Transcript Telecommunications Engineering Topic 2: Modulation and FDMA

Telecommunications
Engineering
Topic 5: Wireless
Architectures
James K Beard, Ph.D.
[email protected]
http://astro.temple.edu/~jkbeard/
April 25, 2005
Topic 5
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Essentials
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Text: Simon Haykin and Michael Moher, Modern
Wireless Communications
SystemView
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Web Site
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Use the full version in E&A 603A for your term project
URL http://astro.temple.edu/~jkbeard/
Content includes slides for EE320 and EE521
SystemView page
A few links
Office Hours
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E&A 349
Hours Tuesday afternoons 3:00 PM to 4:30 PM
MWF 10:30 AM to 11:30 AM
Others by appointment; ask by email
April 25, 2005
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Topics
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Architecture topics
 Open
System Interconnection (OSI) model
 Power control
 Handover
 The Network Layer
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Other areas from earlier chapters are
reviewed
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Open System Interconnection
(OSI) Model
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Seven-layer model
 Physical
layer (modem)
 Data link layer
 Network layer
 Transport layer (packetizing, ACK/NAK)
 Session layer (Service selection and access)
 Presentation layer (encryption, compression)
 Application layer (HMI)
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Layers designed together as a system
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Example 7.1: E-mail and the
Seven-Layer Model
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Application layer – e-mail client software
Presentation layer – compression and
encryption (SSH)
Session layer – interface with host
Transport layer – TCP interface, IP addressing
Network layer – routing, adds header
Data link layer – adds header and addresses of
host, adds CRD bits; medium access layer
(MAC) selects free channel and passes to…
Physical layer – FEC and modulation, yet
another header
April 25, 2005
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Power Control Architectures
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Open Loop
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Closed Loop
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Mobile terminals measure strength of pilot channel
Transmit power decreased for strong pilot channels
Fast and simple, but must be approximate
Base station measures mobile terminal signal strength
Mobile station receives signal strength by downlink
Accurate but delay and averaging must be smaller than channel
coherence time
Outer Loop Control
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Base station uses expected signal strength in control algorithm
Complexity can result in a slow loop
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Power Control: Summary
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Power control minimizes SINR in busy cells
Handset power control minimizes SINR in the
base station but not at the mobile terminal
Methods still evolving
Next generation standards will implement
 Newer
techniques such as outer-loop control
 Base station power control for SINR control at the
mobile station
April 25, 2005
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Example 7.3: The Near-Far
Problem
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Mobile terminal distance to base station varies
from 100 m to 10 km
Power differences
 Given
a path loss exponent of 4
 Difference in received power at base station is 80 dB
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Spreading rate of 128 million required to prevent
jamming of weaker user
Solution is power control
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Handover Issues
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Purpose
 Address
operational transition of mobile
terminals between cells
 Maintain continuity of calls
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Calls are dropped in handover because
 Mobile
station signal strength drops too low
before handover is completed
 The new cell doesn’t have a free channel
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Handover Techniques
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Start handover when signal strength is
decreasing but a margin still exists
 Common technique with first-generation systems
 Margin can be small with second-generation systems
that switch cells quickly
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Mobile assisted
 Use base station signal strength in handover logic
 Avoids cell dragging in which mobile station operates
well into another cell, and causes interference with
other mobile stations
April 25, 2005
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Handover Multiple-Access
Issues
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FDMA and TDMA
 Mobile
station must change signaling channels and
traffic channels in handover
 Called hard handover
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CDMA
 Signaling channels are
 Called soft handover
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the same during handover
SDMA
 Switch
stations when mobile station transitions
between beam boundaries
 Can become complex when base station tracks users
with steerable beams
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The Network Layer
Components
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Base station
 RF
links to mobile terminals
 RF, wire, fiber or other links to mobile switching center
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Switching Center
 Handles
billing and authorization
 Executes interconnects between base stations, other
networks, or land line telecommunications
April 25, 2005
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Mobile Switching Center
Functions
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Billing and authorization
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Counts the minutes
Determines roaming status and finds home station/account
Rings the cash register
Modifies routing where appropriate
Interface between cellular and public land line telephone
