WO2004073275A1

WO2004073275A1 - Space-time-frequency diversity for multi-carrier systems

Info

Publication number: WO2004073275A1
Application number: PCT/EP2003/001440
Authority: WO
Inventors: Gerhard Bauch
Original assignee: Docomo Communications Laboratories Europe Gmbh
Priority date: 2003-02-13
Filing date: 2003-02-13
Publication date: 2004-08-26
Also published as: JP2006517749A; EP1593245A1; JP4373339B2; AU2003205761A1

Abstract

The present invention relates to a method of establishing transmit diversity in a multi-carrier communication system using multiple antennas. The method comprises a transformation step (S10) applied to a sequence of transmission symbols for generating a plurality of transmission streams according to a pre-determined transformation rule. A step (S12) relates to the assignment of transmission stream elements in each transmission stream, both in frequency and time to multiple carriers for non-differential type transmit diversity or at least in frequency for differential type transmit diversity. Therefore, the present invention allows for a simple and flexible diversity scheme for reliable data transmission and reception at high data rates.

Description

Space -Time -Frequency Diversity For Multi - Carrier Systems

FIELD OF THE INVENTION

The present invention relates to the field of transmit diversity and reception diversity in multi-carrier communication systems, and in particular to the field of differential and non-differential space-time-frequency transmit and/or reception diversity for such multi- carrier systems.

BACKGROUND ART

In multi-carrier communication systems wireless communication channels may suffer from fading of the received signal due to destructive interference resulting from, e.g., multi-path propagation. It is commonly known in the art that transmit antenna diversity methods are an option to mitigate the detrimental effects of fading phenomena. The basic idea is to transmit the same communication signal via a plurality of transmit antennas and to then use the fact that sub-channels from each transmit antenna to the receiver fade independently, i.e. the probability that all sub-channels are fading in the same way simultaneously is small.

All transmit antennas will be used simultaneously to preserve bandwidth efficiency. For this reason, a preprocessing at the transmitter must ensure that signals transmitted from different multiple antennas may be separated at the receiver. As is known in the art, this may be achieved through transmission of orthogonal signals from different transmit antennas. One such orthogonal transmit diversity scheme is known as space- time block codes from orthogonal designs. A disadvantage of this diversity technique is that it requires that channels be constant over at least one space-time block coded block.

Further, the channel should be frequency flat in order to avoid intersymbol interference. This can be accomplished by orthogonal frequency division multiple access OFDM which is a multi-carrier system. Frequency division multiple access OFDM relies on the use of a broadband channel which is sub-divided into a number of narrowband sub-channels operating with different carrier frequencies to avoid intersymbol interference. While a space-time block code may be applied at each of the subchannels, nevertheless, the symbol duration in each of the sub-channels will be increased in comparison to a single carrier broadband system. This makes the requirement of a channel being constant during transmission of a space-time block code more critical.

The problems outlined so far become even more severe for differential transmit diversity in multi-carrier systems . The reason for this is that information is contained in a difference of two successive blocks and that the channel needs to be constant over not only one but two blocks. However, channel estimation is difficult in multiple-input multiple-output communication systems, as a plurality of sub-channels has to be estimated and the energy of pilot symbols will be distributed over multiple antennas .

In more detail, known solutions to transmit diversity from orthogonal designs have been proposed for two transmit antennas in S. Alamouti, A simple transmitter diversity technique for wireless communications, IEEE Journal on Selected Areas of Communications, Special Issue on Signal Processing for Wireless Communications, 16 (8) : 1451-1458, 1998 and further been generalized to more transmit antennas in V. Tarokh, H. Jafarkhani, and A.R. Calderbank, Space-time block codes from orthogonal designs, IEEE Transactions on Information Theory, 45 (5) : 1456-1467, June 1999 as space-time block coding.

Further, the application of space-time block codes to orthogonal frequency division multiple access OFDM such that a space-time block code is applied to each sub- channel has been proposed, e.g., in B. Lu, X. Wang, and Y. Li, Iterative receivers for space-time block-coded OFDM systems in dispersive fading channels, IEEE Transactions on Wireless Communications, 1 (2) :213-225, April 2002.

Another approach to coding across sub-channels is space- frequency coding, as proposed in H. Bδlcskei and A.J. Paulraj , Space-frequency coded broadband OFDM systems, in Wireless Communications and Networking Conference (WCNC) , pages 1-6, September 2000. This reference also suggests to distribute symbols of an orthogonal design, as given in the Alamouti paper referred to above, over neighboring sub-channels as space-frequency code. Further, it has been proposed to construct space- frequency codes having a code word length equal to the number of sub-channels in order to obtain maximum frequency diversity.

Another approach to reception diversity in multiple carrier systems has been proposed in A.F. Molisch, M. Win, and J. Winters, Space-time-frequency-coding for MIMO-OFDM systems, in European Personal Mobile Communications Conference (EPMCC) , Vienna, Austria, 2001. It is proposed to use complex codes without an orthogonal structure to achieve the diversity using a single code only.

Yet another approach to transmit diversity is related to a so-called differential transmit diversity from orthogonal design. It is described for two transmit antennas in V. Tarokh, H. Jafarkhani, and A.R. Calderbank, A differential detection scheme for transmit diversity, IEEE Journal on Selected Areas in Communications, 18 (7) : 1169-1174, July 2000, and further generalized to more antennas in H. Jafarkhani and V.

Tarokh, Multiple transmit antenna differential detection from generalized orthogonal designs, IEEE Transactions on Information Theory, 47 (6) : 2626-2631, September 2001.

Further differential transmit diversity schemes have been proposed in B. Hochwald and W. Swelden, Differential unitary space-time modulation, IEEE Transactions on Communications, 48 (12) :2041-2052, December 2000, and B.L. Hughes, Differential space-time modulation, IEEE Transactions on Information Theory,

46 (7) :2567-2578, November 2000. However, in contrast to V. Tarokh and H. Jafarkhani, A differential scheme for transmit diversity, IEEE Journal on Selected Areas in Communications, 18 (7) : 1169-1174, July 2000, these proposals do not rely on transmission symbols constituting an orthogonal design.

Further, in S.N. Diggavi, N. Al-Dhahir, A. Stamoulis, and A.R. Calderbank, Differential space-time coding for frequency-selective channels, IEEE Communications

Letters, 6 (6) :253-255, June 2002, there is described a pure differential space-time diversity scheme. It is proposed to use orthogonal frequency division multiple access in order to decompose a frequency-selective broadband channel to a set of flat fading narrowband channels having different carrier frequencies and then to apply a differential space-time block code on each carrier.

From the above, it becomes clear that dedicated time- frequency diversity schemes are available for multiple carrier communication systems, nevertheless, they suffer from high complexity and a low flexibility. While simple schemes are available for pure space-time or pure space- frequency diversity, however, communication channels need to be constant over many sub-carriers and a long time period.

