Transcript Slide 1

IT351: Mobile & Wireless Computing
Wireless Radio Communications
Objectives:
– To study the wireless radio communication medium, spectrum and signals.
– To study antennas and their role in wireless communications.
– To study the process of wireless signal propagation.
– To introduce basic issues in signal processing and signal modulation.
– To study signal modulation techniques including ASK, FSK and PSK.
– To detail the concept of spread spectrum and study its techniques; FHSS, DSSS.
– To study issues in radio resource management and detail the cellular concept of
channel allocation.
Outline
•
•
•
•
•
•
•
•
The radio spectrum
Signals
Antennas
Signal propagation problems
Multiplexing
Modulation
Spread spectrum
Radio Management
Wireless communications
• The physical media – Radio Spectrum
– There is one finite range of frequencies over which
radio waves can exist – this is the Radio Spectrum
– Spectrum is divided into bands for use in different
systems, so Wi-Fi uses a different band to GSM, etc.
– Spectrum is (mostly) regulated to ensure fair access
Frequencies for communication
•
VLF = Very Low Frequency
•
•
LF = Low Frequency (submarine)
MF = Medium Frequency (radio AM)
link)
HF = High Frequency (radio FM & SW)
UV = Ultraviolet Light
VHF = Very High Frequency (analog TV broadcast)
•
•
•
UHF = Ultra High Frequency (DAB, dig-TV,
mobile phone, GSM)
SHF = Super High Frequency (satellite)
EHF = Extremely High Frequency (direct
Frequency and wave length
–  = c/f
– wave length , speed of light c  3x108m/s, frequency f
twisted
pair
coax cable
1 Mm
300 Hz
10 km
30 kHz
VLF
optical transmission
100 m
3 MHz
LF
MF
HF
1m
300 MHz
10 mm
30 GHz
VHF
SHF
UHF
EHF
100 m
3 THz
infrared
1 m
300 THz
visible light UV
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, beam forming
– large bandwidth available
• Wireless LANs use frequencies in UHF to SHF range
– 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.
Frequencies and regulations
• ITU-R holds auctions for new frequencies, manages frequency bands
worldwide (WRC, World Radio Conferences)
Examples
Europe
USA
Japan
Cellular phones
GSM 880-915, 925960, 1710-1785,
1805-1880
UMTS 1920-1980,
2110-2170
AMPS, TDMA,
CDMA, GSM 824849, 869-894
TDMA, CDMA, GSM,
UMTS 1850-1910,
1930-1990
PDC, FOMA 810-888,
893-958
PDC 1429-1453,
1477-1501
FOMA 1920-1980,
2110-2170
Cordless
phones
CT1+ 885-887, 930932
CT2 864-868
DECT 1880-1900
PACS 1850-1910,
1930-1990
PACS-UB 1910-1930
PHS 1895-1918
JCT 245-380
Wireless LANs
802.11b/g 24122472
802.11b/g 24122462
802.11b 2412-2484
802.11g 2412-2472
Other RF
systems
27, 128, 418, 433,
868
315, 915
426, 868
Wireless communications
• Signals
– Physical representation of data is the signal
– Signals are function of time and location
– In wireless sine waves are used as the basic signal:
• Amplitude: strength of the signal
• Frequency: no of waves generated per second
• Phase shift: where the wave starts and stops
– These factors are transformed into the exactly
required signal by Fourier transforms (complicated
equations that parameterise the sine wave)
Signals
• Sine wave representation
– signal parameters:
parameters representing
the value of data
– signal parameters of
periodic signals:
period T, frequency
f=1/T, amplitude A, phase
shift 
• sine wave as special
periodic signal for a
carrier:
s(t) = At sin(2  ft t + t)
Amplitude
Phase Shift
t
Frequency
Fourier representation of periodic signals
1
1
0
0
t
ideal periodic signal
t
real composition
(based on harmonics)
• It is easy to isolate/ separate signals with different
frequencies using filters
Antennas
• Sending and receiving signals is performed via
antennas
• Role: Radiation and reception of electromagnetic
waves, coupling of wires to space and vice versa for
radio transmission
• Isotropic radiator: equal radiation in all directions
(three dimensional) - only a theoretical reference
antenna
y
z
z
y
x
x
ideal
isotropic
radiator
Antennas
• Real antennas do not produce radiate signals in
equal power in all directions. They always have
directive effects (vertically and/or horizontally)
• Radiation pattern: measurement of
radiation around an antenna
• Most basic antenna is the dipole
– Two antennas both of length /4
(/2 in total)
– Small gap between the two antennas
– Produces an omni-directional signal in
one plane of the three dimensions
Source: Wikipedia
Antennas
• Omni-Directional Antennas are wasteful in areas where
obstacles occur (e.g. valleys)
• Directional antennas reshape the signal to point
