ofdm versus cdma comparison for cellular...
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OFDM versus CDMA comparison for OFDM versus CDMA comparison for
cellular systemscellular systems
Anand DabakDMTS, Manager,
Mobile Wireless Research, DSPS R&D Center, Texas Instruments
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Introduction
• Wireless standards comparison: OFDM versus CDMA
• TI’s proposal to 3GPP2 EV-DO broadcast multicast system (BCMCS)
• TI’s MIMO proposal to IEEE 802.16(e)
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Standards Comparison
200420062005TBD20052002Deployment
FDDFDD/TDDFDDFDDFDDFDDDuplexing
~ 1 million
gates
~ 1.6 million
gates
~ 1 million
gates
~1.4 million
gates
~1.2 million
gates
~1 million
gates
BB complexity
(incl. memory)
BTSBTSRNC/PDSNRNC/PDSNRNC/PDSNRNC/PDSNIP termination
+ OFDM
+ PHY HARQ
+ IP at BTS
+ OFDM
- MAC HARQ*
+ IP at BTS
- CDMA
+ PHY HARQ
- IP at RNC**
- CDMA
+ PHY HARQ
- IP at RNC**
- CDMA
+ Low latency
- IP at RNC**
- CDMA
- High latency
- IP at RNC**
Bandwidth
efficiency features
ProprietaryYesYesYesYesYesStandards
compatibility
Single
company
SharedSharedSharedSingle
company
Single
company
Intellectual
property model
No
153 Kbps
2.4 Mbps
1.25 MHz
DO Rel 0
NoNoYesYesNoCircuit switched
voice support
900 Kbps20 Mbps2 Mbps1.8 Mbps1.8 MbpsUL peak data rate
3.1 Mbps
1.25 MHz
70 Mbps
20 MHz
14.4 Mbps
5 MHz
3.09 Mbps
1.25 MHz
3.07 Mbps
1.25 MHz
DL peak data rate
Bandwidth
F-OFDMWiMax
(802.16(e))
HSDPAEV-DV Rel DDO Rel A
* : 802.16 (d) has MAC level HARQ ** : Most networks are evolving to all IP
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Comparing system alternatives: OFDM vs. CDMAComparing system alternatives: OFDM vs. CDMA
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Advantages of OFDMAdvantages of OFDM
• No need for time-domain equalizer:
– CDMA needs chip equalizer even for channels with 1-2 µs delay spread
• Naturally more robust in frequency selective channels:
– Orthogonality (DFT)
– Cyclic prefix (GI)
• Natural roadmap to achieve higher data rate in the next releases
– MIMO more natural for OFDM: each tone is flat fading
• Convergence with other wireless technologies. Examples:
– WLAN
– WiMAX
– 4G (VSF-OFCDM)
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Throughput studyThroughput study
• Per carrier sector throughput comparison is done for CDMA & OFDM systems:
– ITU PedA and PedB channels
– Data-only
– Max C/I scheduling
– 10 and 50 active users per sector
• Assess potential (maximum) performance throughput gain of OFDM over
CDMA for packet data
Sampling rate = 1.2288 McpsChip rate = 1.2288 Mcps
AMCS: QPSK r=1/4 to 16QAM r=3/4 with
adaptive rate control & scheduling per tone
AMCS: QPSK r=1/4 to 16QAM r=3/4 with
adaptive number of codes
Basic FFT based OFDM receiver 1) Rake receiver
2) 10-tap (perfect) MMSE chip equalizer
No. FFT pts=256, maximum no. data tones=224SF=32, maximum no. Walsh codes=28
OFDMCDMA
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Geometry distributionGeometry distribution
• FR=1:geometry limited to 17dB due to interference from neighboring sectors
• Ior: desired base station power; Ioc: Interfering base station power
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Simulation results: Simulation results: PedAPedA (1 receive antenna)(1 receive antenna)
• OFDM comes very close to CDMA when channel is close to 1-path
• 10 tap equalizer for CDMA assumed, 1 receive antenna
