3D MIMO Scheme for Broadcasting Future Digital TV in Single Frequency Networks

1 3D MIMO Scheme for Broadcasting Future Digit al TV in Single Frequency Networks Youssef Nasser, Jean-François Hélard and Matthieu Crussière Abstract This letter introduces a 3D space- time-space block code for future terrestrial digital TV systems. The code is based on a double layer structure designed for inter-cell and intra-cell transmission s in single freque ncy networks. Witho ut increasing the com plexity of the receiver, the proposed code is very efficient to co pe with equal and unequal received powers in single frequency network scenarios. 1 Introduction Nowadays, on e of the most promi sing technologies for the seco nd generation of future terrestrial digital TV, concer ned with flexibility, high bit rate and, portable and mobile reception is th e combination of multiple-input multiple- output (MIMO) and orthogonal frequency division multiplexing (OFDM) techniques. To increase area covera ge, single frequency networks (SFN) [1] 2 are used for br oadcasting terrest rial digital TV. SF N are based on the sim ple addition of lower power transmitters at various sites throug hout the coverage area. In an SFN, several transmitters tr an smit at the same moment the same signal on the same frequency. Because it is desirable to deploy SFN with lower transmitted powers, increased bit rates and b etter performance, new MIMO-OFDM systems have to be designed to ens ure such transm ission conditions. In this letter, we present a 3-dime nsion (3D) space -time-space block code (STSBC) for MIMO- OFDM systems in SFN with mobile and portable reception. The use of a second space di m ension is due to SFN. The proposed code is based on the combination of 2 layers: one laye r corresponds to an inter-cell ST coding, the second corresponds to an intra-cell ST co ding. In the following, we first present the scenario of m obile and portable reception with single layer re ception. Then, we introd uce our proposed code as a double layer code and adapt it to the SFN environment. 2 3D code Consider a MIM O-OFDM comm unication system using (2× M T ) trans mit antennas (Tx) and M R receive antennas (Rx) fo r a downlink communication. In this letter, we propose to apply a distributed MIMO scheme in an SFN architecture. Such a sy stem could be impl emented on 2 different sites using M T Tx by site as sh own in Figure 1. The transmission coul d therefore be seen as a double layer scheme in the space dom ain. The first layer is seen between the 2 sites separated by D km (distribute d MIMO scheme). The second l ayer is seen between the antennas sep arated by d m within one site. For the first 3 layer, an STB C encoding schem e is applied betwee n the 2 signals trans mitted by each site antenna. In the second la yer, we use a second STBC encoder for each subset of M T signals transmitted from the same site. For the first layer, the STBC encoder t akes L sets of Q data complex sym bols each ( s 1 ,…, s Q ) and transforms them into a 2× U output matrix acco rding to the ST BC scheme. In the second layer (the seco nd step), the encoder tra nsforms each component of the first layer mat rix into M T × T output matrix according to the second layer S TBC scheme. The num ber of rows of the e ncoding matrix in the first layer is equal to two since, the STBC scheme is ap plied between the signals of two different sites. The output signal of eac h site is fed to M T OFDM modul ators, each using N sub-carriers. The reader could construct a double lay er Alamouti code, for exam ple, by considerin g 2 sets of 2 symbols each and then, by applying Alamouti en coding between t he 2 symbols’ sets and another Al amouti encodi ng between the signal s in each site. More generally, the double layer encoding matrix is describ ed by: (2 ) (2) (1) 11 1 (2 ) (2 ) 21 2 U U ⎛⎞ = ⎜⎟ ⎝⎠ XX X XX L L ,1 1 1 ,1 1 (2) ,1 1 , 1 ( , ... ) ( , ... ) ( , ... ) ( , ... ) TT ij Q ij T Q ij ij M Q ij M T Q fs s f s s f ss f ss ⎛⎞ ⎜⎟ = ⎜⎟ ⎜⎟ ⎝⎠ X K MO M L (1) In (1), the superscript indicat es the layer, ,1 ( , ... ) ij mt Q f ss is a function of the input complex symbols s q and depends on the STBC encoder schem e. The time dim ension of the result ing 3D code is equal t o UT × and the resulting 4 coding rate i s QL R UT × = × . In order to have a fair anal ysis and comparison between diffe rent STBC codes, the sig nal power at the output of the ST encoder at each site is normalized by 2× M T . In the following, we will compare different STBC schem es assuming that a portable or mobile terminal receives signals from the 2 site s with unequal powers. It is a real case in SFN where the terminal receives signals from the 2 sites transm itters. We will assume that the relative po wer imbalance factor between the received signals from the two sites is equal to β . At the receiving side, we assume that a sub-optimal iterative r eceiver i s used for non-orthogonal STBC schemes. The sub-optimal solution proposed h ere consists of an iterative receiver where the ST detector and th e channel decoder exchan ge extrinsic information in an iterative wa y until the algorithm converges [2]. 2.1 Single layer case: in ter-cell ST coding In the single layer case i.e. M T =1, the second layer matrix X (2) resumes to one element . The MIMO transmission i s therefore achieved by the set of one antenna in each site. Due to the mobility of the terminal i.e. different assumed positions, the first layer ST scheme must be efficient face to unequal recei ved powers. In this letter, we consider the orthogonal Alamou ti code [3], the space multiplexing (SM) scheme [4] and the Golden code [5] with M R = 2 Rx antennas. Without loss of generality, we assume that the transmission from a transmitting antenna i to a receiving an tenna j is achieved for each sub-carrier n through a fre quency non-selecti ve Rayleigh fading cha nnel. 5 Figure 2 gives t he required E b /N 0 to obtain a bit error rate (BER) equal to 10 -4 for different values of β a nd a spectral efficiency η =4 [b/s/Hz]. As expec ted, this figure shows that the Gold en code presents the best performance when the 2 Rx antennas receive the sam e power from the 2 sites (i.e. β =0 dB). When β decreases howeve r, the Alamouti scheme is the m ost efficient since it presents onl y 3 dB loss in term s of required E b /N 0 with respect to the case of equal received powers. Indeed, the transmission scenario becom es equivalent t o a transmission scena rio with one Tx antenna for very small values of β . 2.2 Double layer case In the case of a double layer reception, the code construction is base d on the single layer re sults. We restrict our st udy to M T = 2 T x a n t e n n a s b y s i t e a n d M R =2 Rx antennas. We construct th e first layer with the Alamouti scheme, since it is the most resistant for th e case of unequal received powers. In a compleme ntary way, we propose to cons truct the second layer with the Golden code si nce it offers the best results in the case of equal received powers. After combination of the 2 layers, (1) yields: () ( ) ( ) ( ) () () () () () () () () () () () () 12 34 56 78 34 12 78 56 * * ** * * ** * * ** * * ** 56 78 1 2 34 * * ** * * ** * * ** * * ** 78 56 34 1 2 1 5 ss s s s s s s js s s s js s s s X s s s ss ss s j ss ss j ss ss αθ α θ α θ α θ αθ α θ α θ αθ αθ αθ αθ αθ αθ αθ αθ αθ ⎛⎞ ++ + + ⎜⎟ ++ + + ⎜⎟ ⎜⎟ = ⎜⎟ −+ −+ + + ⎜⎟ ⎜⎟ +− + − + + ⎝⎠ (2) where 15 2 θ + = , 1 θ θ =− , 1( 1) j α θ = +− , 1( 1 ) j α θ = +− , j = μ and (.) * stands for com plex conjugate. 6 Figure 3 shows the results in ter ms of required E b /N 0 to obtain a BER equa l to 10 -4 for different values of β and 3 S TBC schemes i.e. o ur proposed 3D code schem e, the 1-Layer Alam outi and the Golden co de schemes. Figure 3 shows t hat our proposed sche me presents the best pe rformance whatever the spectral efficiency and the fact or β . Indeed, it is op timized for SFN systems owin g to the robustness of t he Alamouti schem e to unbalanced received powers a nd the full rank of the Golden code. For β =-12 dB, the proposed 3D code offers a gain e qual to 1.8 dB (respectively 3 dB ) with respect to the Alamouti scheme for η =4 [b/s/Hz] (resp. η =6 [b/s/Hz]). This gain is even greater when it is compar ed to the Golde n code. Moreover, the maximum loss of our c ode due to unba lanced receive d powers is only equal to 3 dB in terms of E b /N 0 . These results confirm that the propo sed 3D code is very robust whatever the spectral e fficiency and the im balance factor β . Eventually, we should note that the factor β could be rel ated to the channel impulse respo nse delay and to the power path loss. The n, it can be used to adjust synchronisation prob lems. 3 Conclusion In this letter, a new 3D STSBC is presented. It is based on a double layer structure defined for inter-cell and intra-cell situations by ad equately combining the Alamouti code and the G olden code perform ance. We showed that our proposed scheme is v ery efficient to cope with equal an d unequal received powers in SFN scenarios. 7 Acknowledgments The authors woul d like to thank the Eur opean CELTIC project “B21C ” for its support of this work. References [1] A. Mattson, “ Single frequency networks in DTV”, IE EE Trans. on Broadcasting, V ol. 51, Issue 4, pp.: 413-422, D ec. 2005. [2] Y. Nasser, J.-F. Hél ard, and M. Crussiere, “On the Influence of Ca rrier Frequency O ffset and Sampling F requency Offset i n MIMO-OFDM Systems for Future Digital TV”, in the proceedings of t he IEEE International Symposium on W ireless and Pervasive computing, pp. 93- 96, May 2008, Santor ini, Greece. [3] S.M. Alamouti, “A simple transmit diversity technique for wireless communications”, IEEE J. on Select ed Areas in Comm unications, vol. 16, no. 8, pp. 14 51-1458, Oct. 19 98. [4] G. J. Foschini, “Layered sp ace-time architecture for wireles s communi cation in a fading envir onment when u sing multi-elem ent antenna,” Bell Labs Tech. J., vol. 1, no. 2, pp . 41–59, 1996. [5] J.-C. Belfiore, G. Rekaya, and E. Viterbo, “The golden code: a 2 × 2 full-rate space-time code with non-vanishing determinants,” IEEE Trans. in Information Theory, vo l. 51, no. 4, pp. 1432–1436, Apr. 2005. 8 Author’s affiliation Youssef Nasser, member IEEE , youssef.nasser@insa-rennes.fr, (Institute of Electronics a nd Telecommunications of Rennes, INSA Rennes, 20 Avenue de Buttes des Coesmes, 35043 Renne s cedex, France). Jean-Françoi s Hélard, Senior membe r IEEE , jean-francois .helard@insa- rennes.fr, (Inst itute of Electronics and Telec ommunications of Rennes, I NSA Rennes, 20 A venue de Buttes des C oesmes, 35043 Rennes cedex, France). Matthieu Crussière, member IEEE , matthieu.crussiere@insa-ren nes.fr, (Institute of Electronics and Telecomm unic ations of Rennes, INSA Rennes, 20 Avenue de Buttes des Coe smes, 35043 Renne s cedex, France). Figure Caption Figure 1- SFN netw ork with unequal received powers ( M T =2) Figure 2- Requir ed Eb/N0 to obtain a BER=10 -4 , single layer case, DVB-T parameters , conv olutional encode r (171,133) o η =4 [b/s/Hz] : Alamouti code : 64-QAM, R=1, cha nnel encoding rate R c =2/3. Other schem es: 16-QAM, R=2, R c =1/2. 9 Figure 3- Re quired Eb/N0 to obtain a BER=10 -4 , double layer case, DVB-T parameters, η =4 [b/s/Hz]: Alamouti code: 64-QAM, R=1, R c =2/3. Other schem es: 16-QAM, R=2, R c =1/2. η =6 [b/s/Hz]: Alamouti code: 256-QAM, R=1, R c =3/4. Other schem es: 64-QAM, R=2, R c =1/2. D d d P β P M T ant ennas M T antennas D d d P β P D d d P β P M T antennas M T antennas Figure 1 : 10 -12 -10 -8 -6 -4 -2 0 6 8 10 12 14 16 18 β [d B] E b /N 0 [dB ] Ala mou ti SM Golden 3dB Figure 2 : -12 -10 -8 -6 -4 -2 0 6 8 10 12 14 16 18 20 β [d B] E b /N 0 [dB ] 3D-code A l amout i Golden 1. 8 dB η = 4 [ b/s /H z] 3 dB η = 6 [ b/s /H z] Figure 3 :