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© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 2
2. Wireless Transmission
Frequencies and SignalsMultiplexing
Modulation and Spread Spectrum
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 3
Frequencies for Communication (1)
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:λ = c/f wave length λ, speed of light c ≅ 3x108 m/s, frequency f
1 Mm300 Hz
10 km30 kHz
100 m3 MHz
1 m300 MHz
10 mm30 GHz
100 µm3 THz
1 µm300 THz
visible lightVLF LF MF HF VHF UHF SHF EHF infrared UV
optical transmissioncoax cabletwisted pair
2
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 4
Frequencies for Mobile Communication (2)
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, focusing– Large bandwidth available
Wireless LANs use frequencies in UHF to SHF spectrum– 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.
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 5
Frequencies and Regulations
ITU-R holds auctions for new frequencies and manages frequency bands worldwide (WRC, World Radio Conferences) E urope U S A Jap an
C ellu lar P hon es
G SM 450-457, 47 9-4 86/460 -467,489-4 96, 890-915/935-9 60, 1 710-1785/1805-1 880 U M TS (F DD ) 1920-1 980, 2110-2190 U M TS (T DD ) 1900-1 920, 2020-2025
AM P S , TD M A , C D M A 824-849, 869-894 TD M A , C DM A , G S M 1850-191 0, 1930-199 0
P D C 810-826, 940-956, 1429-146 5, 1477-151 3
C ordless P hon es
C T1 + 885 -887 , 930-9 32 C T2 8 64-8 68 D E C T 1 880-1900
P AC S 1850-1910, 1930-1990 P AC S -U B 191 0-1930
P H S 1895-191 8 JC T 254-380
W ire less L AN s
IE E E 802.11 2 400-2483 H IP ER L AN 2 5 150-5350, 5470-5 725
902-928 IE EE 802.11 2400-248 3 5150-535 0, 5725 -5825
IE EE 802.11 2471-249 7 5150-525 0
O th ers R F-C ontro l 2 7, 128 , 418 , 433 , 8 68
R F-C ontro l 315, 915
R F-C ontro l 426 , 868
3
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 6
Signals (1)
Physical representation of dataFunction 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, and phase shift ϕ– Sine wave as special periodic signal for a carrier:
s(t) = At sin(2 π ft t + ϕt)
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 7
Different representations of signals: – Amplitude (amplitude domain)– Frequency spectrum (frequency domain)– Phase state diagram (amplitude M and phase ϕ in polar coordinates)
Composed signals transferred into frequency domain using Fourier transformationDigital signals need:– Infinite frequencies for perfect transmission – Modulation with a carrier frequency for transmission (analog signal!)
Signals (2)
f [Hz]
A [V]
ϕ
I= M cos ϕ
Q = M sin ϕ
ϕ
A [V]
t[s]
4
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 8
Fourier Representation of Periodic Signals
)2cos()2sin(21)(
11nftbnftactg
nn
nn ππ ∑∑
∞
=
∞
=
++=
1
0
1
0t t
Ideal periodic signal Real composition(based on harmonics)
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 9
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 always have directive effects:– Vertically and/or horizontally
Radiation pattern: – Measurement of radiation around an antenna
zy
x
z
y x Ideal isotropic radiator
5
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 10
Signal Propagation Ranges
Distance
Sender
Transmission
Detection
Interference
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
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 11
Signal Propagation
Propagation in free space always like light (straight line)Receiving power proportional to 1/d² – d = distance between sender and receiver
Receiving power additionally influenced by:– Fading (frequency dependent)– Shadowing (Abschattung)– Reflection (Reflexion) at large obstacles– Refraction (Brechung) depending on the density of a medium– Scattering (Streuung) at small obstacles– Diffraction (Beugung) at edges
reflection scattering diffractionshadowing refraction
6
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 12
Real World Examples
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 13
Signal can take many different paths between sender and receiver due to:– Reflection, scattering, and diffraction
Time dispersion: signal is dispersed over timeInterference with “neighbor” symbols, Inter Symbol Interference (ISI)
The signal reaches a receiver directly and phase shiftedDistorted signal depending on the phases of the different parts
Multi-path Propagation
Signal at senderSignal at receiver
LOS pulsesMulti-pathpulses
7
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 14
Effects of Mobility
Channel characteristics change over time and location: – Signal paths change permanently– 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
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 15
Multiplexing in 4 dimensions:– Space (si)– Time (t)– Frequency (f)– Code (c)
Goal:– Multiple use of a shared medium
Important: guard spaces needed!
