JP4255820B2 - Multi-carrier CDMA system and its transmitting and receiving stations - Google Patents

Description

  The present invention relates to a multi-access spreading code allocation method in a multi-carrier CDMA system applied to mobile communications, a transmission station and a receiving station to which the allocation method is applied, and a system using them.

  The multi-carrier CDMA (hereinafter referred to as MC-CDMA) system, which combines OFDM (Orthogonal Frequency Division Multiplexing) and CDMA (Code Division Multiple Access System), utilizes high tolerance to delay waves and frequency orthogonality found in OFDM. In recent years, research and development has been actively promoted as having both the characteristics of high frequency utilization efficiency and the characteristics of high frequency utilization efficiency in a multi-cell environment found in CDMA.

  MC-CDMA can be broadly divided into two types: time-domain spread MC-CDMA using DSSS (Direct Sequence Spread Spectrum) and frequency-domain spread MC-CDMA in which one chip of a spreading code is assigned to each subcarrier. it can. In the latter case, when the OFDM symbol is asynchronous between the allocation targets as in the uplink, the characteristics deteriorate due to the influence of asynchronous reception at the receiver, and when the frequency selective fading is different for each allocation target, the receiver , The cross correlation (inter-code interference; ICI) with different spreading codes included in the despread output increases, and the characteristics deteriorate. Even in the downlink, there are cases where ICI cannot be ignored when synthesizing data symbols spread in the frequency domain in the despreading stage, thereby degrading characteristics (Non-Patent Documents 1 and 2).

  In any MC-CDMA, when the allocation target or data is multiplexed, the orthogonality of the spreading code is used to perform the process of allocating the specified spreading code to each allocation target or each data block. At that time, a process of assigning a code randomly selected from the defined spreading codes is generally performed.

E. A. Sourour and M. Nakagawa, “Performance of orthogonal multicarrier CDMA in a multipath fadingchannel,” IEEE Trans. Commun., Vol.44, pp.559-569, March 1996 S. Hara and R. Prasad, “Overview of multicarrier CDMA,” IEEE Commun. Mag., Pp.126-133, Dec. 1997 Takashi Shono, Tomoyuki Yamada, Kiyoshi Kobayashi, Katsuhiko Araki, Satoshi Hirose “Multiple access interference in frequency-selective fading channels on uplink MC-CDMA,” IEICE Technical Report, RCS2003-180, Nov. 2003

  As described above, when frequency domain spread MC-CDMA is applied to mobile communication, ICI occurs when despreading is performed in the receiver due to various factors. Although this ICI can be quantitatively evaluated as DUR (desired wave to interference wave power ratio), it has been shown that DUR varies greatly depending on the combination of spreading codes assigned at the time of multiple access (Non-patent Document 3). Accordingly, an object of the present invention is to maximize system performance by minimizing ICI (that is, maximizing DUR) in frequency domain spread MC-CDMA multiple access.

To achieve the above object, the present invention comprises a replicator that duplicates a symbol stream, a frequency domain spreader that multiplies a spreading code in the frequency domain, and an inverse discrete Fourier transformer or an inverse fast Fourier transformer. A transmission station, a discrete Fourier transformer or a fast Fourier transformer, a channel estimator that estimates and corrects distortion due to a propagation path, a frequency domain despreader that multiplies the spreading code in the frequency domain and adds the spread code. A multi-carrier CDMA system configured to determine the spreading code based on a DUR standard, that is, a procedure for evaluating system performance from a desired signal-to-interference power ratio , wherein N is 2 or more as and spreading factor S smaller than natural number, when assigning N number of the spreading codes, the DUR norms, unions assignable _S C _N Street From relate the N combinations of the spreading code obtained, the desired power of spreading code k and D _k, U _kl spreading codes l is interference power to be supplied to spreading code k, U _k a non-spreading code k N- when each one of said spreading code is the sum of the interference power given to the spreading code k, (the number of claim 1, a signal to interference power DUR shown by (D / U) _k for the spreading code k 1) by obtained, the calculating the DUR for N combinations resulting from the combination of street _S C _N, in the set of _S C _N Street, of N based on equation (1) It defines a multi-carrier CDMA system that selects the combination of spreading codes that maximizes the minimum value of the DUR.

