HK1177344B

HK1177344B - Method and device for generating transmission signals or ofdm symbols in a communication system

Info

Publication number: HK1177344B
Application number: HK13104148.8A
Authority: HK
Inventors: Costa Elena; Haas Harald; Schulz Egon; Rohling Hermann; Galda Dirk
Original assignee: Siemens Aktiengesellschaft
Priority date: 2001-09-18
Filing date: 2013-04-03
Publication date: 2016-04-08

Description

Method and apparatus for generating a transmission signal or an OFDM symbol in a communication system

This application is a divisional application of the following international patent applications, international application numbers, which have been filed on 9/16/2002: PCT/EP02/10382, national application number: 02818230.8, title of the invention: method for generating or processing OFDM symbols in a transmission system with spread user data and communication system equipment "

Technical Field

The present invention relates to a method for generating a transmission signal, in particular an OFDM symbol therefor, in a communication system, and to a communication system device implementing the method.

Background

In modern communication systems, in particular according to the standard GSM (global system for mobile communications) or UMTS (universal mobile telecommunications system), user data is encoded and allocated to carriers before a transmission signal is generated.

In fourth generation communication systems, multiple access methods with OFDM transmission systems (OFDM: orthogonal frequency division multiplexing) were devised. The method allocates user data for a large number of sub-carriers (OFDM-TDMA) or a certain number of different sub-carriers (OFDM-FDMA) in an OFDM symbol, or data of each determined data source. In OFDM-FDMA (FDMA: frequency division multiple access), for example, the subcarriers represent separate, respective adjacent frequency bands in a larger frequency range. It is also known, for example, to allocate user data exclusively to a large number of complete OFDM symbols, as in OFDM-TDMA (TDMA: time division multiple access), to allocate user data transmitted via an interface between different communication stations in time-sequential order in OFDM symbols, in particular to allocate user data of one station to one or more OFDM symbols each in direct sequential order. By this OFDM transmission method, inter-symbol interference (ISI: inter-symbol interference) can be avoided. And further, Multiple Access Interference (MAI) in the two Multiple Access methods is also avoided.

According to an alternative method, the user data can be spread by applying orthogonal code words over a predetermined number of sub-carriers and/or OFDM symbols using an orthogonal matrix, as is known from OFDM-CDMA or MC-CDMA (CDMA: code division multiple access; MC: multi-carrier). With the OFDM-CDMA method, all users' data are allocated all available frequencies, with codewords being used for discrimination. In the case of mutually parallel input data and in particular in the case of the investigation of a large number of users or data sources, undesirable multiple access interference can occur if no further measures are taken.

To avoid this problem, the subcarriers are therefore only allocated to a plurality of users at the same time to achieve spreading of the user data over the plurality of subcarriers, which is referred to as OFDM-FDMA with user-specific spreading data. The spreading and allocation of the subcarriers is thus achieved in a common and comprehensive method step. Spreading is mainly done using so-called Walsh-Hadamard-matrix of different sizes as described in the paper "multi-Carrier CDMA Controlled Equalization in indoor leishmaniing channels" (Controlled Equalization of multi-Carrier CDMA in an indoor ricia facing Channel) "published by m.yee, j. -p.linnartz at IEEE VTC conference held by schdulkel, sweden in 1994, or" Performance of Coherent OFDM-CDMA for wideband mobile communication "(Performance of Coherent OFDM-CDMA for broadband mobile communication" published by t.muller, k.br. ninghauce, h.rohling, in kluwer academic press 1996. Wherein different users are each assigned a number of orthogonal code words between 1 and N, where N is the number of available sub-carriers, according to the required data transmission rate.

Also, "Multi-carrier spread Spectrum and its relation to Single-carrier transmission" (Multi carrier spread Spectrum and Spectrum correlation to Single-carrier transmission) "published by k.br. sunning, h.rohling, at ottawa eevtc' 98, canada 1998, discusses in general the reduction of Peak-to-Average ratio (PAR: Peak-to-Average-ratio) by spreading with FFT (FFT: fast fourier transform) matrix, but does not take multiple access into account.

Disclosure of Invention

The technical problem underlying the present invention is to provide an alternative method of providing OFDM symbols, and a suitable communication station for performing the method.

