epl 657 wireless environment and mobility issues
TRANSCRIPT
EPL 657
Wireless Environment
and Mobility Issues
Panayiotis Kolios, Dept. of
Computer Science, UCY
1
Overview
• Why study?
• Frequency bands
• The wireless environment
• Signal distortion – wireless channels
2
Why study?
3
Why study?
• In a wireless environment (open space) carrying
data using radio signals, over given frequency
bands:
– Many additional complexities in
comparison to fixed media transmission,
(as e.g. electrical signals in copper, or
optical in fibre), which can seriously
degrade the performance of wireless
networking systems
4
Wireless networks compared to
fixed networks
• Higher loss-rates due to interference, plus signal attenuation
– RF 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., 9.6kbit/s with GSM
• 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
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)
Mobile communication
• Two (wishful?) 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 stationary computer
notebook in a hotel with fixed access
wireless LANs in historic buildings
Personal Digital Assistant (PDA)
• The demand for mobile communication creates the need for
integration of wireless networks into existing fixed networks:
– local area networks: standardization of IEEE 802.11
– Internet: Mobile IP extension of the internet protocol IP
– wide area networks: e.g., internetworking of 3G/4G and PSTN
Challenges for wireless / mobile
networks
• 2 grand challenges (beyond those for traditional
fixed networks)
– Wireless link
• Capacity of link affected by many factors, e.g. (dynamic)
spectrum allocation
• Quality of link connection is subjected to many (environmental)
factors and can vary substantially
– Mobility
• Wireless link quality is adversely affected by device location
(distance) from transmitting / receiving source ( where a
varies between about 2 to 4)
• Device / node portability
1ad
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 or ? as alternative
Challenges in wireless / mobile
communication • Wireless Communication
– transmission quality (bandwidth, error rate, delay)
– modulation, coding, interference
– media access, regulations
– ...
• Mobility
– location dependent services
– location transparency
– quality of service support (delay, jitter, security)
– ...
• Portability
– power consumption
– limited computing power, sizes of display, ...
– usability
– ...
• Addressability (especially for Internet connected devices) and security
– Internet addresses are linked to the Network Point of Attachment (NPA) which has physical
meaning
– In sensor networks a different meaning of addressing
Simple reference model used
here; not always ‘applicable’
Application
Transport
Network
Data Link
Physical
Medium
Data Link
Physical
Application
Transport
Network
Data Link
Physical
Data Link
Physical
Network Network
Radio
Trend toward all-IP networks cross
layering?
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
• Application layer
• Transport layer
• Network layer
• Data link layer
• Physical layer
The wireless environment
13
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:
λ = c/f
wave length λ, frequency f
speed of light c ≅ 3x108m/s,
14
Some frequencies are strictly
controlled (pre-assigned by regulating
bodies), others are open to use (even
by different applications), subject to
some given constraints, e.g. Max.
Transmit Power
λ
f
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, focusing
large bandwidth available
• Wireless LANs use frequencies in UHF to SHF spectrum smaller antenna
some systems planned up to EHF
limitations due to absorption by water and oxygen molecules
(resonance frequencies)
15
‘optimum’ antenna size can be related to λ
Recall: Signals
• 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 ϕ sine wave as special periodic signal for a carrier:
s(t) = At sin(2 π ft t + ϕt)
16
Transmitted signal <> received signal!
• Wireless transmission distorts any transmitted signal – Received <> transmitted signal; results in uncertainty at receiver
about which bit sequence originally caused the transmitted signal
– Abstraction: Wireless channel describes these distortion effects
• Sources of distortion – Attenuation – energy is distributed to larger areas with increasing distance
– Reflection/refraction – bounce of a surface; enter material
– Absorption – energy is absorbed without any reflection
– Diffraction – start “new wave” from a sharp edge
– Scattering – multiple reflections at rough surfaces
– Doppler fading – shift in frequencies (loss of center)
17
Example wireless signal strength in a
multi-path environment
• Brighter color = stronger signal
• Obviously, simple (quadratic)
free space attenuation formula
is not sufficient to capture
these effects
18
© Jochen Schiller, FU Berlin
Source /
access point
Distortion effects: Non-line-of-sight
paths • Because of reflection, scattering, …, radio communication is not
limited to direct line of sight communication (good or bad?)
