An AM-Compatible Hybrid Modulation for Broadcasting

Wei Wang School of Information Science and

Engineer Southeast University

Nanjing, China richardwangseu@gmail.com

Lenan Wu School of Information Science and

Engineer Southeast University

Nanjing, China wuln@seu.edu.cn

Bingcheng Zhu School of Information Science and

Engineer Southeast University

Nanjing, China 241851989@qq.com

Abstract—A novel method of digital-analog hybrid

modulation compatible with current amplitude modulation broadcasting is proposed. This approach is based on double sideband with reserved carrier (DSB-RC) modulation, which is commercially used by broadcast stations. The major modification of DSB-RC is to replace the sine carrier with a well-designed quasi-carrier—the one that is modulated by M-ary Phase Position Shift Keying (MPPSK). This quasi-carrier, on one hand, guarantees a qualified modulation index as the sine carrier does, on the other hand, can convey digital information. Computer simulation demonstrates that the hybrid modulation has good backward compatibility and excellent bit error rate performance.

Keywords-Hybrid Modulation; DSB-RC; MPPSK; Backward Compatibility

I. INTRODUCTION

In recent years, the number of wireless devices increased dramatically, which brings about higher demand for data volume and channel capacity. In analog amplitude modulation (AM) broadcasting, the double sideband with reserved carrier (DSB-RC) scheme has been utilized till now due to its simple and cheap receiver. However, the transmitted sine carrier of this scheme is a huge waste of power and baseband audio signal in AM broadcasting is limited to 50~5000 Hz[1]. To excavate the value of the wasted power and idle band, a quasi-carrier —Mary Phase Position Shift Keying (MPPSK), as well as its unique demodulation scheme, is proposed[2]. MPPSK concentrates its most energy into the idle band of AM broadcasting and have less interference with signals out of the idle band. In other words, MPPSK scheme and AM scheme can harmoniously coexist in the same bandwidth.

The method proposed in this paper can serve as a transitional scheme in the progress of broadcasting digitization. Subsystem MPPSK transmits digital information, while subsystem DSB-RC can still transmit analog audio that is accessible for current radio sets. Further digitization can be achieved by replacing the analog audio with band signal generated by Orthogonal Frequency Division Multiplexing (OFDM)[3]method.

The rest of the paper is organized as follows: Section II introduces MPPSK and its waveform optimization. In Section III, system model of the hybrid modulation is illustrated. Section IV gives computer simulation results and performance analysis. Finally, Section V concludes this paper.

II. M-ARY PHASE POSITION SHIFT KEYING

Quasi-carrier is a key prerequisite for the hybrid modulation. A qualified quasi-carrier should have these fundamental properties: (1) Fast power spectrum convergence; (2) Constant envelop; (3) Good Bit Error Rate (BER) performance. MPPSK is equipped with all these properties.

A. The Spectrum Analysis of MPPSK The MPPSK is evolved from Extended Binary Phase Shift

Keying (EBPSK)[4][5], a binary modulation that features in solitude-peak low-sideband power spectrum density (PSD). The modulated MPPSK signal is defined as follows:

0 0 0 11 , 0 1 1 1 Where, T is the carrier period,T 1/ , K is the carrier

number per time slot, N is the carrier number per symbol. Then N/K (N/K>M-1) is the slot number in each symbol. θ refers to phase shift, and r (0 r 1 ) is the guard interval(GI) factor. Apparently, the variables M, K, N and θ form a parameter set to comprehensively adjust the bandwidth and data rate. When K/N or θ turns small, MPPSK signal becomes more similar to pure sine waveform. If θ = 0, MPPSK equals sine and therefore no information can be transferred. Hence, small values of K/N and θ result in sine-like waveform, which presents in solitude-peak low-sideband PSD appearance and occupies very limited bandwidth. On the other hand, increased M brings about more time slots are utilized.

The work is supported by the National Natural Science Foundation of China (NSFC) under the grant 60872075.

2012 7th International ICST Conference on Communications and Networking in China (CHINACOM)

978-1-4673-2699-5/12/$31.00 © 2012 IEEE197

Figure 1 shows the PSD of a modulated MPPSK signal. We can see it is made up of continuous part and discrete part. And the discrete spectra consist of high order harmonic components and carrier frequency component. For a qualified quasi-carrier, its PSD should be solitude-peak and low-sideband, thus the high order harmonic components are unwanted.

