3g cdma-qualcom


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– There are others : IS95 HDR, EDGE, etc.
» Direct Spread CDMA TDD
» Direct Spread CDMA FDD
» Multi-carrier CDMA FDD
Global 3G comprises of 3 modes :
– Marketed as Global 3G CDMA implying a single unified standard. In reality,
– Mostly dominated by Direct Sequence CDMA.
– Market is expected to be fragmented amongst several competing
IMT2000 guidelines defined by the ITU.
– Analog was 1G. GSM/IS95 were 2G. Next is 3G.
What is 3G ?
across the world.
Envisioned as a single Global standard allowing seamless roaming
Used interchangeably with IMT2000 although there are some specific
A loosely defined term referring to next generation wireless systems.

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3g cdma-qualcom

  1. 1. Page 1Jan 5, 2000<3rd generation CDMA wireless systems><Avneesh Agrawal, Qualcomm>www.TempusTelcosys.com
  2. 2. Page 2Jan 5, 2000Overview• What is 3G ?• A brief overview of IS95• Key design choices for CDMA 3G systems.– Bandwidth– Modulation– Coding– Power Control– Transmit Diversity– Base Station Synchronization.– Acquisition– Beam Forming– Multi-user detection– Peak To Average Power• Objective is not to provide detailed justifications, but instead providesome insight into the level of optimization that went into designing thephysical layer of the next generation wireless systems.www.TempusTelcosys.com
  3. 3. Page 3Jan 5, 2000What is 3G ?• A loosely defined term referring to next generation wireless systems.– Analog was 1G. GSM/IS95 were 2G. Next is 3G.• Used interchangeably with IMT2000 although there are some specificIMT2000 guidelines defined by the ITU.• Envisioned as a single Global standard allowing seamless roamingacross the world.– Market is expected to be fragmented amongst several competingstandards.– Mostly dominated by Direct Sequence CDMA.– Marketed as Global 3G CDMA implying a single unified standard. In reality,Global 3G comprises of 3 modes :» Multi-carrier CDMA FDD» Direct Spread CDMA FDD» Direct Spread CDMA TDD– There are others : IS95 HDR, EDGE, etc.www.TempusTelcosys.com
  4. 4. Page 4Jan 5, 200080 81 95 99 00 0201 03 04Indicative timeline ofcommercial launch92IS-41The Big PictureAMPSGSMHCSDGPRSEDGE IIEDGECDMA-95CDMA-DSFDDCDMA-MC1xCDMA-MC3xCDMA-TDDCDMA-95BGSM-MAPIPIS95 HDRwww.TempusTelcosys.com
  5. 5. Page 5Jan 5, 2000IS95 Forward Link (BS to mobile)Walsh 0Pilot Channel(all 0s)User m ForwardTraffic ChannelRate Set 1Walsh mMuxAdd CRCAdd 8 tail bitsConv CodeRate 1/2K = 9SymbolRepetitionBlockInterleaverLong CodeGeneratorDecimator Decimator42 bit Long Code1.2288 Mcps19.2 ksps 800 Hz8.6 kbps4.0 kbps2.0 kbps0.8 kbps9.6 kbps4.8 kbps2.4 kbps1.2 kbps19.2 ksps9.6 ksps4.8 ksps2.4 ksps19.2 kspsPower ControlBits800 bpsWalsh jMuxAdd CRCAdd 8 tail bitsConv CodeRate 1/2K = 9SymbolRepetitionand PunctureBlockInterleaverLong CodeGeneratorDecimator Decimator42 bit Long Code1.2288 Mcps19.2 ksps 800 Hz13.35 kbps6.25 kbps2.75 kbps1.05 kbps14.4 kbps7.2 kbps3.6 kbps1.8 kbps28.8 ksps14.4 ksps7.2 ksps3.6 ksps19.2 kspsPower ControlBits800 bpsUser j ForwardTraffic ChannelRate Set 2AAASync ChannelPaging Channel (x n)Other users traffic channelAwww.TempusTelcosys.com
  6. 6. Page 6Jan 5, 2000IS95 Forward Link (contd.)A s(t)ΣBasebandFilter XBaseband Filter XI-channel PN sequence1.2288 McpsQ-channel PN sequence1.2288 Mcpscos (2π f t)sin (2π f t)www.TempusTelcosys.com