– Effects depend strongly on frequency, thus different behavior at higher frequencies
• Different paths have different lengths = propagation time
– Results in delay spread of the wireless channel
– Closely related to frequency-selective fading properties of the channel
– With movement: fast fading
19
Line-of-
sight path
Non-line-of-sight path
signal at receiver
LOS pulses multipath
pulses
© Jochen Schiller, FU Berlin
Gain, Attenuation and path
loss
20
Attenuation results in path loss • Effect of attenuation: received signal strength is a function of the
distance d between sender and transmitter
• Captured by Friis free-space equation
– Describes signal strength at distance d relative to some reference
distance d0 < d for which strength is known
– d0 is far-field distance, depends on antenna technology
21
Power received is inversely proportional to distance (free space)
Suitability of different frequencies –
Attenuation
• Attenuation depends on the
used frequency
• Can result in a frequency-
selective channel
– If bandwidth spans frequency
ranges with different
attenuation properties
22
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Generalizing the attenuation
formula • To take into account stronger attenuation than only
caused by distance (e.g., walls, …), use a larger exponent
> 2
– is the path-loss exponent
– Rewrite in logarithmic form (in dB):
• Take obstacles into account by a random variation
– Add a Gaussian random variable with 0 mean, variance 2 to dB
representation
– Equivalent to multiplying with a lognormal distributed r.v. in metric
units ! lognormal fading
23
Range and coverage
See tutorial
24
•range “maximum distance at which two radios can operate and maintain a
connection.”
•can use simple geometry to determine the coverage area of an Access
Point using the formula to determine the area of a circle (π)r2 where the
radius (r) is the range of the Wi-Fi signal.
•The coverage area of an Access Point is often referred to as a cell and these
terms are usually used interchangeably.
Link formulas
25
Range Basics • Function of data rate (tradeoff) – the higher the data
rate, the shorter the range. • determining the range of an Access Point,
– a few terms need to be defined and a basic understanding of the mathematics that goes into determining the distance by which a radio signal will travel needs to be provided.
• In an open environment, or what is referred to as Free Space, Power varies inversely with the square of the distance between two points (the receiver and the transmitter). – The stronger the Transmit Power, the higher the signal strength or
Amplitude. Antenna Gain also increases Amplitude and will be further discussed.
• While Gain and Power increase the distance a wireless signal can travel, the expected signal loss (Path Loss) between the transmitter and a receiver reduces it.
26
Path Loss and RSSI
• Path Loss is the reduction in signal strength that a signal experiences as it travels through the air or through objects between the transmitter and receiver.
• relative strength of that signal at the receiver is measured as the Received Signal Strength Indicator (RSSI). RSSI is normally expressed in dBm or as a numerical percentage. – For clarification purposes, a dB (Decibel) is a measure of the
ratio between two quantities (10Log10 x/y) while dBm is a Decibel with respect to milliwatts of power.
– An overall Link Budget can be defined by taking into account all the gains and losses of a signal as it moves from a transmitter to a receiver.
27
dBm (sometimes dBmW) is an abbreviation for the power ratio in decibels (dB) of the measured power referenced to
one milliwatt (mW)—note 0dBm is equivalent to 1 milliwatt. It is used in radio, microwave and fiber optic networks as a
convenient measure of absolute power because of its capability to express both very large and very small values in a
short form.
By comparison, the decibel (dB) is a dimensionless unit, used for quantifying the ratio between two values, such as
signal-to-noise ratio.