In Claim 2 , the replicator which duplicates a symbol stream, The frequency domain spreader which multiplies a spreading code by a frequency domain, The transmission station which has an inverse discrete Fourier transformer or an inverse fast Fourier transformer, A discrete Fourier transform Or a fast Fourier transformer, a channel estimator that estimates and corrects distortion due to a propagation path, and a receiving station that includes a frequency domain despreader that multiplies and adds the spreading code in the frequency domain. A multi-carrier CDMA system that determines the spreading code based on a DUR standard, that is, a procedure for evaluating system performance from a desired wave-to-interference wave power ratio , wherein N is a natural number greater than 2 and smaller than a spreading factor S , when assigning N number of the spreading codes, the DUR norms, the N diffusion marks resulting from the combination of assignable _S C _N Street The present invention relates to a combination, the desired power of spreading code k and D _k, the interference power U _kl spreading codes l gives the spreading code k, respectively (N-1) of the spreading codes other than the spreading code k and U _k is When the sum of the interference powers given to the spreading code k is given, the desired wave-to-interference wave power DUR for the spreading code k is expressed by (D / U) _k and is obtained by the equation (2), Based on the equation (2), the DUR is calculated for N combinations obtained from the _S C _N combinations, and the average value of the N DURs becomes the maximum in the _S C _N combinations. It defines a multi-carrier CDMA system that selects the combination of spreading codes.

In claim 3 , a specific spreading code is selected and assigned to each of the assigned number N of spreading codes N = 2, 3,..., S from among the combinations of spreading codes obtained by the DUR norm. 3. The multi-carrier CDMA system according to claim 1 or 2 , wherein k is a natural number greater than or equal to 2 and smaller than a spreading factor S, and a specific combination of a specific spreading code corresponding to N = k and a specific number corresponding to N = k + 1 The combination of specific spreading codes is defined by claim 3 , wherein the combination of spreading codes is compared, and if the number of N = k + 1 combinations that do not include N = k combination elements is R _k , the specific spreading code combination is The multi-carrier CDMA system is selected so that the value of the equation (3) is minimized.

According to a fourth aspect of the present invention, in the multi-carrier CDMA system according to the first or second aspect , when a combination element of the selected spread code is assigned to a spread code assignment target, a preset high priority is provided. It defines a multi-carrier CDMA system that allocates spreading codes with a high DUR in order from the allocation target.

According to a fifth aspect of the present invention, in the multi-carrier CDMA system according to the first or second aspect , elements of the combination of the selected spreading codes are sequentially switched in synchronization with a predetermined time interval as a spreading code allocation target. It stipulates the configuration.

In claim 6 is a multi-carrier CDMA system that comprises a replicator for duplicating the symbol stream, and the frequency domain spreader for multiplying the spreading codes in the frequency domain, an inverse discrete Fourier transformer or Fast Fourier Transform It defines the transmitting station.

In claim 7 , a discrete Fourier transformer or a fast Fourier transformer, a channel estimator that estimates and corrects distortion caused by propagation, a frequency domain despreader that multiplies and adds a discrete code in the frequency domain, and Is defined for a multi-carrier CDMA system receiving station.

  According to the multicarrier CDMA apparatus and spreading code allocation method of the present invention, ICI generated by frequency selective fading in frequency domain spreading MC-CDMA can be minimized by the spreading code allocation method. This makes it possible to maximize the performance of frequency domain spread MC-CDMA, which has better characteristics than time domain spread MC-CDMA.

