W-CDMA for UMTS – Principles W CDMA for UMTS Principles Introduction Introduction CDMA Background UMTS Standardization K P t dD l i Md Key Parameter and Duplexing Modes Code Division Multiple Access (CDMA) Why CDMA ? CDMA Principles / Spreading Codes Multi-path Radio Channel and Rake Receiver Problems to Solve Problems to Solve Macro Diversity and Soft Handover Near-Far Problem and Power Control UMTS General Requirements UMTS General Requirements FDD vs. TDD Spectrum Allocation Summary
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W-CDMA for UMTS – PrinciplesW CDMA for UMTS Principles
IntroductionIntroductionCDMA BackgroundUMTS StandardizationK P t d D l i M dKey Parameter and Duplexing Modes
Code Division Multiple Access (CDMA)Why CDMA ?CDMA Principles / Spreading CodesMulti-path Radio Channel and Rake Receiver
Problems to SolveProblems to SolveMacro Diversity and Soft HandoverNear-Far Problem and Power Control
UMTS General RequirementsUMTS General RequirementsFDD vs. TDDSpectrum Allocation
Summary
References
H Holma A Toskala (Ed ) “WCDMA for UMTS” 4th edition Wiley 2007H. Holma, A. Toskala (Ed.), WCDMA for UMTS , 4th edition, Wiley, 2007.
T. Benkner, C. Stepping, UMTS – Universal Mobile Telecommunications System, J. Schelmbach 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 SpectrumR.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 C i i ” A h H 1998Communication”, Artech House, 1998.
R. Prasad, W. Mohr, W. Konhäuser, “Third Generation Mobile Communications Systems”, Artech House, March 2000.y , ,
ETSI SMG2 air interface standardization for UMTS:ETSI SMG2 air interface standardization for UMTS:five groups evaluated competing concepts until end of 1997 (research was driven by FRAMES project)
No decision at SMG#24 on Dec 15 1997; result of the vote was 58 45% for W-CDMANo decision at SMG#24 on Dec 15, 1997; result of the vote was 58.45% for W-CDMA, 41.55% for TD-CDMA
Two proposals selected at SMG#24bis on Jan 28, 1998,W-CDMA (61 1%) and TD-CDMA (38 7%)W-CDMA (61.1%) and TD-CDMA (38.7%) consensus decision: W-CDMA for the paired frequency bands, TD-CDMA for the unpaired bands
3rd Generation Partnership Project (3GPP) formed Jan 1999 (ETSI ARIB T1P1 TTA3rd Generation Partnership Project (3GPP) formed Jan 1999 (ETSI, ARIB, T1P1, TTA, …); CDMA proposals: W-CDMA in Europe & Japan (3GPP), and CDMA2000 in the US (3GPP2)
Pressure from ITU and operators to harmonize W-CDMA:Pressure from ITU and operators to harmonize W-CDMA:OHG (Operator Harmonisation Group) proposal accepted by 3GPP on July 6-7, 1999, i.e. CDMA standard with 3 modes (W-CDMA, TD-CDMA and multi-carrier CDMA); chip rate = 3.84Mcpsrate 3.84Mcps
In June 1999 work on narrowband TDD started (TD-SCDMA); main proponent: China; chip rate = 1.28Mcps
Based upon spread spectrum technique developed for military anti-jam applications
Wide bandwidth needed to support high bit rates and to combat fading in pp g gmulti-path radio channels
Many users share the same RF carrier
E h i i d i d d diff t t d i t lEach 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) increase
Spreading codes keep channels apart such that the same carrier can be used in the next cell (frequency re-use is 1)in the next cell (frequency re use is 1)
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 ca use t e o e ba d dt o t e t a s ss o c a eeach sender has a unique random number, the sender XORs the signal with this random numberthe receiver can “tune” into this signal if it knows the pseudo randomthe receiver can tune into this signal if it knows the pseudo random number, tuning is done via a correlation function
Advantages: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 codedinterference (e.g. white noise) is not codedforward error correction and encryption can be easily integrated
Disadvantages:Disadvantages:higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is a signal)ll i l h ld h th t th t i ( t l)
all signals should have the same strength at a receiver (power control)
Spread spectrum technology:
Problem of radio transmission: frequency dependent fading can wipe out narrowProblem 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 codecode
⇒ protection against narrow band interference
detection at
interferencespread signal
signal (despreaded)
spreadinterference
power power
Side effects:
detection atreceiver
interference
f f
coexistence of several signals without dynamic coordinationtap-proof
Alternatives: Direct Sequence (UMTS)F eq enc Hopping (slo FH GSM fast FH Bl etooth)
XOR of the signal with pseudo random number (chipping sequence)XOR of the signal with pseudo-random number (chipping sequence)many chips per bit (e.g., 128) result in higher bandwidth of the signal
Advantagesreduces frequency selective fading
tbg
in cellular networks base stations can use the same frequency range
user data
0 1 XORtc
(data rate)
q y gseveral base stations can detect and recover the signalsoft handover
FDD mode: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)
• TDD: each time frame contains at least one time slot for the uplink and one time slot for the downlink
• Only spreading factors between 1 and 16 are used resulting in the same transmission rates as in FDDsame transmission rates as in FDD
• As in FDD, FDMA can be used if a Network operator provides of further , p pfrequency channels
S it hi i t ld b ft h ti l t thi ld l i i i• Switching points could be after each time slot, this would also minimize transmission delay, puts however high demands on hardware in NodeB and UE.