However, generally broadband communication channels are frequency-selective and vary over time, which is a major problem as the transmission symbol duration on each sub- carrier increases for a higher number of sub-carriers as proposed for future communication systems .

SUMMARY OF INVENTION

In view of the above, the object of the invention is to provide a simple and flexible diversity scheme for reliable data transmission and reception at high data rates.

According to a first aspect, this object is achieved by a method of establishing space-time-frequency transmit diversity in a multi-carrier communication system using multiple antennas . The method comprises as first step a transformation of a sequence of transmission symbols into a plurality of transmission streams for supply to each of the multiple antennas according to a predetermined transformation rule. A second step of the inventive method relates to the assignment of transmission stream elements in each transmission stream, both in frequency and time to multiple carriers available at each antenna.

Therefore, the first aspect of the present invention relates to transmit diversity, where transmit symbols are distributed, over successive transmission time slots at different sub-carriers. While the use of multiple antennas allows to achieve space diversity, the distribution of different transmission stream elements over different time slots allows for time diversity, while the use of a plurality of sub-carriers allows for frequency diversity.

An advantage of the present invention is a relaxation of requirements for a constant communication channel in time and frequency. In other words, requirements regarding constant channel coefficients are relaxed in time and frequency compared to existing proposals.

Another advantage of the present invention is the separation of space diversity from time and frequency diversity. This forms the basis for very simple and efficient transmit diversity schemes which exploit space, time, and frequency diversity.

Yet another advantage of the present invention is the reduction in delay compared to conventional space-time transmit diversity schemes and in the overall overhead. It is important to note that no restriction exists on the type of distribution of transmission stream elements in time and frequency which increases flexibility, as in dependence of existing fading conditions an appropriate mapping scheme may be flexibly applied and varied over time .

According to a second aspect of the present invention the object outlined above is achieved through a method of establishing differential space-time-frequency transmit diversity in a multiple carrier communication system using multiple antennas. In a first step, a sequence of transmission bits is transformed into a plurality of transmission streams for supply to the multiple antennas according to a predetermined differential transformation rule. Then, the transmission stream elements in each transmission stream are assigned in frequency and time to multiple carriers available at each antenna.

Therefore, further to the advantages given above with respect to the first aspect, the present invention is also adapted to a differential transmit type diversity scheme. Here, it should be noted that further to the non-differential transmission type diversity scheme outlined above, diversity may also be achieved as dedicated differential space-frequency diversity without mapping of transmission stream elements onto a plurality of time slots which, nevertheless, is a preferred embodiment of the differential space-frequency diversity according to the present invention.

Yet another advantage achieved according to the second aspect of the invention is that the overhead caused by a transmission of a reference matrix according to existing differential type diversity schemes becomes is significantly reduced.

According to a preferred embodiment of the non- differential type transmit diversity scheme, the predetermined transformation rule is an orthogonal design, and according to a further preferred embodiment a differential orthogonal design.

Therefore, according to this preferred embodiment of the present invention spatial diversity is achieved through exploitation of orthogonal designs and a simple combination step at the receiver side, whereas time and frequency diversity may be handled at the receiver side, e.g., using an outer error correcting code. This separation makes the system simple and very flexible.

In view of the above, only the multiplexing component and/or the orthogonal design or the differential orthogonal design have to be modified when system parameters like the number of sub-carriers, the modulation scheme or the number of transmit antennas are changed .

This is of particular importance for highly adaptive systems like future mobile communication radio systems . Here, the complexity is significantly higher than with pre-existing proposals. An important contribution according to the present invention is that requirements regarding constant channels over time and frequency are significantly relaxed compared to space-time block codes or space-frequency block codes.

Further, another important advantage of the different diversity schemes explained so far is that the way different transmission stream elements are distributed on different sub-carriers and/or time slot(s) at each sub-carrier is fully flexible. The only rule to be observed is not to map two different transmission stream elements to one and the same time slot on the same sub- carrier.

A further preferred embodiment of the present invention being particularly related to the differential type diversity scheme uses a mapping of transmission stream elements onto different sub-carriers and time slots, such that the transmission stream elements are mapped continuously with respect to successive orthogonal designs onto the sub-carriers and related time slot(s) . Here, also this continuous mapping scheme may be applied to any type of transmission scheme with sub-channels showing a correlation in two dimensions such as time and frequency.

A third aspect of the present invention relates to a method of achieving space-time-frequency diversity reception in a multiple carrier communication system using multiple antennas.

In a first step, a plurality of transmission streams and related transmission stream elements are received at each of a plurality of antennas at a receiver. Here, it is assumed that transmission stream elements have been assigned in frequency and time to multiple carriers at a transmitter to achieve space-time-frequency transmit diversity in the sense outlined above. In a second step transmission stream elements are de-mapped in frequency and time to a stream of output symbols as reverse operation to the mapping at the transmitter side. A third step relates to forwarding of the stream of output symbols for subsequent processing.

A fourth aspect of the present invention relates to a method of achieving differential space-time-frequency diversity reception in a multiple carrier communication system using multiple antennas.

In a first step a plurality of transmission streams and related transmission stream elements are received at each of the multiple antennas. Similar to the non- differential type diversity reception, also for the differential type diversity reception it is assumed that transmission stream elements have been assigned at least in frequency to multiple carriers at a transmitter side to achieve differential space-frequency transmit diversity. A second step relates to a de-mapping of transmission stream elements at least in frequency to a stream of output symbols as reverse operation to the assignment achieved at the transmitter side. Then, in a third step, the stream of output symbols is forwarded for subsequent processing in the receiver.

From the above, it becomes clear that the space-time- frequency diversity according to the present invention is not only supported at the transmitter side but also at the receiver side. The same advantages as outlined above with respect to the transmission diversity, either non-differential or differential, are also achieved at the receiver side, i.e. increased flexibility and reduced complexity of system design for diversity reception.

According to a further preferred embodiment being related to non-differential or differential type diversity reception, it is proposed to forward generated output symbols to a combiner for subsequent estimation of transmitted symbols in a turbo detection step. In particular, it is proposed to provide a turbo feedback from a forward error correction FEC decoder to a space-time block code detector to further improve the performance and the diversity level. While previously it was assumed that a turbo feedback from the forward error correction FEC decoder to the diversity combiner cannot be effective due to the orthogonal structure of the space-time block code. Therefore, according to the present invention there is proposed a bit-wise feedback and a special mapping of bits to related modulation symbols.

Further, it is proposed to use information generated in the turbo detection as a-priori information for a soft- output space-time-frequency transmit diversity detector rather than for a channel estimator. It is also proposed to use a mandatory special mapping of bits to symbols in an anti-Gray modulation scheme, where constellation points of a modulation scheme having minimum Euklidean distance differ in as many bits as possible.

