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© Ammar Abu-Hudrouss Islamic University Gaza ١
Wireless CommunicationsWireless Communications
Slide 2Wireless Communications
Course SyllabusCourse Syllabus
Text Book Andrea Goldsmith, Wireless Communications, Cambridge
University Press 2005.References1. Rappaport, Wireless Communications: Principles and Practice,
Prentice Hall 2nd Ed 2. D. N. C. Tse and P. Viswanath, Fundamentals of Wireless
Communication, Cambridge, U.K., 2005
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Slide 3Wireless Communications
Course SyllabusCourse Syllabus
Course Content:
Statistical multipath channel models (ch. 3).
Capacity of wireless channel: (ch 4).
Diversity techniques for the receiver and the transmitter (ch 7).
Multiple antenna and space time communications (ch. 10)
Orthogonal Frequency Division Multiplexing (OFDM) (ch. 12)
Spread spectrum : DSSS and FSSSS (ch. 13)
Multiple access, Random access, power control (ch. 14)
Cellular systems and infrastructure-based wireless networks (ch. 15)
Slide 4Wireless Communications
Course SyllabusCourse SyllabusGrading PolicyThe final course grade will be distributed as follows:
Quizzes and class activity 10 %Programming and simulation assignments 20 %Research topic and presentation 15%Midterm exam 25 %Final exam 30 %
Plagiarism will not be tolerated at any case. Copying homework from your colleagues or project from any source will lead to severe consequences.
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Slide 5Wireless Communications
Wireless Communications
Satellite TV Cordless phone Cellular phone Wireless LAN, WIFI Wireless MAN, WIMAX Bluetooth Ultra Wide Band Wireless Laser Microwave GPS Ad hoc/Sensor Networks
Slide 6Wireless Communications
Basic Concepts
Simplex, half-duplex, and full duplex Base Station Mobile Station Subscriber Transceiver Mobile Switching centre Control Channel Roamer Handoff Page
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Slide 7Wireless Communications
Pager System
Slide 8Wireless Communications
Pager System
Broad coverage for short messaging
Message broadcast from all base stations
Simple terminals
Optimized for 1-way transmission
Overtaken by cellular
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Slide 9Wireless Communications
Cordless phone
DC2 and DECT standards
Slide 10Wireless Communications
Bluetooth
Cable replacement RF technology (low cost) Short range (10m, extendable to 100m) 2.4 GHz band (crowded) 1 Data (700 Kbps) and 3 voice channels, up to 3 Mbps Widely supported by telecommunications, PC, and consumer
electronics companies
Few applications beyond cable replacement
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Slide 11Wireless Communications
IEEE 802.15.4 / ZigBee Radios
Low-Rate WPAN Data rates of 20, 40, 250 Kbps Support for large mesh networking or star clusters Support for low latency devices CSMA-CA channel access Very low power consumption Frequency of operation in ISM bands
Focus is primarily on low power sensor networks
Slide 12Wireless Communications
Satellite Systems
Cover very large areas Different orbit heights
GEOs (39000 Km) versus LEOs (2000 Km) Optimized for one-way transmission
Most two-way systems struggling or bankrupt Global Positioning System (GPS) use growing
Satellite signals used to pinpoint locationPopular in cell phones, PDAs, and navigation devices
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Slide 13Wireless Communications
Cellular System
Mobile identification number (MIN) electronic serial number (ESN)
Slide 14Wireless Communications
2G to 3G evolution
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Slide 15Wireless Communications
4G and LTE (long term evolution)
OFDM/MIMO Much higher data rates (50-100 Mbps) Greater spectral efficiency (bits/s/Hz) Flexible use of up to 100 MHz of spectrum Low packet latency (<5ms). Increased system capacity Reduced cost-per-bit Support for multimedia
Slide 16Wireless Communications
Wireless Local Area Networks (WLANs)
WLANs connect “local” computers (100m range) Breaks data into packets Channel access is shared (random access) Backbone Internet provides best-effort service
Poor performance in some apps (e.g. video)
01011011InternetAccessPoint
0101 1011
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Slide 17Wireless Communications
Wifi Networks (Supporting Multimedia)
802.11n++
Wireless HDTVand Gaming
• Streaming video• Gbps data rates• High reliability• Coverage in every room
