layered-division-multiplexing theory and practice

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IEEE TRANSACTIONS ON BROADCASTING 1

Layered-Division-Multiplexing: Theory and PracticeLiang Zhang, Senior Member, IEEE, Wei Li, Member, IEEE, Yiyan Wu, Fellow, IEEE,

Xianbin Wang, Senior Member, IEEE, Sung-Ik Park, Senior Member, IEEE,

Heung Mook Kim, Member, IEEE, Jae-Young Lee,

Pablo Angueira, Senior Member, IEEE, and Jon Montalban

AbstractAs the next generation digital TV (DTV) stan-dard, the ATSC 3.0 system is developed to provide significantimprovements on the spectrum efficiency, the service reliability,the system flexibility, and system forward compatibility. One ofthe top-priority requirements for the ATSC 3.0 is the capabil-ity to deliver reliable mobile TV services to a large variety ofmobile and indoor devices. Layered-division-multiplexing (LDM)is a physical-layer non-orthogonal-multiplexing technology to effi-ciently deliver multiple services with different robustness andthroughputs in one TV channel. A two-layer LDM structure isaccepted by ATSC 3.0 as a baseline physical-layer technology.This LDM system is capable of delivering robust high-definition(HD) mobile TV and ultra-HDTV services in one 6 MHz chan-nel, with a higher spectrum efficiency than the traditionaltime/frequency-division-multiplexing (T/FDM)-based DTV sys-tems. This paper presents a detailed overview on the LDMtechnology, and its application in the ATSC 3.0 systems. First,the fundamental advantages of the LDM over the traditionalTDM/FDM systems are analyzed from information theory pointof view. The performance advantages of the LDM are thenconfirmed by extensive simulations of the ATSC 3.0 system. Itis shown that, LDM can realize the potential gain offered bysuperposition coding over the TDM/FDM systems, by properlyconfiguring the transmission power, channel coding, and modu-lation, and using different multiple antenna technologies in themultiple layers. Next, the efficient implementation of LDM inthe ATSC 3.0 system is presented to show that the performance

advantages of the LDM are obtained with small additional com-plexity. This is achieved by carefully aligning the transmissionsignal structure and the signal processing chains in the multiplelayers. Finally, we show that the LDM can be further integratedwith different multiple antenna technologies to achieve furthertransmission capacity.

Manuscript received August 21, 2015; revised October 28, 2015 andNovember 13, 2015; accepted November 16, 2015. This work was supportedby the ICT Research and Development Program of MSIP/IITP under GrantR0101-15-294 through Development of Service and Transmission Technologyfor Convergent Realistic Broadcast.

L. Zhang, W. Li, and Y. Wu are with the Communications Research CentreCanada, Ottawa, ON K2H 8S2, Canada (e-mail: [email protected];

[email protected]; [email protected]).X. Wang is with the Department of Electrical and Computer

Engineering, Western University, London, ON N6A 5B9, Canada (e-mail:[email protected] ).

S.-I. Park, H. M. Kim, and J.-Y. Lee are with the Broadcasting SystemResearch Department, Electronics and Telecommunications Research Institute,Daejeon 305-700, Korea (e-mail: [email protected]; [email protected];

[email protected]).P. Angueira and J. Montalban are with the Department of Communications

Engineering, University of Basque Country, Bilbao 48013, Spain (e-mail:[email protected]; [email protected]).

Color versions of one or more of the figures in this paper are availableonline athttp://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TBC.2015.2505408

Index TermsCloud transmission, layered-division-multiplexing, LDM, OFDM, UHDTV, channel estimation,successive signal cancellation, mobile service, multiple-layertransmission, MIMO, MISO, SIMO, SISO.

I. INTRODUCTION

THE BROADCASTING industry is facing a severe chal-

lenge due to the reallocation of some existing terrestrial

TV spectrum to meet the rapidly increasing demand for com-

mercial broadband wireless services. To address this challenge,

the Advanced Television Systems Committee (ATSC) started

the process to develop the standard of the next generation

digital broadcasting system, the ATSC 3.0 system, to pro-

vide higher data capacity, more robust performance, better

spectrum efficiency, and to enable new services [1]. One of

the primary requirements for the next generation DTV is the

capability of delivering high data rate multimedia services to

mobile terminals, including mobile HDTV and non-real-time

data streaming services. Delivering mobile services over a

large coverage area requires the transmission be robust in both

low Signal to Noise Ratio (SNR) and high Doppler distortion

environments.

To address these challenges, Layered-Division-

Multiplexing (LDM), which was code-named Cloud

Transmission (Cloud Txn), was proposed in [2] for next

generation digital TV (NG-DTV) systems to achieve more

efficient spectrum utilization and to provide more versatile

broadcasting services [3]. Due to its significant performance

advantages and the proposed simple implementation, a

two-layer LDM was accepted as a Physical Layer (PHY)

baseline technology for ATSC 3.0. Different from the

traditional time-division-multiplexing (TDM) or frequency-

division-multiplexing (FDM), LDM is a Non-Orthogonal

Multiplexing (NOM) technology. In an LDM system, a

layered transmission structure is used to simultaneously

transmit multiple signals with different power levels androbustness for different services: mobile, multiple HDTV or

Ultra HDTV (UHDTV). In a two-layer LDM system, the

upper layer (UL), with higher power allocation, is used to

deliver mobile services to indoor, portable and handheld

receivers. The lower layer (LL) is designed to deliver high

data rate services, such as UHDTV or multiple HDTV

services to fixed reception terminals, where the operational

SNR is usually high due to the large and possibly directional

receive antennas. It has been shown in [4] that a properly

designed LDM system can deliver in the UL a 720p or

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2 IEEE TRANSACTIONS ON BROADCASTING

1080p HDTV service to mobile/handheld receivers with

very robust performance, and an UHDTV service in the LL

which requires 20 to 30 dB SNR (4k or 8k HD resolution)

for successful reception. One advantage of LDM is that it

can co-exist with all the other emerging PHY technologies,

such as the multiple antenna technologies, Non-Uniform

constellations (NU-QAM), Bit-Interleaved-Coded-

Modulation (BICM), Peak-to-Average-Power-Ratio (PAPR)

reduction technologies, etc.

In information theory, the concept of LDM has been des-

cribed as a form of superposition coding [5]. However, its

application in practical systems has been prohibited by the

extra implementation required for successive signal cancel-

lation. A similar concept has also be proposed as a non-

orthogonal multiple user access technology in [6][8], where

it is shown that the actual application of this technology is

also limited by the implementation complexity.

The key contribution of the proposed LDM structure for

ATSC 3.0 is the identification of the layered system structure

which not only achieves the significant performance improve-

ment, but also provides an efficient implementation with lowcomplexity. Firstly, by properly configuring the transmission

power, signal coding and modulation, and using different mul-

tiple antenna technologies in the multiple layers, LDM can

realize the potential capacity gain offered by superposition

coding over the TDM/FDM systems. Secondly, by carefully

aligning the transmission signal structure and the baseband

signal processing in the multiple layers, highly accurate suc-

cessive signal cancellation can be achieved with low additional

signal processing complexity and low additional memory

requirement. In other words, the LDM realizes the potential

gain of the superposition coding with affordable complexity

for consumer devices.

This paper is organized as follows: Section II brieflydescribes the LDM system structure. In Section III, chan-

nel capacity analysis is performed to reveal the fundamental

advantage of the LDM systems over the TDM/FDM systems.

SectionIV presents extensive simulation results on two-layer

ATSC 3.0 systems to demonstrate the performance advantages

of LDM. In Section V, the efficient implementation of LDM

in ATSC 3.0 standard is described, including the additional

modules required for both the transmitters and the receivers.

In Section VI, a coverage analysis is presented to show that

the LDM provides a different paradigm for mobile and fixed

service coverage as compared to the traditional single-layer

systems. SectionVII briefly discusses the application of mul-

tiple antenna technologies in LDM systems for further capacityand robustness improvement. Section VIII gives more poten-

tial application scenarios of LDM in ATSC 3.0 system for

future investigation. Finally, SectionIX gives the conclusions.

