3G (WideBand CDMA)

  • 5MHz bandwidth vs 1.25MHz

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  • A subscriber has a time slot and a frequency

  • Hopping can occur between frequencies

    • More Secure

CDMA

Separate users to share frequencies
Multiple Antenna arrays
Beamforming
Polarized antennas
5G

Β 

  • Spatial separation of users

  • Multi-beam antennas

  • CDMA, TDMA or FDMA can still be used for each beam

Cell Selection

Device is turned on
Searching for signals
Check for signal level
Attach to cell

  • Range 100m

  • Packetization

  • Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)

  • Transmit when channel is clear

  • All transmissions are acknowledged
    Collision occurred when no ACK

  • Retries after a random waiting interval

Wireless Lan

Bluetooth

Range only up to 10 meters
1 Data channel (700Kbps)
3 Voice Channels
Master-Slave

  • Piconet
  • Scatternet

TDMA

IOT WIRELESS PROTOCOLS

ZigBee

IEEE 802.15.4
20-250 kbps
Low Power
IoT
Low latency devices

802.11ah (HaLow)
347 Mbps
Low Power
Comparable to Bluetooth
IoT
Β 

γ€ž

Competition between cellular (5G) and Wi-Fi 6 is not a new debate

– Write a small literature review

Satellite Communication

Can broadcast a signal in large areas anywhere and anytime.

TYPES OF EARTH ORBITS

By Inclination
By Shape
By Altitude

KNOWN ORBITS

Geostationary Orbit (GEO) Satellites

Medium Earth Orbit (MEO) Satellites

Low Earth Orbit (LEO) Satellites

LEO

500-1500 km Height

Very Expensive to maintain

RTT = 10-30ms

MEO

5000-12000 km Height

Expensive to maintain

RTT = 70-200ms

GEO

358000 km Height

Relative Cheap to maintain

RTT = 0.5s

SATELLITE PROPERTIES

  • Large Bandwidth
  • Expensive relative to coverage

Fibres

  • Large Coverage
  • Limited Bandwidth
  • Prone to Errors

Satellite

  • Easy to engineer a fixed data rate over point-to-point links
  • Can be expensive to deploy, over distances
  • Doesn’t readily support mobility or broadcast

Wired & FIbre

  • Easy and inexpensive to deploy

  • Naturally supports mobility

  • Naturally supports broadcast

  • Transmissions interfere and must be managed

  • Signal strengths hence data rates vary greatly

Wireless

Physical Layer

By Alexios Louridas

Physical Layer

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