Soft Cyclic Delay Diversity and its Performance for DVB-T in Ricean Channels

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2 Soft Delay Diversity and its Performance for DVB-T in Ricean Channels Armin Dammann, Ronald Raulefs and Simon Plass German Aerospace Center (DLR) Institute of Communications and Navigation Oberpfaffenhofen, Wessling, Germany Abstract delay diversity (CDD) provides additional diversity in Rayleigh fading channels, and therefore, improves the system performance. For line-of-sight (LOS) propagation, e.g., the additive white Gaussian noise channel, the implementation of CDD yields to a performance loss. The power distribution among the transmit (TX) antenna branches is a further parameter which can freely be chosen for optimizing the system performance and allows to switch on/off CDD softly. The idea is to feed different power levels into the multiple TX antenna branches rather than distributing the TX power uniformly among the TX antennas. We exemplarily implement the soft CDD principle to the terrestrial digital video broadcasting system (DVB-T). We consider a Ricean multipath fading channel, which allows to control the ratio of LOS and non-los propagation power via the Ricean factor for simulations. Simulation results for 2-TX and 4-TX antenna CDD show that antenna power weighting significantly reduces the SNR loss in LOS propagation by the cost of a slight degradation of the SNR gain in non-los scenarios. I. INTRODUCTION Multiple antenna transmission schemes have gained a high attraction since they offer a capacity which rises proportional to the minimum of the number of transmit (TX) and receive (RX) antennas from information theory point of view. In the recent years, several approaches have been proposed, which take advantage of multiple TX- respectively RX-antennas. One representative of multiple antenna schemes is lic delay diversity (CDD) []. CDD is a variant of delay diversity (DD) [2] and adapted to communications systems with lic extensions as guard intervals such as orthogonal frequency division multiplexing (OFDM) for instance. Signal delays in DD may cause intersymbol interference. In contrast, CDD prevents such additional intersymbol interference by using lic signal shifts. Typically, multi TX/RX-antenna techniques like Space-Time coding [3], [4] require signal processing in both the transmitter and the receiver. However, CDD as well as DD can be implemented solely at the transmitter, the receiver or both sides. The fact that the counterpart e.g. the RX in case of a TX sided implementation needs not to be aware of the implementation makes these techniques standard compatible. I.e. they can be implemented as an extension for already existing systems without changing the standard. However, applying CDD as multiple antenna scheme does not increase the channel capacity in information theoretic sense. CDD has extensively been investigated for Rayleigh fading channels. It can be shown that CDD transforms the multipleinput/single-output (MISO) channel into an equivalent SISO channel. This transformation increases the number of propagation paths and the frequency selectivity of the channel, which improves the system performance in multipath Rayleigh fading scenarios. Non-LOS propagation is most likely for cellular wireless communications systems in many environments and scenarios. The situation completely changes if we focus on broadcast systems like DVB-T. Here the transmit antenna is located on a high mast and for fixed reception often rooftop mounted antennas as legacy equipment from analogue TV are used for reception. Therefore, we typically observe strong LOS propagation. Line-of sight e.g. the additive white Gaussian noise (AWGN) channel as an extremal representative of such a channel would be transformed into a static multipath channel by the use of CDD, which definitely decreases performance. Therefore, it is of high interest to investigate the performance of CDD for propagation environments where both LOS and non-los (Rayleigh) components with different weighting occur. For this reason we use a Ricean multipath propagation channel for our investigations, where its Ricean factor describes the power ratio of LOS and non-los propagation paths. We implement CDD for the terrestrial digital video broadcasting standard (DVB-T) [5] and investigate the bit error performance in Ricean multipath channels with different Ricean factors. To combat the system degradation of CDD for LOS propagation we propose and show the performance of soft lic delay diversity, which is a method to softly control the additional amount of diversity introduced to the system. II. SYSTEM DESCRIPTION Compared to wireless communications systems, LOS propagation in TV broadcasting is more distinct. Transmit antennas are typically located on high masts and for fixed reception users often install rooftop antennas. Therefore, the propagation conditions for TV broadcasting cover a large variety of scenarios, described by multipath Rayleigh fading channel models with non-los, Ricean fading channels (mixed LOS and non- LOS) and, as pure LOS propagation, the AWGN channel. Subsequently, we show the application of CDD to DVB-T and define the channel model, which is used for our investigations.