Slide 18Wireless Communications
Wimax (802.16)
Wide area wireless network standardSystem architecture similar to cellularHopes to compete with cellular
OFDM/MIMO is core link technology Operates in 2.5 and 3.5 MHz bands
Different for different countries, 5.8 also used.Bandwidth is 3.5-10 MHz
Fixed (802.16d) vs. Mobile (802.16e) WimaxFixed: 75 Mbps max, up to 50 mile cell radiusMobile: 15 Mbps max, up to 1-2 mile cell radius
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Slide 19Wireless Communications
Characteristic of Wireless Channel
Slide 20Wireless Communications
Channel Impulse Response
)(tx
Channel
)(ty
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Slide 21Wireless Communications
Channel Impulse Response Response of channel at t to impulse at t-:
t is time when impulse response is observedt- is time when impulse put into the channelis how long ago impulse was put into the channel for the current observation path delay for MP component currently observed
))(()(),(1
)( tettc n
N
n
tjn
n
Slide 22Wireless Communications
Received Signal Characteristics
Received signal consists of many multipath components Amplitudes change slowly Phases change rapidly
Constructive and destructive addition of signal componentsAmplitude fading of received signal (both wideband and narrowband signals)
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Slide 23Wireless Communications
Major Categories of Fading
Large Scale Fading :This is the loss that propagation models try to
account for mostly dependant on the distance from the transmitter to the receiver also known as Large Scale Path Loss, Log-Normal Fading or Shadowing
Small Scale Fading :Could be 20-30 dB over a fraction of a wavelength.
It is Caused by the superposition or cancellation of multipath propagation signals, the speed of the transmitter or receiver or the bandwidth of the transmitted signal. It is also known as Multipath Fading or Rayleigh Fading
Slide 24Wireless Communications
Small Scale Fading:
The type of fading experienced by a signal propagating through a channel can be determined by the nature of the transmitted signal with respect to the characteristics of the channel.
Factors influencing small scale fading:Factors influencing small scale fading:• Multipath propagation.• Speed of the mobile.• Speed of the surrounding objects.• Transmission bandwidth of the signal.
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Slide 25Wireless Communications
Inter symbol interference
A
B
C
D = A+B+C
A
Slide 26Wireless Communications
Power delay Profile
Rec
eive
d S
igna
l Lev
el (
dBm
)
-105
-100
-95
-90
-90
0 50 100 150 200 250 300 350 400 450
Excess Delay (ns)
RMS Delay Spread () = 46.4 ns
Mean Excess delay () = 45 ns
Maximum Excess delay < 10 dB = 110 ns
Noise threshold
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Slide 27Wireless Communications
Delay Spread:
Mean excess delay RMS delay spread Excess delay spread
Mean excess delay is the first moment of the power delay profile and is defined by the equation
hk
hkk
kk
kkk
P
P
a
a
)(
)(
2
2
Slide 28Wireless Communications
Maximum excess delay is defined as the ,where , is the first arriving signal and is the maximum delay at which a multipath component is within X dB of the strongest arriving multipath signal.
RMS delay spread is the square root of the second central moment of the power delay profile and is defined by the equation:
where
22 )(
hk
hkk
kk
kkk
P
P
a
a
)(
)( 2
2
22
2
0 xx0
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Slide 29Wireless Communications
Example (Power delay profile)
-30 dB
-20 dB
-10 dB
0 dB
0 1 2 5
Pr()
(µs)
s 38.4]11.01.001.0[
)0)(01.0()2)(1.0()1)(1.0()5)(1(_
22222_
2 07.21]11.01.001.0[
)0)(01.0()2)(1.0()1)(1.0()5)(1(s
s 37.1)38.4(07.21 2
1.37 µs4.38 µs
Slide 30Wireless Communications
RMS delay spread and coherence b/w
RMS delay spread and coherence b/w (Bc) are inversely proportional
1cB
.501
cB For 0.9 correlation
.51
cB For 0.5 correlation
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Slide 31Wireless Communications
Coherence Bandwidth
)(tx
Time domain view
High correlation of amplitudebetween two different freq.components
Range of freq overwhich response is flat
Bc delay spread
)( fX
Freq. domain view
Slide 32Wireless Communications
Time dispersive nature of channel
RMS delay spread () Coherence b/w (Bc)
Time domain view Freq domain view
Delay spread and coherence bandwidth are parameters whichdescribe the time dispersive nature of the channel.
