1. Department of Electrical and Computer Engineering
ECE 4331, Fall, 2009
                                                           
Zhu Han
Department of Electrical and Computer Engineering
Class 25
Nov. 17th, 2009

2. 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.

3. PN Sequence Generator Pseudorandom sequence
Randomness and noise properties
Walsh, M-sequence, Gold, Kasami, Z4
Provide signal privacy 3

4. 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.

5. Code Division Multiple Access (CDMA)
In CDMA, the narrowband message signal is multiplied by a very large bandwidth signal called spreading signal (code) before modulation and transmission over the air. This is called spreading.
CDMA is also called DSSS (Direct Sequence Spread Spectrum). DSSS is a more general term.
Message consists of symbols
Has symbol period and hence, symbol rate
Spreading signal (code) consists of chips
Has Chip period and and hence, chip rate
Spreading signal use a pseudo-noise (PN) sequence (a pseudo-random sequence)
PN sequence is called a codeword
Each user has its own cordword
Codewords are orthogonal. (low autocorrelation)
Chip rate is oder of magnitude larger than the symbol rate.
The receiver correlator distinguishes the senders signal by examining the wideband signal with the same time-synchronized spreading code
The sent signal is recovered by despreading process at the receiver. 5

6. CDMA Advantages Low power spectral density.
Signal is spread over a larger frequency band
Other systems suffer less from the transmitter
Interference limited operation
All frequency spectrum is used
Privacy
The codeword is known only between the sender and receiver. Hence other users can not decode the messages that are in transit
Reduction of multipath affects by using a larger spectrum
Random access possible
Users can start their transmission at any time
Cell capacity is not concerete fixed like in TDMA or FDMA systems. Has soft capacity
Higher capacity than TDMA and FDMA
No frequency management
No equalizers needed
No guard time needed
Enables soft handoff 6

7. 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 7

8. 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

9. Direct Sequence Spreading9

10. Spreading & Despreading

11. 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 11

12. CDMA Principle Represent bit 1 with +1 Represent bit 0 with -1 1 1
One bit period (symbol period) 1 1
Data
Coded
Signal
Input to the modulator (phase modulation)
Chip period 12

13. Processing Gain
Main parameter of CDMA is the processing gain that is defined as:
Gp: processing gain
Bspread: PN code rate
Bchip: Chip rate
R: Data rate
IS-95 System (Narrowband CDMA) has a gain of 64. Other systems have gain between 10 and 100.
1.228 Mhz chipping rate
1.25 MHz spread bandwidth 13

14. 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 14

15. DSSS Transmitter Message Baseband sss(t) + BPF m(t) Transmitted Signal
p(t)
PN Code
Generator
Oscillator fc
Chip Clock 15

16. DSSS Receiver Phase Shift Keying IF Wideband Demodulator Filter
Received
Data
Received
DSSS Signal
at IF
PN Code
Generator
Synchronization
System 16

17. Spectra of Received Signal
Spectral Density
Spectral Density
Interference
Signal
Interference
Signal
Frequency
Frequency
Output of Correlator after
dispreading,
Input to Demodulator
Output of Wideband filter 17

18. 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. 18

19. CDMA Example – transmission from two sources
A Data A
Codeword
A Signal
B Data B
Codeword
B Signal
Transmitted
A+B
Signal 19

20. 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 20

21. 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 21

22. 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! 22

23. 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

24. 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. 24

25. 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. 25

26. 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
1999
2000
2001
2002
cdmaOne
IS-95A 3X
No 3X
IS-95B 1X 2G
2.5G
3G Phase 1
3G Phase 2

27. 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

28. 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)

29. 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

30. 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

31. 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)

32. 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

33. 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)

34. OFDM bit loading Map the rate with the sub-channel condition
Water-filling

35. OFDM Time and Frequency Grid
Put different users data to different time-frequency slots

36. 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.

37. OFDM Transmitter and Receiver

38. 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.

39. 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.

40. 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