1. IEEE P802.22 Wireless RANs Date: 2006-01-18
Month Year
doc.: IEEE yy/xxxxr0
January 2006
System Level Ranging
IEEE P Wireless RANs Date:
Authors:
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Jeff Zhuang, Motorola
John Doe, Some Company

2. Month Year
doc.: IEEE yy/xxxxr0
January 2006
Abstract
Ranging is a critical PHY element. It provides timing advance information for the CPE and a means of requesting initial access or bandwidth resources, among other things. The accuracy of the timing advance is important since errors must be absorbed in the cyclic extension along with the channel delay spread. Detection of bandwidth requests is important for minimizing latency and effectively allocating resources.
This contribution proposes evaluation criteria for ranging and inquires whether improvements can be made over existing techniques.
Jeff Zhuang, Motorola
John Doe, Some Company

3. Goal Understand the system-level functional requirement for ranging
January 2006
Goal
Understand the system-level functional requirement for ranging
Understand the performance of ranging in relevance to other system parameters
Discuss the possible improvement under deployment scenarios
Jeff Zhuang, Motorola

4. January 2006
Background (1)
All wireless networks need to allow unsolicited access, with CPEs requesting initial registration, resource allocation, timing/power adjustment, and so on.
“Random access” transmission fulfills critical system functionalities:
Initial Access: Enter the system for the first time or re-enter after loss of sync
Bandwidth Request: Ask for resource allocation for uplink data
Handover Access: Register with another sector (unnecessary for .22)
Maintenance functions: Maintain periodic timing/power adjustment, etc.
Jeff Zhuang, Motorola

5. January 2006
Background (2)
A “good” random access scheme directly impacts user experience
Reliable: high success rate/low collision rate
Efficient: low overhead
3G CDMA:
Sequence-based single carrier transmission
IEEE (OFDM systems):
Binary PN sequence on 144 dispersed subcarriers
Jeff Zhuang, Motorola

6. Evaluation Criteria (1)
January 2006
Evaluation Criteria (1)
Detection and estimation performance are most relevant
Detection performance
Success rate: average ranging requests that can be successfully detected
Miss rate: average ranging requests that are present but not detected
Collision rate: average ranging requests that share the same code
False alarm: average number of false detections
Considerations:
Number of users will vary in any given frame (traffic dependent)
Number of ranging opportunities allocated (available codes × physical resources)
Received signal power (CPE Tx power likely power controlled)
Jeff Zhuang, Motorola

7. Evaluation Criteria (2)
January 2006
Evaluation Criteria (2)
Estimation performance
Timing offset accuracy (for initial ranging)
Considerations
Cyclic extension: the longer the CE, the more inaccuracy can be tolerated (but lower efficiency)
Number of ranging subcarriers (ratio to FFT size)
Tradeoff: the more ranging subcarriers, the better is the resolution, but most likely with less received signal power per subcarrier
Ranging subcarrier distribution:
Dispersed across the whole band: zero stuffing results in unpredictable false peaks
Contiguous (sub-band): no false peaks but loss in resolution (multiple proximate rays can collapse to one)
Jeff Zhuang, Motorola

8. Possible areas for improvement over 802.16e
January 2006
Possible areas for improvement over e
Design a flexible and adjustable scheme after anticipating future traffic’s impact on ranging
Initial ranging will not be as frequent as in mobile deployment
More simultaneous ranging with heavy VoIP usage
Allocate more ranging subcarriers (particularly for FFT size > 2K)
Better timing offset estimation accuracy (power constraint is not that serious)
Increase the maximum round-trip delay (RTD) allowed for larger cell size
35 km = 233 µs, 100 km = 667 µs
Use contiguous (rather than dispersed) subcarriers
Better timing offset estimation
Better detection performance
Consider codes with better cross-correlation and low PAPR
Have observed significant performance degradation when two or more codes are occupied (no collision, but interfering with each other)
Jeff Zhuang, Motorola

9. January 2006
References
IEEE C802.16e-04/143r1, Ranging Improvement for e OFDMA PHY, 7 July 04, Xiangyang (Jeff) Zhuang, Kevin Baum, Vijay Nangia, Mark Cudak
Jeff Zhuang, Motorola