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Comments on CID 1599

Comments on CID 1599. Authors:. Date: 2007-06-27.

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Comments on CID 1599

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  1. Comments on CID 1599 Authors: Date: 2007-06-27 Notice:This document has been prepared to assist IEEE 802.11. 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. John Ketchum, Qualcomm Incorporated

  2. Abstract This presentation provides background for editorial disposition of 11-07/0544r6 in D2.03. John Ketchum, Qualcomm Incorporated

  3. Summary • CID 1599 addresses minor issue in 20.3.10.8.1 (Space Time Block Coding (STBC)) • Comment: There is no explicit statement which symbols STBC can be applied to. • Proposed change: Add statement such as; "STBC or hybrid STBC/SM can be applied only for Data OFDM symbols, i.e., the SERVICE field and PSDU.“ • 544r6 also addresses an ad hoc comment that was forward to the PHY ad hoc after LB97 was complete and comments were organized: “We are confused about correct sequence for the following two operations: 90-degree phase rotation of 40 MHz upper subcarriers and STBC mapping. “When we look at section 20.3.4 in the draft 2.0 for the sequence in the TX, it states that the rotation should be done (step m) before the STBC is applied (step n). “However, when we check OFDM modulation section 20.3.10.10, particularly 20.3.10.10.3 “Transmission in 40 MHz HT Format”, it implies the reverse order: STBC mapping first, followed by 90-degree rotation. Equation 20.59 (copied below) in this section indicates that ãk multiplication (90-degree rotation for upper tones) is applied to STBC-mapped subcarriers . This latter conclusion makes more sense in terms of properly demodulating STBC symbols. ” John Ketchum, Qualcomm Incorporated

  4. More Summary • 544r6 attempts to address both of these comments by the following: Comment resolution Section 20.3.4 (“Overview of PPDU encoding process”) should provide along with figures 305 and 307 enough information on the fact that STBC should be applied only to Data field constellation complex symbols. Besides section 20.3.10.8.1 is a subclause of section 20.3.10 which only deals with the Data field. With the proposed modifications (see below) provides further clarification as a resolution to this CID. Two modifications are thus proposed: • swap step m) and n) in section 20.3.4 to comply with equation 104 and make the required edits for the swapping since step m) deals with the OFDM symbol formation which is used in step n) • amend section 20.3.10.8.1 for removing OFDM carrier reference since it is introduced later John Ketchum, Qualcomm Incorporated

  5. Steps (m) and (n) in D2.0 • Divide the complex number string for each of the resulting NSS spatial streams into groups of NSD complex numbers, where the value of NSD is determined from the CH_OFFSET field of TXVECTOR and the CH_BANDWIDTH field of TXVECTOR. Each such group is associated with one OFDM symbol in one spatial stream. In each group, the complex numbers are indexed 0 to NSD-1 and mapped hereafter onto OFDM subcarriers as described in 20.3.10.10.2 (Transmission in 20 MHz HT format), 20.3.10.10.3 (Transmission in 40 MHz HT Format), 20.3.10.10.4 (Transmission in HT duplicate format), and 20.3.10.11 (Non-HT duplicate transmission). • If space time block coding (STBC) or hybrid space time block coding/spatial multiplexing (STBC/SM) is to be applied, as indicated by the STBC field in the TXVECTOR, operate on the complex number in each data subcarrier in sequential pairs of OFDM symbols as described in 20.3.10.8.1 (Space-Time Block Coding (STBC)) to generate NSTS OFDM symbols for every NSS OFDM symbols associated with the NSS spatial streams. If STBC or hybrid STBC/SM is not to be used, the number of space time streams is the same as the number of spatial streams, and the sequences of OFDM symbols in each space time stream are composed of the sequences of OFDM symbols in the corresponding spatial stream. John Ketchum, Qualcomm Incorporated

