JP3970092B2 - Symbol timing synchronization apparatus and symbol timing synchronization method - Google Patents

Description

となる。ここで、Rsは単位周期内のiサンプル目の受信信号波形サンプル、lは単位周期に付した通し番号、Ncは単位周期長のサンプル点数、Nsは同相加算期間の単位周期数、αは重み係数である。
【００３３】
このとき、kサンプル目の相関値をCr(k)とすると、同相加算された受信信号波形とパイロットシンボルの送信信号レプリカ波形との複素共役の積の和で求めることができる。すなわち、
【数２】

となる。ここで、jはパイロットシンボルの送信信号レプリカのサンプル値に付けた通し番号、Rはパイロットシンボルの送信信号レプリカのサンプル数、rep^*(j)は、パイロットシンボルの送信信号レプリカのj番目のサンプル値の複素共役である。
なお、同相加算期間の中央にある受信信号に最も大きな重みを付け、端に行くに従って小さくなるようにした重みを付けるという同相加算は、同相加算する受信信号が過去および現在のものであるか、あるいは、過去、現在および未来のものであるかなどに関係しない。すなわち、現在位置が同相加算期間中のいずれにあるかに関係しない。
【００３４】
ところで、参照信号は、図２，図３を参照して説明したように、基本的にはパイロットシンボルの送信信号レプリカであるので、１シンボル長である。しかし、参照信号をそれよりも短くしてもよい。すなわち、パイロットシンボルの送信信号レプリカの一部分との部分相関を検出してもよい。
この参照信号の長さ（レプリカ長）に応じて相関検出の長さも短くなる。例えば、図３（ｈ）に示したパイロットシンボルの送信信号レプリカを、第1サンプルから第３サンプルまでの計３サンプル長とする。この場合、例えば、図３（ｃ）〜図３（ｇ）に示した同相加算信号サンプル0〜4（厳密には、その複素共役）の、先頭から３サンプルタイミング長との乗算を行うことにより、相関検出の長さが３サンプルの相関検出を行う。
【００３５】
上述したようなパイロットシンボルの送信信号レプリカの一部分との部分的な相関検出を採用することにより、相関検出器が相関検出する長さが短くなる。その結果、乗算回数が減少するので、計算量およびコストを削減できる。
なお、同相加算を行わない従来技術の場合にも、パイロットシンボルの送信信号レプリカの一部分との相関検出を行ってもよい。しかし、参照信号の長さを短くすると、その分、シンボルタイミングの同期精度は低下する。これに対し、同相加算を行う単位周期数を増やすことにより、参照信号の長さをある程度短くしてもシンボル同期タイミングを十分な精度で検出することができる。
【００３６】
上述したように、受信信号波形を同相加算したものとパイロットシンボルの送信信号レプリカとの相互相関値を検出した後は、相互相関値に基づいて、例えば、相互相関値が最高となるときの受信信号のシンボル先頭タイミング、あるいは、このシンボル先頭タイミングよりも前方へ時間オフセットさせたタイミングなどを改めてシンボル先頭タイミングとして、これより１シンボル長の有効シンボル期間を切り出せばよい。
このような有効シンボル期間を設定することにより、有効シンボル期間の設定精度を向上させるとともに、マルチパスをＧＩ期間内に位置させることができるので、隣接シンボル干渉の発生を低減させることができる。
なお、マルチパス伝搬路特性の変動に応じて、相関検出処理を、受信の進行とともに任意の検出間隔で繰り返し行い、相関値を逐次更新してもよい。
上述した説明では、本発明をセルラー方式の、基地局から各移動局に向けた下りリンクにおける移動局受信に適用した場合を説明したが、同様な作用効果を奏するシンボルタイミング同期であれば、必ずしもこのような場合に限られるものではない。
【００３７】
【発明の効果】
上述した説明から明らかなように、請求項１，４に記載の発明によれば、符号多重されたユーザデータによる符号間干渉、その他、隣接セルなどからの干渉波やノイズ等の影響を低減して、シンボルタイミングの同期精度を向上させることができるとともに、相関検出に要する計算量およびコストを削減できるという効果がある。
その結果、サブキャリアに分離する際に発生するおそれのある隣接シンボル間干渉を低減させ、通信品質の向上を図ることが可能となる。
加えて、請求項２，５に記載の発明によれば、さらに計算量およびコストが削減できるという効果がある。加えて、請求項３，６に記載の発明によれば、高速フェージングにも対応可能であるという効果がある。
【図面の簡単な説明】
【図１】本発明の実施の一形態を説明するための受信機側のブロック構成図である。
【図２】本発明の実施の一形態を説明するための波形図である。
【図３】同相加算された受信信号から相関値列を出力する具体例の説明図である。
【図４】 MC-CDMA方式の説明図である。
【図５】従来のMC-CDMA方式の送信機の概要を示す構成図である。
【図６】従来のMC-CDMA方式の受信機の概要を示す構成図である。
【図７】従来のシンボルタイミング同期装置を説明するための波形図である。
【図８】マルチパス環境における隣接シンボル間干渉の説明図である。
【符号の説明】
１…相関検出部、２…同相加算器、３…相関検出器、４…参照信号出力部、５…有効シンボル期間切り出し部、６…シンボル同期位置決定部、７…FFT部[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a symbol timing synchronization apparatus and synchronization method in a multicarrier code division multiple access (MC-CDMA) system.
