W-CDMA for UMTS – Principles
Introduction
CDMA Background/ History
Code Division Multiple Access (CDMA)
Why CDMA ?
CDMA Principles / Spreading Codes
Multi-path Radio Channel and Rake Receiver
Problems to Solve
Macro Diversity and Soft Handover
Near-Far Problem and Power Control
UMTS General Requirements
FDD vs. TDD
Key Parameters
Spectrum Allocation
Cellular Communication Networks 2 Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2015
CDMA History
Pioneer Era (Spread Spectrum)
40s and 50s: Spread Spectrum technique for military anti-jam applications
1949: Claude Shannon and Robert Pierce develop basic ideas of CDMA
1970s: Several developments for military systems (e.g. GPS)
Narrow-band CDMA Era
1993: IS-95 standard (mainly driven by Qualcomm)
1992–1995: RACE project CODIT (UMTS Code Division Testbed, PKI, Ericsson, Telia, etc.)
Wide-band CDMA Era
1995–1999: ACTS project FRAMES: FMA Mode 1 (TD/CDMA), FMA Mode 2 (W-CDMA)
1995: cdma2000 1x/ 3x (USA)
1998: UMTS (Rel.-99): FDD and TDD mode
1999: Harmonization: W-CDMA, TD-CDMA and multi-carrier CDMA (chip rate: 3.84 Mchip/sec)
1999: Narrowband TDD mode (TD-SCDMA), chip rate: 1.28 Mchip/sec
High-Speed CDMA Era
since 2000: HSDPA (Rel.-5/ 2000), E-DCH (Rel.-6/ 2002), HSPA+ (Rel.-7/ 2005)
cdma2000 1x EV-DO/DV
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Spread Spectrum Technology
Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference
Solution: spread the narrow band signal into a broad band signal using a special code
protection against narrow band interference
Side effects:
coexistence of several signals without dynamic coordination
signal can only be detected if the spreading process is known
Alternatives:
Direct Sequence (UMTS)
Frequency Hopping (slow FH: GSM, fast FH: Bluetooth)
detection at
receiver
interference spread
signal
signal (despreaded)
spread
interference
f f
power power
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Spreading and Frequency Selective Fading
FDMA: Relatively small bandwidth on each channel
Guard bands to avoid interference between the users
Channels maybe (temporary) unavailable due to channel selective fading
CDMA: relatively large bandwidth of the spread signal
Frequency selective fading causes only some reduction in the level of the received signal
Users are separated by the spreading sequence
2 2
2 2
2
frequency
channel quality
1
spread signals
frequency
channel quality
1 2
3
4
5 6
small bandwidth guard band
Cellular Communication Networks 5 Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2015
CDMA Multiple Access
CDMA (Code Division Multiple Access)
all terminals send on the same frequency probably at the same time and can use the whole bandwidth of the transmission channel
each sender has a unique random number (spreading sequence), the sender modulates the signal with this random number
the receiver can “tune” into this signal if it knows the pseudo random number, tuning is done via a correlation function
Advantages:
all terminals can use the same frequency, less planning needed
huge code space (e.g. 232) compared to frequency space
interference is not coded (acts like white noise)
forward error correction and encryption can be easily integrated
Disadvantages:
higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is a signal)
all signals should have the same strength at a receiver (power control)
Cellular Communication Networks 6 Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2015
CDMA Multiple Access (contd.)
