DVB-T/H Digital Television Transmission and its Simulation over Ricean and Rayleigh Fading Channels

Transcription

1 DVB-T/H Dgtal Televson Transmsson and ts Smulaton over Rcean and Raylegh Fadng Channels Ladslav Polák, Tomáš Kratochvl Department of Rado Electroncs, Brno Unversty of Technology, Purkyňova 118, 612 Brno, Czech Republc Emal: Abstract Ths paper deals wth the smulaton of the transmsson of the dgtal televson sgnal accordng to the DVB- T/H standard over fadng channels for the moble recepton. Applcaton for ths smulaton was developed n MATLAB and enables a complete analyss of transmtted and receved data. The smulaton results of the broadcastng over Gaussan (AWGN), Rcean (RC 2 ANX B) and Raylegh (RL 2 ANX B) channels are presented and dscussed. The BER before and after Vterb decodng and MER were compared n all smulated transmsson condtons. Fnally, obtaned results are evaluated and dscussed. 1 Introducton In 1998, the DVB (Dgtal Vdeo Broadcastng) Project started to study the possblty of moble recepton of the sgnals defned n the up and comng DVB-T (Terrestral) standard. The overall concluson was that t was feasble f a number of updates and extensons were ncluded n the DVB-T standard [1]. Therefore, work started toward a new verson of the standard. The resultng standard was called DVB-H (Handheld). It was publshed n November 24 as ETSI (European Telecommuncatons Standards Insttute) standard EN [2]. The DVB-H system can be precsely defned as a transmsson system bult out of several DVB standards amng at effcent terrestral broadcastng of dgtal multmeda data to handheld devces [1]. It s an extenson of DVB-T wth some backwards compatblty,.e. t can share the same transmsson multplex wth DVB-T servces. Ths compatblty s an mportant feature of DVB-H, as many DVB-T systems have already been launched n Europe and all over the world [3]. Therefore, DVB-H creates a brdge between the classcal broadcast systems and the world of cellular rado networks [3], [4]. Moreover, standard DVB-H uses for the data transmsson method Tme slcng, whch enables lower average power consumpton for the battery-powered devces. More nformaton about ths technology t s possble to fnd n [1], [5]. Moble use of broadcastng servces s related to demandng rado channel condtons. Multpath recepton leads to fadng of transmtted sgnal. As a consequence, the sgnal s mpacted by short-term dropouts wth burst errors n the demodulated data stream. Therefore, standard DVB-H needs for the transmsson of selected data better error protecton. Suffcent protecton for transmsson over a fadng rado channel s secured by MPE-FEC (Mult-Protocol Encapsulaton - Forward Error Correcton). Ths FEC (nner and outer FEC) stage contans the tme nterleavng scheme regarded here. MPE-FEC used advanced RS (Reed-Solomon coder). Ths encoder RS (255,191) adds 64 correcton bytes to 191 nput bytes and t s able to correct up to 32 erroneous bytes. For the transmsson, the DVB-H system as a DVB-T uses OFDM (Orthogonal Frequency Dvson Multplex). Ths has been adopted because of ts hgh data capacty and for applcatons such as moble broadcastng. Standard DVB-T uses modes 2K and 8K. The 2K mode provdes the best moble recepton condtons because of ts larger nter carrer spacng. However, t s only sutable for small-sze SFNs (Sngle Frequency Network) and DVB-H networks. The 8K mode can be used both n SFNs and MFNs (Multple) networks. It provdes a Doppler tolerance allowng for hgh-speed recepton. Standard DVB-H also defnes an optonal 4K mode; t s desgned specfcally wth the requrements of DVB-H. The 4K mode provdes a compromse between the sze of the transmsson cells (best acheved wth the 2K mode) and the resstance to echo effects (optmal n the 8K mode). Introducton and usng of ths OFDM mode s the second bg dfference between DVB-H and DVB-T. Ths paper s focused on classfcaton, analyss, smulaton and assessment of the sgnal fadng n the scope of DVB-H recepton. Sgnal transmsson between the transmtter and recever s defned by channel profle models. For the smulaton of the behavor of the transmtted sgnal n ths channel models was used program envronment MATLAB. In ths program was created an approprate algorthm wtch allows smulate the sgnal processng across the entre communcaton chan of DVB-H broadcastng, ncludng ts transmttng and recevng parts. Ths paper s organzed as follow. In secton 2, we ntroduce the man feature of the tested fadng channels and focuses on ther parameters. The fadng channel profles of both model and ther possbltes of mplementaton for the programmng n MATLAB are presented and dscussed n secton 3. The created program n MATLAB for the smulaton of the complete transmsson over fadng channels s presented n secton 4. Parameters for the smulaton are presented n secton 5. Secton 5 shows the dfferences between the constellaton dagrams of receved and corrected (after equalzaton) sgnals and also ncludes dependences of BER (Bt Error Rato) before and after Vterb decodng and MER (Modulaton Error Rato) on C/N rato whch are obtaned and compared. Fnally, acheved results are dscussed n secton

2 2 Fadng Transmsson Channels 2.1 Gaussan Channel (AWGN) Model of the Gaussan channel descrbes a case, whch s based on a drect sgnal path from transmtter to recever only. In ths case the receved sgnal s only attenuated and ncludes a defnte level of nose. Ths channel s overlad wth AWGN (Addtve Whte Gaussan Nose), whch s manly produced n the recever tself. The best condton for the receved data s defned as Gaussan channel. 2.2 Rcean Channel When reflected sgnals are added to a drect sgnal then the qualty of the recepton get worse. Multpath propagaton of the sgnals makes a varance of sgnal ntensty and causes ISI (Inter Symbol Interferences). These stuatons are modeled n channel, whch s defned as a Rce channel. In ths type of channel the Gaussan channel and ts characterstcs also exsts. The sgnal s completed wth reflecton of sgnals from dfferent ways. Influence of the channel on receved sgnal s possble descrbed wth the followng mathematcal equaton (1) ρx y( t) = N e j2πθ ( t) + ρ e x( t τ ) = 1 N e = ρ 2, (1) where ρ s the attenuaton n the drect sgnal path, Ne s the certan number of echoes, ρ s the attenuaton n echo path, θ s the phase rotaton n echo part I, and τ s the relatve delay tme n echo part. The Rce factor K denotes the rato of the sgnal n the drect path to the sum n all echo paths (2) K = N e ρ = 2 ρ Raylegh Channel. (2) There are very often cases, when the drect sgnal path s not secured as domnate sgnal at the antenna of the recever. Channel model whch smulates these cases, has a drect sgnal fully attenuated. The transmsson channel model wth only echo sgnals s called Raylegh channel. Ths channel model presents worst condtons for the gans of sgnals on the antenna recever. The mathematcal equaton, whch descrbed the nfluence of Raylegh channel on the sgnal, s that same as a (1), only the part, whch descrbes the drect propagaton of the sgnal, s omtted [6], [7], [8]. Table 1: Rcean channel profle (RC 2 ANX B) and Raylegh channel profle (RL 2 ANX B) specfcaton [8] Path number ρ τ θ [rad] [-] [us] Transmsson Channel Smulaton For explorng the behavor of transmtted sgnal n all mentoned transmsson channels where used a RC 2 ANX B and RL 2 ANX B profles [6]. Booth of these channel profles defnes 2 ndrect paths. Each of paths, resp. reflectons, has defned level of gan, delay and phase shft. In case of Rce channel n fact we have 2 ndrect paths and 1 drect path. Ths path has zero gan, delay and phase shft. Factor K equal to 1 was used n smulaton. Used values of gans, delays and phase shfts for each path are n Tab. 1 and accord to [6]. 