Baseband Transceiver Design for the IEEE 802.16a OFDM mode

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NTU Confidential

Baseband Transceiver Design for the

IEEE 802.16a OFDM mode

Advisor : Tzi-Dar Chiueh

Student : Sang-Jung Yang

Date : December 15

th

, 2003

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Outline

• Review of 802.16a System

Review of 802.16a System

• Channel Model

_{Channel Model}

• Transceiver Architecture

Transceiver Architecture

–

Coarse Symbol Boundary Detection

–

Fractional and Integer part CFO Estimation

–

Tracking Residual CFO

–

Tracking TFO

• Encountered Problem

Encountered Problem

• Conclusion

Conclusion

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Scope of 802.16a (1/3)

• 802.11 drives demand for 802.16a

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Scope of 802.16a (2/3)

Subscriber

Station

Base

Station

• 802.16a is an IEEE Standard for Local and metropolitan a

rea networks (MAN), and specifies an air interface for

f

ixed broadband wireless access

systems operating between

2 to 11 GHz.

• 802.16a defined 3 non-interoperable PHYs :

Single Carrie

r 、 OFDM and OFDMA

. The MAC is TDMA or FDMA.

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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 Simulation

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Channel Model (1/3)

• Channel profile :

Model Tap1 Tap2 Tap3 K factor

SUI-1

Delay (us) 0 0.4 0.9 3.3 Power (dB) 0 -15 -20 Doppler Frequency (Hz) 0.4 0.4 0.4

SUI-2

Delay (us) 0 0.4 1.1 1.6 Power (dB) 0 -12 -15 Doppler Frequency (Hz) 0.2 0.15 0.25

SUI-3

Delay (us) 0 0.4 0.9 0.5 Power (dB) 0 -5 -10 Doppler Frequency (Hz) 0.4 0.3 0.5

SUI-4

Delay (us) 0 1.5 4 0.2 Power (dB) 0 -4 -8 Doppler Frequency (Hz) 0.4 0.4 0.4

SUI-5

Delay (us) 0 4 10 0.1 Power (dB) 0 -5 -10 Doppler Frequency (Hz) 2 1.5 2.5

SUI-6

Delay (us) 0 14 20 0.1 Power (dB) 0 -10 -14 Doppler Frequency (Hz) 0.4 0.3 0.5

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Channel Model (2/3)

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Channel Model (3/3)

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Transceiver Block Diagram

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Transceiver Block Diagram

-- Transmitter

Random Generator Random Generator QAM Mapper QAM Mapper Pilot Insertion Pilot Insertion Frame Shaping Frame Shaping IFFT (256-point) IFFT (256-point) Scrambler Scrambler RS Encoder RS

Encoder Convolution_Encoder Convolution

Encoder InterleaverInterleaver

: Simulink : C++

802.16a OFDM mode Transmitter Block Diagram

802.16a OFDM mode Transmitter Block Diagram Use r Dat a To DA C Inner Transmitter

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Transceiver Block Diagram

-- Receiver

802.16a OFDM mode Receiver Block Diagram

802.16a OFDM mode Receiver Block Diagram Fro m AD C Long Preamble Extraction Long Preamble Extraction WLS Estimator WLS Estimator De-rotator

De-rotator (256-Point)_(256-Point)FFTFFT

Coarse Symbol Boundary Detection

and Fractional Part CFO Acquisition

Fine Symbol Boundary Detection

and Integer part CFO Acquisition

and Integer part

CFO Acquisition LPFLPF FFT Window

FFT

Window _ExtractionPilot Pilot Extraction Channel Estimation Channel Estimation FEQ

FEQ Slicer_Slicer

NCO NCO Integrator Integrator Interpolator Interpolator To FEC Scaling Scaling

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Coarse Symbol Boundary Detection

and Fractional Part CFO Acquisition

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Coarse Symbol Boundary Detection (1/3)

Short Preamble

Guard

Interval IntervalGuard

64 64 64 64

Long Preamble

128

Signal Detection, AGC, ……

PN Sequence

period = 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

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Coarse Symbol Boundary Detection (2/3)

• We can simply use delay correlator to detect the reception of shor t preamble. From [1], we compute the following equations:

• Where rn is the received signal, P(d) is the delay correlator of l

ength L (for our case, L=64 ), R(d) is the power sum of L consecut ive 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 opt

imum position for M(d). )

2 2 1 0 2 ) ( 2 2 1 0 1 0 * )) ( ( ) ( ) ( ) ( ) ( ) ( d R d P d M r d R e r r r d P L m d m L L T f j L m d m L m d L m d m s    



