Abstract:
An apparatus for recovering a decision-directed carrier according the present invention comprises a) a first conjugate complex sample generating part for generating a first conjugate complex sample in accordance with an received complex sample; b) a frequency recovering part for receiving the first conjugate complex sample and for recovering a carrier frequency of the first conjugate complex sample by compensating a carrier frequency offset of the input signal; c) a phase recovering part for receiving the first conjugate complex sample and for recovering a carrier phase of the input signal by compensating a carrier phase offset of the input signal; and d) a symbol decison part for selecting a symbol in accordance with the output value from said means c) and for outputting the selected symbol to said means a).

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates to an apparatus for recovering carrier, particularly relates to an apparatus for recovering decision-directed carrier of which baseband signal is processed in complex domain. 
     2. Description of the Prior Art 
     In a general demodulator of digital radio communication system, carrier recovery means the compensation of carrier frequency offset and phase offset contained in sample data in order to decide symbol data. 
     Carrier contained in a received signal is removed by a local oscillator in an intermediate frequency converter through a radio frequency receiving part. Even though the carrier is removed, there are the frequency offset and the phase offset of the carrier. Therefore, the frequency offset and the phase offset are detected and compensated in the baseband in order to simply implement demodulator. 
     In satellite communication, since high frequency is used for the intermediate and the radio frequency, the received signal is much affected by the frequency shift when bandwidth of transmission signal is narrow. When a narrow band signal such as voice signal is transmitted, the received signal is to be shifted by multiple numbers of the bandwidth. Therefore, there needs a frequency synthesizer which converts the received signal to an appropriate baseband signal. 
     Wide band satellite communication is not much affected by frequency shift. For example, in a satellite communication which transmits the signal having transmission rate more than 45 Mbps by Quadrature Phase Shift Keying (QPSK), if frequency shift is within 5000 PPM(Particles Per Million), the frequency shift may be under the control by using an accurate frequency oscillator. 
     Some methods have been proposed in order to improve carrier recovery performance. 
     In 1983, A. J. Viterbi and A. M. Viterbi proposed Mth Powering Algorithm (hereinafter, which is referred to “V&amp;V algorithm”) for detecting carrier phase for MPSK (M-ary Phase Shift Keying) signal (See, A. J. Viterbi and A. M. Viterbi, ‘Nonlinear Estimation of PSK Modulation Carrier Phase with Application to Burst Digital Communication’, IEEE Trans. Infor. Theory, vol. IT-32, July 1983). However, the V&amp;V algorithm has a shortcoming that noise to signal considerably increases when M increases. 
     In 1991, Fitz analyzed vagueness of V&amp;V algorithm and proposed solution (See, M. P. Fitz, ‘Equivocation in nonlinear digital carrier synchronizers’, IEEE Trans. On Comm., Vol. COM-39, No.11, November 1991). 
     Classen proposed a decision-directed method and the analyzed performance of the method (See, F. Classen, H. Meyer and P. Sehier, ‘An all feedforward synchronization unit for digital radio’, Proc. of VTC&#39;93, 1993). In Classen&#39;s method, though VCO (Voltage Controlled Oscillator) is not used, ROM is used in order to represent the detected carrier phase by complex value. 
     Also, Fitz proposed a decision-directed burst mode carrier synchronization techniques applicable to TDMA (See, M. P. Fitz, ‘Dicision-Directed Burst-Mode Carrier Synchronization Techniques’, IEEE Trans. On Comm., Vol. COM-40, No.10, October 1992). This Fitz&#39;s technique needs a divider in order to detect carrier frequency instead of VCO. Using the Fitz&#39;s technique, frequency recovery may be obtained in broad range; however, it is difficult for high speed communication to use the divider, thereby being unsuitable to perform the frequency recovery. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a decision-directed carrier recovering apparatus which is apt for high rate satellite communication and rapidly recovers carrier phase and carrier frequency by processing baseband signals in complex domain. 
     According to the first aspect of the present invention, this object is accomplished by providing an apparatus for recovering carrier of an input signal from outside, the apparatus comprising: a) means for generating a first conjugate complex sample in accordance with a received complex sample; b) means for receiving the first conjugate complex sample and for recovering a carrier frequency of the first conjugate complex sample by compensating a carrier frequency offset of the input signal; c) means for receiving the first conjugate complex sample and for recovering a carrier phase of the input signal by compensating a carrier phase offset of the input signal; and d) means for selecting a symbol in accordance with output value from said means c) and for outputting the selected symbol to said means a). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawing in which: 
     FIG. 1 is a block diagram of the decision-directed carrier recovering apparatus in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments of the present invention will be described with reference to the accompanying drawings. 
     FIG. 1 shows a block diagram of the decision-directed carrier recovering apparatus in accordance with the present invention. 
     Referring FIG. 1, the decision-directed carrier recovering apparatus comprises a conjugate complex sample generating part  110 , a frequency recovering part  120 , a phase recovering part  130  and a symbol decision part  140 . 
     The conjugate complex sample generating part  110  receives a complex sample and outputs a conjugate complex sample of the complex sample. 
     The frequency recovering part  120  receives the conjugate complex sample from the conjugate complex sample generating part  110  and recovers a decision-directed carrier frequency by compensating carrier frequency offset of an input signal from outside. The frequency recovering part  120  comprises a delayer  121 , multipliers  122  and  125 , a phase offset detector  123  and a conjugate complex sample generating part  124 . 