Principle of CDMA Communication
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DSSS (Direct Sequence Spread Spectrum) I
Modulation of the signal with pseudo-random number (code sequence)
Many chips per bit (e.g. 128) result in higher bandwidth of the signal
Spreading factor SF: ratio between chip rate RC and data symbol rate RS
RC = RS · SF
TC = TS / SF
Processing Gain
GS = 10 · log10(SF)
user data (data rate)
code sequence (chip rate)
resulting signal (chip rate)
1
0
=
Tc
Ts
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DSSS (Direct Sequence Spread Spectrum) II
X
user data
code
sequence
modulator
radio
carrier
spread
spectrum
signal transmit
signal
transmitter
demodulator
received
signal
radio
carrier
X
code
sequence
baseband
signal
receiver
integrator
products
decision
data
sums
correlator
Cellular Communication Networks 9 Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2015
CDMA Principle (Downlink)
Code 0
Code 1
Code 2
S
data 0
data 1
data 2
Code 0
Code 1
Code 2
data 0
data 1
data 2
sender (base station) receiver (terminal)
Transmission over
air interface
Cellular Communication Networks 10 Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2015
CDMA Principle (Uplink)
Code 0
Code 1
Code 2
S
data 0
data 1
data 2
Code 0
Code 1
Code 2
data 0
data 1
data 2
sender (terminal) receiver (base station)
transmission over
air interface
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UMTS Spreading
Constant chip-rate of 3.84 Mchip/s (FDD)
Variable data rates are realized by different spreading factors of the orthogonal channelization codes
Higher data rates: less chips per bit (and vice-versa)
Senders are separated by unique, quasi-orthogonal scrambling codes
Simple code management: each station can reuse the same orthogonal channelization codes
No need for precise synchronization as the scrambling codes remain quasi-orthogonal
data1 data2 data3
scrambling code1
chan. code3
chan. code2
chan. code1
data4 data5
chan. code4
chan. code1
sender1 sender2
scrambling code2
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Functionality of Channelization and Scrambling Codes
Channelization Code Scrambling Code
Usage UL: Separation of physical data (DPDCH) and control channels (DPCCH) from same terminal
DL: Separation of DL connections to different users within one cell
UL: Separation of terminals DL: Separation of sectors/cells
Length 4 – 256 chips (1.0 – 66.7 µs) UL+DL: 10ms = 38400 chips
Number of codes Number of codes under 1 scrambling code = spreading factor (SF)
UL: several millions
DL: 256
Code Family Orthogonal Variable Spreading Factor
Long 10 ms code: Gold code
Spreading Yes, increases transmission bandwidth
No, does not affect transmission bandwidth
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OVSF-Coding Tree
1
1,1
1,-1
1,1,1,1
1,1,-1,-1
X
X,X
X,-X 1,-1,1,-1
1,-1,-1,1
1,-1,-1,1,1,-1,-1,1
1,-1,-1,1,-1,1,1,-1
1,-1,1,-1,1,-1,1,-1
1,-1,1,-1,-1,1,-1,1
1,1,-1,-1,1,1,-1,-1
1,1,-1,-1,-1,-1,1,1
1,1,1,1,1,1,1,1
1,1,1,1,-1,-1,-1,-1
SF=1 SF=2 SF=4 SF=8
SF=n SF=2n
...
...
...
...
In UMTS, spreading factors (SF) from 4 – 512 (DL) / 4 – 256 (UL) are used:
4 x SF4, 8 x SF8 …………………… 256 x SF256, 512 x SF512
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Downlink Dedicated Channel Symbol and Bit Rates
Spreading factor
Channel symbol rate
(kbps)
Channel bit rate (kbps)
DPDCH channel bit rate range
(kbps)
Maximum user data rate with
1/2-rate coding (approx.)
512 7.5 15 3-6 1-3 kbps
256 15 30 12-24 6-12 kbps
...
16 240 480 432 215 kbps
8 480 960 912 456 kbps
4 960 1920 1872 936 kbps
4, with 3 parallel codes
2880 5760 5616 2.3 Mbps
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CDMA in Theory
Sender A
sends Ad = 1, code sequence Ac = +1 –1 +1 –1 –1 +1 +1
sending signal As = Ad Ac = (+1, –1, +1, –1, –1, +1, +1)
Sender B
sends Bd = –1, code sequence Bc = –1 +1 +1 –1 +1 –1 +1
sending signal Bs = Bd Bc = (+1, –1, –1, +1, –1, +1, –1)
Both signals superimpose in space
interference neglected (noise etc.)
