telecommunications engineering topic 5: wireless architectures

April 25, 2005 Topic 5 1

Telecommunications EngineeringTopic 5: Wireless ArchitecturesJames K Beard, Ph.D.

[email protected]

http://astro.temple.edu/~jkbeard/

Topic 5 2April 25, 2005

Essentials Text: Simon Haykin and Michael Moher, Modern

Wireless Communications SystemView

Use the full version in E&A 603A for your term project Web Site

URL http://astro.temple.edu/~jkbeard/ Content includes slides for EE320 and EE521 SystemView page A few links

Office Hours 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


Topics

Architecture topicsOpen System Interconnection (OSI) modelPower controlHandoverThe Network Layer

Other areas from earlier chapters are reviewed


Open System Interconnection (OSI) Model Seven-layer model

Physical layer (modem)Data link layerNetwork layerTransport layer (packetizing, ACK/NAK)Session layer (Service selection and access)Presentation layer (encryption, compression)Application layer (HMI)

Layers designed together as a system


Example 7.1: E-mail and the Seven-Layer Model 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


Power Control Architectures

Open Loop Mobile terminals measure strength of pilot channel Transmit power decreased for strong pilot channels Fast and simple, but must be approximate

Closed Loop 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

Base station uses expected signal strength in control algorithm Complexity can result in a slow loop


Power Control: Summary

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


Example 7.3: The Near-Far Problem 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

Spreading rate of 128 million required to prevent jamming of weaker user

Solution is power control


Handover Issues

PurposeAddress operational transition of mobile

terminals between cellsMaintain continuity of calls

Calls are dropped in handover becauseMobile station signal strength drops too low

before handover is completedThe new cell doesn’t have a free channel


Handover Techniques

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 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


Handover Multiple-Access Issues FDMA and TDMA

Mobile station must change signaling channels and traffic channels in handover

Called hard handover CDMA

Signaling channels are the same during handover Called soft handover

SDMA Switch stations when mobile station transitions

between beam boundaries Can become complex when base station tracks users

with steerable beams


The Network Layer Components Base station

RF links to mobile terminals RF, wire, fiber or other links to mobile switching

center

Switching Center Handles billing and authorization Executes interconnects between base stations, other

networks, or land line telecommunications


Mobile Switching Center Functions Billing and authorization

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 Power control functions Handover Provide data capability to mobile terminals


Indoor LANs

TerminologyCells are service setsUser terminals are stationsBase stations are stations

PeculiaritiesOften design and growth is ad hoc without

planningDissimilar packet sizes through networkWired and 802.11 terminals on same station


Physical Layer for Various MAC Standards (Table 7.2 p. 470) GPRS WCDMA IEEE 802.11b Bluetooth IEEE 802.11a Frequency Band

935-960 MHz (F) 890-915 MHz (R)

1920-1980 MHz (F) 2110-2117 MHz (R)

2.4 GHz 2.4-2.4385 GHz

5.2 GHz

Channel BW 200 kHz 5 MHz 50 MHz 80 MHz 20 MHz Modulation GMSK QPSK BPSK/QPSK

FH or DS GMSK/FH BPSK,…,64-

QAM/OFDM Data rates Up to 116 kbps Up to 2 Mbps Up to 11 Mbps M 1 Mbps Up to 54 Mbps Access strategy

FDMA/TDMA FDMA/CDMA/FDD FDMA/CDMA/TDD

CSMA/CA FH/TDD FDMA/CSMA

Cell size Up to 35 km < 35 km 1-20 m 1-1- m 1-100 m FEC Variable, including

rate-1/2 convolutional

Variable, including rate-1/2, 1/3 convolutional

Rate ½, 1/3 convolutional

Variable; repetition, Hamming, ARQ

Rate ½, 1/3, 3/4 convolutional

Frame size 4.61 ms 10 ms Up to 20 ms n X 625 μs n=1,2,3,4,5

24 μs to 5 mx

Diversity Frequency hopping Space-time block coding with transmit diversity

Dual antenna None Dual antenna

Network topology

Point-to-multipoint/cellular

Point-to-multipoint/cellular

Point-to-multipoint

Point-to-point connection and connectionless

Point-to-multipoint


Physical Layer for Various Data Network Standards (Table 7.3) DECT GSM IS-95 WCDMA Frequency Band 1880-1900 MHz 935-960 MHz (F)

