06-04-2011, 12:22 PM
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Mobile communication
Two aspects of mobility:
user mobility: users communicate (wireless) “anytime, anywhere, with anyone”
device portability: devices can be connected anytime, anywhere to the network
Wireless vs. mobile Examples
û û stationary computer
û ü notebook in a hotel
ü û wireless LANs in historic buildings
ü ü Personal Digital Assistant (PDA)
Typical application: road traffic
Mobile devices
Effects of device portability
Power consumption
limited computing power, low quality displays, small disks due to limited battery capacity
CPU: power consumption ~ CV2f
C: internal capacity, reduced by integration
V: supply voltage, can be reduced to a certain limit
f: clock frequency, can be reduced temporally
Loss of data
higher probability, has to be included in advance into the design (e.g., defects, theft)
Limited user interfaces
compromise between size of fingers and portability
integration of character/voice recognition, abstract symbols
Limited memory
limited value of mass memories with moving parts
flash-memory
Wireless networks in comparison to fixed networks
Higher loss-rates due to interference
emissions of, e.g., engines, lightning
Restrictive regulations of frequencies
frequencies have to be coordinated, useful frequencies are almost all occupied
Low transmission rates
local some Mbit/s, regional currently, e.g., 53kbit/s with GSM/GPRS
Higher delays, higher jitter
connection setup time with GSM in the second range, several hundred milliseconds for other wireless systems
Lower security, simpler active attacking
radio interface accessible for everyone, base station can be simulated, thus attracting calls from mobile phones
Always shared medium
secure access mechanisms important
Simple reference model used here
Influence of mobile communication to the layer model
service location
new applications, multimedia
adaptive applications
congestion and flow control
quality of service
addressing, routing, device location
hand-over
authentication
media access
multiplexing
media access control
encryption
modulation
interference
attenuation
frequency
Mobile Communications
Chapter 2: Wireless Transmission
Frequencies for communication
VLF = Very Low Frequency UHF = Ultra High Frequency
LF = Low Frequency SHF = Super High Frequency
MF = Medium Frequency EHF = Extra High Frequency
HF = High Frequency UV = Ultraviolet Light
VHF = Very High Frequency
Frequencies for mobile communication
VHF-/UHF-ranges for mobile radio
simple, small antenna for cars
deterministic propagation characteristics, reliable connections
SHF and higher for directed radio links, satellite communication
small antenna, beam forming
large bandwidth available
Wireless LANs use frequencies in UHF to SHF range
some systems planned up to EHF
limitations due to absorption by water and oxygen molecules (resonance frequencies)
weather dependent fading, signal loss caused by heavy rainfall etc.
Signals I
physical representation of data
function of time and location
signal parameters: parameters representing the value of data
classification
continuous time/discrete time
continuous values/discrete values
analog signal = continuous time and continuous values
digital signal = discrete time and discrete values
signal parameters of periodic signals:
period T, frequency f=1/T, amplitude A, phase shift j
sine wave as special periodic signal for a carrier:
s(t) = At sin(2 p ft t + jt)
Signals II
Different representations of signals
amplitude (amplitude domain)
frequency spectrum (frequency domain)
phase state diagram (amplitude M and phase j in polar coordinates)
Composed signals transferred into frequency domain using Fourier transformation
Digital signals need
infinite frequencies for perfect transmission
modulation with a carrier frequency for transmission (analog signal!)
Signal propagation ranges
Transmission range
communication possible
low error rate
Detection range
detection of the signal
possible
no communication
possible
Interference range
signal may not be
detected
signal adds to the
background noise
Signal propagation
Propagation in free space always like light (straight line)
Receiving power proportional to 1/d² in vacuum – much more in real environments
(d = distance between sender and receiver)
Receiving power additionally influenced by
fading (frequency dependent)
shadowing
reflection at large obstacles
refraction depending on the density of a medium
scattering at small obstacles
diffraction at edges
Multipath propagation
Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction
Time dispersion: signal is dispersed over time
è interference with “neighbor” symbols, Inter Symbol Interference (ISI)
The signal reaches a receiver directly and phase shifted
è distorted signal depending on the phases of the different parts