                  ) ( 1 ) ( ˆ opt opt d M d M R N S  

Normalized CFO

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Fine Symbol Boundary Detection and

Integer Part CFO Acquisition

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Fine Symbol Boundary Detection and

Integer Part CFO Acquisition (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 1 2 256 , 64 1 ) ( 2 64 ) ( 2 ) 64 ( 2 ) ( i f i f s i f s N N T f T f d P of Phase                                 

Subcarrier spac

ing

Sample

time

Integer part

CFO

Fractional 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∆ ∆

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Fine Symbol Boundary Detection and

Integer Part CFO Acquisition (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 and

Integer Part CFO Acquisition (3/3)

• The procedure of Symbol Bound ary Detection and CFO Estimat ion 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 correlati on, we find the Fractional CFO.

(iii) When the moving Average drops to half of the peak value ， we set the position 128 sample s right to the peak position as the Coarse Sy

mbol Boundary

(iv) Start finding Fine Symbol Boundary at the pos ition of ±16 samples from Coarse Symbol Bo undary.

(v) Use Long Preamble Correlator Bank at the sear ching window. The set with peak occurs indic ates the correct Integer part CFO, and the pea k position is then the

Fine symbol boundary.

(vi) To handle the situation that “The first path is n ot 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 o f 64 consecutive received samples.

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Simulation Result of Symbol Boundary

Detection and CFO Estimation (1/2)

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Simulation Result of Symbol Boundary

Detection and CFO Estimation (2/2)

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Tracking Residual

CFO---WLS Estimator, LPF,NCO

WLS Estimator, LPF,NCO

WLS Estimator WLS Estimator De-rotator De-rotator LPF LPF NCO NCO

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WLS Estimation

[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

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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:

1 2 1

1 )

(

_







z

C

z

F

X

D

C2 C1 Input Output

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Low Pass Filter (2/3)

Without AWGN C1=0.5, C2=0.5 Without AWGN C1=0.5, C2=0.25 Without AWGN C1=0.5, C2=0.125 Without AWGN C1=0.25, C2=0.125

bes

t

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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

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2 7 NTU Confidential Pilot Extraction Pilot Extraction FEQ

FEQ Slicer_Slicer

Pilot Extraction, FEQ and Slicer

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Simulation 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

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2 9 NTU Confidential Integrator Integrator Interpolator Interpolator Scaling Scaling

Tracking

TFO---Scaling, Integrator, and Interpolator

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Scaling 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.

CFO

Spacing

Subcarrier

ppm

TFO

Frequency

Carrier



(

)

_

_

MOD

f













BW(MHz)

BW(MHz) Tb(us)Tb(us) ∆∆f(kHz)f(kHz) CFO (CFO (∆∆f)f) Case 1 Case 1 1.75 128 125*(2)-4 _10.93632 Case 2 Case 2 3.5 64 125*(2)-3 5.46816 Case 3 Case 3 7 32 125*(2)-2 _2.73408 Case 4 Case 4 14 16 125*(2)-1 1.36704 Case 5 Case 5 28 8 125 0.68352

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Interpolator (1/4)

• We use Farrow Structure piecewise parabolic

Interpolator to resample signal.

)

1 (

)

(

)

1 (

)

2 (

)

(

1 0 1 2























_ _ k k k k

m

x

c

m

x

c

m

x

c

m

x

c

k

y

Structure

Farrow

for

c

k k k k k k k k

5 .

0

1 )

1 (

)

1 (

2 1 2 0 2 1 2 2





























 

















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Interpolator (2/4)

• If TFO < 0, i.e. Receiver clock period < Transmitter

clock period)

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Interpolator (3/4)

• If TFO > 0, i.e. Receiver clock period > Transmitter

clock period)

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Interpolator (4/4)

• We can modify our interpolator as shown below :

If TFO < 0 and



_k

Overflows

S

h

if

t

R

e

g

is

te

rs



_k

If TFO > 0 and

not Overflow yet



_k

Overflows

_k TF_O (m -1,m0,m1,m 2) 0~ 1 >0 <0 (d₁,d₂,d₃, d₄) >1 >0 (d2,d3,d4,d 5) <0 (d0,d1,d2,d 3)

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Encountered Problem…

• Since we use Farrow structure to model the effect of TFO, and

compensate TFO, the imperfect property of Farrow structure be

come serious especially when



k

≈ 0.5

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Conclusion 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!

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Reference

• [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 tim ing offset for OFDM systems over fading channels

Pei-Yun Tsai; Hsin-Yu Kang; Tzi-Dar Chiueh;

Vehicular Technology Conference, 2003. VTC 2003-Spring. The 57t h 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