[0002]
[Prior art]
Currently, the fourth generation method is being studied with the goal of higher speed and higher capacity than the third generation mobile communication method (W-CDMA). In the fourth generation scheme, further improvement in communication quality is demanded by further improving the frequency utilization rate and overcoming propagation delay, and the MC-CDMA scheme is considered promising as a scheme for realizing this.
FIG. 4 is an explanatory diagram of the MC-CDMA system. In the cellular system, it is used for the downlink from the base station to each mobile station.
FIG. 4A is a frame configuration diagram. The horizontal axis is time, the vertical axis (front of the page) is frequency (subcarrier), and the vertical axis is the number of the multiplexed code. Both the data symbol series and the pilot symbol series are shown separated by one symbol length.
In this MC-CDMA system, each data symbol sequence and pilot symbol sequence of one or a plurality of users are code-multiplexed with different spreading codes. The pilot symbol sequence is constantly transmitted. This code multiplexing is performed for each subcarrier. The carrier frequencies of the subcarriers have frequency arrangements that are orthogonal to each other, similar to Orthogonal Frequency Division Multiplexing (OFDM).
[0003]
FIG. 4B is a configuration diagram of a pilot symbol sequence. A spreading code having one symbol length as one period is assigned to the pilot symbol sequence, and the pilot symbol sequence is code-spread in the time axis direction. In order to simplify the explanation, a spreading code length of 4 chips is shown. In the example shown in the figure, as the spreading code to be assigned to the pilot symbol sequence, a code in which each chip is “1” in any subcarrier is used.
FIG. 4C is a configuration diagram of code spread pilot symbols.
The pilot symbol may be an arbitrary value for each subchannel, but it is better if one having a good autocorrelation characteristic is selected from various random patterns. In the example shown in the figure, a value of 1 or −1 on the IQ phase plane is taken and assigned to each subchannel at random, which is 1, -1,1,1, -1,. As described above, if all of the chips of the spread code are “1”, the value of the pilot symbol after code spread will continue to take the same value in each subcarrier as shown in FIG. Become. That is, the pilot symbols have the same time series waveform.
[0004]
Returning to FIG. 4 (a), a code spreading method for a data symbol sequence of a plurality of users will be described.
Each data symbol sequence of a plurality of users includes a method of code spreading in the time axis direction and a method of code spreading in the frequency axis direction. Even if only one of them is adopted, the method may be switched according to propagation path fluctuation.
When code spreading is performed in the time axis direction, the data symbol sequence is sequentially distributed to each subcarrier by serial-parallel conversion. In each subcarrier, the distributed data symbol is spread by multiplying the spread data code assigned to each user. At this time, as the spreading code assigned to each user, the same code may be assigned to any subcarrier, but may be different depending on each subcarrier.
[0005]
On the other hand, when spreading in the frequency axis direction, for example, the same symbol of each data symbol series is supplied in common to each subcarrier. The spreading code assigned to each user distributes the value of each chip to each subcarrier. In each subcarrier, the supplied data symbol is spread by multiplying the value of the chip of the spreading code assigned to each user.
As a spreading code to be assigned to each user, an orthogonal code such as an orthogonal variable spreading factor (OVSF) code or a Walsh code may be used. At this time, since the spreading code in which each chip of the spreading code is all “1” is assigned to the pilot symbol sequence shown in the figure, it is not assigned to the data symbol sequence.
In this way, when each data symbol sequence for a plurality of users is code-spread in the time axis direction, the pilot symbol sequence and each data symbol sequence for a plurality of users are code-multiplexed in the time axis direction. Become. On the other hand, even when each data symbol sequence of a plurality of users is code-spread in the frequency axis direction, the pilot symbol sequence and each data symbol sequence of a plurality of users are code-multiplexed in the frequency axis direction.
[0006]
FIG. 4D is a configuration diagram of a transmission signal.
A guard interval (hereinafter referred to as GI) is provided at the head of each one symbol section. A copy of a part of the tail of each transmission signal waveform generated based on the frame structure shown in FIG. 4A is copied as a GI, and the beginning of the original one symbol section and its It is inserted between the end of the previous symbol interval.
When separating the received signal into subcarriers by performing Fourier transform, this GI allows a shift in the window position for cutting out an effective symbol period of one symbol length. Also, in a multiple propagation path (multipath) environment, a wave transmitted from a single transmission station is received as a plurality of arrival waves having different arrival times by passing through a plurality of propagation paths (paths). . GI reduces inter-symbol interference caused by a plurality of incoming waves when performing Fourier transform.
As described above, in the MC-CDMA system, a pilot symbol sequence is code-multiplexed and transmitted together with a data symbol sequence. On the receiver side, the timing synchronized with the symbol of the transmission signal is determined based on the pilot symbol sequence component included in the received signal, and the position of the FFT window for extracting the effective symbol period of one symbol length is determined. By performing Fourier transform on the received signal within this effective symbol period, the received signal is separated by subcarrier to obtain a data symbol sequence and a pilot symbol sequence.
[0007]
FIG. 5 is a block diagram showing an outline of a conventional MC-CDMA transmitter.
The case where the data symbol series is spread in the time axis direction is shown.