890-915 MHz (R) 869-894 MHz (F) 824-849 (R)

1920-1980 MHz (F) 2110-2117 MHz (R)

Channel BW 1.728 MHz 200 kHz 1.25 MHz 5 MHz Modulation GMSK GMSK BPSK QPSK Data rates 1.152 Mbps 270.8 kbps 1200-9600 bps Up to 2 Mbps Access strategy FDMA/TDMA/TDD FDMA/TDMA/FH FDMA/CDMA FDMA/CDMA/FDD

FDMA/CDMA/TDD Cell size < 300 m < 35 km < 35 km < 35 km FEC None (16-bit CRC) Variable, including

rate-1/2 convolutional



Frame size 10 ms 4.61 ms 20 ms 10 ms Voice encoding ADPCM at 32 kHz RELP at 13 kbps CELP at 9.6 kbs

and 14.4 kbps Adaptive multirate ACELP 4.75 to 12.2 kbps

Traffic channels per RF channel

12 8 Up to 63 Depends on data rate

Diversity Antenna diversity at base station

Frequency hopping Spread spectrum with RAKE receiver

Space-time block coding with transmit diversity


Theme Example 5: 802.11(Wi-Fi) Pages 328-331 Timeline

User station (STA) logs onto local base station (AP), AP authenticates STA and provides ID

STA listens 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 Random time access (like Ethernet)


(5)(7) Convolutional Code with Hard Decoding

SystemView

0

0

2

2

4

4

6

6

8

8 1.00e+0

1.00e-1

1.00e-2

1.00e-3

1.00e-4

BER

Eb/No in dB

w3, PSK (coherent)


(5)(7) Convolutional Code with Soft Decoding

SystemView

0

0

2

2

4

4

6

6

8

8 1.00e+0

1.00e-1

1.00e-2

1.00e-3

1.00e-4

BER

Eb/No in dB

w3, PSK (coherent)


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?


Factor Comment Antenna gain More directionality means more frequency reuse Modulation 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.

Filters Match filtering improvements allow for steeper band rolloffs and a more compact spectral shape

Oscillators Crystal oscillators allow operation in the high GHz range. Stability and accuracy of oscillators also help improve compactness of spectrum.

Semiconductor technology

Improved switching speeds and good immunity to RFI and EMI. Circuit design methods have improved, thus reducing susceptibility to interference.

Digital processing 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.

Coding 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.

Factors That Increase Spectral Usage


Problem 3.2 Page 110

Consider the sinusoidal modulating signal

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?

cos 2m mm t A f t


Solution for Problem 3.2

cos 2

cos 2 cos 2

1cos 2 cos 2

2

c c

c m m c

c m c m c m

s t A m t f t

A A f t f t

A A f f f f


Polynomial Arithmetic Modulo 2

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


Examples

Two polynomials

Multiplying them

Taking the result modulo 2

5 3

4 3 2

1 101011

1 011101

x x x

x x x

9 8 7 6 5 4 3 22 3 2 3 1x x x x x x x x x

9 8 6 5 3 2 1 1101101111x x x x x x x


Finite Fields

Example Integer arithmetic modulo 7Elements 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 All integer arithmetic modulo a prime

defines a finite field


Vector Extensions of Finite Fields 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


Defining Characteristics of Galois Fields Successive multiplication by x

Begin with 1 Steps through all N elements except zero A sequence of length N-1

A reciprocal Defined as producing 1 as a product Always exists

Division is defined as multiplication by reciprocal


Special Case for Signal Processing 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


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


Example

2xx 3x

Generating Polynomial:

3 1x x


Definitions of Orthogonality

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 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

Orthogonal signals

1,

0,i j

i js t s t dt

i j


Next Time

Assignment:Look at the study guideGo over the slides to dateLook particularly at Chapters 4 and 7Make up a list of questionsSend them to me by email:

[email protected]

telecommunications engineering topic 5: wireless architectures

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