I I . LAYERED-D IVISION-M ULTIPLEXING(LDM)

The block diagrams of the transmitters and the receivers

in a two-layer LDM system are plotted in Fig. 1. Different

from the traditional TDM/FDM systems, in the LDM sys-

tems, multiple digital broadcasting/multicasting services are

transmitted in different signal layers within one RF channel,

Fig. 1. Two-Layer LDM Transmitter and Receiver.

while each layer occupies the full frequency spectrum at full

time duration. Therefore, this structure inherently provides full

frequency diversity and time diversity to the signals in all

layers.In the currently proposed ATSC 3.0 system, the transmis-

sion signals in different layers use OFDM-based physical-layer

waveforms, and all layers share the same OFDM signal struc-

ture. These include the OFDM FFT size, the cyclic prefix (CP)

duration, as well as the pilot structure. While using the

same OFDM structure is not mandatory for LDM systems

in general, it can greatly simplify the signal detection at the

LDM receivers and therefore allows a low-complexity and

power-efficient receiver implementation.

For the rest of this paper, for all layers in an LDM sys-

tem, the signals are assumed to use an OFDM structure with

N subchannels, among which Kactive subchannels carry sig-

nal power and the others are left empty (null subchannels)to provide adjacent channel interference protection. Among

the K active subchannels, M subchannels are allocated for

data transmission and P subchannels are used to carry pilot

symbols.

In an LDM system, transmissions in different layers have

different characteristics for the delivery of different services.

In general, the UL signal is designed to deliver robust

mobile broadcasting services to portable, handheld and indoor

receivers. Delivering reliable mobile broadcasting services has

proven very challenging in the past. First of all, there is lim-

ited antenna gain and directivity for most portable receivers.

Secondly, mobile devices can frequently move into shadowed

or indoor areas. Both result in very low received signal power.Secondly, fast moving mobile receivers experience fast time-

varying channels. Designing receivers for reliable detection

in fast-fading channels has always been a challenging task,

especially for OFDM-based systems.

To deliver robust mobile services with decent coverage

areas, the UL signal in an LDM system is designed to have

higher transmission power and use very strong channel cod-

ing and modulation, e.g., rate- 14

Low-Density-Parity-Check

(LDPC) code with QPSK modulation. This signal is optimized

for robustness in harsh wireless channel environments and can

provide good detection at very low SNR (lower than 0 dB),
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ZHANG et al.: LAYERED-DIVISION-MULTIPLEXING: THEORY AND PRACTICE 3

and therefore provides a large coverage area and less service

outages that are caused by multipath distortion and shadow-

ing. It can be further shown that, designing a robust receiver

for the UL signal in fast fading channels is no longer a signif-

icant challenge [9]. In ATSC 3.0 systems, new LDPC codes

were designed to provide even better performance specifically

under low SNR conditions [10][13].

The LL is usually designed to deliver UHDTV or multiple

full HDTV services, which usually require much higher SNRs

for successful reception. These services are intended for fixed

receivers, which have large antennas at high locations (e.g.,

rooftop), and sometimes directional antennas that are capable

of providing significantly higher antenna gain as compared to

mobile devices. Therefore, the LL is usually designed to have

lower transmission power, weaker channel coding and larger

signal constellation (i.e., 256QAM or higher). When desired,

more transmission layers can be added to deliver other types

of services.

As shown in Fig. 1.(a), for a two-layer LDM system, the

data of each service is first processed by its own physical-

layer signal processing modules, including channel encod-ing, interleaving etc.. The interleaved bit sequence is then

modulated into QPSK/mQAM symbols. A block of M sym-

bols are allocated to the data subchannels of one OFDM

symbol, where pilot symbols are inserted into the pilot

subchannels.

The frequency-domain (FD) LDM signal is generated as the

superposition of the FD signals from the two layers as,

X(k)= XUL(k)+gXLL(k) (1)

where XUL(k) and XLL(k) are FD symbols from the UL and

the LL, X(k) is the combined LDM symbol, and kis the sub-

channel index. The injection level, g

, defines the power levelof the LL signal relative to the UL signal.

The injection level, g, determines the power allocation

among the two layers. Since the UL signal is designed to

have higher power, g has a real value in [0, 1), where a g = 0

results in a single-layer system.

The block diagram of an LDM receiver is shown

in Fig. 1.(b). The received LDM signal can be expressed as,

Y(k) = XUL(k)H(k)+gXLL(k)H(k)+N(k) (2)

where Y(k) is the received symbol in the kth subchannel, and

N(k) contains AWGN noise and other additive interferences.

To decode the UL signal, the lower-power LL service is

treated as an additional interference. The impact of this inter-

ference is controllable by using different injection levels. For

example, a 5 dB injection level sets the LL signal 5 dB lower

than the UL signal. The injection level of an LDM system is

usually selected to meet the service requirements of the two

layers.

To decode the LL signal, the receiver first needs to cancel

the UL signal. From (2), the decision symbols of the LL signal

can be obtained as,

YLL(k) = 1

g

Y(k) XUL(k) H(k)

(3)

where XUL(k) is the estimate of the UL transmission symbol

in the kth subchannel, and H(k) is the channel estimate.

To perform the signal cancellation in (3), the receiver

needs to obtain the estimates of the UL transmission symbols,XUL(k). This is achieved by performing a regular UL signal

detection including equalization, demodulation, deinterleav-

ing, and channel decoding, generating an accurate decision bit

sequence. The receiver then performs channel encoding, inter-

leaving and modulation to re-construct the UL transmission

symbols. Although this cancellation process involves complex-

ity to perform UL channel decoding, it can provide the most

reliable UL symbol estimates.

The LL is typically designed to deliver high data rate ser-

vices to fixed receivers at high SNRs, which should easily

guarantee perfect UL signal detection, i.e., XUL(k)= XUL(k).

Furthermore, at the high SNRs required for LL service detec-

tion, the LDPC decoding for the UL services can usually

achieve the perfect detection with small number of iterations,

which results in very low additional complexity to perform

UL signal detection [14].

Simpler signal cancellation can be performed by mak-ing hard decisions on the UL transmission symbols without

involving the channel decoding process. This, however, will

inevitably introduce cross-layer interference (CLI) since the

error performance of the UL signal without channel decoding

will not be perfect even at very high SNRs.

III. ACHIEVABLE C APACITIES OF THEM ULTIPLEL AYERS

IN LDM-BASEDS YSTEMSI NFORMATION

THEORETICAL A NALYSIS

In this section, we conduct an information theoretical ana-

lysis on the achievable transmission capacities of the different

layers in an LDM system, and prove the fundamental advan-tage of the LDM system over the TDM/FDM systems when

delivering mixed multiple services with different requirements

on the throughput and the robustness.

The fundamental difference of the LDM from the

TDM/FDM is to use power allocation to the multiple layers to

perform the service multiplexing, where all layers occupy the

entire time-frequency resource of the channel. This concept

is called superposition coding in the literature [5] and [15],

where it was proved that, for two transmissions over one chan-

nel, the best performance is achieved by using a superposition

code operating in the full available band (or time duration),

rather than by using separate superposition codes operating

in disjoint smaller subbands (i.e., FDM) or sub-time-durations(i.e., TDM). This shows that LDM strictly outperforms the

TDM/FDM in delivering multiple mixed TV services in one

channel.

In [16], the authors investigated the achievable rates of the

mobile and fixed services in an LDM system with non-capacity

achieving transmissions. It is shown that the performance

of LDM can be worse than those of TDM/FDM systems.

However, this only happens when the mobile service in the

UL has an SNR threshold close to that of the fixed service

in the LL. We would like to point out that, for practi-

cal ATSC 3.0 system deployment, these scenarios will be
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Fig. 2. Equivalent Single-Layer Signal Model of a Two-Layer LDM System.

strictly avoided since they require more complicated signal

cancellation techniques and result in large signal cancellation

noise for LL signal detection.

