Presented at ICTC2018(9th International Conference on Information and Communication Technology Convergence) Date : Oct 18, 2018 Place : Jeju, Korea DOI) 10.1109/ICTC.2018.8539461 URL) https://ieeexplore.ieee.org/document/8539461 [ URL of the paper/preprint ] https://www.researchgate.net/publication/328364760_Boosting_the_Performance_of_Nested_Spatial_Mapping_with_Unequal_Modulation_in_80211n [ Prior works of Nested Spatial Mapping without Unequal Modulation(UEQM) ] https://www.slideshare.net/ealwanlee/nested-mimo-lectures-in-2017-seoul [ List of the articles related with this slide ] https://www.linkedin.com/pulse/list-articles-nested-spatial-mapping-wlan80211n-ealwan-lee/

1. GCT Semiconductor, Inc.http://www.gctsemi.com
Boosting the Performance of Nested Spatial Mapping
with Unequal Modulation in 802.11n
Ealwan Lee
GCT Semiconductor, Inc.
Session : WLAN, WPAN and WBAN (P-2)
Oct 18, 2018 (08:30 ~ 10:10)

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GCT Semiconductor, Inc.ICTC 2018
Table of Contents
 Introduction (p. 2)
 History & Background
✓ Hierarchical Modulation, LDM(ATSC-3.0), NOMA
✓ Higher order modulation, MIMO
 Review of prior works of Nested Spatial Mapping and Overview of this work
✓ 256-QAM with Nested Spatial Mapping (IP-SOC 2016)
 Generalization, extension and improvement of prior arts (pp. 3 ~ 11)
 Unequal modulation (pp. 3 ~ 8)
✓ 64QAM+QPSK for improved 256-QAM
✓ 64QAM+16QAM for spoofing of 1024-QAM introduced in 802.11ax
 Improvement of channel estimator (p. 9)
 Balancing the LLR of SIC demapper between the two streams (pp. 10 ~ 11)
 Application (pp. 12 ~ 14)
 AMC
 Backward compatibility with 802.11n.
 Revision points in F/W for its adoption
 Conclusion (p. 15)

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1. Introduction : Background & History
 Prior works related with Nested Spatial Mapping - But, not the same.
 Hierarchical Modulation in Next-Gen Broadcasting System : LDM in ATSC-3.0
✓ Respective channel encoder per stream(higher/lower priority)
✓ UMB, NOMA in cellular wireless network.
✓ Modulation of low layer is dependent on the state of upper layer modulator.
 Modulation order increases
✓ 256-QAM in 802.11ac and LTE-A
✓ 1024-QAM in 802.11ax since the publication of Nested Spatial Mapping in 2016.
 Multiple antenna device is not preferred in recently emerging IoT connectivity solution.
✓ Bulky volume, Large power consumption, High implementation cost
 Initial works of Nested Spatial Mapping (Prior works of the author)
 Tx/Rx of dual 16-QAM stream from a single channel encoder through a single physical antenna.
 Maximal exploitation of 802.11n standard to raise the effective modulation order.
802.11ac
802.11n
802.11ax
256-QAM
1024-QAM
Hierarchical Modulation
Unequal
Modulation
Performance boosting
of 256-QAM
Nested
Spatial Mapping

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GCT Semiconductor, Inc.ICTC 2018
2.1. Generalization of Nested Spatial Mapping
 In UEQM of 802.11
 Higher order is placed before the lower order with equal RMS power to each antenna.
 In Nested Spatial Mapping,
✓ (c=0) : High power is assigned to the higher order modulation(1st stream)
✓ (c=1) : High power is assigned to the lower order modulation(2nd stream)
 Normalization of the composite signal is carried out for fair comparison.
IFFT DAC+
IFFT DAC
~
X
X
RF-PLL
s2-c[n]
s1+c[n]
y(t)
GI
GI
FLT
FLT
0ns
-400 ns
x1+c[n]
X 𝜶 𝒎 𝟏+𝒄,𝒎 𝟐−𝒄
−𝟏
=
𝑲 𝒎 𝟏+𝒄
𝑲 𝒎 𝟐−𝒄
∙
𝟐−𝒎 𝟐−𝒄
𝟏 + 𝜹 𝒎 𝟏+𝒄,𝒎 𝟐−𝒄
CSD
CSD
X
𝒈 𝒎 𝟏+𝒄,𝒎 𝟐−𝒄
= ൗ𝜶 𝒎 𝟏+𝒄,𝒎 𝟐−𝒄
𝟏 + 𝜶 𝒎 𝟏+𝒄,𝒎 𝟐−𝒄
𝟐
CC
enc
bn
𝒙 𝟏+𝒄 𝒏 = 𝒈 𝒎 𝟏+𝒄,𝒎 𝟐−𝒄
∙ 𝒔 𝟏+𝒄 𝒏 + 𝜶 𝒎 𝟏+𝒄,𝒎 𝟐−𝒄
−𝟏
∙ 𝒔 𝟐−𝒄 𝒏
MCS 1st
QAM
2nd
QAM
12(2,2) 16 16
36(2,1) 16 4
37(3,1) 64 4
38(3,3) 64 16
c=0
c=1