• Zero dBm equals one milliwatt. A 3 dB increase
represents roughly doubling the power, which means that
3 dBm equals roughly 2 mW. For a 3 dB decrease, the
power is reduced by about one half, making −3 dBm equal
to about 0.5 milliwatt. To express an arbitrary power P as x
dBm, or go in the other direction, the following equations
may be used:
• or, where P is the power in W and x is the power ratio in
dBm.
dBm
28
http://en.wikipedia.org/wiki/DBm
29
dBm level Power Notes
80 dBm 100 kW Typical transmission power of FM radio
station with 50 km range
60 dBm 1 kW = 1000 W
Typical combined radiated RF power of
microwave oven elements Maximum
allowed output RF power from a ham radio
transceiver (rig) without special permissions
50 dBm 100 W Typical thermal radiation emitted by a
human body Typical maximum output RF
power from a ham radio transceiver (rig)
40 dBm 10 W Typical PLC (Power Line Carrier) Transmit
Power
37 dBm 5 W Typical maximum output RF power from a
hand held ham radio transceiver (rig)
36 dBm 4 W Typical maximum output power for a
Citizens' band radio station (27 MHz) in
many countries
33 dBm 2 W Maximum output from a UMTS/3G mobile
phone (Power class 1 mobiles) Maximum
output from a GSM850/900 mobile phone
30 dBm 1 W = 1000 mW
Typical RF leakage from a microwave oven
- Maximum output power for DCS 1800 MHz
mobile phone Maximum output from a
GSM1800/1900 mobile phone
27 dBm 500 mW Typical cellular phone transmission power
Maximum output from a UMTS/3G mobile
phone (Power class 2 mobiles)
26 dBm 400 mW Access point for Wireless networking
http://en.wikipedia.org/wiki/DBm
Below is a table summarizing useful cases:
30
24 dBm 250 mW Maximum output from a UMTS/3G mobile
phone (Power class 3 mobiles)
23 dBm 200 mW Maximum output in interior environment
from a WiFi 2.4Ghz antenna (802.11b/g/n).
22 dBm 160 mW
21 dBm 125 mW Maximum output from a UMTS/3G mobile
phone (Power class 4 mobiles)
20 dBm 100 mW
Bluetooth Class 1 radio, 100 m range
Maximum output power from unlicensed
AM transmitter per U.S. Federal
Communications Commission (FCC) rules
15.219 [1]. Typical wireless router
transmission power.
15 dBm, 10 dBm, 6 dBm, 5 dBm, 4 dBm
32 mW, 10 mW, 4.0 mW,
3.2 mW, 2.5 mW
Typical WiFi transmission power in laptops.
3 dBm 2.0 mW Bluetooth Class 2 radio, 10 m range
More precisely (to 8 decimal places)
1.9952623 mW
http://en.wikipedia.org/wiki/DBm
31
0 dBm 1.0 mW = 1000 µW Bluetooth standard (Class 3) radio, 1 m range
−1 dBm 794 µW
−3 dBm 501 µW
−5 dBm 316 µW
−10 dBm 100 µW Typical maximum received signal power (−10 to
−30 dBm) of wireless network
−20 dBm 10 µW
−30 dBm 1.0 µW = 1000 nW
−40 dBm 100 nW
−50 dBm 10 nW
−60 dBm 1.0 nW = 1000 pW The Earth receives one nanowatt per square
metre from a magnitude +3.5 star[2]
−70 dBm 100 pW
Typical range (−60 to −80 dBm) of wireless
received signal power over a network (802.11
variants)
−80 dBm 10 pW
−100 dBm 0.1 pW
−111 dBm 0.008 pW = 8 fW Thermal noise floor for commercial GPS single
channel signal bandwidth (2 MHz)
−127.5 dBm 0.178 fW = 178 aW Typical received signal power from a GPS
satellite
−174 dBm 0.004 aW = 4 zW Thermal noise floor for 1 Hz bandwidth at room
temperature (20 °C)
−192.5 dBm 0.056 zW = 56 yW Thermal noise floor for 1 Hz bandwidth in outer
space (4 kelvins)
−∞ dBm 0 W Zero power is not well-expressed in dBm (value
is negative infinity)
http://en.wikipedia.org/wiki/DBm
Antennas: isotropic radiator
• How do we get signals through space? E.M radiation.