A. Achievable Capacities of the Multiple Layers in

LDM-Based DTV Systems in Typical Selective

Fading Channels

In this section, we present the achievable transmission

capacities in the different layers in LDM-based DTV systems.The analysis assumes the selective fading channels as more

typical scenarios as comparing to AWGN channel.

For a two-layer LDM system, which delivers a mobile ser-

vice in the UL and a fixed service in the LL, the mobile

receivers decode the UL signal treating the LL signal as addi-

tional interference, while the fixed receivers need to first cancel

the UL signal component from the received signal and then

perform the LL service detection.

When assuming perfect UL signal cancellation, the trans-

missions in the two layers of the LDM system can be modeled

as two parallel independent transmissions as shown in Fig. 2.

The SNRs for the UL and LL signals are calculated as,

UL = pUL |H|2

pLL |H|2 +pN

LL = pLL |H|

2

pN(4)

where pUL and pLL are the transmission powers of the two

layers, His the channel response, and pN is the noise power.

Following (4), the achievable capacities for the UL and LL

signals are calculated as,

CUL = E

log2

1+

PUL |H|2

PLL |H|2 +Pn

CLL = E

log2

1+PLL |H|

2

Pn

(5)

For the LDM system delivering mobile services in the UL

and fixed services in the LL, the UL and LL capacities are

the channel capacities available for mobile and fixed services,

i.e., Cm = CUL and Cf = CLL. For this scenario, (5) shows

that, the achievable capacities of the mobile and fixed services

in an LDM system are nonlinearly dependent on the power

allocation of the two services.

In existing DTV systems, multiplexing of fixed and mobile

services in one RF channel is implemented by TDM in the

DVB-T2/NGH systems [17], [18], and by FDM in the ISDB-T

systems. For these systems, the channel capacity available for

one service is directly proportional to its time or frequency

allocation. For a TDM system to deliver a mobile service and

a fixed service, the capacity allocated for the two services are

calculated as,

Cm = CTm

Tt

Cf = CTf

Tt(6)

where Cm and Cf are the available capacities for the mobile

and fixed services, Tm and Tf are the times allocated for the

mobile and fixed services, and Ttis the total time duration.

For selective fading channels, the total channel capacity, C,

is calculated as [19]

C= E

log2

1+

Ps |H|2

Pn

(7)

where Ps and Pn are the signal and noise powers.

It is easy to understand that(7) is directly applicable to FDMsystems, where the corresponding time-durations (Tm,Tf,Tt)

are replaced by the spectrum occupancies (Fm, Ff, Ft).

Eq. (6) shows that the achievable capacities of the mobile

and fixed services in TDM/FDM systems are linearly depen-

dent on the power allocation of the two services.

Lets assume a DTV system simultaneously delivering a

mobile service and a fixed service in one RF channel. The

mobile service is configured to have robust performance with

a low SNR threshold of 0 dB, while the fixed service is con-

figured to have a high SNR threshold of 20 dB, which is a

reasonable value for existing fixed receivers usually equipped

with roof-top antennas. In Fig. 3, we plotted the achievable

capacities for the mobile and fixed services for both LDMand TDM/FDM systems based on (5) a n d (6). The total

transmission power is constant and shared between the two

services. Fig. 3 shows the achievable transmission rates of

the mobile and fixed services with different power allocations.

The left-most point of each curve represents a single-service

transmission where all the power is allocated for the mobile

service, while the right-most point for the single-service sce-

nario where only the fixed service is delivered with all the

power. A similar curve for AWGN channel was shown in [16].

Due to the difference in the SNR thresholds of the two

services, Fig. 3 show an uneven tradeoff between the mobile

and fixed service transmission rates, i.e., the fixed service can

obtain higher additional capacity with the same amount ofpower increase. With all the transmission power, the system

can provide a single mobile service of 0.86 b/s/Hz, or a single

fixed service of 5.9 b/s/Hz.

When comparing the two curves for LDM and TDM/FDM,

it is clearly shown that the LDM always provides better trans-

mission capacities than the TDM/FDM. The advantage of

LDM is further illustrated by the two scenarios in Fig. 3,

corresponding to system requirements for mobile service

throughputs of 0.5 b/s/Hz and 0.7 b/s/Hz. For the mobile

capacity of 0.5 b/s/Hz, using LDM provides an additional

capacity of 2.0 b/s/Hz for the fixed service over the TDM/FDM
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Fig. 3. Achievable Mobile and Fixed Service Capacities for LDM VersusTDM/FDM.

Fig. 4. Achievable Service Capacities with Different Fixed Service SNR

Thresholds.

systems; while for the higher 0.7 b/s/Hz mobile capacity, this

advantage becomes more than 2.5 b/s/Hz.

In Fig. 4, we compare the achievable data rates for the

mobile and fixed services of the LDM and TDM/FDM sys-

tems for a constant mobile service SNR threshold of 0 dB, and

different fixed service SNR thresholds from 0 dB to 30 dB.

It is observed that LDM offers better performance than

TDM/FDM in all scenarios. The advantage of LDM over

TDM/FDM is quite small when the fixed service has sim-

ilar SNR threshold as the mobile service. The LDM curve

almost overlaps the TDM/FDM curve for the fixed serviceSNR threshold of 0 dB; while for the fixed service SNR thresh-

old of 30 dB, significant capacity advantage is observed. The

higher the SNR threshold of the fixed service, the larger the

advantage of the LDM systems. For example, for a mobile

throughput of 0.5 b/s/Hz, LDM provides additional capacity

of {0.04, 0.6, 2.0, 3.8} b/s/Hz for fixed service delivery, for

the fixed service SNR thresholds of {0, 10, 20, 30} dB.

One likely deployment scenario for the ATSC 3.0 system is

to first deliver a fixed DTV service which has similar or bet-

ter quality as comparing to existing DTV services delivered

by the ATSC 1.0 system. The rest of the system capacity is

Fig. 5. Mobile Service Capacity Advantage of LDM Over TDM/FDM withDesired Fixed Service Throughputs.

then allocated to deliver one or multiple mobile DTV services.

For this scenario, the advantage of LDM over TDM/FDM sys-

tem is measured by comparing the achievable mobile servicedata rates for specific fixed service data rates. In Fig. 5, the

extra mobile service capacities offered by the LDM system

over the TDM/FDM systems are plotted with three desirable

fixed service data rates: {2.0, 3.0, 4.0} b/s/Hz, correspond-

ing to about {10, 15, 20} Mbps fixed service throughputs. An

SNR threshold of 15 dB is assumed for the fixed services.

The x-axis is the mobile service SNR threshold, and the y-

axis is the additional mobile service capacity offered by the

LDM system over the TDM/FDM systems, i.e., Mobile =

Cm(LDM)Cm(TDM/FDM) b/s/Hz.

It is shown in Fig. 5 that, for all fixed service throughputs,

using LDM always provides higher mobile service through-

put than that for TDM/FDM. The advantage of LDM is thehighest for the mobile service SNR thresholds from 4 dB to

6 dB. For moderate fixed service data rates (i.e., 2-3 b/s/Hz),

LDM provides over 50% mobile service throughput increase

when comparing to TDM/FDM. The advantage of LDM over

TDM/FDM becomes smaller when the fixed service data rate

is closer to the limit, 4.3 b/s/Hz, which is achieved when all

the power is allocated to the fixed service.

For scenarios where providing mobile services is of higher

priority, the system is deployed to first guarantee a desirable

mobile service throughput and allocate the rest of the capacity

to fixed services. The performance advantage of the LDM sys-

tem is then measured by the additional fixed service capacity

offered by LDM over the TDM/FDM systems. In Fig. 6, theadditional fixed service capacities for the LDM system over

the TDM/FDM systems are plotted for four desirable mobile

service data rates: {0.2, 0.4, 0.6, 0.8} b/s/Hz, corresponding

to about {1.0, 2.0, 3.0, 4.0} Mbps mobile services. For each

curve, the x-axis is the fixed service SNR threshold, and the y-

axis is the additional fixed data rate of LDM over TDM/FDM

systems, i.e., Fix= Cf(LDM)Cf(TDM/FDM) b/s/Hz.