3.1 Implementaton of the Fadng Channel Models n MATLAB Applcaton whch allows smulaton of the transmsson of the DVB-H standard n varous types of channels was created n MTLAB. Ths applcaton follows the structure of the transmtter of the DVB-H standard. The recever was programmed as nverted transmtter. Resources for the smulaton of the transmsson and sgnal processng are ncluded n Matlab n functons of the Communcaton Toolbox. Ths toolbox also ncludes models of dfferent types of transmsson channels wth many adjustable parameters. In MATLAB, the transmsson channel wth fadng can be appled to the sgnal wth flterng usng functons rcanchan and rayleghchan. But for usng these MAT- LAB functons t s necessary to use a flter, whch s also n ths case, wth low samplng very slow. Moreover, these functons do not enter the phase shft of paths, whch s defned n the standard [6]. Therefore, the modelng of channel 74 2

3 Fg. 1: Man wndow of the created MATLAB applcaton for the DVB-T/H. envronments wth fadng that was used s a custom algorthm based on equatons (1), (2) and (3). 4 MATLAB Applcaton Descrpton Man wndow of the created applcaton n MATLAB for the smulaton of complete transmsson n standard DVB-T/H can be seen n Fg. 1. Before the smulaton, user must defne the type of nput sequence. The created applcaton allows selectng between two optons. In the frst case the user can enter the number of bts for generatng nput bt sequence. When checkbox Pcture s actve, that presented the second opton. In ths case DVBH.bmp pcture s loaded and converted nto a sequence of ones and zeros. The Code Rate opton allows control of the error protecton of the transmtted data. User cans choce before a smulaton between these code rates (accordng to DVB-T/H specfcaton): 1/2 (no puncturng), 2/3, 3/4, 5/6 and 7/8 (mnmum error protecton). The Type of Modulaton and OFDM mode optons enable choose of modulaton. DVB-T/H standard defnes tree type of modulatons: QPSK, 16QAM and 64QAM. User can set the followng OFDM modes: 2k, 4k (DVB-H) and 8k. The opton Guard Interval enables set values as are defned n [2]: 1/4 (longest), 1/8, 1/16 and 1/32 (shortest). It s vsble n the Fg. 1 that n table Settngs of SNR t s gven the value of the Sgnal-to-Nose Rato n db. The table Type of Channel gves a choce for a user selected between the Gaussan (AWGN) and Multpath channel. The button Path settngs enables set up dfferent types of fadng channels. Opton Decson of Vterb Decoder allows choosng of the type of decson n Vterb decodng process. Here exst two cases: soft decson and hard decson. The table Results of smulaton shows the MER, BER before (pre) and after (post) Vterb decodng. The outputs of each smulaton are also the constellaton dagrams. Applcaton shows the receved and corrected constellaton dagram. Of course, user can show these dagrams n separate wndows. For ths step serves buttons n the table Plot Settngs. 5 Parameters of the smulaton For the smulaton of the DVB-T/H transmsson was used the followng settngs: mode: 2k (used n moble recepton) code rato: 1/2 (presents robust transmsson) nner modulaton: QPSK, 16QAM, 64QAM guard nterval: 1/32 (used n small DVB-H networks) transmsson channel: Gaussan (AWGN), Rcean (RC 2 ANX B), Raylegh (RL 2 ANX B) K-Factor: 1 (factor used n Rcean channels) Vterb decodng decson: soft. 74 3

4 1,E+ QPSK - Gaussan QPSK - RC 2 ANX B QPSK - RL 2 ANX B 1,E+ QPSK - RL 2 ANX B 1,E-5 1,E-5 Fg. 2: Bt error rato BER before Vterb decodng as a functon channels RC 2 ANX B, RL 2 ANX B (mode 2k nner modulaton QPSK). Fg. 5: Bt error rato BER after Vterb decodng as a functon channels RC 2 ANX B, RL 2 ANX B (mode 2k nner modulaton QPSK). 1,E+ 16QAM - Gaussan 16QAM - RC 2 ANX B 16QAM - RL 2 ANX B 1,E+ 16QAM - Gaussan 16QAM - RC 2 ANX B 16QAM - RL 2 ANX B 1,E-5 1,E-5 Fg. 3: Bt error rato BER before Vterb decodng as a functon channels RC 2 ANX B, RL 2 ANX B (mode 2k nner modulaton 16QAM). Fg. 6: Bt error rato BER after Vterb decodng as a functon channels RC 2 ANX B, RL 2 ANX B (mode 2k nner modulaton 16QAM). 1,E+ 64QAM - Gaussan 64QAM - RC 2 ANX B 64QAM - RL 2 ANX B 1,E+ 64QAM - Gaussan 64QAM - RC 2 ANX B 64QAM - RL 2 ANX B 1,E-5 1,E-5 Fg. 4: Bt error rato BER before Vterb decodng as a functon channels RC 2 ANX B, RL 2 ANX B (mode 2k nner modulaton 64QAM). Fg. 7: Bt error rato BER after Vterb decodng as a functon channels RC 2 ANX B, RL 2 ANX B (mode 2k nner modulaton 64QAM). 74 4

5 a) b) c) d) e) f) g) h) ) Fg. 8: Transmtted, receved and corrected constellaton dagrams for dfferent type of modulatons used n DVB-T/H: a) to c) QPSK n the Gaussan channel, d) to f) 16QAM n the Rcean channel, g) to ) 64QAM n the Raylegh channel (carrer-to-nose rato C/N = 2 db n all smulatons). 6 Smulaton results Transmsson of 1 kbts of the data was smulated for varyng C/N rato n the Gaussan channel (AWGN) and n the fadng channels Rcean (type RC 2 ANX B) and Raylegh channel (type RL 2 ANX B). The QEF (Quas Error-Free) operaton s defned as a bt error rate after Vterb decodng less or equal to [8]. Then the BER rato after Reed-Solomon decodng s less or equal to The Fg. 2 to Fg. 4 llustrate BER before Vterb decodng n Gaussan and two fadng channel models RC and RL. BER after Vterb decodng for all tested communcaton channel are llustrate n the Fg. 5. to Fg. 7. Fg. 5 shows the dependence of the BER after Vterb decodng on C/N rato n case of the QPSK modulaton. How t can be seen n the Fg. 2, that there were obtaned three BER curves: one for the Gaussan channel, one for the Rcean channel and one for the Raylegh channel. But after Vterb decodng, just one BER curve has been obtaned only for the Raylegh channel (Fg. 5.). Ths s because the modulaton QPSK s the most robustness to fadngs. The second reason s that the DVB-H uses advanced RS encoder. It can be seen that for QEF n Raylegh channel and 2k mode, mn. 4 db s needed. Results of the smulaton when t was used 16-QAM modulaton for the transmsson va Gaussan, Rcean and Raylegh channel are show Fg. 3 and Fg. 6. It s vsble n Fg. 3, as well as at nfluence of nose n all tested channel models, that lower number of states n multstate modulatons (n ths case 16-QAM) lead to a lower BER. That means that the hgher number of states n multstate modulatons are needed for hgher achevement - hgher C/N for smaller BER. Accordng to the results of the smulatons t s possble to conclude, that n case of Rcean channel and QEF t s needed to ncrease value of C/N by 1dB n comparson wth values n the Gaussan channel (see Fg. 5). In Raylegh channel there s needed ncrease of C/N by 4.2 db. 74 5

