WO2009078538A1

WO2009078538A1 - Phase detection method and apparatus for carrier synchronization in high order quadrature amplitude modulation

Info

Publication number: WO2009078538A1
Application number: PCT/KR2008/004970
Authority: WO
Inventors: Doh-Wook Kim; Eung Don Lee; Dongwoen Yoon; Yun Jeong Song; Soo In Lee
Original assignee: Electronics And Telecommunications Research Institute
Priority date: 2007-12-17
Filing date: 2008-08-25
Publication date: 2009-06-25
Also published as: KR100930794B1; KR20090065035A

Abstract

A phase detection method for carrier synchronization in a high order Quadrature Amplitude Modulation (QAM), the phase detection method including: compensating a signal received in a carrier recovery loop into compensated signal, the compensated signal being compensated through a predetermined process; determining reference decision symbol on the compensated signal; calculating weight on the compensated signal; calculating phase error using the compensated signal and reference decision symbol; and calculating and outputting weighted phase error using the weight and phase error.

Description

PHASE DETECTION METHOD AND APPARATUS FOR

CARRIER SYNCHRONIZATION IN HIGH ORDER

QUADRATURE AMPLITUDE MODULATION

Technical Field

[1] The present invention relates to a method of stabilizing frequency acquisition of carrier synchronization, which is one of synchronization technologies of a high order Quadrature Amplitude Modulation (QAM), a method of reducing hardware complexity, and a modem apparatus for enabling the methods above. More particularly, the present invention relates to a phase detection method using a full constellation to improve stability of frequency acquisition, a method of reducing hardware complexity using the phase detection method, and a modem apparatus for enabling the methods above.

[2] This work was supported by the IT R&D program of MIC/IITA. [2006-S-019-02,

The Development of Digital Cable Transmission and Receive System for lGbps Downstream] Background Art

[3] Phase detection methods in association with a Quadrature Amplitude Modulation

(QAM) in a conventional art include a decision-directed phase detection (DDPD) scheme and a reduced constellation phase detection (RCPD) scheme. The DDPD scheme determines a transmitted symbol as a constellation point nearest to a received signal using all transmitted symbols. The RCPD scheme determines a transmitted symbol as a reference constellation point nearest to a received signal taking only a portion of all transmitted symbols. The RCPD scheme uses a reduced constellation.

[4] The RCPD scheme includes Sari & Moridi, Jablon, Kim & Choi scheme, and the like. The Sari & Moridi scheme determines a transmitted symbol as a diagonal constellation point nearest to a received signal taking only diagonal symbols of transmitted symbols. The Jablon scheme determines a transmitted symbol as a constellation point nearest to a received signal taking only four corner symbols of transmitted symbols. The Kim & Choi scheme determines a transmitted symbol as a diagonal constellation point nearest to a received signal taking only symbols with high power of transmitted symbols.

[5] In the above-described DDPD scheme, phase tracking may not be appropriately performed due to a symbol which does not exist on a diagonal axis. Accordingly, as a QAM order increases, a frequency acquisition performance may be remarkably degraded. [6] Also, in the Sari & Moridi and Jablon schemes, as a QAM order increases, the number of updates of a phase detector decreases. Accordingly, the Sari & Moridi and Jablon schemes are not suitable for 64 QAM and over.

[7] Also, the Kim & Choi scheme uses a reduced constellation which uses only symbols with high power, and thereby may be affected by a randomizer and power threshold. Disclosure of Invention Technical Problem

[8] The present invention provides a phase detection method and apparatus for carrier synchronization in a high order Quadrature Amplitude Modulation (QAM) uses a full constellation phase detector (FCPD) using a full constellation, instead of a phase detector using a conventional reduced constellation in a carrier recovery loop of a high order QAM, and thereby may stabilize frequency acquisition of carrier synchronization.

[9] The present invention also provides a phase detection method and apparatus for carrier synchronization in a high order QAM which uses a phase detector using a full constellation, and thereby may reduce hardware complexity.

[10] The present invention also provides a phase detection method and apparatus for carrier synchronization in a high order QAM which controls only loop filter gain, and thereby may rapidly reduce a tracking jitter and provide a large carrier frequency acquisition. Technical Solution

[11] According to an aspect of the present invention, there is provided a phase detection method for carrier synchronization in a high order Quadrature Amplitude Modulation (QAM), the phase detection method including: compensating a signal received in a carrier recovery loop for phase through a predetermined process; determining reference decision symbol on the compensated signal; calculating weight on the compensated signal; calculating phase error using the compensated signal and reference decision symbol; and calculating and outputting weighted phase error using the weight and phase error.

