Download Basics of lower layers

Basics of lower layers
Dmitri A. Moltchanov
E-mail: [email protected]
http://www.cs.tut.fi/kurssit/TLT-2636/
TLT-2636: Wireless networks
D.Moltchanov, TUT, 2011
OUTLINE
• Next generation networks
• The electromagnetic spectrum
• Radio propagation
• Modulation techniques
• Two-way communications
• Centralized multiple access schemes
• Random multiple access schemes
• Error control
• Channel adaptation techniques
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1. NG networks
What a next generation (NG) scenario might or should be?
Network 1
IP BACKBONE
Network N
Network 2
Network 4
Network 3
Figure 1: The foreseen configuration of NG network: a number of interconnected technologies.
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WLAN
AD HOC NETWORKS
NGN BACKBONE
WMAN
3G MOBILE SYSTEMS
BAN/PAN
Figure 2: NG networks: wireless access is the integral part.
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1.1. All-IP concept
NGN: packet-switched, based on TCP/IP protocol suit.
5
Applications
4
TCP/UDP
3
IP
2
Data-link
FEC, ARQ...
1
Physical
ASK, FSK, PSK...
HTTP, FTP, E-MAIL...
TCP, UDP, RTP
Figure 3: Advantages of IP protocol.
IP is characterized by:
• IP allows integration of voice, video, data in a single network;
• IP is independent of data-link and physical layers;
• IP is independent of transport and application layers.
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What advantages IP may bring:
• immediately allows a rich set of applications;
• allows core network to evolve independently;
• allows wireless access to evolve independently.
IP networks:
• beginning of 90s: IP over E1/E3 (T1/T3 in NA);
• middle and late 90s: IP over ATM
• beginning of 00s: IP over SDH
• middle of 00s: IP over MPLS over SDH
Access networks
• wired: ADSL/Cable modem;
• wireless: 802.11b/g/n, EDGE, UMTS, LTE, 802.16d/e
• some via gateways, some with seamless connectivity...
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1.2. Classification of wireless networks
Based on the coverage areas:
• wireless body areas network (WBAN);
• wireless personal area network (WPAN);
• wireless local area network (WLAN);
• wireless wide area network (WWAN).
BAN
PAN
~1 m
~10 m
LAN
WAN
~500 m
WAN
>10 km
Figure 4: Classification of wireless networks based on their coverage area.
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WBAN networks
• what: BAN is the network consisting of wearable computers;
• aim: is to provide the connectivity between wearable computers: headphones, displays, etc.
WPAN networks
• what: PAN is a network in the environment around the person;
• aim: PAN connects BAN devices to other mobile and stationary devices.
WLAN networks
• what: usually, is a network of laptops;
• aim: provide connectivity and Internet access in the high density areas.
WWAN/WMAN networks
• what: is a network of arbitrary mobile devices;
• aim: is to provide connectivity between remote mobile devices.
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2. The electromagnetic spectrum
Wireless communications: broadcast and reception of electromagnetic waves:
• frequency, f :
– number of cycles per second of the wave, measured in Hz.
• wavelength, λ:
– the length of the cycle, measured in meters.
The speed of propagation waves:
• varies from medium to medium;
• in vacuum equals to the speed of light.
The following relation holds:
r = λf,
(1)
• r is the speed of the wave;
• r = c in vacuum.
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Frequency bands defined by International Telecommunication Union (ITU) is shown in Fig. 5.
mostly usable for communications
Infrared
Frequency (Hz)
100
102
104
106
Ultraviolet
108
1010 1012 1014 1016 1018 1020 1022 1024 1026 1028
100
10-2 10-4 10-6
Radio
108
106
104
102
Wavelength (m)
10-8 10-10 10-12 10-14 10-16 10-18 10-20
Visible
Microwave
X-ray
Gamma ray
not usable:
- affect health
- difficult to modulate
- do not propagate through obstacles
Figure 5: Frequency bands in the electromagnetic spectrum.
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Frequency determines properties of the transmission:
• low frequency: pass through the obstacles;
• higher frequency: more prone to absorbtion by rain or fog;
• higher frequency: reflected by obstacles.
Radio waves:
• relatively easy to generate and modulate;
• have the ability to path through the buildings;
• may travel very long distances;
• radio transmission is omni-directional.
Microwave:
• tend to travel in a straight lines;
• can be narrowly focused and concentrated in a small beam;
• cannot pass through obstacles.
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Infrared waves:
• cannot pass through obstacles;
• relatively directional and inexpensive to implement;
• used in short range communications.
Visible light:
• may provide very high bandwidth at a very low cost using optical laser signalling;
• hard to focus a very narrow uni-directional laser;
• cannot penetrate through rain and fog.
Allocation of waves:
• electromagnetic spectrum is a common resource;
• international agreements have been drawn to allocate it;
• national agreements may override them;
• note: remember military usage!
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3. Radio propagation
Radio waves experience the following propagation mechanisms:
• Reflection:
– when: wave hits an objects which is very large compared to its wavelength;
– result: phase shift of 180 degrees between the incident and the reflected rays.
• Diffraction:
– when: wave hits an object that is comparable to its wavelength;
– result: wave bends at the edges of the object, propagating in different directions.
• Scattering:
– when: wave goes through a medium with objects that are small compared to its wavelength;
– result: wave gets scattered into several weaker outgoing signals.
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Figure 6: Illustration of the radio propagation.
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3.1. Propagation models
We distinguish between:
• large-scale propagation models:
– predict the average received signal strength at a given distance from transmitter;
– what: capture path loss component;
– application: estimation of the radio coverage area around the transmitter.
• small-scale propagation models:
– characterize the rapid fluctuations of the received signal strength;
– what: capture influence of multipath components;
– application: performance evaluation of data transmission over the wireless channels.
Historic aspects:
• most are designed for applications in cellular networks;
• some are applicable to other wireless networks (e.g. ad hoc, vehicular).
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Classification:
• analytical models:
– capture path loss based on analytical representation of propagation phenomenons;
– +: allows to get predictions very quickly;
– −: often too complicated;
– −: limited to the complexity of mathematics.
• empirical model:
– based on fitting empirical formulas to a set of statistical data;
– +: implicitly include all propagation phenomenons;
– −: cannot be derived without measurements;
– −: always specific to those environment in which measurements have been carried out.
One more classification:
• indoor models;
• outdoor models.
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3.2. Large-scale propagation (path loss) models
Figure 7: Illustration of the areas with different received local average signal strength.
Examples:
• free-space propagation model;
• two-ray ground reflection model.
See: T. Rappaport, ’Wireless communications,’ 2nd edition, Prentice Hall, 2002.
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3.3. Small-scale propagation (fading) models
Used to represent rapid changes of the received signal strength.
Figure 8: Attenuation due to distance and rapid fluctuations of the received signal strength.
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3.4. Small-scale (fading) models
Depending on presence of LOS received signal strength has:
• LOS: Rician distribution;
• No-LOS: Rayleigh distribution
The Rician distribution is given by:
µ
¶ µ ¶
2
2
γ
(γ + A )
Aγ
p(γ) = 2 exp −
I
,
0
σ
2σ 2
σ2
As A → 0 the Rician distribution degenerates to Rayleigh one:
µ
¶
2
γ
γ
p(γ) = 2 exp − 2 , γ ≥ 0,
σ
2σ
γ ≥ 0,
(2)
(3)
Signal-to-noise (SNR) ratio:
• received signal strength and
• noise (local, interference).
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Nakagami-m fading:
µ
¶
mm γ m−1
mγ
p(γ) =
exp −
,
(E[γ])m Γ(m)
E[γ]
γ ≥ 0,
(4)
• E[γ] is the average received SNR
• Γ(m) is the gamma function
• m is the Nakagami fading parameter
Nakagami-m covers almost all special cases:
• m = 0.5 we get the worst possible fading case
• m = 1 we get Rayleigh statistical fading model
• m > 1 resulting in Rician fading channels
• m → ∞ no fading
Note: how got get symbol (bit) error rate?
• Simon, M. and Alouini, M. ”Digital Communication Over Fading Channels”, Wiley, 2005.
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4. Modulation techniques
Modulation:
• why: information cannot be transferred as is;
• what: converting data into electromagnetic waves;
• how: altering certain properties of the carrier wave.
Classification based on the nature of the data to be transmitted :
• analog modulation techniques:
– amplitude modulation;
– frequency modulation;
– phase modulation.
• digital modulation techniques:
– amplitude shift keying;
– frequency shift keying;
– phase shift keying.
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4.1. Analog modulation
Characteristics:
• used to transmit analog data (e.g. voice);
• perform superimposing analog data signal x(t) on a predefined carrier signal c(t).
Amplitude modulation:
• frequency and phase of the modulating signal remains the same;
• amplitude varies with that of information signal.
Frequency modulation:
• amplitude and phase of the modulating signal remains the same;
• frequency varies with that of information signal.
Phase modulation:
• amplitude and frequency of the modulating signal remains the same;
• phase varies with that of information signal.
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Figure 9: Illustration of the analog amplitude modulation.
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Figure 10: Illustration of the analog frequency modulations.
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Figure 11: Illustration of the analog phase modulations.
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4.2. Digital modulation
Characteristics:
• used to transmit binary data (e.g. PCM voice);
• alters certain properties of transmitting data;
• difference: changes occur at discrete time instants.
There are a number of digital modulation techniques:
• amplitude shift keying (ASK);
• frequency shift keying (FSK);
• phase shift keying (PSK).
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Amplitude shift keying
• ∆t: transmission time of the symbol;
• 1: presence of a carrier for ∆t;
• 0: absence of a carrier ∆t.
Mathematically ASK is represented as:

A cos2πf t, for 1,
c
c
s(t) =
0,
for 0.
(5)
Figure 12: Illustration of the amplitude shift keying.
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Frequency shift keying
Let fc be the carrier frequency and ∆f be the small frequency offset. Then according to FSK:
• 1: presence of carrier with frequency fc + k for a certain time;
• 0: presence of carrier with frequency fc − k for the same time.
Two-levels: binary FSK (BFSK):

A cos2π(f + k)t, for 1,
c
c
s(t) =
Ac cos2π(fc − k)t, for 0.
(6)
Figure 13: Illustration of the binary frequency shift keying (BFSK).
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Phase shift keying
Binary PSK (BPSK): change in phase by π
• 0: presence of carrier with frequency fc for a certain time;
• 1: presence of carrier with a phase difference of π.
Mathematically, two-level PSK (binary PSK, BPSK) is given by:

A cos(2πf t + π), for 1,
c
c
s(t) =
Ac cos2πfc t,
for 0.
(7)
Multiple phase deviation can also be used to encode multiple bits.
Quadrature PSK (QPSK): change in phase by π/2:

π

A
cos(2πf
t
+
),

c
c
4



A cos(2πf t + 3π ),
c
c
4
s(t) =

Ac cos(2πfc t + 5π
),


4



Ac cos(2πfc t + 7π
),
4
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for 10,
for 11,
for 01,
(8)
for 00.
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4.3. x-QAM
Characteristics:
• BPSK, QPSK, 16-QAM, 64-QAM, 256-QAM, etc.
• combines advantages of amplitude and phase keying.
Figure 14: Illustration of 64-QAM signal constellations.
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5. Two-way communications
Two approaches:
• Time division duplex: uplink and downlink on the same frequency;
• Frequency division duplex: uplink and downlink on different frequencies.
downlink
uplink
downlink
uplink
FRAME
FRAME
FRAME
FRAME
time
frequency
Figure 15: Illustration of TDD and FDD principle.
Shortcomings:
• −: guard bands for synchronization: inefficient use of spectrum;
• −: assigned slot may not always be in use: inefficient use of spectrum.
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6. Centralized multiple access techniques
Why we need multiple access scheme:
• bandwidth is a scarce resource at the air interface;
• there are a number of users that want to transmit.
Centralized vs. random access:
• centralized: master controls assignment (WWAN/WMAN);
• random: stations compete for access (WLAN/WPAN/WBAN).
Basically, there are four centralized multiple access schemes:
• frequency division multiple access (FDMA);
• time division multiple access (TDMA);
• code division multiple access (CDMA).
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6.1. Frequency division multiple access
What is the basis:
• shares available bandwidth in the frequency domain;
• available bandwidth is divided into a number of channels;
• there should be a guard band between adjacent channels;
• each tranbsmitter/receiver pair is assigned the same channel for operation.
F1
F2
Fn
Bandwidth (Hz)
Figure 16: Illustration of FDMA principle.
• −: guard bands: inefficient use of spectrum;
• −: assigned slot may not always be in use: inefficient use of spectrum.
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6.2. Time division multiple access
What is the basis:
• shares the available bandwidth in the time domain;
• frequency band is divided into a number of time slots;
• a set of periodically repeated time slots is known as TDMA frame;
• each node is assigned a slot in each frame and transmits only in this slots.
time slot assigned for a transmitter/receiver pair
4
...
4
...
TDMA frame
Figure 17: Illustration of TDMA principle.
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6.3. Spread spectrum techniques
What is the basis:
• every user uses the entire spectrum;
• individual transmission are encoded with a pseudo-random sequences;
• assigned codes are orthogonal so that the simultaneous transmissions are possible.
There are two types of spreads spectrum techniques available:
• frequency hopping spread spectrum (FHSS);
– spectrum is divided into many subchannels;
– two communicating systems hop on same frequencies.
• direct sequence spread spectrum (DSSS).
– stations are assigned orthogonal codes;
– use these codes for transmission;
– other stations transmissions appears an noise.
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7. Random access schemes
What is the basis:
• provide access to a channel for multiple concurrent stations;
• is not needed when there is a centralized control;
• required when the access is distributed;
• can be used for decentralized access in TDMA and FDMA channels.
Basic techniques:
• ALOHA and slotted ALOHA;
• Carrier Sense Multiple Access (CSMA);
• Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA);
• Carrier Sense Multiple Access with Collision Detection (CSMA/CD).
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7.1. ALOHA
Pure ALOHA
• a terminal transmits whenever the user data is ready;
• if the sender finds that the packet get collided:
– it waits for a random period of time;
– sends the packet again.
Throughput: low, depends on stations and traffic they generate.
Slotted ALOHA
• time is slotted, length on the slot is the time to transmit a packet;
• node starts transmission in the beginning of slots only;
• if collision occurs:
– sender waits for a random number of slots;
– transmits packet again.
Throughput: higher than ALOHA but still low.
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7.2. Carrier Sense Multiple Access (CSMA)
Why we need:
• throughput of ALOHA is very low;
• what to do: listen for packet transmissions.
In general, there are three different CSMA schemes:
• 1-persistent CSMA;
• non-persistent CSMA;
• p-persistent CSMA.
1-persistent CSMA
• when the packet is ready for transmission the sender listens to the channel;
• if the channel is free packet is immediately transmitted;
• if not the senders continues to listen till the channel becomes free.
Probability of starting transmission when the channel is free: 1.
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Two bad effects:
• Wrong ’channel free’ effect;
• Synchronization effect
Wrong ’channel free’ effect
• an arbitrary node starts transmitting;
• a node near the destination sense the channel and finds it free since packet has not yet arrived.
Synchronization effect
node 3
start of sensing
t
node 2
start of sensing
t
node 1
t
Figure 18: Illustration of the synchronization effect.
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Non-persistent CSMA
This scheme was introduced to combat with the synchronization problem. It works as follows:
• when the packet is ready for transmission the sender listens the channel;
• if the channel is busy the sender goes in the waiting state for a randomly chosen time;
• after this time the sender sense the channel again.
random waiting
node 3
node 2
start of sensing
start of sensing
random waiting
t
t
node 1
t
Figure 19: Illustration of non-persistent CSMA.
• advantage: probability of collision is less than for 1-persistent CSMA.
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p-persistent CSMA
• the channel is slotted;
• transmission is a free channel is performed with probability p.
The scheme operates as:
• when the packet is ready for transmission the sender listens the channel;
• if the channel is busy the sender keeps listen the channel until it finds the channel idle;
• if the channel is idle:
– the sender transmits the packet in this slot with probability p;
– defers transmission to the next slot with probability q = 1 − p.
p1>p
node 2
node 1
both have a packet
p1<p
p1<p
t
t
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7.3. Carrier sense multiple access with collision detection
What is new:
• capability to detect collisions.
bandwidth is wasted
collision is detected
node 2
node 2
t
node 1
t
node 1
t
t
Figure 20: Illustration of collision detection advantages.
The algorithm operates as follows:
• if the collision is detected the nodes immediately aborts its current transmission;
• then, the node sends a brief jamming signal;
• any other transmitting node on hearing the jamming signal abort their transmissions;
• after transmitting the jamming signal the node waits for a random time and repeats the CSMA.
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8. Error control
BER can be high (around 10E − 4 ∼ 10E − 2):
• using channel coding (adding additional bits;)
• using protocols with retransmission.
We distinguish between following channel coding:
• coding using error detecting codes:
– cyclic redundancy check (CRC).
• coding using error correcting codes:
– block codes (BCH);
– convolutional codes, turbo codes, etc.
Protocols with retransmissions:
• stop-and-wait ARQ (SW-ARQ);
• go-Back-N ARQ (GBN-ARQ);
• selective-repeat ARQ (SR-ARQ).
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9. Channel adaptation mechanisms
Why do we need something else?:
• propagation conditions are too diverse;
• classic error control may not be sufficient;
• classic error control can be redundant.
Which techniques?
• power control
• automatic modulation and coding;
• hybrid ARQ (HARQ, various types);
• spatial diversity (MIMO);
• spatial multiplexing (OFDM, MIMO);
• source rate adaptation.
Note: these techniques makes wireless systems very complicated.
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