1. Jul 12, 2010
07/12/10
Project: IEEE P Working Group for Wireless Personal Area Networks (WPANs)
Submission Title: Proposed resolution for CID 180
Date Submitted: September 2011
Source: Tim Schmidl, Texas Instruments
Contact:
Voice:
Re:
Abstract: Proposed resolution to CID 180
Purpose: Smart Utility Networks
Notice: This document has been prepared to assist the IEEE P 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 acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P
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2. 07/12/10
Jul 12, 2010
CID 180
Comment:
Data whitening in the MR-FSK mode is as expected, namely the payload is whitened after FEC. In the MR-OFDM mode, the whitening (called scrambling) happens on the un-coded (raw) data. Scrambling the coded data instead will also scramble the parity. In the MR-O-QPSK mode, whitening takes place at chip-level on the DS spread data.
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3. MR-FSK with FEC and Data Whitening
07/12/10
Jul 12, 2010
MR-FSK with FEC and Data Whitening
16 bit PHY header including DW bit
Variable length PSDU
3 tail bits
5 or 13 pad bits
Packet before FEC
32 bits
2 x length of PSDU 6
10 or 26
Packet after FEC
No whitening
Data whitening is applied here
Data whitening
Receiver does not know whether data whitening is used or not until DW bit is decoded. The DW bit is the 5th bit with 11 length bits following in the header. If the receiver tries to FEC decode using only the header, then the traceback length is very short. If the receiver wants to use additional PSDU bits, then it does not know whether or not data whitening is used.
The traceback length should be at least 7 to 8 times the constraint length if the traceback is started from an arbitrary state, so this would be 28 to 32 information bits. There are not enough bits in the header to allow for this traceback length. In fading channels longer traceback lengths are needed. Performing the data whitening before the FEC solves this problem.
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4. Proposed Resolution to CID 180
Jul 12, 2010
07/12/10
Proposed Resolution to CID 180
Revised.
MR-FSK signals whether or not whitening is used by sending the DW bit in the PHY header. When FEC is employed, the whitening when used is applied to the PSDU after the FEC. The receiver does not know in advance whether or not whitening is used, so it does not know whether or not to remove the whitening prior to FEC decoding. The problem can be solved by performing whitening prior to the FEC.
Implement this change by doing the following:
In Figure 115 make a first concatenator which concatenates the PHR and PSDU. Move the FEC and bypass after the first concatenator. Then make a second concatenator which concatenates the SHR and output of the FEC or bypass. Move the data whitening text ahead of the FEC text, and make the corresponding changes in the text to match the updated figure.
For MR-OFDM, the payload is whitened and then the FEC is computed. There is no need to whiten the parity bits since the payload has already been whitened. No change required for MR-OFDM.
----- Meeting Notes (17/01/ :38) -----
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