  7. 7. Page 7Jan 5, 2000IS95 Reverse Link (Mobile to Base Station)Add CRCAdd 8 tail bitsConv CodeRate 1/3K = 9ORRate 1/2K=9SymbolRepetitionBlockInterleaver8.6 kbps4.0 kbps2.0 kbps0.8 kbps9.6 kbps4.8 kbps2.4 kbps1.2 kbps28.8 ksps14.4 ksps7.2 ksps3.6 ksps28.8 ksps14.4 kbps7.2 kbps3.6 kbps1.8 kbps13.35 kbps6.25 kbps2.75 kbps1.05 kbps64-aryOrthogonalModulatorData BurstRandomizerLong CodeGeneratorB1.2288 Mcps1.2288 Mcpswww.TempusTelcosys.com
  8. 8. Page 8Jan 5, 2000IS95 Reverse Link (contd.)s(t)BBasebandFilter XI-channel PN sequence1.2288 McpsQ-channel PN sequence1.2288 Mcpscos (2π f t)Baseband Filter XΣsin (2π f t)D1/2 PN ChipDelay = 406.9 nswww.TempusTelcosys.com
  9. 9. Page 9Jan 5, 20003G CDMAwww.TempusTelcosys.com
  10. 10. Page 10Jan 5, 20003G Standards• Focus on 2 systems– WCDMA FDD and CDMA2000– Expected to be the dominant 3G standards, although IS95 HDR is gainingpopularity.– HDR is a data only system.• WCDMA (CDMA-Direct Sequence)– Strongly pushed by ETSI (Europe) and ARIB (Japan)– CDMA Air interface (3.84 Mcps), GSM protocol stack.– NTT DoCoMo (under pressure from IS95 deployment by DDI/IDO in Japan)is targeting initial deployment in Fall, 2001.• CDMA2000 (CDMA - Multicarrier)– An evolution over IS95– Two versions : 1x (1.2288 MHz) and 3x ( 3 carriers at 1.2288 MHz each)• There seems to be little debate on which system has higher capacity(as technically, the two systems are very similar)• Success depends largely on cost, time to market and political factors.• Focus of this talk is on Physical Layerwww.TempusTelcosys.com
  11. 11. Page 11Jan 5, 2000WCDMA Forward LinkS/P Cch ,1DPDCH1/DPCCHS/P Cch ,2DPDCH2S/P Cch ,NDPDCHNΣΣ..........*jI+jQIQCscrambOVSF Codes = BitReverse(Walsh Codes)Root Raised CosineFilter (roll-off = .22)IQ11 00223344556677889917171616151514141313121211111010Gold CodePN sequencewww.TempusTelcosys.com
  12. 12. Page 12Jan 5, 2000WCDMA Forward LinkTx.Antenna 2MUXDataTx.Antenna 1Channelization code andlong scrambling code C ,spreading length = MAnt1Ant2Ant1Ant2TPCTFIPilotMUXChannelEncoderInterleaverSTTDEncoderRateMatchingQPSK symbolsDiversity PilotOne radio frame, Tf = 10 msTPCNTPC bitsSlot #0 Slot #1 Slot #i Slot #14Tslot = 2560 chips, 10*2kbits (k=0..7)Data2Ndata2 bitsDPDCHTFCINTFCI bitsPilotNpilot bitsData1Ndata1 bitsDPDCH DPCCH DPCCHwww.TempusTelcosys.com
  13. 13. Page 13Jan 5, 2000WCDMA Reverse LinkPilotNpilot bitsTPCNTPC bitsDataNdata bitsSlot #0 Slot #1 Slot #i Slot #14Tslot = 2560 chips, 10*2kbits (k=0..6)1 radio frame: Tf = 10 msDPDCHDPCCHFBINFBI bitsTFCINTFCI bitsΣCch,1DPDCH1βdCch,3DPDCH3βdCch,d5DPDCH5βdChannelization codes gain factorsΣCch,2DPDCH2βdCch,4DPDCH4βdCch,6DPDCH6βdCch,0DPCCH*jCscrambI+jQβcIQwww.TempusTelcosys.com
  14. 14. Page 14Jan 5, 2000Bandwidth• Both systems support wider bandwidth.• Biggest advantage is ability to support higher peak rates.– Although HDR supports the same peak rates in a 1.25 MHz channel.• Other advantages (increased frequency diversity, better interferencestatistics, etc.) have not been properly quantified.• The disadvantage is increased design complexity• WCDMA has a bandwidth of 3.84 Mcps. Big PR effort against IS95 :Wideband vs Narrowband CDMA.• CDMA2000 1x is the same as IS95. 3x MultiCarrier is 3.6864 Mcps.• Both WCDMA and CDMA2000 3x MC support data rates around 2Mbps.– Only a single user (in good channel conditions) / sector can be supported atthese rates, i.e. high rate service is not going to be cheap !www.TempusTelcosys.com