towards a target, e.g. an open street
– Placing directional antennas together can be used to form
cellular reuse patterns
• Antennas arrays can be used to increase reliability
(strongest one will be received)
• Smart antennas use signal processing software to adapt
to conditions – e.g. following a moving receiver (known as
beam forming), these are some way off commercially
Antennas
/4
y
y
z
x
z
side view (yz-plane)
top view (xz-plane)
y
y
z
x
z
side view (yz-plane)
simple
dipole
x
side view (xy-plane)
side view (xy-plane)
/2
x
top view (xz-plane)
directed
antenna
Signal propagation
• In perfect conditions (a vacuum)
wireless signals will weaken
predictably
– Transmission range: receivers can
understand enough of the signal (i.e.
low error) for data
– Detection range: receivers hear the
signal but cannot recover the data (i.e.
high error)
– Interference range: there is a signal
but it is indistinguishable from other
noise
• Wireless is less predictable since it has
to travel in unpredictable substances –
air, dust, rain, bricks
sender
transmission
distance
detection
interference
Signal propagation: Path loss (attenuation)
• In free space signals propagate as light in a straight
line (independently of their frequency).
• If a straight line exists between a sender and a
receiver it is called line-of-sight (LOS)
• Receiving power proportional to 1/d² in vacuum (free
space loss) – much more in real environments
(d = distance between sender and receiver)
• Situation becomes worse if there is any matter
between sender and receiver especially for long
distances
– Atmosphere heavily influences satellite transmission
– Mobile phone systems are influenced by weather
condition as heavy rain which can absorb much of the
radiated energy
Signal propagation
• Radio waves can penetrate objects depending on frequency.
The lower the frequency, the better the penetration
– Low frequencies perform better in denser materials
– High frequencies can get blocked by, e.g. Trees
• Radio waves can exhibit three fundamental propagation
behaviours depending on their frequencies:
– Ground wave (<2 MHz): follow the earth surface and can
propagate long distances – submarine communication
– Sky wave (2-30 MHz): These short waves are reflected at the
ionosphere. Waves can bounce back and forth between the
earth surface and the ionosphere, travelling around the world –
International broadcast and amateur radio
– Line-of-sight (>30 MHz): These waves follow a straight line of
sight – mobile phone systems, satellite systems
Additional signal propagation effects
•
•
•
•
•
Receiving power additionally influenced by
fading (frequency dependent): signals can change as the receiver moves
Blocking/ Shadowing: large objects may block signals (building,..etc)
Reflection: waves can bounce off dense objects
Refraction: waves can bend through objects depending on the density of
a medium
• scattering : small objects may reflect multiple weaker signals
• diffraction at edges
shadowing
reflection
refraction
scattering
diffraction
Multipath propagation
• Signal can take many different paths between sender
and receiver due to reflection, scattering, diffraction,…
multipath
LOS pulses pulses
signal at sender
signal at receiver
• Different signals use different length paths
• The difference is called delay spread
– Systems must compensate for the delay spread
– Interference with “neighbor” symbols, Inter Symbol
Interference (ISI)
• Symbols may cancel each other out
• Increasing frequencies suffer worse ISI
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
power
fading)
long term
fading
• Additional changes in
– distance to sender
– obstacles further away
short term fading
–  slow changes in the average
power received (long term fading)
t
Multiplexing
• Multiplexing describes how several users can share a
medium with minimum or no interference
• It is concerned with sharing the frequency range amongst
the users
• Bands are split into channels
• Four main ways of assigning channels
– Space Division Multiplexing (SDM) : allocate according to location
– Time Division Multiplexing (TDM): allocate according to units of time
– Frequency Division Multiplexing (FDM): allocate according to the
frequencies
– Code Division Multiplexing (CDM) : allocate according to access
codes
• Guard Space: gaps between allocations
Multiplexing
channels ki
• Multiplexing in 4 dimensions
–
–
–
–
space (si)
time (t)
frequency (f)
code (c)
k1
k2
k3
k4
k5
k6
c
t
c
t
s1
f
s2
• Goal: multiple use
of a shared medium
• Important: guard spaces needed!
