wireless communication ebook
TRANSCRIPT
1
Wireless Communication
Lecture 1
Wireless Fundamentals
Ammar Karim
2
Course Division
1. Fundamentals
Evolution of wireless systems, various impairments in wireless channels.
Spreading: FHSS, DSSS, Spreading sequences
Understanding of FDMA-TDD/FDD, TDMA-FDD/TDD and CDMA-FDD/TDD Systems.
Equalization
2. Wireless Data Networks
Data networks, IEEE 802.11 WLANS their design and operation, Random Access Methods.
Mobile IP.
WLLs: MMDS/LMDS, Wi-MAX
Bluetooth
3. Cellular System
Cellular Fundamentals: Cellular systems, cellular operations, Handoffs & Cluster size Relationship between C/I and Cluster Size, Derivation of expressions to link the Re-Use ratio (D/R) to the Cluster Size (N) , Power control, cellular hierarchy, AMPS and AMPS architecture, Call establishment and control
Frequency planning & re-use, Radio Propagation effects, Adjecent Interference, Cell splitting
Tele traffic engineering
GSM: architecture, entities, channels, signal processing, handoff, call control, roaming, security
CDMA
GPRS
4. Overview of Cutting-edge Technologies: 3G and Beyond
3
Recommended Books
David Parsons, The Mobile Radio Propagation Channel, 2nd Edition, John Wiley & Sons; ISBN: 047198857
T. S. Rappaport, Wireless Communications, 2nd Edition, 2002, Pearson Education; ISBN: 81-7808-648-4
Simon Haykin, Communication Systems, 4th edition, May 2000, John Wiley & Sons; ISBN: 0471178691
Lecture Notes
4
Evaluation Criterion
Assignments = 5%
Quizzes = 10%
Mids = 30% (15% each)
Final Exam = 55%
5
Age of Information Communications
Blackberry 8705g Nokia DVB-H phone Mobile MM
6
Evolving Communication Networks
Core and Access Networks
7
Wireless communication
Early wireless communication:
in the 400-900 TeraHertz Band!
150 BC smoke signals (Greece)
1794, optical telegraph
What is wireless communication:
Any form of communication that does not require a transmitter and receiver to be in physical contact
Electromagnetic waves propagate through free space
Radar, RF, Microwave, IR, Optical
8
Types of Communication
Simplex
one-way communication
radio, TV, etc
Half-duplex:
two-way communication but not simultaneous
push-to-talk radios, etc
Full-duplex:
two-way communication
cellular phones
Frequency-division duplex (FDD)
Time-division duplex (TDD): simulated full-duplex
9
Forms of Communication
Analogue & Digital
Which one is Better?
Digital?
Why? Digital Data has inherited frequency reuse property
Lesser noise and interference as compared to analogue communication
Lower transmit power is required
1/0’s can transmit anything : sound, picture, video etc.
10
Types of Media/Environments used for Communication
Wireless & Wired
Why Wireless is better than Wired ?
User Mobility
Reduced Cost (cheap infrastructure) Cabling very critical
Developing nations utilize cellular telephony rather than
laying twisted-pair wires to each home
Flexibility Can easily set-up temporary LANs
Disaster situations
Office moves
Only use resources when sending or receiving a signal
11
Wired Vs. Wireless Communication
Wired Wireless Each cable is a different channel One media (cable) shared by all
Signal attenuation is low High signal attenuation
No interference High interference
noise; co-channel interference; adjacent
channel interference
12
Why wireless different than wired?
