ITU - Telecommunication Standardization Sector
`
`Temporary Document 09XC-100
`
`STUDY GROUP 15
`
`Original: English
`
`Xian, China — 27 — 31 July 2009
`
`Question: 4/15
`
`SOURCE!': CopperGate Communications
`
`G.hn: Using Two Symbols for the Header of a PHY frame on Coax
`TITLE:
`
`
`ABSTRACT
`
`The contribution proposes that the number of symbols used for the header of the PHY frame on
`coax (baseband and RF)befixed to two exactly. The reason for this proposalis that a single symbol
`does not provide adequate reliability for the 50 MHz case, as shownbelow.
`
`1.
`
`Introduction
`
`The PHY-header of a PHY-frameis defined in G.9960 [1] to contain important information
`necessary in order to process the payload. The information of the header is encoded in a single FEC
`block of 168 bits with code rate 1/2. This information is modulated unto either one or two OFDM
`symbols using QPSK modulation. The effective numberof repetitions depends on the numberof
`active subcarriers and on the number of header symbols (1 or 2).
`
`It is not specified what the conditions for choosing one or two symbols are. The intention is that
`there will be a sufficient numberofrepetitions of the FEC block so that some minimum level of
`robustness is guaranteed.
`
`A straightforward calculation can be carried out in order to assess the required number of OFDM
`symbols. Let us consider the more challenging setting of 50 MHz devices on coax. Both for
`baseband and RF, the numberofsubcarriers is 256. Since the header is modulated using QPSK and
`rate 1/2 a single copy of the PHY-header FEC block takes 168 subcarriers. Obviously, there is only
`room for a single complete copy of the FEC block. This situation is problematic, because the header
`does not have any advantage over the payload in terms of robustness.
`Decoding a 136-bit FEC block with a BLERlevel of 10° requires an SNR of 5 dB or so. Onthe
`other hand, decoding a 520-byte FEC block with a BLERlevel of 10° requires an SNR ofabout 3.5
`dB — some1.5 dBless.
`
`It is reasonable to approximate that if there are N copies of a FEC block then it acquires an extra
`coding gain of 10logi9(N), even if Nis non integer. If we assumethat 0.9 of the bandis utilized then
`this means there are 256x0.9/168 = 1.4 repetitions per symbol, which provide a gain of 1.5 dB. This
`
` Contact: Boaz Kol,
`CopperGate Communications
`
`
`
`INTEL-1020
`10,079,707
`
`INTEL-1020
`10,079,707
`
`

`

`makes the header aboutas robust as a payload encoded using a FEC block of 520 bytes with code
`rate 1/2 and QPSK modulation.
`
`This contribution proposes to always use two header symbols on coax. The numberof subcarriers in
`two OFDM symbols is 512, whichis barely sufficient to accommodate 3 full copies of the header,
`which require 3x 168 = 504 subcarriers. Note that in practice some guard bands are expected near
`the band edges. In this case, even 3 full repetitions will notfit in.
`
`2. Proposal
`
`It is proposed to incorporate the following text additions to [2] G.hn: Draft text for G.9960
`Amendment | — version 4.0.
`
`<<Start of Text>>
`
`7.1.3.4 Header encoder
`
`The functional model of the header encoderis presented in Figure 7-10. It contains an FEC encoder
`and a Header Repetition Encoder (HRE).
`
`Headerbits
`
`Header FEC
`encoder
`
`encoder Header Encoder
`
`Header
`repetition
`
`Encoded
`headerblock
`
`Figure 7-10/G.9960 — Functional diagram of the Header Encoder
`
`The bits of the PHY-frame headershall input the Header FEC encoderin their original order and
`encoded as described in §7.1.3.2. The size of the FEC codeword andthe coding rate of the Header
`FEC encoderare described in Table 7-19.
`
`The FEC codeword enters the HRE. The HRE shall operate as follows:
`
`
`
`- The FEC codewordshall be first copied M times, where M=ceiling (Dxkx/Nrec), kx is the
`numberofbits to be loaded onto the OFDM symbolcarrying the header, and D is the
`number of symbols to be used for header transmission
`
`-
`
`The encoded headerblockshall be formed by concatenation of M copies of the header FEC
`encoder output. As each codewordis cascadedto one another, the bits (6;) within a
`codeword shall be cyclically shifted by 2 bits as follows:
`o The 1° FEC codeword copy shall be formed as {o, bi, ..., byezc.2, burec-i}-
`o The 2™ FEC codeword copy shall be formed as {bo, 63, ..., bvrzci, 50, 61}.
`o The 3 FEC codeword copy shall be formed as {h4, bs, ..., byrec-1, bo, 51, b2, bs}.
`
`

`

`o The M™ FEC codeword copy, where M> 3, shall be formed as {bexi4-2), bexi-t), ---
`byrec.1, bo, b1, -.-, bex4)s bax}.
`Valid values of D are either 1 or 2. Details £ : are for further study.
`
`
`
`NOTE:Since the coding rate used for header encodingis 2, the numberofbits in the FEC
`codeword is always even, and the numberofbits in the encoded header blockis even.
`
`<<End of Text>>
`
`3. References
`
`[1] G.9960 LC commentresolution draft text R7.doc “ITU-T Recommendation G.9960, Unified
`high-speed wire-line based home networking transceivers - Foundation”, Editor for G.hn,
`Baltimore Maryland, 18 — 26 May 2009.
`
`[2] 0O9XC-R12a, “G-.hn: Draft text for G.9960 Amendment 1 — version 4.0”, Xian, China — 27 —
`31 July 2009.
`
`4. Summary
`
`It is proposed to agree on the following new items.
`
`a symbols for coax (baseband and RF)? fy ae202
`
`SOT a201
`
`— adopted into 09XC-R12a?
`
`Open
`Open
`
`Should the PHY-frame header include exactly two OFDM
`Should the text changes proposed in Sec 2 of09XC-100 be
`
`09XC-100
`09XC-100
`
`