C80216m-07-231r2Enhanced Hierarchical Modulation for IEEE 802.16m

IEEE 802.16 Presentation Submission Template (Rev. 9)Document Number:

IEEE S802.16m-07/231r2Date Submitted:

2008-01-23Source:

Shu WANG, Li-Hsiang SUN, Sang G. KIM and Ki-Dong LEE Voice: +1-858-635-5305

LG Electronics Mobile Research USA E-mail: swang@lge.com10225 Willow Creek RoadSan Diego, CA 92131*<http://standards.ieee.org/faqs/affiliationFAQ.html>

Venue:Re : IEEE 802.16m-07/047. Contribution pertains to: multiple access and broadcast.multicast services

Base Contribution:IEEE C802.16m-07/231

Purpose:For discussion and approval by TGM

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C80216m-07-231r2Introduction• Hierarchical modulations are used for digital broadcast system design as well as

multiuser access multiplexing– Dedicated network: DVB-T, Media-FLO, UMB-BCMCS.– Hierarchical network: DVB Multiplexing.

• Hierarchical modulations can help – provide different QoS or UEP to users with different profiles, e.g. higher

throughput for users with advanced receiver,– multiplexing users in a high efficient way,– upgrade system to provide better service to new users with advanced receiver

with keeping existing users unchanged.• The enhanced hierarchical modulation scheme by rotating enhancement layer(s) is

investigated here for the next generation system in terms of an information theoretic perspective: achievable throughputsa signal-processing perspective: inter-layer interference, effective SNR, effective power, modulation efficiency.an implementation perspective: peak-to-average power ratio (PAPR)

• These criteria can be used for optimizing and evaluating layered/hierarchical transmissions in the future too.

C80216m-07-231r2

Superposition Precoding and Hierarchical Modulation

• Optimal broadcast channel capacity is achievable by superposing two users’ signal together.

• Superposition precoding with interference cancellation outperforms TDM and FDM schemes in most time.

• Hierarchical modulation is one of the popular implementations ofsuperposition precoding.

Achievable rates, ( Bergmans and Cover, 1974 ).

Base Layer: QPSK

Enhancement Layer: rotated QPSK QPSK/QPSK Hierarchical Modulation

θ

θ

01 00

11 10

0100

1110

0111

0011

0010

0110

1110

00010000

01010100

11111101

1100

10101011

10011000

b1b0

e1e0

α

β

2α

2β

b1e1b0e0

C80216m-07-231r2

Hierarchical Modulation in Standards (1/2)

Media-FLO supports hierarchical transmission of base/enhancement layers• Extends coverage with layered source

coding• Provides a more graceful degradation

of reception.

C80216m-07-231r2Hierarchical Modulation in Standards (2/2)

• Besides using a dedicated DVB-H network, DVB-H service can also be embedded into DVB-T network using hierarchical modulation.– DVB-H service use the HP input while DVB-T services use LP.– The HP input can offer increased robustness in mobile environment

over the LP input– The LP input can serve higher bit-rate for fixed reception service

DVB-T -- QPSK/64QAM, 13.6/4.5Mbps with coding rate ¾ and ½.

C80216m-07-231r2

Enhanced Hierarchical Signal Constellation

Base Layer: QPSK

Enhancement Layer: rotated QPSKEnhanced QPSK/QPSK

Hierarchical Modulation

ba

θ

θc

01 00

11 10

0100

1110

0111

0011

0010

0110

1110

00010000

01010100

11111101

1100

10101011

10011000

b1b0

e1e0

α

β2α

2β

∆1

δ

∆2

a’b’

c’

c”

There are a couple of ways to find the best rotation angle:if the target SNR’s are known, maximizing the sum capacity of

the two layers. if only the power splitting ratio is known, optimizing Euclid

distance profile.another practical approach is to find the best angle by

simulations.

The key advantage: minimum complexity increase.

