1. EE 6331, Spring, 2009 Advanced Telecommunication
                                                           
Zhu Han
Department of Electrical and Computer Engineering
Class 22
Apr. 16th, 2009

2. Outline Review CDMA OFDM 2G-3G-4G Exam2 until this class
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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3. Example 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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4. Generator sequences
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5. Representing convolutional codes compactly: code trellis and state diagram
Input state ‘1’
indicated by dashed line
State diagram
Code trellis
Shift register states
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6. Distance for some convolutional codes
Lower the coding rate, larger the L, then larger the distance
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7. Puncture 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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8. The largest metric, verify that you get the same result!
Note also the Hamming distances!
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9. The 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 its efficiency via concentrating into survivor paths of the trellis
Decoder’s output sequence
for the m:th path
Received code
sequence
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10. The maximum likelihood path
Smaller accumulated
metric selected
After register length L+1=3
branch pattern begins to repeat 1 1
(Branch Hamming distances
in parenthesis)
First depth with two entries to the node
The decoded ML code sequence is whose Hamming
distance to the received sequence is 4 and the respective decoded
sequence is (why?). Note that this is the minimum distance path.
(Black circles denote the deleted branches, dashed lines: '1' was applied)
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11. Parallel 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.
Systematic Output
Input
Encoder #1
MUX
Parity
Output
Interleaver
Encoder #2
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12. Iterative 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.
Deinterleaver
APP
APP
Interleaver
systematic
data
Decoder #1
Decoder #2
hard bit
decisions
parity
data
DeMUX
Interleaver
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13. Performance as a Function of Number of Iterations
K=5, r=1/2, L=65,536
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14. LDPC 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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15. Pro 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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16. Trellis Coded Modulation
Combine both encoding and modulation. (using Euclidean distance only)
Allow parallel transition in the trellis.
Has significant coding gain (3~4dB) without bandwidth compromise.
Has the same complexity (same amount of computation, same decoding time and same amount of memory needed).
Has great potential for fading channel.
Widely used in Modem
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17. Set Partitioning
Branches diverging from the same state must have the largest distance.
Branches merging into the same state must have the largest distance.
Codes should be designed to maximize the length of the shortest error event path for fading channel (equivalent to maximizing diversity).
By satisfying the above two criterion, coding gain can be increased.
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18. 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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19. PN Sequence Generator Pseudorandom sequence
Randomness and noise properties
Walsh, M-sequence, Gold, Kasami, Z4
Provide signal privacy
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20. Direct 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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21. Direct 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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22. Spreading & Despreading
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
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23. Direct Sequence Spreading
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24. Spreading & Despreading
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25. CDMA – 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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26. CDMA Example R Receiver (a base station) Data=1011… Data=0010… A B
Transmitter (a mobile)
Transmitter
Codeword=101010
Codeword=010011
Data transmitted from A and B is multiplexed using CDMA and codewords.
The Receiver de-multiplexes the data using dispreading.
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27. CDMA Example – transmission from two sources
A Data A
Codeword
A Signal
B Data B
Codeword
B Signal
Transmitted
A+B
Signal
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28. CDMA Example – recovering signal A at the receiver
A+B
Signal
received A
Codeword at
receiver
Integrator
Output
Comparator
Output
Take the inverse of this to obtain A
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29. CDMA Example – recovering signal B at the receiver
A+B
Signal
received B
Codeword at
receiver
Integrator
Output
Comparator
Output
Take the inverse of this to obtain B
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30. CDMA Example – using wrong codeword at the receiver
A+B
Signal
received
Wrong
Codeword
Used at
receiver
Integrator
Output
Comparator
Output X
Noise
Wrong codeword will not be able to decode the original data!
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31. Near 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
pr(M) M M M M
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32. Frequency 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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33. Hybrid 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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34. Hybrid 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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35. 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)
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36. OFDM bit loading Map the rate with the sub-channel condition
Water-filling
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37. OFDM Time and Frequency Grid
Put different users data to different time-frequency slots
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38. 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.
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39. OFDM Transmitter and Receiver

40. Pro 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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41. OFDM Applications
ADSL and VDSL broadband access via telephone network copper wires.
IEEE a and g 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 or WiMax Wireless MAN standard.
The IEEE 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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42. The IEEE a/g Standard
Belongs to the IEEE system of specifications for wireless LANs.
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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43. 4G Road Map cdma2000 IS 95 B GSM TDMA EDGE UWC-136 GPRS W-CDMA
1XRTT/3XRTT
cdma2000
CDMA
(IS 95 A)
IS 95 B
GSM
TDMA
EDGE
UWC-136
GPRS
W-CDMA 4G
1999
2000
2001
2002
cdmaOne
IS-95A 3X
No 3X
IS-95B 1X 2G
2.5G
3G Phase 1
3G Phase 2
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44. 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
MHz
Downlink Frequency
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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45. 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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46. 3G 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 MHz
Convolutional Codes
Turbo Codes for high data rates
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47. 3G: CDMA2000 (2000) CDMA 1xEV-DO CDMA 1xEV-DV Channel Bandwidth:
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 Mbps
Modulation Scheme
QPSK in downlink
BPSK in uplink
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48. 3G: CDMA2000 Spreading Codes
Downlink
Variable length orthogonal Walsh sequences for channel separation
M-sequences 3x215 for cell separation (different phase shifts)
Uplink
M-sequences 241 for user separation (different phase shifts)
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49. 3G: W-CDMA (2000) Stands for “wideband” CDMA Channel Bandwidth:
5, 10 or 20 MHz
Chip rate at Mbps
Modulation Scheme
QPSK in downlink
BPSK in uplink
Downlink
Variable length orthogonal sequences for channel separation
Gold sequences 218 for cell separation
Uplink
Gold sequences 241 for user separation
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50. 4G 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
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