EE 6331, Spring, 2009 Advanced Telecommunication Zhu Han Department of Electrical and Computer...
-
Upload
ross-henderson -
Category
Documents
-
view
216 -
download
3
Transcript of EE 6331, Spring, 2009 Advanced Telecommunication Zhu Han Department of Electrical and Computer...
EE 6331, Spring, 2009
Advanced Telecommunication
Zhu Han
Department of Electrical and Computer Engineering
Class 22
Apr. 16th, 2009
ECE6331
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
ECE6331
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
ECE6331
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
ECE6331
Distance for some convolutional codesDistance for some convolutional codes
Lower the coding rate, larger the L, then larger the distance
ECE6331
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.
ECE6331
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!
ECE6331
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
ECE6331
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)
ECE6331
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
ECE6331
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
ECE6331
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
ECE6331
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
ECE6331
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
ECE6331
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
ECE6331
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.
ECE6331
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.
ECE6331
PN Sequence GeneratorPN Sequence Generator
Pseudorandom sequence– Randomness and noise properties
– Walsh, M-sequence, Gold, Kasami, Z4
– Provide signal privacy
ECE6331
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.
ECE6331
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
ECE6331
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
ECE6331
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
ECE6331
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.
ECE6331
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
ECE6331
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
ECE6331
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
ECE6331
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!
ECE6331
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)
ECE6331
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
ECE6331
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.
ECE6331
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.
ECE6331
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 -
-->
ECE6331
OFDM Time and Frequency GridOFDM Time and Frequency Grid
Put different users data to different time-frequency slots
ECE6331
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
ECE6331
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.
ECE6331
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.
ECE6331
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
ECE6331
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
ECE6331
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
ECE6331
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)
ECE6331
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
ECE6331
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
ECE6331
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)
ECE6331
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
ECE6331
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