A Comparative Study of Costas Loop and DPLL for Carrier Synchronization

This paper presents comparison of a simple costas loop and DPLL for carrier recovery. Synchronization is basically the process of frequency and phase aligning of remote oscillators. The first task is carrier acquisition which includes forming a frequency and phase-matched replica of the local oscillator for the process of down conversion. A local oscillator in detector is forced to oscillate in both phase and frequency with respect to the carrier oscillator at the transmitter. Timing recovery is the second task focused on collecting a set of aligned input samples time and the replica template. The presence of carrier phase error causes signal to rotate. For larger rotation, the symbol signal space projections lie in a wrong decision region. Hence there occurs a decision error even if a perfect timing recovery scheme is applied for zero additive noise. Carrier synchronization aims to detect the phase of carrier. Simulation results are presented for evaluating the behavior of costas loop and DPLL. Keyword—Synchronization, Carrier recovery, Costas loop, DPLL, Frequency estimator, Phase Estimator I.INTRODUCTION Generally transmitted signal is modified by the channel. Here the channel is assumed to be an additive white Gaussian noise (AWGN) channel. A delay also occurs at the receiver due to the time required for propagation of signal. It leads to sampling at instances in suboptimal time. Along with the relative movement between the transmitter and receiver causes a Doppler shift, imperfections in their local oscillators also tend signal to rotate in the signal space by a constant frequency offset [5]. The received signal is hence a modified version of transmitted signal due to these effects. The role of synchronizer is to estimate the fequency offset and delay occurred at channel, along with the phase offset and then to remove or reduce these modifications. Sent data symbols are then detection by the synchronizer [7]. II. THE COSTAS LOOP In today‟s fast paced world, increase in the communication systems demand for high data rate tends operating frequency to millimeter wave region. Hence with modulation schemes like ASK and PSK, high data rate at larger bandwidth is applicable. Communication system has a hierarchy of synchronization problem when dealing with these high data rates and high frequencies is the problem of carrier recovery. Synchronizer detection includes recovery of carrier offset present in RF signal with respect to a reference phase and frequency. Baseband signal requires high speed A/D-converters which in turn increases power consumption as well as design complexity [7]. For frequency exceeding several GHz, delay line will be in tens of millimeters. The length of delay elements determines the data rate [8]. For BPSK signal circuit is required which works with twice the carrier frequency since a frequency multipler is used. Hence for millimeter range of frequency costas loop is used [6]. A. Operation J. Costas proposed the Costas loop as a phase tracker for AM signals [9, 5] which was later modified to demodulate PSK signals [2], [6]. The operation of the Costas loop is shown in Fig. 3. BPSK modulated RF signal X is given by: X = I(t)sin (ωt + θ) + Q(t)cos(ωt + θ) (1) Where I(t) and Q(t) are symols having value of ±1, which varies at symbol rate. This signal is then passed through a multiplier which multiplies signal with LO of same frequency and an instantaneous phase of θ′ and a LO with 90◦ phase shift. After passing through loop filter the signals are: ZI(t) = I(t)cos φ − Q(t)sin φ (2) ZQ(t) = I(t)sin φ + Q(t)cos φ (3) With φ = θ − θ′. The Output of the limiters for phase error of |φ| < 45 ° is: International Journal of Engineering Research & Technology (IJERT) IJ E R T IJ E R T ISSN: 2278-0181 www.ijert. org Vol. 3 Issue 7, July 2014