A Comparative Study on Synchronization Algorithms for Various Modulation Techniques in Gsm

International Journal of Computer Engineering and Technology (IJCET), ISSN 0976-6367(Print),

ISSN 0976 - 6375(Online), Volume 6, Issue 2, February (2015), pp. 38-44 © IAEME

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A COMPARATIVE STUDY ON SYNCHRONIZATION

ALGORITHMS FOR VARIOUS MODULATION

TECHNIQUES IN GSM

Samarth M Kerudi1, Dr.P.Sri Hari

2

1Asst. Professor, Dept. of ECE, STJIT, Ranebenur, Karnataka, India

2Professor-HOD, Dept. of EIE, GITAM University, Hyderabad, Telangana, India

ABSTRACT

The study on various modulation techniques implemented in GSM is the main area of interest

to address the problem of estimating synchronization parameters. On estimation of synchronization

parameters, a robust synchronization algorithm can be developed for GSM standards. Later in this

paper a fast synchronization algorithm for Gaussian Minimum Shift Keying (GMSK) modulation

technique is proposed for GSM in mobile devices. GMSK is the spectrally efficient modulation

technique with constant envelope property. This synchronization algorithm for GMSK results in low

complexity and low signal-to-noise ratio (SNR) for GSM standards. Symbol-by-symbol (SBS)

demodulator is used for demodulating GMSK in GSM which can outperform the traditionally high

complexity Viterbi Algorithm, but SBS assumes perfect synchronization in designing the

demodulator. The proposed method will extend to practical scenario by considering imperfect

synchronization caused by synchronization algorithms at the receiver.

Index Terms: CPM, GMSK, GSM, SNR, Synchronization Parameters

I. INTRODUCTION

GSM is a standard developed to describe protocols for second generation (2G) digital cellular

networks used by mobile phones. GSM being operated in more than 219 countries and territories,

became default global standard for mobile communications as of 2014 amidst more than 90% market

share. [1]. Designing any service for mobile devices using GSM is moderately complex due to its

limited resources availability like small battery capacity, limited input/output capabilities and also

weak radio transmitters. GSM architecture can be divided into three groups – mobile station (MS),

base station subsystem (BSS), and network subsystem.

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ISSN 0976 – 6367(Print)

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Volume 6, Issue 2, February (2015), pp. 38-44

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Physical layer in GSM is responsible for the modulation of signals. The main goal of

modulation is to process the periodic waveform (the data to be transported) by varying certain

parameters in order to use that signal to convey the message with the best possible quality while

occupying the least of radio spectrum. GMSK is a form of continuous-phase FSK (Frequency Shift

Keying) in which the phase change is changed between symbols to provide a constant envelope.

Despite the attractive features of CPM, the complexity at receiver is high due to the implicit memory

of modulation, and it requires maximum likelihood sequence detection (MLSD) for the best

performance [2]. Compared with other modulation techniques for GSM, GMSK showed high

spectral efficiency than QPSK, BPSK, 8-PSK, 16-PSK, MSK [2]. In the GMSK, the side lobe levels

of spectrum are reduced by passing the modulation data waveform through a pre modulation

Gaussian pulse shaping filter [2].

In any networking systems, modulation techniques are quintessential for system to be in

synchronization. In the GSM systems, BCCH (Broadcast Control Channel) carriers provide the

necessary information for synchronization. Synchronization in GSM occurs in time division multiple

access (TDMA) and frequency division multiple access (FDMA). Synchronization is a multi-

parameter estimation problem. The multi-parameters are symbol timing offset (STO), carrier

frequency offset (CFO), and carrier phase offset (CPO). Considering the limited resource availability

and burst transmission characteristics in GSM, fast synchronization algorithms with low complexity

are necessary [3].

