On Enhancing Hierarchical Modulations - Semantic Scholar

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TSG-C NTAH Title:

On Enhancing Hierarchical Modulations

Source:

Shu Wang, Soonyil Kwon and Sukwoo Lee +1-858-635-5305 {swang, sykwon, sugoo}@lge.com LG ELECTRONICS Inc. Abstract:

This contribution discusses hierarchical modulations for next-generation standards in terms of achievable rates, modulation efficiency, symbolerror rate, PAPR, etc.

Date:

August 27, 2007

Recommendation:

FYI

Notice LG Electronics, Inc. grant a free, irrevocable license to 3GPP2 and its Organizational Partners to incorporate text or other copyrightable material contained in the contribution and any modifications thereof in the creation of 3GPP2 publications; to copyright and sell in Organizational Partner's name any Organizational Partner's standards publication even though it may include all or portions of this contribution; and at the Organizational Partner's sole discretion to permit others to reproduce in whole or in part such contribution or the resulting Organizational Partner's standards publication. LG Electronics, Inc. is also willing to grant licenses under such contributor copyrights to third parties on reasonable, non-discriminatory terms and conditions for purpose of practicing an Organizational Partner’s standard which incorporates this contribution. This document has been prepared by LG Electronics, Inc. to assist the development of specifications by 3GPP2. It is proposed to the Committee as a basis for discussion and is not to be construed as a binding proposal on LG Electronics, Inc. LG Electronics, Inc. specifically reserve the right to amend or modify the material contained herein and to any intellectual property of LG Electronics other than provided in the copyright statement above.

On Enhancing Hierarchical Modulations Shu Wang, Soonyil Kwon and Sukwoo Lee LG Electronics Mobile Research, USA

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Introduction •

•

•

•

Hierarchical modulations are widely used in digital broadcast system design such as • Dedicated network: DVB-T, Media-FLO, UMB-BCMCS. • Hierarchical network: DVB Multiplexing. Hierarchical modulations can help • provide different QoS’s to users with different profiles, e.g. higher throughput for users with advanced receiver. • provide unequal protection on different contents, e.g., video, audio, text. • update 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 throughputs a 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.

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Superposition Precoding and Hierarchical Modulation 01

00

0011

0000 0001

0010

b 1b 0

α

2β

0110

10

11

θ

0111

0100 0101

Base Layer: QPSK 1111

00 01

1100 1101

1110

β

θ

e 1e 0

2α

10 11

Enhancement Layer: rotated QPSK

1011 1010

1000 1001

QPSK/QPSK Hierarchical Modulation

Achievable rates, ( Bergmans and Cover, 1974 ).

•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 of superposition precoding.

b1e1b0e0

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Hierarchical Modulation in Standards (1/2)

ƒMedia-FLO supports hierarchical transmission of base/enhancement layers • Extends coverage with layered source coding • Provides a more graceful degradation of reception.

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Hierarchical Modulation in Standards (2/2)

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

•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

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Enhanced Hierarchical Signal Constellation 01

00

ƒ The key advantage: minimum 0011

b1b0

α

0001

0010 0111

c’

10

11

0000

∆1

θ

c b

a 2β

0110

∆2

0100 0101

δ

Base Layer: QPSK

complexity increase. ƒ The major gain: higher throughput on the base layer ƒ The extra benefit: lower bit-error rate on the enhancement layer

1111

00

1100

a’

1101

b’

1110

01

β

θ

c”

e1e0

10

1010

2α

1011

1000 1001

11

Enhancement Layer: rotated QPSK

Enhanced QPSK/QPSK Hierarchical Modulation

There are a couple of ways to find the best rotation angle: ƒ if the target SNR’s are known, maximizing the sum capacity of the two layers. ƒ if only the power splitting ratio is known, optimizing Euclid distance profile. ƒ another practical approach is to find the best angle by simulations.

