Baseband Transceiver Design for the IEEE 802.16a OFDM mode
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Transcript of Baseband Transceiver Design for the IEEE 802.16a OFDM mode
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Baseband Transceiver Design for the Baseband Transceiver Design for the IEEE 802.16a OFDM mode IEEE 802.16a OFDM mode
Advisor : Tzi-Dar ChiuehStudent : Sang-Jung Yang
Date : December 15th , 2003
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OutlineOutline
• Review of 802.16a SystemReview of 802.16a System• Channel ModelChannel Model• Transceiver ArchitectureTransceiver Architecture
– Coarse Symbol Boundary DetectionCoarse Symbol Boundary Detection– Fractional and Integer part CFO EstimationFractional and Integer part CFO Estimation– Tracking Residual CFOTracking Residual CFO– Tracking TFOTracking TFO
• Encountered ProblemEncountered Problem• ConclusionConclusion• ReferenceReference
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Scope of 802.16a (1/3)Scope of 802.16a (1/3)
• 802.11 drives demand for 802.16a
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Scope of 802.16a (2/3)Scope of 802.16a (2/3)
Subscriber Station
Base Station
• 802.16a is an IEEE Standard for Local and metropolitan area networks (MAN), and specifies an air interface for fixed broadband wireless access systems operating between 2 to 11 GHz.
• 802.16a defined 3 non-interoperable PHYs : Single Carrier、 OFDM and OFDMA. The MAC is TDMA or FDMA.
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Scope of 802.16a (3/3)Scope of 802.16a (3/3)
RF frequency 2-11GHz
FFT Size 256
Effective subcarriers 192
Bandwidth (MHz) BW
Guard time (us) Tg
Data time (us) Tb
Symbol time (us) Tg + Tb
Subcarrier spacing (kHz) ∆f
Sampling rate (MHz) Fs = BW x 8/7
Maximum data rate (Mbps)(BW=28MHz, 64QAM, code rate 3/4)
104.73
• System specifications of 802.16a OFDM mode.ETSI (fs/BW=8/7)
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Channel Model for SimulationChannel Model for Simulation
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Channel Model (1/3)Channel Model (1/3)• Channel profile :
Model Tap1 Tap2 Tap3 K factor
SUI-1
Delay (us) 0 0.4 0.9
3.3Power (dB) 0 -15 -20
Doppler Frequency (Hz)
0.4 0.4 0.4
SUI-2
Delay (us) 0 0.4 1.1
1.6Power (dB) 0 -12 -15
Doppler Frequency (Hz)
0.2 0.15 0.25
SUI-3
Delay (us) 0 0.4 0.9
0.5Power (dB) 0 -5 -10
Doppler Frequency (Hz)
0.4 0.3 0.5
SUI-4
Delay (us) 0 1.5 4
0.2Power (dB) 0 -4 -8
Doppler Frequency (Hz)
0.4 0.4 0.4
SUI-5
Delay (us) 0 4 10
0.1Power (dB) 0 -5 -10
Doppler Frequency (Hz)
2 1.5 2.5
SUI-6
Delay (us) 0 14 20
0.1Power (dB) 0 -10 -14
Doppler Frequency (Hz)
0.4 0.3 0.5
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Channel Model (2/3)Channel Model (2/3)
BW = 1.75MHz (for subcarrier index -127~128 )
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Channel Model (3/3)Channel Model (3/3)
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Transceiver Block DiagramTransceiver Block Diagram
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Transceiver Block DiagramTransceiver Block Diagram-- Transmitter -- Transmitter
RandomGenerator
RandomGenerator
QAMMapper
QAMMapper
PilotInsertion
PilotInsertion
FrameShaping
FrameShaping
IFFT(256-point)
IFFT(256-point)
ScramblerScrambler RSEncoder
RSEncoder
ConvolutionEncoder
ConvolutionEncoder InterleaverInterleaver
: Simulink
: C++
802.16a OFDM mode Transmitter Block Diagram802.16a OFDM mode Transmitter Block Diagram
User Data
To DAC
Inner Transmitter
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Transceiver Block DiagramTransceiver Block Diagram-- Receiver-- Receiver
802.16a OFDM mode Receiver Block Diagram802.16a OFDM mode Receiver Block Diagram
From ADC
LongPreamble Extraction
LongPreamble Extraction