f
c
t
s3
f
Space Division Multiplexing (SDM)
• Space Division
– This is the basis of
frequency reuse
– Each physical space is
assigned channels
– Spaces that don’t
overlap can have the
same channels
assigned to them
– Example: FM radio
stations in different
countries
channels ki
k1
k2
k3
k4
k5
k6
c
t
c
t
s1
f
s2
f
c
t
s3
f
Frequency Division Multiplexing (FDM)
• Separation of the whole spectrum into smaller non
overlapping frequency bands (guard spaces are needed)
• A channel gets a certain band of the spectrum for the
whole time – receiver has to tune to the sender
frequency
• Advantages
k
k
k
k
k
k
– no dynamic coordination
necessary
– works also for analog signals
• Disadvantages
– waste of bandwidth
if the traffic is
distributed
unevenly
t
– inflexible
1
2
3
4
5
6
c
f
Time Division Multiplexing (TDM)
• A channel gets the whole spectrum for a certain amount
of time
• Guard spaces (time gaps) are needed
• Advantages
– only one carrier in the
medium at any time
– throughput high even
for many users
• Disadvantages
– precise clock
synchronization
necessary
t
k1
k2
k3
k4
k5
k6
c
f
Time and frequency multiplexing
• Combination of both methods
• A channel gets a certain frequency band for a certain
amount of time
• Example: GSM
• Advantages
k
k
k
k
– better protection against
tapping
– protection against frequency
selective interference
• but: precise coordination
required
t
1
2
3
4
k5
k6
c
f
Code Division Multiplexing (CDM)
• Code Division
– Instead of splitting the channel, the receiver is told
which channel to access according to a pseudo-random
code that is synchronised with the sender
– The code changes frequently
– Security: unless you know the code it is (almost)
impossible to lock onto the signals
– Interference: reduced as the code space is huge
– Complexity: very high
Code multiplexing
• Each channel has a unique code
k1
k2
k3
k4
• All channels use the same spectrum
at the same time
• Advantages
k6
c
– bandwidth efficient
– no coordination and synchronization
necessary
– good protection against interference
and tapping
• Disadvantages
k5
f
t
– precise power control required
– more complex signal regeneration
• Implemented using spread spectrum technology
Modulation
• Definition: transforming the information to be transmitted
into a format suitable for the used medium
• The signals are transmitted as a sign wave which has three
parameters: amplitude, frequency and phase shift.
• These parameters can be varied in accordance with data or
another modulating signal
• Two types of modulation
– Digital modulation: digital data (0, 1) is translated into an analog
signal (baseband signal)
– Analog modulation: the center frequency of the baseband signal
generated by digital modulation is shifted up to the radio carrier
Why we need digital modulation?
• Digital modulation is required if digital data has to be
transmitted over a medium that only allows analog
transmission (modems in wired networks).