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GCT Semiconductor, Inc.ICTC 2018
2.2. Nominal Nesting Factor of UEQM
 Unlike EQM, it is no more in the form of powers-of-two.
2(Km1 -7/a1,3 K3) 2(Km1 -3/a2,2 K2)
2(Km1 -1/a3,1 K1)
2/a1,3K3
2/a2,2 K2
2/a3,1 K1
𝟐 𝟑
∙ 𝑲 𝟑 𝟐 𝟐
∙ 𝑲 𝟐
𝟐 𝟏
∙ 𝑲 𝟏
𝒎 𝟐 = 𝟑, 𝑲 𝟑 = Τ𝟏 𝟒𝟐 𝒎 𝟐 = 𝟐, 𝑲 𝟐 = Τ𝟏 𝟏𝟎 𝒎 𝟐 = 𝟏, 𝑲 𝟏 = Τ𝟏 𝟐
𝟐
෥𝜶 𝒎 𝟏,𝒎 𝟐
∙ 𝑲 𝒎 𝟐
= 𝟐 ∙ 𝑲 𝒎 𝟏
−
𝟐 𝒎 𝟐 − 𝟏
෥𝜶 𝒎 𝟏,𝒎 𝟐
∙ 𝑲 𝒎 𝟐
෥𝜶 𝒎 𝟏,𝒎 𝟐
= 𝟐 𝒎 𝟐 ∙
𝑲 𝒎 𝟐
𝑲 𝒎 𝟏
≠ 𝟐 𝒎 𝟏
Found only in case c=1

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GCT Semiconductor, Inc.ICTC 2018
2.2. Nesting Factor & Nesting Margin
 Unlike hierarchical modulation adopted by the standardization body,
 Nested Spatial Mapping suffers a two-bit change across the streams.
 A slight lowering of the nesting factor, nesting margin, is expected to optimize the performance.
erfc
1 + (2𝑚2−𝑐 − 1) ⋅ δ 𝑚1+𝑐,𝑚2−𝑐
𝜎 𝑛
= erfc
1 − δ 𝑚1+𝑐,𝑚2−𝑐
𝜎 𝑛
1+𝜇
inter-stream error intra-stream error
  [0 ~ 1] :
effective discrepancy between intra-stream BER
and inter-stream BER.
Dependent on MCS.
Even implementation loss counts in.
𝜎 𝑛 = 10−
𝑆𝑁𝑅
20
1 + (2𝑚2−𝑐 − 1) ⋅ δ 𝑚1+𝑐,𝑚2−𝑐
2
≈ 1 + 𝜇 ⋅ 1 − δ 𝑚1+𝑐,𝑚2−𝑐
2
− 𝜎 𝑛
2
⋅ ln
𝜎 𝑛
𝑢
𝜋
+ ln
1 + (2𝑚2−𝑐 − 1)δ 𝑚1+𝑐,𝑚2−𝑐
1 − δ 𝑚1+𝑐,𝑚2−𝑐
1+𝜇
Large number asymptote applied ( n << 1 )
𝛿 𝑚1+𝑐,𝑚2−𝑐
=
𝜇 − 𝜎 𝑛
2
⋅ 𝑙𝑛 Τ𝜎 𝑛
𝜇
𝜋
2𝑚2−𝑐 + 𝜇) ⋅ (2 + 𝜎 𝑛
2
≈
𝜇
2𝑚2−𝑐 + 𝜇 ⋅ 2
Robust/insensitive to CSI(e.g. SNR)
𝛿 𝑚1+𝑐,𝑚2−𝑐
=
𝛼 𝑚1+𝑐,𝑚2−𝑐
෤𝛼 𝑚1+𝑐,𝑚2−𝑐
− 1

