Improved polarized 2x2 MIMO spatial multiplexing method for DVB-NGH system

Recently terrestrial digital broadcasting systems have experienced a growth with the demand of high-data rate. In order to meet such demand, the multiple-input multiple-output (MIMO) technology has received wide attention. This paper proposes a pre-coding method, which provides high-space channel correlation for the improved performance over terrestrial broadcasting channels when the MIMO spatial multiplexing (SM) is adopted for digital video broadcasting-next generation handheld systems. When signals with two different modulation orders are transmitted through two antennas, a method that is based on nonuniform power is also proposed for improved reception performance. To optimize the proposed method, phase shifting values for the pre-coding method and appropriate non-uniform power ratio are obtained. These obtained parameters are applied to a terrestrial broadcasting system, and then the performance improvement over the conventional SM is shown through computer simulations.

Improved Polarized 2x2 MIMO Spatial Multiplexing

Method for DVB-NGH System Jae Hyun Seo, Tae Jin Jung, Heung Mook Kim, and Dong Seog Han

Abstract—Recently terrestrial digital broadcasting systems have

expe-rienced a growth with the demand of high-data rate In order to meet

such demand, the multiple-input multiple-output (MIMO) technology has

received wide attention This paper proposes a pre-coding method, which

provides high-space channel correlation for the improved performance

over terrestrial broadcasting channels when the MIMO spatial

multi-plexing (SM) is adopted for digital video broadcasting-next generation

handheld systems When signals with two different modulation orders

are transmitted through two antennas, a method that is based on

non-uniform power is also proposed for improved reception performance To

optimize the proposed method, phase shifting values for the pre-coding

method and appropriate non-uniform power ratio are obtained These

obtained parameters are applied to a terrestrial broadcasting system, and

then the performance improvement over the conventional SM is shown

through computer simulations.

Index Terms—DVB-NGH, DVB-T2, OFDM, MIMO, terrestrial,

spatial multiplexing.

I INTRODUCTION

IN RECENT terrestrial digital broadcasting systems, there has been

effort to provide a 3DTV service in addition to multiple channel

SDTV and HDTV Moreover, due to the recent demand of UHDTV

service, it is required to provide higher data rates to provide these

services In order to meet such demand, transmission technologies

that provide spectrum efficiency within limited bandwidth, and the

multiple-input multiple-output (MIMO) technology has been

consid-ered as one of the tractable methods However, current broadcasting

systems do not provide a return channel as LTE, WiMAX, and

other wireless communication systems, and their transmitters cannot

receive channel information from receivers [1] Furthermore, because

terrestrial broadcasting systems provide wide coverage areas in

gen-eral, most channels include LOS (line of sight), which has high space

channel correlation

The DVB-T2 (digital video broadcasting-terrestrial 2ndgeneration)

system, the second generation terrestrial DTV standard in Europe,

has adopted a variety of technologies to increase data rates over

the first generation DVB-T system One simple technologies is to

increase modulation orders of OFDM systems for improved

spec-trum efficiency There are other technologies such as increasing the

Manuscript received March 25, 2015; revised June 23, 2015; accepted

July 13, 2015 This work was supported by the ICT Research and

Development Program of MSIP/IITP “Development of Service and

Transmission Technology for Convergent Realistic Broadcast” under Grant

R0101-15-294.

J H Seo and H M Kim are with the Department of Broadcasting

Systems Research, Electronics and Telecommunications Research Institute,

Daejeon 305-700, Korea.

T J Jung is with the School of Electronics and Computer Engineering,

Chonnam National University, Gwangju 500-757, Korea.

D S Han is with the School of Electronics Engineering, Kyungpook

National University, Daegu 702-701, Korea (e-mail: dshan@ee.knu.ac.kr ).

Color versions of one or more of the figures in this paper are available

online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TBC.2015.2459659

FFT size given the same length of guard interval, decreasing the rate

of pilot or training signal, and adopting channel coding methods that provide high efficiency [2] In the DVB-T2 system, the modulation order is increased up to 256-QAM, the FFT size is increased up

to 32k, and the rate of pilot is reduced down to 1% In addition, the new channel coding methods, the LDPC and BCH codes, are adopted, so that the reception performance and spectrum efficiency are improved over the past standard using the convolutional and Reed-Solomon codes [3] Based on the ATSC (advanced television systems committee) system, an ADT (augmented data transmission) system has been introduced in order to increase data rate of the existing system By using the hierarchical modulation, DTV signal cancellation, and advanced error correction code, the ADT sys-tem can offer additional data capacity of up to a few Mbps This system does not require additional RF spectrum for the increased data rate, but also guarantee backward compatibility with legacy receivers [4]