The input data of the user k is modulated in advance by the QPSK method or the like and is input to the serial / parallel converter 21 as a data symbol series. The serial / parallel converter 21 parallelizes (distributes) the data symbol series. The parallelized data symbol sequence is multiplied by the multiplier 22. ₀ ~ 22 _Nc-1 In each of the subcarriers # 0 to # Nc-1, _{k, 0} (T) -c _{k, Nc-1} (T) is multiplied and spread processing is performed. Nc is the number of subcarriers.
Multiplexer 23 ₀ ~ 23 _Nc-1 , The code symbol-spread data symbol sequence of the user k is multiplexed with the code-spread data symbol sequence and the code-spread pilot symbol sequence of the other users, and the result is sent to the fast inverse Fourier transform (IFFT) unit 24. Is output. The IFFT unit 24 frequency-multiplexes and creates one series of time series waveforms. The guard interval adding unit 25 inserts a GI between the head of each symbol of the time series waveform and the tail of the immediately preceding symbol and outputs an MC-CDMA transmission signal.
[0008]
FIG. 6 is a block diagram showing an outline of a conventional MC-CDMA receiver.
The case where the user symbol series is spread in the time axis direction is shown.
In the OFDM scheme, a method for detecting autocorrelation of a received signal waveform and a method for detecting a cross-correlation between a received signal waveform and a replica are known as symbol synchronization detection methods. In the method of detecting autocorrelation, the fact that the GI and the copy source symbol tail section are the same is used, or a repeated pattern is inserted into the preamble at the beginning of the burst. In the method of detecting the cross-correlation, a predetermined pattern is inserted into the preamble at the beginning of the burst, and the cross-correlation between the received signal waveform and a replica (replica) of the predetermined pattern is detected.
[0009]
Hereinafter, in the MC-CDMA system described above, a case will be described in which a cross-correlation with a replica (replica) of a time-series waveform of pilot symbols is detected.
The MC-CDMA reception signal is converted into a complex baseband signal (I-phase signal, Q-phase signal) by a quadrature demodulator (not shown), A / D converted, and converted into a digital value. A correlation detection unit 31 detects a cross-correlation value of the received signal with a reference signal output from the reference signal output unit 4 over a period (hereinafter referred to as a unit period) in which a guard interval is added to one symbol period. And is supplied to the effective symbol period cutout unit 5.
A pilot symbol transmission signal replica is used as the reference signal. The pilot symbol transmission signal replica means a replica of the transmission signal waveform when only the code-spread pilot symbol sequence is frequency-multiplexed in the IFFT unit 24 in the transmitter shown in FIG. The length is one symbol length and does not include the GI section.
The reference signal output unit 4 is realized by, for example, a memory that stores a reference signal. The correlation detection unit 31 is realized by a sliding correlator, a matched filter, or the like.
[0010]
The symbol synchronization position determination unit 6 determines the symbol synchronization position based on the timing at which the correlation value peak is detected from the correlation detection unit 31. The effective symbol period cutout unit 5 cuts out a reception signal in the effective symbol period in the FFT window based on the determined symbol synchronization position, and outputs the cut signal to the FFT unit 7.
In the FFT unit 7, the received signal in the FFT window period is Fourier transformed and separated into subcarriers # 0 to # Nc-1. The separated received signals are respectively correlated with the correlation detector 32. ₀ ~ 32 _Nc-1 The spreading code c assigned to user k _{k, 0} (T) -c _{k, Nc-1} The received signal is despread by detecting the correlation with (t), and the data symbol sequence of user k is restored.
[0011]
On the other hand, the pilot symbol sequence in each subcarrier is despread and demodulated in channel estimation unit 33. By detecting the phase variation of the demodulated pilot symbol of each subcarrier, the propagation path characteristics of each subcarrier are estimated. A compensation signal is created for each subcarrier based on the estimated value of the propagation path characteristic. Multiplier 34 ₀ ~ 34 _Nc-1 , Phase compensation or the like is performed by multiplying the data symbol sequence of user k by the compensation signal.
The parallel-serial converter 35 rearranges the compensated data symbol sequence of the user k in each subcarrier into one sequence and outputs the data symbol sequence of the user k. The data symbol sequence of user k is demodulated to become a received data sequence of user k.
[0012]
FIG. 7 is a waveform diagram for explaining a conventional symbol timing synchronization apparatus. It is an explanatory waveform diagram and does not show an accurate waveform.
FIG. 7A shows a waveform of data symbol sequence components synthesized for all users included in the MC-CDMA transmission signal.
FIG. 7B is a waveform of a pilot symbol sequence component included in the MC-CDMA transmission signal.
FIG. 7C shows a waveform of a noise component added in the propagation path.
FIG. 7D is a waveform of the MC-CDMA reception signal obtained by synthesizing the waveforms of FIGS. 7A to 7C.
FIG. 7 (e) to FIG. 7 (g) show the waveform of the transmission signal replica of the pilot symbol with respect to the reception signal within the unit period (GI + 1 symbol). _e , Τ _f , Τ _g Shows only delayed ones.
[0013]
The correlation detector 31 shown in FIG. 6 cuts out a waveform of a unit period from the MC-CDMA received signal waveform shown in FIG. The cut-out start timing is random. The correlation between the cut-out received signal and the pilot symbol transmission signal replica output from the reference signal output unit 4 is detected.