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GCT Semiconductor, Inc.ICTC 2018
3.1. Improvement of 256-QAM Spoofing with UEQM
 Prior works : MCS12(16QAM+16QAM)
 Application of Unequal Modulation : MCS37(QPSK+64QAM, 64QAM+QPSK)
 SNR = 28 dB
 c = 1 is recommended for spoofing of 256-QAM.
0 6 12 18 24 30
10
-3
10
-2
10
-1
10
0
nesting factor, 20  log am
1+c
,m
2-c
(dB)
BLER(%)
MCS37(c=1)
MCS12
MCS37(c=0)
𝜹 𝟏,𝟑 𝜹 𝟐,𝟐 𝜹 𝟑,𝟏
𝟐 𝟑 ∙
𝑲 𝟑
𝑲 𝟏
𝟐 𝟐 ∙
𝑲 𝟐
𝑲 𝟐
𝟐 𝟏 ∙
𝑲 𝟏
𝑲 𝟑
Optimal Nesting Factor =
23
∙
𝐾3
𝐾1
< 21
< 22
∙
𝐾2
𝐾2
< 23
< 21
∙
𝐾1
𝐾3
Nominal Nesting Factor
Nesting Margin
𝜇
2 ∙ 3 + 𝜇 ∙ 2
<
𝜇
2 ∙ 2 + 𝜇 ∙ 2
<
𝜇
2 ∙ 1 + 𝜇 ∙ 2
Nominal Nesting Factor
+
Nesting Margin

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GCT Semiconductor, Inc.ICTC 2018
3.2. Spoofing of 1024-QAM comparable to 802.11ax
 Application of Unequal Modulation : MCS38(16QAM+64QAM, 64QAM+16QAM)
 SNR = 37 dB.
 c = 0 is recommended for spoofing of 1024-QAM.
 Possibly limited by unknown impairment of the data path optimized for 802.11n as of today.
0 6 12 18 24 30
10
-3
10
-2
10
-1
10
0
nesting factor, 20  log am
1
,m
2
(dB)
BLER(%)
MCS38(c=1)
MCS38(c=0)
𝜹 𝟐,𝟑
𝟐 𝟑 ∙
𝑲 𝟑
𝑲 𝟐
𝜹 𝟑,𝟐
𝟐 𝟐 ∙
𝑲 𝟐
𝑲 𝟑
23
∙
𝐾3
𝐾2
< 22
< 23
< 22
∙
𝐾2
𝐾3
Nominal Nesting Factor
Nesting Margin
𝜇
2 ∙ 3 + 𝜇 ∙ 2
<
𝜇
2 ∙ 2 + 𝜇 ∙ 2
Optimal Nesting Factor = Nominal Nesting Factor
+
Nesting Margin

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GCT Semiconductor, Inc.ICTC 2018
3.3. Operating range : (SNR, Nesting Factor)
 Around the required Tx EVM(~SNR), the operating range is wide enough
 Condition : 100+ Mbps
 State-of-the-art technology ascertains the Tx EVM above 40 dB as of today.
nesting factor, 20  log a3,2
(dB)
SNR(dB)
15 18 24 30
30
36
42
48
10
20
30
40
50
60
70
80
90
100
Colormapforthroughput(Mbps)
p. 12
p. 7
Excerpt from p. 556 of [3] in the paper

10. 9/15
GCT Semiconductor, Inc.ICTC 2018
4.1. Refined Channel estimate for Nested Spatial Mapping
 Exploit the fact that
 ℎ𝑖2 = 𝛼 𝑚1,𝑚2
⋅ ℎ𝑖1
 Strategy for synchronizing the value of nesting factor over the link
 Simply fix it based on the MCS selected!
✓ The optimal performance is insensitive to the variation of nesting margin
 Estimate by summing over the channel estimate.
✓ Adaptive control of nesting margin with discrete step can be applied if needed.
෨ℎ𝑖1[𝑛] ←
1 ∙ ℎ𝑖1[𝑛] + ො𝛼 𝑚1,𝑚2
∙ ℎ𝑖2[𝑛]
1 + ො𝛼 𝑚1,𝑚2
2
෨ℎ𝑖2[𝑛] ← ො𝛼 𝑚1,𝑚2
∙ ෨ℎ𝑖1[𝑛]
𝛾 =
σ 𝑛=1
𝑁 𝑠𝑐
ℎ𝑖2
∗
[𝑛] ∙ ℎ𝑖2[𝑛]
σ 𝑛=1
𝑁𝑠𝑐
ℎ𝑖1
∗
[𝑛] ∙ ℎ𝑖1[𝑛] + ℎ𝑖2
∗
[𝑛] ∙ ℎ𝑖2[𝑛] ො𝛼 𝑚1,𝑚2
=
𝑅𝑒(𝛾)
1 − 𝑅𝑒(𝛾)
i is index for receiver path.
Multiple Receiver chain serves only to MRC.
frequency(n)
ℎ𝑖1[𝑛]
ℎ𝑖2[𝑛]
𝛼 𝑚1,𝑚2
≈ ො𝛼 𝑚1,𝑚2
𝛼 𝑚1,𝑚2