• Max C/I scheduler assumed
• Ior: desired base station power; Ioc: Interfering base station power
OFDM performance similar to CDMA
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Simulation results: Simulation results: PedBPedB (1 receive antenna)(1 receive antenna)
• OFDM ~ 20% better than CDMA in Ped B
• 10 tap equalizer for CDMA assumed, 1 receive antenna
• Max C/I scheduler assumed
• Ior: desired base station power; Ioc: Interfering base station power
OFDM to CDMA gain < 20 %
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Simulation results: Simulation results: PedBPedB ( 2 receive antennas)( 2 receive antennas)
• OFDM ~10% better than CDMA in Ped B
• 10 tap equalizer for CDMA assumed, 2 receive antennas
• Max C/I scheduler assumed
• Ior: desired base station power; Ioc: Interfering base station power OFDM to CDMA gain ~ 10%
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Simulation results: Simulation results: VehVeh A (10 users)A (10 users)• OFDM ~15% better than CDMA with 1 receive antenna, ~ 5 % gain with 2 receive antennas
• 10 tap equalizer for CDMA assumed
• Max C/I, Round robin schedulers assumed
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Simulation results: Simulation results: VehVeh A (50 users)A (50 users)• OFDM ~15% better with 1 receive antenna and ~ 5 % better with 2 receive antennas
• 10 tap equalizer for CDMA assumed
• Max C/I, Round robin schedulers assumed
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TITI’’ss proposal to 3GPP2 EVproposal to 3GPP2 EV--DO broadcast multicast DO broadcast multicast
system (BCMCS)system (BCMCS)
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Review of OFDM EBM (proposed by Qualcomm, June Review of OFDM EBM (proposed by Qualcomm, June ‘‘2004)2004)
• Prior work:
– OFDM enhanced broadcast-multicast (EBM) system was proposed by Qualcomm.
– OFDM EBM has 4x the peak rate of current BCMCS.
• Features of OFDM EBM:
– OFDM modulation for data, CDMA modulation for TDM pilot and MAC.
– Content dependent spreading
– Data rate loss due to insertion of cyclic prefix and pilot tones is offset from gains due
to ISI avoidance (240 data symbol tones, 80 cyclic prefix tones, 80 pilot/guard tones)
� 40% overhead
Data
400 chips
MAC 64
chips
½ slot = 1024 chips ½ slot = 1024 chips
Pilot
96
chips
Data
400 chips
Data
400 chips
Data
400 chips
Pilot
96
chips
MAC 64
chips
MAC
64
chips
MAC
64
chips
OFDM OFDM OFDM OFDMCDMA CDMA
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Implications of OFDMImplications of OFDM--EBM EBM
• Impact of OFDM EBM Introduction in EV-DO Systems:
– Transmitter: EV-DO operators need to implement the new OFDM chain in
addition to the existing CDMA infrastructure only to support BCMCS.
– Receiver: OFDM and CDMA require different receiver functions. A new
additional receiver chain is required.
EncodedData
OFDMTransmitter
EV-DOTransmitter Demodulated
DataOFDMReceiver
EV-DOReceiver
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Source of OFDM EBM Gains Source of OFDM EBM Gains
• Main reasons for OFDM EBM performance improvement
– content vs sector dependent spreading
– ISI-limited Rake receiver for current BCMCS
• CDMA TIA-1006 can incorporate:
– content dependent spreading for soft combining across multiple sectors
– use an equalizer to suppress ISI
� Gain of OFDM EBM over TIA-1006 is due to design aspects and not due
to any fundamental advantage of OFDM over CDMA in EVDO.