  15. 15. Page 15Jan 5, 2000Modulation• Reverse Link– IS95 used 64-ary orthogonal modulation» This allowed non-coherent demod at the receiver. Coherent demodulation atreceiver was considered risky.» Peak data rate (I.e. 14.4 kbps) was much lower than the signal bandwidth (1.2288Mcps).» Assumed a conventional receiver (I.e single user CDMA receiver) at the basestation. This implies that primary objective is to reduce transmit power at themobile.» So, with this system, objective is to use the best (given the constraints ofimplementation complexity) rate 1/p code, where p = processing gain.» IS95 used a convolutional code (rate 1/2 or 1/3) followed by a (6,64) orthogonalblock code, followed by a repetition code.www.TempusTelcosys.com
  16. 16. Page 16Jan 5, 2000Modulation• Forward Link– IS95 used BPSK because» More tolerant to phase errors. Performance in fast fading channels was aconcern.» General view was that IS95 was interference limited and hence more efficientmodulation was not necessary.• Clearly, increased bandwidth would have allowed more powerful lower ratecodes, and hence could have increased capacity.• In benign channel conditions (e.g. wireless local loop), the number ofavailable walsh channels was limiting forward link capacity.www.TempusTelcosys.com
  17. 17. Page 17Jan 5, 2000Modulation• 3G– WCDMA, CDMA2000 : both are going for QPSK modulation (on both links)with pilot for phase reference.» Increased capacity, lower rate codes.» Coherent demodulation not perceived as a problem. In fact, the overhead of piloton the up link more than compensated by improvements in synchronization andpower control.» Supporting higher data rates. Hence, there is insufficient processing gain for 64-ary orthogonal modulation.– HDR is using adaptive modulation (upto 16 QAM) and adaptive processinggain to improve capacity.www.TempusTelcosys.com
  18. 18. Page 18Jan 5, 2000Coding• IS95– Convolutional codes only.– Rate 1/2 or 1/3 on uplink, K = 9.– Rate 1/2 or 3/4 on downlink. The rate 3/4 code is used for the highest datarate (14.4 kbps), and is not a good code …• 3G– Same conv codes for CDMA2000, WCDMA and HDR, except that the rate3/4 code has been removed and a rate 1/3 code on the downlink has beenintroduced.– Turbo Codes for data.» CDMA2000 and WCDMA use the same parallel Concatenated Codes. K = 4, rate1/3. The turbo interleavers are different» HDR : Serial Concatenated codes. K = 5, rate 1/2 outer code followed by K = 3,rate 1/2 inner codewww.TempusTelcosys.com
  19. 19. Page 19Jan 5, 2000Power Control• IS95– Fast Reverse Link Power Control at 800 Hz– Very slow Forward Power Control» IS95 A forward power control was a few Hz.» IS95 B increased it to 50 Hz– Slow Forward Power Control big limitation.– In order to guarantee voice quality, base station has to put a floor onminimum transmit power.– Generally, the forward link is the capacity limiting link.• 3G– CDMA2000 uses 800 Hz for both uplink and downlink.– WCDMA uses 1500 Hz for both links.– Improved forward power control has a significant improvement on systemcapacity.– HDR uses rate control instead of power control.www.TempusTelcosys.com
  20. 20. Page 20Jan 5, 2000Transmit Diversity• No transmit diversity for IS95• CDMA2000 uses 2 forms:– OTD : Orthogonal Transmit Diversity.» Transmit consecutive symbols on adjacent antennas using orthogonal codes.– STS : Space Time Spreading» Ant 1 : S1 x W1(t) - S2* x W2(t)» Ant 2 : S1* x W2(t) + S2 x W1(t)» W1(t), W2(t) are orthogonal sequences.• WCDMA supports several forms of Transmit Diversity– STTD : Space Time Transmit Diversity» Ant 1 : transmit S1 S2 , S1 & S2 are complex symbols» Ant 2 : transmit -S2* S1*» For STS & STTD, performance equivalent to two antenna receive diversity in flatfading environment.www.TempusTelcosys.com
