1 1 Wireless Communication Orthogonal Frequency Division Multiplexing and Multi-Carrier CDMA Jean-Paul Linnartz 1,2 & Nathan Yee 1 1 Dept. of Electrical Engineering and Computer Science University of California, Berkeley CA 94720-1770 USA 2 Natuurkundig Laboratorium, Philips Research, 5656 AA Eindhoven, The Netherlands
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Orthogonal Frequency Division Multiplexing and Multi-Carrier CDMA · 2012-04-01 · 1 Wireless Communication Orthogonal Frequency Division Multiplexing and Multi-Carrier CDMA Jean-Paul
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11
W i r e l e s s C o m m u n i c a t i o n
Orthogonal Frequency Division Multiplexing and Multi-Carrier
CDMA
Jean-Paul Linnartz1,2 & Nathan Yee11D e p t. of Electr ical Engineering and Com p u ter Science
U n iversity of California, Berke ley CA 94720-1770 USA
2N a tuurkundig Laborator ium , Philips Research,
5656 AA Eindhoven, The Netherlands
22
Multi-Carrier Modulation Signal Spectra
• MCM Transmit Signal Spectrum
• MCM Received Signal Spectrum
• Subcarrier orthogonality is not eroded by dispersive multi-path propagation if and
Tb Trms»
fD 1 Tb⁄«
Frequency
Frequency
33
W i r e l e s s C o m m u n i c a t i o n
What is MC-CDMA?
• A digital modulation / multiple access technique
• A combination of OFDM and DS-SS• A CDMA method using the FFT of DS-SS
signals• A spread spectrum modulation technique in
which each bit is modulated on multiple subcarriers with relative phase polarity according to a spreading code
44
W i r e l e s s C o m m u n i c a t i o n
channel
frequency
signal
Comparison of Modulation Technique Spectrums
• Narrowband (BPSK)
• SS-CDMA
• MC-CDM
55
W i r e l e s s C o m m u n i c a t i o n
Motivation
• Narrowband susceptible to flat fading in frequency selective channels
• CDMA-SS spreads signal energy over large bandwidth for frequency diversity
• However, wideband signals more sensitive to delay spreads because of inter-chip interference
• Use multi-carrier modulation with narrowband subcarriers
• Lower spreading factor required
66
W i r e l e s s C o m m u n i c a t i o n
MC-CDMA signal
Frequency
OFDM signal
Frequency
77
W i r e l e s s C o m m u n i c a t i o n
Time - Frequency Diagram
Narrowband Wideband OFDM
Time
Frequency
a[0]
a[1]
a[0]
a[1]
1/Tb
TbTb/N
N/Tb
a[0]
a[4]
1/Tb
a[1]
a[5]
a[2]
a[6]
a[3]
a[7]
Tb
88
W i r e l e s s C o m m u n i c a t i o n
Time - Frequency (cont.)
DS-CDMA MC-CDMA
Time
Frequency
Tb/NN/Tb
a[0] c2Tb
1/Tb
a[0]
c0
a[0]
c1
a[0]
c2
a[0]
c3
a[1]
c3
a[1]
c2
a[1]
c1
a[1]
c0
a[1] c0
a[0] c1
a[0] c0
a[1] c3
a[1] c2
a[1] c1
a[0] c3
Tb
1/Tb
a[0]
a[2]
a[1]
FH-SS
99
W i r e l e s s C o m m u n i c a t i o n
X
cos{2π(fc + (N-1)F/Tb)t}
X
cos{2π(fc + F/Tb)t}
X
cos{2πfct}
X
ci, N-1
X
ci, 1
X
ci, 0
ai[k]
Σ
Transmitter Model
......
si(t)
1010
W i r e l e s s C o m m u n i c a t i o n
Implementation Aspects
• MC-CDMA requires no chip synchronization. It requires only carrier and bit synchronization.
• MC-CDMA may use FFT devices• Charged-capacitor domain detector circuit?• MC-CDMA receiver might be simpler than
DS-SS receiver with full MRC for all resolvable paths. MC-CDMA does not require multiple despreaders
• Subcarrier amplitude estimation can be simple for the downlink
1111
Synchronization & Estimation in Downlink
• “Interfering” signals to other users help• Rapid acquisition of synchronization and channel
estimation• Improvement: Use PN sequence in Frequency
Domain
NNNN
NN
NNNN
NN
-N-N-N-N
-N-N
-N-N-N-N
-N-N
amcm N,∑
amcm 2,∑amcm 1,∑
Freq
uenc
y
Timesyncword
identical for all usersMC-CDMA
1212
Joint Signal in Downlink
• Interference helps during acquisition
φ2 I
Q
Subcarrier 2
φ1 I
Q
Subcarrier 1
1313
W i r e l e s s C o m m u n i c a t i o n
Simplified Channel Model
• Frequency selective channel with
• Narrowband behavior of subcarrier
•
• Rayleigh / Rician amplitude and uniform phase• IID fading at the subcarriers
1Tb------ B« Wc
FTb------«
Hm fci
Tb----F+
ρm i, ejθm i,=
ChannelSignal
1414
W i r e l e s s C o m m u n i c a t i o n
frequency
1515
W i r e l e s s C o m m u n i c a t i o n
Uplink vs. Downlink
ρm i, θm i,,{ }m 0=
M 1– ρm i, ρ0 i,=
θm i, θ0 i,= m∀
USER 0
USER 1
USER 2
BASESTATION
USER 0
USER 1
USER 2
BASESTATION
1616
W i r e l e s s C o m m u n i c a t i o n
X
cos{2πfct + φi,0}
X
cos{2π(fc + F/Tb)t + φi,1}
X
cos{2π(fc + (N-1)F/Tb)t + φi,N-1}
d0,0
d0,1
d0,N-1
Receiver Model
......