This technical problem is solved by a method of generating OFDM symbols as set forth below.

In addition, the technical problem is also solved by a communication system apparatus as defined below.

In this case, the data of the data source for forming the OFDM symbols ordered in time, in particular by Inverse Fast Fourier Transformation (IFFT), are transformed and allocated to the subcarriers, in particular in a data-source-specific manner, wherein: (i) said allocation is performed in a data source-specific manner independently of the above-mentioned transformation, (ii) said transformation, in particular spreading, is performed with an orthogonal matrix or with a fast fourier transform (FET) or a Discrete Fourier Transform (DFT), and (iii) the transformation results are transmitted on the above-mentioned subcarriers such that: data of the data source is exclusively allocated to a plurality of subcarriers.

In an advantageous manner, it is possible to allocate subcarriers, on which the orthogonal transformation matrix or the extended transformation result is transmitted, in a data-source-specific manner, i.e. exclusively for each user or user data, which is complete in the ideal case, although the transformation is applied so as to avoid undesired multiple access interference.

In an advantageous manner, the orthogonal matrix is therefore initially selected arbitrarily. In addition, it can also be implemented in any way to first assign a transform result to a particular subcarrier of a user or data source.

It is particularly advantageous to distinguish spread user-specific data or data source-specific data and to distinguish sub-carrier allocation transformed or spread data spread into one or more OFDM symbols when a Discrete Fourier Transform (DFT) or Fast Fourier Transform (FFT) is used for spreading instead of, for example, a walsh-hadamard transform.

The above-described technical problem is not solved by the method of generating OFDM symbols defined below.

In this case, the OFDM symbol can also be generated in a simple manner when combining the fourier transform and the inverse fourier transform, since the corresponding steps can be omitted in its generation. Two successive fourier transforms can act to cancel effects including, inter alia, frequency offset. In particular, the method according to the invention for generating OFDM symbols in a communication system, wherein data of one or more data sources (user 1-user M) are mapped on a large number of orthogonal subcarriers (Nc), to which the data of the data sources for forming the OFDM symbols ordered in time are allocated in a data source-specific manner, wherein the OFDM symbols are formed when input data are allocated to output data by repeating a symbol sequence corresponding to the number of allocated subcarriers and a subsequent frequency offset. The communication system apparatus according to the present invention has: a data input of at least one data source; a control device for operating the communication system device and for processing data; at least one storage device and/or processing module for temporarily storing and processing data of at least one data source; wherein the control device is designed for generating OFDM symbols such that: data of a data source for forming OFDM symbols ordered in time by Inverse Fast Fourier Transform (IFFT) can be transformed in a data source-specific manner and can be allocated to the subcarriers, and wherein: the allocation is made in a data source specific manner independent of the transformation, the transformation can be implemented using orthogonal matrices, and the transformation results are transmitted on the subcarriers such that: data of the data source can be exclusively allocated to a plurality of subcarriers. Another communication system apparatus according to the present invention has: a data input of at least one data source; a control device for operating the communication system device and for processing data; at least one storage device and/or processing module for storing and processing data of at least one data source over time; wherein the control device is designed for generating OFDM symbols such that: the data for the data source, which form, in particular, OFDM symbols ordered in time by means of Inverse Fast Fourier Transformation (IFFT), can be transformed in a data-source-specific manner and can be allocated to the subcarriers mentioned, and wherein: the assignment is made in a data source specific manner independent of the above-mentioned transform, which can be implemented using a fast fourier transform (FET) or a Discrete Fourier Transform (DFT), and the transform results are transmitted on the above-mentioned subcarriers such that: data of the data source can be exclusively allocated to a plurality of subcarriers.

A large amount of redundancy is at first glance generated by a sequence of temporally repeated data in the OFMD symbols. However, since the signals generated by a large number of stations are superimposed during transmission over the radio interface, this redundancy of the overall system under full load is not provided in a data-source-specific manner. In triple redundancy, for example, of the data source, this corresponds to 4 cycles or one occupation of every fourth subcarrier. The four data sources are capable of simultaneously accessing the air interface without interference.