WLS Estimator
WLS Estimator
De-rotator
De-rotator
FFT(256-Point)
FFT(256-Point)
Coarse Symbol Boundary Detection and Fractional Part
CFO Acquisition
Coarse Symbol Boundary Detection and Fractional Part
CFO Acquisition
Fine Symbol Boundary Detection
and Integer part CFO Acquisition
Fine Symbol Boundary Detection
and Integer part CFO Acquisition LPFLPF
FFT Window
FFT Window
PilotExtraction
PilotExtraction
ChannelEstimation
ChannelEstimation
FEQFEQ SlicerSlicer
NCONCO
IntegratorIntegrator
InterpolatorInterpolator
To FEC
ScalingScaling
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Coarse Symbol Boundary DetectionCoarse Symbol Boundary Detectionand Fractional Part CFO Acquisitionand Fractional Part CFO Acquisition
Coarse Symbol Boundary Detection and Fractional Part
CFO Acquisition
Coarse Symbol Boundary Detection and Fractional Part
CFO Acquisition
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Coarse Symbol Boundary Detection Coarse Symbol Boundary Detection (1/3)(1/3)
Short PreambleGuardInterval
GuardInterval
64 64 64 64
Long Preamble
128 128
Signal Detection, AGC, ……
PN Sequenceperiod = 64
PN Sequenceperiod = 128
• Since the first several samples are used for Signal detection, AGC, ……, we can not sure how many periods(64 samples) of short preamble can be used for symbol boundary detection.
• Assume that we can get at least 2 complete periods of short preamble.
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Coarse Symbol Boundary Detection Coarse Symbol Boundary Detection (2/3)(2/3)
• We can simply use delay correlator to detect the reception of short preamble. From [1], we compute the following equations:
• Where rn is the received signal, P(d) is the delay correlator of length L (for our case, L=64 ), R(d) is the power sum of L consecutive received samples, M(d) is the delay correlator normalized by R(d).
• The reason of computing M(d) is that, from [1], we have
So we can estimate SNR by computing this equation. (dopt is the optimum position for M(d). )
2
2
1
0
2
)(221
0
1
0
*
))((
)()(
)(
)()(
dR
dPdM
rdR
errrdP
L
m Lmd
LTfjL
m mdLmd
L
m mds
)(1
)(ˆ
opt
opt
dM
dMRNS
Normalized CFO
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Fine Symbol Boundary Detection and Fine Symbol Boundary Detection and Integer Part CFO AcquisitionInteger Part CFO Acquisition
Fine Symbol Boundary Detection
and Integer part CFO Acquisition
Fine Symbol Boundary Detection
and Integer part CFO Acquisition
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Fine Symbol Boundary Detection Fine Symbol Boundary Detection and Integer Part CFO Acquisition and Integer Part CFO Acquisition
(1/3)(1/3)• For 802.16a, MAX CFO = 10.68GHz * ±4ppm = 85.44KHz
≒12.5*(minimum Subcarrier spacing 6.84KHz) Need Integer part CFO Acquisition
• For 802.16a, CFO can be derived from the following equation:
)(4
12
256,641
)(2
64)(2
)64(2
)(
if
if
sif
s
NN
Tf
Tf
dPofPhase
Subcarrier spacing Sample time
Integer part CFOFractional part CFO
• If CFO=3.2 ∆f, Phase of P(d) will be 2πx 0.8 = 1.6π= -0.4π ( tan∵ -1 lies in (-π,π] )• ∴0.5πx (f + i ) = -0.4π, we have (f + i ) = -0.8 ∆f, so we compensate 0.8 ∆f• So the total CFO will be 3.2+0.8=4 ∆f
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Fine Symbol Boundary Detection Fine Symbol Boundary Detection and Integer Part CFO Acquisition and Integer Part CFO Acquisition
(2/3)(2/3)• Therefore, for CFO lies in [-2,2] ∆f, we compensate it to 0 ∆f
and for CFO lies in [2,6] ∆f, we compensate it to 4 ∆f, and so on…
• The following figure illustrates the compensation of fractional CFO :
• Since for 802.16a, the Maximum CFO can be ±12.5 ∆f, the resulting Integer part CFO can be {-12,-8,-4,0,4,8,12} ∆f• We adopt correlator bank with 7 sets of correlator to find the correct integer part CFO.