• Digital signals, i.e. 0/1, can be sent over wires using voltages
• Wireless must use analogue sine waves
• This translation is performed by digital modulation
–
–
–
–
digital data is translated into an analog signal (baseband)
Shift Keying is the translation process
Amplitude, Freq., Phase Shift Keying (ASK/FSK/PSK)
differences in:
• spectral efficiency: how efficiently the modulation scheme utilizes the
available frequency spectrum
• power efficiency: how much power is needed to transfer bits
• Robustness: how much protection against noise, interference and multipath propagation
Why we need analogue modulation ?
• Analogue modulation then moves the signal into the
right part of the channel
– Motivation
• smaller antennas (e.g., /4)
• Frequency Division Multiplexing
• medium characteristics – path loss, penetration of objects,
reflection,..etc
– Basic schemes
• Amplitude Modulation (AM)
• Frequency Modulation (FM)
• Phase Modulation (PM)
Modulation and demodulation
digital
data
101101001
digital
modulation
analog
baseband
signal
analog
modulation
radio transmitter
radio
carrier
analog
demodulation
radio
carrier
analog
baseband
signal
synchronization
decision
digital
data
101101001
radio receiver
Digital Modulation - Amplitude Shift
Keying (ASK)
• Amplitude Shift Keying (ASK)
– 0 and 1 represented by different amplitudes
• i.e. a basic sine wave
– Problem: susceptible to interference
– Constant amplitude is hard to achieve
– ASK is used for optical transmissions such as infra-red
and fibre (simple + high performance)
– In optical  light on = 1 light off = 0
Digital Modulation - Frequency Shift
Keying (FSK)
• Frequency Shift Keying (FSK)
– 0 and 1 represented by
different frequencies
– Switch between two oscillators
accordingly
– Twice the bandwidth but more
resilient to error
Digital Modulation - Phase Shift Keying
(PSK)
• Phase Shift Keying (PSK)
– 0 and 1 represented by different (longer) phases
– Flip the sine wave 180 to switch between 0/1
– Better still than FSK but more complex
• Other modulation schemes are mostly complex variants of
ASK, FSK, or PSK…
Digital modulation - summary
• Modulation of digital signals known as Shift Keying
1
0
1
• Amplitude Shift Keying (ASK):
– very simple
– low bandwidth requirements
– very susceptible to interference
t
1
0
1
• Frequency Shift Keying (FSK):
– needs larger bandwidth
– more error resilience than AM
• Phase Shift Keying (PSK):
– more complex
– robust against interference
t
1
0
1
t
Analog modulation
• Definition: Impress an information-bearing analog
waveform onto a carrier waveform for transmission
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
– Advantage: protection against narrow band interference
power
interference
power
spread
signal
detection at
receiver
f
signal
spread
interference
f
• Side effects:
– coexistence of several signals without dynamic coordination
– tap-proof
Spread spectrum
• Basic idea
–
–
–
–
Spread the bandwidth needed to transmit data
Lower signal power, more bandwidth, same energy
Resistant to narrowband interference
Steps
•
•
•
•
•
•
Apply spreading (convert narrow band to broadband)
Send low power spread signal
Signal picks up interference
Receiver can de-spread signal
Signal is more powerful than remaining interference
Signal is therefore able to be interpreted
Effects of spreading and interference
dP/df
dP/df
i)
user signal
broadband interference
narrowband interference
ii)
f
sender
dP/df
f
dP/df
dP/df
iii)
iv)
f
receiver
v)
f
f
Spreading and frequency selective fading
channel
quality
2
1
5
3
6
narrowband channels
4
frequency
narrow band
signal
guard space
channel
quality
1
2
2
2
spread
spectrum
2
2
spread spectrum channels
frequency
Spread spectrum problems
• Spread spectrum problems:
– Increased complexity of receivers
– Raising background noise
• Spread spectrum can be achieved in two different
ways:
– Direct Sequence
– Frequency Hopping
Spread Spectrum – Direct Sequence
Spread Spectrum (DSSS)
• Each bit in original signal is represented by multiple
bits in the transmitted signal
• Spreading code spreads signal across a wider
frequency band
• 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
DSSS
• Chipping sequence appears
like noise, to others
• Spreading factor S = tb /tc
• If the original signal needs a
bandwidth w, the resulting
signal needs s*w
• The exact codes are
optimised for wireless
– E.g. for Wi-Fi 10110111000
(Barker code)
– For civil application spreading
code between 10 and 100
– For military application the
spreading code is up to 10,000
tb
user data
0
1
XOR
tc
chipping
sequence
01101010110101
=
resulting
signal
01101011001010
tb: bit period
tc: chip period
DSSS
• New modulation process:
– Sender: Chipping  Digital Mod.  Analog Mod.
