On the efficiency of PAPR reduction schemes deployed for DRM systems
 Sanam Moghaddamnia^{1}Email author,
 Albert Waal^{2},
 Martin Fuhrwerk^{1},
 Chung Le^{1} and
 Jürgen Peissig^{1}
https://doi.org/10.1186/s1363801607475
© The Author(s) 2016
Received: 22 April 2016
Accepted: 5 October 2016
Published: 26 October 2016
Abstract
Digital Radio Mondiale (DRM) is the universally, openly standardized digital broadcasting system for all frequencies including LW, MW, and SW as well as VHF bands. Alongside providing high audio quality to listeners, DRM satisfies technological requirements posed by broadcasters, manufacturers and regulatory authorities and thus bears a great potential for the future of global radio. One of the key issues here concerns green broadcasting. Facing the need for highpower transmitters to cover wide areas, there is room for improvement concerning the power efficiency of DRMtransmitters. A major drawback of DRM is its high peaktoaverage power ratio (PAPR) due to the applied transmission technology based on Orthogonal Frequency Division Multiplexing (OFDM), which results in nonlinearities in the emitted signal, low power efficiency, and high costs of transmitters. To overcome this, numerous schemes have been investigated for reducing PAPR in OFDM systems. In this paper, we review and analyze various technologies to reduce PAPR providing that the technical feasibility and DRMspecific system architecture and edge conditions regarding the system performance in terms of modulation error rate, compliance with frequency mask, and synchronization efficiency are ensured. All evaluations are carried out with I/Q signals which are monitored in real operation to present the actual performance of proposed PAPR techniques. Subsequently, the capability of the best approach is evaluated via measurements on a DRM test platform, where achieved transmit power gain of 10 dB is shown. According to our evaluation results, PAPR reduction schemes based on active constellation extension followed by a filter prove to be promising towards practical realization of powerefficient transmitters.
Keywords
1 Introduction
The DRM system has been designed particularly to enable a highquality digital transmission over the current analogue radio broadcast bands using the globally existing frequency and bandwidth plan. DRM can be used for a variety of audio contents with the capacity of integrating text and data. It also offers substantial benefits to information broadcast in case of disasters and emergencies. DRM deploys Coded Orthogonal Frequency Division Multiplexing (COFDM) transmission technology. The DRM system specification such as transmitter and receiver structures, coding rates, and constellations has been licensed and released by the European Telecommunications Standards Institute (ETSI). To support robust data transmission at different rates and channel conditions, different operating modes are specified by the DRM standard, which can be divided mainly in DRM30 and DRM+ [1]. The latter is designed to operate in VHF bands (30–300 MHz) including the analogue frequency modulation (FM) band, whereas DRM30 modes are developed to work in the bands below 30 MHz like the analogue amplitude modulation (AM). DRM30 has a variety of modes (A, B, C, and D) allowing different spectrum usage settings and is within scope of investigation. Owing to reliable and high data transmission rates OFDM is an attractive communications technology. However, a major problem of OFDM systems is that OFDM signals exhibit high PAPR and thus being subject to nonlinear effects of RF front ends. To achieve maximum power efficiency and coverage in DRMbased systems, a linear high power Amplifier (HPA) is required. However, if the linear range of the HPA is not sufficient or used inefficiently, large PAPR leads to high outofband emissions and modulation error rate (MER). In other words, unless the HPA is not operated in its linear range with large power backoff, it is impossible to increase the output power without violating the frequency mask and minimum threshold necessary for MER. This results in low power efficiency and expensive transmitters [1, 2]. This paper is motivated by the need arose to determine efficient PAPR reduction approaches being in compliance with system structure and constraints imposed by the DRM standard. From the practical standpoint, an indepth investigation on the actual performance of PAPR reduction technologies has not yet been concluded and thus is the subject matter of our research.
In order to improve the power efficiency of OFDM systems, various PAPR reduction techniques have been proposed, which can be categorized into signal scrambling and signal distortion technologies. Signal distortion schemes such as clipping, windowing and peak cancellation reduce peak power at cost of reduced signal quality and usually spectral regrowth. Some signal scrambling schemes such as coding require additional bits to reduce PAPR at the cost of reduced data rate. Another group of signal scrambling approaches including Selective Level Mapping (SLM), Partial Transmit Sequence (PTS), and Hadamard Transformation either requires side information at the receiver to decode the input signal or modification of the receiver structure; thus, both are not standards compliant. Active constellation extension is another signal scrambling scheme, where the outer points of signal constellations are extended adaptively so that the PAPR of an OFDM symbol is minimized [3–5]. This method does not result in throughput and MER loss and thus is of interest for practical use in many OFDMbased systems.
The contributions of this paper are organized as follows; Section 2 gives a brief review of the OFDM signal characteristic and PAPR formulation. Section 3 presents the DRM transmitter structure and specification including criteria used to choose and to implement PAPR reduction schemes. In Section 4, we investigate the feasibility of techniques applied for PAPR reduction. Section 5 provides an efficiency comparison of all respective schemes in terms of PAPR, MER, compliance with frequency mask, and synchronization accuracy. In Section 6, a number of measurements are performed to verify the capability of PAPR reduction scheme based on ACE. Finally, the main issues of this work are concluded in Section 7.
2 OFDM signal characteristic
where P _{ av }=E[x[n]^{2}] is the average power of x[n] and E[·] denotes the expectation. Due to the superposition of the numerous independent subcarriers of random phase and amplitude, OFDM signals usually exhibits high PAPR values. Therefore, the characteristic of the OFDM signal in terms of PAPR distribution should be given in evaluating the performance of any PAPR reduction scheme. The distribution of PAPR bears stochastic characteristics and thus usually is expressed in terms of the Complementary Cumulative Distribution Function (CCDF), indicating the probability that PAPR exceeds a predefined threshold γ (Pr[PAPR>γ]). Note that the passband PAPR is about twice of the baseband PAPR considered in our investigation [6].
where I _{ k } and Q _{ k } are the ideal I/Q components and \(\tilde {I}_{k}\) and \(\tilde {Q}_{k}\) are the actual I/Q components of the data symbol of the kth subcarrier [7].
3 DRMspecific selection criteria of PAPR algorithms