[12] In this instance, the reference decision symbol is a diagonal constellation point nearest to the compensated signal. The weighted phase error is represented as a function of weight and the phase error between reference decision symbol, which is a diagonal constellation point, and the compensated signal.

[13] According to another aspect of the present invention, there is provided a phase detection apparatus for carrier synchronization in a high order QAM, the phase detection apparatus including: a phase compensator rotating a signal received in a carrier recovery loop into compensated signal, the compensated signal being com- pensated through a predetermined process; a reference detector determining reference decision symbol on the compensated signal; a weight calculator calculating weight on the compensated signal; a phase detector calculating phase error using the compensated signal and reference decision symbol; and a weighted phase detector calculating and outputting weighted phase error using the weight and phase error. Brief Description of the Drawings

[14] FIG. 1 is a block diagram illustrating a configuration of a phase detection apparatus for carrier synchronization in a high order Quadrature Amplitude Modulation (QAM) according to an embodiment of the present invention;

[15] FIG. 2 is a block diagram illustrating a configuration of a phase detection apparatus for carrier synchronization in a high order QAM according to another embodiment of the present invention;

[16] FIG. 3 is a block diagram illustrating a configuration of a phase detector using a full constellation according to an embodiment of the present invention;

[17] FIG. 4 is a diagram illustrating phase ambiguities in a first quadrant constellation of

256 QAM according to an embodiment of the present invention;

[18] FIG. 5 is a graph illustrating S-curve characteristics depending on weight degrees of a proposed phase detector with respect to 256 QAM according to an embodiment of the present invention;

[19] FIG. 6 is a graph illustrating S-curve characteristics depending on a phase detection method according to an embodiment of the present invention and phase detection methods in a conventional art, with respect to 256 QAM;

[20] FIG. 7 is a graph illustrating S-curve characteristics depending on a phase detection method according to an embodiment of the present invention and phase detection methods in a conventional art, with respect to 256 QAM when diagonal symbols are excluded;

[21] FIG. 8 is a graph illustrating S-curve characteristics depending on signal to noise ratios (SNRs) of a proposed phase detector with respect to 256 QAM according to an embodiment of the present invention;

[22] FIG. 9 is a graph illustrating an average loop filter output of a carrier recovery loop according to an embodiment of the present invention;

[23] FIG. 10 is a graph illustrating root-mean- square (RMS) phase error of a carrier recovery loop according to an embodiment of the present invention; and

[24] FIG. 11 is a flowchart illustrating a phase detection method for carrier synchronization in a high order QAM according to an embodiment of the present invention. Mode for the Invention

[25] Hereinafter, embodiments of the present invention are described in detail by referring to the figures.

[26] FIG. 1 is a block diagram illustrating a configuration of a phase detection apparatus for carrier synchronization in a high order Quadrature Amplitude Modulation (QAM) according to an embodiment of the present invention. FIG. 2 is a block diagram illustrating a configuration of a phase detection apparatus for carrier synchronization in a high order QAM according to another embodiment of the present invention.

[27] According to the present invention, a full constellation phase detector (FCPD) is used in a carrier recovery loop as illustrated in FIG. 1, and thus an effect of a randomizer may be reduced and frequency acquisition of carrier synchronization may be stabilized. Also, only loop filter gain is controlled as illustrated in FIG. 2, and thus complexity of the carrier recovery loop may be reduced due to a single operation mode while maintaining a carrier acquisition and tracking performance.

[28] As described above, according to the present invention, the phase detector may stabilize the frequency acquisition of carrier synchronization and reduce the complexity of the carrier recovery loop. A configuration of the phase detector according to an embodiment of the present invention is described below.

[29] FIG. 3 is a block diagram illustrating a configuration of a phase detector using a full constellation according to an embodiment of the present invention.

[30] A phase compensator rotates an arbitrary signal (r(n)) received in a carrier recovery loop into compensated signal (q(n)). The compensated signal (q(n)) is compensated through a predetermined process.

[31] In this instance, the compensated signal (q(n)) is a signal when compensating the received signal (r(n)) through the phase compensator, as illustrated in FIGS. 1 and 2.