Correspondingly, according to the invention, uplink communication is also possible, for example between a mobile subscriber station and a base station, with the data arriving from the individual data sources being distributed or combined, if necessary after usual coding, scrambling and modulation, directly at the respective OFDM symbol positions. Thereby advantageously simplifying the structure of the transmitting communication station. For this purpose, the base station can remain unchanged and also possess the structure of an OFDM receiver in order to exploit the advantages specific to OFDM, such as being able to correct the received signal and distinguish subscriber station data or users individually.

Preferred embodiments are given by the further disclosure of the present application.

Particularly in combination with subcarriers which are distributed at equal distances on the frequency axis, OFDM-FDMA systems are produced for different user data or data sources, in which the time signal or OFDM symbol has a constant envelope after a usual inverse fourier transformation (IFFT). This is a significant advantage since it is not necessary if various smoothing methods are currently applied.

Drawings

Embodiments of the invention are explained in detail below with the aid of the figures. Shown in the figure are:

FIG. 1: general diagram of multiple access, wherein the structure of an OFDM-FDMA system with user-specific data symbol spreading is described;

FIG. 2: a possible implementation of an OFDM-FDMA system is illustrated in which a particularly advantageous FFT spreading matrix and user data are allocated to equidistant sub-carriers;

FIG. 3: illustrating the allocation of extended user data to equidistant sub-carriers, an

FIG. 4: particularly preferred embodiments for carrying out the method.

Detailed Description

As is evident from fig. 1, the method for generating OFDM symbols comprises different individual steps, the incoming user data of a single user, user station, or data source being single data t ' 1, t ' 2, … t ' y arriving in time order. For example, the input of data from M different users or user stations or data sources (user 1-user M) is described. These user data are then individually pre-processed in a single block, typically including, for example, encoding, Interleaving (Interleaving), and modulation. This first coding involves channel coding and is performed in a manner specified by the user. Further, conversion from serial signals to parallel signals is achieved, and data input before and after are distributed or mapped (Mapping) on L different data paths or transmission lines. The Y time values are thus mapped encoded by the data input, where each user data symbol comprises a single value of L complex values. These may be elements of any arbitrary symbol group, such as PSK or QAM modulated data (PSK: Phase Shift keying); QAM: quadrature amplitude modulation).

The actual transformation or spreading on the individual frequency sub-carriers and encoding of the different user data is described in the following blocks.

In a first step, the data input for each user on L data paths are expanded, i.e. a total of L data values are to be allocated to the P data values. This is preferred for using signals that propagate statistically independently on different sub-carriers (frequency diversity). In a preferred embodiment, the number of individual data values or lines is not changed, although it is also possible that L composite data values per user exist before and after expansion. Similar to the spreading of data with walsh-hadamard matrices in OFDM-CDMA, L corresponds to the number of codewords allocated to the subscriber station and P corresponds to the number of orthogonal codeword symbols available for use. In this case the extension conforms to a fully loaded system.

In a next step, the mapping on the individual subcarriers is carried out, with preference being given to channel adaptive mapping (channelidaptive mapping). During the introduction of P or L data lines per user in the symbol, a line or data path is derived from the symbol on behalf of a single subcarrier Nc. Here, the uppermost line represents a transmission line having a subcarrier f1 … fN of the lowest frequency f1, and the line shown at the lowermost or lowermost data path represents the highest frequency fN available in the frequency band.

After allocating data on the different frequency subcarriers, in a third, illustrated block an inverse fourier transform, e.g. an inverse fast fourier transform, IFFT, is first performed. The parallel data is then converted to serial data. According to current general regulations, generating a time sequence of data values t1 … ti, ti +1 … t2i, …, i.e. a cycle of setting a guard interval (guardlnterval) before it as a time signal in order to form an OFDM symbol, continues.

In an OFDM-FDMA system architecture describing user-specific spreading with data symbols, there is intentionally a distinction between the above-mentioned spreading or transformation on the one hand and mapping on the subcarriers on the other hand.

This distinction is made in order to be able to transform or spread the data of the individual users in a first step first with orthogonal matrices. The result of the transformation is then transmitted on subcarriers in the following step, wherein the subcarriers are exclusively allocated for the individual users. In principle, therefore, any allocation of subcarriers is possible, wherein the system preferably allocates the first subcarrier to the first user and the last subcarrier to the last user. This is not absolutely necessary, however, in particular there is no expectation, for example, in the allocation of matching channels. The system also preferably maps and transmits user data on sub-carriers allocated on the frequency band at equal distances.