Noisy, time-varying channel BER varies by orders of magnitude
Environmental conditions affect transmission
Shared medium Other users create interference
Must develop ways to share the channel
Bandwidth is limited spectrum allocated by state rules
13
Classification of Wireless Systems
Mobile Wireless Systems
GSM, TDMA, CDMA
WLAN, Ad-hoc, Bluetooth, Home RF
Fixed Wireless Systems
MMDS, LMDS, Satellite
WiMax(IEEE 802.16a)
Infrastructure Dependent Wireless Systems
Cellular, WLAN,
WLL, WiMAX, Satellite
Ad Hoc Wireless Systems
Packet Radios
Sensor dust, mesh
14
Satellite – Wide coverage and high mobility
Cellular networks – High mobility
Wireless LANs, Wireless Local Loop, etc – Low/None mobility
Wireless Networks - Infrastructure
15
Wireless Networks - Ad Hoc
16
Evolution of Wireless Networks
1st generation: analog - voice
AMPS with manual roaming
Cordless phones
Packet radio
2nd generation: digital - voice, data
Cellular & PCS with seamless roaming and integrated paging (IS-95, IS-136, GSM)
Multizone digital cordless
wireless LANs (IEEE 802.11), MANs (Metricom), and WANs (CDPD)
17
The 3nd Generation
Wide-area mobile voice/data
2.5G: GPRS, EDGE
3G standards: UMTS,/IMT2000, Wideband CDMA, CDMA2000
Wireless Local Loop (IEEE 802.16)
LMDS (local multipoint distribution) 24-28GHz
MMDS below 5 GHz
WiMAX
Higher-speed WLAN
802.11b (2.4GHz, 11 Mbps), IEEE 802.11a (5GHz, 54 Mbps & higher)
HyperLAN
Personal area networks
Bluetooth, 802.15
Wireless device networks
Sensor networks, wirelessly networked robots
18
Evolution Path
19
Attributes of Wireless Access
20
New Paradigm in Wireless Design
21
Wireless Channel
22
Wireless Channel
Pr ~ 1/r2
23
Multi-path Propagation
Received signal is made up of several paths which can be classified as:
1. Direct Path
2. Reflected Path
3. Scattered Path
4. Diffracted Path
1
2
3
4
Line Of Sight (LOS) Non Line Of Sight (NLOS)
24
Other Basic Propagation Mechanisms
Reflection: It occurs when a propagating electromagnetic wave intrudes upon an object which has very large dimensions when compared to the wavelength of the propagating wave. Reflection occurs from the surface of the earth and from buildings and walls.
Diffraction: It occurs when the radio path between the transmitter and receiver is obstructed by a surface that has sharp irregularities (edges). The secondary waves resulting from the obstructing surface are present throughout the space and even behind the obstacle, giving rise to a bending of waves around the obstacle, even if the line of sight path does not exist between the transmitter and the receiver.
Scattering: It occurs when the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength, and where the number of obstacles per unit volume is large. Scattered waves are produced by rough surfaces, small objects, or by other irregularities in the channel.
25
LOS & NLOS Scenarios
LOS (Line of Sight): The equations shown below hold only for LOS scenarios, where direct paths of electromagnetic rays exist. Since, the received signal is directly received at the receiver the effects such as reflection, diffraction and scattering doesn’t affect the signal reception that much.
NLOS (Non Line of Sight): When the direct LOS between transmitter and receiver is lost the effects such as reflection, diffraction and scattering become very important as in the absence of direct path they become the main contributors to the received signal at the receiver.
1
2
3
4
Line Of Sight (LOS) Non Line Of Sight (NLOS)
26
Shadowing – Slow Fading
27
Slow Fading (Shadowing)
Shadowing: It is the term given to the slow variations in received signal power as the user moves through the environment, especially behind large buildings or near by hills. These variations occur approx. 1 -2 times per second, that’s why Slow Fading!
Reflected
Scattered Path
Diffracted Path
3
4
28
Shadowing: Behavior Prediction and
Mathematical Modeling Behavior of the Constraint
P & 1/d4
Equipment Developed
Receiver and transmit Antennas
Amplifier (at the transmitter to increase the power)
Factors affecting this behavior
PT (Transmit power)
GT (Transmit Antenna Gain)
GR (Receiver Antenna Gain)
Effective Area of Antenna
Note: This effect can be mitigated by increasing the power using Amp.
29
Slow Fading (Shadowing)
PR= PT GT GR (λ / 4 π) 2 x 1/d4
PT = Transmit power (Watts)
PR = Received Power (watts)
GT = Transmit Antenna Gain – relative to isotropic source (no unit)
GR = Receiver Antenna Gain – relative to isotropic source (no unit)
λ = Carrier’s Wavelength (λ = c / f) (meters)
d = Distance between transmitter and receiver (meters)
3
4
Non Line Of Sight (NLOS)
30
The Effects of Multipath Propagation
31
Types of Fading
32
Fast Fading in Mobile terrestrial Channel
1
2
3
4
0o 180o
270o
90o
200o 300o
This can be attributed to the phasor addition of various multi-path signals.100-200 times/sec, that’s why Fast Fading!
33
Fast Fading in Mobile terrestrial Channel
Constructive interference takes place when two or more rays arrive in-phase (or almost in-phase) with each other
Destructive interference takes place when two or more rays arrive anti-phase (or almost out-of-phase) with each other. This also means rays arriving 180o apart from each other
Semi-constructive/destructive
34
Non-line-of-sight case (k=0)
Rayleigh Fading
35
Line-of-sight case (k>1)
Rician Fading
36
K-Factor
K-Factor is the ratio of power of a dominant (LOS) path to the power of the random components (/scatter)
For cases where LOS component is week (Rayleigh), the K-factor will be small (in some cases negative). However, if the line of sight dominates (Rician), the K-factor will normally take positive values between 5 and 10 dB.