channel 1
channel 2
channel 3
Sig
nal
Cha
nnel
Symbol Time (Ts) Signal bandwidth (Bs)
signal 1
signal 2
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Slide 33Wireless Communications
Revisit Example (Power delay profile)
-30 dB
-20 dB
-10 dB
0 dB
0 1 2 5
Pr()
(µs)
s 38.4_
2_2 07.21 s
s 37.1
1.37 µs4.38 µs
kHzBcoherence c 146.51)%50(
Signal bandwidth for Analog Cellular = 30 KHzSignal bandwidht for GSM = 200 KHz
Slide 34Wireless Communications
Coherence bandwidth:
It is the range of frequencies over which two frequency components have a potential for amplitude correlation.
If two sinusoids with a frequency separation of greater than Bc are propagating in the same channel, they are affected quite differently by the channel.
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Slide 35Wireless Communications
Doppler Effect
Slide 36Wireless Communications
Doppler Shift
cosvf v
Doppler shift
Example- Carrier frequency fc= 1850 MHz (i.e. = 16.2 cm)- Vehicle speed v = 60 mph = 26.82 m/s
- If the vehicle is moving directly towards the transmitter
- If the vehicle is moving perpendicular to the angle of arrival of the transmitted signal
Hzf 165162.082.26
0f
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Slide 37Wireless Communications
Doppler Spread and Coherence Time
Doppler spread and Coherence Time take into account the relative motion between mobile and base station, or by movements of objects in the channel.
They describe the time varying nature of the channel in a small scale region.
Slide 38Wireless Communications
Doppler Spread Bd:When a signal of frequency fc is transmitted,
the received signal spectrum, called the Doppler spectrum, will have components fc - fd to fc + fd, where fd is the Doppler shift.
Coherence time Tc:It is used to characterize the time varying
nature of the frequency depressiveness of the channel in the time domain
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Slide 39Wireless Communications
For high correlation
For correlation above 0.5
Mean of the previous two equation is usually used in digital communication systems
mc fT 1
mc fT
169
mc fT 423.0
Slide 40Wireless Communications
Time varying nature of channel
Coherence Time (TC) Doppler spread (BD)
Symbol Time (TS) Signal bandwidth (BS)
Time domain view Freq domain view
Doppler spread and coherence time are parameters whichdescribe the time varying nature of the channel.
channel 1
channel 2
channel 3
Sig
nal
Cha
nnel
signal 1
signal 2
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Slide 41Wireless Communications
Small Scale Fading:
Different types of transmitted signals undergo different types of fading depending upon the relation between the
Signal Parameters: Bandwidth, Symbol Periodand
Channel Parameters: RMS Delay Spread,Doppler Spread
In any mobile radio channel a wave can be dispersed either in Time or in Frequency.
These time and frequency dispersion mechanisms lead to four possible distinct effects which depend on the nature of transmitted signal, the channel and the velocity.
Slide 42Wireless Communications
Flat Fading:
A received signal is said to have underwent Flat Fading if “The Mobile Radio Channel has a constant gain and linear phase response over a Bandwidth which is greater than the Bandwidth of the transmitted Signal”
Fading in which all frequency components of a received radio signal vary in the same proportion simultaneously
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Slide 43Wireless Communications
Here the multipath structure of the channel is such that spectral characteristics of the transmitted signal are preserved at the receiver But due to the fluctuations in the gain of the channel caused by multipath, the signal strength varies with time
Slide 44Wireless Communications
From the figure we can note that if the channel gain varies with time, a change of amplitude of the received signal occurs.
From the figure we can note that the spectrum of the received signal r (t) is preserved even though there is a change in gain.
Flat fading channels are also referred as amplitude varying channels or narrow band channels, since the bandwidth of the applied signal is narrow as compared to the channel flat fading bandwidth.
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Slide 45Wireless Communications
Typical Flat fading channels cause deep fades To achieve low bit error rates during times of deep fades, Flat fading channels operate at 20 to 30dB more transmitter power compared to the systems operating over non-fading channels.
Rayleigh distribution is the most common amplitude distribution.
According to this distribution, Rayleigh Flat fading channel model assumes that the channel induces an amplitude which varies in time.
