EE 6331, Spring, 2009

Advanced Telecommunication

Zhu Han

Department of Electrical and Computer Engineering

Class 22

Apr. 16th, 2009

                                                           

                                                           

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OutlineOutline Review

– Convolutional code Encoder Decoder: Viterbi decoding

– Turbo Code

– LDPC Code

– TCM modulation

CDMA

OFDM

2G-3G-4G

Exam2 until this class

Project 2 due on the exam

                                                           

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ExampleExample Convolutional encoder, k = 1, n = 2, L=2

– Convolutional encoder is a finite state machine (FSM) processing information bits in a serial manner

– Thus the generated code is a function of input and the state of the FSM

– In this (n,k,L) = (2,1,2) encoder each message bit influences a span of C= n(L+1)=6 successive output bits = constraint length C

– Thus, for generation of n-bit output, we require n shift registers in k = 1 convolutional encoders

                                                           

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Generator sequencesGenerator sequences

                                                           

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Representing convolutional codes Representing convolutional codes compactly: code trellis and state diagramcompactly: code trellis and state diagram

Shift register states

Input state ‘1’ indicated by dashed line

Code trellisState diagram

                                                           

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Distance for some convolutional codesDistance for some convolutional codes

Lower the coding rate, larger the L, then larger the distance

                                                           

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Puncture CodePuncture Code A sequence of coded bits is punctured by deleting some of

the bits in the sequence according to some fixed rule.

The resulting coding rate is increased. So a lower rate code can be extended to a sequence of higher rate codes.

                                                           

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Note also the Hamming distances!

correct:1+1+2+2+2=8;8 ( 0.11) 0.88

false:1+1+0+0+0=2;2 ( 2.30) 4.6

total path metric: 5.48

The largest metric, verifythat you get the same result!

                                                           

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The Viterbi algorithmThe Viterbi algorithm Problem of optimum decoding is to find the minimum distance path

from the initial state back to initial state (below from S0 to S0). The minimum distance is the sum of all path metrics

that is maximized by the correct path

Exhaustive maximum likelihood method must search all the paths in phase trellis (2k paths emerging/entering from 2 L+1 states for an (n,k,L) code)

The Viterbi algorithm gets itsefficiency via concentrating intosurvivor paths of the trellis

0ln ( , ) ln ( | )jm j mjp p y x

y x

Received codesequence

Decoder’s output sequencefor the m:th path

                                                           

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The maximum likelihood pathThe maximum likelihood path

The decoded ML code sequence is 11 10 10 11 00 00 00 whose Hamming distance to the received sequence is 4 and the respective decoded sequence is 1 1 0 0 0 0 0 (why?). Note that this is the minimum distance path.(Black circles denote the deleted branches, dashed lines: '1' was applied)

(1)

(1)

(0)

(2)

(1)

(1)

1

1

Smaller accumulated metric selected

First depth with two entries to the node

After register length L+1=3branch pattern begins to repeat

(Branch Hamming distancesin parenthesis)

                                                           

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Parallel Concatenated CodesParallel Concatenated Codes

Instead of concatenating in serial, codes can also be concatenated in parallel.

The original turbo code is a parallel concatenation of two recursive systematic convolutional (RSC) codes.– systematic: one of the outputs is the input.

Encoder#1

Encoder#2In

terle

aver MUX

Input

ParityOutput

Systematic Output

                                                           

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Iterative DecodingIterative Decoding There is one decoder for each elementary encoder.

Each decoder estimates the a posteriori probability (APP) of each data bit.

The APP’s are used as a priori information by the other decoder.

Decoding continues for a set number of iterations.– Performance generally improves from iteration to iteration, but

follows a law of diminishing returns.

