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W-CDMA for UMTS – Principles
IntroductionCDMA Background/ HistoryKey Parameters
Code Division Multiple Access (CDMA)Why CDMA ?CDMA Principles / Spreading CodesMulti-path Radio Channel and Rake Receiver
Problems to SolveMacro Diversity and Soft HandoverNear-Far Problem and Power Control
UMTS General RequirementsFDD vs. TDDSpectrum Allocation
UMTS Networks 2Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
References
H. Holma, A. Toskala (Ed.), “WCDMA for UMTS”, 5th edition, Wiley, 2010.T. Benkner, C. Stepping, UMTS – Universal Mobile Telecommunications System,
J. Schlembach Fachverlag, 2002.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”, Prencice-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.
UMTS Networks 3Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
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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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 neededTDD mode: Synchronization needed
Multi-rate/Variable-rate scheme
Variable-spreading factor + Multi-codeSpreading factor: 4 – 256 (FDD) and 1 – 16 (TDD)
Channel coding scheme
Convolutional coding (rate 1/2 – 1/3)Turbo coding
UMTS Networks 5Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
CDMA Key Characteristics
Based upon spread spectrum technique developed for military anti-jam applicationsWide bandwidth needed to support high bit rates and to combat fading in multi-path radio channelsMany users share the same RF carrierEach user is assigned a unique random code different to and approximately orthogonal to other codesInterference limited systems; quality degrades as number of users on a channel (carrier) increasesSpreading codes keep channels apart such that the same carrier can be used in the next cell (frequency re-use is 1)
UMTS Networks 6Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
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 XORs the signal with this random numberthe 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 neededhuge code space (e.g. 232) compared to frequency spaceinterference (e.g. white noise) is not codedforward 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)
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Spread Spectrum Technology
Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interferenceSolution: 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 coordinationtap-proof
Alternatives: Direct Sequence (UMTS)Frequency Hopping (slow FH: GSM, fast FH: Bluetooth)
detection atreceiver
interferencespread signal
signal (despreaded)
spreadinterference
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 usersChannels 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 signalUsers are separated by the spreading sequence
22
22
2
frequency
channelquality
1
spreadsignals
frequency
channelquality
1 23
4
5 6
small bandwidth guard band
UMTS Networks 9Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
DSSS (Direct Sequence Spread Spectrum) I
XOR 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 rate Rb
RC = Rb · SFtb = tC · SF
Processing GainGS = 10 · log10(SF)
user data
codesequence
resultingsignal
0 1
0 1 1 0 1 0 1 01 0 0 1 11
XOR
0 1 1 0 0 1 0 11 0 1 0 01
=
tb
tc
tb: bit durationtc: chip duration
(data rate)
(chip rate)
(chip rate)
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DSSS (Direct Sequence Spread Spectrum) II
Xuser data
codesequence
modulator
radiocarrier
spreadspectrumsignal
transmitsignal
transmitter
demodulator
receivedsignal
radiocarrier
X
codesequence
basebandsignal
receiver
integrator
products
decisiondata
sums
correlator
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CDMA Principle (Downlink)
Code 0
Code 1
Code 2
data 0
data 1
data 2
Code 0
Code 1
Code 2
data 0
data 1
data 2
sender (base station) receiver (terminal)
Transmission overair interface
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CDMA Principle (Uplink)
Code 0
Code 1
Code 2
data 0
data 1
data 2
Code 0
Code 1
Code 2
data 0
data 1
data 2
sender (terminal) receiver (base station)
transmission overair 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 codesNo need for precise synchronization as the scrambling codes remain quasi-orthogonal
data1 data2 data3
scramblingcode1
chan.code3
chan.code2
chan.code1
data4 data5
chan.code4
chan.code1
sender1 sender2
scramblingcode2
UMTS Networks 14Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
Functionality of Channelization and Scrambling Codes
Channelization Code Scrambling CodeUsage UL: Separation of physical data
(DPDCH) and control channels (DPCCH) from same terminalDL: 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 us) UL+DL: 10ms = 38400 chips
Number of codes Number of codes under 1 scrambling code = spreading factor (SF)
UL: several millionsDL: 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,-1,11,-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 kbps8 480 960 912 456 kbps
4 960 1920 1872 936 kbps
4, with 3 parallel codes
2880 5760 5616 2.3 Mbps
UMTS Networks 17Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
CDMA in Theory