The cut out received signal is stored in, for example, a memory, and the waveform of the transmission signal replica of the pilot symbol and the waveform obtained by sequentially delaying (cyclically shifting) the waveform within a unit period that is a GI + 1 symbol period (strictly speaking, Multiplication with (complex conjugate). As shown in FIGS. 7E to 7G, multiplication of the delayed pilot symbols with the transmission signal replica is performed. No multiplication is performed in the GI section on the transmission signal replica side of the pilot symbol. The position of the symbol synchronization timing is known according to the delay τ of the transmission signal replica when the cross-correlation value is maximized.
FIG. 7H shows the output of the correlation detection unit 31. For example, 41e, 41f, and 41g are correlation values with the pilot symbol transmission signal replicas shown in FIGS. 7 (e) to 7 (g), respectively. The output timing of the correlation value is displayed in accordance with the timing (post-GI timing) of the symbol head portion of each transmission signal replica.
In the example shown in the figure, the waveform of the pilot symbol component shown in FIG. 7B and the waveform of the transmission signal replica of the pilot symbol shown in FIG. Accordingly, the correlation value 41f in FIG.
[0014]
In the above description, the basic case in which only one wave from the transmitting station reaches the receiving station has been described. In a multipath environment, since a plurality of incoming waves reach a receiving station from one transmitting station, interference between adjacent symbols occurs.
FIG. 8 is an explanatory diagram of interference between adjacent symbols in a multipath environment.
An example will be described where delay wave 1 to delay wave 3 arrive after the first preceding arrival wave. A combination of these incoming waves is a received signal. n-1, n, and n + 1 are numbers assigned to the symbols. The hatched portion is GI.
The output timing of the correlation value is displayed in accordance with the symbol head portion of the transmission signal replica of the pilot symbol. 51 ₀ , 51 ₁ , 51 ₂ , 51 _Three Are correlation values output by correlation with the preceding wave, the delayed wave 1, the delayed wave 2 and the delayed wave 3, respectively. In the example shown, the correlation value 51 ₁ Is the largest. This means that the reception level of the delay wave 1 is the highest. In mobile communication, non-line-of-sight communication is common, and the peak position of the correlation value does not always indicate the preceding wave.
[0015]
In such a situation, the correlation value 51 having the highest reception level ₁ Is a symbol synchronization timing, if the Fourier transform of the nth symbol is performed in the period of the effective symbol period (FFT window c), a delay wave 1 and a delay having a delay within GI from the delay wave 1 As for the wave 2, both are cut out from the nth symbol, and no intersymbol interference occurs. However, if there is a preceding wave as shown in the figure that precedes the delayed wave 1 even slightly, the GI of the (n + 1) th symbol is included in this waveform, so Interference occurs.
Therefore, the symbol head of the delayed wave 1 having the largest correlation value is not set as the symbol synchronization position, but the timing slightly ahead of this is set as the symbol synchronization position.
[0016]
As an example of OFDM, Akinori Hira et al., “Timing synchronization method of OFDM communication system in frequency selective fading environment”, IEICE Transactions B, vol.J84-B, No.7, (2001- 7), pp.1255-1264, the following method is adopted. That is, the correlation value sequence is searched forward from the timing of the peak sample indicating the maximum correlation value in the correlation value sequence, and among the correlation values exceeding 1 / a (a is a parameter) of the maximum correlation value, The position of the correlation value at the forefront is defined as the symbol synchronization position.
In the example of FIG. 8, the correlation value 51 ₀ Is used as the symbol synchronization timing, and the Fourier transform of the nth symbol is performed in the period of the effective symbol period (FFT window b). Therefore, with respect to the subsequent delayed wave, the allowable delay amount that does not cause interference with the preceding symbol (n−1) is reduced correspondingly, but with respect to the preceding wave, interference with the subsequent (n + 1) symbol is reduced. An allowable delay amount that does not occur can be obtained.
In general, it is considered that incoming waves are scattered before and after a delayed wave having the largest cross-correlation value. In such a case, interference between adjacent symbols due to a plurality of incoming waves can be reduced.
[0017]
In the OFDM scheme, since the preamble transmission interval and the user data symbol transmission interval are temporally different, there is no intersymbol interference between them. However, in the MC-CDMA system as shown in FIG. 4, since the pilot symbol sequence and the user data symbol sequence are code-multiplexed and transmitted simultaneously, the intersymbol interference is large. In the cellular system, interference waves from adjacent cells cannot be ignored.
Therefore, simply calculating the cross-correlation between the pilot symbol and the transmission signal replica as described with reference to FIG. 7 includes a lot of noise and interference waves in the correlation value output. Detection accuracy is degraded. As a result, since the effective symbol period cannot be accurately cut out, in either case of single path or multipath, when separating into subcarrier components, there is a possibility that interference between adjacent symbols may occur and reception signal quality may deteriorate. is there.
[0018]
[Problems to be solved by the invention]
The present invention has been made in order to solve the above-described problems, and includes a guard interval in which each data symbol sequence and pilot symbol sequence of one or a plurality of users is code-division multiplexed in each of a plurality of orthogonal subcarriers. An object of the present invention is to provide a symbol timing synchronization apparatus and a symbol timing synchronization method capable of improving the accuracy of symbol timing synchronization and reducing the amount of calculation and cost in a multicarrier CDMA system using a transmission signal to which is added. To do.