– Receiver: Demod.  Chipping  Integrator  Decision?
• At the receiver after the XOR operation (despreading), an
integrator adds all these products, then a decision is taken
for each bit period
• Even if some of the chips of the spreading code are
affected by noise, the receiver may recognize the
sequence and take a correct decision regarding the
received message bit.
DSSS (Direct Sequence Spread Spectrum)
spread
spectrum
signal
user data
transmit
signal
X
modulator
chipping
sequence
radio
carrier
transmitter
correlator
lowpass
filtered
signal
received
signal
demodulator
radio
carrier
data
X
chipping
sequence
receiver
sampled
sums
products
integrator
decision
Spread Spectrum – Frequency Hopping
Spread Spectrum (FHSS)
• Uses entire bandwidth for signals
• Signal is broadcast over seemingly random series of radio
frequencies
– A number of channels allocated for the FH signal
– Width of each channel corresponds to bandwidth of input signal
• Signal hops from frequency to frequency at fixed intervals
– Transmitter operates in one channel at a time
– At each successive interval, a new carrier frequency is selected.
Pattern of hopping is the hopping sequence
– Time on each frequency is the dwell time
– Fast hopping = many hops per bit
– Slow hopping = many bits per hop
– Fast hopping is more robust but more complex
– FHSS is used in Bluetooth - 1600 hops/s, 79 channels
Frequency Hopping Spread Spectrum
(FHSS)
• Process 1 - Spreading code modulation
– The frequency of the carrier is periodically modified
(hopped) following a specific sequence of frequencies.
– In FHSS systems, the spreading code is this list of
frequencies to be used for the carrier signal, the
“hopping sequence”
– The amount of time spent on each hop is known as
dwell time and is typically in the range of 100 ms.
• Process 2 - Message modulation
– The message modulates the (hopping) carrier, thus
generating a narrow band signal for the duration of
each dwell, but generating a wide band signal if the
process is regarded over periods of time in the range
of seconds.
FHSS
• Discrete changes of carrier frequency
– sequence of frequency changes determined via
pseudo random number sequence
• 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
• Disadvantages
– not as robust as DSSS
– simpler to detect
FHSS
tb
user data
0
1
f
0
1
1
t
td
f3
slow
hopping
(3 bits/hop)
f2
f1
f
t
td
f3
fast
hopping
(3 hops/bit)
f2
f1
t
tb: bit period
td: dwell time
FHSS (Frequency Hopping Spread
Spectrum)
narrowband
signal
spread
transmit
signal
user data
modulator
modulator
frequency
synthesizer
transmitter
received
signal
narrowband
signal
data
demodulator
hopping
sequence
hopping
sequence
frequency
synthesizer
demodulator
receiver
Resource Management
• Radio Resource Management
– Channel Access
– Channel Assignment
• Power Management
• Mobility Management
– Location Management
– Handoff/Handover: the term handover or handoff
refers to the process of transferring an ongoing call or
data session from one channel to another
• Example: The cellular System
The cellular system: cell structure
• Channel allocation: Implements space division multiplexing
(SDM)
– base station covers a certain transmission area (cell)
– Cellular concept: channel reuse across the network prevents
interference, improves the likelihood of a good signal in each cell
• Mobile stations communicate only via the base station
• Advantages of cell structures
–
–
–
–
f5
f
f2
f
4
6
higher capacity, higher number of users
f1
f3
f7
less transmission power needed
f2
more robust, decentralized
base station deals with interference, transmission area etc. locally