The present invention relies on the insight that due to the orthogonal structure of the space transmit diversity scheme a turbo iteration on the basis of a Gray mapping, where constellation points with minimum Euklidean distance differ in as few bits as possible, would not be effective as constellation points with minimum Euklidean distance would differ in one bit only. It should be noted that irrespective of the type of transmit diversity and/or diversity reception referred to so far, it may be applied to any multi-carrier system, e.g. OFDM or multiple carrier CDMA.

According to another preferred embodiment of the present invention there is provided a computer program product directly loadable into the internal memory of a transmission/reception apparatus comprising software code portions for performing the inventive non- differential or differential diversity process when the product is run on a processor of the transmission/reception apparatus .

Therefore, the present invention is also provided to achieve an implementation of the inventive method steps on computer or processor systems. In conclusion, such implementation leads to the provision of computer program products for use with a computer system or more specifically a processor comprised in, e.g., a multiple carrier communication system.

These programs defining the functions of the present ^' invention can be delivered to a computer/processor in many forms, including, but not limited to information permanently stored on non-writable storage media, e.g., read only memory devices such as ROM or CD ROM discs readable by processors or computer I/O attachments; information stored on writable storage media, i.e. floppy discs and harddrives; or information convey to a computer/processor through communication media such as network and/or Internet and/or telephone networks via modems or other interface devices . It should be understood that such media, when carrying processor readable instructions implementing the inventive concept represent alternate embodiments of the present invention.

BRIEF DESCRIPTION OF DRAWINGS

In the following, the present invention will be explained with reference to the drawing, in which:

Fig. 1 shows basic principles underlying space- time block codes as technological background to the present invention;

Fig. 2 shows further examples of a space-time block code as further technological background to the present invention;

Fig. 3 shows an overview on multiple input multiple output communication systems as further technological background to the present invention;

Fig. 4 shows a schematic diagram of an apparatus for achieving non-differential transmission diversity according to the present invention;

Fig. 5 shows a schematic diagram of the assignment unit according to the present invention as shown in Fig. 4 ;

Fig. 6 shows a flowchart of operation to achieve non-differential transmit diversity according to the present invention;

Fig. 7 shows a further schematic diagram of an apparatus for achieving non- differential transmit diversity according to the present invention;

Fig. 8 shows a schematic diagram of an apparatus for achieving non-differential or differential diversity reception according to the present invention;

Fig. 9 shows a flowchart of operation for achieving non-differential or differential diversity reception according to the present invention;

Fig. 10 shows a further schematic diagram of a post-processing unit as preferred embodiment of the present invention; Fig. 11 shows an example of non-differential transmission diversity according to the present invention;

Fig. 12 shows an example of non-differential diversity reception according to the present invention in compliance with the example of non-differential transmission diversity shown in Fig. 12;

Fig. 13 shows a further example of non- differential transmission diversity according to the present invention;

Fig. 14 shows a further example of achieving non-differential reception diversity in compliance with the example of achieving non-differential transmission diversity shown in Fig. 13;

Fig. 15 shows a basic principle for achieving differential transmit diversity as technological background to the present invention;

Fig. 16 shows a flowchart of operation to achieve differential transmit diversity according to the present invention;

Fig. 17 shows an example of differential space-frequency transmit diversity according to the present invention;

Fig. 18 shows a further example of differential space-time-frequency transmit diversity according to the present invention;

Fig. 19 shows a further example of differential space-time-frequency transmit diversity according to the present invention; and

Fig. 20 shows a further example of achieving differential space-time-frequency transmit diversity according to the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In the following, the best mode of carrying the different aspects of the present invention as well as preferred embodiments thereof will be explained with reference to the drawing.

Before explanation of different aspects of the present invention, initially, basic principles underlying space- time block codes as technological background to the present invention will be explained. Fig. 1 shows a transmit antenna diversity scheme for two transmit antennas using a space-time block code.

As shown in Fig. 1, a source of information generates a stream of bits u forwarded to a modulation unit 12, e.g., 8-phase shift keying unit. Operatively, the modulation unit 12 achieves a mapping of the input bits onto different transmission symbols. For the particular example shown in Fig. 1, bits 010 are mapped to symbol C_j=2, and input bits 111 are mapped to transmission symbol c₂ = l . The generated symbols are forwarded as transmission symbols x_x,x₂ to a space time block code unit 14 operating according to a generalized complex orthogonal design, i.e. a matrix with elements x_l,x₂,-x₂ ^* , and x_x ^* .

Generally, each such orthogonal design is characterized by a matrix.

where the number of columns n_τ corresponds to the number of transmit antennas, and the number of rows P corresponds to the number of time slots.

From the above, it should be clear that while the matrix B is referred to as orthogonal design a synonymous reference may as well be a transformation design as the stream of transmission symbols is mapped into a plurality of transmission streams respectively forwarded to related transmit antennas 16 and 18, as shown in Fig. 1.

The entries b,₇ of the orthogonal design B are elements x_t of an N-ary signal constellation, the complex conjugates x^* or linear combinations hereof. A block of K symbols x_t,t = \,...,K is input to the space-time block coder also referred to as transformation unit 14. The space-time block coder maps the K symbols on the n_T - P entries b.. of the orthogonal design B according to the mapping rule of the space time block code. Then, all entries b_y in the same row of the orthogonal design B are transmitted simultaneously from the n_τ antennas, e.g. antennas 16 and 18 for the examples shown in Fig. 1.

Further, entries in the same column of the orthogonal design B are transmitted from the same antenna 16 or 18 in successive time slots so that the columns of the orthogonal design represent space, while the rows represent time, as outlined above.

For the particular example shown in Fig. 1, the orthogonal design is

For the example shown in Fig. 1 a block is derived from two 8-PSK transmission symbols. According to the 8-PSK modulation scheme three source bits are mapped to a complex constellation point x_t . The symbol c_x = 2 is mapped to x_x = j , and the symbol c₂=7 is mapped to x₂

jlyl . The complex transmission symbols x_x and x₂ are written in a Pxn_τ = 2x2 matrix according to the orthogonal design given above.

In the first time slot the transmission symbols in the first row of the orthogonal design are transmitted simultaneously, i.e. x_x ^m = x_x is transmitted from antenna 1 while ,⁽²⁾ = x₂ is transmitted from antenna 2. In the next time slot, the symbol x₂ ⁾ = -x₂ ^* is transmitted from antenna

1 while the symbol x₂ ²⁾ = x_x ^* is transmitted from antenna 2.

Due to the orthogonality of the orthogonal design, equivalently referred to as space-time block code or transformation rule for a stream of transmission symbols, at the receiver side a simple combination allows for reception diversity. As will be explained in more detail in the following. During transmission the channel is required to be constant over the number of time slots or equivalently the number of rows of the space-time block code matrix i.e. during P=2 symbol durations .

