High performance OFDM systems for digital video broadcasting-terrestrial (DVB-T)

Transcription

1 High performance OFDM systems for digital video broadcasting-terrestrial (DVB-T) Mr Y. A. LAFTA 1 and Dr. P. JOHNSON 2 School of engineering, Liverpool John Moores University, Byrom Street, Liverpool, L3 3AF, UK Y.Lafta@28.ljmu.ac.uk 1 Introduction Abstract Digital wireless communication has become one of the most exciting research topics in the electrical and electronic engineering field due to the explosive demands for high-speed wireless services, such as cellular video conferencing. Digital video broadcasting-terrestrialsecond generation (DVB-T2) has been demonstrated to provide services with very high spectral efficiency and improved performance. Also, OFDM systems have been deployed in mobile networks for their spectral efficiency and optimum bit error rate. Among the OFDM systems, wavelet based systems have been demonstrated to have improved bandwidth and channel performance. In this paper the authors demonstrate that very high spectral efficiency, BER and PAPR can be achieved by employing DWT-DAPSK scheme with the DVTB-T2 system. It is demonstrated in this paper that including companding with this system results in further reduction of PAPR. Correspondence should be addressed to YHYA LAFTA on Y.Lafta@28.ljmu.ac.uk 1.1 Orthogonal Frequency Division Modulation (OFDM) Systems The layered transmission technology that is used to support the multi-service in DVB-T (Digital Video Broadcasting- Terrestrial) with different priorities is not suitable for mobile applications because of its low mobility and high-power consumption. DVB-H (Digital Video Broadcasting-Handheld) system addresses those issues, which is especially developed for mobile services and provides highspeed. Recent research investigations identify the OFDM to be suitable for the upcoming DVB-H system and wireless communication systems due to its excellent robustness to frequency-selective fading channels [4] - [6]. The OFDM scheme provides an efficient means to handle high-speed data streams. This is because the OFDM system uses multi-carrier modulation (MCM) technique which divides the entire data stream into several small numbers of lower data-rate subcarrier data streams, thus reducing the frequency selective fading [15]. Service providers working towards 4G communication systems are continuously 66

2 met with the challenge of accommodating more and more users within a limited available bandwidth [8]. The main advantage of the OFDM systems is that they are immune to multi path fading [11]. However, their major disadvantage is that the transmitted signal has a high peak-toaverage power ratio (PAPR). Many research investigations have led to the conclusion that the wavelet based OFDM is more advantageous than the Fourier based OFDM [1][3] [21]. One of the main reasons being Fourier based OFDM systems require allocation of guard bands between subcarrier frequencies to avoid inter symbol interference, however wavelet based OFDM systems do not require such guard bands and therefore the overall system bandwidth is increased. The discrete wavelet transform (DWT) is represented by a function of a countable set of wavelet coefficients, i.e. individual wavelet functions localized in space [28]. The wavelets are a family of functions constructed from the translation and dilation of signal function ψ called the mother wavelet. Due to an extremely high spectral containment property of the wavelet filters and the fact that extremely less energy is contained in the side lobes, the amount of interference between carriers in wavelet systems is much lower than that in Fourier systems [21]. Therefore, even in the absence of guard bands, DWT-OFDM can combat narrowband interference better than the traditional discrete Fourier transform systems and is inherently more robust in terms of inter carrier interference (ICI) which is the most detrimental feature in DFT-OFDM systems [2]. This research work proposes a combination of DWT with DAPSK modulation scheme in anticipation that this system should have reduced PAPR compared to the other Fourier based OFDM systems. A set of cosinusoidal functions cos(2πnf_ t), n=,1,.., N-1 representing the signal can be used on orthogonal basis to achieve the multi carrier modulation (MCM) scheme that can be synthesized using a discrete cosine transform (DCT) [23]. The minimum frequency spacing_ required to satisfy the DCT based OFDM is 1 2T Hz [25]. The baseband DCT-OFDM signal x (t) is still a real value when the data symbols d_n are obtained using real valued modulation formats, such as pulse amplitude modulation (PAM) and binary phase shift keying ( BPSK). DVB-T could be used to provide portable and mobile services over the 8 MHz channel at the date rate ranging from 4.98 to Mb/s, as well as support indoor and outdoor fixed reception. 1.2 Organization of the paper The organization of this paper is as follows: section 1.3 reviews the QAM modulation scheme and 1.4 reviews related work in the area of DCT (DWT) OFDM systems with QAM modulation for high data rate next generation communication networks. Finally, section 2 presents the simulation and the results of the proposed DWT system with DAPSK and QAM modulation schemes. The performance from the system is evaluated against metrics such as Eb/No, ISI and PAPR and compared with that of DCT system. 1.3 Modulation Schemes Quadrature amplitude modulation (QAM) is more attractive for the highly 67

3 mobile radio environment (e.g., in a car or a train), which is a high-order and noncoherent modulation scheme, employed as a potential means to give further improvement to the spectral efficiency. QAM scheme does not require complicated channel estimation and equalization to be detected by the receiver. Use of pilot symbol can be avoided to more efficiently transmit the informationbearing data. Moreover, the non-coherent modulation can preferably withstand the Doppler Effect caused by the high mobility between the transmitter and receiver [18]. The 16 QAM has a more efficient signal distribution than the 16 DAPSK, in terms of the relative power saving in an AWGN channel [22]. Since highly accurate carrier recovery and phase tracking are required in the receiver, applying coherent detection in the mobile radio environment is a very difficult task. It is also important to accurately track the fading envelope for systems such as 16 QAM. The coded bits are mapped to form symbols which use gray coding in the constellation map. The coding rate is (1/2) for the 16QAM modulation scheme. When the symbol is normalized, the average power is unity irrespective of the modulation scheme used. The data are converted by the process to corresponding value of M-ary constellation, which is a complex word, constituting real and imaginary parts. The bandwidth B =1/ is divided into N equally spaced subcarriers at frequencies (kδf), k=, 1, 2 N-1 with Δf=B/N and as the sampling interval. Grouping and mapping information bits into complex symbols will be carried out at the transmitter. Inverse wavelet Transform IDWT or inverse discrete cosine IDCT will be used to modulate spread data symbol on the orthogonal carriers, as in conventional OFDM. 1.4 Related work As a candidate for next-generation digital television terrestrial broadcasting systems (DTTB) and the 4 th generation mobile receivers, the OFDM systems have attracted considerable research attention. The OFDM system has been successfully implemented by [12] [17] and [13] in the wireless broadcasting applications such as Digital Video Broadcasting (DVB) and Digital Audio Broadcasting (DAB). The above mentioned systems use data rates in the range of 4Mbits/s and with DTTB system could deliver 4 GB contents to unlimited number of local servers. The ETSI-BRAN system employs this transmission technique for HIPERLAN 2 (high performance local area networks), and also for the extension of the IEEE standard for the 5-GHz frequency range. A DVB-T system supports networks covering portable mobile receivers in buses, cars, and receivers indoor [14] [19]. The digital dividend has been designated such as a matter of controversy between broadcasters and mobile communications operators which occupy part of it as desired [9]. Future mobile communication systems with data rates far beyond the Universal Mobile Telecommunication system (UMTS) can be realized using OFDM as a suitable transmission technique. 2 Simulation and Analysis In this section, the simulations results for the DWT and DCT-OFDM systems with 64DAPSK/64QAM modulation schemes and for two different input types are presented. The input types considered 68

4 for the simulations are uncoded random binary system and coded signal DVB-T. This simulation also considered AWGN and Rayleigh transmission channels within the system. The results are presented in figures 1 to 4. The simulation parameters for the number of sub carriers N is 64, and the number of samples for the symbols is 1. If the number of the sub carriers and symbols were larger, the time for running the simulations will be longer. The simulation has been run for a larger set of system parameters and no significant difference to the results was noticed. The figures 1a to 1c represent the power spectral density of the received signal for DWT (and DCT)-OFDM systems with 64QAM modulation scheme using AWGN channel with Gaussian spectrum and Rayleigh channel with Jakes spectrum. higher signal power than that for the DCT based OFDM system using the same modulation scheme. It is also clearly seen that the power spectral density (PSD) for DWT-OFDM is 2-4dB/rad/sample better than the DCT-OFDM system when using the 64QAM scheme modulation via AWGN transmission channel type. This huge difference in PSD between the received signals which were transmitted with the same input power leads to the observation that DWT system has much better signal to noise ratio. Whereas, the DWT-OFDM system with 64QAM using Rayleigh with Jakes spectrum demonstrates to have higher bandwidth efficiency when compared to the DCT OFDM system under exactly the same conditions. The DCT-OFDM seems to have better than PSD signal using the AWGN and Rayleigh with Gaussian spectrum. This means that for binary input stream, the spectrum for this system will have smaller side-lobes and so will be immune to inter-symbol interference (ISI). Fig1a Power spectral densities for DWT and DCT systems with 64QAM modulation schemes via A Fig1b Power spectral densities for DWT and DCT systems with 64QAM modulation schemes via Rayleigh (Gaussian) channel The simulation results indicate that the DWT-OFDM with 64QAM scheme has Fig1d Power spectral densities for DWT system with 64DAPSK modulation schemes via AWGN channel The figures 1d to 1f represent the power spectral density of the received signal for DWT -OFDM systems with 64DAPSK modulation scheme using AWGN or Rayleigh channels with Gaussian and Jakes spectrum respectively. The DWT- OFDM system with 64DAPSK has 75dB 69