This is advantageous for obtaining a constant envelope in the uplink. It may still be advantageous to allocate exactly adjacent subcarriers to the same subscriber station.

Although applying a transform can completely avoid Multiple Access Interference (MAI), the following benefits are achieved based on allocating orthogonal subcarriers at different subscriber stations to avoid multiple access interference. Spreading is the specific processing of the subscriber station where the received data is affected by the same channel as in OFDM-CDMA with respect to the code symbol number of the uplink.

Therefore, the orthogonal matrix can be arbitrarily selected in principle. And preferably the well-known walsh-hadamard transform or fourier transform, especially the discrete or fast fourier transform.

As is apparent from fig. 2 in the exemplary embodiment, a fast fourier transform FFT is applied as a preferred form of transformation or spreading and subsequent transmission on a single subcarrier. The fourier transformation FFT is therefore carried out in a user-defined manner, i.e. the input data symbols of the user are each time fed to their own fourier transformation.

The data resulting from the fourier transformation specified by the respective user is then distributed over the respective frequency sub-carriers f1 … fN, wherein the data of the users are preferably transmitted on sub-carriers which are not directly adjacent to each other. But in principle it is absolutely possible to implement this allocation on the individual subcarriers by any technique and means.

As is apparent from fig. 3, the individual data of the user-specified fourier transform are particularly preferably allocated to subcarriers in equidistant order on the frequency axis. In the described embodiment, L user data or data symbols with L user data values are introduced in an L-valued fourier transform (L-Point-FFT). According to a preferred embodiment, but this is not necessary, the L data values mapped on the N subcarriers are generated again by fourier transformation.

Mapping of transform values is implemented on the current N-24 frequency channels as subcarriers f1 … f24, where the number of L data values corresponds to L-8. As described above, the first data value is allocated on the first subcarrier f1 when allocated equidistantly on the first subcarrier f1, the second data value is allocated on the fifth subcarrier f5, and so on or transmitted.

The following is an inverse Fourier transform (Nc-PointFFT) of the Nc value of the IFFT as shown in the above figure. After conversion into a serial data sequence, a time sequence of individual data values to be transmitted via an interface is generated. After a preceding guard interval GI, an OFDM symbol is generated here from the guard interval GI and 24 temporally successive data values t1 … t 24. As mentioned above, in the equidistant allocation described above, the individual data values are allocated over 4 segments or periods, wherein all periods of each user time signal are equal. This generates a periodic repetition of the data sequence.

If a mobile station transmitter is involved as shown in fig. 4, all other subcarriers remain unoccupied because these subcarriers in the uplink are used by other mobile users in other mobile stations. In the downlink, the transmission signal is formed by the superposition of signals periodically transmitted by all users. If the same number L of subcarriers is allocated to all users, the added signal and the signal of each individual user have the same periodicity. If different users are assigned different numbers of subcarriers, the summed signal does not necessarily have a periodic signal distribution. In the uplink, the transmitted signal possesses a constant envelope, which is usually no longer constant in the downlink due to superposition of signals of different users or subscriber stations, but has a significantly smaller peak than a conventional OFDM signal. The corresponding fig. 2 depicts the downlink situation with many users per transmitter and the uplink situation shown in fig. 3.

Here summarized in simple data, the mapping on the sub-carrier f1 … fN is defined in the mapping on the data symbol length L of the input user data and the corresponding transformation result of the price L, where N is the available sub-carrier. After the inverse fourier transform, the OFDM symbol has a length N plus the length of the guard interval GI. The individual data values are allocated here at equidistant allocation over N/L data periods, which can also be referred to here as data blocks or data segments.

The spreading matrix formed by the fourier transform FFT is combined with equidistant allocation on the frequency axis, which combination is particularly advantageous in that a time signal with a constant envelope is formed in the uplink after the inverse fourier transform and a time signal with a significantly reduced peak value compared to a conventional OFDM system is formed in the downlink. This constant envelope is produced because the peak-to-average-ratio (PAR) is significantly smaller than other OFDM symbol generation methods. Just before the transformation, the PAR of the modulation symbols is fit. If one PSK modulation is inserted here, PAR becomes 1.