3 CDD Extension Pilot & TPS Signals COD CC(7,33) MOD Frame Adaptation IFFT 2 Inner DVB-T Transmission System (a) Transmitter with 2-TX-antenna CDD frontend remove FFT Pilot Symbols Deframing CE Data Symbols CSI DEMOD DECOD CC(7,33) (b) Receiver Fig.. Block diagram of the inner DVB-T system A. DVB-T and Delay Diversity The physical layer of a DVB-T transmitter comprises three main parts, which are (i) MPEG-2 source coding and multiplexing, (ii) outer coding with interleaving and (iii) inner coding, interleaving, framing and modulation. The DVB-T standard defines a target bit error rate (BER) of 2 4 after decoding of the inner channel code, which yields to a quasi error free data stream after decoding of the outer Reed-Solomon code. Therefore, we are interested in the inner DVB-T and model the data stream at the input of the inner system as pseudo random binary sequence. Figure (a) shows the block diagram of a DVB-T transmitter with 2-TX-antenna CDD. The binary data is encoded, using a convolutional code with generator polynomials (7, 33) 8. The mother code of rate R = /2 may be punctured in order to achieve higher code rates. After interleaving and modulation, the complex valued data symbols together with scattered pilot symbols, continuous pilot carriers (CPC) and transmission parameter signalling (TPS) data are arranged in an OFDM frame, which consists of 68 OFDM symbols. The OFDM symbols are transformed into a time domain signal by an inverse fast Fourier transformation (IFFT). The time domain signal is normalized and split into the TX-antenna branches in such a way, that the overall transmitted power is independent of the number of TX-antennas. In each TX-antenna branch the signal is shifted lically by δ i before the guard interval as lic prefix is added. Considering one OFDM symbol, the antenna specific TX signals are s i (k) = NT s(k δ i mod ) N FFT = NT l= e j2π δ l i S(l) e j2π k l () For the time interval k = N G,..., we get the OFDM symbol together with the lic prefix. S(l) are the complex valued frequency domain symbols, carrying data or pilots. The receiver is shown in Fig. (b). First, the guard interval is removed from the received time domain baseband signal r(k) = T i= max m= h i (m) s i (k m) + n(k). (2) n(k) denotes complex valued AWGN with variance σ 2 and N max is the maximum channel delay spread. The remaining OFDM time domain symbol is transformed into frequency domain by an FFT, which yields to N FFT R(l) = r(k) e j2π k l NFFT k= N T = S(l) H i (l) e j2π δ NT i= l i }{{} = H(l) +N(l). (3) with H i (l) = k= h i (k) e j2π k l and the AWGN term N(l) again with variance σ 2. Eq. (3) shows that CDD can be described as an equivalent channel transfer function H(l). For this reason, a receiver cannot distinguish whether a propagation path results from CDD or the channel itself. The deframing unit separates data- and pilot symbols. From the pilots the complex valued channel fading coefficients for each subcarrier are estimated. With this estimation and the received data symbols, the demodulator provides soft information in form of log-likelihood (LL) ratios by applying the MAX-Log- MAP algorithm [6]. The deinterleaved LL values are used as soft input for the Viterbi algorithm [7], which provides a maximum likelihood estimation of the information bits. Table I shows possible coding and modulation parameters according to the DVB-T standard.