[0019]
[Means for Solving the Problems]
According to the present invention, the guard interval is added to the data symbol sequence and the pilot symbol sequence of one or a plurality of users code-multiplexed in a plurality of orthogonal subcarriers. A symbol timing synchronization apparatus in a multicarrier CDMA system using a transmission signal, comprising reference signal output means, an in-phase adder, a correlation detector, and symbol synchronization timing determination means, wherein the reference signal output means is the pilot symbol sequence 1 symbol interval is output as a reference signal for the transmission signal when only the signal is transmitted, and the in-phase adder uses the interval in which the guard interval is added to the 1 symbol interval of the transmission signal as a unit period. , Adding the received signal in phase over a plurality of periods of the unit period, A function detector that outputs a correlation value between the received signal that has undergone in-phase addition by the in-phase adder and the reference signal that is output from the reference signal output means; and the symbol synchronization timing determination means that includes the correlation detector The timing of symbol synchronization with the transmission signal is determined on the basis of the output timing of the correlation value output by the.
Therefore, even if there is inter-symbol interference due to the user data symbol sequence due to the in-phase addition over a plurality of unit cycles, the components of the pilot symbol sequence are emphasized, and the synchronization accuracy of the symbol timing can be improved. At the same time, since the amount of calculation for correlation detection is larger than that for in-phase addition, the amount of calculation and cost can be reduced as compared with the case where the correlation value is added in phase after detecting the correlation over a plurality of unit cycles.
[0020]
According to the second aspect of the present invention, a transmission signal in which a guard interval is added to each data symbol sequence and pilot symbol sequence of one or a plurality of users code-multiplexed in a plurality of orthogonal subcarriers is used. A symbol timing synchronization apparatus in a carrier CDMA system, comprising a reference signal output means, an in-phase adder, a correlation detector, and a symbol synchronization timing determination means, wherein the reference signal output means transmits only the pilot symbol sequence For the transmission signal, a part of one symbol period is output as a reference signal, and the in-phase adder uses a period obtained by adding the guard interval to the one symbol period of the transmission signal as a unit period. The received signals are added in phase over a plurality of periods of the unit period, and the correlation detection is performed. A correlation value between the received signal that has been subjected to the in-phase addition by the in-phase adder and the reference signal that is output from the reference signal output means, and the symbol synchronization timing determination means that is output from the correlation detector The timing of symbol synchronization with the transmission signal is determined based on the output timing of the correlation value.
Therefore, as in the first aspect of the invention, the symbol timing synchronization accuracy can be improved, and the calculation amount and cost can be reduced.
In addition, the length of correlation detection by the correlation detector is shortened according to the shortening of the length of the reference signal. As a result, since the number of multiplications is reduced, the amount of calculation and cost can be reduced. Since in-phase addition is performed, sufficient synchronization accuracy of the symbol timing can be obtained even if the length of the reference signal is shortened.
[0021]
According to a third aspect of the present invention, in the symbol timing synchronization device according to the first or second aspect, the in-phase adder has the unit in the center with respect to the received signal of each unit period to be subjected to in-phase addition. The in-phase addition is performed by assigning the largest weight to the period and assigning a smaller weight to the end.
Therefore, since the phase change at the same sample position in the unit period increases as the distance between the reception signals to be added in phase increases, the weight is determined. It becomes possible to prevent the detection accuracy of symbol synchronization timing from deteriorating.
[0022]
The invention described in claims 4 to 6 is a symbol timing synchronization method described from the viewpoint of the method invention using the means constituting the invention described in claims 1 to 3 as a processing step. The same effects as those of the described invention are achieved.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a block diagram of a receiver side for explaining an embodiment of the present invention. In the figure, the same parts as those in FIG.
As in the prior art, the present invention also detects a pilot symbol by detecting a cross-correlation between a reception signal waveform and a replica representing a transmission waveform of a pilot symbol, and performs symbol synchronization with the transmission signal. . In the prior art, cross-correlation detection is performed in a unit period (GI + 1 symbol). On the other hand, the present invention performs correlation detection after performing in-phase addition over a plurality of unit cycles.
The correlation detection unit 1 includes an in-phase adder 2 and a correlation detector 3. The correlation detector 3 corresponds to the correlation detector 31 shown in FIG.
The received signal is input to the in-phase adder 2, where the received signal is subjected to in-phase addition over a plurality of unit periods, and the received signal subjected to in-phase addition is output. Specifically, in-phase addition is performed in units of unit periods for each I-phase component and Q-phase component of the received signal that is a complex baseband signal. In-phase addition is performed in a range where the propagation path does not change significantly. The correlation detector 3 outputs a correlation value with a reference signal (a transmission signal replica of a pilot signal) output from the reference signal output unit 4 for the reception signal subjected to the in-phase addition.
[0024]
FIG. 2 is a waveform diagram for explaining an embodiment of the present invention. Corresponding to FIG. 7 relating to the prior art, like FIG. 7, it is an explanatory waveform diagram and does not show the waveform accurately.
2A to 2D are the same as FIGS. 7A to 7D.
FIG. 2 (e) shows the received signal of FIG. ₁ ~ 11 _Four As shown in FIG. 4, the waveform is divided into unit periods at an arbitrary cut-out timing and in-phase added over four periods as an example. In-phase addition is performed by adding the I-phase and Q-phase components of the received signal.