Decoder#1

Decoder#2

DeMUX

Interleaver

Interleaver

Deinterleaver

systematic data

paritydata

APP

APP

hard bitdecisions

                                                           

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Performance as a Function of Number of IterationsPerformance as a Function of Number of Iterations

K=5, r=1/2, L=65,536

0.5 1 1.5 210

-7

10-6

10-5

10-4

10-3

10-2

10-1

100

Eb/N

o in dB

BE

R

1 iteration

2 iterations

3 iterations6 iterations

10 iterations

18 iterations

                                                           

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LDPC IntroductionLDPC Introduction

Low Density Parity Check (LDPC) History of LDPC codes

– Proposed by Gallager in his 1960 MIT Ph. D. dissertation– Rediscovered by MacKay and Richardson/Urbanke in 1999

Features of LDPC codes– Performance approaching Shannon limit– Good block error correcting performance– Suitable for parallel implementation

Advantages over turbo codes– LDPC do not require a long interleaver– LDPC’s error floor occurs at a lower BER– LDPC decoding is not trellis based

                                                           

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Pro and ConPro and Con ADVANTAGES

– Near Capacity Performance .. Shannon’s Limit – Some LDPC Codes perform better than Turbo Codes– Trellis diagrams for Long Turbo Codes become very complex and

computationally elaborate … and make my head hurt !– Low Floor Error – Decoding in the Log Domain is quite fast.

DISADVANTAGES– Long time to Converge to Good Solution– Very Long Code Word Lengths for good Decoding Efficiency– Iterative Convergence is SLOW

Takes ~ 1000 iterations to converge under standard conditions.

– Due to the above reason transmission time increases

i.e. encoding, transmission and decoding– Hence Large Initial Latency

(4086,4608) LPDC codeword has a latency of almost 2 hours

                                                           

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Trellis Coded ModulationTrellis Coded Modulation

1. Combine both encoding and modulation. (using Euclidean distance only)

2. Allow parallel transition in the trellis.

3. Has significant coding gain (3~4dB) without bandwidth compromise.

4. Has the same complexity (same amount of computation, same decoding time and same amount of memory needed).

5. Has great potential for fading channel.

6. Widely used in Modem

                                                           

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Set PartitioningSet Partitioning

1. Branches diverging from the same state must have the largest distance.

2. Branches merging into the same state must have the largest distance.

3. Codes should be designed to maximize the length of the shortest error event path for fading channel (equivalent to maximizing diversity).

4. By satisfying the above two criterion, coding gain can be increased.

                                                           

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Spread-spectrum transmission Spread-spectrum transmission

Three advantages over fixed spectrum – Spread-spectrum signals are highly resistant to noise and

interference. The process of re-collecting a spread signal spreads out noise and interference, causing them to recede into the background.

– Spread-spectrum signals are difficult to intercept. A Frequency-Hop spread-spectrum signal sounds like a momentary noise burst or simply an increase in the background noise for short Frequency-Hop codes on any narrowband receiver except a Frequency-Hop spread-spectrum receiver using the exact same channel sequence as was used by the transmitter.

– Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference. The spread-spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. As a result, bandwidth can be utilized more efficiently.

                                                           

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PN Sequence GeneratorPN Sequence Generator

Pseudorandom sequence– Randomness and noise properties

– Walsh, M-sequence, Gold, Kasami, Z4

– Provide signal privacy

                                                           

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Direct Sequence (DS)-CDMADirect Sequence (DS)-CDMA

It phase-modulates a sine wave pseudo-randomly with a continuous string of pseudo-noise code symbols called "chips", each of which has a much shorter duration than an information bit. That is, each information bit is modulated by a sequence of much faster chips. Therefore, the chip rate is much higher than the information signal bit rate.

It uses a signal structure in which the sequence of chips produced by the transmitter is known a priori by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal.