For better understanding, it is necessary to formally illustrate that this involves a generally linear transformation, and that there is no direct frequency-space-to-time-space relationship when applying fourier transforms for user-specific data transformations and expansions, as is typically considered in fourier transforms.

According to a further embodiment, for the case in which the two fourier transforms theoretically act as counteractive effects for one another, it is conceivable to consider the fourier transform as a spreading matrix and to determine the subsequent inverse fourier transform for the transmission of the individual frequency subcarriers over the time data sequence. As a result, a particularly advantageous implementation for generating OFDM symbols can be converted therefrom. As shown in fig. 4, which depicts e.g. transmissions in the uplink from a subscriber station to a station on the network side, such transitions cancel each other out when only one user transmits.

These blocks can be transformed by fourier transform FFT and by assigning transform sequences whose effects cancel each other after assignment on the individual subcarriers f1 … fN of the inverse fourier transform IFFT, or by repeating symbol sequences corresponding to the assigned number, it being currently assumed that the subcarriers are replaced and then frequency shifted when assigning input data on the output data.

The repetition of the time signal is formed in that the subcarriers not allocated to the user remain unmodulated. Each individual period of the OFDM signal finally corresponds to the inverse discrete fourier transform of the applied subcarrier. In principle, therefore, in the ideal conversion and later error-free transmission via the interface, the reception of the OFDM signal after the guard interval GI at the receiver end involves a large number of periods with identical data sequences. The time signal thus corresponds to a repeating sequence of the original input in a discrete fourier transform, as is done according to the embodiment of fig. 2.

As is apparent from the lower region shown in fig. 4, an equivalent system results in which the blocks can be replaced by repeating a sequence of symbols and correspondingly implemented frequency offsets, the blocks being provided with data converted from serial to parallel, user-specific fourier transforms, mapping and transmission on subcarriers, inverse fourier transforms and data converted from parallel to serial.

Claims

1. A method for generating OFDM symbols in a communication system, wherein data of one or more data sources is mapped on a large number of orthogonal sub-carriers,

it is characterized in that the preparation method is characterized in that,

transforming and allocating data of a data source for forming OFDM symbols ordered in time by inverse fast fourier transform IFFT to the above subcarriers in a data source specific manner, wherein:

-said assigning is done in a data source specific manner independent of the above-mentioned transformation,

-performing said transformation with an orthogonal matrix, and

-transmitting the transformed result on said sub-carriers such that: data of the data source is exclusively allocated to a plurality of subcarriers.

2. The method of claim 1, wherein

Transmitting the transform result on the above subcarriers such that: the first sub-carrier is allocated to the data source of the first user and the last sub-carrier is allocated to the data source of the last user.

3. The method of claim 2, wherein

Only one data source is allocated to each of the first to last subcarriers, respectively.

4. The method of claim 1, wherein

The data of the data source is channel coded and modulated before being mapped onto a large number of sub-carriers.

5. The method of claim 1, wherein

The data of each data source is allocated to subcarriers that are equally distributed over the carriers.

6. The method of claim 1, wherein

The transformed data of the individual data sources are allocated to subcarriers of a plurality of subsequent OFDM symbols.

7. The method of claim 1, wherein

After channel coding and modulation, the data of the data source are inserted in blocks after a guard interval in order to form time-ordered OFDM symbols, wherein each block is formed by a complete data sequence.

8. The method of claim 7, wherein

When input data is allocated to output data by repeating a symbol sequence corresponding to the allocated number of subcarriers and a subsequent frequency offset, data is inserted into an OFDM symbol.

9. The method of claim 1, wherein the transforming is expanding.

10. A method for generating OFDM symbols in a communication system, wherein data of one or more data sources is mapped on a large number of orthogonal sub-carriers,

it is characterized in that the preparation method is characterized in that,

-performing said transformation using a fast fourier transform FET or a discrete fourier transform DFT, and

11. The method of claim 10, wherein

12. The method of claim 11, wherein

13. The method of claim 10, wherein

14. The method of claim 10, wherein

15. The method of claim 10, wherein

16. The method of claim 10, wherein

17. The method of claim 16, wherein

18. The method of claim 10, wherein the transforming is expanding.

19. A method for generating OFDM symbols in a communication system, wherein data of one or more data sources is mapped on a large number of orthogonal sub-carriers,

it is characterized in that the preparation method is characterized in that,

the data of the data source for forming the temporally ordered OFDM symbols, which are formed when the input data are allocated to the output data by repeating a symbol sequence corresponding to the number of allocated subcarriers and a subsequent frequency offset, are allocated to the above subcarriers in a data source-specific manner.