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CDMA-based EBM (proposed by TI Oct’2004)
• The CDMA EBM proposal:
– introduces content dependent spreading in each data field
– adds 96 BCMCS TDM pilot chips within each data field
• CDMA EBM has lower overhead (24%) than OFDM EBM (40%)
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CDMA EBM Receiver
� Receiver chain functionality similar to 1xEV-DO/TIA-1006
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BCMCS Modes for CDMA EBMBCMCS Modes for CDMA EBM
0.842116QAM, R=1/52.45761
0.421116QAM, R=1/51.22882
0.5614QPSK, R=1/50.81923
4096
0.631616QAM, R=1/51.84321
0.6316QPSK, R=1/50.92162
0.4211QPSK, R=1/50.61443
3072
0.421116QAM, R=1/51.22881
0.4211QPSK, R=1/50.61442
0.2807QPSK, R=1/50.40963
2048
0.4211QPSK, R=1/50.61441
0.2105QPSK, R=1/50.30722
0.1404QPSK, R=1/50.20483
1024
Effective code
rate
Modulation &
base code rate
Data rate
(Mbps)
No. slotsPayload (bits)
Highest rate mode in CDMA EBM
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OFDM OFDM vsvs CDMA EBM: 1 Receiver AntennaCDMA EBM: 1 Receiver Antenna
CDMA and OFDM EBMs achieve similar coverage at 0.4 Mbps and 0.6 Mbps.
OFDM EBM has ~10% coverage gain at 0.9216 Mbps for 0.1% PER.
CDMA EBM performance can be improved with better pilot code design
~10%
OFDM results match with Qualcomm’s. Q =
1.0 assumed for OFDM.
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OFDM vs CDMA EBM: 2 Receiver Antennas
OFDM, CDMA have similar coverage for data rates below 1.2 Mbps
CDMA EBM performs significantly better than OFDM EBM at 1.8432 Mbps
� Lower overhead gives improved performance at higher rates for CDMA EBM
OFDM results match with Qualcomm. Q =
1.0 assumed for OFDM.
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• Backward Compatibility to EV-DO and TIA-1006
– Same CDMA modulation (length 16 Walsh code) for data fields.
– Same transmit signal spectrum and characteristics as EV-DO.
– Similar transmit chain functionality as EV-DO.
– Similar receive chain functionality as EV-DO
• Similar or Better Performance with 2 receiver antennas
– Similar coverage at low to medium data rates
– Significantly better coverage at high rates
• Similar or Slightly Worse Performance with 1 receiver antenna
– Similar coverage at low to medium data rates
– Slightly worse (10%) coverage at high rates
– CDMA EBM performance can be improved with better pilot code design
• Higher Peak Rate
– OFDM EBM peak rate: 1.84 Mbps
– CDMA EBM peak rate: 2.45 Mbps
Advantages of CDMA EBM over OFDM EBMCre
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TITI’’ss MIMO proposal to IEEE 802.16(e)MIMO proposal to IEEE 802.16(e)
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Closed Loop MIMO pre-coding
• Adaptive basis transformation at TX
• Select V from a finite predetermined set S adaptive to channel
realization H
sP
s1
V1
P
1
Q
(HV)H SICz1
zPH
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Basis selection for 2X2
• Based upon Givens rotations
• Example of set generation for P=2 ( |S | = NM )
• BER-optimal V selection:
• Minimum feedback requirement for FDD: log2 NM
M
mφ
N
nθ
θθ
eθeθmn
nn
jφ
n
jφ
nmm )1(2
,2
)1( ,
cossin
sincos −=
−=
−=
ππV
( )VHV
,minmax,,1
pPp
SINRΛ=
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Simulation results (2X2)
• Approximately 1.5 dB gain over no pre-coding
~ 1.5 dB gain
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Simulation results (4X2)
• Approximately 4 dB gain over no pre-coding for 2X2
~ 4 dB gain
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Conclusions
• OFDM throughput gain over CDMA with equalizer for a bandwidth of 1.25 MHz is < 20 %.
• With larger channel bandwidths, OFDM offers advantage over CDMA because of simplified receiver processing: 10 MHz, 20 MHz.
• OFDM combined with MIMO can give good gains over CDMA systems.
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