     The delayer  121  delays the input signal from outside. The multiplier  122  multiplies the delayed signal from the delayer  121  by a conjugate complex sample from the conjugate complex sample generating part  110 . The frequency offset detector  123  detects a frequency offset from an output signal of the multiplier  122 . The conjugate complex sample generating part  124  receives the detected frequency offset signal from the frequency offset detector  123  and outputs the conjugate complex sample. The multiplier  125  multiplies the carrier of the input signal from outside by a conjugate complex sample from the conjugate complex sample generating part  124  and outputs the result to the phase recovering part  130 . 
     The phase offset detector  123  may be implemented by a low pass filter which filters the output signal of the multiplier  122  and detects frequency offset. 
     The phase recovering part  130  receives a conjugate complex sample from the conjugate complex sample generating part  110  and recovers phase offset of the output signal of the frequency recovering part  120 . The phase recovering part  120  comprises a plurality of delayers  131 ,  132  and  137 , multipliers  133  and  136 , a phase offset detector  134  and a conjugate complex sample generating part  135 . 
     The delayer  131  and  132  respectively delay the output signal of the multiplier  125  and the delayer  131 . The multiplier  133  multiplies a conjugate complex sample from the conjugate complex sample generating part  110  by the output signal of the delayer  132 . The phase offset detector  134  detects phase offset of the output signal of the multiplier  133 . The conjugate complex sample generating part  135  receives a output signal of the phase offset detector  134  and outputs the conjugate complex sample. The multiplier  136  multiplies the output signal of the delayer  131  by the output signal of the conjugate complex sample generating part  135 . The delayer  137  delays the output signal of the multiplier  136  and outputs the delayed signal to the symbol decision part  140 . 
     The phase offset detector  134  may be implemented by a low pass filter which filters the output signal of the multiplier  133  and detects phase offset. 
     The symbol decision part  140  selects a complex sample apt for a value of the output signal of the phase recovering part  130  and outputs the selected complex sample to the conjugate complex sample generating part  110 . 
     Operations of the decision-directed carrier recovering apparatus as described above will be explained. 
     The frequency recovering part  120  is a frequency tracking loop and recovers a decision-directed carrier frequency by compensating carrier frequency offset of an input signal from outside. 
     An input signal from an outside is delayed for a certain time by the delayer  121  and transferred to the multiplier  122 . The multiplier  122  multiplies the delayed carrier signal from the delayer  121  by a conjugate complex sample from the conjugate complex sample generating part  110  and outputs the result to the frequency offset detector  123 . 
     The frequency offset detector  123  implemented by a low pass filter filters the output signal of the multiplier  122 , detects frequency offset and outputs the detected frequency offset to the conjugate complex sample generating part  124 . Here, the frequency offset is represented by a complex sample. 
     Receiving a detected frequency offset signal from the frequency offset detector  123 , the conjugate complex sample generating part  124  converts representation form of the frequency offset from the complex sample to the conjugate complex sample and outputs the conjugate complex sample to the multiplier  125 . 
     The multiplier  125  multiplies the carrier of the input signal from outside by a conjugate complex sample from the conjugate complex sample generating part  124  and outputs the result to the delayer  131  of the phase recovering part  130 . 
     The phase recovering part  130  is a phase tracking loop and recovers phase offset of the output signal of the frequency recovering part  120 . 
     The signal of which frequency is recovered by the frequency recovering part  120  is delayed by the delayer  131 . The delayed signal is transferred to the delayer  132  and the multiplier  136 . The delayed signal from the delayer  132  is multiplied by the conjugate complex sample from the conjugate complex sample generating part  110  by the multiplier  133 . 
     The phase offset detector  134  implemented by a low pass filter filters the output signal of the multiplier  133 , detects a phase offset and outputs the detected phase offset to the conjugate complex sample generating part  134 . Here, the frequency offset is represented by a complex number. 
     Receiving a detected frequency offset signal from the phase offset detector  134 , the conjugate complex sample generating part  135  converts representation form of the phase offset from the complex sample to the conjugate complex sample and outputs the conjugate complex sample to the multiplier  136 . 
     After detecting the carrier phase offset of the input signal, the multiplier  136  multiplies the output signal of the delayer  131  by the phase offset from the conjugate complex sample generating part  135  and outputs the multiplication result to the delayer  137 . 
     Receiving the signal delayed by the delayer  137  for a certain time, the symbol decision part  140  compares the delayed signal with a pre-determined reference signal, selects a complex sample in accordance with the comparison result and outputs the selected complex sample to the conjugate complex sample generating part  110 . 
     The phase offset detector  134  may be implemented by a low pass filter which filters the output signal of the multiplier  133  and detects phase offset. 
     The symbol decision part  140  selects a symbol apt for a value of the output signal of the phase recovering part  130  and outputs the selected symbol to the conjugate complex sample generating part  110 . 
     Operation of the decision-directed carrier recovering apparatus as described above will be explained by equations. 
     The carrier x k  of the input signal to MPSK signal from outside is expressed as equation 1. 
     