An embodiment of a multicarrier CDMA apparatus according to the present invention will be described below with reference to the drawings. In the communication system to which the present invention is applied, transmission is performed from a plurality of transmitting stations serving as terminals to a receiving station serving as a base station (uplink), and vice versa. Multi-carrier CDMA (MC-CDMA), which is a combination of Orthogonal Frequency Division Multiplexing (OFDM) and Code Division Multiple Access (CDMA) in communication between these stations when communicating with a certain receiving station (downlink) Is adopted. In such a communication system, the receiving station in the present invention has a multiple access control unit that controls the connection between the receiving station and a plurality of transmitting stations.
1) When a transmitting station transmits a frame, while transmitting a channel estimation preamble included in the frame, the frame transmission of a terminal other than the terminal is suspended, and after transmission of the preamble is completed, a terminal other than the terminal It has a function to resume frame transmission.
2) A multi-carrier CDMA control unit that accommodates transmission / reception stations serving as a plurality of terminals that can be connected to the receiving station is provided. At the same time, the multi-carrier CDMA control unit also time-divides uplink and downlink in the terminal. A time division duplex control unit that performs duplex (TDD) and a pilot signal transmission control unit that transmits a pilot signal common to all allocation targets using a specific downlink slot at a specific period. In addition, the wireless terminal
1) It has a channel transfer function deriving unit for deriving a channel transfer function based on the received pilot signal.
2) In addition, a channel transfer function notifying unit that notifies the radio base station of the derived channel transfer function is provided.

  FIG. 1 is a block diagram of a frequency-domain spread MC-CDMA transmission station. In FIG. 1, a data stream (Information data) as an input signal is modulated by a modulator 11 to become a bit stream, and then multiplexed together with a pilot signal (Pilot) by a multiplexer 12. Since this multiplexed output is a bit stream of a serial signal, it is converted into Np sequences by a serial-to-parallel converter (S / P) 13 and further spread by replicas (Copier) 141 to 14n in each sequence. Duplicated into SF symbol streams corresponding to the rate. Thereafter, the duplicated symbol stream is multiplied by a spreading code from the spreader 15 in the frequency domain. Here, when generating a spreading code, spreading code assignment is controlled by a spreading code assigner 150. This multiplication result is inversely transformed into a signal in the time domain by an inverse fast Fourier transformer (IFFT) 16 (or inverse discrete Fourier transformer), and then a guard interval is inserted by a guard interval inserter (GI Insertion) 17. It is transmitted as a transmitted signal (Transmitted signal). Here, the guard interval GI is inserted to remove the influence of multipath fading that causes inter-code interference.

FIG. 2 is a configuration diagram of a frequency-domain spread multi-carrier CDMA receiving station. A received signal (Received signal) is detected by a symbol timing detector 211, and then a guard interval remover (GI) is detected.
removal) 212 to remove the guard interval. The signal thus obtained is converted into data in the frequency domain divided into Nc pieces by a fast Fourier transformer (FFT) 22, a distortion in the propagation path is estimated by a channel estimator 23, and multiplication processing is performed. This corrects this distortion. Thereafter, it is multiplied by a spreading code which is the output of the frequency domain despreader 24. Here, when generating the spreading code, the spreading code assigner (Spreading code assigner) 241 controls the spreading code assignment. Furthermore, the data is collected into data in each SF frequency domain corresponding to the transmitting station, and further added by adders 251 to 25n to form a symbol stream, and a parallel signal is serially converted by a parallel-serial converter (P / S) 26. After the bit stream is obtained, recovered data is obtained by an inverse signal point converter (Demodulator) 27.

(Embodiment 1)
FIG. 3 is a sequence diagram of the spreading code assignment method in the spreading code assigner in the MC-CDMA system. In FIG. 3, for each of the available S spreading codes, a DUR (desired wave-to-interference power ratio) between two spreading codes of the own spreading code and a spreading code other than itself is obtained (step 301). For the calculation of DUR, when N spreading codes (N is a natural number greater than or equal to 2 and smaller than spreading factor S) are assigned, the DUR for spreading code k is (D / U) _k and D _k is spread. The desired power of the desired wave in the code k, U _kl is the interference power that the spreading code l gives to the spreading code k, and Uk is the sum of the interference power that each of the N−1 spreading codes other than the spreading code k gives to the spreading code k It can be calculated by the following equation (Equation 4). Here, (Expression 4) is the same as (Expression 1) described in claim 1.