  6. New Steps (m) and (n) in 11-07/544r6 • If space time block coding (STBC) is to be applied, as indicated by the STBC field in the TXVECTOR, then apply STBC encoding to the sequential pairs of complex constellation symbols generated at the output of the constellation mappers as described in 20.3.10.8.1 to produce NSTS complex constellation symbols for every NSS complex constellation symbols associated with the NSS spatial streams. If STBC is not to be used, the number of space time streams is the same as the number of spatial streams, and the sequences of complex constellation symbols in each space time stream are composed of the sequences of complex constellation symbols in the corresponding spatial stream. • Divide the complex number string for each of the resulting NSTS spatial streams into groups of NSD complex numbers, where the value of NSD is determined from the CH_OFFSET field of TXVECTOR and the CH_BANDWIDTH field of TXVECTOR. Each such group is associated with one OFDM symbol in one spatial stream. In each group, the complex numbers are indexed 0 to NSD-1 and mapped hereafter onto OFDM subcarriers as described in 20.3.10.10.2 (Transmission in 20 MHz HT format), 20.3.10.10.3 (Transmission in 40 MHz HT Format), 20.3.10.10.4 (Transmission in HT duplicate format), and 20.3.10.11 (Non-HT duplicate transmission). John Ketchum, Qualcomm Incorporated

  7. The Problem with the new text • The primary problem is with the following text fragment from the new step (m): • “…then apply STBC encoding to the sequential pairs of complex constellation symbols generated at the output of the constellation mappers…” • In D2.0 a blocking of NSD constellation symbols was performed prior to STBC coding to account for the later formation of the OFDM symbol (multiplexing) which is missing in 544r6 step m) indicating that STBC coding is no longer applied on sequential pairs of OFDM symbols. This goes clearly against D2.0 and also against the common understanding of STBC encoding process which generally is applied on a per subcarrier basis over two sequential OFDM sybols. • This issue has to be fixed otherwise it will cause serious interoperability issues with D2.0 based implementations for anyone who tries to naively implement the PPDU encoding process only referring to 20.3.4: “summary of the PPDU encoding process”. • The 544r6 modification would create an inconsistency between section 20.3.4 (summary of the PPDU encoding process) and section 20.3.10.8 (constellation mapping and STBC) • Subsidiary problem: • Reversing the order of (m) and (n) and removing references to OFDM symbols from the new step (m) further obscures the fact that modulation symbols are demuxed into OFDM subcarriers prior applying STBC. • This is arguably an editorial issue rather than a technical issue, but in the interests of providing an accurate and clear explanation of PPDU encoding with STBC, this should also be fixed. John Ketchum, Qualcomm Incorporated

  8. Comment • Changes in 544r6 are in large part driven by the comment that • “When we look at section 20.3.4 in the draft 2.0 for the sequence in the TX, it states that the rotation should be done (step m) before the STBC is applied (step n).” referring to the 90-degree phase rotation required in the upper half of a 40 MHz signal. • In fact, there is no such explicit statement in step (m) of D2.0, although it also does not explicitly state that the phase rotation is done after STBC encoding. Instead, step (m) refers to the sections where the explicit mathematical definitions of the 40 MHz signal are given, including the phase rotation. • The changes in 544r6 create much bigger problems than any that might have been reported in the above comment. • Should clean up original text to take into account lack of clarity about where the phase rotation takes place, and other cleanup that was included in 544r6. • Clean-up should improve clarity about the relative ordering of demuxing to OFDM bins, STBC operation, and inverse FFT • Includes keeping original ordering of steps (m) and (n) John Ketchum, Qualcomm Incorporated

