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Interleaver for Nss>4. Date: 2011-01-18. Authors:. Background. BCC interleaver permutations are characterized by three parameters N ROW , N COL and N ROT as in 11n. In 11ac, N ROW , N COL and N ROT need to be modified to accommodate higher bandwidths and/or more streams.
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Interleaver for Nss>4 Date: 2011-01-18 Authors:
Background • BCC interleaver permutations are characterized by three parameters NROW, NCOL and NROT as in 11n. • In 11ac, NROW, NCOL and NROT need to be modified to accommodate higher bandwidths and/or more streams. • Column depth for 80MHz is 26, i.e., (NCOL = 26, NROW = 9 * NBPSCS) • NROT and Stream Permutaions: • When Nss <=4 20/40MHz: NROT and permutation are the same as 11n. • When Nss <=4 80MHz: NROT = floor(234/4)=58, so cyclic shift [0, 2, 1, 3]* NROT*NBPSCS) for Nss=1,2,3,4 respectively (as in current spec framework). • When Nss >4: the SS permutation and NROT are TBD.
Best set of SS permutation and NROT for Nss >4 based on largest D (1/4) • Given BW and Nss >4, find SS permutation and NROT such that average subcarrier distance (D) of two adjacent SS is maximized. Specifically, where ``” means the third interleaver permutation, i.e., freq. rotation based on SS permutation and NROT, A is the normalization factor, i is the data tone index before the third permutation, k,m are the exact tone index after the permutation. |a| is to take the absolute value of a. Note: Objective criterion of the best set of SS permutation and NROTis to provide best PER performance.
Best set of SS permutation and NROT for Nss >4 based on largest D (2/4) One practical issue: More than one set of SS permutation and Nrot give the same D value. For example, for (80M, 7SS), 58*[0 6 4 1 3 5 2], and 58*[0 2 5 3 1 4 6] give the same D of 115.6. Break the tie:Check the average distance of the j-th adjacent SS, where j =2,3… until the tie is resolved. Specifically, Using the above method, we get one best set of SS permutation and Nrot for most of pair (BW, Nss), except few case such as (80M, 5 ss). For (80M, 5 ss), 47*[0 2 4 1 3] and 47*[ 0 3 1 4 2] give the same D’s for all possible j.
80M 8ss Best D corresponds to 28*[0 5 2 7 3 6 1 4] Best set of SS permutation and NROT for Nss >4 based on largest D (3/4) • One example of the simulation curves: two dimensional search for best SS permutation and NROT for (80M, 8 ss): each ``*” corresponds the largest D (j=1) for a given Nrot among all the possible SS permutations.
Best set of SS permutation and Nrot for Nss >4 based on largest D (4/4) Some considerations • The criterion based on largest D requires different Nrot and SS permutation for each pair of (BW, Nss). • This criterion may not provide best PER performance, but can be used as a reference of a good PER curve. • Nss>4 has stringent SNR requirement and already very high complexity in basic Tx/Rx HW, interleaver design hence should focus on the PER performances.
Candidate Nrot and SS permutation for Comparisons • Selection based on largest D for each (BW, Nss) combination. • Selection based on the second largest D for each (BW, Nss) combination • Subset of Nrot*[0 4 2 6 1 5 3 7] for 5~8SS. • Nrot=floor(Ndata-tone/8) • Extension of exiting 4ss selection, i.e., subset of floor (N_data_carr/4) *[ 0 2 1 3 0 2 1 3] • Subset of Nrot*[0 5 2 7 3 6 1 4], Nrot = 28, 13, 6 for 80M, 40M and 20M, respectively. • i.e. optimize Nrot and permutation based on 8SS for different BW respectively, then take subset for 4<Nss<8.
Simulation setup • Packet length: 1000 byes • MIMO: 5x8, 6x8, 7x8, 8x8 for 5, 6, 7, 8ss, respectively • Channel: 11ac DNLOS • 1000 channel realizations for each PER curve • Therefore, PER > 1% is accountable • MCS3 (16QAM ½), and MCS7 (64QAM 5/6) for different (BW, Nss) combinations. • 5 sets of Nrot and SS permutation for each (BW, Nss, MCS), as previously discussed.
Summary of simulation results (1/2) • Nrot and SS permutation based on larges D provide best and close to best PER performance in all cases. • Bottom line: Their PER performance can serve as the reference for good selection of Nrot and SS permutation. • PER performance depends on many factors including BW, Nss, MCS, etc, in addition to the selection of Nrot and SS permutation. • None of the 5 sets of Nrot and SS permutation under test provide the best PER performance in all cases. But some sets show severe degradation in certain configurations, such as Set 4 for (80M, 5ss, MCS3).
Summary of simulation results (2/2) By checking with the simulation results available, we propose Set 5 to be the selection of Nrot and SS permutation for Nss>4. • Recall: Set 5 is the subset of Nrot *[0 5 2 7 3 6 1 4], Nrot =28, 13 and 6 for 80M, 40M and 20M, respectively. Specifically, Set 5 has following advantages: • Good PER performance in most simulated cases, compared with the reference Set 1 and other sets • No severe degradation in any case • Systematic and Simple formulation by using the subset of 8SS.
Math Expressions of the Proposal • At interleaver: if Nss>4, • At deinterleaver: if Nss>4
Pre-motion #1 • Do you agree to insert the following text in 3.2.4.3.3 Frequency interleaver of the spec framework? • “For BCC encoding, when Nss>4, the third permutation of the interleaver uses the following parameters: • Permutation as the first Nss values of [0 5 2 7 3 6 1 4]*Nrot *NBPSCS. • NROT =28, 13 and 6 for 80MHz, 40MHz and 20MHz, respectively.” -Yes -No -Abstain
