1. CS441 – Mobile & Wireless Computing Communication Basics
Department of Computer Science
Southern Illinois University Carbondale
CS441 – Mobile & Wireless Computing
Communication Basics
Dr. Kemal Akkaya
Mobile & Wireless Computing

2. Transmission Fundamentals
How to relay information?
Electromagnetic Signals
TV, Radio, Internet etc.
Signal
A function of time
Has 3 components:
Amplitude (A) : Signal strength
Frequency (f) : # of cycles
Phase ( ) : Relative position
Sine wave for the signal
s(t) = A sin(2 f t +  )
Either analog or digital
Wavelength (λ):
Distance occupied by 1 cycle
λ = c*T = c / f
Analog: No breaks in the signal
Digital: Signal intensity is discrete
1 Cycle
Wavelength
Amplitude
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3. Effects on Signal Attenuation: Distortion: Noise: Error:
Decrease in amplitude of signal along transmission
Distortion:
Interference of different frequency components of a signal
Noise:
In the absence of signal, there is random mixture of frequencies on the channel called channel noise
Error:
When digital signals are combined with noise, some bits can be received in error
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4. Signal/Bandwidth/Data Rate
sin(2ft)+(1/3) sin(23ft)
Signal may include many frequencies
Combination of sinusoids
Spectrum:
Range of frequencies a signal contains
The signal in the figure contains frequencies between f and 3f
Bandwidth:
Width of the spectrum is called bandwidth
Bandwidth for the figure : 3f – f = 2f
Increasing the bandwidth makes the wave look like more square (i.e. digital signal)
Hence, increasing the bandwidth helps to reduce the distortion at the receiver side.
Bandwidth = 7f – f = 6f
How much data can we communicate with a certain bandwidth?
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5. Channel Capacity Channel Capacity: 1 Noise: Nyquist Bandwidth:
Noise, attenuation, distortion etc. limit the data rate that can be achieved in a channel.
The maximum rate at which data can be transmitted over a given communication path is called Channel capacity
Noise:
Should be minimized to get more data rate
Nyquist Bandwidth:
Assumes an error free channel (no noise)
In case of noise: Shannon Capacity Formula
SNR :
Signal to Noise Ratio : Signal Power / Noise Power (often represented in decibels)
Sets upper bound on achievable data rate 1
T=1sec, f=1/1=1Hertz
Data rate = 2 bits/sec
C= 2 B logM
Channel capacity
Bandwidth
Voltage levels
C = Blog2(1+SNR)
Theoretical upper bound
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6. Analog and Digital Data Transmission
How analog and digital signals are transmitted?
Analog signals (continuous) can be propagated through
Wire, twisted pair, coaxial cable, fiber optic cable and atmosphere
Digital signals (discrete) can only propagated through
Wired medium – No wireless since it requires infinite frequencies
How to propagate digital signals then?
Digital data can be represented as analog signals:
What does a Modem do?
Modem converts a series of binary pulses into an analog signal by modulating a carrier frequency. They use voice spectrum to represent digital data and hence can use the telephone lines. At the other end, the analog signal is converted to digital signal again and received.
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7. How to do that encoding? Modulation is the solution: Motivation:
Modulate digital data so that an analog signal is generated
Modem would be the classical example
Motivation:
When only analog transmission facilities are available, modulation is required to convert digital data into analog signals
How to do digital modulation?
Operation in on or more of the 3 characteristics of a signal
These are amplitude, frequency and phase
Three main techniques
ASK: Amplitude Shift Keying – digital data over optical fiber
FSK: Frequency Shift Keying – on LANs that use coaxial cable
PSK: Phase Shift Keying – Networks
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8. ASK, FSK and PSK
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9. Other digital modulation techniques
Binary Frequency Shift Keying (BFSK)
Uses two different frequencies
Multiple Frequency Shift Keying (MFSK)
More than two frequencies are used
Gaussian Frequency Shift Keying (GFSK)
Two level shift from base frequency : Bluetooth uses this
Binary Phase Shift Keying (BPSK)
Two phrases used to represent bits : In Satellite Systems
Differential Phase Shift Keying (DPSK)
Phase shift with reference to previous bit
Four-level (QPSK) and Multilevel Phase Shift Keying
Each element represents more than 1 bit
Differential QPSK (DQPSK) is used in b networks
Quadrature Amplitude Modulation (QAM)
Combination of ASK and PSK
Two different signals sent simultaneously on the same carrier frequency
Started to be used in Wireless Sensor Networks
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10. Modulation of Analog Signals
Digital Modulation had a motivation
What was that?
What is analog modulation and the idea behind it?
Sometimes a higher frequency may be needed for transmission
Modulation will help to provide frequency division multiplexing
3 types of analog modulation
Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase Modulation (PM)
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11. AM and FM Example FM AM
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12. Digitization Converting analog data into digital signals
Digital data can then be transmitted using NRZ-L
NRZ-L a way to transmit digital signals
Digital data can then be transmitted using code other than NRZ-L
Digital data can then be converted to analog signal
Analog to digital conversion done using a codec
Pulse Code Modulation (PCM)
Delta Modulation (DM)
Original signal
Sample value
D/2
3D/2
5D/2
7D/2
-D/2
-3D/2
-5D/2
-7D/2
Approximation
3 bits / sample
Rs = Bit rate = # bits/sample x # samples/second
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13. Multiplexing Carrying multiple signals on a single medium
Capacity of transmission medium usually exceeds capacity required for transmission of a single signal
More efficient use of transmission medium:
Combine multiple signals
Increased data rate provides cost efficiency
Transmission and reception equipment
Analog multiplexing
Frequency Division Multiplexing (FDM)
Digital Multiplexing
Time Division Multiplexing (TDM)
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14. FDM Example Combining analog signals
Takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal
Transmission
3 Channels
1 Link
Multiplexer
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15. TDM Example Digital technique to combine data
Takes advantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signal
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16. Transmission Media for Signals
It is the physical path between transmitter and receiver
Guided media: Solid media such as copper, optical fiber etc.
Unguided media: Atmosphere or outer space: Wireless Transmission
Here is the electromagnetic spectrum for telecommunications:
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17. General Frequency Ranges
Microwave frequency range
1 GHz to 40 GHz
Used for satellite communications
Radio frequency range
3 KHz to 300 GHz
Can be analog : TV, Radio
Or digital: Cell phones, wireless networks
Infrared frequency range
Roughly 3x1011 to 2x1014 Hz
Useful in local point-to-point multipoint applications within confined areas
Medium Wave Radio
Electric
Waves
Radio
Infra-red
Visible
Light
Ultra
Violet
X-Rays
Gamma
Rays
Cosmic 30 3
300
Long Wave Radio
FM Radio
GSM 3G
Microwave Radio Links TV
VLF LF MF HF
VHF
UHF
SHF
EHF
Radio Spectrum
kHz
MHz
GHz
DECT
WiFi Bluetooth
TETRA
LMDS
“Sweetspot”
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18. Frequency Regulations
Federal Communications Commission (FCC)
Charged with regulating interstate and international communications by radio, television, wire, satellite and cable
Prevent interferences between different devices
Current Allocation of the Radio Spectrum by frequency
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