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Fine Symbol Boundary Detection Fine Symbol Boundary Detection and Integer Part CFO Acquisition and Integer Part CFO Acquisition
(3/3)(3/3)• The procedure of Symbol Boundary Detection and CFO Estimation is:
(i) Compute Normalized Delay Correlation M(d) and its moving average with length equals to guard interval.
(ii) Find the peak of the moving average, and from the phase of its corresponding delay correlation, we find the Fractional CFO.
(iii) When the moving Average drops to half of the peak value, we set the position 128 samples right to the peak position as the Coarse Symbol Boundary
(iv) Start finding Fine Symbol Boundary at the position of ±16 samples from Coarse Symbol Boundary.
(v) Use Long Preamble Correlator Bank at the searching window. The set with peak occurs indicates the correct Integer part CFO, and the peak position is then the
Fine symbol boundary.
(vi) To handle the situation that “The first path is not the strongest path”, we use a threshold to find the peak. The threshold is set to be the “half of R(d)”, which is half of the power sum of 64 consecutive received samples.
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Simulation Result of Symbol Boundary Simulation Result of Symbol Boundary Detection and CFO Estimation (1/2)Detection and CFO Estimation (1/2)
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Simulation Result of Symbol Boundary Simulation Result of Symbol Boundary Detection and CFO Estimation (2/2)Detection and CFO Estimation (2/2)
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Tracking Residual CFO---Tracking Residual CFO---WLS Estimator, LPF,NCOWLS Estimator, LPF,NCO
WLS Estimator
WLS Estimator
De-rotator
De-rotator
LPFLPF
NCONCO
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WLS EstimationWLS Estimation
atan
atan
abs
abs
+
-
Divider
D
D
kth pilot in theprevios symbol
kth pilot in thecurrent symbol
To LPF
Joint WLSEPhase difference inconsecutive blocks
Weighting calculation
yk
wk
[4]
WLSE Block Diagram
k
k
y : Phase Difference between 2 Symbols of k'th sub-carrier index
w : Weighting factor of k ' th sub carrier index
GR : Guard Interval Ratio
• According to the Spec of 802.16a [2] , there’s only 1 oscillator in the receiver. Therefore, we can adopt the Joint WLSE method [3] to find the residual CFO and TFO.
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Low Pass Filter (1/2)Low Pass Filter (1/2)• From [5], we adopt the PI control LPF. Its transfer function is
• We can adjust the values of C1 and C2 to make a trade-off between convergence speed and jitter.
• The block diagram of LPF is shown below:
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1 1)(
z
CCzF
X
X
DC2
C1
Input Output
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Low Pass Filter (2/3)Low Pass Filter (2/3)Without AWGNC1=0.5, C2=0.5
Without AWGNC1=0.5, C2=0.25
Without AWGNC1=0.5, C2=0.125
Without AWGNC1=0.25, C2=0.125
best
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Low Pass Filter (3/3)Low Pass Filter (3/3)
• Simulation under SUI-3, CFO= -12.5∆f, C1=0.25,C2=0.125, Residual CFO= -0.05∆f
SNR=12dB SNR = 20dB
LPF Output LPF Output
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PilotExtraction
PilotExtraction
FEQFEQ SlicerSlicer
Pilot Extraction, FEQ and SlicerPilot Extraction, FEQ and Slicer
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Simulation ResultSimulation Result
• Simulation under SUI-3, SNR=25dB, CFO= -0.68352 ∆f, Residual CFO=0.05 ∆f, 16QAM, 500 OFDM Symbol transmitted ( 384,000 data bits ), BER=4.87x10-3
Pilots are modulated with BPSK
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IntegratorIntegrator
InterpolatorInterpolator
ScalingScaling
Tracking TFO---Tracking TFO---Scaling, Integrator, and Scaling, Integrator, and
InterpolatorInterpolator
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Scaling CFO to get TFOScaling CFO to get TFO• Since we have only 1 oscillator in receiver, we have
• If the total estimated CFO is ,we can get TFO () by scaling , i.e.