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GCT Semiconductor, Inc.ICTC 2018
4.2. SIC demapper (1/2) : Balancing of LLR between streams
 SIC demapper can be used in replace of ML demapper.
 Linear de-correlation type receiver such as ZF, MMSE cannot decode NSM(rank = 1)
 c, ordering info of SIC demapper, can be detected with the sign of ℎ1 − ℎ2 .
 In Tx side, c is fixed to a specific value.
 Double-check and re-adjust of the LLR for the balancing between the two streams.
 Each BICM demapper output should be independent of 𝐾 𝑚.
 Or the output should be re-adjusted by following re-scaler(addressed in the paper).
BICM
demapper
X
𝒉 𝟏+𝒄
∗
𝑹[𝒏]
+ X
𝒉 𝟐−𝒄
∗
= 𝜶 𝒎 𝟏+𝒄,𝒎 𝟐−𝒄
−𝟏
⋅ 𝒉 𝟏+𝒄
∗𝑹[𝒏] − 𝒉 𝟏+𝒄 ∙ ො𝒔 𝟏+𝒄[𝒏]
X𝒉 𝟏+𝒄
BICM
demapper
ො𝒔 𝟏+𝒄[𝒏]
ො𝒔 𝟐−𝒄[𝒏]
X
𝝀 𝟏[𝒏]
𝝀 𝟐[𝒏]
𝜷 𝒎 𝟏+𝒄,𝒎 𝟐−𝒄
−𝟏
= 𝑲 𝒎 𝟐−𝒄
/𝑲 𝒎 𝟏+𝒄
To channel decoder
𝑲 𝒎 𝟏+𝒄
⋅ 𝝀 𝟏+𝒄[𝒏]
𝒄
𝒄 = 𝒉 𝟏 > 𝒉 𝟐 ? 𝟎: 𝟏
𝒉 𝟏
+
-
𝒉 𝟐
𝑲 𝒎 𝟏+𝒄
⋅ 𝒉 𝟏+𝒄
𝟐
Errata in the paper corrected !
𝑲 𝒎 𝟐−𝒄
⋅ 𝒉 𝟐−𝒄
𝟐
BICM
demapper
X
𝒉 𝟏+𝒄
∗
/𝑲 𝒎 𝟏+𝒄
𝒉 𝟏+𝒄
𝟐
ො𝒔 𝟏+𝒄[𝒏]
𝝀 𝟏+𝒄[𝒏]

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GCT Semiconductor, Inc.ICTC 2018
4.2. SIC demapper (2/2) : Comparison with ML demapper
 No need for the concern about the balancing or re-adjust of prior works : EQM(MCS12)
 𝛽2,2 = ൗ𝐾2
𝐾2
= 1
 Balanced LLR or re-adjustment of LLR for given SNR reduces
 the performance gap with ML demapper.
0 6 12 18 24 30
10
-3
10
-2
10
-1
10
0
nesting factor, 20  log am
1+c
,m
2-c
(dB)
BLER(%)
MCS37,MLD(c=1)
MCS37,SIC(c=1)
MCS12,MLD
MCS12,SIC
MCS38,MLD(c=0)
MCS38,SIC(c=0)
SNR = 28dB for 256-QAM
SNR = 37dB for 1024-QAM
𝜷 𝟑,𝟐 =
𝑲 𝟑
𝑲 𝟐
𝜷 𝟏,𝟑 =
𝑲 𝟏
𝑲 𝟑
𝜷𝒊,𝒋 = 𝟏
Errata in the paper corrected !
N.B.
The performance gap of MCS37(c=1)
with ML demapper is much narrower
than that of MCS12.