• Problems
–
–
–
–
f3
Expensive
fixed network needed for the base stations
handover (changing from one cell to another) necessary
interference with other cells
f5
f4
f1
Frequency planning
• Frequency reuse only with a certain distance
between the base stations
• Cell sizes from some 100 m in cities to, e.g., 35 km
on the country side (GSM) - even less for higher
frequencies
• Cells are combined in clusters
• All cells within a cluster use disjointed sets of
f
frequencies
f
• The transmission power of a sender has to f f f
f
f
be limited to avoid interference
f
• Standard model using 7 frequencies
• To reduce interference further, sectorized antennas
can be used especially for larger cell radii
3
5
4
f2
f5
6
1
3
7
2
f4
f1
Frequency planning
f3
f3
f2
f1
f2
f1
f3
f2
f1
f3
f2
f2
3 cell cluster
f3
f2
f3
f5
f4
f1
f1
f3
f3
f2
f6
f1
f3
f3
f5
f4
f7
f1
f3
f2
f6
f7
f5
f2
7 cell cluster
f2
f2
f2
f1 f
f1 f
f1 f
h
h
3
3
3
h1 2
h1 2
g2 h3 g2 h3
g2
g1
g1
g
1
g3
g3
g3
3 cell cluster
with 3 sector antennas
Radio resource management
• Channel Allocation
– Channel Allocation is required to optimise frequency
reuse
• Fixed Channel Allocation
• Dynamic Channel Allocation
• Hybrid Channel Allocation
3
1
6
1
Frequency Reuse
6
5
2
7
2
4
3
7
Radio resource management : Channel
allocation
• Fixed Channel Allocation (FCA)
– Permanent or semi-permanent allocation
– Certain frequencies are assigned to a certain cell
– Problem: different traffic load in different cells
– Methods:
• Simple: all cells have same number of channels
• Non-uniform: optimise usage
according to expected traffic
3
• Borrowing: channels can be
6
reassigned if underused
1
5
(BCA)
Frequency Reuse
4
3
1
7
2
7
2
6
Radio resource management
• Dynamic Channel Allocation (DCA)
– Gives control to base stations / switches to adapt
– Channels are assigned as needed, not in advance
– Base station chooses frequencies depending on the
frequency already used in neighbour cells
– Channels are returned when user has finished
– More capacity in cells with more traffic
– Assignment can also be based on interference
measurements
– Affecting factors include:
• Blocking probability
• Usage patterns and reuse distance
• Current channel measurement
Radio resource management
• Hybrid Channel Allocation (HCA)
– Fixed schemes are not flexible enough
– Dynamic schemes are too complex / difficult
– Hybrid Schemes:
• Split resources into pools of fixed and dynamic
channels
• Assign core of fixed channels then allocate rest
dynamically
• Altering the ratio may optimise the system
– E.g. produce the lowest blocking rate
Radio resource management
• Overlapping Cells
– Cells are naturally overlap (ideal shape is circular)
– System may push some users into adjacent cells
– Cost: increased handoff rate
• Handoff
– Two types of channel assignment: new calls,
handoff
– New calls have lower priority than handoff calls
– QoS Channel Access Control should favour
handoff over new
Radio resource management
• Macrocell/Microcell Overlay
– Smaller cells increases frequency of handoff
– Overlaying large cells on top of small ones:
•
•
•
•
•
Fast moving terminals are assigned channels in Macrocells
Slow moving terminals can use microcells
Overlap can be used to handoff during congestion
Increases the capacity (area)
But, increases the complexity
CDM cellular systems: Cell breathing
• CDM instead of FDM. Do not need elaborate channel
allocation schemes and complex frequency planning.
• Cell size depends on current load: cell breathe
• Additional traffic appears as noise to other users
• If the noise level is too high users drop out of cells