20. A communication system apparatus having:

-a data input of at least one data source,

-a control device for operating the communication system device and for processing data,

at least one storage device and/or processing module for temporarily storing and processing data of at least one data source,

it is characterized in that the preparation method is characterized in that,

the control device is intended to generate OFDM symbols, the control device being designed such that: the data of the data source for forming the OFDM symbols ordered in time by inverse fast fourier transform IFFT can be transformed in a data source specific manner and can be allocated to subcarriers,

wherein:

-said transformation can be implemented using orthogonal matrices, an

-transmitting the transformed result on said sub-carriers such that: data of the data source can be exclusively allocated to a plurality of subcarriers.

21. The communication system apparatus of claim 20,

it is characterized in that the preparation method is characterized in that,

the control device is designed such that: by transmitting the transform result on the above subcarriers, the first subcarrier can be allocated to the data source of the first user, and the last subcarrier can be allocated to the data source of the last user.

22. The communication system apparatus of claim 21,

it is characterized in that the preparation method is characterized in that,

the control device is designed such that: it is possible to allocate only data of one data source to each of the first to last subcarriers, respectively.

23. The communication system apparatus of claim 20,

it is characterized in that the preparation method is characterized in that,

the control device is designed such that: the data of the data source can be channel coded and modulated before being mapped onto a large number of sub-carriers.

24. The communication system apparatus of claim 20,

it is characterized in that the preparation method is characterized in that,

the control device is designed such that: the data of each data source can be allocated to subcarriers that are equally spaced on the carrier.

25. The communication system apparatus of claim 20,

it is characterized in that the preparation method is characterized in that,

the control device is designed such that: the transformed data of the individual data sources are allocated to subcarriers of a plurality of subsequent OFDM symbols.

26. The communication system apparatus of claim 20,

it is characterized in that the preparation method is characterized in that,

the control device is designed such that: after channel coding and modulation, the data of the data source are inserted in blocks after a guard interval in order to form time-ordered OFDM symbols, wherein each block is formed by a complete data sequence.

27. The communication system apparatus of claim 26,

it is characterized in that the preparation method is characterized in that,

the control device is designed such that: when input data is allocated to output data by repeating a symbol sequence corresponding to the number of allocated subcarriers and a subsequent frequency offset, the data is inserted into an OFDM symbol.

28. The communication system apparatus of claim 20,

wherein the transforming is expanding.

29. A communication system apparatus having:

-a data input of at least one data source,

it is characterized in that the preparation method is characterized in that,

wherein:

-said transformation can be carried out using a fast fourier transform FET or a discrete fourier transform DFT, and

30. The communication system apparatus of claim 29,

it is characterized in that the preparation method is characterized in that,

31. The communication system apparatus of claim 30,

it is characterized in that the preparation method is characterized in that,

32. The communication system apparatus of claim 29,

it is characterized in that the preparation method is characterized in that,

33. The communication system apparatus of claim 29,

it is characterized in that the preparation method is characterized in that,

34. The communication system apparatus of claim 29,

it is characterized in that the preparation method is characterized in that,

35. The communication system apparatus of claim 29,

it is characterized in that the preparation method is characterized in that,

36. The communication system apparatus of claim 35,

it is characterized in that the preparation method is characterized in that,

37. The communication system apparatus of claim 29,

wherein the transforming is expanding.

38. A communication system apparatus having:

-a data input of at least one data source,

it is characterized in that the preparation method is characterized in that,

the control device is intended to generate OFDM symbols, said control device being designed such that: data of a data source for forming temporally ordered OFDM symbols can be allocated to subcarriers in a data source-specific manner, wherein the OFDM symbols can be formed when input data is allocated to output data by repeating a symbol sequence corresponding to the number of allocated subcarriers and a subsequent frequency offset.