On the reuse of DVB-T transmitter infrastructure for DVB-T2

Transcription

1 MM On the reuse of DVB-T transmitter infrastructure for DVB-T F. Fraile 1, Student Member, IEEE, C. Nader,3,4, Student Member, IEEE, J.C. Guerri 1 and N. Börsell Abstract The new DVB-T standard was specifically designed to allow the reuse of DVB-T infrastructure if required. This paper presents measurement results that verify the correct performance of a commercial DVB Power Amplifier with DVB- T waveforms. Furthermore, the paper includes configuration guidelines and performance evaluation of the Tone Reservation PAPR reduction algorithm included in the DVB-T standard. PAPR reduction is key technology when reusing DVB-T amplifiers for DVB-T transmissions without penalizing amplifier efficiency. Index Terms DVB-T, Efficiency, PAPR Reduction, Tone Reduction. T I. INTRODUCTION HE ETSI standard DVB-T [1] is a revision of the DVB- T norm, the most widely used terrestrial broadcast standard. The primary obective of DVB-T is to provide broadcasters with a more advanced, more efficient alternative to current terrestrial broadcast standards, as a complementary system that allows a more efficient usage of broadcast spectrum, rather than a substitute to the extremely cost efficient DVB-T []. Among the different commercial requirements that shaped the DVB-T specification, one stated that the new norm must be able to reuse infrastructure originally meant for its precursor, DVB-T. Clearly, this could be a very compelling argument in cases where new DVB-T services supersede DVB-T services over updated broadcast licenses. In such scenarios, broadcasters could benefit from some of the benefits of the new standard (improved coverage, higher capacity, etc.) with very little investments in the radio transmitter, given that the radio frequency (RF) chain designed for DVB-T can be reused with DVB-T. A key element in an RF chain is the power amplifier (PA) that holds the highest power level in the system. Its power added efficiency (PAE) directly influences the power consumption of the wireless system. Also, its input-output signal nonlinearity is important for in-band error and out-ofband interference. Combining the nonlinear behavior of the 1 Universidad Politécnica de Valencia, ES-4601, Valencia, Spain, ffraile@iteam.upv.es, phone: Center for RF Measurement Technology, University of Gävle, Gävle, SE , Sweden, charles.nader@hig.se, avierferrercoll@hig.se 3 Signal Processing Lab, Royal Institute of Technology, Stockholm, SE , Sweden 4 Department ELEC, Vrie Universiteit Brussel, Brussels, B-1050, Belgium PA with the orthogonal frequency division multiplexing (OFDM) signals used in both DVB standards, characterized by high peak-to-average power ratio (PAPR), peak clipping will occur, causing in-band and out-of-band (OOB) emissions. The in-band errors can lead to an increase in the bit-error-rate while the OOB emissions will interfere with adacent channels and break the spectral mask. As a consequence, the operating power level of the amplifier needs to be reduced, i.e. a large number of dbs equivalent to the PAPR level needs to be backed-off. A drawback of such power back-off is the decrease in the efficiency of the amplifier, as a large amount of supplied energy will dissipate as heat. Hence, a trade-off arises between linearity and efficiency. In order to deal with the high PAPR and its consequences on the PA performance, DVB-T incorporates two different PAR reduction techniques, namely Tone Reduction (TR) [3] and Active Constellation Extension (ACE) [4]. Thus, through PAPR reduction, DVB-T transmissions can outperform the power efficiency of DVB-T transmissions when they both use equivalent OFDM configurations. Furthermore, DVB-T incorporates new FFT modes and new Guard Intervals (GI) aimed at reducing the Guard Interval Insertion overhead [5]. Since denser FFT modes have longer symbols periods, they require of lower relative Guard Interval periods to achieve the same robustness against channel delay spread. Thus, DVB-T defines shorter GI durations for the denser FFT modes, improving the GI insertion overhead and in turn the overall capacity. However, an increment of the FFT size results in an increase in the signal PAPR, deteriorating PA performance, i.e. the PA has to work with higher back-off margins to avoid peak clipping which results in lower efficiency. In such cases, PAPR reduction can alleviate the power efficiency penalty inherent to dense FFT modes. This paper focuses on the performance of TR PAPR reduction and its impact on the power efficiency of commercial DVB-T Power Amplifiers. The research evaluates the case where the DVB-T transmitter infrastructure, except for the modulator, is reused for DVB- T transmissions. The rest of the paper is structured as follows: Next section describes the DVB-T OFDM modulation, as well as presenting the difference metrics involved in the evaluation. Section III describes the main