Slide 46Wireless Communications
Summary
Signal undergoes Flat Fading if: Bs<<Bc
where Bs is bandwidth and Bc is the coherence bandwidth of the channel
And Ts>>
whereTs is the reciprocal bandwidth and rms delay spread.
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Slide 47Wireless Communications
Frequency Selective Fading:
The channel creates frequency selective fading on the received signal when the channel possesses a constant gain and linear phase response over a bandwidth, which is smaller than the bandwidth of the transmitted signal
Under these conditions the channel impulse response has a multipath delay spread which is greater than the reciprocal bandwidth of the transmitted message waveform So the received signal includes multiple versions of the transmitted waveform, which are attenuated and delayed in time, and hence the received signal is distorted.
Slide 48Wireless Communications
Frequency selective fading is much difficult to model than flat fading channels because each multipath signal must be modeled and the channel must be considered to be a linear filter.
It is for this reason that wideband multipath measurements are made and models are developed from these measurements.
When analyzing mobile communication systems, statistical impulse response models such as the 2-ray Rayleigh model or computer generated or measured impulse responses are generally used for analyzing frequency selective small-scale fading.
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Slide 49Wireless Communications
Slide 50Wireless Communications
For frequency selective fading, the spectrum S(f) of the transmitted signal has a bandwidth which is greater than the coherence bandwidth Bc of the channel.
Frequency selective fading is caused by multipath delays which approach or exceed the symbol period of the transmitted symbol.
These channels are also known as wideband channels since the bandwidth of the signal s(t) is wider than the bandwidth of the channel impulse response.
As time varies, the channel varies in gain and phase across the spectrum of s(t),resulting in time varying distortion in the received signal r(t)
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Slide 51Wireless Communications
Summary
Signal undergoes Frequency Selective Fading if:
Bs>Bc
where
Bs is bandwidth and
Bc is the coherence bandwidth of the channel
And
Ts<where
Ts is the reciprocal bandwidth and
rms delay spread.
Slide 52Wireless Communications
Small scale fading
Multi path time delay
Doppler spread
Flat fading BC
BS
Frequency selective fading BC
BS
TC
TSSlow fading
Fast fading TC
TS
fading
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Slide 53Wireless Communications
Rayleigh Fading Distribution:
Rayleigh Fading Distribution in mobile radio channels is commonly used to describe the statistical time varying nature of the received envelope of a flat fading signal or the envelope of an individual multipath component.
where is the rms value of the received voltage signal before
envelope detection,is the time-average power of the received signal before
envelope detection.
2
0 (r < 0)
)0(2
exp)( 2
2
2
rrrrp
Slide 54Wireless Communications
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Slide 55Wireless Communications
The variance of the Rayleigh distribution is given byr
which represents the ac power in the signal envelope.
The rms value of the envelope is 2
2)(][][
2
0
2222
drrprrErEr
22 4292.02
2
Slide 56Wireless Communications
The median value of r is found by solving
Note: It is customary to use median values instead of the mean values, since fading data are usually measured in the field and a particular distribution cannot be assumed. By using median values instead of mean values it is easier to compare different fading distributions which have widely varying means
medianr
drrp0
)(21
177.1medianr
٢٩
Slide 57Wireless Communications
Ricean Fading Distribution:
When there is a dominant stationary (nonfading) signal component present, such as a line-of-sight propagation path, the small scale-scale fading envelope distribution is Ricean.
Random multipath components arriving at different angles are superimposed on a stationary dominant signal
At the output of an envelope detector this has the effect of adding a dc component to the random multipath
Slide 58Wireless Communications
The effect of a dominant signal arriving with many weaker multipath signals gives rise to Ricean distribution
As the dominant signal becomes weaker, the composite signal gives resembles a noise signal which has an envelope that is Rayleigh
Thus, the Ricean distribution degenerates to a Rayleigh distribution when the dominant component fades away
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Slide 59Wireless Communications
The Ricean distribution is given by
Where A denotes peak amplitude of the dominant signalIo(.) is the modified Bessel function of the first kind and
zero order
202
)(
22
22
)(
ArIerrpAr For A 0 ,r 0
0 For r< 0
Slide 60Wireless Communications
Ricean Factor K completes determines the Riceandistribution.
As A 0, K - dB, and as the dominant path decreases in amplitude, the Ricean distribution degenerates to Rayleigh distribution