II. LITERATURE REVIEW

In [4] Morelli and D’Amico, proposed a joint maximum likelihood (ML) estimator for

synchronization parameters like symbol timing, carrier phase and frequency offset estimation in

AWGN channels. Their work operated on alternating BPSK symbols for training sequence which

simplifies the likelihood function significantly leading to a rather simple estimation algorithm. From

the work presented in [4], we can conclude that the training sequence not only affects the estimation

performance but it may also affect the complexity of the estimation algorithm.

From the work presented by Zhao et al. [5], they have presented an efficient phase and timing

synchronization algorithm which can be applied to all continuous phase modulation techniques in

communications. Despite its good performance in fading channels, the complexity of the algorithm is

high in computation and prior to synchronization frequency ambiguity has to be resolved.

With respect to the paper presented by Gunther and Moon in [6], they have proposed

synchronization algorithm for burst-mode QPSK signals, which includes frame synchronization in

addition to timing offset, frequency offset and phase offset. The algorithm presented in [6] is based

kurtosis, which is a statistical measure.

Kurtosis is related to second and fourth order moments of random variable. Bit Error Rate

(BER) of frequency estimation in low SNR regions is highlighted in this work. The results obtained

from simulation shows that the errors in frequency offset estimation leads to errors in symbol timing

and carrier phase estimation thus proportional to the increase in BER.

The work carried out in [7] by Huber and Liu, proposed that the delay occurred in the transformation

of the CPM signal into frequency domain using non-orthogonal function results in phase offset that

is predicted using ML algorithm, along with carrier phase. This algorithm works only for timing

delays which are smaller than the symbol duration, and hence this [7] proposed system can be used

in phase locked loop structure encountering with false clocks.

Nezami in his work [8], proposed a synchronization method for minimum shift keying

modulation technique in communication systems based on Discrete Fourier Transform (DFT) of the

received preamble. Even though the alternating training sequence estimates all three synchronization

parameters, overall performance in low SNR region is poor.



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In [9], Lee et al. has proposed a maximum likelihood algorithm for synchronizing frame

bursts of PSK signals in the existence of frequency offsets. Carrier phase and frequency offset are

treated as undesired equally distributed parameters in the derivation of likelihood function. In [9], the

author mentioned that the work requires known training sequence and hence these are well suited

with data-aided (DA) techniques.

The work carried out by Ehsan Hosseini and Erik Perrins in [10], shows that the algorithm

which is implemented in feed forward manner estimates the frequency offset via two Fast Fourier

Transform (FFT) operations. After the frequency estimation, symbol timing and carrier phase can be

computed via closed-form expressions.

A feed forward technique for DA joint symbol timing and frequency estimation algorithm for

GMSK signals is proposed by Huang et al. [11]. The performance of GMSK modulation in

estimating synchronization parameters depends on the error rate of frequency offset and symbol

timing.

III. PROPOSED SYSTEM

Synchronization offers the potential for increase in capacity of cellular networks based on

GSM and Enhanced Data rates for GSM Evolution (EDGE). Tight frequency reuse and fractional

loading together allows synchronization in the network to be minimized through the intelligent share

of frequency hopping parameters. Modulation index of 90° for the traditional GMSK technique is

known to have a very narrow power spectrum. For fast and efficient synchronization in GSM mobile

devices, both sender and receiver which mean both modulation and demodulation techniques must

perform optimally well over radio channel. SNR is calculated to know the efficiency of signal

transmission, lower the SNR value higher is the efficiency of the system. Bit error rate (BER), is

estimated to know the percentage of error bits that occur while transmitting the data. Lower the

percentage of BER, higher is the throughput of the system. Synchronization parameters which are

evaluated for robust synchronization method are symbol timing offset (STO), carrier phase offset

(CPO), and carrier frequency offset (CFO). Continuous phase modulation technique is used in our

work for developing fast synchronization algorithm for GSM communication systems.