Figure1. Power Spectrum Density of MPPSK

Great efforts were done to optimize MPPSK’s spectrum, in [5] the high order harmonic components of discrete spectra are removed by subtracting. However, the waveform of optimized signal is no longer constant envelop, making it unfit for DSB-RC anymore. In next subsection, we firstly analyses the formula of MPPSK’s PSD and then explore for a new method of optimization.

B. The Optimization of MPPSK When a sequence of M-ary source symbol is modulated,

the transmitted MPPSK signal can be defined as

S t 2

When M=2, the PSD has been figured out in conditions of “0” and “1” distributed in equal probability [6]. S 14 |G G | 14 δ mT |G G | 3

Where T NT is the symbol rate of MPPSK, and G f

and G f are the PSD of g and g . In equation (3), the first part corresponds to the continuous component, we use S to represent itand the second part to the discrete component, we use S to represent it, thus S f 14T |G f G f | 4 S f 14T δ f mT |G f G f | 5

According to (5), the sum of G and G decides the discrete spectra and high order harmonic components of 2PPSK. As is mentioned above, these components of spectrum are unwanted with the exception of the peak component . To backward induce proper parameters of optimized 2PPSK, we compulsorily let all the high order harmonic components (m 0) of S be zero,

S 14 δ mT |G G | 14T δ mT |F g g | 14T |G 0 G 0 | 6

Where F · refers to the Fourier transform. Obviously, when g t g t , the discrete spectra will disappear. However, in that case, 2PPSK turns into Binary Phase Shift Keying (BPSK), which lacks the property of solitude-peak low-sideband PSD appearance. Hence, the sum of g t and g t should not be constant zero. This conclusion inspires us to find a function whose zero points coincide with the values of ∑ δ f T , except for m 0. Amazingly, as figure2 shows, sinc functionhas this feature.

Figure2. Sinc Function and Impulse Function

Thus, the MPPSK should satisfy the following properties

|G G | · 2 /2 / 7

g g · 2 0 0 8

Where k is a multiplicative coefficient.

Similarly, this conclusion applies to the condition of M>2,

G · 2 /2 / 9

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g · 2 00 10

Generally, the equation (10) can be concluded as follows. Phase jump should fill all the time slots of a symbol in equal probability. With this guide, we can completely remove the discrete spectra and high order harmonic components of MPPSK. Figure 3 shows the PSD of an optimized MPPSK signal.

Figure3. PSD of an optimized MPPSK modulated signal

III. SYSTEM MODEL

Figure 4 shows the structure of hybrid modulation system. It combines two different systems. One is modified AM system and the other is MPPSK system. Similarly, the source data of hybrid system is divided into two streams, data stream “I” is analog audio transmitted through modified AM system, and data stream “II” is digital data transmitted through MPPSK system.

Assuming original AM signal as follows, YAM f A m t M cos 2πf t 11

Where the baseband analog audio m t is data stream “I”, A is the amplitude of carrier and M represents the modulation index. In hybrid modulation system, we replace the sine carrier of DSB-RC transmitter with the MPPSK modulated quasi-carrier, which conveys data stream “II”. Thus, the hybrid AM-MPPSK signal is YH f A m t M · S t A m t M · ∑ g t iNT 12

To receive merely data stream “I”, the present AM receivers need not be modified. While, to receive both data stream “I” and “II”, a parallel MPPSK demodulator should be installed after the IF filter. The MPPSK demodulator consists of four modules:

1)Clipper Amplifier. This module clips the hybrid AM-MPPSK signalthatexceeds a predetermined threshold and then amplifies the cut signal. By doing this, the modulated MPPSK signal reverts to be constant envelop. Besides, the

phase jump, as well as jump position, of the former MPPSK signal is preserved.

. Figure4. Structure of Hybrid Modulation System

2)Impacting Filter. A special IIR band-pass filter, its extreme narrow notch at the center frequency equips it with frequency-selective characteristics that drastically responses to the phase jump of MPPSK by producing spurious AM impulse [6].

3) Detector/Decision Module. This module discriminates the M-ary symbol by position of the impulse raised in envelop of the impacting filter’s response.

4) Bit Synchronization Module. This module is responsible for the symbol synchronization of MPPSK signal.

Detailed introduction of MPPSK demodulator can be derived in [5].