The major gain: higher throughput on the base layer

The extra benefit: lower bit-error rate on the enhancement layer

QPSK/16QAM Hierarchical Modulation

C80216m-07-231r2

Channel Capacity using N-ary Modulation

• The capacity of a general N-ary modulation can be written by

n denotes normally distributed complex-valued noise with variance σ2

( ) ∑ ∑−

=

−

=

−−+

⎪⎭

⎪⎬⎫

⎪⎩

⎪⎨⎧

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−−=

1

0

1

022

12 2

22

exploglogN

j

N

i

nsns

NNij

ENCσ

Signal Constellation and Euclid Distances Profile

C80216m-07-231r2

16QAM: Hierarchical Modulation Perspective

0 5 10 15 20 25 300

0.5

1

1.5

2

2.5

3

3.5

4

4.5

SNR γ (dB)

Ach

ieva

ble

Rat

e, b

it/Sy

mbo

l

Shannon BoundC16QAM(γ)CQPSK(γ)C16QAM(γ)-CQPSK(0.2γ)CQPSK(0.8γ)CQPSK(0.2γ)

16-QAM

QPSK

C16QAM

(γ)-CQPSK(0.2γ)

CQPSK

(0.8γ)

CQPSK

(0.2γ)

log2(1+γ)

C16QAM

(γ)-CQPSK(0.2γ)

capacity degradation dueto inter-layer interference

Δ

C80216m-07-231r2

Achievable Rates: 16QAM/QPSK

P/σ2=(P1+P2)/σ2= 23dB0 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

0

1

2

3

4

5

6

7

Base-Layer P ow er Ratio β

Ach

ieva

ble

Rat

e R

, bit/

sym

bol

R16Q AM/QPSK of the regu lar h ierarch ical modu lationR16Q AM/QPSK of the enhanced h ierarch ical modu lation

R16Q AM/QPSKB of the regu lar h ierarch ical modu lation

R16Q AM/QPSKB of the enhanced h ierarch ical modu lation

R16Q AM/QPSKE , the Q PSK enhancement-layer rate

rate increase by rotating enhancement-layer symbol

capacity loss due to the interferencefrom enhancement layer

C80216m-07-231r2

Inter-Layer Interference

0 2 4 6 8 10 12 14 16 18 2010

-6

10-5

10-4

10-3

10-2

10-1

SNR (dB)

BER

pe

0 2 4 6 8 10 12 14 16 18 2010

-6

10-5

10-4

10-3

10-2

10-1

SNR (dB)

QPSK16QAMQPSK/QPSK ζ=-1.9dBOptimized QPSK/QPSK ζ=-1.9dBBase-Layer QPSKOptimized Base LayerEnhancement-Layer QPSKOptimized Enhancement Layer

QPSK16QAMQPSK/QPSK ζ=-6.0dBOptimized QPSK/QPSK ζ=-6.0dBBase-Layer QPSKOptimized Base LayerEnhancement-Layer QPSKOptimized Enhancement Layer

the regular hierarchical modulation andits base layer & enhancement layer

the optimized hierarchical modulation andits base layer & enhancement layer

16QAM

16QAM

Enhancement Layers

Base Layers

QPSK

QPSK

C80216m-07-231r2

Effective Signal-to-Noise Ratio

• Effective SNR γeff is defined as the SNR necessitated when the base layer signal is sent alone with the same power.– Effective SNR always is less than the actual SNR. – The required symbol energy for achieving the same BER is called

effective power, which is smaller than actual base-layer signal power.– For example, For QPSK/QPSK hierarchical modulation, the effective

SNR of base-layer BER pe is given by

( ) ( ) ( ) 2base1

QPSK2

2ff2

eff Pσεγ

σσε

σγ =≤== −e

e p

Due to inter-layer interference, effective SNR or effective power is less than actual SNR or power. Stronger inter-layer interference is and smaller effective SNR/power becomes

C80216m-07-231r2

Modulation Efficiency (1/2)

( )base

2eff

base

2

eff εσε

εσ

γη ==ηη

σ 02lim→

∞ =

• Modulation efficiency of a modulated signal is defined by the ratio between effectiveSNR and actual SNR.