5 higher PSD than that of the DWT-OFDM system with 64QAM scheme, though with less bandwidth efficiency. using Rayleigh Channel and AWGN channels, the DCT-64QAM is shown to have at least 15dB lower PSD with exponential transform when compared to that with or without companding. Fig1e Power spectral densities for DWT system with 64DAPSK modulation schemes via Rayleigh (Jakes) channel The PSD of the DWT and DCT OFDM systems were also simulated with and without -law companding, and exponential transforms. The results are shown in figures 2a and 2b. Figure 2a representing the DWT-OFDM system with 64DAPSK scheme using Rayleigh Channel shows that the signal with nocompanding suffers from out-of-band interference (OBI) compared to the signal that has gone through companding or exponential transforms. Fig.2b PSD for DWT with 64DAPSK channel AWGN Fig.3 PSD for DCT with 64QAM Rayleigh s channel (Jakes and Gaussian) and AWGN Fig.2a PSD for DWT with 64DAPSK channel Raleigh ( Jakes and Gaussian) The DWT-64DAPSK system in Fig.2a using Rayleigh channel with Jakes or Gaussian spectrum is shown to have at least 32dB lower PSD with or without companding when compared to that with exponential transform. Fig.3 represents DCT-OFDM system with 64QAM scheme Fig.4 CCDF for DWT/DCT with 64DAPSK/64QAM for no companding, companding and exponential signals In figure 4, the continuous line graphs represent the complementary cumulative distribution (CCDF) vs. PAPR performance for DWT systems, whereas the dashed lines represent the same for DCT systems. It can be observed from the 7

6 Power/frequency (db/hz) Power/frequency (db/hz) Power/frequency (db/hz) Power/frequency (db/hz) Power/frequency (db/hz) International Journal of Digital Information and Wireless Communications (IJDIWC) 2(1): graph that both DCT and DWT systems with companding have at least 1.9 db less PAPR than the systems without companding and the systems with exponential transform have 1.3 db less PAPR than that without any transform When the input data is changed to that of DVB-T, created by using a trellis code with length = 256 and Fs=1, the DWT based systems seem to possess better bandwidth efficiency and PAPR than the DCT based systems. This effect is depicted in the simulation results shown in figure 5a to 5d Fig.5b Power spectral densities for DCT systems with 64QAM modulation via AWGN using trellis code Fig.5a Power spectral densities for DWT systems with 64QAM modulation via AWGN using trellis code Fig.5c Power spectral densities for DWT systems with 64DAPSK modulation via AWGN using trellis code Fig.5b Power spectral densities for DCT systems with 64QAM modulation via AWGN using trellis code Fig.5d Power spectral densities for DCT systems with 64DAPSK modulation via AWGN using trellis code 71