6 MER [db] QPSK - Gaussan QPSK - RC 2 ANX B QPSK - RL 2 ANX B Fg. 9: Modulaton error rato MER as a functon of C/N rato n the Gaussan channel and n the fadng channels RC 2 ANX B, RL 2 ANX B (mode 2k nner modulaton QPSK). MER [db] QAM - Gaussan 16QAM - RC 2 ANX B 16QAM - RL 2 ANX B worst. Therefore for achevng the QEF n Rcean channel t s needed 13 db and 16.4 db for the Raylegh channel. The transmtted, receved and corrected constellaton dagrams of all type of modulatons are shown n Fg. 7. The frst lne represented the case, when we used QPSK modulaton and the channel has only Gaussan feature. How t can see, the equalzaton, appled on the receved sgnal, has not any effect. The constellaton dagrams of modulaton 16QAM n Rcean channel are shown n the second lne n Fg. 7. The receved sgnal has many noses, but appled the equalzaton, t can be corrected. The reason of ths s that n the Rcean channel already exts one drect path between the transmtted and receved sgnal and that the modulaton 16QAM s very robust aganst fadngs. In the thrd lne n Fg. 7 are shown the constellaton dagrams of 64QAM modulaton before and after equalzaton n Raylegh transmsson channel. The correctons of the receved sgnal n ths case are the hardest. Ths s because that the Raylegh channel conssts of only ndrect paths and 64QAM modulaton has the smallest resstance to fadngs. The MER s a measure of the sum of all nterference effects occurrng on the transmsson lnk. Lke the C/N rato, t s usually specfed n db [8]. Influence of the fadng on MER n all mentoned modulatons s evdent (see Fg. 9 to Fg. 11). A lesser C/N rato leads to a lower MER n db. 7 Acknowledgement 5 Fg. 1: Modulaton error rato MER as a functon of C/N rato n the Gaussan channel and n the fadng channels RC 2 ANX B, RL 2 ANX B (mode 2k nner modulaton 16QAM) QAM - Gaussan 64QAM - RC 2 ANX B 64QAM - RL 2 ANX B Ths contrbuton was supported by the grant project of Research programme of Brno Unversty of Technology MSM Electronc Communcaton Systems and New Generaton Technology (ELKOM) and grant projects of the Czech Scence Foundaton no. 12/8/H27 Advanced methods, structures and components of the electronc wreless communcaton, no. 12/8/P295 Analyss and Smulaton of the Transmsson Dstortons of the Dgtal Televson DVB-T/H and no. 12/1/132 Research and modelng of advanced methods of mage qualty evaluaton. 8 References MER [db] Fg. 11: Modulaton error rato MER as a functon of C/N rato n the Gaussan channel and n the fadng channels RC 2 ANX B, RL 2 ANX B (mode 2k nner modulaton 64QAM). The same results, but n case of 64-QAM modulaton n all mentoned channel envronments, are shown n Fg. 3 and Fg. 6. When t was used 64-QAM modulaton for the transmsson than the data stream, whch s transferred, are the largest. But, the resstances of transmtted data n fadng channels are [1] BORKO, A., SYED, A., Handbook of Moble Broadcastng, DVB-H, DMB, ISDB-T and MEDIAFLO. Taylor & Francs Group, LCC, 28, 726 p., ISBN [2] ETSI, Techncal Specfcaton 32 34, Dgtal Vdeo Broadcastng (DVB); Transmsson System for Handheld Termnals (DVB-H), V1.1.1, 24. Avalable on: < 34/1.1.1_6/en_3234v111p.pdf> [3] POLAK, L., KRATOCHVIL, T., Transmsson of the DVB-H n Moble TV Mode over Fadng Channels, n the 33 rd Internatonal Conference on Telecommuncaton and Sgnal Processng TSP 21, Baden near Venna, Austra, pp

7 [4] KORNFELD, M., REIMERS, U., DVB-H the emergng standard for moble data communcaton. In EBU Techncal Revew, vol. III, 25, pp Avalable on: < [5] FISHER, W., Dgtal Vdeo and Audo Broadcastng Technology. A Practcal Gude for Engneers. 2 nd ed. Berln: Sprnger Verlag, 28, 586 p., ISBN [6] KRATOCHVIL, T., ŠTUKAVEC, R., DVB-T Dgtal Terrestral Televson Transmsson over Fadng Channels. Radoengneerng. 25, vol. 17, no. 3, pp [7] POLAK, L., Smulaton of Multpath Propagaton of Multstate Modulatons, n Proceedngs of the 17 th Conference Techncal Computng Prague 29. Prague, 29, pp.1-6. [8] PÄTZOLD, M., Moble fadng Channels. USA, John Wlley & Sons, 22, 43 p., ISBN