4 TABLE I MAIN DVB-T CODING AND MODULATION PARAMETERS Parameter Specified Values FFT length 248 (2k), 892 (8k) Rel. guard interval lengths N G / /4, /8, /6, /32 Inner conv. code rates R /2, 2/3, 3/4, 5/6, 7/8 Modulation 4-, 6-, 64-QAM H 2 - Power [db] Fig db db -4.3 db -6.5 db -3 db -5.2 db -2.7 db Delay [ns] B. Channel Model Power-delay profile of the Indoor Commercial Channel B For our investigations, we use Indoor Commercial Channel B model as defined in [8]. This 7-path multipath Rayleigh fading channel models large open centers, such as shopping malls and airports. Its power-delay profile is shown in Fig. 2. The fading processes for the several propagation paths are statistically independent, where the Doppler spectrum is uniform (rectangular) with bandwidths in the range of f Dmax = Hz. For multi TX antenna configurations we assume statistical independence of the channel fading processes from the different TX antennas to the RX antenna as well. Subsequently, we are interested in propagation scenarios, which contain LOS, therefore we define an additional propagation path at delay zero with power h 2. The average channel impulse response power of the resulting multipath Ricean fading channel model is normalized to one, i.e., P h 2 + E{ h p 2 } =!, (4) p= where E{ h p 2 } is the average power of the fading process h p for propagation path p. We define the power ratio of LOS and non-los propagation paths as the Ricean factor K = log h 2 (4) h 2 NP = log p= E{ h p 2 } h 2 [db] (5) Note that K = results in the original indoor Rayleigh fading model (Fig. 2), whereas K = + yields to an AWGN channel. III. SOFT CYCLIC DELAY DIVERSITY LOS propagation is a severe problem for CDD since the constant (LOS) paths of the channel are transformed into a -2 -TX 2-TX, P = db 2-TX, P = 3dB 2-TX, P = 6dB 2-TX, P = db 2-TX, P = 2dB -3 2 Frequency [MHz] Fig. 3. Equivalent CTF H(f) 2 for pure LOS (AWGN), 2-TX CDD, δ =.9 µs ( samples) IFFT Fig. 4. N T Front end of a generic OFDM Transmitter Delay Diversity Extension NT Principle of antenna power weighting sn ( ) T k s ( k) s ( k) static frequency selective one. Using 2-TX-antenna CDD, for instance, transforms an AWGN channel with CTF H i (l) = into an equivalent channel with an absolute square CTF H(l) 2 = + cos(2π δ f/ ) according to (3). This is depicted in Fig. 3 for δ =.9 µs. We can clearly observe deep fades (see graph for ), which degrade the system performance compared to the -TX antenna case. The reason for these deep fades is the equal power distribution among the TX-antennas. A solution to overcome this problem is to weight the signal at each TX-antenna branch by different factors α i. The implementation principle is shown in Fig. 4. To keep the transmitted power independent of the number of TX-antennas yields to the normalization T i= E{ α i 2 }! =. (6) First of all, the implementation shown in Fig. 4 allows a flexible allocation of power to the different TX-antenna branches with several degrees of freedom. In order to describe the power distribution by one parameter, we define P = log α 2 (N T ) α 2 [db] (7) as the TX power ratio between the first TX antenna and the average power of the CDD extension, i.e., TX-antennas

5 ... N T. The parameter P allows to switch on/off CDD softly. Therefore, we call this principle soft CDD. For 2-TX antenna CDD definition (7) provides a unique description of the power distribution among the antennas. In this case the equivalent CTF for pure LOS (AWGN) is H(l) 2 = + 2 P lin + P lin cos ( 2π δ f ), (8) which is shown in Fig. 3 for δ =.9 µs and different TX antenna power ratios. P lin = P is the linear representation of P. For more than 2 TX antennas the power distribution within the CDD extension is a further degree of freedom. Its optimization is not in the scope of this paper. Here, we use a uniform distribution, i.e., E{ α 2 } = E{ α 2 2 } =... = E{ α NT 2 } for the first approach. In this case, P = yields to the original definition of CDD, where the transmission power is equally distributed among the TX antennas. Note that P + completely switches off the CDD extension. IV. RESULTS In this section, we provide simulation results for CDD with antenna power weighting applied to the inner DVB-T system as introduced in Section II-A. We use the 2k mode with a subcarrier spacing of f = 4464 Hz, 6-QAM modulation and an inner code rate of R = 3/4. The guard interval length is N G = /32 = 64 samples, which equals 7 µs for 8 MHz channels. This parameter set results in a net bit rate after the outer Reed-Solomon decoder of 8. Mbit/s for 8 MHz channels. We consider 2-TX- and 4-TX antenna CDD with linear delay increment δ i = i.9 µs. Results in [9] have shown that no further gain is achievable for the considered channel model if we further increase the lic delays δ i. The Doppler spectrum of the Rayleigh (non- LOS) components is uniform with a bandwidth of f Dmax = Hz, which is.% of the subcarrier spacing and thus negligible in terms of intercarrier interference. At the receiver we assume exact knowledge of the channel states, i.e., channel estimation is perfect. In Rayleigh fading channels, CDD provides additional propagation paths, which increases the available diversity. In pure LOS (AWGN), however, these additional propagation paths are static, and thus, transform the AWGN channel into a static frequency selective one, which degrades the system performance. So, the SNR gain turns into a loss if we increase the LOS component in a Ricean channel. This can clearly be seen in Fig. 5. For the indoor Rayleigh fading environment, we get an SNR gain of 3.5 db at BER = 2 4 for 2-TX antenna CDD compared to the -TX antenna case (Fig. 5(a)). For the AWGN channel (Fig. 5(b)), however, we observe an SNR loss of 6 db. As CDD is softly switched off, i.e., P is increased, the SNR loss decreases significantly and almost vanishes for P = 2 db. For the Rayleigh fading channel the SNR gain These DVB-T parameters are used in the UK for instance decreases. For the SNR loss reduces by 5.3 db to.7 db, whereas the SNR gain for the Rayleigh channel is still 2.3 db. For P + no signal power is transmitted over the 2 nd TX antenna, which yields finally to the single-tx antenna case. With soft CDD we have got the ability to find a compromise between SNR gains and losses in non-los respectively LOS scenarios. Figure 5 has shown the extremal cases (K = ± ) of the Ricean channel, introduced in Section II-B. Our interest now is in on the SNR gain/loss in Ricean channels for different Ricean factors K. Numerical results for the SNR gain at BER = 2 4 versus the Ricean factor K are shown in Fig. 6(a) for 2-TX antenna CDD. As the the power of the LOS propagation path increases (The Ricean factor K increases), the SNR gain vanishes and turns over into an SNR loss. This turnover point is at about K = 6.5 db for and shifts to higher Ricean factors with increasing P. Results in Fig. 5 have already indicated the ability of soft CDD to reduce LOS losses significantly. However, the price to pay is a reduction of the SNR gain in non-los scenarios, which is relatively small compared to the reduction of the LOS loss. Fig. 6(b) shows the SNR gain versus the Ricean factor K for a 4-TX antenna CDD system with δ i = i.9 µs, i.e., a lic delay increment of samples from one TX antenna to the next. The tendency of the results are similar to the 2-TX antenna case. By increasing P, the SNR losses in strong LOS propagation decrease faster than the SNR gains for non-los. For P = 2 db there is still a gain of 2 db for Rayleigh propagation, whereas the SNR loss for AWGN almost vanishes. Due to the higher diversity, introduced by 4 TX antennas, the maximum achievable gain is about 5.5 db for and K. V. CONCLUSIONS delay diversity (CDD) is a standard conformable antenna diversity technique, which improves the system performance of OFDM systems in Rayleigh fading (non-los) propagation environments. For LOS propagation, however, there are severe problems in terms of performance losses. With soft CDD it is possible to control the diversity, which is additionally inserted into the system. Investigations have been done for a DVB-T system with 2-TX and 4-TX antenna soft CDD for a Ricean multipath propagation channel. Simulation results have shown that soft CDD significantly reduces the SNR loss in LOS propagation. This comes along with a slight degradation of the diversity gain in non-los propagation environments. Nevertheless, soft CDD allows to find good system performance compromises for systems which have to deal with a wide range of propagation conditions in terms of mixed LOS/non-LOS propagation, such as broadcast systems, e.g., DVB-T. Further investigations for outdoor scenarios, which might have a stronger relation to LOS propagation, are of high interest. For soft CDD with more than 2 TX antennas, the power distribution among the CDD antenna branches is a further system property which provides room for further optimization.

