W-CDMA for UMTS – Principles Introduction CDMA Background/ History Key Parameters 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 Spectrum Allocation
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”, AddisonWesley, 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
Andreas Mitschele-Thiel, Jens Mueckenheim
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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 UMTS Networks
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W-CDMA for UMTS – Summary of Key Parameters
Multiple-Access
DS-CDMA (TD-CDMA)
Duplex scheme
FDD (TDD) 3.84 MChip/s (TDD: 1.28/ 3.84/ 7.68 MChip/s) Flexible in the range 4.6 – 5.0 MHz (200 kHz carrier raster) 1920 – 1980 / 2110 – 2170 paired (FDD) 1900 – 1920 and 2010 – 2025 unpaired (TDD) 10 ms / (15 time slots) FDD mode: No accurate synchronization needed TDD mode: Synchronization needed Variable-spreading factor + Multi-code Spreading factor: 4 – 256 (FDD) and 1 – 16 (TDD) Convolutional coding (rate 1/2 – 1/3) Turbo coding
Chip rate Carrier spacing Frequency bands Frame length Inter-BS synchronization Multi-rate/ Variable-rate scheme Channel coding scheme
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CDMA Key Characteristics Based upon spread spectrum technique developed for military anti-jam applications Wide bandwidth needed to support high bit rates and to combat fading in multi-path radio channels Many users share the same RF carrier Each user is assigned a unique random code different to and approximately orthogonal to other codes Interference limited systems; quality degrades as number of users on a channel (carrier) increases Spreading codes keep channels apart such that the same carrier can be used in the next cell (frequency re-use is 1)
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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 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 (e.g. white noise) is not coded 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) UMTS Networks
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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 interference
power
power
spread signal
signal (despreaded) spread interference
detection at receiver
Side effects:
f
f
coexistence of several signals without dynamic coordination tap-proof Alternatives: Direct Sequence (UMTS) Frequency Hopping (slow FH: GSM, fast FH: Bluetooth) UMTS Networks
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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
channel quality
1
2
6
4 frequency small bandwidth
guard band
channel quality
Frequency selective fading causes only some reduction in the level of the received signal Users are separated by the spreading sequence
1
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2
2
2
2
2
frequency
spread signals
UMTS Networks
5
3
Nov. 2011
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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
tb user data
(data rate)
0
Spreading factor SF: ratio between chip rate RC and data rate Rb RC = Rb · SF tb = tC · SF
1 tc
code sequence 01101010110101
(chip rate)
= resulting signal
Processing Gain GS = 10 · log10(SF)
XOR
01101011001010
(chip rate)
tb: bit duration tc: chip duration
UMTS Networks
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DSSS (Direct Sequence Spread Spectrum) II spread spectrum signal
user data X
transmit signal modulator
code sequence
radio carrier transmitter
correlator baseband products signal
received signal
sums data
demodulator radio carrier
X
integrator
decision
code sequence receiver
UMTS Networks
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CDMA Principle (Downlink) sender (base station)
receiver (terminal)
Code 0
Code 0
data 0
data 0 Code 1
Code 1 Transmission over air interface
data 1
data 1 Code 2
Code 2
data 2
data 2
UMTS Networks
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11
CDMA Principle (Uplink)
sender (terminal) Code 0
receiver (base station) transmission over air interface
Code 0
data 0
data 0 Code 1
Code 1
data 1
data 1 Code 2
Code 2
data 2
data 2
UMTS Networks
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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
data4
data5
chan. code1
chan. code2
chan. code3
chan. code1
chan. code4
scrambling code1
scrambling code2
sender1 UMTS Networks
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sender2 Nov. 2011
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Functionality of Channelization and Scrambling Codes
Channelization Code
Scrambling Code
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
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 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
Usage
UMTS Networks
DL: Separation of sectors/cells
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OVSF-Coding Tree 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
X,X
1,-1,1,-1,1,-1,1,-1
X,-X SF=2n
...
1,1,-1,-1,-1,-1,1,1
1
X
SF=n
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
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 UMTS Networks
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Downlink Dedicated Channel Symbol and Bit Rates
Spreading Channel factor 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
...