The first observation is that, for all mobile data rates, the

advantage of LDM becomes more significant for fixed ser-

vices with higher SNR thresholds. Secondly, the advantage of

LDM is small for low mobile service data rate, and it becomes
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Fig. 6. Fixed Service Capacity Advantage of LDM over TDM/FDM withDesired Mobile Service Throughputs.

larger for higher mobile service data rates. When the desired

mobile service data rate approaches the limit (0.9 b/s/Hz as

shown in Fig. 4), the advantage of LDM becomes smaller

again. This is consistent with the achievable data rates of LDMand TDM/FDM systems shown in Fig.4.

B. Channel Capacity Distribution for LDM With

Specific Power Allocation

In this section, we take a closer look at the channel capacity

distributions between the two layers of an LDM system when

specific power allocation. Considering the general analysis in

previous section, a specific power allocation corresponds to

one specific point in the achievable mobile/fixed capacity pairs

in Fig. 3.

The power allocation of a two-layer LDM system is con-

trolled by the injection level. For practical LDM systems, suchas the ATSC 3.0 system, it is important to select a proper

inject level. First of all, the injection level needs to be prop-

erly configured so that the UL can provide a desired mobile

service throughput with strong robustness in the challenging

mobile channels faced by the large variety of mobile receivers.

Secondly, the LL injection level needs to be low enough to

guarantee that a sufficiently low cross-layer-interference (CLI)

can be achieved with low complexity from signal cancellation.

In the current proposals, proper injection levels are defined

from 4 dB to 10 dB, based on the extensive simulation

results.

In this section, we show the capabilities of the LDM sys-

tem to deliver mixed mobile and fixed service with the typicalinjection level of5 dB, and compare them to the TDM/FDM

systems. These results will directly confirm the simulation per-

formances and the comparisons to the TDM/FDM counterpart

systems as shown in [3].

1) Channel Capacity Distribution in AWGN Channels:

The channel capacity (b/s/Hz) of an AWGN channel is

calculated as,

C= log2

1+

Ps

Pn

(8)

where Ps and Pn are the signal and noise powers.

Fig. 7. Mobile Channel Capacities, AWGN Channel.

The channel capacities of the two layers are calculated as,

CUL =log2

1+

pUL

pLL+ pn

CLL =log2

1+ pLLpn

(9)

Taking (8) and (9), it is easily verified that,

CUL+ CLL = C (10)

Eq. (10) shows that, in an LDM system, the distribution

of the capacity in the multiple layers is solely controlled by

the LL injection level. Furthermore, in the LDM system, the

transmission capacity of each layer is a non-linear function

of its power allocation, which occurs inside the logarithm

calculation in(9).

To demonstrate the impact of the non-linear capacity dis-

tribution in the different layers, the channel capacities ofthe UL and LL are calculated for a two-layer LDM system

and compared to the TDM/FDM-based systems which carry

approximately the same mobile and fixed service throughputs.

A two-layer ATSC 3.0 system is assumed with an LL injec-

tion level of5 dB, i.e., the LL has a power level 5 dB lower

than the UL, which means that 76% of the transmission power

is allocated to the UL mobile service and 24% to the LL fixed

service.

For fair comparison, the TDM/FDM-based systems are con-

figured to carry one mobile service and one fixed service in a

single RF channel. Three TDM/FDM systems are considered,

with time/frequency allocations of {25%, 33%, 50%} for the

mobile service.The TDM/FDM scenario with 75% time/frequency allocated

to the mobile service has the same power allocation as the

LDM system with5 dB injection. However, this TDM/FDM

system is extremely inefficient in delivering mobile services

with reasonable throughput. It is therefore ignored in our

comparisons.

The channel capacities allocated to the mobile and fixed ser-

vices in the two-layer LDM and the single-layer TDM/FDM

systems are calculated and plotted in Fig.7. It is shown that,

at low SNRs from 10 to 10 dB, the LDM system allo-

cates higher channel capacity to mobile service than the three
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Fig. 8. Fixed Channel Capacities, AWGN Channel.

TDM/FDM systems. For a bandwidth efficiency of 0.4 b/s/Hz,

the SNR threshold of the mobile service in the LDM system

is {1.8, 4.0, 6.0} dB better than those of the FDM/TDM sys-

tems with {25%, 33%, 50%} mobile service allocation. For

0.8 b/s/Hz, the SNR advantages of the LDM-based systembecome {1.8, 4.5, 7.5} dB.

The channel capacities allocated for the fixed services in

the LDM system and in the TDM/FDM systems are plotted

in Fig. 8. It is shown that, the LDM system provides higher

fixed service capacity than the TDM/FDM systems at SNRs

higher than {10, 17, 25} dB for the {50%, 67%, 75%} fixed

service allocations.

In Fig.7,it is shown that the TDM/FDM system with 50%

mobile service allocation suffers only 0.2 b/s/Hz lower mobile

service capacity than the LDM system. However, Fig. 8 shows

that the same TDM/FDM systems suffer fixed service capac-

ities losses of {0.5, 1.2, 2.0} b/s/Hz at SNRs of {15, 20,

25} dB, respectively.2) Capacity Distribution in Selective Fading Channels:

For wireless fading channels, when the channel selectivity

is significant in time-domain, frequency-domain or time-

frequency-domains, one can average over many independent

channel fades by performing channel decoding over a large

number of coherent time or frequency intervals. Therefore, it

is always possible to achieve reliable communication at the

average channel capacity over independently fading channel

intervals. Detailed analysis on the channel capacity of fast

fading channels can be found in [19].

This scenario is equivalent to an OFDM transmission in an

ideal frequency-selective channel where each subchannel expe-

riences an independent fading process. The channel capacity

of the kth subchannel is calculated as,

C(k)= log2(1+k)

= log2

1+

ps

pn|H(k)|2

(11)

where H(k) is the channel response.

The average capacity is calculated as the mean of all

subchannels,

C= E

log2

1+

ps

pn|H(k)|2

(12)

Fig. 9. Mobile Channel Capacities, Selective Fading Channel.

Fig. 10. Fixed Channel Capacities, Selective Fading Channel.

Assuming a Wide Sense Stationary (WSS) Rayleigh fading

channel, H(k) can be approximated as a complex Gaussianrandom variable with unit power, i.e.,

H(k) = Hr(k)+jHi(k) (13)

where Hr/Hi N(0, 1/2).

This analysis assumes independent fading in different

time-frequency units, which is not the case of most practi-

cal scenarios, where the multipath fading channels usually

have both non-zero coherent bandwidth and coherent time.

Therefore, there is usually strong correlation between two

channel responses closely located in either time or frequency

domain.

On the other hand, most of the wireless communications

systems are designed with an interleaver and forward errorcorrection (FEC) coding. An interleaver with sufficient inter-

leaving depth makes the received symbols appear to have

passed a channel with random fading in both time and fre-

quency domains before they are passed to the FEC decoder.

With a powerful FEC, such as the LDPC code, the chan-

nel capacities shown in (11) and (12) and using (13) can be

approached.

The capacity allocations to the mobile and fixed services

in selective fading channels are plotted in Fig. 9and Fig. 10,

for the same two-layer LDM and the three TDM/FDM-base

systems as described in last section. Similar to the observation
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in AWGN channels, the LDM-based system allocates higher

channel capacity to the mobile services at low SNRs and to

the fixed services at high SNRs.

For a mobile service with a bandwidth efficiency of

0.4 b/s/Hz, i.e., 2.2 Mbps in a 6 MHz channel, the LDM

system provides {2.0, 5.0, 7.0} dB better performance over

the TDM/FDM systems allocating {25%, 33%, 50%} power

to the mobile service. For a mobile capacity of 0.8 b/s/Hz,

using LDM brings improvements of {1.8, 5.3, 8.3} dB over

the three TDM/FDM systems.