–It is not allowed to violate the DRM standard such as making modifications to receiver structure, data rate, cellinterleaving and pilot cells.

–The quality of the audio signal has to be maintained. This means that the MER should not fall below a predetermined level [8].

– The frequency spectrum mask is required to conform to the DRM standard. The deterioration of spectral properties should remain within a reasonable limit [9].

– Synchronization performance should not be compromised. In other words, executing the PAPR reduction algorithms should not affect the efficiency of synchronization.
In view of aforementioned criteria, there is a limited number of feasible possibilities to integrate a PAPR reduction module in the DRM transmitter structure [10].
4 Compatibility of PAPR reduction schemes with DRM system
A number of different techniques of PAPR reduction has been reported in the literature. A distinction is mainly drawn between signal distortion and signal scrambling techniques. First, we briefly outline the main principle behind these techniques. Subsequently, it is reviewed whether the PAPR reduction approaches are compatible with the DRM system requirements that the standard demands.
4.1 Signal distortion
Techniques base on the signal distortion reduce high peaks by nonlinearly distorting the OFDM signal. Due to the simple implementation together with the high PAPR gains, an approach based on signal distortion has become a favored PAPR reduction technology. According to literature, the major drawback of these techniques lies in the BER performance degradation [11]. However, it is possible to improve the BER performance via upsampling and conditional filtering as reported in [6]. In the following, we provide a brief overview of PAPR reduction algorithms based on the signal distortion.
4.1.1 Nonlinear companding transform
Different companding schemes have been proposed in [11] and [12], each having advantageous in terms of PAPR reduction or spectral properties. Here, the companding noise is regarded as a major problem.
4.1.2 Peak reduction carrier
Here, the PAPR reduction is enabled using a portion of subcarriers served as peak reduction carriers, the socalled bearing peak reduction carriers, at the cost of transmission efficiency [13].
4.1.3 Random phase shuffling
This approach brings about a PAPR reduction by allocating a random phase to each subcarrier which without corresponding receiver results in a substantial deterioration of BER performance [14].
4.1.4 Envelop scaling
This method facilitates the PAPR reduction by adjusting the envelope of a couple of subcarriers. However, this scheme is merely applicable to PSKmodulated signals [11].
4.1.5 Clipping and filtering
Here, b _{ l } is the peak scaling function and w(n) represents the aligned pulse cancellation function at the position l. Ideally, a sinc function can be used as the pulse function. The efficiency of this approach depends on time and frequency characteristics of the pulse function [11].
4.2 Signal scrambling
Signal scrambling techniques differ from each other in how the signal scrambling is performed to decrease the PAPR. Reducing PAPR via signal scrambling causes no inband interferences and MER degradation. However, most of these approaches imply the use of a portion of subcarriers/bits or transmission of additional information. Furthermore, to obtain the optimum PAPR reduction gain a search of all possible combinations of the bits is needed which requires a high computational effort [11]. In the following, a brief outline of the signal scrambling schemes is given.
4.2.1 Hadamard transform
This approach with a requirement of applying Hadamard and inverse Hadamard Transformation at the transceiver promises a PAPRreduction of about 2 dB without affecting the spectral efficiency [18].
4.2.2 Dummy sequence insertion