Fig. 2 shows further examples of space-time block codes where for the left example the number of antennas is four and the number of time slots where the transmission channel is expected to be constant is equally four, while for the right hand side example the number of transmit antennas is three while the number of time slots where the transmission channel is assumed to be constant is again four.

Irrespective of the example explained above with reference to Fig. 1 and 2, within the framework of the present invention it is assumed that transmission symbols are constellation elements of a modulation scheme, e.g., a quadrature amplitude shift modulation scheme QAM or a phase shift modulation scheme PSK. It should be noted that according to the present invention there exists no restriction of the type of modulation scheme. For n_τ transmit antennas, the symbols x_k are mapped to n_τ transmission streams according to the orthogonal design or transmission rule having dimension

P - n_r .

Further to the above, it is assumed that transmission is achieved in a multi-carrier communication system, where data is transmitted simultaneously via N_τ sub-carriers at each antenna, such each sub-carrier corresponds to one of a plurality of carrier frequencies. Here, the channel coefficients will vary in time and frequency . direction. However, the channel coefficients in successive time slots as well as for neighbouring sub- carriers are correlated.

As the receiver side requires the channel coefficients during transmission of an orthogonal design to be constant, according to the present invention the elements of one orthogonal design will be assigned appropriately as will be explained in detail in the following. Also at the receiver the received transmission symbols will be de-mapped according to a reverse operation of the assignment achieved at the transmitter side as will also be explained in detail in the following.

Before explanation of such details the channel model underlying multiple input multiple output communication channels MIMO will be explained as technological background to the present invention with respect to Fig, 3.

As shown in Fig. 3, it is assumed that a multiple carrier communication system has n_τ transmit antennas and n_R receive antennas. At each antenna, the transmission symbols will be transmitted using multiple sub-carriers, i.e. the channel is divided into N_s orthogonal sub-channels. In the following, each plurality of transmission symbols transmitted at a point in time at each of the sub- carriers will be referred to as transmission symbol, or orthogonal frequency division multiplex symbol OFDM.

As shown in Fig. 3, at each of the transmission branches

\,...,n_τ there is provided a serial/parallel conversion unit 20-1, 20-nτ to map the set of forwarded elements in the orthogonal design onto a plurality of sub-carriers. Then, the Inverse Fast Fourier Transformation units 22- 1, 22 - nt execute an Inverse Fourier Transformation on the output values of the serial/parallel conversion units 20-1, 20-n-p. The inverse Fast Fourier Transformation units 22-1, 22-nτ are provided to achieve a decoupling of related sub-carriers before forwarding related transformation results to parallel/serial conversion units 24-1, 2 -n^. Then, at each antenna

\,...,n_τ there is inserted a guard interval through a related guard interval unit 26-1, 26-nτ.

As shown in Fig. 3, at the receiver side the guard interval will be removed in a guard interval removing unit 28-1, ..., 28-nR as reverse operation. Then, there follows a serial/parallel conversion in a related serial/parallel converter unit 30-1, 30-n_r to generate the reception elements on each of the sub-carriers which are forwarded to related Fast Fourier Transformation units 32-1, ..., 32-nR as reverse operation to related

Inverse Fast Fourier Transformation at the transmitter side. Hereafter, the results of Fast Fourier Transformation at each reception antenna will be forwarded to a related parallel/serial conversion unit 34-1, ..., 34-np_ for subsequent processing of the generated output reception streams, i.e. a generation of an output bit stream.

As shown in Fig. 3, for each sub-carrier operated between the transmitter and the receiver, there may be derived a transfer function from each transmit antenna to each receiver antenna. These transfer functions allow to model a flat fading multiple input multiple output MIMO channel used in the multiple carrier communication system.

In particular, the addition of a cyclic prefix as guard interval GI allows to decompose a frequency-selective broad band channel into a plurality of N_s parallel frequency-flat MIMO channels, as shown in the lower part of Fig. 3. Here, a block of N_s - n_τ symbols being transmitted on N_s sub-carriers at each of the n_τ transmit antennas is referred to as OFDM symbol in the following.

It should be noted that the symbol duration on each sub- carrier is increased by a factor of N_s compared to single carrier systems having the same bandwidth. It should also be noted that the present invention as outlined in the following is not restricted to frequency-flat multiple input multiple output channels as discussed so far but may as well be applied to frequency-selective MIMO channels which are decomposed into a set of flat-fading channels.

In view of the technical background of the present invention as explained above with reference to Fig. 1 to 3 in the following different aspects of the present invention will be explained referring to Fig. 4 to 15.

According to the present invention, non-differential type transmit diversity is achieved by assigning symbols generated through a transformation rule, e.g., an orthogonal design, to a plurality of sub-carriers at each antenna and also over successive time slots yielding space-time frequency transmit diversity from a transformation rule.

Here, it should be noted that the present invention is not restricted to the application of an orthogonal design but applies to any type of transformation starting from a stream of transmission symbols generating a plurality of transmission streams for supply to the plurality of transmit antennas. The distribution of elements on the transformation rule both in time and frequency reflects the requirements for a constant channel, both, in time and frequency direction. It should also be noted that all kinds of assignment both in time and frequency direction covered by the present invention. Further, the assignment is the same for all transmit antennas to achieve compatibility with existing reception schemes to be used at the reception side. All transmit antennas at the transmitter side transmit simultaneously.

Fig. 4 shows a schematic diagram of an apparatus for achieving non-differential transmission diversity according to the present invention.

The apparatus 36 comprises an assignment unit 38 and a transmission unit 40. The input values to the assignment unit 40 are determined through a transformation of transmission symbols into a plurality of transmission streams using a predetermined transformation rule, as outlined above.

Fig. 5 shows a schematic diagram of the assignment unit 40 shown in Fig. 4.

The assignment unit 38 comprises a selection unit 42 for selecting a sub-carrier and a time slot of transmission elements before transmission and a mapping unit 44 for achieving the assignment of each transmission element determined by the selection unit 42.

Fig. 6 shows a flowchart of operation of the transmission apparatus shown in Fig. 4 and 5. As also shown in Fig. 6, step S10 is related to a transformation of transmission symbols into a plurality of transmission streams. Step S12 is related to an assignment of transmission stream elements at each antenna in frequency and time to multiple sub-carriers available at each antenna. In other words, for each element in each transmission stream there is selected a sub-carrier and a related time slot on the sub-carrier for the transmission stream element through the selection unit 42. Then the transmission stream element is mapped accordingly in the mapping unit 44.

Further, operatively, the transmission unit 4o shown in Fig. 4 will transmit transmission stream elements via the plurality of transmit antennas 1, ..., n^ after mapping.

Fig. 7 shows a further detailed schematic diagram of an apparatus for achieving non-differential transmit diversity according to the present invention. Those elements being identical to the ones explained previously with respect to Fig. 4 to 6 will be denoted using the same reference numerals.

Fig. 7 relates to the application of non-differential transmit diversity and space, time, and frequency using an orthogonal design. As shown in Fig. 7, the transformation of a stream of transmission symbols into a plurality of transmission streams 1, ..., _j> is achieved through the orthogonal design B . The transmission symbols forwarded for transformation are generated using previously existing units, i.e. a forward error correction FEC encoder unit 46, an interleaver unit 48, and a modulator unit 50. It should be noted that the particular type of generation of the transmission symbols is non-binding to the different aspects of the present invention.

As shown in Fig. 7, the transformation unit 38 generates the plurality of transmission streams for forwarding to each of the transmit antennas 1, ..., nT- Before transmission each transmission stream element will be assigned to one of the sub-carriers and one of the available time slots on each sub-carrier. This is achieved before executing the Inverse Fast Fourier Transformation, parallel/serial conversion, insertion of the guard interval.

While above different aspects of achieving non- differential transmit diversity have been explained with reference to Fig. 4 to 7, in the following related aspects of achieving non-differential diversity reception at the receiver side according to the present invention will be explained with reference to Fig. 8 to 10. Fig. 8 shows a schematic diagram of an apparatus for achieving non-differential diversity reception according to the present invention.

As shown in Fig. 8, it is assumed that at the receiver n, ... , n^ receiving antennas are used. The signal received by each receipt antenna 1, ..., np> is forwarded to a related reception unit 46-1, ..., 46-n^ generating a plurality of reception symbol streams. Each single reception symbol stream is related to a sub-carrier operated at each of the antennas 1, ..., n^. At each reception unit 46-1, ... 46-n^ the related output streams are forwarded to a related de-mapping unit 48-1, ...,

48-nR which achieves a de-mapping of the elements in the reception streams of each sub-carrier as reverse operation to the assignment of transmission stream elements at the transmitter side. Then, related de- mapped reception stream elements are forwarded to a processing unit 50 for generating a stream of output symbols.

Fig. 9 shows a flowchart of operation for achieving non- differential diversity reception according to the present invention.

As shown in Fig. 9, each reception unit 46-1, ..., 6-n^ is adapted to generate a reception transmission stream for each sub-carrier. Therefore, each reception 46-1, ... , 46-nR operatively achieves removal of a guard interval, a serial/parallel conversion and a Fast Fourier Transformation.

As also shown in Fig. 9, each de-mapping unit 48-1, ..., 48-nR operatively achieves a de-mapping of transmission stream elements as reverse operation to the assignment of transmission stream elements both in frequency in time to multiple sub-carriers and related time slots. Each de-mapping unit 48-1, ..., 48-n^ therefore de-maps the transmission stream element in frequency and time to a stream of output symbols as reverse operation to the assignment at the transmitter for each antenna. Then, in a step S20, the post-processing unit 50 processes the de-mapped reception streams, e.g., for generation of maximum likelihood estimates of output bits.

Fig. 10 shows a preferred embodiment of the postprocessing unit 50 according to the present invention.

As shown in Fig. 10, the post-processing unit 50 comprises a combiner unit 52, an a posteriori probability de-mapping unit 54 adapted to deliver soft outputs with respect to code bits c_k , a de-interleaver unit 56 adapted to achieve reverse interleaving of the soft outputs, and an forward error correction FEC decoder 58. Further, the post-processing unit 50 comprises a first superposition element 60, an interleaving unit 62 and a second superposition unit 6 . As alternative, the post processing unit may contain the combiner unit 52 only.

Operatively, the a posteriori probability de-mapping unit 54 shown in Fig. 10 derives log-likelihood ratios of code bits from receiver combiner outputs. The de- interleaver unit 56 de-interleaves the log-likelihood ratios of the code bits, for subsequent de-coding in the FEC decoder unit 58.

Further, the FEC decoder unit 58 decodes to source bits. The FEC decoder unit 58 will also output a-posteriori-

likelihood ratios about the code bits L^d(ck) from which the input log-likelihood ratios L^d (c_k) of code bits are subtracted in the superposition unit 60 for generation

of extrinsic log-likelihood information L_e(ck) . This extrinsic log-likelihood information is forwarded to the interleaver unit 62. The output of the interleaver unit 62 is substrated from the log-likelihood ratio provided by the a posteriori probability de-mapping unit 54 through the second superposition unit 64.

Further, the output of the interleaving unit 62 is forwarded as a posteriori log-likelihood information

L_a (ck ) to the a-posteriori probability de-mapping unit 54. According to the present invention it is proposed to use the feedback as a-priori information for the soft output space-time-frequency transmit diversity detector rather than for a channel estimator. It is also proposed to achieve a constellation with an outer FEC code and an optional turbo feedback from the FEC decoding unit 58 to the soft-output diversity combiner.

Therefore, regarding the turbo feedback, it is proposed to perform a bit-wise feedback. It is proposed to use a special non-Gray-mapping of code bits to constellation symbols of a higher order modulation scheme.

In other words, the mapping of constellation symbols is different from a Gray-mapping such that constellation symbols with minimum Euklidian distance differ in as many bits as possible. This non-Gray mapping of code bits allows for an improvement over previous existing space-time turbo-coding schemes where turbo iterations did not improve a detection accuracy due to the orthogonality of the transmit diversity scheme.

While above different aspects of the present invention with respect to non-differential transmit diversity and diversity reception have been explained with reference to Fig. 4 to 10, in the following further details of assignment transmission steam elements in each transmission stream in frequency and time to multiple carriers will be explained. As already outlined above, the assignment of transmission stream elements in frequency and time to multiple carriers may be achieved in any suitable way as long as different transmission stream elements are not assigned to the same sub-carrier and the same time slot at the same time.

Those skilled in the art will readily understand that any type of representation of such assignment is well covered by the present invention. For reasons of illustration, one such type of representation of an assignment of transmission stream elements in frequency and time to multiple carriers will be explained.

Heretofore, it is assumed that the transmission symbols are arranged into a set of N transmission stream elements as vector

A ^t >

Considering that the number of sub-carrier is N_s the step of selecting a carrier and time slot for a set of

N transmission stream elements t_ =[t_N, ..., t is achievable through specification of a set of carrier indices

C = { _x , ..., _N},a_t el,...,Ν_s; wherein a_t is the carrier selected for the i - th transmission stream element. The selection of a time slot may be represented through specification of a set of time slot indices

S = {β_x , ..., /? }, ?,. el,...,Γ_;

wherein T is the number of available time slots on each sub-carrier and β_t is the time slot selected for the i- th transmission stream element.