For fixed services, the LDM-based system provides higher

capacity than the TDM/FDM systems with {50%, 67%, 75%}

fixed service allocation at SNRs higher than {10, 19, 27} dB.

3) Capacity Distribution in Slow Fading Channels: In

slow-fading channels, the instantaneous channel capacity is

calculated as,

Cinst= log2

1+|H|2

(14)

where H is the instantaneous channel response and = pspn

is

the SNR.

Since the channel response, H, has non-zero probabilityof being close to zero, there is no definite capacity as the

maximum rate of reliable communications supported by the

channel.

When the transmitter does not know the channel state

information (CSI), which is the typical scenario in digital

broadcasting systems, the transmitter encodes data at a rate

of R (b/s/Hz). There is a non-zero probability for the chan-

nel gain, H, to have a value that is too small to make the

detection error probability arbitrarily small. When the instan-

taneous channel capacity is lower than the encoded data rate R,

the system is called to be in outage.

For slow-fading channels, the outage probability is calcu-

lated as,

Pout(R)= P

log2

1+ |H|2

(15)

The transmission capacity of the slow-fading channels is

measured by channel outage capacity, which is defined as the

transmission rate, RC, at which the outage probability, Pout, is

equal to a pre-defined threshold, , i.e.,

Pout(RC)= (16)

Taking (14) and (15), we have,

Pout(RC)= P|H|2 = 2RC 1

= P

H2r +H2i =

2RC 1

= P

rH=

2RC 1

= (17)

where for Rayleigh fading channels, Hr and Hi are Gaussian

random variables N

0, 12

, and rH is a Rayleigh random

variable.

Fig. 11. Mobile Channel Capacities, Slow-Fading Channels.

Fig. 12. Fixed Channel Capacities, Slow-Fading Channels.

The -Outage capacity can be calculated as,

RC() = log2

1+

1()

2 (18)

where (x)= P {rHx}.

For a two-layer LDM-based system, the instantaneous chan-

nel capacities for the UL and LL are calculated as,

CULinst =log2

1+

|H|2 pUL

|H|2 pLL+ pn

CLLinst =log2

1+

|H|2 pLL

pn

(19)

The -Outage capacity for the UL is now calculated as,

RULC () = log2

1+

pUL1()

2

pn+ pLL1()

2

(20)

and for LL,

RLLC () = log2

1+

1()

2pLL

pn

(21)

For an outage probability of = 0.1, the -Outage capaci-

ties allocated for the mobile and fixed services in slow fading

channels are plotted in Fig. 11 and Fig. 12, for the same

LDM and TDM/FDM systems. Similar to AWGN and selec-

tive fading channels, the LDM system provides higher capacity
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for mobile services at low SNRs and for fixed services at

high SNRs.

For a mobile service of 0.4 b/s/Hz (about 2.2 Mbps in a

6 MHz channel), the LDM system provides {2.0, 5.5, 7.5} dB

better performance over the TDM/FDM systems with {25%,

33%, 50%} power allocations for the mobile service. For a

mobile service of 0.8 b/s/Hz (about 4.4 Mbps), the LDM

brings improvements of {2.0, 5.3, 8.3} dB over the three

TDM/FDM systems.

In slow-fading channels, the LDM system provides higher

fixed service capacity than the TDM/FDM systems with

{50%, 67%, 75%} fixed service allocation at SNRs higher

than {19, 27.5, 35} dB.

4) Summary on Capacity Distributions of LDM vs

TDM/FDM Systems: The most important observation here is

that the LDM system simultaneously provides higher trans-

mission capacity for both mobile service at low SNR and

fixed service at high SNR. Therefore, the LDM system pro-

vides more efficient use of the spectrum in the scenario of

simultaneously delivering a robust mobile HDTV service and

a high-data-rate fixed UHDTV service (or multiple HDTVservices) in one RF channel.

This advantage of the LDM system is mainly because of

the nonlinear distribution of the channel capacity with respect

to the power distribution. As shown in (5), the capacity of

different layers is calculated with signal power distribution

inside the logarithm operation; while for TDM/FDM systems,

the capacities of different services are calculated in (6) with

the signal power distribution outside the logarithm operation.

An intuitive explanation is given as following. For

TDM/FDM systems, the SNR for the mobile receivers is

designed for the edge of the coverage area. All mobile

receivers inside the coverage area have higher SNR, where

the additional power is wasted, resulting in the waste of chan-nel capacity. On the other hand, in the LDM system, this

wasted capacity is re-allocated to the fixed service by con-

trolling the SNR for the mobile service at a constant value

(i.e., the injection level) inside the coverage area.

C. Cross-Layer Interference (CLI)

Thus far, perfect UL signal cancellation is assumed in all the

channel capacity analysis. This assumption has been shown to

be directly applicable to the typical scenarios of the two-layer

ATSC 3.0 system with LDM, where the UL delivers a mobile

service with strong channel coding and modulation scheme

and the LL delivers fixed services to receivers with rooftop

antennas. When decoding the LL services, the signal SNR

is usually quite high, where the UL signal has even higher

SNR due to the stronger transmission power, which is usu-

ally sufficient to guarantees perfect UL signal detection and

re-construction.

In this case, the decision symbols for the LL signal becomes,

YLL(k) = XLL(k)H(k)

+ 1

g

XUL(k)H(k)XUL(k)H(k)

+1

gN(k)

=XLL(k)H(k)+IUL(k)+1

gN(k) (22)

where IUL(k) is the CLI from the UL into the LL decision

symbols, which is calculated as,

IUL(k) = 1

gXUL(k)

H(k) H(k)

=

1

gXUL(k)H(k) (23)

In (23), the XUL is a random process containing random

modulation symbols. The H is the channel estimation error,

which can be well approximated by a Gaussian-distributed

random process with zero mean and a variance depending

on the channel estimation algorithm. Since the XUL and the

Hare independent, the CLI can be modeled as a Gaussian-

distributed random process, with zero mean and a variance

that is calculated as,

2I = 2H

g2 (24)

assuming 2X=1. The 2

His the mean square error (MSE) of

the channel estimates.

Eq. (24) shows that the CLI power is directly proportionalto the MSE of the channel estimates. In [20], it was shown that

a properly designed channel estimation modules can provide a

an estimate MSE lower than 30 dB. Assuming a LL injection

level of 5 dB, the CLI is 25 dB lower than the LL signal,

which is much lower than the noise thresholds of most typical

LL services, and can be safely ignored. This 30 dB cancella-

tion performance is obtained in the Typical Urban (TU) [21]

channels, which are much severer than the typical channels

encountered by most fixed receivers with rooftop antennas.

In most practical cases with Line-Of-Sight (LOS) or strong

Rician channels, the CLI power should be much lower.

In summary, for typical ATSC 3.0 scenarios, the CLI can

be safely ignored in performance analysis. This justifies thatthe above capacity analysis is directly applicable for ATSC

3.0 system performance estimation.

IV. PERFORMANCE A DVANTAGES OF

LDM-BASEDATSC 3.0 SYSTEMS

In this section, we present the performance advantage of

LDM for delivering mixed mobile and fixed services in one TV

channel by extensive computer simulations of the ATSC 3.0

system. Specifically, the simulation results show that LDM is

especially effective in providing high-data-rate robust mobile

services in severe mobile fading channels, including both the

slow-fading for pedestrian-speed receivers and the very fastfading channels for high vehicle-speed receivers.

A. Simulation Performances of LDM and TDM/FDM for

Mixed Services Delivery

In this section, we show the performance of LDM-based

ATSC 3.0 system to deliver mixed services with different sys-

tem configurations. The simulated ATSC 3.0 system has two

layers and delivers one robust mobile service and one high

data-rate fixed service. The injection level is 5 dB. Different

channel coding and modulation schemes were considered

in the simulations to achieve different service throughput
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TABLE ISNR REQUIREMENTS FOR G OO DP ERFORMANCE, LDM V S TDM/FDM

and robustness. The simulations were performed with 16k-FFT

mode, a CP ratio of 1/16, and an in-band pilot pattern

following the PP2 defined for the DVB-T2 system [17].