As the name suggest, the PAPR reduction is achieved via a dummy sequence such as complementary and correlation sequences before inverse fast Fourier transformation (IFFT), which should be removed at the receiver side [11].
4.2.3 SLM and PTS
Selective Level Mapping (SLM) and Partial Transmit Sequence (PTS) schemes require transmission of additional information as well as the modification of the DRM receiver (Demapper) [6].
4.2.4 Interleaving and block coding
Both schemes are based on similar principles to SLM and PTS and require corresponding deinterleaving and decoding at the transmitter [11].
4.2.5 Tone reservation
Here, unused carriers are deployed to generate a time signal which is added to the original signal. Effective PAPR reduction is provided if sufficient subcarriers are available [6].
4.2.6 Tone insertion
The PAPR reduction is performed here via expanding the constellation points, the socalled polar or rectangular mapping and accordingly the corresponding demapping is required at the receiver [6].
4.2.7 Active constellation extension
where C _{ k } is the extension vector for the given subcarrier constellation and subject to the above mentioned constraint. In other words, only the components of C _{ k } within an allowable extension area are considered. This is carried out iteratively until PAPR is reasonably minimized. In summary, the algorithm works as follows. Data symbols of a given subcarrier X _{ k } are transformed into a timedomain signal x using IFFT. Any x[n]>CL is clipped in magnitude and the clipped portion can be obtained by c[n]=(CL−x[n])e ^{ j φ[n]}. Using FFT c[n] is transformed into the extension vector C _{ k }. The algorithm is iterated until PAPR is essentially decreased or a given iteration time is reached. Another issue concerns the clipping level choice being of particular importance to achieve the best PAPR reduction. Specifying the ideal clipping level is a difficult task, because this depends on various factors as reported in [19].
DRMcompatible PAPR reduction schemes
Technique  Method  Standard conformity  Explanation 

Signal distortion  Nonlinear companding transform  No  Need for modification of the receiver 
Peak reduction carrier  No  Need for modification of the transceiver  
Random phase shuffling  No  Need for modification of the receiver  
Envelop scaling  No  Applicable only to PSKmodulated signals  
Clipping and filtering  Yes  No need for modifications  
Signal scrambling  Hadamard transform  No  Need for modification of the receiver 
Dummy sequence insertion  No  Need for modification of the receiver  
SLM and PTS  No  Need for modification of the receiver  
Interleaving and block coding  No  Need for modification of the transceiver  
Tone reservation  No  Need of Additional subcarriers  
Tone insertion  No  Need for modification of the receiver  
ACE  Yes  No need for modifications 
5 Simulation results
In order to determine the most efficient PAPR reduction approach for the DRM, we tested potential schemes with real I/Q signals generated by a DRM modulator at the sampling frequency of f _{ s }=48 kHz. We evaluated the oftenused DRM30 in robustness mode B with 10kHz bandwidth and N=206 subcarriers, where 64QAM and 4QAM mapping are used for the MSC and the FAC/SDC, respectively. Both realization and evaluation of all signal processing methods were carried out in MATLAB. The performance evaluation includes PAPR, distance to frequency mask and synchronization accuracy analysis. The CCDF curves are based on 10^{5} random baseband OFDM symbols. Notice that at least an average MER of about 35 dB at the transmitter side is tolerable for high quality data transmission. Therefore, the PAPR reduction is subject to the condition that the minimum MER and DRMspecific frequency mask are met. Finally, the influence of the utilized schemes on the synchronization is investigated.
5.1 PAPR analysis
Specification of the lowpass equiripple FIRfilter
Parameter  Prefilter  Postfilter 