  21. 21. Page 21Jan 5, 2000Transmit Diversity– Feedback Mode Transmit Diversity» WCDMA provides fast feedback (upto 1500 Hz) mode transmit diversity.» Allows receiver to control the amplitude and phase of the two antennas.– Time Switched Transmit Diversity» Signal is transmitted alternately from two antennas using predetermined pattern.S1 S2STTD encoderS1 S2-S2*S1*T 2T0 T 2TAnt 1Ant 2MobileAntennaPath 1Path jNdataSTTDwww.TempusTelcosys.com
  22. 22. Page 22Jan 5, 2000Base Station Synchronization• CDMA systems generally have a frequency reuse factor of 1, andhence do not require any frequency planning.• However, they do need to do code planning in order to ensure that theydo not allocate the same PN codes to adjacent base stations.• In IS95 and CDMA2000, different base stations use a different offset ofthe same PN sequence.– Base stations are synchronized using GPS. Hence, having different offsetsensures that the PN sequences from different base stations will not coincidewith one another. The offsets are at a minimum of 256 chips apart.• WCDMA does not require synchronization.– Mostly a political issue as some governments do not want to have theircommunications infrastructure rely on a US defense program.– Once again, this was a big PR effort against IS95 & CDMA2000.– Most of the initial deployments are expected to be in synchronous mode.www.TempusTelcosys.com
  23. 23. Page 23Jan 5, 2000Base Station Synchronization• Async. Systems cannot use offsets of the same PN sequence fordifferent base stations and hence we need an efficient way to generatemultiple PN sequences.• WCDMA uses Gold codes for PN sequences. Gold codes areconstructed as linear combinations (in GF(2) ) of two m-sequences.– Cyclic shifts of one sequence with respect to another create different codes.– IS95 & CDMA2000 use an m-sequence (I.e. maximal length LFSR) forgenerating the PN sequence.• Asynchronous base stations have some problems :– Initial Acquisition» Instead of searching for a single PN sequence, with async. Systems, the mobilehas to search for multiple PN sequences.– Handoff searching.» Every handoff search is like initial acquisition.» In contrast, for sync. Systems, handoff searching is simpler. E.g. for IS95, theinitial acquisition window size is 215 chips. For handoff searching, the uncertaintyis much less (= max delay spread)www.TempusTelcosys.com
  24. 24. Page 24Jan 5, 2000Acquisition• Fast acquisition is very important for a mobile user in a multi-cellularenvironment.– Even more important for CDMA systems where minimizing transmit powerto close the link is a key determinant of system capacity.– So, phone should always try to lock onto the strongest pilot.• CDMA2000 uses a continuous pilot like IS95.• WCDMA uses a 3 step hierarchical search process to reduceacquisition time.www.TempusTelcosys.com
  25. 25. Page 25Jan 5, 2000WCDMA Searching• Total of 512 Gold Codes divided into 64 groups of 8 codes each.• In addition, there are 2 Synchronization sequences, SCH1 and SCH2.• SCH1 is a 256 chip PN code common to all base stations. Repeatsevery slot (1 slot = 2560 chips)• SCH2 can be one 16 different sequences. Code length is 256 chipsand it is time aligned with SCH1. Sequence length is 15 slots (10 ms).– Sequence is sub-set of a Reed Solomon Code.– Comma Free Property. That means, no cyclic shift of a code is a valid code.– So, receiver can unambiguously determine start of 15 slot sequence.– 64 different sequences, each representing one code group• Step 1 : Use 256 chip match filter to determine modulo ‘slot’ (I.e. 2560chips) timing.• Step 2 : Identify code group and derive frame timing (10 ms timing)• Step 3 : Exhaustive search against 8 possible codes in a code group.www.TempusTelcosys.com