The LDM performance is compared to those of TDM sys-

tems, which use 32k mode for fixed service and 8k mode for

mobile service. Three TDM systems are considered, which

allocate {55%, 40%, 30%} of time for mobile service delivery.

Table I summarizes the simulation performances of LDM

and TDM systems for different mobile and fixed service

throughputs. The performances are characterized by the

required SNRs for error-free detection performance.

These simulation results confirm that the LDM system

indeed provides better performance for both fixed and mobile

services. It is further shown that, while LDM provides signif-

icant performance improvement relative to the TDM systems

for mobile service delivery, its advantage for fixed service

delivery is less obvious. A TDM system can be configured to

provide similar mobile service quality as comparing to LDM.

This, however, results in significant loss in its performance for

fixed service delivery.

B. Advantage of LDM for Robust Mobile Services

The ATSC 3.0 system uses OFDM to more efficiently over-

come the multipath fading and to provide easy deployment of

single-frequency-network (SFN). One well known vulnerabil-

ity of OFDM systems is their sensitivity to fast time-varying

channels, which are usually encountered by fast moving

receivers. The channel time-variation causes Doppler spread

in the received signal, resulting in a power leakage among

the signals carried in adjacent subchannels. This power leak-

age becomes an additional interference for the signal detection

and is called inter-carrier-interference (ICI). The impact of the

Fig. 13. Impact of Doppler for Mobile Receivers.

ICI is a bit error rate (BER) floor at the receiver which can-

not be reduced simply by increasing the transmission power.

This puts a limitation on the receiver mobility and results

in a much smaller mobile service coverage for fast moving

receivers [22], [23].The power of the ICI is directly proportional to the normal-

ized Doppler spread, fd/fU, where fd is the Doppler spread

and fUis the subchannel bandwidth. The Doppler spread can

be calculated as,

fd= v

cfc (25)

where v is the receiver speed, c = 3 108 m/s is the light

speed, and fc is the carrier frequency.

A tight upper-bound of the ICI power is derived in [24] as,

2

ICI

1

122 fd

fU (26)

where 1 = 12

for the Classical Jakes Doppler spectrum.

Eq. (26) tells us that the larger the subchannel bandwidth,

the lower the ICI power. Therefore, to avoid severe ICI, the

existing DVB-T/T2 systems deliver the mobile services using

OFDM transmission modes with large subchannel bandwidth.

In[25], we have shown that the impact of ICI can be demon-

strated as a deterioration in the SNR of the received signal,

which can be translated into an increase in the SNR thresh-

old for successful signal detection. In Fig. 13, the increases

of the SNR threshold due to the ICI are plotted for three

OFDM transmission modes, {8k, 16k, 32k}, for which the

subcarrier spacings are {837, 419, 209} Hz, respectively.A 6 MHz TV channel at 600 MHz is assumed. In addition,

two mobile services are considered with significantly different

SNR thresholds, 0 dB and 10 dB.

The first observation in Fig. 13 is that, wider subcarrier

spacing brings more robustness performance to fast moving

receivers, i.e., 8k mode provides the best performance and

32k mode brings the worst. Lets define the 3-dB speed limit

of the receiver as the speed that causes a 3 dB increase in the

SNR threshold. The speed limits for {8k, 16k, 32k} modes

are {320, 160, 80} km/h for the mobile service with an SNR

threshold of 10 dB.
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The second observation is that the impact of ICI is much

less for mobile services designed with lower SNR thresholds.

In Fig.13, the impact of ICI to the mobile service with an SNR

threshold of 0 dB is much less than that for the service with

10 dB threshold. The 3-dB speed limits for this mobile service

are {> 400, 330, 165} km/h for the {8k, 16k, 32k} modes.

Therefore, this mobile service is significantly more robust for

high speed receivers.

Fig. 13 gives a comparison between the mobile

services in LDM and traditional TDM systems

(DVB-T2/NGH) [26], [27]. In DVB-T2/NGH systems,

multiple mobile services are transmitted, and each is allocated

with a small percentage of time. This makes it necessary to

use weaker channel codes and higher modulation to deliver

a decent service data rate. However, this results in higher

SNR thresholds, such as the 10 dB curve in Fig.13. Because

the higher SNR threshold makes the mobile service more

sensitive to ICI, the most robust 8k mode has always been

the choice for DVB-T2/NGH for mobile service delivery in

the past.

On the other hand, for the ATSC 3.0 system with LDM, themobile services are delivered in the UL, with high transmission

power, strong channel codes and robust QPSK modulation,

which results in very low SNR thresholds. The typical value

is the 0 dB shown in Fig. 13.Therefore, LDM provides much

more robust performance and larger coverage for fast moving

receivers. To provide good mobility while keeping high spec-

trum efficiency (small GI percentage), 16k mode is currently

recommended to delivery both mobile and fixed services in

the two layers.

In addition, Fig.13shows that, using LDM makes it possi-

ble to use the most vulnerable 32k mode to deliver mobile and

fixed services in one TV channel, since the 3-dB speed limit

is 165 km/h, which is more than sufficient for most vehicularreceivers. Detailed analysis on the performance impact of FFT

size can be found in [28].

In[9], the capability of the LDM system to deliver mobile

services were investigated in detail. In Fig.14,the performance

of an LDM-UL mobile service is plotted, with QPSK and

rate- 415

LDPC code, which provides a throughput of 2.7 Mbps.

The LL injection level is 5 dB. The performance is com-

pared to those of TDM systems with different LDPC code

rates and different modulation schemes to provide similar

throughput.

It is clearly shown that, due to the strong coding and

modulation, the LDM UL provides robust performances for

receivers at all speeds up to 300 km/h. For the pedestrianspeed at around 3 km/h, the required SNR is 3.5 dB for good

service detection, while it only requires an SNR of 5 dB for a

very fast receiver moving at 300 km/h. For the medium-speed

receivers moving at 20 to 150 km/h, the required SNRs are

much lower, close to 2.0 dB.

For a TDM system which allocates 50% of the time to

deliver a mobile service of 2.7 Mbps (QPSK and rate- 815

LDPC

code), the required SNR is 6.0 dB for the receivers moving at

pedestrian speed, 5.0 dB for the medium-speed receivers, and

19.0 dB for the high-speed receivers at 300 km/h. For a TDM

system allocating 40% time for a mobile service with the same

Fig. 14. Mobile Performances of LDM vs TDM Systems, 16k.

throughput, the required SNRs for the pedestrian speeds and

the medium speeds are [8.5, 6.8] dB. The receivers moving

at 250 km/h or faster can no longer obtain zero-free detec-

tion performance. In addition, it requires an SNR of 13 dB or

higher for receivers moving at 225 km/h. The performance

becomes worse for the TDM systems allocating less time

percentage for the mobile services.

Interested readers are referred to [9] and [29] for detailed

and comprehensive results on the mobile and indoor perfor-

mances of LDM-based ATSC 3.0 systems.

V. EFFICIENT I MPLEMENTATION OF

LDM-BASEDATSC 3.0 SYSTEMS

Although the concept of LDM has existed as superposition

coding for a long time, its application in practical systemshas been prohibited by the extra complexity required for suc-

cessive signal cancellation. The key of the proposed LDM

structure for ATSC 3.0 lies in the simple implementation,

which makes the benefit of superposition coding feasible for

consumer-level devices. In this section, we describe the imple-

mentation modules specifically for LDM in the ATSC 3.0

system and show that there is relatively small additional

complexity required to achieve the significant performance

improvement.

The following sections will briefly present the critical imple-

mentation challenges and their low complexity realizations.

A thorough and comprehensive description and analysis on

the low-complexity implementation of LDM in the ATSC 3.0

system is presented in [14].