Since the data symbol changes arbitrarily, as shown in FIG. 2A, the waveform of the data symbol sequence component differs for each unit period. The same applies to the noise component shown in FIG. On the other hand, the same waveform of the pilot symbol sequence component shown in FIG. 2B is repeated in a unit cycle. Therefore, as shown in FIG. 2 (e), the received signal subjected to the in-phase addition has an enhanced pilot symbol sequence component, and data symbol sequence component, noise component, and other interference components are reduced.
[0025]
As a result, the correlation detector 2 shown in FIG. 1 receives the in-phase added reception signal of FIG. 2 (e) and is output from the reference signal output unit 4. The pilot symbol shown in FIG. By outputting the correlation value with the transmission signal replica, a correlation value in which the influence of noise and the influence of intersymbol interference with the user data symbol sequence is reduced can be obtained.
Similar to the prior art described with reference to FIG. 8, correlation values are output at a plurality of timings according to the multipath delay time. The symbol synchronization position determination unit 4 determines a symbol synchronization timing suitable for reducing intersymbol interference from a plurality of timings at which a large correlation value is output, as in the prior art.
[0026]
FIG. 3 is an explanatory diagram of a specific example in which a correlation value sequence is output from a reception signal subjected to in-phase addition.
FIG. 3A shows a received signal, and FIG. 3B shows a received signal 11 cut out from the received signal by dividing it by a unit cycle. ₁ , 11 ₂ , 11 _Three , 11 _Four It is. For convenience, the unit period is set to 5 sample timings and the 0th sample is set to GI so as to match the number of chips of the spreading code. It is assumed that the reception signal cut-out start timing is performed in the fourth sample.
FIG. 3C shows an in-phase addition signal sample 0 obtained by in-phase addition of the cut-out received signal over four periods. 3 (d), 3 (e), 3 (f), and 3 (g), the in-phase addition signal sample 0 is sequentially shifted to the left (1) within one unit period (5 sample timings). In-phase addition signal samples 1 to 4 are cyclically shifted by one sample timing in the past) direction.
FIG. 3H shows a pilot symbol transmission signal replica output by the reference signal output unit 4.
[0027]
The correlation detector 3 shown in FIG. 1 has, for example, each symbol length of the in-phase addition signal samples 0 to 4 shown in FIGS. 3 (c) to 3 (g) (in the example shown, 4 samples from the head). 3) and the transmission signal replica (strictly speaking, its complex conjugate) of the 4-sample pilot symbol shown in FIG. Detection is performed, and a correlation value sequence as shown in FIG. In the illustrated example, since the pilot symbol component included in the in-phase addition signal sample 1 shown in FIG. 3E matches the transmission signal replica of the pilot symbol, the in-phase addition signal sample 2 outputs the maximum correlation value. It will be.
As a result, it can be seen that the head of the symbol is two sample timings after the cut-out start sample timing of the cut-out received signal.
[0028]
In the above description, the in-phase addition signal sample 0 is cyclically shifted. Instead, the in-phase addition signal sample 0 remains as it is and is multiplied by the pilot symbol transmission signal replica that is cyclically shifted within one unit period as shown in FIGS. 7 (e) to 7 (g). By doing so, cross-correlation may be detected.
In the above description, real-time processing has been described. That is, as shown in FIG. 3B, a past received signal 11 that has already been received. ₁ ~ 11 _Three And the current received signal 11 for which the symbol synchronization timing is determined and reception processing such as subsequent FFT processing is performed. _Four , And in-phase addition and correlation detection.
Instead of the above-described past received signal, or in addition to the above-described current and past received signals, a future, for example, received signal 11 (not shown) in the future after the current received signal to be subjected to reception processing. _Five ~ 11 ₇ Up to in-phase addition and correlation detection, _Four On the other hand, the symbol synchronization timing may be determined and the subsequent reception process may be performed.
In order to simplify the description, the correlation detection process has been described as being performed with 4 samples per symbol. In general, the cross-correlation with the transmission signal replica is detected at a sampling interval in the FFT unit 7 that is sufficiently shorter than this.
[0029]
The correlation detection unit 1 described above performs correlation detection after in-phase addition of received signals. Therefore, the correlation detection over a plurality of unit cycles can be realized by a single correlation detection process with the reference signal.
On the other hand, when the correlation detection is performed a plurality of cycles (4 cycles in the illustrated example) and the correlation detection values are added in phase over a plurality of unit cycles, the correlation detection including multiplication is compared with the in-phase addition. Since the amount of calculation increases, the amount of calculation and cost can be reduced by detecting the correlation after adding the received signals in-phase first.
[0030]
In the above description, the received signal 11 cut out in the unit period in the in-phase adder 2. ₁ , 11 ₂ , 11 _Three , 11 _Four Were added in phase. Even when the number of in-phase addition periods, in other words, the in-phase addition period is set within a range of several tens of msec where the multipath environment does not change, if the maximum Doppler frequency related to the moving body speed is large, Also, the phase variation of the received pilot symbol component increases, and the effect of the in-phase addition decreases. At this time, it is presumed that the phase change at the same sample position in the unit period becomes larger as the distance between the reception signals of a plurality of periods to be added in-phase increases.
