1. IEEE P802.22 Wireless RANs Date: 2006-06-22
June 2006
doc.: IEEE /0xxxr0
June 2006
Uninterrupted Frame Synchronization and Channel Estimation after a Quiet Period/Frame
IEEE P Wireless RANs Date:
Authors:
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Baowei Ji, Samsung
Baowei Ji, Samsung

2. June 2006
doc.: IEEE /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

3. 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

4. Q1: Why remedy is needed? (1/3)
June 2006
doc.: IEEE /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

5. 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

6. 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

7. 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 and is contention-based, which in turn requires one-shot synchronization.
The channel access in and 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

8. 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 a:
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

9. Q4: Is this a real problem?
June 2006
Q4: Is this a real problem?
Yes.
In d/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

10. 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