[32] A reference detector 210 determines reference decision symbol on the compensated signal (q(n)).

[33] For example, when it is assumed that, in the received signal (r(n)), time synchronization and gain control are completely achieved, the compensated signal (q(n)) and reference decision symbol (d(n)) may be represented as,

[34] [Equation 1]

¹³⁵¹ q(n) = |<7(»)| exp{jθ_q(n)} = q, (n) + jq_Q{n)

[36] Here, I denotes an in-phase symbol of the received signal (r(n)), and Q denotes a quadrature-phase symbol of the received signal (r(n)). [37] Also, the reference decision symbol (d(n)) is a diagonal constellation point nearest to the compensated signal (q(n)) and is determined by the reference detector 210. The reference decision symbol (d(n)) may be represented as, [38] [Equation 2]

[39] ά{ή) = sgnfø₇ (n)} + j sgn{q_Q(n)}

[40] In this instance, sgn(x) outputs + 1 or - 1 depending on a sign of an factor x, a phase error between the compensated signal (q(n)) and reference decision symbol (d(n)) may be given by,

[41] [Equation 3]

[43] In this instance, a range of Θ(n) is [-π/4, π/4], and Im{ } is an operator selecting only imaginary part. Accordingly, Im{ } may be extracted as, [44] [Equation 4]

[45] j [ q{n) \ ₌ {q_Q O) ^• dj j^ή) - ff₇ QQ ^• d_Q Qn) } m [^<X)J I d{n) I²

[46] Accordingly, a phase detector 230 may output phase error ( ψ

(n)) using the compensated signal (q(n)) and reference decision symbol (d(n)). The phase error (

(njj may be given by, [47] [Equation 5]

¹⁴⁸¹ „ T,,l W,I _ — A„. J fa_g ^(»)- -ft^(»M_c ^(»)}

[49] Here, K_d denotes a gain of the phase detector 230.

[50] According to the present invention, a transmitted symbol is determined by the reference detector 210 as the reference decision symbol (d(n)) which is the diagonal constellation point nearest to the compensated signal (q(n)). Accordingly, when the transmitted symbol is applied to a full constellation, an accurate phase error with respect to a transmitted symbol which does not exist on a diagonal axis may not be acquired.

[51] Accordingly, the transmitted symbol is required to be predicted using power of the compensated signal (q(n)). Also, the phase detector 230 is required to be corrected based on phase ambiguity between constellation points where the transmitted symbol exists and the reference decision symbol (d(n)) which is the diagonal constellation point. [52] FIG. 4 is a diagram illustrating phase ambiguities in a first quadrant constellation of

256 QAM according to an embodiment of the present invention. [53] As illustrated in FIG. 4, it is assumed that an area between I axis and Q axis is divided into L regions and transmitted symbols exist in every region, phase ambiguity may be an average phase angle between constellation points included in an arbitrary region and their diagonal axis. [54] Accordingly, a weight calculator 220 may calculate weight on compensated signal.

The weight may be calculated by, [55] [Equation 6]

[57] In this instance, N(

h

) denotes a number of constellation points in a region where the compensated signal exists. The region satisfies

X_x < ! q(n) I < τ _y

. d_k(n) denotes a k_th constellation point in a region where the compensated signal exists. (d_k(n)} denotes a phase angle between the diagonal axis and the k^ constellation point d_k(n). M denotes a weight degree, and L denotes a number of divided regions of a quadrant where the compensated signal exists.

[58] Accordingly, as constellation points of a region where the transmitted symbols exist are close to the diagonal axis, a weight is close to 1. As the constellation points are far from the diagonal axis, the weight is close to 0.

[59] A weighted phase detector 240 may output weighted phase error based on the phase ambiguity. The weighted phase error may be given by,

[60] [Equation 7]

¹⁶¹¹ ψ_G in^ = W{q{ή)} ψ{n) Ψ G

(n) denotes weighted phase error, W{q(n)}denotes weight, and ψ

(n) denotes the phase error.)

[63] A simulation result when features of the phase detector 200 according to the present invention are applied to a data-over-cables service interface specifications(DOCSIS) downstream cable modem system is described below.