s2
s3
s1
Multiplexing
f
t
c
k2 k3 k4 k5 k6k1
f
t
c
f
t
c
Channels ki
8
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 16
Frequency Multiplexing
Separation of the entire spectrum into smaller frequency bandsA channel gets a certain band of the spectrum for the full time
Advantages:– No dynamic coordination
necessary– Works also for analog signals
Drawbacks:– Waste of bandwidth
if the traffic is distributed unevenly
– Inflexible– Guard spaces
k2 k3 k4 k5 k6k1
f
t
c
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 17
Time Multiplexing
A channel gets the whole spectrum for a certain amount of time
Advantages:– Only one carrier in the
medium at any time– Throughput high even
for many users
Drawbacks:– Precise
synchronization necessary
– Common clock
f
t
c
k2 k3 k4 k5 k6k1
9
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 18
f
Time and Frequency Multiplexing
Combination of both methodsA channel gets a certain frequency band for a certain amount of time– Example: GSM
Advantages:– Better protection against tapping– Protection against frequency
selective interference– Higher data rates compared
to code multiplex
But: – Precise
coordination required
t
c
k2 k3 k4 k5 k6k1
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 19
Code Multiplexing
Each channel has a unique code
All channels use the same spectrum at the same timeAdvantages:– Bandwidth efficient– No coordination and synchronization necessary– Good protection against interference and
tapping
Drawbacks:– Lower user data rates– More complex signal regeneration
Implemented using spread spectrum technology
k2 k3 k4 k5 k6k1
f
t
c
10
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 20
Modulation
Digital modulation:– Digital data is translated into an analog signal (baseband)– ASK, FSK, PSK - main focus in this chapter– Differences in spectral efficiency, power efficiency, robustness
Analog modulation:– Shifts center frequency of baseband signal up to the radio carrier
Motivation for the necessity of analog modulation in wireless networks:– Smaller antennas (e.g., λ/4)– Frequency Division Multiplexing– Medium characteristics
Basic analog modulation schemes:– Amplitude Modulation (AM)– Frequency Modulation (FM)– Phase Modulation (PM)
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 21
Modulation and Demodulation
SynchronizationDecision
DigitalData
AnalogDemodulation
radiocarrier
AnalogBase-band
Signal
101101001 Radio Receiver
DigitalModulation
Digital Data
AnalogModulation
RadioCarrier
AnalogBase-band
Signal
101101001 Radio Transmitter
11
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 22
Digital Modulation
Modulation of digital signals known as Shift Keying
Amplitude Shift Keying (ASK):– Very simple– Low bandwidth requirements– Very susceptible to interference
Frequency Shift Keying (FSK):– Needs larger bandwidth
Phase Shift Keying (PSK):– More complex– Robust against interference
1 0 1
t
1 0 1
t
1 0 1
t
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 23
Advanced Frequency Shift Keying
Bandwidth needed for FSK depends on the distance between the carrier frequenciesSpecial pre-computation avoids sudden phase shifts:
MSK (Minimum Shift Keying)
Bit-separated into even and odd bits, the duration of each bit is doubled Depending on the bit values (even, odd) the higher or lower frequency, original or inverted is chosenThe frequency of one carrier is twice the frequency of the otherEquivalent to offset QPSKEven higher bandwidth efficiency using a Gaussian low-pass filter:
GMSK (Gaussian MSK), used in GSM
12
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 24
Advanced Phase Shift Keying
BPSK (Binary Phase Shift Keying):– Bit value 0: sine wave (0° shift)– Bit value 1: inverted sine wave (180° shift)– Very simple PSK– Low spectral efficiency– Robust, e.g., used in satellite systems
QPSK (Quadrature Phase Shift Keying):– 2 bits coded as one symbol– Symbol determines shift of sine wave– Needs less bandwidth compared to BPSK– More complex (regular synchronization, pilot signal)
Often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK (American IS-136, Japanese PHS)
11 10 00 01
Q
I01
Q
I
11
01
10
00
A
t
BPSK
QPSK
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 25
Quadrature Amplitude Modulation
Quadrature Amplitude Modulation (QAM): – Combines amplitude and phase modulation
It is possible to code n bits using one symbol:– 2n discrete levels, n=2 identical to QPSK– Bit error rate increases with n, but less errors compared to comparable PSK
schemes
Example: 16-QAM (4 bits = 1 symbol)
Symbols 0011 and 0001 have the same phase, but different amplitude. 0000 and 1000 have different phase, but same amplitude.
used in standard 9600 bit/s modems
0000
00010011
1000
Q
I
0010
13
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 26
Hierarchical Modulation
DVB-T (Digital Video Broadcast Terrestrial) modulates two separate data streams onto a single DVB-T streamHigh Priority (HP) embedded within a Low Priority (LP) streamMulti-carrier system, about 2000 or 8000 carriersQPSK (2 first bits), 16 QAM, 64 QAMExample: 64 QAM– Good reception: resolve the entire
64 QAM constellation– Poor reception, mobile reception:
resolve only QPSK portion– 6 bit per QAM symbol, 2 most
significant determine QPSK– HP service coded in QPSK (2 bit),
LP uses remaining 4 bit
Q
I
00
10
000010 010101
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 27
Spread Spectrum Technology
Problem of radio transmission: – Frequency-dependent fading can wipe out narrow band signals for duration of the
interference
Solution: – Spread the narrow band signal into a broad band signal using a special code
Protection against narrow-band interference
Side effects:– Coexistence of several signals without dynamic coordination– Tap-proof
Alternatives: Direct Sequence, Frequency Hopping
Detection atReceiver
InterferenceSpread Signal
Signal
SpreadInterference
f f
Power Power
14
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 28
Spreading and Frequency Selective Fading
f
ChannelQuality
1 23
45 6
Narrow-band Signal Guard Space
22222
f
ChannelQuality
1
SpreadSpectrum
Narrow-band Channels
1. 6 signals in FDM2. Quality changes over time3. Spot view4. 1, 2, 5, 6 ok, 3, 4 not ok5. Apply SS onto 6 signals!