Length [sample]  0.1  0.2 
Passband edge [kHz]  8.3  4.7 
Stopband edge [kHz]  8.7  5.2 
Passband ripple [dB]  0.001  0.001 
Stopband atten. [dB]  60  15 
5.2 Spectral property
5.3 Synchronization efficiency
How the PAPR reduction algorithms affect the efficiency of synchronization is a further important question so far left untreated within PAPR reduction studies. In this work, we mainly focus on the synchronization schemes proposed for DRM, where the receiver has the full knowledge of the pilot cell position and phase. During the acquisition phase the receiver performs a search for a known pilot pattern of each OFDM symbol by calculating the correlation between the known pilot pattern P and the received OFDM Symbols R.
where MF _{org} and MF _{PAPR} are the MF of the original and PAPR reduced DRM signal, respectively.
The results of MSE calculated for the MF of original and PAPR reduced signals
Method  MSE [dB] 

Clippingpostfiltering  −83 
Prefilteringpostclipping  −88 
ACE postfiltering  −59 
5.4 Power efficiency
Accordingly, the PAPR reduction results in either power savings with a fixed P _{out} or increased transmit output power having a fixed P _{DC}. It is often desired to improve the coverage and thus P _{out} having a fixed P _{DC}. Based on the evaluation results of PAPR at a CCDF of 10^{−4}, the HPA efficiency of the original DRM signal is about 2 %, whereas the one achieved by the most promising schemes, prefilteringpostclipping and ACE postfiltering, is about 10 and 8.5 %, respectively.
6 Measurements
7 Conclusions
The main focus of this work is on identifying DRMcompatible PAPR reduction schemes and their actual efficiency in real operation. On the basis of criteria prescribed by the DRM standard, all PAPR reduction schemes applicable to DRM are determined and the underlying parameters are adjusted individually to achieve the best tradeoff between the PAPR gain, MER loss, spectral pollution, and synchronization accuracy. According to our investigation, the ACE postfiltering approach provides the superior performance in terms of average PAPR value, robustness to spectral pollution, and modulation quality. The simple procedure of prefiltering and postclipping promises a superior PAPR gain in comparison to schemes based on the postfiltering structure and thus serves as another alternative to reduce PAPR. Moreover, it is possible to achieve almost equal amount of maximum and average PAPRs via this approach, which implies that the occurrence of nonlinearities can be entirely prevented by the proper adjustment of amplifier operation range. However, the PAPR gain achieved by the prefiltering and postclipping other than the ACE postfiltering strongly depends on the applied filter properties. In other words, adjusting the filter parameters to achieve compliance with the frequency mask results in PAPR deterioration. Furthermore, none of these approaches have any impact on the synchronization efficiency. All this taken together, both approaches are worth considering from a practical point of view for PAPR reduction in DRM systems. According to our investigations, the ACE postfiltering is seen as the most promising PAPR reduction approach applicable to DRM, which has been verified by measurements. In particular, combining the ACE postfiltering scheme and the issues of adaptive equalization of amplifier nonlinearities bears a great potential for green broadcasting and is subject of our future work.
Declarations
Competing interests
The authors declare that they have no competing interests.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License(http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Authors’ Affiliations
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