Here, it should be noted that clearly such a set notation is used for purpose of illustration only and any other type of data structure representation of selecting a carrier and time slot for a set of transmission stream elements is well covered by the present invention. One typical example would be a linked list of selected sub-carriers and time slots, a table representation, etc. Nevertheless, assuming the set notation as outlined above, the step of assigning transmission stream elements to selected carriers in time slot on each sub-carrier may be represented through a carrier vector c₍. representing the carrier of the i -th transmission stream element t₍. as column vector according to

a time slot vector s, representing the time slot of the i - th transmission stream element t₍. as row vector according to

and a product of the transmission stream element t_i and a dyad of carrier vector c_; and time slot vector s_t according to

t, £1 s_:

Using this vector notation and generation of dyads for representing a transmission stream element assignment, the constraint that no two transmission stream elements will be assigned to the same sub-carrier and time slot may be expressed such that for two different transmission stream elements the difference of the related dyads must be unequal to the zero matrix

fa{l....ΛN l,...*}^1' ≠ i → £lll - ^Cj j ^{≠ 0}

The final assignment result is achievable through superposition of all dyads representing the assignment of the transmission stream elements to different sub- carriers and related different time slots according to Dτ = ∑*t C, - S; =ι

In compliance with the above notation, also the functionality achieved at the receiver side may be represented in a non-binding way using the same representation .

In particular, the function of de-mapping transmission stream elements from different sub-carriers and related time slots on output symbols is achievable through arranging T transmission stream elements on each of the

N_j carriers into a diversity reception matrix D_R at each antenna, determining a carrier vector c₍. from the set of carrier indices C to represent the carrier of the i - th transmission stream element t_i as column vector according to

{1, = a, ,j ≠ a_i

determining a time slot vector s. from the set of time slot indices S to represent the time slot of the i -th transmission stream element t_t as row vector according to

calculating a product of transposed carrier vector c^ , the diversity reception matrix D_R , and the transposed time slot vector s^ according to

T T y_i = £_i - D_T - S_J

Using the illustrative notion outlined above, the following examples for achieving non-differential type transmission type transmission diversity and diversity reception will be explained with reference to Fig. 11 to 14.

Fig. 11 shows a first example of non-differential type transmission diversity according to the present invention.

As shown in Fig. 11, the first example relates to non- differential type transmission diversity with two transmit antennas and two sub-carriers used at each such antenna. Further, the transformation rule applied is in an orthogonal design according to the matrix B .

As shown in Fig. 11, in particular two such orthogonal designs

and

are considered for space-time-frequency diversity assignment of transmission stream elements.

As also shown in Fig. 11, at each of the n_τ = 2 transmit antennas a number of N_s = 2 sub-carriers are available for frequency diversity and T = 2 time slots are used for a time diversity. A sequence of n=4 transmission stream elements is considered for assignment in frequency in time over the different sub-carriers and time slots. The assignment shown in Fig. 11 may be represented using the carrier vectors and slot vectors as outlined above according to

1 0 1 0 c, = ^ / ₄ — 0 ' £2 ~ , c 1 0 1

. = [0 1]

*₂ = [l 0]

£₃ = [0 1] ₄ = [i 0]

Applying these carrier and slot vectors to the set of transmission stream elements

the final result for the diversity transmission at the first antenna is

Applying the same set of carrier and slot vectors to the set of transmission stream elements

. = k 2j

the final diversity result achieved for the second transmit antenna will be

Assuming that the generated transmission diversity is used for transmission of related transmission stream elements, in the following a corresponding example of non-differential diversity reception according to the present invention in compliance with the example of non- differential transmission diversity shown in Fig. 11 will be explained. Heretofore, Fig. 12 shows a plurality of reception antennas 1, ..., n& and related functionality as explained previously with reference to Fig. 8 to 10. In the most general sense one may assume, e.g., that the transmission stream elements at the first reception antenna are received according to an order

Applying the transposed of the carrier vector and slot vector given above with respect to the transmission side according to

y_t = i ' D_R - s ,i = l,...,N

leads to the result

^τ r>^{! T} —

—2 ^* R ^' —2 — y-.

£3 ' -^R ^' ^3 y

for the first reception antenna.

Further, the result second reception antenna will be achieved according to

£x -D R ²-s^τX ^■yx

^■ rι2 i-2 Di = y₂

T τ.2 T 2

£yD_R-s__A =y₄

Fig. 13 shows a second example of non-differential transmission diversity according to the present invention with a number of n_τ=3 of transmission antennas and a number of N_s=4 sub-carriers at each antenna. Further, the number of time slots used for a time diversity on each sub-carrier is T = 2, and N = 8 transmission stream elements are forwarded to each antenna for mapping on different sub-carriers and related time slots. As shown in Fig. 13, transmission stream elements arranged in two orthogonal designs

and

are considered for distribution of related transmission stream elements onto different sub-carriers and time slots. The set of carrier vectors and slot vectors for the example illustrated in Fig. 13 are

and

s₂ = [l 0]

-is = [0 1]

5τ =[ 1] £.=[1 0]

At the first antenna the application of these sub- carriers and time slots to the set of transmission stream elements

= |θ x^* -x₅ ^* x_Λ -x, x

achieves the following transmission diversity

Similarly, the application of the same assignment to the set of transmission stream elements

t — I ^* n ^{* *} n ^* I

supplied to the second antenna achieves the following transmission diversity

Similarly, applying the same set of carrier vectors and slot vectors, or in other words, the same assignment of transmission stream elements also to the transmission stream elements supplied to the third antenna

achieves the diversity result

-A', x₃ 0

M = _* _* 0

Similar to the previous example shown in Fig. 11 and 12, also for the example shown in Fig. 13 at the receiver side appropriate de-mapping can be executed to achieve a related non-differential reception diversity, as shown in Fig. 14.

In particular, Fig. 14 shows that the reverse operation to transmit diversity will put reception stream elements into an appropriate order for subsequent processing.

In conclusion, considering the non-differential type space-time-frequency block diversity scheme as outlined above for the transmitter side and the receiver side it is possible to overcome the problems associated with space-time block codes, i.e. fast fading of communication channels, and also of space-frequency block codes, i.e. frequency selectivity. This is achieved by distributing elements of transmission streams derived from, e.g., an orthogonal design both in time and frequency in order to relax the requirements for constant channel coefficients in both dimensions.

The approach to non-differential transmit diversity is particularly advantageous when the number of transmit antennas is increased and the channel coefficients have to be constant for a higher number of transmission stream elements .

Further, there are many possibilities to distribute transmission stream elements at an antenna in time and frequency. E.g., it is also possible to realize an asymmetric assignment where different transmission stream elements derived within one transformation step are assigned in a different manner.

Also, the type of assignment of transmission stream elements to different sub-carriers and time slots may vary over time. In other words, the processing of different sets of transmission stream element vectors derived from transmission stream symbols according to a predetermined transformation rule vary over time.