2 MM11-87 aspects of the methodology and the measurement setup. Section IV includes the results, consisted of parameter optimization of the TR PAPR reduction algorithm, together with power performance assessment of the PA. II. DVB-T: AN OVERVIEW In this section, an overview of the OFDM signal used in DVB-T is presented and some important metrics used in the process of this work are given. In addition, a part of this section is dedicated to the tone reservation technique for PAPR reduction and its important figure of merits. A. DVB-T OFDM The baseband signal of a DVB-T frame is defined in [1] as: The most noticeable difference with respect to the definition of the baseband signal for DVB-T [] is the existence of a preamble p 1 t (1) ( ) of duration T p1 at the beginning of every frame. As in DVB-T, the rest of the frame consists of OFDM symbols of period, where T U is the useful part of the symbol and the Guard Interval (GI) duration. Thus, within a frame, each OFDM symbol of index l modulates K total carriers with complex values C l = [ c l,k min,...,c l,k max ], which could be either values of BPSK modulated pilots or QAM constellation points in the case of data carriers. In DVB-T, all data carriers are modulated with the same QAM constellation diagram, either QPSK, 16-QAM or 64- QAM. For the case of DVB-T, several adustments have been included: an additional 56-QAM mode is added; the concept of rotated constellations is included; and most importantly the concept of Physical Layer Pipes (PLP) is introduced, i.e., K total can be divided in subsets of carriers whose modulation parameters can be configured independently of each other. There are many parameters that assess the quality of the OFDM modulation, but the most widely used in broadcasting is the Modulation Error Ratio (MER) [6], defined as: MER 10log 10 Km 1 Km 1 I I Q Q Where I and Q are the real and imaginary components of each of the K m data carrier symbols while di and dq are () the distances between the received real and imaginary components and the ideal values of each data carrier symbol. As for the FFT mode, in DVB-T, there are three possible different values of K total, given by the FFT sizes N K,4K, 8K, whereas in DVB-T, K total is determined by the FFT size N 1 K,K,4K,8K,16K, 3K and the possibility of using an extended mode for the denser FFT modulations, hence a total of 9 different values for K total are available. Since the occupied bandwidth is kept constant with N (except for the Extended Mode configurations), T U is directly proportional to the FFT size. Similarly, in DVB-T the guard interval duration can only take a number of values, corresponding to a fraction of T U, ,,, U T. As mentioned, the GI prevents Inter Symbol Interference caused by delay spread in the channel. The Guard Interval is configured to handle multipath echoes as well as to support Single Frequency Network (SFN) radio networks. Longer GIs provide extra protection against multipath fading and allow for greater SFN maximum sizes, but at the expense of decreasing the overall system throughput, due to the overhead caused by GI insertion. Noting that the actual duration of is proportional to N, denser FFT modes can achieve the same absolute GI duration with less GI overhead. For this reason, DVB-T introduces new values of for the denser FFT modes, namely,,,,,, U T. Through these new GIs, it is possible to reduce the GI 1 insertion overhead. For instance, TU in the 3K 18 mode has the same absolute duration as 1 T U in the K 8 mode and therefore both allow for the same SFN sizes and provide same robustness against reflections, but they represent an overhead of roughly 0.8% and 1.5% respectively. B. Tone Reservation PAPR Reduction A maor drawback of using denser FFT modes is the probability increase in having high peaks in the signal, and hence higher PAPR. The PAPR of OFDM is defined in literature as: t[0, T ) x( t) max t [0, T ) x U Pav max ( t) PAPR (3) E x(t) As indicated in [7], the PAPR may also be computed from the discrete samples defined in (1) as long as the oversampling ratio is at least 4, in order to have accurate

3 MM estimate of the peaks. The Tone Reservation technique for PAPR reduction relies on the creation of a signal that iteratively cancels peaks in the time domain. The peak-canceling signal is synthesized from a set of reserved carriers, not available to transmit data when TR PAPR reduction is activated. In the method standardized for DVB-T, the peak-canceling signal is an impulse-like kernel signal obtained directly from standardized sets of reserved carriers. The algorithm is iterative: in every loop, the kernel signal is scaled, shifted and then combined with the original signal so that the detected peak is reduced down to a pre-defined value of V P. The PAPR is reformulated for TR as follows: analysis of the effect of the two configuration parameters of the TR algorithm, as well as the study of the effect of TR PAPR reduction in the Power Added Efficiency (PAE) of a general purpose Power Driver Amplifier (PDA). These studies require of high degrees of control over the DVB-T OFDM and TR configuration parameters. Besides, some of the metrics, such as the D PAPR, require of access to interfaces which are not provided in commercial modulators or receivers. For these reasons, the authors decided to use software baseband DVB-T OFDM modulation and demodulation, implemented in Matlab. The next diagram shows the setup used during the measurements: max PAPR_ r t[0, T ) E x(t) x( t) c( t) (4) Where c(t) is the combination of shifted, scaled kernels obtained in every iteration of the algorithm. The PAPR reduction experienced by a symbol is: PAPR PAPR _ r (5) PAPR It should be mentioned that the average power increase, Ex(t) E x(t) c( t ), caused by c(t) counteracts PAPR reduction and therefore needs to be kept small. For the presented PAPR reduction strategy to be successful, some aspects need to be considered. The clipping threshold, V P, and the maximum number of iterations per symbol, i, need proper configuration. If V P is too large, there will be no PAPR reduction, hence it should be upper bounded. In addition, kernel signals are not perfect impulse signals and can increase the power of other samples. Such behavior of peaks regrowth is more acute for large number of iterations. It is also important to note the trade off between complexity and number of iterations found in practical implementations of the algorithm. Finally, the Power Added Efficiency (PAE) of an amplifier is defined as the ratio between the Gain of an amplifier and the power that it consumes: Gain PAE 100*( ) (6) P DC III. METHODOLOGY The main obective of the measurements is to evaluate the performance of commercial DVB-T PA with DVB-T waveforms and the effect of TR PAPR reduction in this scenario. As partial results, this paper also presents an Figure 1. Simplified diagram of the transmitter and the measurement setup The first partial result, the analysis of the TR PAPR algorithm specified in [1] is performed through simulations with the DVB-T software implementation, obtaining the PAPR reduction and Power Increase for different configurations of the TR algorithm. The tests check the mean symbol PAPR Reduction and Power Increase for different configurations of the PAPR TR algorithm. Once the optimal parameters are achieved, the Complementary Cumulative Distribution Function (CCDF) of the symbol PAPR is compared to the reference DVB-T symbol PAPR CCDF. In order to evaluate the performance of PAs, the baseband DVB-T signals are up-converted with an arbitrary waveform signal generator. Every measurement requires the configuration of the signal levels and RF frequencies of the input signal to the amplifier. The output of the amplifier under test is then connected to a vector signal analyzer, used to down-convert the RF signal to a low frequency and then digitize it through a highresolution analog-to-digital converter. The digital baseband signals are then treated in Matlab to obtain the amplifier gain, the signal MER (as defined above) and other relevant metrics such as the Adacent Channel Power (ACP) emission. As for the commercial DVB-T PA, it implements a power control unit that adapts the input signal level in order to

4 MM guarantee a constant output level. The input stage also implements linear pre-correction to improve the amplifier response. Lastly, the transmitter implements pass band filters at the central frequency in order to reduce OOB. Figure 1 also depicts a simplified block diagram of the transmitter. In order to protect the unit, the control circuit shut downs the input stage if operated outside a designated range. For some of the tests, it is required that the symbol power exceeds the input range of the power limiter in the power control unit. In those cases, the power sweeping strategy described in [5] is used. The strategy thereby presented consists on modulating the amplitude of the baseband signal with a staircase representing different power steps. Each step of the staircase is formed by an integer multiple of OFDM symbols and the period of the staircase signal is considerably smaller than the integration window of the power control unit. Such method offers a constant average power of the OFDM signal within the integration window of the PA power control unit, while at the same time, allows the instantaneous power of symbols to exceed the input range of the amplifier control unit, hence enabling the measurement of different power levels outside the input power range of the PA. IV. RESULTS AND ANALYSIS In this section, figure of merits regarding the TR PAPR reduction algorithm and the PA performance are studied and evaluated. A. Optimization of TR PAPR Reduction parameters Figure shows the mean symbol PAPR reduction (dbs in primary y axis) and the Power Increase (dbs in secondary y axis) for different configurations of the TR algorithm. The X axis represents the Clipping Ratio (db), defined as V p / P a. The DVB-T FFT N is equal to 3K and the data carriers apply QPSK modulation. decreases, the Power Increase due to peak regrowth and the PAPR reduction become equivalent. It can also be noted how peak regrowth is more noticeable as the number of iterations increases. There is also some coupling between the configurations of both parameters, since the Clipping Ratio that gives the best mean PAPR reduction is different for every number of iterations. It is around this optimum value that the algorithm should be configured to maximize PAPR reduction and minimize the Power Increase. Figure 3. Mean Symbol PAPR Reduction and Power Increase against for different FFT sizes and QAM types. Figure 3 represents again the PAPR reduction (db) and the Power Increase (db) for different values of the FFT size and QAM modulation types. The figure represents the values for i =10 against different values of V P. The graph shows how the FFT size affects PAPR reduction and Power Increase: the improvements achieved will be in the same order of magnitude, regardless of the FFT size, although the optimal values depend on the FFT size. Figure. Mean Symbol PAPR Reduction and Power Increase versus the clipping ratio for different number of iterations.of the TR algorithm. The DVB-T parameters are FFT size 3K and QPSK The Figure shows that when the Clipping Ratio is too large, there will be no effect on the signal, neither PAPR reduction nor Power Increase). As the clipping ratio Figure 4. Symbol PAPR CCDF for DVB-T N = K and N = 3K mode with and without TR PAPR Reduction. Figure 4 represents the CCDF of the signal PAPR with and without PAPR Reduction for DVB-T signals with N = K

5 Power Added Efficiency (%) Modulation Error Rate (db) MER (db) Gain (db) MM and 3K. The number of iterations of the TR algorithm is set to 10 ( i =10) and the clipping ratios are configured to the optimum values that maximize PAPR reduction. The graph shows that, for the K mode, the implementation achieves a PAPR reduction higher than 1 db 99.9% of the time and slightly below 1 db 99.9% for the more challenging 3K mode. B. Effect on Power Added Efficiency of Driver Amplifier The next figure shows the PAE of a DPA together with the MER of the output of the device against the Back-Off level used at the input. The MER showed corresponds to the DVB- T, 56 QAM mode with and without TR PAPR reduction. The TR algorithm is configured for maximum PAPR reduction with 10 iterations Input Back-Off (db) Figure 5. Power Added Efficiency and Modulation Error Rate for DVB-T 3K mode with and without TR PAPR reduction. The figure shows the normal behavior of a transistor amplifier: as the input is backed-off, the PAE and signal clipping decrease, hence a trade-off between the amplifier efficiency and the quality of the output signal exists. The measurement over the PAPR reduced signal clearly shows that it requires an Input Back-Off level of around 9dB to avoid clipping. Without PAPR reduction, the signal is already clipped at this level. With higher back-off levels, there is a noticeable difference between the quality of both signals for a given PAE level. However, the MER of the PAPR reduced signal rapidly converges to the MER of its equivalent non PAPR reduced signal as the amplifier starts clipping its peaks. This is the expected behavior for the TR algorithm: since higher peaks are reduced by the algorithm, the amplifier clips many samples once the signal reaches its non-linear region. C. Performance of the Power Amplifier PAE MER MER-TR Figure 5 represents the MER (db, primary y axis) measured at the output of the amplifier for different values of the power input level. The secondary axis represents the overall gain in db, including the Gain of the DPA. The measurement covers a power input range equal to the power input range of the amplifier. As explained, the amplifier monitors the input power level and adapts the gain so that the output power is kept constant. It can be seen how the amplifier Gain is linear in the input range to achieve a constant output power. The MER at the output of the amplifier draws a curve in which two different regions are clearly distinguished. When the input power is too low, the MER increases with the input power. At about -1 db input power, the MER enters a linear region where the MER remains constant at around 33 db, which is an acceptable value for the broadcast signal MER KQPSK No TR MER 3K QPSK TR Gain KQPSK No TR Gain 3K QPSK TR Power_In_Level (db) Figure 5. MER and amplifier Gain against input power V. CONCLUSION The results show that the RF system design supports DVB-T signals with the 3K mode when the TR PAPR reduction algorithm is used. Commercial amplifiers are complex systems and to obtain higher signal quality through PAPR requires knowledge about how its different components behave in normal operation. Results also show that the parameters of the algorithm need proper configuration to obtain any benefits from it. ACKNOWLEDGMENT The authors would like to thank the staff members of the Electronic department of the University of Gävle for the great atmosphere that surrounded this research work. REFERENCES [1] ETSI, Digital Video Broadcasting (DVB); Implementation guidelines for a second digital terrestrial television broadcasting system (DVB-T), ETSI TR v0.9.6, January

6 MM [] ETSI, Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial television (DVB-T), ETSI EN v.1.6.1, January 009. [3] J. Tellado-Mourelo, Peak to average power reduction for multicarrier modulation Ph.D. thesis, Stanford University, Stanford, Calif, USA, September [4] B. S. Krongold and D. L. Jones, PAR reduction in OFDM via active constellation extension, IEEE Transactions on Broadcasting, vol. 49, no. 3, pp , 003. [5] ETSI, Digital Video Broadcasting (DVB); Implementation guidelines for a second generation digital terrestrial television broadcasting system (DVB-T), ETSI TS V.1.1.1, October 010. [6] ETSI, Measurement guidelines for DVB systems, ETSI TR V1..1, January 001. [7] Harif M., Gharavi-Alkhansari M., Khala B.H.: On the peak- to-average power of OFDM signals based on oversampling, IEEE Trans. Commun., 003, 51, (1), pp [8] C. Nader, H. Altahir, O. Anderse, N. Börsell, E. Condo, N. Keskitalo, H. de la Rosa: Automated Multidimensional Characterization of Power Amplifier for Design and Production, International Instumentation and Measurement Technology Conference, Singapore, May 009.