– 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 e.m. radiation around an antenna
32
See tutorial
Antennas: directed and sectorized
• Often used for microwave connections or base
stations for mobile phones (e.g., radio coverage
of a valley)
33
Antennas: directed and sectorized
34
Cell
sizes
Antenna gain • Gain (also known as Amplification) improves range of an antenna
– extends range of a Wi-Fi network. – Gain refers to an increase of the Amplitude or Signal Strength
• One of the advantages of a directional antenna (e.g. a dipole) is greater antenna Gain; this is a result of the RF energy pattern being focused vs. an isotropic design. Other types of antennas are more directional in design taking their radiated energy and squeezing it into a very narrow pattern.
– good analogy: think of the isotropic antenna like a light bulb radiating energy equally in all directions, and the directional antenna like a flash light with the light focused in one direction
– the energy of the directional antenna is concentrated in a particular direction, enabling the beam to travel much farther than an isotropic antenna.
• Antenna Gain is bi-directional so it will amplify the signal as it is being transmitted and as it is received. So if a directional antenna is providing 6db Gain on transmit, it will also increase received sensitivity an equal amount so the
– antenna design of the Wi-Fi Access Point plays a critical role in the amount of range (coverage) delivered.
35
Antenna gain basics
36
dBi
dB(isotropic) – the forward gain of an antenna compared with the hypothetical isotropic antenna, which uniformly
distributes energy in all directions. Linear polarization of the EM field is assumed unless noted otherwise.
dBd
dB(dipole) – the forward gain of an antenna compared with a half-wave dipole antenna. 0 dBd = 2.15 dBi
Attenuation
37
RF signal strength is reduced as it passes through various materials.
This effect is referred to as Attenuation.
As more Attenuation is applied to a signal, its effective range will be
reduced. The amount of Attenuation will vary greatly based on
the composition of the material the RF signal is passing through.
Note: A change in
power ratio by a factor
of two is approximately
a 3 dB change
20dB is a factor of 100
EIRP
• EIRP - Effective Isotropic Radiated Power
EIRP = Power out (dBm) + antenna gain (dBi)
– cable loss (dB)
• EIRP Regulations
38
Simplistic Range Calculations
• The Model
For indoor environment the signal power at the
receiver SRx is related to the transmit power
TRx as shown below (this model will be used as the reference analysis model)
Where C=speed of light, f=center frequency, N: path loss coefficient. ITU recommends N=3.1 for 5-GHz and N=3 for 2.4-GHz
• IEEE 802.11b (with N=3)
• With EIRP of 30dBm max range=154m
• With EIRP of 19dBm max range=66.4m
• With EIRP of 15dBm max range=48.4m
• IEEE 802.11a (with N=3.1)
• With EIRP of 18dBm range=14m with 54Mbits /s
• With EIRP of 23dBm range=30m with 54Mbits/s
Simplistic Range Calculations
Receiver Sensitivity
• For IEEE 802.11b receiver should be able to detect -
76dBm with BER of min 10e-5 in the absence of Adjacent
Chanel Interference (ACI). If ACI is present the receiver
must be able to detect -70dBm
• For IEEE 802.11a as follows
Link Budget
42
Example: Consider a WLAN access point (AP) transmitting to an
AP 1.5 km away
Transmistting antenna gain = 13.5 dBi
transmitting power = 100 mW
Distance to receiver AP = 1500 metres
Receiving AP antenna gain =13.5 dBi
Rx sensitivity = -82 dBi.
The free space path loss = 104.3 dB.
The Rx Power Level = 20.0 + 13.5 - 104.3 + 13.5 = -57.3
The Loss Budget equals -(-82) - 57.34 – 10 (safety margin) = 14.7
Because 14.7 is greater than 0, the link will work.