While above, principles of non-differential space-time- frequency transmit diversity and related diversity reception have been explained with reference to Fig. 4 ^■ to 14, in the following aspects of the present invention being related to a differential transmit diversity and diversity reception will be explained with reference to Fig. 15 to 20.

Principles of differential transmit antenna coding from orthogonal designs will be explained with reference to Fig. 15. Principles of differential transmit diversities have been proposed in V. Tarokh and H. Jafarkhani, A differential detection scheme for transmit diversity, IEEE Journal on Selected Areas in Communications, 18 (7) :1169-1174, July 2000, incorporated herein by reference. Further, a generalization of differential transmit diversity to more than one transmit antennas is described in H. Jafarkhani and V. Tarokh, Multiple

Transmit Antenna Differential Detection from Generalized Orthogonal Designs, IEEE Transactions on Information Theory, 47 (6) : 2626-2631, September 2001, incorporated herein by reference.

As shown in Fig. 15, differential transmit diversity relies on a mapping of bits u_k which are transmitted using, e.g., an identical orthogonal design as outlined above. The mapping is achieved onto complex constellation points A_k and B_k . The vector (x₂,_{+2 >} ^x _2t+χ) which is transmitted in a time slot has unit length according to

^v2M-2 + \ X 2. t+X It should be noted, that this requirement is introduced for reasons of differential detection. The mapping of bits onto constellation points may be obtained starting from an M-ary phase shift keying PSK constellation with constellation points

d_k e^J2m/M÷φ0,n = l,....,M - l

and applying

A_k = d_2Md(0)^* + d_2t+2d(0)^'

β_k ^:= -^d2M^d(⁰) + ^d2_t+2^d(⁰)

The reference symbol d(0) may be chosen randomly from the M-ary PSK constellation. As log₂( ) bits are mapped on each of the PSK constellation symbols d_2t+x and d_2M according to an arbitrary mapping, e.g., a Gray mapping, the constellation points A_k and B_k are determined by

2-log₂( ) bits, an important property of the mapping is that the vector A_k , B_k has unit length, i.e.

Similar as in differential modulation, a reference space-time block code matrix x₂

* _* x₂ X,

is transmitted first. The space-time block code matrix contains arbitrary symbols x and x₂ taken from the M-PSK constellation, so that for the coding of the first bits a reference to a previous code matrix, i.e. the reference code matrix is possible. The following symbols are obtained according to

\^X2t+X ^X2t+2 ) ⁼ ₂t-X ^X2t P~ "k ^X2t ^X2t-X ) "

Therefore, orthogonal designs will be transmitted over the communication channel allowing to separate the transmission symbols transmitted simultaneously from different antennas through simple combination at the receiver side.

In view of the general frame work for differential transmit diversity outlined so far, Fig. 16 shows a flowchart of operation to achieve differential transmit diversity according to the present invention.

It should be noted that this operation is achieved in each of the assignment units 50-1, 50-nrr explained above with reference to Fig. 4 and 7, respectively. In other words, while the overall structure of the transmit diversity apparatus is the same for a non-differential transmit diversity and transmit diversity, nevertheless for differential type transmit diversity the operation of the assignment unit is changed.

As shown in Fig. 16, in a first step S22 a plurality of transmission streams are generated through transformation. A set of input transmission bits is transformed in a plurality of transmission streams using a pre-determined differential transmission rule, e.g., as outlined above.

Further, in a step S24 at each antenna the generated flow of transmission stream elements is assigned at least in frequency to multiple sub-carriers available at the antenna. Then, and optionally, in a step S26 transmission stream elements are also assigned in time to different time slots at each available sub-carrier.

It should be noted that according to the present invention and within the frame work of differential type transmit diversity according to the present invention it is proposed to achieve an assignment in frequency alone. Such an approach will be referred to differential space- frequency transmit diversity in the following. Therefore and contrary to the non-differential type of transmission diversity, for the differential type of transmit diversity the present invention also covers an assignment of transmission stream elements in frequency only. Optionally, and further to the above, the present invention also covers an assignment of transmission stream elements generated from a differential transmission rule to multiple time slots of sub- carriers. This aspect of the present invention will be referred to as differential space-time-frequency block code in the following.

In the following, a first example of differential type space-frequency diversity will be explained with reference to Fig. 17.

Usually, the detector of a differential type space time block coder assumes that channel coefficients are constant during transmission of two space time block code matrixes. However, due to the relatively long symbol duration of a transmission symbol, the channel coefficients may not be constant during transmission of two space-time block code matrixes.

To avoid the problem of fast channel variations in time different transmission symbols are assigned to the same time slot of different sub-carriers, or in other words to the same OFDM symbols rather than on the same sub- carrier of a sequence of OFDM symbols. From the example shown in Fig. 17, it should be clear that this reduces transmission delay.

However, the communication channel needs to be approximately constant over four neighbouring sub- carriers which is true for communication channels with low frequency selectivity and may further be accomplished by using a large number of sub-carriers to make the sub-carriers spacing very narrow.

Referring to the example shown in Fig. 17, after mapping according to the differential space-frequency block code on n_τ streams associated with the transmit antennas a simple serial to parallel assignment may be used at each transmit antenna to achieve space-frequency differential type transmit diversity. Accordingly, at the receiver side a parallel to serial de-mapping may be used for diversity reception.

As outlined above, for differential type space-frequency block codes a reference matrix may be transmitted in each OFDM symbol which reduces the data rate. However, as shown in Fig. 17, this may be avoided when differential coding is performed over several OFDM symbols such that the first N_^ transmission stream elements are allocated to sub-carriers \,...,N_S of the first OFDM symbol and the next N_s transmission stream elements are allocated to sub-carriers N_s ,...,l of the second OFDM symbol, etc.

Further to the above, it should be noted that the type of representation of assignment of transmission stream elements on different sub-carriers for differential type space-frequency transmit diversity may also be achieved and represented as outlined above for the non- differential type transmit diversity and diversity reception.

In other words, also with differential type space- frequency assignment of different transmission stream elements to different sub-carriers a related representation may rely on carrier vectors and slot vectors, as outlined above.