  9. D2.04 Text • Divide the complex number string for each of the resulting NSS spatial streams into groups of NSD complex numbers, where the value of NSD is determined from the CH_OFFSET field of TXVECTOR and the CH_BANDWIDTH field of TXVECTOR. Each such group is associated with one OFDM symbol in one spatial stream. In each group, the complex numbers are indexed 0 to NSD-1 and these indices have an associated one-to-one correspondence with subcarrier indices as described in 20.3.10.10.2 (Transmission in 20 MHz HT format), 20.3.10.10.3 (Transmission in 40 MHz HT Format), 20.3.10.10.4 (Transmission in HT duplicate format), and 20.3.10.11 (Non-HT duplicate transmission). • If space time block coding (STBC) is to be applied, as indicated by the STBC field in the TXVECTOR, operate on the complex number associated with each data subcarrier in sequential pairs of OFDM symbols as described in 20.3.10.8.1 (Space-Time Block Coding (STBC)) to generate NSTS OFDM symbols for every NSS OFDM symbols associated with the NSS spatial streams. If STBC is not to be used, the number of space time streams is the same as the number of spatial streams, and the sequences of OFDM symbols in each space time stream are composed of the sequences of OFDM symbols in the corresponding spatial stream. In each group of NSD resulting complex numbers in each space time stream, the complex numbers indexed 0 to NSD-1 are mapped onto OFDM subcarriers as described in 20.3.10.10.2 (Transmission in 20 MHz HT format), 20.3.10.10.3 (Transmission in 40 MHz HT Format), 20.3.10.10.4 (Transmission in HT duplicate format), and 20.3.10.11 (Non-HT duplicate transmission). • Steps (m) and (n) (change marks removed for clarity): John Ketchum, Qualcomm Incorporated

  10. D2.04 Text • Step (o) – added sentence at end to clarify where 90 degree phase shift occurs in 40 MHz waveform • Determine whether 20 MHz or 40 MHz operation is to be used from the CH_BANDWIDTH, CH_OFFSET field in the TXVECTOR. For 20 MHz operation, insert four subcarriers as pilots into positions –21, –7, 7, and 21. The total number of the subcarriers, NST, is 56. For 40 MHz operation (with the exception of MCS 32 and non-HT duplicate format ), insert six subcarriers as pilots into positions -53, -25, -11, 11, 25, and 53, resulting in a total of NST = 114 subcarriers. See 20.3.10.10.4 (Transmission in HT duplicate format) for pilot locations when using MCS 32, and 20.3.10.11 (Non-HT duplicate transmission) for pilot locations when using non-HT duplicate format. The pilots are modulated using a pseudo-random cover sequence. Refer to 20.3.10.9 (Pilot subcarriers) for details. For 40 MHz operation, apply a +90 degree phase shift to the complex value in each OFDM subcarrier with an index greater than 0, as described in 20.3.10.10.3 (Transmission in 40 MHz HT Format), 20.3.10.10.4 (Transmission in HT duplicate format), and 20.3.10.11 (Non-HT duplicate transmission). John Ketchum, Qualcomm Incorporated

  11. Other Changes • 544r6 text for 20.3.10.8.1 was modified some to be consistent with changes to 20.3.4 • Striving for clarity of exposition of order of operations: • Symbol demux  STBC  IFFT • Disconnect between symbol indexing (0 to NSD-1) and subcarrier indexing in OFDM symbol needs to be handled carefully • Column headings in Table 194 modified to remove reference to OFDM symbols. John Ketchum, Qualcomm Incorporated

  12. Next Steps • PHY ad hoc should review text in D2.04 associated with 544r6 and CID 1599 that has been modified for: • Accuracy • Clarity • PHY ad hoc options: • Approve CID 1599 EMR: Vote to accept changes in D2.04 associated with 544r6 and CID1599 and forward to TGn for voting • Consider any further modifications to the associated text and approve new resolution to CID1599 • Reject all changes and return to original text in D2.0 • Not a good idea: • Reject editor’s modifications to previously approved resolution and instruct editor to implement 544r6 without modification. John Ketchum, Qualcomm Incorporated

  13. References • 11-07/544r6, ‘Resolution proposal to CIDs 1599, 1600’ Jeremy Gosteau, Marc de Courville, Stéphanie Rouquette Léveil, Motorola. John Ketchum, Qualcomm Incorporated

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