37
BER for Various Fading Conditions
38
Types of Small-scale Fading
Fading Effects Due to Multipath Time Delay Spread-Flat fading
It is the most common type of fading described in the technical literature.
The spectral characteristics of the transmitted signals are preserved at the receiver, however the strength of the received signal changes with time.
Flat fading channels are known as amplitude varying channels or narrow-band channels.
39
Types of Small-scale Fading
Fading Effects Due to Multipath Time Delay Spread- Frequency Selective Fading
Frequency selective fading is due to time dispersion of the transmitted symbols within the channel. Thus the channel brings on inter-symbol-interference.
Computer generated impulse responses are used for analyzing frequency selective small-scale fading.
Frequency selective fading channels are known as wideband channels since the BW of the signal is wider than the BW of the channel impulse response.
40
Types of Small-scale Fading
Flat fading channel characteristics
,ths(t) r(t)
s(t) ,th
r(t)
t t t 0 0 0 Ts Ts+τ τ τ<<Ts
fc
S(f)
f fc
f
R(f)
f fc
H(f)
41
Types of Small-scale Fading
Frequenecy selective fading channel characteristics.
,ths(t) r(t)
s(t) ,th
r(t)
t t t 0 0 0 Ts Ts+τ τ τ<<Ts
fc
S(f)
f fc
f
R(f)
f fc
H(f)
Ts
42
Doppler Effect/Shift
The Doppler effect, named after Christian Doppler, is the change in frequency and wavelength of a wave that is perceived by an observer moving relative to the source of the waves.
For waves, such as sound waves, that propagate in a wave medium, the velocity of the observer and of the source are reckoned relative to the medium in which the waves are transmitted.
The total Doppler effect may therefore result from either motion of the source or motion of the observer.
Example: As the train approaches the station sound pitch is increased and as it leaves pitch starts decreasing.
This Phenomenon of sound waves was discovered by the Dutch scientist Christoph Hendrik Diederik Buys Ballot in 1845. Later Hippolyte Fizeau discovered independently the same phenomenon on electromagnetic waves in 1848.
43
Doppler Effect/Shift
For waves that travel at the speed of light, the mathematical model of this phenomenon is as follows:
fdoppler = fv/c cos θ
f’= f + fdoppler
Where
f’ = observed frequency (Hz)
fdoppler = Doppler Frequency (Hz)
V = the velocity of the transmitter relative to the receiver (meters/second)
θ = Arrival angle (degrees)
c = speed of light = 3 x 108 (meters/second)
44
Doppler Effect
45
Error Compensation Mechanisms in Wireless Channels
46
Adaptive Equalization
47
ARQ & FEC
Error detection codes
Detects the presence of an error
Automatic repeat request (ARQ) protocols
Block of data with error is discarded
Transmitter retransmits that block of data
ACK and NACK
Retransmissions --> excessive delay
Retransmission strategy not conveniently implemented
Error correction codes, or forward correction codes (FEC)
Designed to detect and correct errors
48
Error Detection Process
Transmitter
For a given frame, an error-detecting code (check bits) is calculated from data bits
Check bits are appended to the data bits
Receiver
Separates incoming frame into data bits and check
bits
Calculates check bits from received data bits
Compares calculated check bits against received check bits
Detected error occurs if mismatch is found
49
Error Detection Process
50
Code Rate and Redundancy
In case of block codes, encoder transforms each k-bit data block into a larger block of n-bits called code bits or channel symbol
The (n-k) bits added to each data block are called redundant bits, parity bits or check bits
They carry no new information
Ratio of redundant bits to data bits: (n-k)/k is called redundancy of code
Ratio of data bits to total bits, k/n is called code rate
51
Block Error Correction Codes
Transmitter
Forward error correction (FEC) encoder maps each
k-bit block into an n-bit block codeword
Codeword is transmitted
Receiver
Incoming signal is demodulated
Block passed through an FEC decoder
52
Forward Error Correction Process
53
FEC Decoder Outcomes
No errors present
Codeword produced by decoder matches original codeword
Decoder detects and corrects bit errors
Decoder detects but cannot correct bit errors;
reports un-correctable error
Decoder detects no bit errors, though errors are present