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Response to Comments on Optional Enhanced ACR and AACR Values

Response to Comments on Optional Enhanced ACR and AACR Values. Authors:. Date: 2008-08-15. Abstract. Responses to Comments Concerning Clause 17 Optional Enhanced ACR and AACR Values, including suggested modifications to the 802.11p Draft.

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Response to Comments on Optional Enhanced ACR and AACR Values

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  1. Response to Comments on Optional Enhanced ACR and AACR Values Authors: Date: 2008-08-15 Carl Kain, Noblis (USDOT)

  2. Abstract Responses to Comments Concerning Clause 17 Optional Enhanced ACR and AACR Values, including suggested modifications to the 802.11p Draft. Carl Kain, Noblis (USDOT)

  3. Response to Comments Concerning Clause 17 Enhanced ACR and AACR Values CIDs 395, 398, 399, 400, 401, 404, 403, 404 Carl Kain, Noblis (USDOT)

  4. Comment Summary Carl Kain, Noblis (USDOT)

  5. Comment Summary Carl Kain, Noblis (USDOT)

  6. Comment Summary • Most commenters want optional enhanced ACR and AACR reduced by 10 dB • Some Commenters want justification for tighter values • There is a more significant issue related to the transmission mask of the interfering signal used in the test described in the base document. • This issue defines what enhanced values should be reasonable, and should address the concerns of the commenters Carl Kain, Noblis (USDOT)

  7. Test defined in Current Base Standard (Clause 17) • The nonadjacent channel rejection shall be measured by setting the desired signal’s strength 3 dB above the rate-dependent sensitivity specified in Table 17-13, and raising the power of the interfering signal until a 10% PER occurs for a PSDU length of 1000 octets. • Example in this document for illustrative purposes will be BPSK R=1/2; AACR for 10 MHz Channel • Receiver Sensitivity is -85 dB • Desired Signal = -85 dBm + 3dB = -82 dBm • The power difference between the interfering and the desired channel is the corresponding nonadjacent channel rejection. • AACR from Table 17-13 for this example is 32 dB • The interfering signal in the nonadjacent channel shall be a conformant OFDM signal, unsynchronized with the signal in the channel under test. For a conformed OFDM PHY, the corresponding rejection shall be no less than specified in Table 17-13. Carl Kain, Noblis (USDOT)

  8. Test in Current Base Standard AACR=32 Carl Kain, Noblis (USDOT)

  9. Some Additional Information • The base standard specifies a 5 dB implementation margin • If the adjacent (or alternate adjacent) interferer has a transmission mask that allows in-channel interference higher than the desired signal, the error rate will be irreducible. • The desired situation is to have the interfering signal be below the noise floor in the middle of the desired channel (where you cannot filter it out), and build filters in the receiver to handle the adjacent and alternate adjacent channel interference. • In the example on the previous slide, that is exactly the case in the base standard. • 802.11p needs an enhanced ACR and AACR because there are possible geometries and situations where adjacent and alternate adjacent channels may be in use in close proximity (same as for any 802.11); however, the potential interference in the base standard is not acceptable for vehicle safety applications. Carl Kain, Noblis (USDOT)

  10. Some Additional Background • The current base standard assumes the transmission mask, A, so if you set the ACR and AACR for the least stringent mask (Mask A), any other mask will make standards compliance easier. • The real problem is not that the values are too stringent, it is that to be consistent with the base standard, and provide improved performance for ITS applications, the transmitters operating at the medium and higher power levels (in the US) need to use the Mask C, which is identical to the Mask M that is already in the base standard (Annex I, Figure I.2-Transmit spectrum masks for the U.S. 4.9 GHz public safety band) • Next slide (Courtesy of Jerry Landt, Transcore) illustrates the current problem Carl Kain, Noblis (USDOT)

  11. Optional AACR Example with Alternate Adjacent Interferer using Mask A AACR=47 15 dB above noise floor Carl Kain, Noblis (USDOT)

  12. Proposed Solution • Make the enhanced ACR and AACR values testing with conformant signals that use the transmit Mask C (same as Mask M in base standard) • Mask C out of band values are set to 8 dB below the test signal (5dB for implementation loss + 3 dB above receiver sensitivity level as specified in base standard) • Reduction in current (802.11p D4.0) enhanced values would be between 2-6 dB, but by specifying the transmit mask of the interferer, the filter design constraints are significantly relaxed– a good compromise that is in agreement with the concerns of the commenters. • Implementers desiring better performance for safety applications will use the optional enhanced ACR/AACR values AND the transmit Mask C. Carl Kain, Noblis (USDOT)

  13. Changes required in 802.11p • Replace the existing optional enhanced performance ACR and AACR values in table 17-13a with the following reduced values: modulation coding rate ACR Non-Adjacent Channel Rejection BPSK 1/2 34 42 BPSK 3/4 33 41 QPSK 1/2 31 39 QPSK 3/4 29 37 16-QAM 1/2 26 34 16-QAM 3/4 22 30 64-QAM 1/2 18 26 64-QAM 3/4 17 25 • Insert the following as an additional paragraph at the end of both 17.3.10.2 and 17.3.10.3 An optional enhanced performance specification is provided for systems requiring low bit error rate (BER). If dot11ACRType = 2, the optional enhanced receiver performance specifications given in Table 17-13a shall apply. The interfering signal in the adjacent channel shall be a conformant OFDM signal, using transmit mask M (See Figure I.2) (note: mask M is equivalent to mask C), unsynchronized with the signal in the channel under test. For a conformant PHY meeting the optional enhanced receiver performance specifications, the corresponding rejection shall be no less than specified in Table 17-13a. The corresponding minimum receiver sensitivities for each modulation and coding rate are the same as in Table 17-13. Carl Kain, Noblis (USDOT)

  14. Resolution and Motion • We agree with the commenters that the enhanced receiver ACR and AACR were too stringent • We recognize that the enhanced values should only apply to conformant signals using transmitter Mask M, and relaxed the values accordingly • The interfering signal under test (using Mask M) will inject less out of band interference, thus simplifying receiver filter design • The official resolution to the comments is “counter”; referring to this document • Move to accept the proposed changes in 802.11p D4.01 as proposed in this document and instruct the editor to make such changes. • Moved: Carl Kain • Second: Francois Simon • For :10 • Against 0 • Abstain 1 Carl Kain, Noblis (USDOT)

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