Signals in noise and interference
43
Signal-to-Noise Ratio (SNR) • The range of an Access Point is a function of data rate.
– notion that higher data rates do not appear to “travel” as far as the lower data rates is a function of the Signal to Noise Ratio (SNR) and not because the Access Point and the client can’t necessarily “hear” each other.
• SNR is the ratio of the desired signal to that of all other noise and interference as seen by a receiver. SNR is important as it determines which data rates can be correctly decoded in a wireless link.
• It is expressed in dB as a ratio. – The received signal and the noise level, determine the SNR.
– As data rates increase from 6 Mbps to 54 Mbps, more complex modulation and encoding methods are used that require a higher SNR to properly decode the signal.
• E.g. a 54 Mbps per second signal requires 25 db of SNR: signal will not be properly decoded at greater distances because as the signal moves further from the source, a greater amount of path loss occurs (the signal is attenuated). Lower data rate transmissions, can be more easily decoded and as a result appear to “travel” farther.
• E.g. in an outdoor environment with just free space loss, a 6 Mbps signal can actually be decoded 7 times further away than a 54 Mbps.
44
SNR for
different
modulation
schemes
45
The more complex (and
higher efficiency)
modulation schemes
require higher SNR to
decode signal
Noise and interference • So far: only a single transmitter assumed
– Only disturbance: self-interference of a signal with multi-path “copies” of itself
• In reality, two further disturbances – Noise – due to effects in receiver electronics, depends on
temperature
• Typical model: an additive Gaussian variable, mean 0, no correlation in time
– Interference from third parties
• Co-channel interference: another sender uses the same spectrum
• Adjacent-channel interference: another sender uses some other part of the radio spectrum, but receiver filters not good enough to fully suppress it
• Effect: Received signal is distorted by channel, corrupted by noise and interference – What is the result on the received bits?
46
Symbols and bit errors
• Extracting symbols out of a distorted/corrupted wave
form is fraught with errors
– Depends essentially on strength of the received signal compared
to the corruption
– Captured by signal to noise and interference ratio (SINR)
• SINR allows to compute bit error rate (BER) for a given
modulation
– Also depends on data rate R (# bits/symbol) of modulation
– E.g., for simple DPSK, data rate corresponding to bandwidth:
47
Examples for SINR ! BER
mappings
48
1e-07
1e-06
1e-05
0.0001
0.001
0.01
0.1
1
-10 -5 0 5 10 15
Coherently Detected BPSKCoherently Detected BFSK
BER
SINR
Signal Important quantities
• Important quantities to measure the strength of the signal to the
receiver, noise, interference e.g.
SNR . Signal to Noise Ratio in dB
SIR = Signal to Interference Ratio; received power of reference user
in dBm/received power of all interferers in dBm
C/I . Carrier over Interference in dB
Carrier Power (dBm) / received power of all interferers in dBm
49
Signal Important quantities -
Examples • SNR – Signal to Noise Ratio
Assumptions to simplify things:
- All the users are equally distributed in the coverage area so that they have equal
distances to the TRX Antenna
- The power level they use is the same thus the interference they cause is on the same
level.
- All the UEs use the same Baseband rate e.g. 60 kbits/sec for Streaming Video.
If assumed that there are X users under the same TRX Coverage (in the same
Cell) and the above assumptions are applied, it means that there are X – 1 users
causing interference to one (1) user. This indicates the Signal to Noise Ratio and
when expressed in mathematical format the outcome is the following equation:
Where P is the power required for information transfer in one channel and is a
multiple of the energy used per bit (Eb) and the Baseband rate ( P = Eb x
Baseband rate)
)1(
XP
PSNR
50
Bit Error Rate
• IEEE 802.11b for BER better than 10e-5 then min S/N
• IEEE 802.11a for BER better than 10e-5 then min S/N
Signal Important quantities -
Examples
• SIR – Signal to Interference Ratio – The Signal to Interference Ratio (SIR) at the receiver is considered as a
quality parameter and is determined by the ratio of the desired signal power to the total interference power from all the other users.