（数４）により、_ＳＣ_Ｎ通りのＤＵＲの組（Ｎ個で構成）を算出（ステップ３０２）する。

Based on (Equation 4), _S C _N DUR sets (composed of N pieces) are calculated (step 302).

Then, among the set of streets _S C _N, the smallest value among the N DUR selects a set of spreading codes having a maximum (step 303). The selected spreading code obtained in this way is arbitrarily assigned to the assignment target connected to this system (step 304). If the number of multiple connections, that is, the number of connection allocation targets does not change (No in step 305), this state is maintained, and if it changes (Yes in step 305), the process returns to the input side of step 302 and assigns spreading codes again. To start.

(Embodiment 2)
In the above processing, the combination of spreading codes that maximizes the smallest value among the N DURs is obtained, but an average value may be used instead of the minimum value of DURs. That is, in this case, a combination of spreading codes that maximizes the average value of N DURs is selected. FIG. 4 shows a sequence diagram in this case, and the processing of step 403 is changed from “minimum DUR value” to “average value”.

(Embodiment 3)
FIG. 5 shows a third embodiment of the present invention. First, all DUR calculation results are created and held in advance in a spread code assigner as a table, and the number of multiple access changes. The table is called and referred to, and a spreading code to be assigned is newly determined (step 506).

  In FIG. 6, the average value of the DUR is obtained from the table created as described above (step 603), and a process of selecting a spreading code set that maximizes the average value is performed (step 603).

  FIG. 7 and FIG. 8 show the results of trial calculation of the effectiveness of the present invention, particularly when a combination of spreading codes that maximizes the minimum value of N DURs is selected. FIG. 7 shows an uplink system using a Walsh code, in which EGC (Equal Gain Combining) or MRC (Maximum Ratio Combining) is applied at the stage of despreading processing. FIG. 8 is a comparison of the system in the case of applying the MRC in the uplink system using the Walsh code or the orthogonal Gold code. FIG. 9 shows the effect of selecting a combination of spreading codes that maximizes the average value of N DURs. FIG. 9 shows an uplink system using a Walsh code and a system when MRC is applied. In any case, it can be seen that the performance is improved from the viewpoint of DUR as shown by the DUR maximization (plot filled in black). Further, FIG. 8 shows that the Walsh code is superior to the orthogonal Gold code among the orthogonal codes.

(Embodiment 4)
Next, an example of the spread code allocation method will be described with reference to the schematic diagram of FIG. FIG. 10 shows the change of the spreading code assigned to each allocation target in accordance with the change in the allocation target number N. The horizontal array indicates the allocation target number and the vertical array indicates the allocation target number. In FIG. 10, c ₂ , c ₄ , c _5, etc. symbols c _i indicate individual spreading codes to be assigned. One example of these arrays in FIG. 10 is shown, and the allocation target number N = k (k is This represents a change in the spread code assigned to each assignment target in accordance with a change in the natural number (2 or more and smaller than the spreading factor S).

That is, for each of the assigned numbers of spreading codes N = 2, 3,..., S, the DUR selection method defined by the above equation (1), that is, among the combinations of spreading codes obtained by the DUR norm. The multi-carrier CDMA system according to the first embodiment or the second embodiment, in which a specific spreading code is selected and assigned from N = k (k is a natural number greater than or equal to 2 and smaller than the spreading factor S). A combination of a specific spreading code and a specific spreading code combination corresponding to N = k + 1 are compared, and the number of N = k + 1 combinations that do not include the elements of the combination of N = k is expressed as R _k. For example, the combination of the specific spreading codes is

（数５）式を最小とするように選択することを特徴としている。ここで（数５）式は請求項４に記載の（数３）式と同じである。上記（実施の形態１）または（実施の形態２）を満足する拡散符号の組は１個以上である。つまり、選択できる候補が複数存在することがある。多元接続数が変化した場合に、変化前の割当拡散符号の組を考慮せず新規に拡散符号を割り当てる処理を行うより、変化前の割当拡散符号を可能な限り変えないで、新しい割当対象に対してのみ拡散符号を割り当てる方が処理のオーバーヘッドが少なくて済む。