• Modulation efficiency is not greater than 1.• Modulation efficiency, as well as effective SNR and effective power, is the parameter

proposed by us for evaluating the performance of the whole transceiver chain, including modulation and demodulation.

• Asymptotic modulation efficiency is the ratio when the SNR becomes very large and interference becomes dominant.

• Asymptotic modulation efficiency is proposed by us for evaluating the interference resistance capability of both hierarchical modulation scheme and demodulation scheme.

C80216m-07-231r2

Modulation Efficiency (2/2)

0 2 4 6 8 10 12 14 16 18 200

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

SNR (dB)

Mod

ulat

ion

Effic

ienc

y η

Base Layer ζ=-6.0dBOptimized Base Layer ζ=-6.0dBEnhancement Layer ζ=-6.0dBOptimized Enhancement Layer ζ=-6.0dBBase Layer ζ=-1.9dBOptimized Base Layer ζ=-1.9dBEnhancement Layer ζ=-1.9dBOptimized Enhancement Layer ζ=-1.9dBQPSK

C80216m-07-231r2

PEP and Minimum Euclid Distance

• A upper bound for pairwise error probability (PEP) can be derived with assuming– The Hamming distance is d << K: two codeword c and c’ differ in d

bits.– Perfect interleaving.

Observation: PEP is dominated by the terms with the smallest squared Euclid distance in high SNR region

{ }2min24

2

241

1

'

1

2

21 '|'Pr

Δ−

=

−−

=

≤≤⎟⎠

⎞⎜⎝

⎛−=→ ∏∑ σσ

σ

dii

iieeQ

d

i

d

ikk

ss

ssccc

Minimum Euclid distancepairwise error probability

C80216m-07-231r2Minimum Euclid Distance: QPSK/16QAM

0 5 10 15 20 25 30 35 40 450

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Rotation Angle θ (degree)

Min

imum

Euc

lid D

ista

nce

Δ =

min

{|a-c

|,|b-

c|}

P2/P1=0.08

P2/P1=0.09

P2/P1=0.10

P2/P1=0.11

P2/P1=0.12

P2/P1=0.13

P2/P1=0.14

P2/P1=0.15

C80216m-07-231r2

PAPR of OFDM

8PSK, L=128-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Real Part of OFDM Signal

Imag

inar

y Pa

rt o

f OFD

M S

igna

l

6.53dB

3.01dB

( )( )

( )LOtsE

tsPAPR ≈= 2

2max

2 4 6 8 10 12 14 160

0.05

0.1

0.15

0.2

0.25

Peak-to-Average-Power Ratio (dB)

Prob

abili

ty D

ensi

ty F

unct

ion

C80216m-07-231r2

Hierarchical Modulation with Rotation

0 2 4 6 8 10 1210-4

10-3

10-2

10-1

100

Peak-to-Average Power Ratio (dB)

CC

DF Gaussian Approximation, L=128

Regular QPSK/QPSK, P2/P1=0.01, L=128Regular QPSK/QPSK, P2/P1=0.09, L=128Regular QPSK/QPSK, P2/P1=0.25, L=128Enhanced QPSK/QPSK, P2/P1=0.01, L=128Enhanced QPSK/QPSK, P2/P1=0.04, L=128Enhanced QPSK/QPSK, P2/P1=0.09, L=128Enhanced QPSK/QPSK, P2/P1=0.16, L=128Enhanced QPSK/QPSK, P2/P1=0.25, L=128

C80216m-07-231r2

Conclusions

• Hierarchical modulation has been adopted in various standards including MediaFLO, DVB-H and UMB.

• The enhance hierarchical modulation is adopted in UMB, the salient features of which include– minimum modulation/demodulation complexity increase.– high bps: channel capacity gain on lower layer(s)– lower BER: signal processing gain.

• The enhanced hierarchical modulation is investigated in terms ofachievable throughputsinter-layer interferenceasymptotic modulation efficiencypeak-to-average power ratio

• The enhanced hierarchical modulation is recommended for 16m