baseband transceiver design for the ieee 802.16a ofdm mode

NTU Confidential

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

2NTU Confidential

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

3NTU Confidential

Scope of 802.16a (1/3)Scope of 802.16a (1/3)

• 802.11 drives demand for 802.16a

4NTU Confidential


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.

5NTU Confidential


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)

NTU Confidential

Channel Model for SimulationChannel Model for Simulation

7NTU Confidential

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


0.2 0.15 0.25

SUI-3

Delay (us) 0 0.4 0.9

0.5Power (dB) 0 -5 -10


0.4 0.3 0.5

SUI-4

Delay (us) 0 1.5 4

0.2Power (dB) 0 -4 -8


0.4 0.4 0.4

SUI-5

Delay (us) 0 4 10

0.1Power (dB) 0 -5 -10


2 1.5 2.5

SUI-6

Delay (us) 0 14 20

0.1Power (dB) 0 -10 -14


0.4 0.3 0.5

8NTU Confidential

Channel Model (2/3)Channel Model (2/3)

BW = 1.75MHz (for subcarrier index -127~128 )

9NTU Confidential

Channel Model (3/3)Channel Model (3/3)

10

NTU Confidential

Transceiver Block DiagramTransceiver Block Diagram

11

NTU Confidential

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

12

NTU Confidential

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


CFO Acquisition

Fine Symbol Boundary Detection

and Integer part CFO Acquisition


and Integer part CFO Acquisition LPFLPF

FFT Window

FFT Window

PilotExtraction

PilotExtraction

ChannelEstimation

ChannelEstimation

FEQFEQ SlicerSlicer

NCONCO

IntegratorIntegrator

InterpolatorInterpolator

To FEC

ScalingScaling

13

NTU Confidential

Coarse Symbol Boundary DetectionCoarse Symbol Boundary Detectionand Fractional Part CFO Acquisitionand Fractional Part CFO Acquisition


CFO Acquisition


CFO Acquisition

14

NTU Confidential

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.

15

NTU Confidential

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

16

NTU Confidential

Fine Symbol Boundary Detection and Fine Symbol Boundary Detection and Integer Part CFO AcquisitionInteger Part CFO Acquisition





17

NTU Confidential

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

18

NTU Confidential


(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.

19

NTU Confidential


(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.

20

NTU Confidential

Simulation Result of Symbol Boundary Simulation Result of Symbol Boundary Detection and CFO Estimation (1/2)Detection and CFO Estimation (1/2)

21

NTU Confidential

Simulation Result of Symbol Boundary Simulation Result of Symbol Boundary Detection and CFO Estimation (2/2)Detection and CFO Estimation (2/2)

22

NTU Confidential

Tracking Residual CFO---Tracking Residual CFO---WLS Estimator, LPF,NCOWLS Estimator, LPF,NCO

WLS Estimator

WLS Estimator

De-rotator

De-rotator

LPFLPF

NCONCO

23

NTU Confidential

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.

24

NTU Confidential

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:

12

1 1)(

z

CCzF

X

X

DC2

C1

Input Output

25

NTU Confidential

Low Pass Filter (2/3)Low Pass Filter (2/3)Without AWGNC1=0.5, C2=0.5

Without AWGNC1=0.5, C2=0.25



best

26

NTU Confidential

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

27

NTU Confidential

PilotExtraction

PilotExtraction

FEQFEQ SlicerSlicer

Pilot Extraction, FEQ and SlicerPilot Extraction, FEQ and Slicer

28

NTU Confidential

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

29

NTU Confidential

IntegratorIntegrator


ScalingScaling

Tracking TFO---Tracking TFO---Scaling, Integrator, and Scaling, Integrator, and


30

NTU Confidential

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

31

NTU Confidential

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

32

NTU Confidential

Interpolator (2/4)Interpolator (2/4)• If TFO < 0, i.e. Receiver clock period < Transmitter clock period)

33

NTU Confidential

Interpolator (3/4)Interpolator (3/4)• If TFO > 0, i.e. Receiver clock period > Transmitter clock period)

34

NTU Confidential

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)

35

NTU Confidential

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

36

NTU Confidential

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……

37

NTU Confidential

Thank you for your Attention!Thank you for your Attention!

38

NTU Confidential

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

baseband transceiver design for the ieee 802.16a ofdm mode

Documents