(Equation 5) is selected so as to minimize the equation. Here, (Expression 5) is the same as (Expression 3) described in claim 4. One or more sets of spreading codes satisfy the above (Embodiment 1) or (Embodiment 2). That is, there may be a plurality of candidates that can be selected. When the number of multiple access changes, rather than changing the assigned spreading code before the change as much as possible, rather than performing the process of assigning a new spreading code without considering the set of assigned spreading code before the change, The allocation of the spreading code only for the processing requires less processing overhead.

The case where the assigned spreading code before the change (N = k) is also included in the selectable spreading code set after the change (N = k + 1), particularly when the inherited code exists. (For example, c ₂ , c ₄ , c _5, etc. at N = 4 in FIG. 10 are inherited codes). These inherited codes are used as they are, and a process of replacing and using only when there is no inherited code is used. Do.
For example, c _{7 at} N = 4 occurs here, and since it does not exist at the next N = 5, it indicates that the process of new → discard is executed, and c ₅ at N = ₅ takes over from N = 4 , N = 6 does not exist, indicating that inheritance → discard processing is performed.

In addition, when assigning an element of the selected combination of spreading codes (that is, spreading code) to the assignment target of spreading code, the operator inputs the priority determined in consideration of the external condition, and this input priority A spreading code with a high DUR is assigned in order from the highest assignment target. Here, the priority refers to the communication quality required by the allocation target, and is referred to as so-called quality of service (QoS) for the customer. By using this method, the elements of the selected spreading code combination (ie, spreading codes c ₂ , c _4, etc.) are sent in the order of c ₂ , c ₄ , c ₅ at some point, At the time of, the above-mentioned QoS can be obtained by switching the order in which the spreading code allocation target is a series of code configurations, such as sending in the order of c ₂ , c ₄ , and c ₅ , and randomizing in synchronization with this unit. Accordingly, it is possible to provide equal communication quality to each allocation target without performing the priority setting process for ensuring the above.

The frequency-domain spreading | diffusion MC-CDMA transmitter block diagram. 1 is a receiver configuration diagram of frequency domain spread MC-CDMA. FIG. FIG. 3 is a sequence diagram of the spreading code assignment method according to the first embodiment. FIG. 11 is a sequence diagram of the spreading code assignment method according to the second embodiment. FIG. 11 is a sequence diagram of the spreading code assignment method according to the third embodiment. The sequence diagram of the other spreading code allocation method in (Embodiment 3). The DUR comparison figure which shows the effect of the spreading code allocation method in (Embodiment 1). DUR which shows the effect of the spreading code allocation method with a different orthogonal code in (Embodiment 1). The DUR figure which shows the effect of the spreading code allocation method in (Embodiment 2). The schematic diagram of the spreading code allocation method by (Embodiment 4).

Explanation of symbols

11: Modulator 12: Multiplexer 13: Series-parallel converter 141-14n: Duplicator 15: Spreader 150: Spreading code assigner 16: Inverse fast Fourier transformer 17: Guard interval inserter 211: Symbol timing detector 212: Guard interval remover 22: Fast Fourier transformer 23: Channel estimator 24: Frequency domain despreader 241: Spread code assigners 251 to 25n: Adder 26: Parallel-serial converter 27: Inverse signal point converter

Claims (7)

