1. Baseband Transceiver Design for the IEEE 802.16a OFDM mode
Advisor : Tzi-Dar Chiueh
Student : Sang-Jung Yang
Date : December 15th , 2003

2. Outline Review of 802.16a System Channel Model
Transceiver Architecture
Coarse Symbol Boundary Detection
Fractional and Integer part CFO Estimation
Tracking Residual CFO
Tracking TFO
Encountered Problem
Conclusion
Reference

3. Scope of a (1/3)
drives demand for a

4. Scope of a (2/3)
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.
Subscriber Station
Base Station

5. Scope of 802.16a (3/3) System specifications of 802.16a OFDM mode.
ETSI (fs/BW=8/7)
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

6. Channel Model for Simulation

7. Channel Model (1/3) Channel profile : SUI-1 SUI-2 SUI-3 SUI-4 SUI-5
Tap1
Tap2
Tap3
K factor
SUI-1
Delay (us)
0.4
0.9
3.3
Power (dB)
-15
-20
Doppler Frequency (Hz)
SUI-2
1.1
1.6
-12
0.2
0.15
0.25
SUI-3
0.5 -5
-10
0.3
SUI-4
1.5 4 -4 -8
SUI-5 10
0.1 2
2.5
SUI-6 14 20
-14

8. Channel Model (2/3)
BW = 1.75MHz (for subcarrier index -127~128 )

9. Channel Model (3/3)

10. Transceiver Block Diagram

11. Transceiver Block Diagram -- Transmitter
User Data
Scrambler RS
Encoder
Convolution
Encoder
Interleaver
: Simulink
: C++ To
DAC
IFFT
(256-point)
Frame
Shaping
Pilot
Insertion
QAM
Mapper
Random
Generator
Inner Transmitter
802.16a OFDM mode Transmitter Block Diagram

12. Transceiver Block Diagram -- Receiver
Interpolator
De-
rotator
FFT
(256-Point)
FEQ
Slicer
To FEC
Coarse Symbol
Boundary Detection
and Fractional Part
CFO Acquisition
NCO
FFT
Window
Pilot
Extraction
Long
Preamble
Extraction
From
ADC
Fine Symbol
Boundary Detection
and Integer part
CFO Acquisition
LPF
WLS
Estimator
Channel
Estimation
Integrator
Scaling
802.16a OFDM mode Receiver Block Diagram

13. Coarse Symbol Boundary Detection and Fractional Part CFO Acquisition

14. Coarse Symbol Boundary Detection (1/3)
Guard
Interval
Short Preamble
Guard
Interval
Long Preamble
PN Sequence
period = 64
PN Sequence
period = 128 64 64 64 64
128
128
Signal Detection, AGC, ……
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. Coarse Symbol Boundary Detection (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). )
Normalized CFO

16. Fine Symbol Boundary Detection and Integer Part CFO Acquisition

17. Fine Symbol Boundary Detection and Integer Part CFO Acquisition (1/3)
For a, MAX CFO = 10.68GHz * ±4ppm = 85.44KHz
≒12.5*(minimum Subcarrier spacing 6.84KHz)
Need Integer part CFO Acquisition
For a, CFO can be derived from the following equation:
Subcarrier spacing
Sample time
Fractional part CFO
Integer 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 (ef + ei ) = -0.4π, we have (ef + ei ) = -0.8 ∆f, so we compensate 0.8 ∆f
So the total CFO will be =4 ∆f

18. 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 a, 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. Fine Symbol Boundary Detection and Integer Part CFO Acquisition (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. Simulation Result of Symbol Boundary Detection and CFO Estimation (1/2)

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

22. Tracking Residual CFO--- WLS Estimator, LPF,NCO
De-
rotator
NCO
LPF
WLS
Estimator

23. WLS Estimation
According to the Spec of a [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.
[4]
WLSE Block Diagram

24. 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: X D C2 C1
Input
Output

25. Low Pass Filter (2/3) best Without AWGN C1=0.5, C2=0.5 Without AWGN

26. Low Pass Filter (3/3)
Simulation under SUI-3, CFO= -12.5∆f, C1=0.25,C2=0.125, Residual CFO= -0.05∆f
LPF Output
LPF Output
SNR=12dB
SNR = 20dB

27. Pilot Extraction, FEQ and Slicer

28. Simulation Result
Simulation under SUI-3, SNR=25dB, CFO= ∆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. Tracking TFO--- Scaling, Integrator, and Interpolator

30. Scaling CFO to get TFO
Since we have only 1 oscillator in receiver, we have
If the total estimated CFO is e ,we can get TFO (d) by scaling e, i.e.
For a with ETSI channelization, we have the following 5 cases when TFO is fixed to -8ppm.
BW(MHz)
Tb(us)
∆f(kHz)
CFO (∆f)
Case 1
1.75
128
125*(2)-4
Case 2
3.5 64
125*(2)-3
Case 3 7 32
125*(2)-2
Case 4 14 16
125*(2)-1
Case 5 28 8
125

31. Interpolator (1/4)
We use Farrow Structure piecewise parabolic Interpolator to resample signal.

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

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

34. Interpolator (4/4) We can modify our interpolator as shown below :mk
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)
If TFO > 0 and
mk Overflows
If TFO < 0 and
mk Overflows
Shift Registers
mk not Overflow yet

35. 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 mk ≈ 0.5
mk≈ 0.5
mk≈ 0
mk≈1

36. Conclusion and Future Work
Several blocks of a 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. Thank you for your Attention!

38. Reference
[1]Robust frequency and timing synchronization for OFDM Schmidl, T.M.; Cox, D.C.; Communications, IEEE Transactions on , Volume: 45 Issue: 12 , Dec , Page(s):
[2] IEEE a draft version 7.
[3]Joint weighted least squares estimation of frequency and timing offset for OFDM systems over fading channels Pei-Yun Tsai; Hsin-Yu Kang; Tzi-Dar Chiueh; Vehicular Technology Conference, 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