To be backward compatible with present AM broadcasting, COFDM modulator and demodulator in the dotted line block are unnecessary. However, when absolute digitization is required, these modules are crucial. Our digitization method is to replace human voice with baseband COFDM signal in transmitter and demodulate it by COFDM demodulator in receiver. By this, FM-comparable sound quality can be obtained on AM band. Specific analysis can be seen in [7]-[9] .

IV. SIMULATION RESULTS

In this section, we provide simulation results for the proposed system. The parameters are as follows. For MPPSK system, M=256, N=510, K=2, and the positions of phase jump are P 2i 1,2i 1 i M 1, i is an integer . For DSB-RC system, the intermediatefrequency is 465 kHz and the modulation index k is 0.9. Besides, the sampling frequency is 4650 kHz.

COFDM Modulator

MPPSK Modulator

Sine Carrier

Digital Data

Analog Audio/ Digital Data

I: DSB-AM

II: MPPSK

Amplifier/Antenna

Current

IF FilterEnvelope Detection

COFDM Demodulator

Analog Audio/Digital Data

Impulse Filter

Clipper Amplifier

Detector/ Decision

Digital Data

BitSynchronization

Sine Carrier

Channel

Up Conversion

Transmitter

Receiver

I: DSB-AM

II: MPPSK

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Firstly, the usage of spectrum by the hybrid system needs to be delved into. Figure 5 shows the PSD of the hybrid AM-MPPSK signals. Clearly, we see that the -60 dB bandwidth of the hybrid signal is the same as the original DSB-RC signal. And the remaining spectral components are too trivial to be regarded as interference. Figure 6shows the PSD of the hybrid signal in Additive White Gaussian Noise (AWGN) Channel and the Carrier-to-Interference Ratio (CIR) is 14dB. We can see that the continuous components of spectrum are completely submerged in the noise.

Figure5. Spectrum of the Hybrid AM-MPPSK Signal

Figure6. Spectrum of the Hybrid AM-MPPSK Signal submerged in AWGN

Secondly, the quality of analog audio demodulated from the hybrid system is another important evaluation standard. Currently, the most accurate method for evaluating voice quality is through subjective listening tests. However, various objective measures are proposed to replace subjective test. Among these measures, the perceptual evaluation of speech quality (PESQ) measure is the most welcome standard and is the one recommended by ITU-T for speech quality assessment of 3.2 kHz (narrow-band) handset telephony and narrow-band speech codecs [10][11]. The PESQ score is computed as a linear combination of the average disturbance value D and the average asymmetrical disturbance values A as follows: PESQ 4.5 0.1D 0.0309A 13

The comparison of different systems is presented in Figure 7. When CIR merely increases by 1-2 dB, the performance of hybrid modulation will be comparable with original DSB-RC modulation. Actually, human ear can hardly tell the difference.

Finally, figure 8 shows the BER performance of the quasi-carrier MPPSK. When CIR =26dB, BER of MPPSK, whose phase jump value is π/5, comes down to almost zero. To improve the BER performance and utility of MPPSK, channel code can be applied. Low Density Parity Check Code (LDPC) decoder based on Support Vector Machine (SVM) is able to improve the BER performance by 10 dB[12]. In addition, the augment of phase jump can improve the BER performance at the cost of MPPSK’s spectrum properties. Figure 8 illustrates phase jump’s effect on the BER.

Figure7. PESQ Scores of Demodulated Audio From

Hybird AM-MPPSK Signal

Figure8. BER of Quasi-carrier MPPSK From

Hybird AM-MPPSK Signal

The bit rate of the optimized MPPSK proposed in this paper can be calculated as:

R fN log M fM 1 K log M 14

Increasing M, on one hand, brings about decreasing bit rate R, on the other hand,adds its similarity to sine wave. Generally, the selection of MPPSK’s parameters means the tradeoff between factors like bit rate, BER and spectrum property. The simulation results prove that the hybrid modulation system are not only backward compatible but also reliable when transmitting digital data.

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V. CONCLUSION

In this paper, a novel method of digital hybrid modulation compatible with analog AM broadcasting system is proposed and MPPSK technique is employed to utilize wasted power and idle band. Simulation results demonstrate that the hybrid modulation has good compatibility with AM broadcasting. Besides, good BER performance of both COFDM and MPPSK can be obtained. In the next step, multi-path fading channel models and advanced channel coding will be introduced into system simulation to further study the system robustness.

ACKNOWLEDGEMENT

The work is supported by the National Natural Science Foundation of China (NSFC) under the grant 60872075.

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