As + Bs = (+2, –2, 0, 0, –2, +2, 0)
Receiver wants to receive signal from sender A
apply sequence AC chipwise (inner product)
Ar = (+2, –2, 0, 0, –2, +2, 0) Ac = 2 + 2 + 0 + 0 + 2 + 2 + 0 = 8
result greater than 0, therefore, original bit was „1“
receiving B
Br = (+2, –2, 0, 0, –2, +2, 0) Bc = –2 –2 + 0 + 0 – 2 – 2 + 0 = –8, i.e. „–1“
wrong sequence CC = +1 +1 –1 –1 +1 +1 –1
Cr = (+2, –2, 0, 0, –2, +2, 0) Cc = 0, decision impossible
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CDMA on signal level I
data A
code A
signal A
Real systems use much longer keys resulting in a larger distance
between single code words in code space
0 1 0 Ad
Ac
As
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CDMA on signal level II
signal A
data B
code B
signal B
As + Bs
0 1 1 Bd
Bc
Bs
As
+1
0
–1
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CDMA on signal level III
Ac
(As + Bs)
• Ac
integrator
output
comparator
output
As + Bs
data A
0 1 0
0 1 0 Ad
+1
0
–1
+1
–1
+1
0
–1
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CDMA on signal level IV
integrator
output
comparator
output
Bc
(As + Bs)
• Bc
As + Bs
data B
0 1 1
0 1 1 Bd
+1
0
–1
+1
–1
+1
0
–1
Cellular Communication Networks 20 Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2015
CDMA on signal level V
Assumptions orthogonality of keys negligance of noise no differences in signal level => precise power control
comparator
output
wrong
code C
integrator
output
(As + Bs)
• C
As + Bs
(1) (1) ?
+1
0
–1
+1
–1
+1
0
–1
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Properties of Spreading Sequences
Cross correlation function (CCF)
Auto correlation function (ACF)
Code sequence #1
Code sequence #2
Required properties of spreading
(properties of the transmitted signals):
• High ACF peak
• Low ACF sidelobe
inter-symbol interference (ISI)
• Low CCF
multi-user interference (MUI)
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Multi-path Transmission
Multi-path components can be resolved due to ACF of codes
Spreader
Spreading
Sequence c(t)
Despreader
(Correlator)
Spreading
Sequence c(t–Td)
Receiver
synchronizes to
each multi-path
component for
de-spreading
Td
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RAKE Receiver
Correlate and track each multi-path component separately
Optimal coherent combining
RAKE receiver with K fingers
• trackers: independent tracking
of dominant paths
• searchers: scan a time window to
search (the pilot channel) for
dominant multi-path components
• time resolution in UMTS approx.
260 ns
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RAKE Receiver – Practical Realization
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Macro-Diversity & Soft Handover
Optimal coherent combining
in the RAKE receiver (at MS)
NodeB 1 NodeB 2
UE
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Multi-user CDMA
Conventional CDMA Receiver (Base Station):
• coherent (amplitude and phase) RF
demodulation at base station
• separate despreading and demodulation of
each signal at base station
• one Rake receiver with K fingers per user
• unsynchronized transmission between the
mobiles
Despreading
(Correlator)
Spreading
Sequence c1(t-Td1)
RAKE 1
Spreading
Sequence c2(t-Td2)
RAKE 2
Spreading
Sequence cn(t-Tdn)
RAKE n
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Near-Far Problem:
• Spreading sequences are not orthogonal
(multi-user interference)
• Near mobile dominate
• Signal to interference ratio is lower for far
mobiles and performance degrades
The problem can be resolved through
dynamic power control to equalize all
received power levels
AND/OR
By means of joint multi-user detection
Near-Far Problem – Power Control
NodeB
UE 1
UE 2
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Interference Cancellation
Multi-user Interference Cancellation (Joint Detection):
Detection mechanism takes into account interference from other users as all signals are known in the receiver (known interference can be canceled)
Multi-user Detector
(Joint Detection/
Interference Cancellation)
Despreading
(Correlator)
c1(t–Td1)
RAKE 1
c2(t–Td2)
RAKE 2
cn(t–Tdn)
RAKE n
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Interference Cancellation – Realization
Subtractive interference cancellation
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FDD vs. TDD Mode
UMTS supports FDD and TDD
FDD mode:
Multiple access scheme: DS-CDMA (Direct Sequence-CDMA)
Symmetric capacity of up- and down-link
Better suited for low bit rate transmission in larger cells (no timing advance, no synchronization from MS required)
TDD mode:
Multiple access scheme: TD-CDMA (JD-CDMA)
Asymmetric capacity allocation for up- and down-link
Strict synchronization required for MS (timing advance)
Relaxed power control and near-far resistance by the use of intra-cell multi-user interference cancellation (spreading factor 1 – 16)
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FDD vs. TDD Mode (contd.)