A duplicator that duplicates the symbol stream, a frequency domain spreader that multiplies the spreading code in the frequency domain, and a transmission station comprising an inverse discrete Fourier transformer or an inverse fast Fourier transformer,
A discrete Fourier transformer or a fast Fourier transformer, a channel estimator that estimates and corrects distortion due to a propagation path, a receiving station that includes a frequency domain despreader that multiplies and adds the spreading code in the frequency domain; Consists of
A multi-carrier CDMA system that determines the spreading code based on a DUR standard, that is, a procedure for evaluating system performance from a desired signal to interference power ratio ,
When N is a natural number that is greater than or equal to 2 and smaller than the spreading factor S, and assigning the N spreading codes,
The DUR norm relates to a combination of N spreading codes obtained from _S C _N possible combinations that can be assigned;
D _k of the desired power of spreading code k, interference power U _kl spreading codes l gives the spreading code k, interference power respectively U _k N-1 pieces of said spreading code other than the spreading code k has on the spreading code k The desired wave-to-interference wave power DUR for the spreading code k is given by (D / U) _k and is given by the equation (1),

Calculate the DUR for N combinations obtained from the _S C _N combinations based on the equation (1),
A multi-carrier CDMA system, wherein a combination of the spreading codes that maximizes a minimum value of N DURs is selected from the _S C _N sets. A duplicator that duplicates the symbol stream, a frequency domain spreader that multiplies the spreading code in the frequency domain, and a transmission station comprising an inverse discrete Fourier transformer or an inverse fast Fourier transformer,
A discrete Fourier transformer or a fast Fourier transformer, a channel estimator that estimates and corrects distortion due to a propagation path, a receiving station that includes a frequency domain despreader that multiplies and adds the spreading code in the frequency domain; Consists of
A multi-carrier CDMA system that determines the spreading code based on a DUR standard, that is, a procedure for evaluating system performance from a desired signal to interference power ratio ,
When N is a natural number that is greater than or equal to 2 and smaller than the spreading factor S, and assigning the N spreading codes,
Relates the combination of the N spreading codes derived from a set together of the DUR norms can be allocated _S C _N Street,
Desired power of code k diffuser expanding the D _k, the interference U _kl spreading codes l interference power given to the spreading code k, the U _k, respectively (N-1) of the spreading codes other than the spreading code k has on the spreading code k When the sum of the power is given, the desired wave-to-interference wave power DUR for the spreading code k is expressed by (D / U) _k and expressed by the equation (2),

Based on the equation (2), the DUR is calculated for N combinations obtained from the _S C _N combinations.
The multi-carrier CDMA system, wherein a combination of the spreading codes that maximizes an average value of N DURs is selected from the _S C _N sets. Allocated number of spreading codes N = 2,3, ..., for each S, assigned to select a particular spreading code from the combinations of the spreading code determined by the DUR normative claim 1 or claim 2 Multi-carrier CDMA system,
A combination of a specific spreading code corresponding to N = k and a combination of a specific spreading code corresponding to N = k + 1, where k is a natural number greater than or equal to 2 and smaller than the spreading factor S, is a combination of N = k + 1 Let R _k be the number that does not contain N = k combination elements in
The combination of the specific spreading codes is selected so that the value of (Expression 3) is minimized.

A multi-carrier CDMA system. The multi-carrier CDMA system of claim 1 or claim 2 , wherein
When assigning the elements of the selected combination of spreading codes to the assignment target of spreading codes,
A multi-carrier CDMA system, wherein spreading codes having a high DUR are assigned in order from the assignment target having a high priority set in advance. The multi-carrier CDMA system of claim 1 or claim 2 , wherein
A multi-carrier CDMA system, wherein elements of a combination of selected spreading codes are sequentially switched in synchronization with a predetermined time interval as a spreading code assignment target. A transmission station used in the multicarrier CDMA system according to any one of claims 1 to 5 ,
A duplicator to duplicate the symbolic Le stream,
A frequency domain spreader that multiplies the spreading code in the frequency domain;
A transmission station for a multicarrier CDMA system comprising an inverse discrete Fourier transformer or a fast Fourier transformer. A receiving station used for the multicarrier CDMA system according to any one of claims 1 to 5 ,
A discrete Fourier transform or a fast Fourier transform;
A channel estimator for estimating and correcting distortion due to the propagation path;
A receiving station for a multicarrier CDMA system, comprising: a frequency domain despreader that multiplies and adds a discrete code in the frequency domain.

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