1. [Adaptive Spreading Scheme]
July 2007
[Adaptive Spreading Scheme]
IEEE P Wireless RANs Date:
Authors:
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Chang-Joo Kim, ETRI

2. July 2007
Abstract
Our proposal can support to secure the at least service to the CPEs that receive the signal of the low SNR (e.g. at the cell edge) and are affected with Intercell Interference. Spread is performed before the IFFT in the frequency domain. The spreaded symbols with adaptive Spreading Factor (SF) are allocated at the subcarriers with the same size of SF (=we will call ‘group’). Several spreaded symbols can be multiplexed with “Spreading Code” in the same ‘group’.
Chang-Joo Kim, ETRI

3. Problems of WRAN System (1)
July 2007
Problems of WRAN System (1)
Low SNR
The CPEs located at the edge of the cell may receive the very low power signal. [CPE 1]
If TV signal is detected in the adjacent channel between N-15 and N+15, the max. transmitted CPE EIRP would have to be limited.
(Refer to the FRD ) [CPE 2]
Chang-Joo Kim, ETRI

4. Problems of WRAN System (2)
July 2007
Problems of WRAN System (2)
Ref.) WRAN_Reference_Model
Base Station Power limits ( < 100W ) [for DTV]
CPE TPC Power limits ( < 4W ) [for DTV]
Chang-Joo Kim, ETRI

5. Problems of WRAN System (3)
July 2007
Problems of WRAN System (3)
Inter-Cell Interference
If the neighboring two WRAN systems use the same frequency, the CPE located between the two WRAN cell that overlap each other would be affected with inter-cell interference.
Chang-Joo Kim, ETRI

6. What’s the Adaptive Spreading Scheme(1)
July 2007
What’s the Adaptive Spreading Scheme(1)
Spreading before IFFT in the frequency domain
Be able to select the specific CPEs that need spread
Some CPE having the low power signal
Some CPE to be affected with intercell interference
Using the ‘Adaptive Spreading Factor(SF)’
1(=OFDMA), 2, 4, 8, 16
Multiplexing with orthogonal spreading code
Adaptively multiplex several symbols according to the required throughput (Max. number of multiplexing is the same size of SF)
Several spread symbols of one CPE or each spread symbols of several CPEs can be multiplexed SF with the different code in the same group
One Group means a collection of subcarriers to be used for spreading symbols of one constellation symbol.
Chang-Joo Kim, ETRI

7. What’s the Adaptive Spreading Scheme(2)
July 2007
What’s the Adaptive Spreading Scheme(2)
Block Diagram : Transmitter
※ Code Multiplexing (C-MUX)
: we can multiplex several spread symbols of one CPE
or each spread symbols of several CPEs
as many as SF with the different code in the same group.
Chang-Joo Kim, ETRI

8. What’s the Adaptive Spreading Scheme(3)
July 2007
What’s the Adaptive Spreading Scheme(3)
Block Diagram : Receiver
Chang-Joo Kim, ETRI

9. What’s the Adaptive Spreading Scheme(4)
July 2007
What’s the Adaptive Spreading Scheme(4)
Frequency Domain : Spreading
Code Domain : Multiplexing
With orthogonal spreading code
< OFDMA >
Using the
Adaptive Spreading Scheme
< Adaptive Spreading Scheme >
Chang-Joo Kim, ETRI

10. What’s the Adaptive Spreading Scheme(5)
July 2007
What’s the Adaptive Spreading Scheme(5)
We can adaptively select the mixture of ‘Spreading Factor’, ‘Modulation Order’ and ‘Code Rate’ according to the channel environments.
Chang-Joo Kim, ETRI

11. Usage of the Adaptive Spreading Scheme(1)
July 2007
Usage of the Adaptive Spreading Scheme(1)
For CPE having the low SNR
CPE 1 : QPSK + SF 2,4,8,16 (Performance is better than QPSK)
CPE 2 : If need, [16QAM + SF+C-MUX] or [64QAM +SF + C-MUX]
CPE 3 : Usually use 64QAM and SF=1(OFDMA)
※ Conventional Design
· Using 64QAM within Contour A
· Using 16QAM within Contour B
· Using QPSK within Contour C
Chang-Joo Kim, ETRI

12. Usage of the Adaptive Spreading Scheme(2)
July 2007
Usage of the Adaptive Spreading Scheme(2)
In the circumstance of the existence of Inter-cell interference
If WRAN use the ‘Adaptive Spreading Scheme’, it is more robust to the effect of inter cell interference.
We can reuse the same spreading code in each WRAN system by allocation of the spreading Group at the different position.
Chang-Joo Kim, ETRI

13. Simulation Conditions
July 2007
Simulation Conditions
FFT size : 2048
Data sub-carriers : 1440
Guard subcarriers : 184 and 183 on the left and right side
Modulation : QPSK
Spreading Factor (SF) : 1,2,4,8
Sequentially allocate the symbol on the frequency domain
AWGN / WRAN Channel Profile B (Rayleigh fading)
Perfect Channel Estimation
Uncoded BER Performance
Chang-Joo Kim, ETRI

14. July 2007
Simulation results (1)
Chang-Joo Kim, ETRI

15. July 2007
Simulation results (2)
Chang-Joo Kim, ETRI

16. Simulation results (3) July 2007
“# of symbol” means the number of symbols to be multiplexed on the sub-carriers.
Chang-Joo Kim, ETRI

17. July 2007
Simulation results (4)
A typical processing gain of 3dB is expected each time spreading factor gets double.
Additional SNR gain can be expected by the diversity gain of the symbol which is spread on the frequency domain.
Various combinations of spreading factors and the number of multiplexed symbols can be considered.
Repetition method is a kind of the adaptive spreading scheme when the code is all 1.
Chang-Joo Kim, ETRI

18. July 2007
Conclusions
‘Adaptive Spreading Scheme’ can support the CPEs receiving the low SNR or located on the cell edge.
In the downstream, WRAN can extend the cell coverage.
‘Adaptive Spreading Scheme’ can multiplex the spread symbols with the orthogonal codes.
‘Adaptive Spreading Scheme’ can mitigate the Inter-Cell Interference when two neighboring WRANs are operated on the same TV band.
Chang-Joo Kim, ETRI