networks
Overall supervision of mobile access control (MAC)
wireless communications network
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Power control functions
Handover
Provide data capability to mobile terminals
April 25, 2005
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Indoor LANs
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Terminology
 Cells
are service sets
 User terminals are stations
 Base stations are stations
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Peculiarities
 Often
design and growth is ad hoc without
planning
 Dissimilar packet sizes through network
 Wired and 802.11 terminals on same station
April 25, 2005
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Physical Layer for Various MAC
Standards (Table 7.2 p. 470)
GPRS
935-960 MHz (F)
890-915 MHz (R)
200 kHz
GMSK
WCDMA
1920-1980 MHz (F)
2110-2117 MHz (R)
5 MHz
QPSK
Data rates
Access
strategy
Cell size
FEC
Up to 116 kbps
FDMA/TDMA
Up to 35 km
Variable, including
rate-1/2
convolutional
Up to 2 Mbps
FDMA/CDMA/FDD
FDMA/CDMA/TDD
< 35 km
Variable, including
rate-1/2, 1/3
convolutional
Frame size
4.61 ms
10 ms
Frequency
Band
Channel BW
Modulation
IEEE 802.11b
2.4 GHz
50 MHz
BPSK/QPSK
FH or DS
Up to 11 Mbps
CSMA/CA
1-20 m
Rate ½, 1/3
convolutional
Up to 20 ms
Bluetooth
2.4-2.4385
GHz
80 MHz
GMSK/FH
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BPSK,…,64QAM/OFDM
Up to 54 Mbps
FDMA/CSMA
M 1 Mbps
FH/TDD
1-1- m
Variable;
repetition,
Hamming,
ARQ
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a
Physical Layer for Various Data
Network Standards (Table 7.3)
Frequency Band
DECT
1880-1900 MHz
Channel BW
Modulation
Data rates
Access strategy
1.728 MHz
GMSK
1.152 Mbps
FDMA/TDMA/TDD
Cell size
FEC
< 300 m
None (16-bit CRC)
Frame size
Voice encoding
10 ms
ADPCM at 32 kHz
< 35 km
Variable, including
rate-1/2
convolutional
4.61 ms
RELP at 13 kbps
Traffic channels per
RF channel
Diversity
12
8
Up to 63
WCDMA
1920-1980 MHz (F)
2110-2117 MHz (R)
5 MHz
QPSK
Up to 2 Mbps
FDMA/CDMA/FDD
FDMA/CDMA/TDD
< 35 km
Variable, including
rate-1/2, 1/3
convolutional
10 ms
Adaptive multirate
ACELP 4.75 to 12.2
kbps
Depends on data rate
Antenna diversity at
base station
Frequency hopping
Spread spectrum
with RAKE
receiver
Space-time block
coding with transmit
diversity
April 25, 2005
GSM
935-960 MHz (F)
890-915 MHz (R)
200 kHz
GMSK
270.8 kbps
FDMA/TDMA/FH
IS-95
869-894 MHz (F)
824-849 (R)
1.25 MHz
BPSK
1200-9600 bps
FDMA/CDMA
< 35 km
Variable, including
rate-1/2, 1/3
convolutional
20 ms
CELP at 9.6 kbs
and 14.4 kbps
Topic 5
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Theme Example 5: 802.11
(Wi-Fi) Pages 328-331
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Timeline
 User
station (STA) logs onto local base station (AP),
AP authenticates STA and provides ID
 STA listens
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Inactive channel – STA sends RTS, AP sends CTS
Active channel – listens for gap and sends packet
 STA fragments
and sends packet
 AP reassembles packet and sends to network layer
 AP disassembles packet from network layer and
sends to STA
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Random time access (like Ethernet)
April 25, 2005
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(5)(7) Convolutional Code with
Hard Decoding
S ys te mV ie w
w3, P S K (c o he re n t)
0
1 .0 0 e+0
2
4
2
4
6
8
6
8
1 .0 0 e-1
BER
1 .0 0 e-2
1 .0 0 e-3
1 .0 0 e-4
0
E b/ No in d B
April 25, 2005
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(5)(7) Convolutional Code with
Soft Decoding
S ys te mV ie w
w3, P S K (c o he re n t)
0
1 .0 0 e+0
2
4
2
4
6
8
6
8
1 .0 0 e-1
BER
1 .0 0 e-2
1 .0 0 e-3
1 .0 0 e-4
0
E b/ No in d B
April 25, 2005
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Problem 2.59 Page 102
When G. Marconi made the first radio
transmission in 1899 across the Atlantic
Ocean, he used all of the spectrum available
worldwide to transmit a few bits per second. It
has been suggested that, in the period since
then, spectrum usage (bits/s/Hz worldwide) has
increased by a factor of a million. List the
factors that have resulted in this substantial
increase. Which factor will likely result in the
largest increase in the future?
April 25, 2005
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Factors That Increase Spectral Usage
Factor
Antenna gain
Modulation
Filters
Oscillators
Semiconductor
technology
Digital processing
Coding
April 25, 2005
Comment
More directionality means more frequency reuse
Increased spectral efficiency leads to a more compact spectrum and
less interference with adjacent channel users. Constant envelope
schemes are immune to amplifier nonlinearities, therefore less
spectral growth. Modulated pulse schemes provide more compact
spectra.
Match filtering improvements allow for steeper band rolloffs and a
more compact spectral shape
Crystal oscillators allow operation in the high GHz range. Stability
and accuracy of oscillators also help improve compactness of
spectrum.
Improved switching speeds and good immunity to RFI and EMI.
Circuit design methods have improved, thus reducing susceptibility
to interference.
Advanced algorithms have been deployed to track phase and gain
with great efficiency resulting in low implementation losses. Also,
advanced algorithms allow operation in environments with higher
levels of interference; thereby increasing the amount of frequency
reuse.
Error correcting codes such as Viterbi and Turbo Codes allow large
improvements in spectral efficiency. Receivers that use ECC can
operate at lower power and in the presence of more interference
than those receivers without ECC.
Topic 5
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Problem 3.2 Page 110
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Consider the sinusoidal modulating signal
m t   Am  cos  2  fm  t 
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Show that the use of double sideband,
suppressed carrier (DSB-SC) modulation
produces a pair of side frequencies, one at fc+fm
and the other at fc-fm, where fc is the carrier
frequency. What is the condition that the
modulator has to satisfy in order to make sure
that the two side-frequencies do not overlap?
April 25, 2005
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Solution for Problem 3.2
s  t   Ac  m  t   cos  2  fc  t 
 Ac  Am  cos  2  fm  t   cos  2  fc  t 