  26. 26. Page 26Jan 5, 2000WCDMA Synchronization ChannelPrimarySCHSecondarySCH256 chips2560 chipsOne 10 ms SCH radio frameacsi,0acpacsi,1acpacsi,14acpSlot #0 Slot #1 Slot #14www.TempusTelcosys.com
  27. 27. Page 27Jan 5, 2000Beam Forming• IS95 only supports fixed sectorization.• Beam Forming is considered important for 3G systems.• All 3G systems (that I am aware of) support beam forming.• Requirement is simple : Each channel with beam forming should havea dedicated pilot for phase reference.• None of the systems provide a mechanism for the phone to provide theCSI (Channel State Information) to the transmitter (with the exceptionof Feedback Mode Transmit Diversity in WCDMA).– Beam form on remote scatterers– Have fixed spot beams for high capacity areas.www.TempusTelcosys.com
  28. 28. Page 28Jan 5, 2000Multi-User Detection• Does not seem to be much interest in multi-user detectors.• A year ago, NTT was a big proponent of multi-user receivers, but latelythere has been little development on that front.• Biggest problem is designing multi-user receivers with reasonablecomplexity for a multi-cellular environment.• WCDMA standard supports short spreading codes (256 chips asopposed to the regular 38400 chips) to aid in multi-user detection.– With long codes, the correlation matrix of the codes changes every symbol.• Schemes such as interference cancellation do not require standardssupport.• In IS95 the downlink was the capacity limiting link. With WCDMA &CDMA2000, the downlink capacity has been improved, but withasymmetric data rates, downlink may still be the capacity limiting link.– Having multi-user receivers on the base station would have little impact oncapacity.www.TempusTelcosys.com
  29. 29. Page 29Jan 5, 2000Peak To Average Power• IS95 uses 2 schemes to reduce Peak To Average Power– Offset QPSK modulation to reduce Peak to Average.– Constant power transmission. For lower data rates, transmission isdiscontinued for some duration. The Peak to average remains the same;however, peak to average when the Power Amplifier is on is reduced.• 3G– HPSK (Hybrid Phase Shift Keying)» c = c1 (w0 + j c2*w1)» c1 = PN sequence changing at chip rate.» c2 = PN sequence changing at half the chip rate.» W0 = { 1 1}; W1 = {1 -1}» phase transitions less than 90 degrees half the time.– Continuous transmission => worse peak to average» Compensated by improved power control, time diversity and receiversynchronization.www.TempusTelcosys.com
  30. 30. Page 30Jan 5, 2000SummaryForward Link Capacity Improvements• Fast Forward Power Control• More spectrally efficient modulation• Turbo codes and lower rate convolutional codes.• Transmit diversity• Dedicated pilots for support of beam forming.• Support higher peak data rates.• Protocol improvements to improve packet data transmission.www.TempusTelcosys.com
  31. 31. Page 31Jan 5, 2000SummaryReverse Link Capacity Improvements• Coherent Reverse Link• Improved synchronization and power control because of Reverse LinkPilot.• Improved time diversity and power control because of continuoustransmission.• QPSK modulation• Turbo codes• Multi-user detection• Faster Power Control (for WCDMA)• Improved Access Channel– Reservation based schemes as opposed to slotted Aloha in IS95www.TempusTelcosys.com