2.1 thanks to prof. dr.-ing. jochen h. schiller for the slides mc - 2013 mobile communications...
TRANSCRIPT
2.1Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013
Mobile CommunicationsChapter 2: Wireless Transmission
• Frequencies• Signals, antennas, signal propagation• Multiplexing, Cognitive Radio• Spread spectrum, modulation• Cellular systems
2.2
Wireless communications
• The physical media – Radio Spectrum
– There is one finite range of frequencies over which radio waves can exist – this is the Radio Spectrum
– Spectrum is divided into bands for use in different systems, so Wi-Fi uses a different band to GSM, etc.
– Spectrum is (mostly) regulated to ensure fair access
Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013
2.3Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013
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
• Frequency and wave length• = c/f • wave length , speed of light c 3x108m/s, frequency f
1 Mm300 Hz
10 km30 kHz
100 m3 MHz
1 m300 MHz
10 mm30 GHz
100 m3 THz
1 m300 THz
visible lightVLF LF MF HF VHF UHF SHF EHF infrared UV
optical transmissioncoax cabletwisted pair
2.4Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013
Example 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.
2.5Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013
Frequencies and regulations
• In general: ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences)
Examples Europe USA Japan
Cellular networks
GSM 880-915, 925-960, 1710-1785, 1805-1880UMTS 1920-1980, 2110-2170LTE 791-821, 832-862, 2500-2690
AMPS, TDMA, CDMA, GSM 824-849, 869-894TDMA, CDMA, GSM, UMTS 1850-1910, 1930-1990
PDC, FOMA 810-888, 893-958PDC 1429-1453, 1477-1501FOMA 1920-1980, 2110-2170
Cordless phones
CT1+ 885-887, 930-932CT2 864-868DECT 1880-1900
PACS 1850-1910, 1930-1990PACS-UB 1910-1930
PHS 1895-1918JCT 245-380
Wireless LANs 802.11b/g 2412-2472
802.11b/g 2412-2462
802.11b 2412-2484802.11g 2412-2472
Other RF systems
27, 128, 418, 433, 868
315, 915 426, 868
2.6Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013
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
2.7
Signals I
• signal parameters of periodic signals: period T: time to complete a wave. frequency f=1/T :no of waves generated per second amplitude A : strength of the signal phase shift :where the wave starts and stops
• sine wave as special periodic signal for a carrier:
s(t) = At sin(2 ft t + t)
• These factors are transformed into the exactly required signal by Fourier transforms.
Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013
2.8Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013
Fourier representation of periodic signals
1
0
1
0
t t
ideal periodic signal real composition(based on harmonics)
• It is easy to isolate/ separate signals with different frequencies using filters
)2cos()2sin(2
1)(
11
nftbnftactgn
nn
n
2.9Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013
Signals II
• Different representations of signals • amplitude (amplitude domain)• frequency spectrum (frequency domain)• phase state diagram (amplitude M and phase 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!)
f [Hz]
A [V]
I= M cos
Q = M sin
A [V]
t[s]
2.10Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013
Antennas: isotropic radiator
• Radiation and reception of electromagnetic waves, coupling of wires to space for radio transmission
• Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna
• Real antennas do not produce radiate signals in equal power in all directions. They always have directive effects (vertically and/or horizontally)
• Radiation pattern: measurement of radiation around an antenna
zy
x
z
y x idealisotropicradiator
2.11Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013
Antennas: simple dipoles
• Real antennas are not isotropic radiators but, e.g., dipoles with lengths /4 on car roofs or /2 as Hertzian dipole shape of antenna proportional to wavelength
• Example: Radiation pattern of a simple Hertzian dipole
• Omni-Directional Antennas are wasteful in areas where obstacles occur (e.g. valleys)
side view (xy-plane)
x
y
side view (yz-plane)
z
y
top view (xz-plane)
x
z
simpledipole
/4 /2
2.12Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013
• Directional antennas reshape the signal to point towards a target, e.g. an open street• Placing directional antennas together can be used to form
cellular reuse patterns
• Gain: maximum power in the direction of the main lobe compared to the power of an isotropic radiator (with the same average power)
• Often used for microwave connections or base stations for mobile phones (e.g., radio coverage of a valley)
• Directed antennas are typically used in cellular systems, several directed antennas combined on a single pole to construct Sectorized antennas.
2.13Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013
Antennas: directed and sectorized
side view (xy-plane)
x
y
side view (yz-plane)
z
y
top view (xz-plane)
x
z
top view, 3 sector
x
z
top view, 6 sector
x
z
directedantenna
sectorizedantenna
2.14Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013
Antennas: diversity
• Multi-element antenna arrays: Grouping of 2 or more antennas• multi-element antenna arrays
• Antenna diversity• switched diversity, selection diversity
• receiver chooses antenna with largest output• diversity combining
• combine output power to produce gain• cophasing needed to avoid cancellation
+
/4/2/4
ground plane
/2/2
+
/2
2.15Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013
• Smart antennas use signal processing software to adapt to conditions –e.g. following a moving receiver (known as beam forming), these are some way off commercially
• E.g. wireless devices direct the electro magnetic radiation away from human body toward a base station to reduce the absorbed radiation.
2.16Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013
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
• Wireless is less predictable since it has to travel in unpredictable substances – air, dust, rain, bricks
distance
sender
transmission
detection
interference
2.17Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013
Signal propagation: Path loss (attenuation)
• In free space signals propagate as light in a straight line (independently of their frequency).
• If a straight line exists between a sender and a receiver it is called line-of-sight (LOS)
• Receiving power proportional to 1/d² in vacuum (free space loss) – much more in real environments(d = distance between sender and receiver)
• Situation becomes worse if there is any matter between sender and receiver especially for long distances• Atmosphere heavily influences satellite transmission• Mobile phone systems are influenced by weather condition
as heavy rain which can absorb much of the radiated energy
2.18Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013
Signal propagation: Path loss (attenuation)
• Radio waves can penetrate objects depending on frequency. The lower the frequency, the better the penetration• Low frequencies perform better in denser materials• High frequencies can get blocked by, e.g. Trees
• Radio waves can exhibit three fundamental propagation behaviours depending on their frequencies:• Ground wave (<2 MHz): follow the earth surface and can
propagate long distances – submarine communication• Sky wave (2-30 MHz): These short waves are reflected at the
ionosphere. Waves can bounce back and forth between the earth surface and the ionosphere, travelling around the world – International broadcast and amateur radio
• Line-of-sight (>30 MHz): These waves follow a straight line of sight – mobile phone systems, satellite systems
2.19Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013
Signal propagation
• Propagation in free space always like light (straight line)• Receiving power proportional to 1/d² in vacuum – much more in
real environments, e.g., d3.5…d4
(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
reflection scattering diffractionshadowing refraction
2.20Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013
Multipath propagation
• Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction
• Time dispersion: signal is dispersed over time• interference with “neighbor” symbols, Inter Symbol
Interference (ISI)• The signal reaches a receiver directly and phase shifted
• distorted signal depending on the phases of the different parts
signal at sendersignal at receiver
LOS pulsesmultipathpulses
LOS(line-of-sight)
2.21Thanks to Prof. Dr.-Ing. Jochen H. Schiller for the slides www.jochenschiller.de MC - 2013
Effects of mobility
• Channel characteristics change over time and location • signal paths change• different delay variations of different signal parts• different phases of signal parts• quick changes in the power received (short term fading)
• Additional changes in• distance to sender• obstacles further away• slow changes in the average
power received (long term fading)
short term fading
long termfading
t
power