The MIMO technology can be another option that provides additional spectrum efficiency within limited bandwidth, and the spatial multiplexing (SM) method is used in general in order to increase data rates After the DVB-T2 system has been developed, research on the MIMO technology based on the DVB-T2 sys-tem has been continued Based on the polarized 2x2 MIMO, the efficient transmission method for fixed reception and the channel estimation method using polarized diversity have been presented [5] In the DVB-T2 system, the MIMO SM method provided the better performance than the input single-output (SISO) under the ideal channel environment [6] Recently, the DVB-NGH (next generation handheld) system, which adopts the MIMO technology as a DVB-T2 based standard, used the SM method based on the polarized 2x2 MIMO in order to increase data rates rather than the diversity performance improvement [7] Furthermore, for reducing complexity of receiver, MIMO decoder simplification method has been presented in DVB-NGH systems [8]

This paper presents a transmission method that increases data rates for terrestrial broadcasting systems An improved polar-ized 2x2 MIMO SM method is proposed for DVB-NGH system, which has adopted a MIMO technology as a terrestrial broadcast-ing standard Usbroadcast-ing the proposed precodbroadcast-ing method, the performance degradation of the conventional SM method in terrestrial chan-nels that contained LOSs with high space channel correlation can

be prevented Furthermore, when two different modulation orders between two antennas are used for polarized 2x2 MIMO systems, the performance can be improved for the designated channel by using the unequal power transmission This unequal power pre-coded 2x2 MIMO SM method was also proposed in DVB-NGH standardization by Electronics and Telecommunications Research Institute (ETRI) [9]

In chapter II, the conventional MIMO SM method is presented

In chapter III, the proposed MIMO SM method is explained Simulation results which provide the improved performance are shown in chapter IV, and conclusion is drawn in chapter V 0018-9316 c  2015 IEEE Personal use is permitted, but republication/redistribution requires IEEE permission.

See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Fig 1 Block diagram of conventional 2x2 MIMO SM for OFDM system.

II CONVENTIONALMIMO SM METHOD

The MIMO SM method that increases the multiplexing rate of

information is to divide the information symbols between the

trans-mit antennas Fig.1shows a block diagram of 2x2 MIMO SM based

on a general OFDM system Note that the 2x2 MIMO SM transmits

two channel encoded signals through two different antennas and

spa-tial multiplexing For example, when QPSK-modulated symbols are

transmitted through two antennas with the same rate as using one

antenna, the transmission with 2 bits can be increased to the

trans-mission with 4 bits, which provides doubled transtrans-mission capacity

Therefore, spectral efficiency is defined by 6 bpc (bits per cell, as the

number of bits assigned per subcarrier), 8 bps, and 10 bpc The

sig-nal constellations for two antennas are QPSK/16QAM for the case of

6 bpc, 16QAM/16QAM for the 8 bpc, and 16QAM/64QAM for the

case of 10 bpc Especially, DVB-NGH provides implementation guide

of a polarized 2x2 MIMO system as an optional profile in order to

exploit the diversity and capacity advantages made possible by the use

of multiple transmission elements at the transmitter and receiver [10]

Also, DVB-NGH is the first broadcasting system to exploit the

diver-sity in the time, frequency and space domains by incorporating at the

physical layer long TI (time interleaver), TFS (time frequency slicing)

and polarized 2x2 MIMO [11], [12]

The condition of correlation in the MIMO channel due to LOS

channel, which happens in terrestrial broadcast transmissions, is

specially affected for the MIMO SM Fig 2shows the signal

con-stellations of transmitted and received antennas when the 2x2 MIMO

channel correlation is 1 and the case of 6 bpc, QPSK/16-QAM

sig-nals are transmitted using the MIMO SM method In this figure, the

QPSK/16-QAM signal constellations are observed at the transmitted

antenna, whereas at the received antenna, the QPSK/16-QAM signals

are duplicated, so that 64 constellation points are observed The

dis-tances of received signal between constellation points are decreased

by MIMO channel environments In order to separate the received

signals from two antennas, the GC (Golden code) can be used for

the improved reception performance, but its complexity is quite large

for implementation [13] The decoding complexity of SM is

propor-tional to O(M2) while that of GC is proportional to O(M 4), where

M is cardinality of the signal constellation (e.g., M= 4 for QPSK)

III PROPOSEDMIMO SM METHOD

In order to improve the reception performance, this paper

pro-poses the MIMO SM method based on a precoder, which shifts

phases of modulated signals transmitted through two antennas Using

this precoder, average distance between such shifted constellations at

a receiver can be increased

Furthermore, when signals from two antennas have different

modulation orders, unequal power ratio is used for performance

improvement With different modulation orders, the different

pow-ers from a transmitter can result in the increased average distance

Fig 2 QPSK/16QAM signal constellations of transmitted and received antennas when channel correlation = 1.

between the combined constellations in received antenna For exam-ple, in the case of two transmitted antennas, if the signal power of higher modulation order is larger than that of lower modulation order, the reception power can be improved Fig 3 shows the proposed unequal power precoded 2x2 MIMO SM

First of all, the unequal power input vector X can be expressed as

X=x1, x2

T

(1)

and unequal power output vector Y can be expressed as

Y=y1, y2

where is defined by (3) and this means power ratio between two transmitted antennas

 =

 √

1− α



(3) This means that the two transmitted antennas produce the equal power when α = 1/2 The precoder output vector R can be

expressed as

R=r1, r2

T

where is defined by (5) and this equation shifts the signal phase

of each transmitted antenna

 =



cosθ − sin θ

sinθ cosθ



(5) From (1) ∼ (5), the transmitted signals r1 and r2 can be expressed as



r1

r2



=



cosθ − sin θ

sinθ cosθ

 √α

0

1− α



x1

x2



(6)

IV SIMULATIONRESULT First of all, simulation was conducted to obtain optimal values of precoder phase shifting and unequal power ratio In this simulation, Ricean channel, which has infinite K value as LOS channel environ-ment, was assumed Figs.4and5, average uncoded BERs were mea-sured when rotation angle of precoder is changed from 0 to 90 degrees with ideal antenna In this case, ideal antenna conditions are assumed

and channel matrix H can be expressed as

H=



h1,1 h1,2

h2,1 h2,2



=



e j θ1,1 e j θ1,2

e j θ2,1 e j θ2,2



(7) whereθ1,1 , θ2,1 , θ1,2 , θ2,2are adopted by random phase

In addition, the performance was measured when the α values,

which represent unequal power ratio, were changed to 1/4, 1/3,

Fig 3 Block diagram of unequal power-precoded 2x2 MIMO SM system.

Fig 4 Performances of average uncoded BER according to rotation angles

of precoder and power ratio when SNR = 7 dB.

Fig 5 Performances of average uncoded BER according to rotation angles

of precoder and power ratio when SNR = 10 dB.

1/2, 2/3, and 3/4 Note that α = 1/2 denotes the same signal

power between QPSK and 16-QAM at each transmitted antenna

and α = 1/3 denotes doubled 16-QAM signal power compared

to QPSK Fig 4 shows average uncoded BERs according to

dif-ferent rotation angles of precoder when SNR= 7 dB In the case of

α = 1/3, the performance of average uncoded BER was the best

when the rotation angles of precoder were 20∼30 or 60∼70 In

the other case, Fig 5 shows average uncoded BERs according to

different rotation angles of precoder when SNR= 10 dB This

sim-ulation result shows that the performance of average uncoded BER

was the best when the rotation angles of precoder were 30∼60 and

also α = 1/3 We find optimal values when α value was 1/3 and

the rotation angle of precoder was about 30 degree Also we found

Fig 6 Minimum distances according to channel phase differences.

another result that the performance of average uncoded BER was the best when α = 1/3 and the rotation angle of precoder was

0 degree In this simulation, we obtained the better performance only using unequal power method when polarized 2x2 MIMO SM system adopted different modulation orders

For the second simulation, minimum distances at the received antenna were computed in the cases of conventional SM, pre-coded SM, and unequal power(UP)-prepre-coded SM methods Prepre-coded

SM shifts the signal phase of each transmitted antenna in (6) and UP-precoded SM is additionally including unequal power ratio in (3)

In this simulation, polarized 2x2 MIMO channel parameters are defined by

θ diff = θ1,1 − θ1,2 orθ2,1 − θ2,2 (8) whereθ diff is defined by (8) and this means channel phase difference Fig 6shows the minimum distances according to channel phase difference, θ diff between h1,1 and h1,2 When the channel phase differences were changed from 0 to 90 degrees, the UP-precoded

SM method had higher average minimum distance than the SM and, precoded-SM from combined signal constellations at the received antenna Partially, precoded-SM method had higher minimum dis-tance than the UP-precoded SM method in channel phase differences ranges were 30∼60 degrees TableI shows the numerical results of minimum and average values of minimum distances for each method According various channel phase difference, the minimum value of conventional SM method had smaller than that of the other methods Also, compared to the conventional SM method, the precoded SM method increased the averaged minimum distance by about 44%, and the UP-precoded SM method had 58% increment Therefore, the UP-precoded SM method achieved the best performance in minimum distance of combined signal constellations at the received antenna These results show the optimal values of precoder’s phase shift and

Fig 7 QPSK/16QAM signal constellations from transmitted and received

antennas when channel correlation= 1, α = 1/3, θ = 30◦.

TABLE II

S IMULATION P ARAMETERS

power ratio on each modulation order under the simulation

config-uration For example, phase shift and power ratio can be slightly

changed by various simulation parameters [14]

Based on the above two simulation results, we have new signal

constellations from transmitted and received antennas with precoder

and unequal power terms Assuming that QPSK and 16-QAM

sig-nals are transmitted through two antennas based on the proposed

MIMO SM method named by UP-precoded SM, Fig 7 shows the

signal constellations of transmitted and received antennas when the

correlation of 2x2 MIMO channel is 1, the precoder phase shifts are

30 degrees and unequal power ratioα = 1/3 As shown in the figure,

64 constellation points in the transmitted antennas are represented by

r1and r2 as in (4) Also, another 64 constellation points are shown

in the received antennas In the transmitted antenna, 64 constellation

points are shown, whereas in the received antenna it can be shown

that the distance between constellations are increased

Following the previous results, the conventional SM, precoded SM

and UP-precoded SM methods were simulated Simulation

parame-ters are shown in Table II The transmission parameters were based

on the DVB-T2 system, and hence, 4K FFT size, 1/4 guard

inter-val, and LDPC code (frame size= 16200 bits, code rate = 4/9, 2/3)

were used For the modulation, QPSK and 16-QAM were used at

the two transmitted antennas For the MIMO channel model of

ter-restrial broadcasting environment, the Helsinki2 outdoor reception

Fig 8 Average uncoded BER performances of conventional SM, precoded

SM and UP-precoded SM (SNR range= 6.5 ∼ 8.5 dB).

was assumed, and channel parameters were Ricean factor K = 1,

XPD= 6 dB, and Doppler frequency = 33.3 Hz This Doppler

fre-quency in Helsinki2 outdoor channel is 33.3 Hz, which corresponds

to 60 km/h for a RF carrier of 600 MHz Note that XPD (cross-polar discrimination) is the ratio between the averaged received powers of co-polarization and cross-polarization, which is defined by

XPD= 10 log10

h1,1

h1,2



 = 10log10



h2,2

h2,1



 (dB). (9) Table III shows the Helsinki2 outdoor reception channel profile used in this simulation This profile consists of 8 multipath and the

relative power magnitudes denote h1,1 , h2,2 gains in the polarized 2x2 MIMO channel [15]

The average coded BER performance for the conventional SM, pre-coded SM, and UP-prepre-coded SM methods are shown and compared

in Figs.8and9 For the precoder,θ = 30◦was used by the precoded

SM and UP-precoded SM For the unequal power,α = 1/3 was used

by the UP-precoded SM The power of 16-QAM signals was twice bigger than that of QPSK signals

Fig 8shows the average uncoded BER performance of the con-ventional SM, precoded SM, and UP-precoded SM when the SNR range was 6.5∼8.5 dB The conventional SM and precoded SM

performed similarly, but the UP-precoded SM method shows better uncoded BER performance than the others Fig.9 shows the aver-age coded BER performance of the conventional SM, precoded SM, and UP-precoded SM when the code rate was 4/9 The conventional

SM and precoded SM performed similarly, and the UP-precoded

SM method performed slightly better than the others In the case

of UP-precoded SM, the SNR was 0.2 dB lower than the conven-tional SM at BER = 10−4 Hence, given the conventional SM,

Fig 9 Average coded BER performances of conventional SM, precoded

SM and UP-precoded SM (code rate = 4/9).

Fig 10 Average uncoded BER performances of conventional SM, precoded

SM and UP-precoded SM (SNR range= 10.5 ∼ 12.5 dB).

Fig 11 Average coded BER performances of conventional SM, precoded

SM and UP-precoded SM (code rate = 2/3).

the use of unequal power provided further gains compared to the use

of precoder In addition, Fig.10shows the average uncoded BER

per-formance of the conventional SM, precoded SM, and UP-precoded

SM when the SNR range was 10.5∼12.5 dB The conventional SM

and precoded SM performed similarly, but the UP-precoded SM method shows better uncoded BER performance than the others Fig 11 shows the average coded BER performance of the conven-tional SM, precoded SM, and UP-precoded SM when the code rate was 2/3 In comparison with the conventional SM, the precoded SM and UP-precoded SM provided the gains of 0.1 dB and 0.4 dB, respectively, at BER= 10−4 Therefore, precoded SM slightly out-performed the conventional SM, and therefore, we could improve additionally SNR gain about 0.2∼0.4 dB by using the unequal power

transmission

V CONCLUSION This paper presented the SM method to increase transmission capacity when the MIMO was used for terrestrial digital broadcasting Especially, when different modulation orders were applied to a polar-ized 2x2 MIMO SM transmitting through two antennas, the proposed method improved the reception performance by using the unequal power ratio In this proposed method, when QPSK and 16-QAM were used with 30 degree phase shift and the power of 16-QAM signals was twice bigger than that of QPSK signals, the SNR perfor-mance was improved by 0.2∼0.4 dB compared to the conventional

SM method

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