IEEE P802.22 Wireless RANs Date: 2006-06-22 June 2006 doc.: IEEE 802.22-06/0xxxr0 June 2006 Uninterrupted Frame Synchronization and Channel Estimation after a Quiet Period/Frame IEEE P802.22 Wireless RANs Date: 2006-06-22 Authors: Notice: This document has been prepared to assist IEEE 802.22. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.22. Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures http://standards.ieee.org/guides/bylaws/sb-bylaws.pdf including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working Group of patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that the draft publication will be approved for publication. Please notify the Chair Carl R. Stevenson as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a draft standard being developed within the IEEE 802.22 Working Group. If you have questions, contact the IEEE Patent Committee Administrator at patcom@iee.org. > Baowei Ji, Samsung Baowei Ji, Samsung

June 2006 doc.: IEEE 802.22-06/0xxxr0 June 2006 Abstract Samsung raised the issue of the interruption of quiet frame/period on the standard receiving process, which was echoed by the proposers group. Logic solutions: It is mandatory to have a remedy method. It could be using empty frame*, or using extended preamble**, or both. *Empty frame: a quiet frame but with regular preamble and broadcasting messages. **Extended preamble: short training sequence (2 OFDMA symbols) plus long training sequence (2 OFDMA symbols). The later is also known as the regular preamble. Baowei Ji, Samsung Baowei Ji, Samsung

Suggested Text When a quiet frame is scheduled opportunistically, June 2006 Suggested Text When a quiet frame is scheduled opportunistically, The regular preamble SHALL be sent as well as FCH and empty MAPs, such that the CPEs can infer that the remaining of the frame is quiet for opportunistic channel sensing. The frame follows the empty frame could use the short preamble besides the long preamble. When a quiet period (could be one frame or multiple frames) is scheduled in advance,: The regular preamble SHALL be sent at the beginning of the first quiet frame; and, The short preamble, besides the long preamble, could be used in the frame immediately following the quiet period. Baowei Ji, Samsung

Q1: Why remedy is needed? (1/3) June 2006 doc.: IEEE 802.22-06/0xxxr0 June 2006 Q1: Why remedy is needed? (1/3) Use of quiet frames has been proposed for in-band channel sensing. For a typical receiver, the insertion of quiet frames would create gaps in the estimation process of key parameters. Reduce the accuracy of frame synchronization. Reduce the accuracy of channel estimation. Affect the estimation of key parameter statistics. Baowei Ji, Samsung Baowei Ji, Samsung

Q1: Why remedy is needed? (2/3) June 2006 Q1: Why remedy is needed? (2/3) Frame Synchronization Frame timing (frame synchronization) is crucial for modem design One of the key considerations for the design of the frame synchronization scheme: Availability of the periodic preambles Quiet frame w/o preamble destroys the preamble periodicity. The updating gap results in larger frame synchronization error residual Preamble Baowei Ji, Samsung

Q1: Why remedy is needed? (3/3) June 2006 Q1: Why remedy is needed? (3/3) Channel Estimation The more accurate, the better performance. A typical receiver improves the channel estimation by interpolating, accumulating and/or averaging the preceding and/or succeeding channel estimates of the coherent frames. A missed preamble interrupts this process, and would reduce the accuracy of channel estimation. Tc Interpolate Interpolate Acc.& Avg. Preamble Receiving Baowei Ji, Samsung

Q2: Why not use the extended preamble for every frame? June 2006 Q2: Why not use the extended preamble for every frame? The channel access in 802.11 and 802.15 is contention-based, which in turn requires one-shot synchronization. The channel access in 802.16 and 802.22 is controlled by the BS who continuously initiates one frame after another. Only regular preamble (i.e., long training sequence) is needed for regular operation. Extended preamble is needed for CPE initiation, or recovering of synchronization after long-time interruption. Baowei Ji, Samsung

June 2006 Q3: Is there any difference for a quiet frame to have a preamble or not? Yes. Preamble offers much more information than pilots. In 802.11a: short training sequence is for packet detection, AGC, coarse frequency-offset correction. Long training sequence is for channel estimation, fine frequency-offset correction, symbol timing, and sample clock tracking. Pilot is mainly for sample clock tracking and/or channel estimation. There is no pilots after the DL bursts in a frame which includes TTG, UL bursts, and RTG. This could occupy half of the frame length. Taking 20ms frame as example, the clock is free running for 30ms rather than 20ms after a quiet frame without a regular preamble. Baowei Ji, Samsung

Q4: Is this a real problem? June 2006 Q4: Is this a real problem? Yes. In 802.16d/e, the clock is required to be no more than ±2ppm. How much does a 4ppm clock drift in 2K FFT OFDMA system? During one symbol, the clock drifts 4*10-6*2048 = 0.82% sample. One symbol is of 1/3ms, (i.e., 2K/6MHz). During 20ms, the clock drifts 49.2% sample. During 30ms, the clock drifts 73.7% sample. Baowei Ji, Samsung

Q5: Why not use the extended preamble after every quiet frame/period? June 2006 Q5: Why not use the extended preamble after every quiet frame/period? Too much overhead could be incurred, especially for the case of opportunistic in-band channel sensing, considering that the extended preamble doubles the length of the regular preamble. The extended preamble is not necessary for recovering synchronization in many cases: Frame/packet detection is not needed because the CPEs know the periodicity of the frame transmission from the BS. Coarse frequency-offset correction is not needed except extremely long interruption. AGC can be done with the long training sequence. Baowei Ji, Samsung