6 - -2 -TX P = 2 db TX P = 2 db -3-3 BER BER SNR [db] (a) K = db (Indoor Commercial Channel B) SNR [db] (b) K = + db (AWGN channel) Fig. 5. BER after the inner Viterbi decoding of -TX and 2-TX CDD, DVB-T 2k-mode, 6-QAM, R=3/4, perfect CE SNR-Gain [db] vs. -TX P = 2 db Rayleigh K [db] AWGN (a) 2-TX CDD SNR-Gain [db] vs. -TX P = 2 db K [db] Rayleigh AWGN (b) 4-TX CDD Fig. 6. SNR gain CDD compared to -TX at BER = 2 4 versus the Ricean factor K, DVB-T 2k-mode, 6-QAM, R=3/4, perfect channel estimation ACKNOWLEDGMENT This work have been performed in the context of the IST project PLUTO (FP6-24-IST ) [], [], which is partly funded by the European Union. REFERENCES [] A. Dammann and S. Kaiser, Standard conformable antenna diversity techniques for OFDM and its application to the DVB-T system, in Proceedings IEEE Global Telecommunications Conference (GLOBECOM 2), San Antonio, TX, USA, Nov. 2, pp [2] A. Wittneben, A new bandwidth efficient transmit antenna modulation diversity scheme for linear digital modulation, in Proceedings IEEE International Conference on Communications (ICC 993), Geneva, Switzerland, May 993, pp [3] V. Tarokh, N. Seshadri, and A. R. Calderbank, Space-time codes for high data rate wireless communication: Performance criterion and code construction, IEEE Transactions on Information Theory, vol. 44, no. 2, pp , Mar [4] S. M. Alamouti, A simple transmit diversity technique for wireless communications, IEEE Journal on Selected Areas in Communications, vol. 6, no. 8, pp , Oct [5] Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial television, European Telecommunications Standard Institute (ETSI), July 999, EN V.2.. [6] P. Robertson, E. Villebrun, and P. Höher, A comparison of optimal and sub-optimal map decoding algorithms operating in the log domain, in Proceedings IEEE International Conference on Communications (ICC 995), Seattle, USA, vol. 2, June 995, pp [7] A. J. Viterbi, A personal history of the Viterbi algorithm, IEEE Signal Processing Magazine, vol. 23, no. 4, pp. 2 42, July 26. [8] Final Report on RF Channel Characterization, Joint Technical Committee on Wireless Access, Sept. 993, JTC(AIR)/ R2. [9] A. Dammann and S. Kaiser, Transmit/receive antenna diversity techniques for OFDM systems, European Transactions on Telecommunications, vol. 3, no. 5, pp , Sept. Oct. 22. [] R. Raulefs, J. Cosmas, K.-K. Loo, M. Bard, G. Pousset, D. Bradshaw, Y. Lostanlen, I. Defee, P. Kasser, and S. Kalli, Physical layer DVB transmission optimisation (PLUTO), in Proceedings 5 th IST Mobile Summit 26, Mykonos, Greece, June 26. [] EU-IST FP6 Project Physical Layer DVB Transmission Optimisation (PLUTO),