[0031]
Therefore, for the received signal period that is the target of in-phase addition, the unit signal is detected after performing the in-phase addition with the largest weight on the received signal sample value at the center and the smaller weight as it goes to the end. Just do it. Specifically, the weight value may be given for each unit period of the received signal. That is, in the example shown in FIG. 3B, each received signal 11 cut out in a unit cycle. ₁ , 11 ₂ , 11 _Three , 11 _Four On the other hand, the received signal 11 of the unit period in the center ₂ , 11 _Three (This is an example where the number of in-phase addition cycles is an even number, so the two received signals are in the center). ₁ , 11 _Four As you go to the end, you can add smaller weights. By such weighting, it is possible to prevent the detection accuracy of the symbol synchronization timing from deteriorating during high-speed fading, and to determine an appropriate effective symbol period.
[0032]
More specifically, a weight coefficient α (0 <α <1) may be used. For example, each received signal 11 cut out in the unit cycle shown in FIG. ₁ , 11 ₂ , 11 _Three , 11 _Four Each with a weight α ¹ , Α ⁰ , Α ⁰ , Α ¹ And then in-phase addition to detect the correlation of the unit period.
An example using a general formula for the above-described weighted in-phase addition and correlation detection is as follows.
If the received signal waveform sample of the i-th sample within the unit period added in phase is Rp (i),
[Expression 1]

It becomes. Here, Rs is the received signal waveform sample of the i-th sample within the unit period, l is a serial number assigned to the unit period, Nc is the number of sample points of the unit period length, Ns is the number of unit periods in the in-phase addition period, and α is a weighting factor It is.
[0033]
At this time, if the correlation value of the k-th sample is Cr (k), it can be obtained by the sum of products of complex conjugates of the reception signal waveform subjected to in-phase addition and the transmission signal replica waveform of the pilot symbol. That is,
[Expression 2]

It becomes. Where j is a serial number assigned to the sample value of the pilot symbol transmission signal replica, R is the number of pilot symbol transmission signal replica samples, rep ^* (j) is a complex conjugate of the j-th sample value of the pilot symbol transmission signal replica.
In addition, the common mode addition that gives the largest weight to the received signal in the center of the common mode addition period and the weight that becomes smaller as it goes to the end is whether the received signal to be added in the common mode is the past or the current one, Or it doesn't matter whether it is in the past, present or future. That is, it does not relate to where the current position is during the in-phase addition period.
[0034]
By the way, as described with reference to FIGS. 2 and 3, the reference signal is basically a transmission signal replica of a pilot symbol, and thus has a length of one symbol. However, the reference signal may be shorter. That is, a partial correlation with a part of the transmission signal replica of the pilot symbol may be detected.
The length of correlation detection is shortened according to the length of the reference signal (replica length). For example, the pilot symbol transmission signal replica shown in FIG. 3 (h) has a total length of 3 samples from the first sample to the third sample. In this case, for example, by multiplying the in-phase addition signal samples 0 to 4 (strictly, its complex conjugate) shown in FIGS. 3C to 3G by the 3 sample timing length from the head. Correlation detection with a correlation detection length of 3 samples is performed.
[0035]
By adopting partial correlation detection with a part of the transmission signal replica of the pilot symbol as described above, the length of correlation detection by the correlation detector is shortened. As a result, since the number of multiplications is reduced, the amount of calculation and cost can be reduced.
In the case of the prior art that does not perform in-phase addition, correlation detection with a part of a transmission signal replica of a pilot symbol may be performed. However, if the length of the reference signal is shortened, the synchronization accuracy of the symbol timing is lowered accordingly. In contrast, by increasing the number of unit cycles for performing in-phase addition, the symbol synchronization timing can be detected with sufficient accuracy even if the length of the reference signal is shortened to some extent.
[0036]
As described above, after detecting the cross-correlation value between the in-phase addition of the received signal waveform and the transmission signal replica of the pilot symbol, based on the cross-correlation value, for example, reception when the cross-correlation value is the highest. An effective symbol period of one symbol length may be extracted from the symbol head timing of the signal or the timing offset in time ahead of the symbol head timing as the symbol head timing.
By setting such an effective symbol period, the setting accuracy of the effective symbol period can be improved and the multipath can be positioned within the GI period, so that the occurrence of adjacent symbol interference can be reduced.
Note that the correlation detection process may be repeatedly performed at arbitrary detection intervals as reception progresses, and the correlation values may be sequentially updated in accordance with fluctuations in multipath propagation path characteristics.
In the above description, the case where the present invention is applied to cellular reception in the downlink from the base station to each mobile station has been described. However, if symbol timing synchronization with the same effect is provided, it is not always necessary. It is not limited to such a case.
[0037]
【The invention's effect】
As is apparent from the above description, according to the first and fourth aspects of the present invention, it is possible to reduce the influence of intersymbol interference caused by code-multiplexed user data and other interference waves and noise from neighboring cells. Thus, the synchronization accuracy of the symbol timing can be improved, and the calculation amount and cost required for correlation detection can be reduced.
As a result, it is possible to reduce the interference between adjacent symbols that may occur when separating into subcarriers and improve the communication quality.
In addition, according to the second and fifth aspects of the invention, there is an effect that the calculation amount and the cost can be further reduced. In addition, according to the third and sixth aspects of the invention, there is an effect that it is possible to cope with high-speed fading.
