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| --- This message came from the IEEE 802.11 Task Group M Technical Reflector ---
I was asked by the Chair to draw the attention of subscribers to this reflector to a few PHY-related comment resolutions that have been agreed in principle by the group during teleconfs, but that could do with review by PHY experts. Please reply (privately if you prefer) if you have any comments. The proposed changes are highlighted in yellow.
Discussion: Assaf KASHER (Intel) has provided the following input: The differential encoding is described in 20.4.3.3.4. The purpose of this fake bit described as the differential encoder initialization is to provide time for a reference waveform (which is the spreading sequence multiplied by either 1 or -1.
We could have added a bit outside the header to do that, but chose to have a waveform corresponding to number of bits which is a multiple of 8. I hope it clarifies the issue. May be inserting the word “dummy” into the description can make it clearer, but I am not sure. […] This [differential encoder initialization] bit and the scrambler initialization bits are not scrambled. […] A receiver may perform non-differential detection and recover d(0) directly. Using the information that d(-1) is 1, it may recover s(0) and therefore c(0). However, the assumption is that
the receiver performs differential decoding. It recovers s(1) by looking at the product of d(1)xd(0). The product is calculated directly on the signals – after correlating the r_DATA(n) with the Ga(32) sequence, the receivers multiplies the peak (or a set
of peaks) in adjacent symbols to recover d(n). This cannot be done for d(0). This is what mean by saying that from the receiver point of view d(0) does not exist. Proposed changes: In the first non-header row of Table 20-11, change “Differential encoder initialization” to “Differential Encoder Initialization” and after “Used to initialize the differential encoding”
append “; c(0) in 20.4.3.3.4. May be set to any value”. Change 20.4.3.3.4 as follows: 20.4.3.3.4 Modulation The scrambled and coded bit stream
c(k), k = 0, 1, 2, …, is converted into a stream of complex constellation points
d(k) using differential binary phase shift keying (DBPSK) as follows. c(k)
NOTE—The scrambling and coding process does not affect the Differential Encoder Initialization field of the DMG control mode header. However, a typical
receiver implementation does not recover d(0) and hence does not recover the value of this field. Proposed resolution: REVISED Make the changes shown under “Proposed changes” for CID 291 in <this document>, which clarify that the Differential Encoder Initialization field is c(0) for the differential encoding process described in 20.4.3.3.4, but that this field
is not recovered by a typical receiver implementation.
Discussion: aPHYHeaderLength is defined at 648.16 as “The current PHY’s header length (in microseconds), excluding aPHYSigTwoLength if present.” For things like DSSS and HR/DSSS it’s all fine (Figures 15-1 and 16-2 respectively): the SERVICE field is sent at a known PHY rate and so the duration of the PHY header is fixed. However for things like OFDM, HT and VHT it’s more problematic (Figures 17-1, 19-1 and 21-4 respectively), because the PHY header includes a SERVICE field that is in the Data field and hence has a non-fixed duration, although Figures 19-1
and 21-4 don’t explicitly indicate what the “PHY header” consists of. Proposed resolution: REVISED At 648.17, after “excluding aPHYSigTwoLength if present” add “and the SERVICE field if it is in the Data field of the PPDU”.
Discussion: The table in 6.5.4.2 claims that the type of aPreambleLength and aPHYHeaderLength is “integer”. However 22.4.4 says: The static TVHT PHY characteristics, provided through the PLME-CHARACTERISTICS service primitive, shall be as shown in Table 19-25 (HT PHY characteristics) except parameters listed in Table 22-25 (TVHT PHY characteristics)
and aPreambleLength, aSTFOneLength, aSTFTwoLength, aLTFOneLength, aLTFTwoLength, aPHYHeaderLength, and aPHYSigTwoLength, which are multiplied by 7.5 for 6 MHz and 7 MHz unit channels and by 5.625 for 8 MHz unit channels. The definitions for these characteristics
are given in 6.5 (PLME SAP interface). The values for aPreambleLength and aPHYHeaderLength in Table 19-25 are 16 µs and 4 µs respectively. The result of multiplying the latter by 5.625 is not an integer. Actually, this might also be true of a[SL]TF{One,Two}Length and aPHYSigTwoLength. Time for a table:
So the non-integers appear only for a[SL]TFTwoLength and aPHYHeaderLength. But for the latter 648.19 already says “If the actual value of the length of the modulated header is not an integer number of microseconds, the value is rounded
up to the next higher value.” So the problem is only for a[SL]TFTwoLength. Proposed resolution: REVISED At 648.12½ add “. If the actual value of the length of the HT-STF is not an integer number of microseconds, the value is rounded up to the next higher value.” to the end of the rightmost
cell. At 648.15½ add “. If the actual value of the length of the Additional HT-LTFs is not an integer number of microseconds, the value is rounded up to the next higher value.” to the end
of the rightmost cell. Mark -- Mark RISON, Standards Architect, WLAN English/Esperanto/Français Samsung Cambridge Solution Centre Tel: +44 1223 434600 Innovation Park, Cambridge CB4 0DS Fax: +44 1223 434601 ROYAUME UNI WWW:
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