Spread Spectrum Channels
1. 6 signals utilize full f2. No f planning required3. Separation by codes per signals4. CDM, unique per signal5. “Extension” of f
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 29
DSSS (Direct Sequence Spread Spectrum) (1)
XOR of the signal with pseudo-random number (chipping sequence):– Many chips per bit (e.g., 128) result in higher bandwidth of the signal
Advantages:– Reduces frequency selective
fading – In cellular networks:
• Base stations can use the same frequency range
• Several base stations can detect and recover the signal
• Soft hand-over
Drawbacks:– Precise power control necessary– Spreading factor s = tb/tc -> s*w bandwidth requirements
User Data
Chipping Sequence
ResultingSignal
0 1
0 1 1 0 1 0 1 01 0 0 1 11
XOR
0 1 1 0 0 1 0 11 0 1 0 01
=
tb
tc
tb: Bit Periodtc: Chip Period
15
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 30
DSSS (Direct Sequence Spread Spectrum) (2)
XUser Data
ChippingSequence Cs
Modulator
RadioCarrier fr
SpreadSpectrum
SignalTransmitSignal
Transmitter
Demodulator
ReceivedSignal
RadioCarrier fr
X
ChippingSequence Cs
Low-passFilteredSignal
Receiver
Integrator
Products
DecisionData
SampledSums
Correlator
Example:IEEE 802.11Chip: 10110111000(Barker Code)1 MHz user datagenerate 11 MHzsignal in 2.4 GHzfrequency
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 31
FHSS (Frequency Hopping Spread Spectrum) (1)
Discrete changes of carrier frequency:– Available bandwidth of small channels separated as in FDM– Sequence of frequency changes determined via pseudo random number (TDM)
Two versions:– Fast Hopping:
several frequencies per user bit– Slow Hopping:
several user bits per frequency
Advantages:– Frequency selective fading and interference limited to short period– Simple implementation– Uses only small portion of spectrum at any time (dwell time: Verweildauer)
Drawbacks:– Not as robust as DSSS– Simpler to detect (no knowledge of a chipping sequence as in DSSS required!)
16
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 32
FHSS (Frequency Hopping Spread Spectrum) (2)
User Data
SlowHopping(3 bits/hop),e.g., GSM
FastHopping(3 hops/bit),e.g., Bluetooth,1600 f changes/sinto 79 carrier f
0 1
tb
0 1 1 tf
f1f2
f3
t
td
f
f1
f2
f3
t
td
tb: Bit Period td: Dwell Time
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 33
Cell Structure
Implements space division multiplex: – Base station covers a certain transmission area (cell)
Mobile stations communicate only via the base stationAdvantages of cell structures:– Higher capacity, higher number of users– Less transmission power needed– More robust, decentralized– Base station deals with interference, transmission area etc. locally
Problems:– Fixed network needed for the base stations– Hand-over (changing from one cell to another) necessary– Interference with other cells
Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM):– Even less for higher frequencies
17
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 34
Frequency Planning (1)
Frequency reuse only with a certain distance between the base stations
Standard model using 7 frequencies:
Fixed frequency assignment:– Certain frequencies are assigned to a certain cell– Problem: different traffic load in different cells
Dynamic frequency assignment:– Base station chooses frequencies depending on the frequencies already used in
neighbor cells– More capacity in cells with more traffic– Assignment can also be based on interference measurements
f4f5f1f3 f2
f6f7
f3 f2f4
f5f1
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 35
Frequency Planning (2)
f1
f2
f3f2
f1
f1
f2
f3f2
f3
f1
f2f1
f3f3
f3f3
f3f4
f5
f1f3
f2
f6
f7
f3f2
f4
f5
f1f3
f5f6
f7f2
f2
f1f1 f1f2f3
f2f3
f2f3h1
h2h3g1
g2g3
h1h2h3g1
g2g3
g1g2g3
3 Cell Cluster
7 Cell Cluster
3 Cell Clusterwith 3 Sector Antennas
18
© 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 36
Cell Breathing
CDM (Code Division Multiplexing) systems: – Cell size depends on current load
• Low load: larger areas• High load: smaller areas
Reason: Additional traffic appears as noise to other users:– Increasing noise due to increasing number of participants– If the noise level is too high users, located at cell boundaries, drop out of cells
“Noisy area”
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