       
           x   k   =e   j(ω     0     k+θ)   d   k   +n   k   (1) 
       
     
     Where, ω 0  is a frequency offset, θ 0  is a carrier phase in range [−π, π], d k  ε{e j2π⊥/M  |l=0,1, . . . , M−1} is data symbols, n k  is a White Gaussian Noise having N 0 /2 double-sided Power Spectrum Density. 
     The output signal from the multiplier  122  for detecting frequency offset can be expressed as equation 2. 
     
       
           ê   w,k   =x   k   {circumflex over (d)}   k *  (2) 
       
     
     The output signal from the multiplier  133  for detecting phase offset can be expressed as equation 3. 
     
       
           ê   θ,k   =x   k   {circumflex over (d)}   k *  (3) 
       
     
     Where, y k , which is the output value of the multiplier  125 , refers to input sample of which the detected frequency offset is compensated. The equation 2 represents a new frequency offset detector for compensating frequency offset. The frequency offset detector detects frequency offset by using only one sample for each symbol. If we assume that symbol decision is always correct, the frequency offset detector  123  is represented by the expectation value of the equation 2 and can be written as equation 4.                        Ω   ^     k     =                E        (       x   k            d   ^     k   *       )                   =                E        (         d   k            d   ^     k   *               j        (         ω   0        k     +     θ   0       )           +       n   k            d   ^     k   *         )                   =                     j        (         ω   0        k     +     θ   0       )                       (   4   )                                
     Here, since the symbol decision is assumed to be correct, the frequency recovering part is not used by time k. Therefore, e jω0 , which is a phase required for compensating frequency offset contained in a next sample x k+1 , remains undecided. After obtaining expectation value in equation 3 for compensating phase offset, phase offset detection value of the phase offset detector  134  may be obtained by equation 5.                        Φ   ^     k     =                E        (       y   k            d   ^     k   *       )                   =                E        (       x   l            d   ^     k   *            Ω   ^       k   -   1     *       )                   =                E        (         d   k            d   ^     k   *               j        (         ω   0        k     +     θ   0       )                   -     j        (         ω   0          (     k   -   1     )       +     θ   0       )             +       n   k            d   ^     k   *         )                   =                     jω   o                     (   5   )                                
     Where, the phase recovering part  130  is affected by no phase offset and by the frequency offset. The output signal of the multiplier  136  containing both of the phase offset and the frequency offset may be written by equation 6.                        R   ^       k   +   1       =                    Ω   ^     k            Φ   ^     k                   =                       j        (         ω   0        k     +     θ   0       )                   jω   0                     =                         j        (         ω   0        k     +   1     )       +     θ   0       )                     (   6   )                                
     Here, since the output signal of the multiplier  136  includes both of the phase offset and the frequency offset as written by the equation 6, the phase offset and the frequency offset included in the next input sample x k+1  are detected and compensated. 
     The decision-directed carrier recovering apparatus recovers carriers by processing the baseband signal in the complex domain without using the existing voltage controlled oscillator and the divider. Therefore, the invention is implemented by simple circuit and considerably decreases noise from complex constitution of circuit. Also, this invention can be used for a high speed satellite communication. 
     Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.