[64] FIG. 5 is a graph illustrating S-curve characteristics depending on weight degrees of a proposed phase detector with respect to 256 QAM according to an embodiment of the present invention. FIG. 6 is a graph illustrating S-curve characteristics depending on a phase detection method according to an embodiment of the present invention and phase detection methods in a conventional art, with respect to 256 QAM. FIG. 7 is a graph illustrating S-curve characteristics depending on a phase detection method according to an embodiment of the present invention and phase detection methods in a conventional art, with respect to 256 QAM when diagonal symbols are excluded. FIG. 8 is a graph illustrating S-curve characteristics depending on signal to noise ratios (SNRs) of a proposed phase detector with respect to 256 QAM according to an embodiment of the present invention.

[65] According to an embodiment of the present invention, since all constellation points in a full constellation are symmetrical on a diagonal axis, DC offset does not exist in the S-curves illustrated in FIGS. 5 through 8. Thus, robustness of a frequency acquisition performance may be provided.

[66] Also, since weight may linearly change through divided regions, the S-curve has a linear characteristic. As illustrated in FIG. 5, as the weight degree M increases, the S- curve has a larger linear characteristic. For example, when M is 20, it may be ascertained that features of the phase detector according to an embodiment of the present invention are identical to those of Sari & Moridi phase detector.

[67] Also, referring to FIGS. 6 and 7, the phase detector may be updated at every symbol using the full constellation, thus a high phase gain is achieved and dependency on diagonal symbols may decrease. Accordingly, a stable frequency acquisition performance may be provided.

[68] According to an embodiment of the present invention, it may be ascertained the phase detection method may provide an improved performance even in a noisy channel as illustrated in FIG. 8.

[69] FIG. 9 is a graph illustrating an average loop filter output of a carrier recovery loop according to an embodiment of the present invention. FIG. 10 is a graph illustrating root-mean-square (RMS) phase error of a carrier recovery loop according to an em- bodiment of the present invention.

[70] As illustrated in FIGS. 9 and 10, for example, when it is assumed that a carrier frequency offset is 200 kHz, a carrier phase offset of 45 degree , L is 32 and M is 4, a frequency offset up to 200 kHz may be recovered, and a tracking jitter performance where an RMS phase error is less than 1 degree in an average of 75000 symbols may be provided.

[71] Also, according to an embodiment of the present invention, a method of stabilizing frequency acquisition of carrier synchronization using a phase detector 200 is provided. The method is described considering a functional aspect of the phase detector 200.

[72] FIG. 11 is a flowchart illustrating a phase detection method for carrier synchronization in a high order QAM according to an embodiment of the present invention.

[73] In this instance, the phase detection method is performed using a phase detector 200, and thus a detailed description of a function of the phase detector 200 is omitted herein.

[74] In operation S 1110, a phase compensator rotates a signal received in a carrier recovery loop into compensated signal. The compensated signal is compensated through a predetermined process.

[75] In this instance, the compensated signal may be a signal where time synchronization and gain control are completely achieved.

[76] Also, the carrier recovery loop may control a loop filter gain in a single mode operation state, and thereby may reduce loop complexity.

[77] In operation Sl 120, a reference detector 210 determines reference decision symbol on the compensated signal.

[78] In operation Sl 120, the reference decision symbol which is a diagonal constellation point nearest to the compensated signal is determined.

[79] In operation Sl 130, a weight calculator 220 calculates weight on the compensated signal.

[80] The weight may be represented as a function between phase ambiguity and weight degree. The phase ambiguity may be an average phase angle between a diagonal axis and at least one constellation point included in a predetermined region.

[81] In operation S 1140, a phase detector 230 calculates phase error using the compensated signal and reference decision symbol.

[82] In operation Sl 150, a weighted phase detector 240 calculates and outputs weighted phase error using the weight and phase error.

[83] In this instance, the weighted phase error may be represented as a function of the phase error and weight between the compensated signal and reference decision symbol. The reference decision symbol may be a diagonal constellation point. [84] The above-described embodiment of the present invention may be recorded in computer-readable media including program instructions to implement various operations embodied by a computer. The media may also include, alone or in combination with the program instructions, data files, data structures, and the like. The media and program instructions may be those specially designed and constructed for the purposes of the present invention, or they may be of the kind well-known and available to those having skill in the computer software arts. Examples of computer- readable media include magnetic media such as hard disks, floppy disks, and magnetic tape; optical media such as CD ROM disks and DVD; magneto-optical media such as floptical disks; and hardware devices that are specially configured to store and perform program instructions, such as read-only memory (ROM), random access memory (RAM), flash memory, and the like. Examples of program instructions include both machine code, such as produced by a compiler, and files containing higher level code that may be executed by the computer using an interpreter. The described hardware devices may be configured to act as one or more software modules in order to perform the operations of the above-described embodiments of the present invention.