UMTS Networks
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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 B sends Bd = 0, code sequence Bc = 0110101 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 Be = (+2, –2, 0, 0, –2, +2, 0)
Bc = –2 –2 + 0 + 0 – 2 – 2 + 0 = –8, i.e. „0“
wrong sequence CC = 1100110 Cr = (+2, –2, 0, 0, –2, +2, 0)
UMTS Networks
Cc = 0, decision impossible
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CDMA on signal level I data A
1
0
Ad
1
key A key sequence A data key
0
1
0
1
0
0
1 0
0
0
1
0 1
1
0
0
1 1
1
0
1
0
1
1
1 0
0
0
1
0 0
0
1
1
0 0
Ak
As
signal A
Real systems use much longer keys resulting in a larger distance between single code words in code space
UMTS Networks
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CDMA on signal level II
As
signal A
data B key B key sequence B data
key
1
0
Bd
0
0
0
0
1
1
0
1
0
1
0
0
0
0
1
0
1
1
1
1
1
1
0
0
1
1
0
1
0
0
0
0
1
0
1
1
1
Bk
Bs
signal B
1 0 As + Bs
UMTS Networks
-1
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CDMA on signal level III data A
1
0
1
Ad 1
As + Bs
0 -1 1
Ak
-1 1 (As + Bs) * Ak
0 -1
integrator output comparator output
UMTS Networks
1
0
Andreas Mitschele-Thiel, Jens Mueckenheim
1
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CDMA on signal level IV data B
1
0
0
Bd 1
As + Bs
0 -1 1
Bk
-1 1 (As + Bs) * Bk
0 -1
integrator output comparator output
UMTS Networks
1
0
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0
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CDMA on signal level V 1 As + Bs
0 -1 1
wrong key K
-1 1
(As + Bs) *K
0 -1
integrator output comparator output
(0)
(0)
Assumptions orthogonality of keys neglectance of noise no differences in signal level => precise power control Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011 UMTS Networks
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Properties of Spreading Sequences Code sequence #1
Auto correlation function (ACF)
Code sequence #2
Required properties of spreading (properties of the transmitted signals): • High ACF peak Cross correlation function (CCF)
UMTS Networks
• 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-T d)
Receiver synchronizes to each multi-path component for de-spreading
UMTS Networks
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RAKE Receiver Correlate and track each multi-path component separately
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
Optimal coherent combining UMTS Networks
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RAKE Receiver – Practical Realization
UMTS Networks
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Macro-Diversity & Soft Handover
NodeB 2
NodeB 1
UE
Optimal coherent combining in the RAKE receiver (at MS)
UMTS Networks
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Multi-user CDMA Conventional CDMA Receiver (Base Station):
Despreader (Correlator)
Spreading Sequence c1(t-T d1)
Spreading Sequence c2(t-T d2)
• 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
Spreading Sequence cn(t-T dn)
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Near-Far Problem – Power Control
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
UE 1
The problem can be resolved through dynamic power control to equalize all received power levels NodeB
AND/OR UE 2
UMTS Networks
By means of joint multi-user detection
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Interference Cancellation Multi-user Interference Cancellation (Joint Detection):
Matched Filter to Sequence c1(t)
MF1 Matched Filter to Sequence c2(t)
MF2 Matched Filter to Sequence cn(t)
Multi-user Detector (Joint Detection Interference Cancellation)
Detection mechanism takes into account interference from other users as all signals are known in the receiver (known interference can be canceled)
MFn
UMTS Networks
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Interference Cancellation – Realization
Subtractive interference cancellation
UMTS Networks
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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)
UMTS Networks
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FDD vs. TDD Mode (contd.)
FDD-Mode (one direction)
TDD-Mode
UMTS Networks
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TDD Mode Switching
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
1 Frame (10ms) of 15 Slots
UMTS Networks
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Global Spectrum Allocations for IMT-2000
IMT-2000
ITU 1885
1930
1880
IMT-2000
2010
1980
2010
PCS A D B E F C
1910
1930
2110
1990
2025
MSS
2170
2170
2110
*Region2 2200
MHz
2200
MHz
2200
MHz
2200
MHz
2200
MHz
MSS
IMT-2000
PCS A D B E F C
1850
2025
MSS
CDMA FDD- MSS WLL 1945 1960 1980 2010
PCS*
2160 IMT-2000
FDD- TDDCDMA WLL WLL 1865 1880 1900 1920
China
2110
MSS 1980
1885 1895 1918.1
USA
1980
1900 PHS
Japan
MSS* 2010 2025
IMT-2000
DECT
Europe
IMT-2000 MSS*
MSS
MSS 2170 MSS
2025
2110
Broadcast Auxilary
MSS 2025
2170
Reserve 2110
MSS 2150
2165
MSS: Mobile Satellite Services
UMTS Networks
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UMTS Spectrum
2200 MHz
Down-link
2100 MHz
2000 MHz
1900 MHz
Up-link
Unpaired Band: 20 + 15MHz (1900-1920 and 2010-2025MHz) for TDD Paired Band: 2 x 60MHz (1920-1980 and 2110-2170MHz) for FDD Details: Uplink
1920 MHz 1
2
3
...
1980 MHz
Downlink
2110 MHz
11 12
1
2
3
...
2170 MHz 11 12
5 MHz
Satellite Band: 2 x 30MHz (1980-2010 and 2170-2200MHz) UMTS Networks
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