                                                           

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Direct Sequence Spread SpectrumDirect Sequence Spread Spectrum

Unique code to differentiate all users

Sequence used for spreading have low cross-correlations

Allow many users to occupy all the frequency/bandwidth allocations at that same time

Processing gain is the system capacity– How many users

the system can support

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Spreading & DespreadingSpreading & Despreading

Spreading– Source signal is multiplied by a PN signal: 6.134, 6.135

Processing Gain:

Despreading– Spread signal is multiplied by the spreading code

Polar {±1} signal representation

DataRate

ChipRate

T

TG

c

sp

                                                           

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Direct Sequence SpreadingDirect Sequence Spreading

                                                           

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Spreading & DespreadingSpreading & Despreading

                                                           

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CDMA – Multiple UsersCDMA – Multiple Users

One user’s information is the other’s interferences

If the interference structure can be explored, multiuser detection– Match filter

– Decorrelator

– MMSE decodor

– Successive cancellation

– Decision feedback

                                                           

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CDMA ExampleCDMA Example

R

A B

Receiver (a base station)

Transmitter (a mobile) TransmitterCodeword=010011 Codeword=101010

Data=1011… Data=0010…

Data transmitted from A and B is multiplexed using CDMA and codewords. The Receiver de-multiplexes the data using dispreading.

                                                           

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CDMA Example – transmission from two sourcesCDMA Example – transmission from two sources

1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0

CodeData

0 1 0 0 1 1 0 1 0 0 1 10 1 0 0 1 1 0 1 0 0 1 1

1 0 1 1 0 0 0 1 0 0 1 1 1 0 1 1 0 0

0 0 1 0

1 0 1 0 1 0

1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 0 1 0 1 0 1

1 0 1 1

1 0 1 1 0 0

TransmittedA+B

Signal

A Data

A Codeword

B Data

B Codeword

CodeDataA Signal

B Signal

                                                           

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CDMA Example – recovering signal A at the receiverCDMA Example – recovering signal A at the receiver

0 1 0 0

A+BSignal

received

A Codeword

atreceiver

CodeB)(A

IntegratorOutput

ComparatorOutput

Take the inverse of this to obtain A

                                                           

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CDMA Example – recovering signal B at the receiverCDMA Example – recovering signal B at the receiver

1 1 0 1

A+BSignal

received

B Codeword

atreceiver

CodeB)(A

IntegratorOutput

ComparatorOutput

Take the inverse of this to obtain B

                                                           

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CDMA Example – using wrong codeword at the receiverCDMA Example – using wrong codeword at the receiver

X 0 1 1 Noise

A+BSignal

received

Wrong Codeword

Used atreceiver

IntegratorOutput

ComparatorOutput

Wrong codeword will not be able to decode the original data!

                                                           

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Near Far Problem and Power ControlNear Far Problem and Power Control At a receiver, the signals may come from

various (multiple sources. – The strongest signal usually captures the

modulator. The other signals are considered as noise

– Each source may have different distances to the base station

In CDMA, we want a base station to receive CDMA coded signals from various mobile users at the same time. – Therefore the receiver power at the base

station for all mobile users should be close to eacother.

– This requires power control at the mobiles.

Power Control: Base station monitors the RSSI values from different mobiles and then sends power change commands to the mobiles over a forward channel. The mobiles then adjust their transmit power.

B

M

MM

M

pr(M)

                                                           

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Frequency Hopping Spread SpectrumFrequency Hopping Spread Spectrum

Frequency-hopping spread spectrum (FHSS) is a spread-spectrum method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver.

Military, bluetooth

                                                           

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Hybrid Spread Spectrum TechniquesHybrid Spread Spectrum Techniques

FDMA/CDMA– Available wideband spectrum is frequency divided into

number narrowband radio channels. CDMA is employed inside each channel.

DS/FHMA– The signals are spread using spreading codes (direct

sequence signals are obtained), but these signal are not transmitted over a constant carrier frequency; they are transmitted over a frequency hopping carrier frequency.

                                                           

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Hybrid Spread Spectrum TechniquesHybrid Spread Spectrum Techniques

Time Division CDMA (TCDMA)– Each cell is using a different spreading code (CDMA

employed between cells) that is conveyed to the mobiles in its range.

– Inside each cell (inside a CDMA channel), TDMA is employed to multiplex multiple users.

Time Division Frequency Hopping– At each time slot, the user is hopped to a new frequency

according to a pseudo-random hopping sequence.

– Employed in severe co-interference and multi-path environments.

Bluetooth and GSM are using this technique.

                                                           

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Orthogonal frequency-division multiplexing Orthogonal frequency-division multiplexing

Special form of Multi-Carrier Transmission.

Multi-Carrier Modulation.– Divide a high bit-rate digital stream into several low bit-rate

schemes and transmit in parallel (using Sub-Carriers)

-6 -4 -2 0 2 4 6

-0.2

0

0.2

0.4

0.6

0.8

Normalized Frequency (fT) --->

Norm

alized A

mplitu

de -

-->

                                                           

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OFDM bit loadingOFDM bit loading Map the rate with the sub-channel condition

Water-filling

                                                           

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OFDM Time and Frequency GridOFDM Time and Frequency Grid

Put different users data to different time-frequency slots

                                                           

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Guard Time and Cyclic Extension...Guard Time and Cyclic Extension...

A Guard time is introduced at the end of each OFDM symbol for protection against multipath.

The Guard time is “cyclically extended” to avoid Inter-Carrier Interference (ICI) - integer number of cycles in the symbol interval.

Guard Time > Multipath Delay Spread, to guarantee zero ISI & ICI.

M ultipath component that does not cause IS I

guard Symbol guard

guard Symbol guard

guard Symbol guard

M ultipath component that causes IS I

                                                           

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OFDM Transmitter and ReceiverOFDM Transmitter and Receiver

                                                           

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Pro and ConPro and Con Advantages

– Can easily be adopted to severe channel conditions without complex equalization

– Robust to narrow-band co-channel interference – Robust to inter-symbol interference and fading caused by multipath

propagation – High spectral efficiency – Efficient implementation by FFTs – Low sensitivity to time synchronization errors – Tuned sub-channel receiver filters are not required (unlike in

conventional FDM) – Facilitates Single Frequency Networks, i.e. transmitter macro-diversity.

Disadvantages– Sensitive to Doppler shift. – Sensitive to frequency synchronization problems – Inefficient transmitter power consumption, since linear power amplifier is

required.

                                                           

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OFDM ApplicationsOFDM Applications ADSL and VDSL broadband access via telephone network copper

wires.

IEEE 802.11a and 802.11g Wireless LANs.

The Digital audio broadcasting systems EUREKA 147, Digital Radio Mondiale, HD Radio, T-DMB and ISDB-TSB.

The terrestrial digital TV systems DVB-T, DVB-H, T-DMB and ISDB-T.

The IEEE 802.16 or WiMax Wireless MAN standard.

The IEEE 802.20 or Mobile Broadband Wireless Access (MBWA) standard.

The Flash-OFDM cellular system.

Some Ultra wideband (UWB) systems.

Power line communication (PLC).

Point-to-point (PtP) and point-to-multipoint (PtMP) wireless applications.

                                                           

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The IEEE 802.11a/g StandardThe IEEE 802.11a/g Standard

Belongs to the IEEE 802.11 system of specifications for wireless LANs.

802.11 covers both MAC and PHY layers.

Five different PHY layers.

802.11a/g belongs to the High Speed WLAN category with peak data rate of 54Mbps

PHY Layer very similar to ETSI’s HIPERLAN Type 2

                                                           

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Road MapRoad Map

1XRTT/3XRTT

cdma2000CDMA

(IS 95 A) IS 95 B

GSM

TDMA EDGE UWC-136

GPRS W-CDMA

3X3X3X3X

No 3XNo 3XNo 3XNo 3X

cdmaOnecdmaOneIS-95AIS-95A

cdmaOnecdmaOneIS-95AIS-95A

1999 2000 2001 2002

1X1X1X1XIS-95BIS-95BIS-95BIS-95B

2G 2.5G 3G Phase 1 3G Phase 2

                                                           

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2G: IS-95A (1995)2G: IS-95A (1995)

Known as CDMAOne

Chip rate at 1.25Mbps

Convolutional codes, Viterbi Decoding

Downlink (Base station to mobile):– Walsh code 64-bit for

channel separation

– M-sequence 215 for cell separation

Uplink (Mobile to base station):– M-sequence 241 for channel

and user separation

Standard IS-95, ANSI J-STD-008

Multiple Access CDMA

Uplink Frequency 869-894 MHz

Downlink Frequency

824-849 MHz

Channel Separation 1.25 MHz

Modulation Scheme BPSK/QPSK

Number of Channel 64

Channel Bit Rate 1.25 Mbps (chip rate)

Speech Rate 8~13 kbps

Data Rate Up to 14.4 kbps

Maximum Tx Power

600 mW

                                                           

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2.5G: IS-95B (1998)2.5G: IS-95B (1998)

Increased data rate for internet applications– Up to 115 kbps (8 times that of 2G)

Support web browser format language– Wireless Application Protocol (WAP)

                                                           

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3G Technology3G Technology

Ability to receive live music, interactive web sessions, voice and data with multimedia features

Global Standard IMT-2000– CDMA 2000, proposed by TIA– W-CDMA, proposed by ARIB/ETSI

Issued by ITU (International Telecommunication Union) Excellent voice quality Data rate

– 144 kbps in high mobility– 384 kbps in limited mobility– 2 Mbps in door

Frequency Band 1885-2025 MHz Convolutional Codes Turbo Codes for high data rates

                                                           

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3G: CDMA2000 (2000)3G: CDMA2000 (2000)

CDMA 1xEV-DO– peak data rate 2.4 Mbps– supports mp3 transfer and video conferencing

CDMA 1xEV-DV– Integrated voice and high-speed data multimedia service up

to 3.1 Mbps

Channel Bandwidth: – 1.25, 5, 10, 15 or 20 MHz

Chip rate at 3.6864 Mbps Modulation Scheme

– QPSK in downlink – BPSK in uplink

                                                           

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3G: CDMA2000 Spreading Codes3G: CDMA2000 Spreading Codes

Downlink – Variable length orthogonal Walsh sequences for channel

separation

– M-sequences 3x215 for cell separation (different phase shifts)

Uplink– Variable length orthogonal Walsh sequences for channel

separation

– M-sequences 241 for user separation (different phase shifts)

                                                           

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3G: W-CDMA (2000)3G: W-CDMA (2000)

Stands for “wideband” CDMA

Channel Bandwidth: – 5, 10 or 20 MHz

Chip rate at 4.096 Mbps

Modulation Scheme– QPSK in downlink

– BPSK in uplink

Downlink – Variable length orthogonal sequences for channel separation

– Gold sequences 218 for cell separation

Uplink– Variable length orthogonal sequences for channel separation

– Gold sequences 241 for user separation

                                                           

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4G OFDM4G OFDM

4G is being developed to accommodate the quality of service (QoS) and rate requirements set by forthcoming applications like wireless broadband access, Multimedia Messaging Service (MMS), video chat, mobile TV, HDTV content, Digital Video Broadcasting (DVB), minimal service like voice and data, and other streaming services for "anytime-anywhere".

Baseband techniques[9] – OFDM: To exploit the frequency selective channel property

– MIMO: To attain ultra high spectral efficiency

– Turbo principle: To minimize the required SNR at the reception side

Adaptive radio interface

Modulation, spatial processing including multi-antenna and multi-user MIMO

Relaying, including fixed relay networks (FRNs), and the cooperative relaying concept, known as multi-mode protocol

3GPP is currently standardizing LTE Advanced as future 4G standard