– For e.g.
– The capacity of CDMA is limited by the amount of interference that can be tolerated from other users.
– System Capacity is maximized if the transmitted power of each terminal is controlled so that its signal arrives at the Base Station with the minimum required SIR.
• If a terminal's signal arrives at the Base Station with a too low received power value then the required QoS of the radio Connection can not be met.
• If the received power value is too high, the performance of this terminal is good, however, interference to all other terminal transmitters sharing the channel is increased and may result is unacceptable performance for other users, unless their number is reduced.
MORE LATER WHEN DISCUSSING RRM TECHNIQUES FOR 3G
52
Signal Important quantities -
Examples
• C/I – Carrier to Interference Ratio
The Wideband Signal to Interference (SIR) Ratio is also called as Carrier to Interference
Ratio (C/I). The Carrier to Interference (C/I) Ratio is very important in Cellular systems
in order to determine the maximum allowed interference level for which the system
will work.
• Eb/No: The Required Eb/No (measured in dB) for a service denotes the value
that the signal energy per bit (Eb) divided by the interference and noise power density
(No) should have for achieving a certain BER (Bit Error Rate) so as to satisfy the
required QoS of a service.
– Eb/No is the measure of signal to noise ratio for a digital communication system. It is measured
at the input to the receiver and is used as the basic measure of how strong the signal is.
– it is the fundamental prediction tool for determining a digital link's
performance. Another, more easily measured predictor of performance is the carrier-to-noise
or C/N ratio
See www.sss-mag.com/ebn0.html
53 fb-bit rate, Bw receiver noise bandwidth
Signal Important quantities
Eb/No . Signal Energy
per bit to noise Power
Density per hertz.
-Eb/No = Signal energy (per
bit ) dBm / noise Power
dBm .Measures how
strong the signal is .
-Different forms of
modulation BPSK, QPSK,
QAM, etc. have different
curves of theoretical bit
error rates versus Eb/No.
54
Eb/No
e.g. For DBPSK/DQPSK
8dB required Eb/No to
achieve a desired BER of
10E-3
db
These curves show the best performance
that can be achieved across a digital link
with a given amount of RF power.
Example calculation • Consider a 12.2 kbps speech service spread over a 5 MHz
Carrier and that an Eb/No of 5.0 dB is required to achieve a 0.01 BER performance.
– After the dispreading in the receiver, the signal power needs to be
typically a few decibels (dB) above the interference and noise power.
– Since an Eb/No of 5.0 dB is enough for efficiently detecting the signal, the required wideband Signal to Interference Ratio (SIR) will be 5.0 dB minus the Processing Gain of 25 dB that can be achieved for the corresponding service (10 x log (WCDMA Chip Rate/Bit Rate)). The chip rate is equal with 3.84 Mcps.
– Thus, the signal power can be 20 dB under the interference and thermal noise power, and the WCDMA receiver can still efficiently detect and interpret the signal correctly.
55
56
The ‘big’ picture ...
57
Effects of Mobility on channel
58
Effects of mobility on channel
• 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)
59
See mobility models papers for modelling
Mobility paper 1, paper 2
Supplementary slides
60
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
61
Signal propagation
• Propagation in free space always like light (straight line)
• Receiving power proportional to 1/dn(d = distance between sender and receiver, n depends on medium, usually 2, but can be higher, e.g. 4, see later)
• 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
62
Real world example
signal coverage
63
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
64
Typical large-scale path loss
65
Measured large-scale path loss
66
Partition losses
67
Measured indoor path loss
68
Measured indoor path loss
69
Measured received power levels over a 605 m 38 GHz fixed wireless link
in clear sky, rain, and hail [from [Xu00], ©IEEE].
70
Measured received power during rain storm at 38 GHz [from [Xu00],
©IEEE].
71