7 PSD(db/rad/sample) PSD(db/rad/sample) PSD(db/rad/sample) International Journal of Digital Information and Wireless Communications (IJDIWC) 2(1): The figures 5a to 5d, depict the power spectral density of the received signal for DWT (and DCT)-OFDM systems with 64DAPSK and 64QAM modulation schemes using AWGN channel transmission for DVB-T input data types. From figures 5a and 5b, it can be seen that the DWT-64QAM system occupies less than half the bandwidth (369Hz) of the DCT-64QAM system (8Hz), while having a superior power spectral density per subband of 65dB/Hz compared to the negligible 4.5 db/hz for DCT. This means that the DWT system presents at least 7 times better BW efficiency than the DCT system. This high BW efficiency ensures immunity to inter-carrier interference (ICI) in the receiver but the DCT has very small side lobs than the DWT system, which means immunity to inter-symbol interference (ISI). From figures 5c and 5d, it can be seen that the DWT-64DAPSK system occupies less than 2.69 times the bandwidth (7Hz) the DCT-64DAPSK system (26Hz). The power spectral density per subband of DWT-64DAPSK is 48dB/Hz and also of DCT-64DAPSK is 88dB/Hz. Therefore, the BW efficiency of the DWT system is 3 times better the DCT system with their DAPSK modulation. This means the DWT system possesses two times better BW efficiency than the DCT system. This high BW efficiency ensures immunity to inter-carrier interference (ICI) in the receiver but the DWT also has very small sidelobs in its spectral density than DCT. This means that the DWT system has better immunity to inter-symbol-interference (ISI) as well PSD of orginal & companded signals Normalized frequency(pi rad/sample) Fig.6a PSD for DWT with QAM via AWGN using trellis code Fig.6b PSD for DCT with QAM via AWGN using trellis code PSD of orginal & companded signals Fig.6cPSD for DCT with 64DAPSK via AWGN using trellis code Figures 6a through 6c compare the BW efficiency performance of the DWT & DCT systems with and without Orginal Proposed Exponential Normalized frequency(pi rad/sample) PSD of orginal & companded signals Normalized frequency(pi rad/sample) Orginal Proposed Exponential No-Companding Companding Exponential 72

8 companding to that of with exponential transform. In figures 6a and 6b, it is shown that DWT-QAM system with exponential transform has 2 times BW efficiency compared with the same system but with no companding. It also has 5 times BW efficiency compared to the system having companding. However, the performance of the DCT with QAM modulation is not as good as the DWT system. It shows 3 times better BW efficiency difference from both nocompanding & companding processes. In fig.6c, it is shown DCT-DAPSK with exponential transform has 4 times BW efficiency compared companding and no companding processes. 3 Conclusions The BW efficiency of the DWT-OFDM system with both 64QAM and 64DAPSK modulation schemes via AWGN transmission channel is shown to be better than the DCT-OFDM system under similar conditions. The DWT system with QAM shows 7 times better BW efficiency than the DCT with QAM modulation. The DWT with DAPSK modulation has two times better BW efficiency than the DCT system with DAPSK. The better BW efficiency possessed by the DWT with DAPSK is an indicator of the system s immunity to inter-carrier interference (ICI) which in turn guarantees high bitrate, high quality data transmission. The DWT system also displays minimal side-lobes which contribute to its immunity to intersymbol interference (ISI). Furthermore, a considerable reduction in PAPR is obtained through companding of the signal. For example, from the simulation results shown above, a reduction of PAPR by 1.9 db has been achieved by the DWT system compared to without companding. With the exponential transform, a reduction of 1.3 db in PAPR has been demonstrated. Hence, in this paper it has been demonstrated that the DWT-OFDM system with 64DAPSK modulation scheme is very suitable for the DVB-T system. References 1. Abdullah, K., and Hussain, Z. M. (27) Performance of Fourier-Based and Wavelet-Based OFDM for DVB-T Systems, Australasian Telecommunication Networks and Applications Conference, pp Ahmed, N. (2) Joint Detection Strategies for Orthogonal Frequency Division Multiplexing, Master thesis, Houston, Texas. 3. Akansu, A.N.,and Xueming, L. (1998) A comparative performance evaluation of DMT (OFDM) and DWMT (DSBMT) based DSL communications systems for single and multitone interference, Proceedings of the IEEE International Conference on Acoustics, Speech and Signal Processing. 4. C. yen Ong et al., (21), Technology and Standards of Digital Television Terrestrial Multimedia Broadcasting System (DVB-T2). ETSI Standard, EN , IEEE Commun. Mag., vol. 48, no. 5, pp Digital Video Broadcasting (DVB), (212), Frame Structure, Channel Coding and Modulation for a Second Generation Digital Terrestrial Television Broadcasting System (DVB-T2), EN , V Error-Correction, (21), Data Framing, Modulation and Estimation Methods for Second Generation Digital Terrestrial Television Broadcasting System Recommendation ITU-R BT, ETSI ES v1.2.2 (23-4) ETSI standard: digital radio mondiale (DRM); system specification. 8. Gupta, D., Vats, B. V., Garg, K. (28) Performance Analysis of DFT-OFDM, DCT-OFDM, and DWT- OFDM Systems in AWGN Channel, IEEE, The Fourth International Conference on Wireless and Mobile Communications, pp Kalogirou, V. P., Nanou, E. D., Capsalis, N. C., Velivasaki, T-H. N., Capsalis, C. N. (29), Compatibility of DVB-T Services and IMT-2 Compliant Mobile Telecommunications in the UHF 73

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