1
  Ac  Am  cos  2   fc  fm    cos  2   fc  fm  
2
April 25, 2005
Topic 5
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Polynomial Arithmetic Modulo 2
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Integer arithmetic modulo 2
 Add,
subtract, multiply integers
 Take this result modulo 2
 Solution is always 0 or 1
 Division? Reciprocal of odd numbers is 1

Polynomial arithmetic modulo 2
 Integers
are coefficients of polynomials
 Perform polynomial arithmetic as usual
 Take coefficients of result modulo 2
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Examples
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Two polynomials
x 5  x 3  x  1  101011
x 4  x 3  x 2  1  011101
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Multiplying them
x9  x8  2  x7  x6  3  x5  2  x 4  3  x3  x 2  x  1
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Taking the result modulo 2
x  x  x  x  x  x  x  1  1101101111
9
April 25, 2005
8
6
5
3
2
Topic 5
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Finite Fields
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Example
 Integer
arithmetic modulo 7
 Elements are {0,1,2,3,4,5,6}
 Reciprocals pairs are (1,1), (2,4), (3,5), (6,6)
 Division is defined as multiplication by
reciprocal
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All integer arithmetic modulo a prime
defines a finite field
April 25, 2005
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Vector Extensions of Finite
Fields
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Sometimes called polynomial fields or Galois
fields
The exist for orders N equal to any power k of a
prime p: N=pk
Arithmetic
 Elements
are characterized as the coefficients of a
polynomial of order k-1
 Addition and subtraction is done modulo p
 Multiplication is defined as modulo a generating
polynomial of order k
April 25, 2005
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Defining Characteristics of
Galois Fields
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Successive multiplication by x
 Begin
with 1
 Steps through all N elements except zero
 A sequence of length N-1
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A reciprocal
 Defined
as producing 1 as a product
 Always exists
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Division is defined as multiplication by reciprocal
April 25, 2005
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Special Case for Signal
Processing
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Galois fields of order 2k
The series of coefficients is a sequence of k
zeros and ones
Addition and subtraction
 Are
identical operations in this field
 Result is a bit-by-bit XOR

Multiplication
 Modulo
a generating polynomial of order k
 Generating polynomial can be added or subtracted
 Table 5.1 page 272 lists some generating polynomials
April 25, 2005
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Generation with Shift Registers
Basis is a shift register with k latches
 Shifting is equivalent to multiplication by x
 A 1 shifted out

 Fed
back in according to the 1s in the
generating polynomial
 Addition is done with an XOR
April 25, 2005
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Example
x

x2
x3
Generating Polynomial:
x  x 1
3
April 25, 2005
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Definitions of Orthogonality
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Vectors with arithmetic modulo 2
 Addition
of two orthogonal vectors gives the zero
vector
 A set of vectors that is closed on addition has the
properties
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Sum of any two is another in the set
The zero vector is always included
 A basis set has the property
 Sum of any two is never another in the basis set or zero
 The opposite of closed on addition
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Orthogonal signals
April 25, 2005
 1, i  j
 si t   s j t   dt  0, i  j
Topic 5
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Next Time
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Assignment:
 Look
at the study guide
 Go over the slides to date
 Look particularly at Chapters 4 and 7
 Make up a list of questions
 Send them to me by email:
[email protected]
April 25, 2005
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