QPSK/16QAM Hierarchical Modulation

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Channel Capacity using N-ary Modulation •The capacity of a general N-ary modulation can be written by Signal Constellation and Euclid Distances Profile

⎧⎪ ⎡ N −1 ⎛ 1 C N = log 2 ( N ) − N ∑ E ⎨log 2 ⎢∑ exp⎜⎜ − j =0 ⎪⎩ ⎢⎣ i =0 ⎝ N −1

s j + n − si

2

2σ 2

−n

2

⎞⎤ ⎫⎪ ⎟⎥ ⎬ ⎟ ⎠⎥⎦ ⎪⎭

n denotes normally distributed complexvalued noise with variance σ2

Achievable Gains 4

Rate Satuaration Points

Achievable Rates (bits/symbol)

3.5

3

2.5

Throughtput Increasable Region

2

Error-Rate Decreasing Region

1.5

1

QPSK/QPSK; ER=2 QPSK/QPSK, Enhanced; ER=2 Shannon Capacity Bound Achiavable Capacity Gain

0.5

0 0

5

10

15

SNR (dB)

20

25

30

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Achievable Rates: 16QAM/QPSK 7

rate in crease b y ro tatin g en h an cem en t-layer sym b o l

Achievable Rate R, bit/symbol

6

5

cap acity lo ss d u e to th e in terferen ce fro m en h an cem en t layer 4

3

2

R 1 6 Q A M/Q P S K of th e reg u lar h ierarch ical m od u lation R 1 6 Q A M/Q P S K of th e en h an ced h ierarch ical m od u lation B

R 1 6 Q A M/Q P S K of th e reg u lar h ierarch ical m od u lation

1

B

R 1 6 Q A M/Q P S K of th e en h an ced h ierarch ical m od u lation E

R 1 6 Q A M/Q P S K , th e Q P S K en h an cem en t-layer rate 0 0

10%

20%

30%

40%

50%

60%

B ase-L ay er P o w er R atio β

70%

80%

90%

P/σ2=(P1+P2)/σ2= 23dB

100%

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Constrained Throughput of Hierarchical Modulations Spectral Efficiency in Good Channel (|h 2| =0.0dB), bit/symbol

6

5.7

the total rate in good reception condition

2

5

4

SPC Bound Shannon Bound TDM Bound Base Layer / 16QAM Enhancement Layer / QPSK Total Rate, 16QAM/QPSK Optimized Base Layer Optimized Enhancement Layer Optimized Total Rate

inter-layer interference

3

2

1

0

the base layer with good reception condition the enhancement layer with good reception condition 0

0.5

1

1.5

2

2.5

3

2

3.5

Base-Layer/16QAM Spectral Efficiency in Bad Channel (|h1| =-6.0dB), bit/symbol

3.75

4

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Inter-Layer Interference Enhancement Layers 10

-1

10

-1

Base Layers 10

-2

10

-2

the regular hierarchical modulation and its base layer & enhancement layer

16QAM

BER p

e

QPSK

10

-3

10

-3

the optimized hierarchical modulation and its base layer & enhancement layer

QPSK 10

10

10

-4

10

QPSK 16QAM QPSK/QPSK ζ =-6.0dB Optimized QPSK/QPSK ζ =-6.0dB Base-Layer QPSK Optimized Base Layer Enhancement-Layer QPSK Optimized Enhancement Layer

-5

-6

0

2

4

6

8

10

12

SNR (dB)

10

-5

-6

14

16

18

16QAM

-4

10 20 0

QPSK 16QAM QPSK/QPSK ζ =-1.9dB Optimized QPSK/QPSK ζ =-1.9dB Base-Layer QPSK Optimized Base Layer Enhancement-Layer QPSK Optimized Enhancement Layer 2

4

6

8

10

12

SNR (dB)

14

16

18

20

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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 2 ( ) ε σ ε base eff 2 −1 = PQPSK ( pe ) ≤ γ = 2 γ eff (σ ) = 2 σ σ 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

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Modulation Efficiency (1/2)

η = γ eff

σ 2 ε eff (σ 2 ) = ε base ε base

η ∞ = lim η σ 2 →0

•Modulation efficiency of a modulated signal is defined by the ratio between effective SNR 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.

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Modulation Efficiency (2/2) 1

Base Layer ζ =-6.0dB Optimized Base Layer ζ =-6.0dB Enhancement Layer ζ =-6.0dB Optimized Enhancement Layer ζ =-6.0dB Base Layer ζ =-1.9dB Optimized Base Layer ζ =-1.9dB Enhancement Layer ζ =-1.9dB Optimized Enhancement Layer ζ =-1.9dB QPSK

0.9

Modulation Efficiency η

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

0

2

4

6

8

10

SNR (dB)

12

14

16

18

20

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PEP and Minimum Euclid Distance •A upper bound for

pairwise error probability (PEP) can be derived with

assuming • The Hamming distance is d