A. Additional Signal Processing Complexity

in the LDM Transmitters

In the currently proposed ATSC 3.0 system, the two layers

of the LDM system share the same OFDM signal struc-

ture, which makes it possible for the two layers to share

the same signal processing modules in both the transmitters

and receivers. This greatly simplifies the implementation and

results in low-complexity design.
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The basic structures of the transmitter and receiver are

shown in Fig. 1. At the transmitter, the additional signal

processing is only the combining process of the two lay-

ers, which is implemented by the simple multiplication and

addition operations. Therefore, there is little extra complex-

ity on the transmitter side for the LDM as comparing to the

single-layer systems.

B. Additional Signal Processing Complexity

in the LDM Receivers

As shown in Fig.1,the implementation of the LDM mobile

receivers is the same as the conventional single-layer wire-

less receivers, consisting of the standard signal processing

modules, e.g., synchronization, OFDM demodulation, chan-

nel estimation and equalization, deinterleaving, and channel

decoding.

On the other hand, to decode the fixed services in the LL, the

receiver needs to decode the UL signal, cancel the UL signal,

and then perform the conventional detection of the LL signal.

Therefore, the implementation of the LL receiver needs to

incorporate the additional complexity for UL signal detection,

re-construction, and cancellation.

1) UL Signal Detection and Cancellation: In order to

decode the LL signal, the receiver needs to decode the UL

signal, re-construct it and cancel it from the received multi-

layer signals. Since LL is configured to deliver fixed services

with high throughput, the SNR for LL signal detection is usu-

ally quite high. If the SNR threshold for the LL service is

15 dB, with an LL injection level of 5 dB, the UL signal

would have an SNR of 20 dB. Since the UL signal is con-

figured to operate at very low SNR threshold (i.e., 0 dB), the

detection at this high SNR should be perfect.

The major additional complexity in decoding the UL signalis the LDPC decoding. The investigations in [14]showed that

the complexity of the LDPC decoding process decreases sig-

nificantly with the higher SNR of the received signal. While

it requires 50 decoding iterations to obtain good performance

at the low SNR threshold of the UL signal, it requires no

more than 5 iterations for perfect decoding at the higher SNRs

above 7 dB. The additional LDPC decoding complexity is less

than 10%.

The reconstruction of the UL signal is performed by re-

encoding, bit-interleaving, and bit-to-symbol re-mapping. The

re-encoding can be accomplished in the LDPC decoding

process without added complexity. The bit-interleaving and

bit-to-symbol re-mapping requires little complexity.To obtain the decision symbols for the LL signal, the UL

signal cancellation is performed as (22). This only requires

one complex multiplication and one subtraction for each

received frequency-domain symbol. The additional complexity

is minimal.

2) Channel Estimation for UL Signal Cancellation:For LL

signal detection, the decision symbols are obtained using (22).

Clearly, the UL signal cancellation needs to be very accurate

in order to provide the high SNR required for LL signal detec-

tion. It is proven in Section III-C that the power of the CLI

is directly proportional to the channel estimation accuracy.

Fig. 15. LDM Receiver Block Diagram with Cross-Layer-Interleaver.

Therefore, a good channel estimator is necessary for the LL

receiver to obtain good performance.

The channel estimation methods to achieve good UL signal

cancellation were investigated in[20]. It was shown that a two-dimensional (2D) channel estimator with frequency-domain

DFT-based interpolation and time-domain Wiener filtering can

provides excellent cancellation performance over 25 dB, i.e.,

the CLI level is 25 dB lower than the LL signal level. This CLI

can usually be ignored when comparing to the noise threshold

of the LL signal.

It was further shown in[20] that the 2D channel estimator

has relative low complexity and is rather simple to implement

for consumer devices, especially for LL receivers which are

not power limited.

C. Reducing Memory Using Cross-Layer InterleaverMemory is an important consideration for DTV receiver

manufactures, since it can significantly increase the cost of

the implementation. For LDM LL signal detection, additional

memory is required to store the received signal for UL signal

cancellation.

In theory, the UL and LL signal processing chains in an

LDM system could be independent, which provides large

flexibility on the type of services carried in the two layers.

However, when the UL and LL use independent interleaving

and de-interleaving processes, the LL signal detection needs

to wait for the deinterleaving process of the UL signal. The

receiver needs to store all the received symbols before each

UL signal deinterleaving process is finished and then performthe signal cancellation. This could require a large amount of

additional memory.

To reduce the memory requirement, the cross-layer inter-

leaver is introduced in [14]. With the cross-layer interleaver,

the block diagram of an LDM receiver is shown in Fig. 15.

It is shown that since the UL and LL are sharing the same

time and cell interleavers, the de-interleaving is performed

prior to the signal cancellation. This significantly reduce the

size of buffers needed for signal cancellation. It has been

shown in[14] that the additional memory requirement is less

than 20%.
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Fig. 16. Coverage Area of Mobile and Fixed Services in LDM Systems.

VI . COVERAGE A NALYSIS OF LDM-BASED N EX T

GENERATIONATSC 3.0 DTV SYSTEM

Due to the capability offered by the LDM to deliververy robust mobile services, the ATSC 3.0 system

could have a quite different mobile/fixed service coverage

paradigm when comparing to the traditional TDM/FDM-based

systems.

To deliver mobile services with traditional TDM/FDM-

based systems, it has been shown in the past attempts that

the major challenge is to achieve a reasonable service cov-

erage [30]. Different from fixed receivers (e.g., roof-top

receivers), the mobile receivers usually have low antenna

installation height, small antenna size, and little antenna

directivity. This results in significantly lower received signal

power at the mobile receivers as compared to fixed receivers,

and therefore a much smaller mobile service coverage. Itbecomes necessary to install many additional transmitters to

obtain a reasonable mobile service coverage in a regular

size city.

For a good understanding of the coverage areas for mobile

and fixed services of the ATSC 3.0 systems, the following

assumptions are made:

A mobile receiver with 1.5m antenna height and a fixed

receiver with 10m antenna height, resulting in 12 dB

difference in field strength.

The fixed-receiver has a directional gain of 8 dB.

With these assumptions, the received signal strength for the

mobile receiver is about 20 dB (100 times) lower than the

fixed receiver.The two-layer LDM system is assumed to deliver a mobile

service in UL with rate- 14

LDPC code and QPSK modulation,

which provides an SNR threshold of about 1.0 dB for fixed

receivers and 2.0 dB for high-speed mobile receivers. For the

fixed services in the LL, the required SNR varies depend-

ing on the required service data rate. The required SNR is

14.4 dB for a throughput of 15 Mbps service and 22.3 dB

for 28 Mbps. Therefore, the UL mobile receivers requires

12.4 dB lower SNR (or signal strength) than the LL fixed ser-

vice of 15 Mbps, and 20.3 dB lower SNR than the LL service

of 28 Mbps.

Fig. 17. Coverage Estimation for Toronto Area.

Taking into account the 20 dB higher antenna gain for fixed

receivers and the 5 dB LL injection level, it is calculatedthat [30]:

The coverage of the UL service for mobile receivers is

slightly smaller than the coverage area of a 15 Mbps

LL service for fixed receivers. The difference is only a

propagation distance of 2.6 dB.

The coverage of the UL service for mobile receivers is

larger than the coverage of a 28 Mbps LL service for

fixed receivers, by a distance for propagation loss of

5.3 dB.

The coverage of the UL service for fixed receivers is

significantly larger than the coverage of 15 Mbps and

28 Mbps LL services for fixed receivers, by a distance

for propagation loss of 20.4 dB and 28.3 dB.This shows that the LDM system can provide a mobile

service coverage very close to that of the fixed-services. In

addition, the UL mobile services have significantly larger

coverage areas for fixed receivers.

The mobile and fixed service coverage areas of the two-layer

ATSC 3.0 system are illustrated in Fig. 16, which shows that

the LDM system can greatly reduce the mobile/fixed service

coverage mismatch. In addition, it delivers the services in UL

to fixed receivers in a significantly larger area.

A coverage estimation of the LDM-based ATSC 3.0 system

is obtained using the CRC-COVLAB coverage prediction soft-

ware for the Toronto area, where the transmitter is installed

on the CN Tower. The coverage predictions for the differentATSC 3.0 services are shown in Fig. 17. It can be seen that

the coverage prediction shown in Fig. 17 agree very well with

those analytical results shown in Fig. 16.