• For 802.16a with ETSI channelization, we have the following 5 cases when TFO is fixed to -8ppm.
CFOSpacingSubcarrier
ppmTFOFrequencyCarrier
)(
MODf
f
BW(MHz)BW(MHz) Tb(us)Tb(us) ∆∆f(kHz)f(kHz) CFO (CFO (∆∆f)f)Case 1Case 1 1.75 128 125*(2)-4 10.93632Case 2Case 2 3.5 64 125*(2)-3 5.46816Case 3Case 3 7 32 125*(2)-2 2.73408Case 4Case 4 14 16 125*(2)-1 1.36704Case 5Case 5 28 8 125 0.68352
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Interpolator (1/4)Interpolator (1/4)• We use Farrow Structure piecewise parabolic Interpolator to resample signal.
)1()(
)1()2()(
10
12
kk
kk
mxcmxc
mxcmxcky
StructureFarrowfor
c
c
c
c
kk
kk
kk
kk
5.0
1)1(
)1(
21
20
21
22
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Interpolator (2/4)Interpolator (2/4)• If TFO < 0, i.e. Receiver clock period < Transmitter clock period)
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Interpolator (3/4)Interpolator (3/4)• If TFO > 0, i.e. Receiver clock period > Transmitter clock period)
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Interpolator (4/4)Interpolator (4/4)• We can modify our interpolator as shown below :
If TFO < 0 and k Overflows
Shift
Reg
ister
s
k not Overflow yetIf TFO > 0 and k Overflows
k TFO (m-1,m0,m1,m2)
0~1>0 <0
(d1,d2,d3,d4)
>1>0 (d2,d3,d4,d5)
<0 (d0,d1,d2,d3)
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Encountered Problem…Encountered Problem…• Since we use Farrow structure to model the effect of TFO, and compensate TFO, the
imperfect property of Farrow structure become serious especially when k ≈ 0.5
k≈ 0 k≈ 0.5 k≈1
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Conclusion and Future WorkConclusion and Future Work
• Several blocks of 802.16a Transceiver have been introduced.• The receiver seems work fine under SUI 1~6 with CFO exists.• However, the way we model TFO seems not ideal enough,
and we can’t have good performance when TFO exists.• The short-term job is to find an appropriate way to model TFO.
– Up-sampling or Using other kind of interpolator
• Other jobs including outer transceiver (in C++), other imperfect channel effect, and OFDMA mode……
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Thank you for your Attention!Thank you for your Attention!
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ReferenceReference• [1]Robust frequency and timing synchronization for OFDM
Schmidl, T.M.; Cox, D.C.; Communications, IEEE Transactions on , Volume: 45 Issue: 12 , Dec. 1997 , Page(s): 1613 -1621
• [2] IEEE 802.16a draft version 7.• [3]Joint weighted least squares estimation of frequency and timing offset for OFD
M systems over fading channels Pei-Yun Tsai; Hsin-Yu Kang; Tzi-Dar Chiueh; Vehicular Technology Conference, 2003. VTC 2003-Spring. The 57th IEEE Semiannual , Volume: 4 , April 22-25, 2003
• [4]Design and Implementation of an MC-CDMA Baseband Transceiver Hsin-Yu Kang; July , 2003• [5] Interpolation in Digital Modems---Part II: Implementation and Performance
Lars Erup, Floyd M.Garden and RobertA. Harris, IEEE Trans. On Comm.1993