Σr(t) ν0
X
c0,0
X
ci,1
X
ci,N-1
...
∫
1717
W i r e l e s s C o m m u n i c a t i o n
ASSUMPTIONS
• LARGE NUMBER OF SUBCARRIERS
• FREQUENCY NON-SELECTIVE FADING FOR SUBCARRIER BANDWIDTH (F/Tb)
• IID RAYLEIGH FADING AT SUBCARRIERS OR RANDOM CODE SEQUENCES
• PERFECT PHASE CORRECTION AT RECEIVER FOR WANTED SIGNAL
1818
Analysis of Performance
• Transmitted Signal of the mth user
• Received Signal
• Decision Variable
sm t( ) cm i, am k[ ] 2πfct 2πiFTb-----t+
cosi 0=
N 1–
∑= pTbt kTb–( )
cm i, 1 1,–{ } cl i, cm i,i 0=
N 1–
∑ Nδl m,=
r t( ) ρm i,i 0=
N 1–
∑ cm i, am
k[ ]m 0=
M 1–
∑= 2πfct 2πiFTb-----+ t θm i,+
n t( )+cos
ν0 a0 k[ ] ρ0 i, d0 i, am k[ ]m 1=
M 1–
∑+i 0=
N 1–
∑=
ρm i, d0 i, θ0 i, θm i,–( ) cm i, c0 i, η+cosi 0=
N 1–
∑×
1919
W i r e l e s s C o m m u n i c a t i o n
Receiver Equalization Techniques
• Contribution to decision variable by mth interferer in downlink is proportional to
BER vs. the # of interferers for MRC (1), EGC (2), and CE for pmin = 0.006 (3),pmin = 0.008 (4), pmin = 0.012 (5), and pmin = 0.018 . The SNR is 10dB, p0 = 0.1, and
1e-05
1e-04
1e-03
1e-02
1e-01
0 50 100
(1)
(2)
(3,4,5,6)
N = 128.
(Rayleigh, SNR=10dB)
32
Downlink BER for CE vs. # of Interferers
BER vs. the # of interferers for MRC (1), EGC (2), and CE for pmin = 0.0008 (3),pmin = 0.0015 (4), pmin = 0.002 (5), and pmin = 0.004 (6). The SNR is 20dB, p0 = 0 .1 , and
N = 1 2 8 .
# of interferers
1e-22
1e-20
1e-18
1e-16
1e-14
1e-12
1e-10
1e-08
1e-06
1e-04
1e-02
1e+00
0 50 100
BER
(1)(2)
(3,4,5,6)
(Rayleigh, SNR=20dB)
33
Downlink BER for CE vs. # of Interferers
BER vs. the # of interferers for MRC (1), EGC (2), and CE for pmin = 0.002 (3),pmin = 0.008 (4), and pmin = 0.014 (5). The SNR is 10dB, p0 = 0 .1 , and N = 128.
# of interferers
BER
(Rician: K=5, SNR=10dB)
1e-05
1e-04
1e-03
1e-02
0 50 100
(1)
(2)
(3,4,5)
34
Downlink BER for CE vs. # of Interferers
BER vs. the # of interferers for MRC (1), EGC (2), and CE for pmin = 0.016 (3),pmin = 0.002 (4), and pmin = 0.008 (5). The SNR is 10dB, p0 = 0 .1 , and N = 128.
# of interferers
BER
(Rician: K=10, SNR=10dB)
1e-05
1e-04
1e-03
1e-02
0 50 100
(1)
(2)
(3,4,5)
3535
W i r e l e s s C o m m u n i c a t i o n
Wiener Filtering
• Optimum weighting vector in a mean-squared error sense
• Weighting vector
• Decision variable
D RY1–
Ra0Y=
ν0 D Y•=
36
Downlink BER for Wiener vs. # of Interferers
Simulation results for the average BER vs. the # of interferers for (1) MRC, (2) EGC,
(Rayleigh, SNR=10dB, N=8)
Avg. BER
# of interferers
2
5
1e-03
2
5
1e-02
2
5
1e-01
0 5 7
(1)
(2)
(3)
1 632 4
and (3) Wiener filtering.
37
Downlink BER for Wiener vs. SNR
Simulation results for the average BER vs. the SNR for (1) Wiener filtering with a,
(Rayleigh, Full Load, N=8)
full load, (2) single subcarrier with noise only, and (3) single subcarrier with noise
Avg. BER
SNR (dB)
3
1e-05
3
1e-04
3
1e-03
3
1e-02
3
1e-01
5 10 152 18
(2)
(3)
(1)
and Rayleigh fading.
3838
W i r e l e s s C o m m u n i c a t i o n
Conclusions
• MC-CDMA is a new, promising CDMA method
• Most effective in downlink• Can operate well with large delay spreads• Controlled equalization better than MRC and
EGC for combating interference• If F>>1, exploit frequency diversity without