TDD-Mode
FDD-Mode
(one direction)
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TDD Mode Switching
1 Frame (10ms) of 15 Slots
multiple switching points, symmetric DL/UL allocation
multiple switching points, asymmetric DL/UL allocation
single switching point, symmetric DL/UL allocation
single switching point, asymmetric DL / UL allocation
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W-CDMA for UMTS – Summary of Key Parameters
Multiple-Access DS-CDMA (TD-CDMA)
Duplex scheme FDD (TDD)
Chip rate 3.84 MChip/s
(TDD: 1.28/ 3.84/ 7.68 MChip/s)
Carrier spacing Flexible in the range 4.6 – 5.0 MHz
(200 kHz carrier raster)
Frequency bands 1920 – 1980 / 2110 – 2170 paired (FDD)
1900 – 1920 and 2010 – 2025 unpaired (TDD)
Frame length 10 ms / (15 time slots)
Inter-BS
synchronization
FDD mode: No accurate synchronization needed
TDD mode: Synchronization needed
Multi-rate/
Variable-rate scheme
Variable-spreading factor + Multi-code
Spreading factor: 4 – 256 (FDD) and 1 – 16 (TDD)
Channel coding
scheme
Convolutional coding (rate 1/2 – 1/3)
Turbo coding
Cellular Communication Networks 34 Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2015
Global Spectrum Allocations for IMT-2000
ITU2010 20251980
MSS MSS*
1930
IMT-2000MSSMSS*
IMT-2000
2160 2170 2200 MHz
*Region2
1885 2110
PHS
20101980 2025
Japan2110 22002170
IMT-2000MSSMSSIMT-2000
18951885 1918.1MHz
1980 2110 22002170
IMT-2000MSS
19001880
DECT
2010
MSSIMT-2000
2025 MHz
Europe
2110 220021652150
Reserve MSSBroadcast Auxilary
1910 1930 1990 2025
MSS
1850
PCS*PCS
A B CD E F
PCS
A B CD E F
MHz
USA
20101980 2025
China
2110 22002170
MSSMSS
1900 1920MHz
1865 1880 1945 1960
CDMA FDD-WLL
FDD-WLLCDMA
TDD-WLL
MSS: Mobile Satellite Services
Cellular Communication Networks 35 Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2015
UMTS Spectrum
2200 M
Hz
2000 M
Hz
2100 M
Hz
1900 M
Hz
Unpaired Band: 20 + 15MHz (1900 – 1920 and 2010 – 2025MHz) for TDD
Paired Band: 2 x 60MHz (1920 – 1980 and 2110 – 2170MHz) for FDD
Up-link Down-link
Satellite Band: 2 x 30MHz (1980 – 2010 and 2170 – 2200MHz)
1 2 3 11 12 . . .
1920 MHz 1980 MHz
1 2 3 11 12 . . .
2110 MHz 2170 MHz
5 MHz
Uplink Downlink
Details:
Cellular Communication Networks 36 Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2015
References
H. Holma, A. Toskala (Ed.), “WCDMA for UMTS”, 5th edition, Wiley, 2010.
A.J. Viterbi, “CDMA, Principles of Spread Spectrum Communication”, Addison-
Wesley, 1995.
R.L. Peterson, R.E. Ziemer, D.E. Borth, “Introduction to Spread Spectrum
Communications”, Prentice-Hall, 1995.
T. Ojanperä, R. Prasad, “Wideband CDMA for Third Generation Mobile
Communication”, Artech House, 1998.
R. Prasad, W. Mohr, W. Konhäuser, “Third Generation Mobile Communications
Systems”, Artech House, March 2000.