Sender A sends Ad = 1, code sequence Ac = 1010011 (assign: “0”= –1, “1”= +1)sending signal As = Ad Ac = (+1, –1, +1, –1, –1, +1, +1)
Sender Bsends Bd = 0, code sequence Bc = 0110101sending 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 Aapply sequence AC chipwise (inner product)
Ar = (+2, –2, 0, 0, –2, +2, 0) Ac = 2 + 2 + 0 + 0 + 2 + 2 + 0 = 8result greater than 0, therefore, original bit was „1“
receiving BBe = (+2, –2, 0, 0, –2, +2, 0) Bc = –2 –2 + 0 + 0 – 2 – 2 + 0 = –8, i.e. „0“
wrong sequence CC = 1100110Cr = (+2, –2, 0, 0, –2, +2, 0) Cc = 0, decision impossible
UMTS Networks 18Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
CDMA on signal level I
data A
key A
signal A
data key
keysequence A
Real systems use much longer keys resulting in a larger distance between single code words in code space
1 0 1
10 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 101 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0
Ad
Ak
As
UMTS Networks 19Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
CDMA on signal level II
signal A
data B
key Bkey
sequence B
signal B
As + Bs
data key
1 0 0
00 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 111 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1
Bd
Bk
Bs
As
10-1
UMTS Networks 20Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
CDMA on signal level III
Ak
(As + Bs) * Ak
integratoroutput
comparatoroutput
As + Bs
data A
1 0 1
1 0 1 Ad
10-1
1
-1
10-1
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CDMA on signal level IV
integratoroutput
comparatoroutput
Bk
(As + Bs) * Bk
As + Bs
data B
1 0 0
1 0 0 Bd
10-11
-110-1
UMTS Networks 22Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
comparatoroutput
CDMA on signal level V
wrongkey K
integratoroutput
(As + Bs) * K
As + Bs
(0) (0) ?
Assumptionsorthogonality of keysneglectance of noiseno differences in signal level => precise power control
10-11
-1
10-1
UMTS Networks 23Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
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)
UMTS Networks 24Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
Multi-path Transmission
Multi-path components can be resolved due to ACF of codes
Spreader
SpreadingSequence c(t)
Despreader(Correlator)
SpreadingSequence c(t-Td)
Receiver synchronizes to each multi-path component for de-spreading
UMTS Networks 25Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
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. 200 ns
UMTS Networks 26Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
RAKE Receiver – Practical Realization
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Macro-Diversity & Soft Handover
Optimal coherent combiningin the RAKE receiver (at MS)
NodeB 1NodeB 2
UE
UMTS Networks 28Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
Multi-user CDMA
Conventional CDMA Receiver (Base Station):
Despreader(Correlator)
SpreadingSequence c2(t-Td2)
• coherent (amplitude and phase) RF demodulation at base station
• separate despreading and demodulation ofeach signal at base station
• one Rake receiver with K fingers per user• unsynchronized transmission between the
mobiles
SpreadingSequence c1(t-Td1)
SpreadingSequence cn(t-Tdn)
UMTS Networks 29Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
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
UMTS Networks 30Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
Interference Cancellation
Multi-user Interference Cancellation (Joint Detection):
Matched Filter toSequence c1(t)
Detection mechanism takes into account interference from other users as all signalsare known in the receiver(known interference can becanceled)
Matched Filter toSequence c2(t)
Matched Filter toSequence cn(t)
MF1
MF2
MFn
Multi-userDetector
(Joint Detection
Interference Cancellation)
UMTS Networks 31Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
Interference Cancellation – Realization
Subtractive interference cancellation
UMTS Networks 32Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
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-linkBetter 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-linkStrict 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)
UMTS Networks 33Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
FDD vs. TDD Mode (contd.)
TDD-Mode
FDD-Mode(one direction)
UMTS Networks 34Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
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
UMTS Networks 35Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
Global Spectrum Allocations for IMT-2000
ITU2010 20251980
MSS MSS*1930
IMT-2000 MSSMSS*IMT-2000
2160 2170 2200 MHz
*Region2
1885 2110
PHS
20101980 2025Japan
2110 22002170
IMT-2000 MSSMSSIMT-2000
18951885 1918.1 MHz
1980 2110 22002170
IMT-2000 MSS
19001880
DECT
2010
MSSIMT-2000
2025 MHz
Europe
2110 220021652150
Reserve MSSBroadcast Auxilary
1910 1930 1990 2025
MSS
1850
PCS*PCSA B CD E F
PCSA B CD E F
MHz
USA
20101980 2025
China2110 22002170
MSSMSS
1900 1920 MHz1865 1880 1945 1960
CDMA FDD-WLL
FDD-WLLCDMA
TDD-WLL
MSS: Mobile Satellite Services
UMTS Networks 36Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
UMTS Spectrum
2200 MH
z
2000 MH
z
2100 MH
z
1900 MH
z
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: