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.
This article has been accepted for inclusion in a future issue of this journal Content is final as presented, with the exception of pagination IEEE TRANSACTIONS ON BROADCASTING 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 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 Index Terms—DVB-NGH, DVB-T2, OFDM, MIMO, terrestrial, spatial multiplexing I I NTRODUCTION N 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 considered as one of the tractable methods However, current broadcasting systems 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 general, most channels include LOS (line of sight), which has high space channel correlation The DVB-T2 (digital video broadcasting-terrestrial 2nd generation) 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 spectrum efficiency There are other technologies such as increasing the I 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 system 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 system 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 single-input singleoutput (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 polarized 2x2 MIMO SM method is proposed for DVB-NGH system, which has adopted a MIMO technology as a terrestrial broadcasting standard Using the proposed precoding method, the performance degradation of the conventional SM method in terrestrial channels 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 precoded 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 This article has been accepted for inclusion in a future issue of this journal Content is final as presented, with the exception of pagination Fig IEEE TRANSACTIONS ON BROADCASTING Block diagram of conventional 2x2 MIMO SM for OFDM system II C ONVENTIONAL MIMO SM M ETHOD The MIMO SM method that increases the multiplexing rate of information is to divide the information symbols between the transmit antennas Fig shows 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 spatial multiplexing For example, when QPSK-modulated symbols are transmitted through two antennas with the same rate as using one antenna, the transmission with bits can be increased to the transmission with bits, which provides doubled transmission capacity Therefore, spectral efficiency is defined by bpc (bits per cell, as the number of bits assigned per subcarrier), bps, and 10 bpc The signal constellations for two antennas are QPSK/16QAM for the case of bpc, 16QAM/16QAM for the 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 diversity 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 shows the signal constellations of transmitted and received antennas when the 2x2 MIMO channel correlation is and the case of bpc, QPSK/16-QAM signals 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 distances 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 proportional to O(M2 ) while that of GC is proportional to O(M ), where M is cardinality of the signal constellation (e.g., M = for QPSK) III P ROPOSED MIMO SM M ETHOD In order to improve the reception performance, this paper proposes 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 powers from a transmitter can result in the increased average distance Fig QPSK/16QAM signal constellations of transmitted and received antennas when channel correlation = between the combined constellations in received antenna For example, 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 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 T = X (2) where is defined by (3) and this means power ratio between two transmitted antennas √ α √ (3) = 1−α 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 = Y (4) where is defined by (5) and this equation shifts the signal phase of each transmitted antenna = cos θ sin θ − sin θ cos θ (5) From (1) ∼ (5), the transmitted signals r1 and r2 can be expressed as √ α √ x1 cos θ − sin θ r1 = (6) sin θ cos θ r2 − α x2 IV S IMULATION R ESULT 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 environment, was assumed Figs and 5, average uncoded BERs were measured when rotation angle of precoder is changed from 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 h2,1 h1,2 h2,2 = ejθ1,1 ejθ2,1 ejθ1,2 ejθ2,2 (7) where θ1,1 , θ2,1 , θ1,2 , θ2,2 are 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, This article has been accepted for inclusion in a future issue of this journal Content is final as presented, with the exception of pagination IEEE TRANSACTIONS ON BROADCASTING Fig 3 Block diagram of unequal power-precoded 2x2 MIMO SM system Fig Performances of average uncoded BER according to rotation angles of precoder and power ratio when SNR = dB Fig 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 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, precoded SM, and unequal power(UP)-precoded SM methods Precoded 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 θdiff = θ1,1 − θ1,2 or θ2,1 − θ2,2 Fig 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 shows average uncoded BERs according to different rotation angles of precoder when SNR = 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 shows average uncoded BERs according to different rotation angles of precoder when SNR = 10 dB This simulation 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 (8) where θdiff is defined by (8) and this means channel phase difference Fig shows the minimum distances according to channel phase difference, θdiff between h1,1 and h1,2 When the channel phase differences were changed from 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 distance than the UP-precoded SM method in channel phase differences ranges were 30∼60 degrees Table I 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 This article has been accepted for inclusion in a future issue of this journal Content is final as presented, with the exception of pagination IEEE TRANSACTIONS ON BROADCASTING TABLE I M INIMUM D ISTANCE FOR E ACH M ETHOD TABLE III P OWER D ELAY P ROFILE OF H ELSINKI O UTDOOR C HANNEL Fig QPSK/16QAM signal constellations from transmitted and received antennas when channel correlation = 1, α = 1/3, θ = 30◦ TABLE II S IMULATION PARAMETERS Fig 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 = dB, and Doppler frequency = 33.3 Hz This Doppler frequency 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 power ratio on each modulation order under the simulation configuration 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 signals are transmitted through two antennas based on the proposed MIMO SM method named by UP-precoded SM, Fig 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 ratio α = 1/3 As shown in the figure, 64 constellation points in the transmitted antennas are represented by r1 and 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 parameters are shown in Table II The transmission parameters were based on the DVB-T2 system, and hence, 4K FFT size, 1/4 guard interval, 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 terrestrial broadcasting environment, the Helsinki2 outdoor reception XPD = 10 log10 h1,1 h2,2 = 10 log10 h1,2 h2,1 (dB) (9) Table III shows the Helsinki2 outdoor reception channel profile used in this simulation This profile consists of 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, precoded SM, and UP-precoded SM methods are shown and compared in Figs and 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 shows the average uncoded BER performance of the conventional 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 shows the average 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 conventional SM at BER = 10−4 Hence, given the conventional SM, This article has been accepted for inclusion in a future issue of this journal Content is final as presented, with the exception of pagination IEEE TRANSACTIONS ON BROADCASTING 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 conventional 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 outperformed the conventional SM, and therefore, we could improve additionally SNR gain about 0.2∼0.4 dB by using the unequal power transmission V C ONCLUSION Fig Average coded BER performances of conventional SM, precoded SM and UP-precoded SM (code rate = 4/9) 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 polarized 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 performance was improved by 0.2∼0.4 dB compared to the conventional SM method R EFERENCES 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 10 shows the average uncoded BER performance of the conventional SM, precoded SM, and UP-precoded SM when the SNR range was 10.5∼12.5 dB The conventional SM [1] J Paulraj, D A Gore, R U Nabar, and H Bolcskei, “An overview of MIMO communications: A key to gigabit wireless,” Proc IEEE, vol 92, no 2, pp 198–218, Feb 2004 [2] L Vangelista et al., “Key technologies for next-generation terrestrial 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with nonvanishing determinants,” IEEE Trans Inf Theory, vol 51, no 4, pp 1432–1436, Apr 2005 [14] S Moon, “Equal/Unequal power transmission for Dual polar MIMO,” DVB Technical Module Sub-Group on Next Generation Handheld, Tech Rep TM-NGH1047, Jun 2011 [15] M Petrov et al., “Configuration for Multi-Path MIMO Channel Simulations in NGH,” DVB Technical Module Sub-Group on Next Generation Handheld, Tech Rep TM-NGH641r8, Feb 2011 ... of conventional 2x2 MIMO SM for OFDM system II C ONVENTIONAL MIMO SM M ETHOD The MIMO SM method that increases the multiplexing rate of information is to divide the information symbols between... 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 spatial multiplexing For example, when... for the case of bpc, 16QAM/16QAM for the 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