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GCT Semiconductor, Inc.ICTC 2018
5.1. Throughput vs. SNR curve for AMC in Boosting Region
 256-QAM
 Optimized MCS37(QPSK+64QAM) outperforms MCS12(16QAM+16QAM) approximately by 1dB.
 PER estimate based on ACK packet can be used for AMC even for 1024-QAM region.
 -greedy algorithm may be used to raise the modulation order.
12 18 24 30 36 42
0
20
40
60
80
100
120
SNR (dB)
Throughput(Mbps)
MCS6
MCS7
MCS37
MCS38
MCS12
MCS36
MCS38
MCS37
MCS12
MCS07
𝑷 𝒆 > 𝟏 −
𝑻𝑷 𝟑𝟕
𝑻𝑷 𝟑𝟖
𝑷 𝒆 > 𝟏 −
𝑻𝑷 𝟎𝟕
𝑻𝑷 𝟑𝟕
𝑷 𝒆 < 𝜽
𝑷 𝒆 < 𝜽

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GCT Semiconductor, Inc.ICTC 2018
5.2. Backward compatibility with existing 802.11n AP
 The format of the OFDM symbol has been modified in 802.11ax
 64-pts FFT(52/56, 3.2us) => 256-pts FFT(234/242, 12.8 us)
 The ratio of shortest guard interval has been reduced from 1/8(0.4 us) to 1/16(0.8 us).
 Nested Spatial Mapping is not exclusive to the use of either 802.11ac or 802.11ax
 It affects and improves 802.11n part only.
Rx
Tx
Wi-Fi 5
(1x1)
Wi-Fi 6
(1x1)
Proposed Wi-Fi 4
2x2EQM UEQM
802.11ac
(1x1)
86.7 86.7 72.2 72.2 72.2
802.11ax
(1x1)
86.7 143.4 72.2 72.2 72.2
NSM
EQM 72.2 72.2 86.7 86.7 86.7
UEQM 72.2 72.2 86.7 108.3 108.3
802.11n
(2x2)
72.2 72.2 72.2 72.2 144.4
256-QAM
1024-QAM
256-QAM
1024-QAM
2x2 64-QAM
Assume
ML demapperhttps://www.wi-fi.org/news-events/newsroom/wi-fi-alliance-introduces-wi-fi-6
Announcement
from wi-fi.org
on [2018-10-03]
Wi-Fi 5 =
Wi-Fi 4 + NSM
Wi-Fi 5 <<
Wi-Fi 4 + NSM
+ UEQM
< Wi-Fi 6
ZF (x)
MMSE (x)

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GCT Semiconductor, Inc.ICTC 2018
5.3. Revision point for indicating NSM functionality
 Rx features
 Rx MCS Bitmask is kept disable not to activate the legacy 2-stream Tx.
 Raise the value of B[89:80] to a proper value such as 86 or 108.
✓ No side effect to the legacy 802.11n device.
 Tx features
 B[96] = 1’b1, B[97] = 1’b1, B[99:98] = 2’b01
 B[100] = 1’b1 if UEQM is enabled.
 No other Out-of-Band information exchange is required.
B[89:80] = 86 > 72 ~ 256-QAM
B[99:98] = 2’b01
1’b0 :
16QAM+16QAM (MCS12)
1’b1 :
16QAM+QPSK (MCS36)
64QAM+QPSK (MCS37)
64QAM+16QAM(MCS38)
B[89:80] = 108 > 72 ~ 1024-QAM
Excerpt from p. 944 of [1] in the paper

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GCT Semiconductor, Inc.ICTC 2018
6. Conclusion
 Nested Spatial Mapping can be improved with Unequal Modulation of 802.11n
 UEQM is available only in 802.11n specification.
✓ No UEQM defined in 802.11ac and 802.11ax specification.
 Required SNR of 256-QAM equivalent modulation has been decreased by 1dB
✓ MCS12(16QAM+16QAM) => MCS37(QPSK+64QAM)
 1024-QAM equivalent modulation is available with MCS38(64QAM+16QAM)
✓ No need to adopt 802.11ax only to achieve 108 Mbps with 20MHz band-width.
 50% increase over 64-QAM of 802.11n, 25% increase over 256-QAM of 802.11ac
 Closed-form Analysis of Optimal Nesting Margin
 Clarification of the nominal nesting factor for UEQM
✓ No power-of-two in case of UEQM.
 Performance variation is insensitive to the nesting margin even in UEQM.
 Recipes shared for efficient hardware implementation
 Refinement of the channel estimate exploiting the channel condition of rank=1.
 SIC demapper
✓ Double-check/Re-adjust of soft demapper output(LLR) narrows the performance gap with ML demapper.