[Brief description of the drawings]
FIG. 1 is a block diagram of a receiver side for explaining an embodiment of the present invention.
FIG. 2 is a waveform diagram for explaining an embodiment of the present invention.
FIG. 3 is an explanatory diagram of a specific example in which a correlation value sequence is output from a reception signal subjected to in-phase addition.
FIG. 4 is an explanatory diagram of an MC-CDMA system.
FIG. 5 is a block diagram showing an outline of a conventional MC-CDMA transmitter.
FIG. 6 is a block diagram showing an outline of a conventional MC-CDMA receiver.
FIG. 7 is a waveform diagram for explaining a conventional symbol timing synchronization apparatus;
FIG. 8 is an explanatory diagram of interference between adjacent symbols in a multipath environment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Correlation detection part, 2 ... In-phase adder, 3 ... Correlation detector, 4 ... Reference signal output part, 5 ... Effective symbol period cutout part, 6 ... Symbol synchronous position determination part, 7 ... FFT part

Claims (6)

A symbol timing synchronization apparatus in a multi-carrier CDMA system using a transmission signal in which each data symbol sequence and pilot symbol sequence of one or a plurality of users is code-division multiplexed in a plurality of orthogonal subcarriers and a guard interval is added. There,
A reference signal output means, an in-phase adder, a correlation detector, and a symbol synchronization timing determination means;
The reference signal output means outputs one symbol interval as a reference signal for the transmission signal when only the pilot symbol sequence is transmitted,
The in-phase adder performs in-phase addition of the reception signal over a plurality of periods of the unit period, with a period obtained by adding the guard interval to the one symbol period of the transmission signal as a unit period,
The correlation detector outputs a correlation value between the received signal that has undergone in-phase addition by the in-phase adder and the reference signal that is output from the reference signal output means,
The symbol synchronization timing determination means determines a timing for symbol synchronization with the transmission signal based on an output timing of the correlation value output by the correlation detector.
A symbol timing synchronizer. A symbol timing synchronization apparatus in a multi-carrier CDMA system using a transmission signal in which each data symbol sequence and pilot symbol sequence of one or a plurality of users is code-division multiplexed in a plurality of orthogonal subcarriers and a guard interval is added. There,
A reference signal output means, an in-phase adder, a correlation detector, and a symbol synchronization timing determination means;
The reference signal output means outputs a part of one symbol period as a reference signal for the transmission signal when only the pilot symbol sequence is transmitted,
The in-phase adder performs in-phase addition of the reception signal over a plurality of periods of the unit period, with a period obtained by adding the guard interval to the one symbol period of the transmission signal as a unit period,
The correlation detector outputs a correlation value between the received signal that has undergone in-phase addition by the in-phase adder and the reference signal that is output from the reference signal output means,
The symbol synchronization timing determination means determines a timing for symbol synchronization with the transmission signal based on an output timing of the correlation value output by the correlation detector.
A symbol timing synchronizer. The in-phase adder performs the in-phase addition with the largest weight on the unit period at the center and the smaller weight toward the end for the reception signal of each unit period to be subjected to in-phase addition,
The symbol timing synchronization apparatus according to claim 1 or 2, wherein A symbol timing synchronization method in a multi-carrier CDMA system using a transmission signal in which each data symbol sequence and pilot symbol sequence of one or a plurality of users is code division multiplexed on a plurality of orthogonal subcarriers and a guard interval is added. There,
A reference signal output step, an in-phase addition step, a correlation detection step, and a symbol synchronization timing determination step,
The reference signal output step outputs, as a reference signal, one symbol period for the transmission signal when only the pilot symbol sequence is transmitted,
The in-phase addition step performs in-phase addition of the received signal over a plurality of periods of the unit period, with a period obtained by adding the guard interval to the one symbol period of the transmission signal as a unit period,
The correlation detection step outputs a correlation value between the received signal that has been added in phase by the in-phase addition step and the reference signal that is output from the reference signal output step,
The symbol synchronization timing determination step determines a timing of symbol synchronization with the transmission signal based on an output timing of the correlation value output by the correlation detection step.
A symbol timing synchronization method. A symbol timing synchronization method in a multi-carrier CDMA system using a transmission signal in which each data symbol sequence and pilot symbol sequence of one or a plurality of users is code division multiplexed on a plurality of orthogonal subcarriers and a guard interval is added. There,
A reference signal output step, an in-phase addition step, a correlation detection step, and a symbol synchronization timing determination step,
The reference signal output step outputs a part of one symbol period as a reference signal for the transmission signal when only the pilot symbol sequence is transmitted,
The in-phase addition step performs in-phase addition of the received signal over a plurality of periods of the unit period, with a period obtained by adding the guard interval to the one symbol period of the transmission signal as a unit period,
The correlation detection step outputs a correlation value between the received signal that has been added in phase by the in-phase addition step and the reference signal that is output from the reference signal output step,
The symbol synchronization timing determination step determines a timing of symbol synchronization with the transmission signal based on an output timing of the correlation value output by the correlation detection step.
A symbol timing synchronization method. The in-phase addition step performs the in-phase addition by assigning the largest weight to the unit period at the center and the smaller weight toward the end for the reception signal of each unit period to be subjected to in-phase addition,
6. The symbol timing synchronization method according to claim 4, wherein the symbol timing is synchronized with each other.

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