[85] According to the present invention, a phase detection method and apparatus for carrier synchronization in a high order Quadrature Amplitude Modulation (QAM) uses a full constellation phase detector (FCPD) using a full constellation, instead of a phase detector using a conventional reduced constellation as a carrier recovery loop of a high order QAM, and thereby may stabilize frequency acquisition of carrier synchronization.

[86] Also, according to the present invention, a phase detection method and apparatus for carrier synchronization in a high order QAM uses a phase detector using a full constellation, and thereby may reduce hardware complexity.

[87] Also, according to the present invention, a phase detection method and apparatus for carrier synchronization in a high order QAM controls only loop filter gain, and thereby may rapidly reduce a tracking jitter and provide a large carrier frequency acquisition.

[88] Although a few embodiments of the present invention have been shown and described, the present invention is not limited to the described embodiments. Instead, it would be appreciated by those skilled in the art that changes may be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.

Claims

[1] A phase detection method for carrier synchronization in a high order Quadrature

Amplitude Modulation (QAM), the phase detection method comprising: compensating a signal received in a carrier recovery loop into compensated signal, the compensated signal being compensated through a predetermined process; determining reference decision symbol on the compensated signal; calculating weight on the compensated signal; calculating phase error using the compensated signal and reference decision symbol; and calculating and outputting weighted phase error using the weight and phase error. [2] The phase detection method of claim 1, wherein the compensated signal is a signal where time synchronization and gain control are completely achieved. [3] The phase detection method of claim 1, wherein the carrier recovery loop controls a loop filter gain in a single mode operation state and reduces loop complexity. [4] The phase detection method of claim 1, wherein the reference decision symbol is a diagonal constellation point nearest to the compensated signal, and calculated by,

[Equation 8] din) = sgnfø («)} + jsgn{q_Q(n)}

(I denotes an in-phase symbol of the signal received in the carrier recovery loop, Q denotes a quadrature-phase symbol of the signal received in the carrier recovery loop, d(n) denotes the reference decision symbol, and q(n) denotes the compensated signal.)

[5] The phase detection method of claim 1, wherein the weighted phase error is represented as a function of the phase error and weight between the compensated signal and reference decision symbol, which is a diagonal constellation point, and is calculated by, [Equation 9] ψ_G (n) = W{q(n)} ψ{n)

(

Ψ G

(n) denotes weighted phase error, W{q(n)} denotes weight, and ψ (n) denotes the phase error.) [6] A phase detection apparatus for carrier synchronization in a high order QAM, the phase detection apparatus comprising: a phase compensator rotating a signal received in a carrier recovery loop into compensated signal, the compensated signal being compensated through a predetermined process; a reference detector determining reference decision symbol on the compensated signal; a weight calculator calculating weight on the compensated signal; a phase detector calculating phase error using the compensated signal and reference decision symbol; and a weighted phase detector calculating and outputting weighted phase error using the weight and phase error, wherein the carrier recovery loop controls a loop filter gain in a single mode operation state and reduces loop complexity. [7] The phase detection apparatus of claim 6, wherein the compensated signal is a signal where time synchronization and gain control are achieved. [8] The phase detection apparatus of claim 6, wherein the reference decision symbol is a diagonal constellation point nearest to the compensated signal, and calculated by,

[Equation 10] d(ή) = SgMq₁ («)} + j sgn{q_Q(n)}

(I denotes an in-phase symbol of a signal received in the carrier recovery loop, Q denotes a quadrature-phase symbol of the signal received in the carrier recovery loop, d(n) denotes the reference decision symbol, and q(n) denotes the compensated signal.) [9] The phase detection apparatus of claim 6, wherein the weighted phase error is represented as a function of the phase error and weight between the compensated signal and reference decision symbol, which is a diagonal constellation point, and is calculated by, [Equation 11] ψ_G (n) = W{q(n)} ' ψ(n)

(

Ψ G

(n) denotes weighted phase error, W{q(n)}denotes weight, and Ψ

(n) denotes the phase error.)