It is shown that, the LDM-based ATSC 3.0 system can

provide a handheld HDTV service and a 4k-UHDTV service

with the same coverage as the legacy fixed HDTV services

which are delivered by the ATSC 1.0 system. The new mobile

HDTV services, shown as the yellow curve, have smaller cov-

erage. Finally, the HDTV service delivered in the UL has an

extremely large coverage for fixed receivers, which is clearly

identified by the blue curve (New fixed HDTV in the figure).
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VII. ACHIEVING F URTHERC APACITYI NCREASE W IT H

MULTIPLEA NTENNAT ECHNOLOGIES INLDM SYSTEMS

Multiple antenna technologies have proven very effective

in providing higher transmission rate, and/or higher service

reliability [31], [32]. Depending on the number of antennas at

the transmitter and the receiver, multiple antenna technologies

can be categorized as

Single-Input-Multiple-Output (SIMO): one transmitterantenna and multiple receiver antennas. SIMO provides

more reliable detection by using receiver antenna diver-

sity combining (e.g., maximum-ratio-combining (MRC),

equal-gain-combining (EGC), selective combining, etc..).

Multiple-Input-Single-Output (MISO): multiple transmit-

ter antennas and one receiver antenna. MISO pro-

vides more reliable signal detection by using transmitter

antenna diversity combining techniques, including space-

time/frequency-coding (ST/FC), etc..

Multiple-Input-Multiple-Output (MIMO): MIMO refers

to a system with multiple antennas at both the transmit-

ters and the receivers. MIMO can be configured in many

different forms to provide higher throughput or higherSNR, or both. In this document, MIMO is used specifi-

cally for the spatial multiplexing (SM) mode to provide

the highest throughput.

Performance of LDM with multiple antenna technologies

was analyzed in [16] in terms of the capacity improvement

with pre-defined percentage of radio resources allocated to

one layer.

In this section, the capacities of the layers in an two-

layer LDM system configured with different multiple-antenna

technologies in different layers are presented based on

information-theoretical analysis. More importantly, the funda-

mental trade-off between the capacities in the two layers are

demonstrated.Due to the lack of return link, the most typical multi-

ple antenna configurations for DTV systems involves two

antennas at the transmitters. We further limit the number

of receive antennas to two (2) for practical implementa-

tion. For broadcasting systems, the 2x2 dual-polarized spatial

multiplexing (SM) was proposed in [33] which has been

shown to provide good throughput increase when applied

to DVB-T systems. In [18], an improved 2x2 polarized

spatial multiplexing, enhanced Spatial Multiplexing with

Phase Hopping (eSM-PH), was proposed for the DVB-NGH

system, which is shown to provide better throughput

increase.

For LDM systems, there is the flexibility to apply dif-

ferent multiple antenna technologies in different layers to

achieve either higher throughput or more robustness. An

obvious option is to deploy spatial multiplexing in the LL

to increase the capacity for the fixed services, which usu-

ally require high SNRs and therefore can obtain good SM

gains; and to transmit the UL signal in both polariza-

tions (with or without STC/SFC) to achieve the highest

robustness.

Because different multiple antenna technologies are used in

the UL and LL, the UL signal cancellation for LDM receiver

needs to be designed accordingly.

Fig. 18. Achievable Mobile and Fixed Service Capacities of a Two-LayerLDM System, SNRm = 0 dB, SNRf =20 dB.

When the throughput of the mobile layer becomes more

important, a second option is to deploy the same SM

pre-coding in both layers in the LDM system. Although it isclaimed that in general using SM does not provide significant

multiplexing gain at low SNRs, it has been shown in [34] that a

2x2 SM with QPSK modulation can achieve an uncoded BER

of 0.05 at an SNR of 2 dB, which is more than sufficient for

error free decoding performance with low-rate LDPC codes.

Therefore, there should be certain multiplexing gain even for

the mobile services with low SNRs.

Following the information theoretical analysis shown in

Section III-A, the achievable capacities of the LDM-based

DTV systems with different multiple antenna configurations

are obtained and plotted in Fig. 18. It is assumed that the

SNR threshold is 0 dB for the mobile services and 20 dB

for the fixed service. With the 2x2 MIMO, the LL capac-ity is almost doubled because of the high SNR threshold. On

the other hand, for UL mobile service, using 1x2 SIMO with

MRC, the capacity is increased by 70%. With ideal decod-

ing of the 2x2 MIMO, the capacity is also almost doubled.

However, performing MRC is simple and easy to implement,

while obtaining ideal MIMO decoding is relatively hard to

achieve.

The derivation of these curves and more investigation results

on the capability of LDM with MIMO will be reported in a

paper to be submitted shortly after.

A superposition MIMO coding for layered sources is intro-

duced in [35], which is essentially a special case of LDM with

different multiple antenna technologies in the multiple layers.

The analysis and simulation results confirmed the capacity

analysis in Fig. 18.

The current ATSC 3.0 standard does not allow the combined

use of LDM with MIMO. However, the above investigations

shows that the LDM-based DTV systems can be flexibly

combined with different multiple antenna technologies in the

different layers to achieve significant performance improve-

ment, in terms of both robustness and transmission capacity.

Therefore, the flexible combination of LDM with MIMO can

be incorporated into the future releases of the ATSC systems

for further system performance enhancement.
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VIII. MORE A PPLICATIONSS CENARIOS OF

LDM I NATSC 3.0 SYSTEMS

In this paper, the advantages of LDM technology in ATSC

3.0 system is mainly demonstrated in terms of delivering

mobile TV services and UHDTV fixed services simultaneously

in one single TV channel.

There are many other scenarios where LDM could find its

place to offer better performance or more diversified servicesin the ATSC 3.0 systems, including

transmitting video streams with scalable video coding to

deliver the same service to different devices;

adding local commercials, news and other services with a

additional layers (i.e., LDM with more than two layers);

performing non-orthogonal multiple access in broadband

wireless communications systems.

These are all the potential areas which are currently under

investigation on the LDM technology.

Another research topic under investigation is local con-

tent/service insertion or local targeted advertisement. With

LDM, one option is using upper layer to transmit mobile

HD and 4K-UHDTV, while using lower layer to deliver localcontent or targeted advertisement. In ATSC 3.0, the strong

modulation/coding schemes are capable of providing system

SNR threshold of lower than 0dB (a negative value). In

this case, the LDM lower layer can provide seamless cov-

erage/service for each SFN transmitter, without the annoying

coverage gaps between SFN transmitter service areas. Since

the advertisement time is typically less that 20% of the pro-

gram time, Non-Real Time (NRT) could be used to play-back

the local content at 5 times the transmission bit rate for

high-definition (audio/video) service quality.

I X. CONCLUSION

In this article, we provide a comprehensive review on the

LDM technology which is accepted as one of the baseline PHY

technologies for the next generation ATSC 3.0 DTV system.

First, it is shown by theoretical analysis that the LDM pro-

vides fundamental performance advantage over the traditional

TDM/FDM systems when delivering multiple services over the

same time-frequency resource. This advantage becomes sig-

nificant when the multiple services have significantly different

SNR thresholds. This corresponds to the important scenario

to simultaneously deliver UHDTV fixed service (or multiple

HDTV fixed services) and extremely robust mobile HDTV ser-

vices in one TV channel. The performance advantage of LDM

for ATSC 3.0 system is further demonstrated by the extensivesimulation results.

With the careful design recommended for the ATSC 3.0

standard, we further show that the performance improvement

brought by LDM is achieved with very limited additional com-

plexity for both transmitters and receivers. The mobile receiver

for an LDM system is the same as those for traditional single-

layer systems; while for fixed receivers capable of decoding

both layers, the added complexity is less than 20% in terms

of both signal processing and memory requirement.

In addition, different multiple antenna technologies can be

incorporated into the different layers in an LDM systems to

achieve either throughput increase or higher robustness, which

could result in more variety of coverage balancing among the

different layers.

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