CPM technique is implemented for mobile communications because of its power- and

bandwidth-efficient signaling scheme. Minimum-shift keying (MSK) and Gaussian MSK (GMSK)

have been implemented in variety of applications which are types of binary CPM. M-ary CPM

systems which mean CPM with multi-level symbol inputs have higher bandwidth efficiency than

binary CPM systems, but due to the complexity in receiver for demodulation and synchronization

problems - the use of m-ary CPM systems is restrained [12]. Complexity of a CPM receiver is

determined by both demodulator complexity and also the complexity of front-end processor (FEP),

which is used to generate sufficient statistics for signal detection and parameter estimation. For m-

ary partial response, an optimal coherent CPM demodulator requires a complex maximum likelihood

(ML) sequence detector [13]. In our proposed system, symbol-by-symbol (SBS) demodulator which

is simpler to implement is carried out as opposed to complex Viterbi algorithm. SBS demodulator

assumes perfect synchronization while designing the demodulator. By implementing SBS

demodulator with the assumption of perfect synchronization, a robust synchronization algorithm can

be developed for GMSK modulation technique by estimating the performance of synchronization

parameters with low SNR value and low BER percentage.

IV. RESULTS

In the GSM standard, Gaussian Minimum Shift Keying with a time-bandwidth product of 0.3

was chosen as a compromise between spectral efficiency and inter symbol interference. For different



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values of SNR, bit error rate (BER) is computed using MATLAB. Figure 1 shows BER vs. SNR

graph for MSK and GMSK modulation schemes. The figure shows us that for higher values of SNR,

GMSK is better compared to MSK.

Fig. 1. BER Vs. SNR graph plot of MSK and GMSK

Figure 2 and Figure 3 shown below provide the comparison of symbol timing and carrier

frequency offset of other proposed systems [14] [15] [3] [16].

Fig. 2. Timing synchronization of GMSK for different methods.

Once the frequenct estimate (CFO) is available, the carrier phase can be computed. The

larger value of observation duration of phase synchronization gives us better phase estimation. The

CPO estimation plot is shown in Figure 4. During data transmission, CPO will be updated accepting

data-decision feedback method.



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Fig. 3. CFO synchronization of GMSK for different methods

Fig. 4. CPO estimation of GMSK for different methods

V. CONCLUSION

Symbol by symbol (SBS) demodulator is implemented for demodulating GMSK signals for

developing an efficient synchronization algorithm for GSM standards in mobile devices. Simple SBS

demodulation scheme is designed with the assumption of perfect training sequence for

synchronization, which provides better results than complex Viterbi Algorithm demodulator in terms

of SNR and BER values. The results of joint estimation of synchronization parameters such as CFO,

STO and CPO, for the proposed systems prove to be better than previous methods for GSM

standards.



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AUTHORS DETAILS

SAMARTH KERUDI has received BE degree in Electronics and Communication

Engineering from STJIT, Bangalore, Karnataka, India in 2008 and M.Tech degree

in Digital Communication and Networks from UBDT, Davanagere, Karnataka,

India in 2010. He has teaching experience of over 5 years. He is currently pursuing

PhD degree from Jawaharlal Nehru Technological University, Hyderabad,

Telangana India.His main area of interest is digital communication.

Dr.PIRATLA SRIHARI has received BE degree in Electronics and

Communication Engineering from SRKR Engineering College, Bhimavaram,

Visakhapatnam, Andhra Pradesh, India in 1988, MS degree in Electronics and

Control from BITS, Pilani, India in 1992, M.Tech in Digital Systems and

Computer Electronics from Jawaharlal Nehru Technological University,

Hyderabad, India in 2001 and PhD in Electronics & Communication Engineering

(Error Control Coding) from JNTU, Hyderabad, Andhra Pradesh, India in 2007. He has teaching

experience of 22 years. Presently he is Professor and Head of the Dept. for Electronics and

Instrumentation Engineering, in GITAM University, Telangana, India. His area of interest are analog

and digital communication. Dr. Srihari is Life Member of Indian Society for Technical Education.

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A Comparative Study on Synchronization Algorithms for Various Modulation Techniques in Gsm