For the particular example shown in Fig. 17 the output of the differential space time block coder is represented by two orthogonal designs according to

-x B(k) = k+X ^Xk

X,. "^■k+X

As shown in Fig. 17, the number of antennas is n_τ = 2 , the number of time slots considered for assignment is T = \ , according to space frequency transmit type diversity, the number of sub-carriers N_s = 4 , and the number of transmission stream elements is N= 8. As also shown in Fig. 17, the first four transmission stream elements are assigned from sub-carrier 1 to sub- carrier N_s while the second set of four transmission stream elements are assigned in reverse order, i.e. from sub-carrier N_s to sub-carrier 1. This may be represented and illustrated using carrier vectors and slot vectors according to

£5

i∑5 ~ §.6 ~ 7 = [1 0]

Here, it should be noted that the reverse ordering of the first four transmission stream elements and the second transmission stream elements may be represented according to

c_M = R - c_i,i = l,...,4 0 0 0 1 0 0 1 0

£i+4 c.,z^' = l,...,4 0 1 0 0 1 0 0 0

It is important to note, that the type of reversal of ordering and assignment to different sub-carriers for differential space-frequency type transmit diversity is not restricted to the representation given above but in a general sense relates to an assignment of related transmission stream elements to related sub carries. Each such assignment will be referred to as continuous assignment in the following and explained considering further examples in the following.

In view of the above, the assignment results achieved at the first antenna may be represented according to

i,-_j x₅ x₄ ^Xl \

-x_*

D_τ =

while the assignment result achieved at the second antenna may be represented according to

t — I ^{* * * *} I x,

D_τ x₆ x₃

Further, it should be noted that the de-mapping or equivalently the de-assignment of the receiver side is similar to that explained previously for non- differential type diversity reception, so that here a repeated explanation will be omitted.

Yet another further aspect of the present invention relates to differential space-time-frequency transmit diversity and diversity reception.

According to the present invention, it is proposed to distribute transmission stream elements of two successive differential type orthogonal designs, both in time and frequency in order to relax the requirements for constant channel coefficients in both dimensions.

Fig. 18 shows an example of differential space-time- frequency transmit diversity according to the present invention.

As shown in Fig. 18, transmission stream elements of one orthogonal design are transmitted using different time slots on the same sub-carrier. Differential encoding is done over frequency, i.e. a successive orthogonal design is transmitted on two successive time slots at neighbouring sub-carriers.

Therefore, the communication channel needs to be constant over two OFDM symbols in time and two sub- carriers in frequency, rather than four OFDM symbols as in usual differential space-time transmit diversity or four sub-carriers as in differential space-frequency transmit diversity.

Generally, differential space-time-frequency transmit diversity may be achieved using two or more OFDM symbols. If it is achieved using two OFDM symbols, a reference matrix has to be transmitted per two OFDM symbols.

For the example shown in Fig. 18, the symbols x_x , x₂ and x₉ , x₁₀ belong to a reference matrix which would reduce the data rate. On the other hand, if differential space- time-frequency transmit diversity is achieved over many OFDM symbols, the channel needs to be constant over the related number of OFDM symbols.

For the example shown in Fig. 18 then, only x, , x₂ would belong to a reference matrix. Further, the channel needs to be constant over four OFDM symbols whenever transmission stream elements are allocated to a new OFDM symbol. For the examples shown in Fig. 18, this is the case for x₇ to x₁₀. Further, it should be noted that for the example shown in Fig. 18 the continuity of assignment of transmission stream elements is represented by arrows such that subsequent orthogonal designs are assigned to different sub-carriers and time slots in close relationship.

Fig. 19 shows a second example of achieving differential space-time-frequency transmit diversity according to the present invention.

For the example shown in Fig. 19 the maximum requirement is that the communication channel is constant over three OFDM symbols or sub-carriers, respectively, rather than four OFDM symbols as in Fig. 18.

In other words, the requirement for time constant communication channels has been relaxed compared to the example shown in Fig. 18 through "vertical" arrangement of the transmission stream elements being related to indices 9 to 12, as indicated by dashed lines in Fig. 19.

Fig. 20 shows a third example of achieving differential space-time-frequency transmit diversity as a combination of the examples shown in Fig. 18 and 19.

For the example shown in Fig. 20, the continuity of assignment of transmission stream elements is represented using again dashed lines. As shown in Fig. 20, the communication channel needs to be constant over two transmission stream elements in time and frequency for most sub-carriers. Only before and after a new OFDM symbol is used, the assignment is changed such that the channel needs to be constant over two OFDM symbols and three sub-carriers rather than four OFDM symbols.

It should be noted that many other mappings are possible. Particularly, if differential space-time-block transformation rules for more than two transmitters are used each transformation rule, e.g., orthogonal design is of dimension Px n_τ , where P ≥ 4 . This offers more degrees of freedom for distribution of transmission stream elements in time and frequency.

Further, referring to the differential type transmit diversity, i.e. differential space-frequency or space- time-frequency transmit diversity, it should be noted that the related diversity reception will be achieved similarly to the non-differential diversity reception outlined above, i.e. through reverse de-mapping at the receiver side in compliance with the assignment of transmission stream elements to sub-carriers in frequency and optionally in time achieved at the transmitter side.

From the above, it should be clear that the present invention relates, both, to non-differential and differential type transmission diversity and diversity reception. All kinds of assignments where transmission stream elements are mapped in both frequency and time for non-differential type transmit diversity and at least in frequency for differential type transmit diversity are covered by the present invention.

Further, the present invention may be applied to any transmission scheme where sub-channels show correlation in two dimensions such as time and frequency. Still further, the present invention may be applied to all kinds of space-time block codes which require constant channel coefficients over a block.

In particular, with respect to a differential type transmit diversity, preferably all kinds of continuous mappings where transmission stream elements of any differential transmit diversity scheme are mapped to at least two sub-carriers and one or more time slots are covered by the present invention. It may be applied to any transmission scheme having sub-channels having a correlation in two dimensions such as time and frequency.

Still further, from a reception side point of view, the present invention also relates to a turbo-detection in a concatenated scheme of an outer forward error correction code and a space-time frequency block code including special anti-Gray mapping of bits to constellation points.

Claims

1. Method of achieving space-time-frequency transmit diversity in a multiple-carrier communication system using multiple antennas, comprising the steps :

transforming a sequence of transmission symbols into a plurality of transmission streams for supply to the multiple antennas according to a pre-determined transformation rule;

assigning transmission stream elements in each transmission stream in frequency and time to multiple carriers available at each antenna.

2. Method according to claim 1, characterized in that the pre-determined transformation rule is an orthogonal design.

3. Method according to claim 2, characterized in that the orthogonal design is set up as complex orthogonal matrix from a group comprising transmission symbols, complex conjugates of transmission symbols, and linear combinations of transmission symbols and complex conjugates of transmission symbols.

4. Method according to claim 3, characterized in that the transmission stream elements are derived from elements of the orthogonal design and different transmission stream elements are assigned to different carriers and different time slots of a carrier.

5. Method of achieving differential space-time- frequency transmit diversity in a multiple-carrier communication system using multiple antennas, comprising the steps:

transforming a sequence of transmission bits into a plurality of transmission streams for supply to the multiple antennas according to a pre-determined differential transformation rule;

assigning transmission stream elements in each transmission stream in frequency to multiple carriers available at each antenna.

6. Method according to claim 5, characterized in that transmission stream elements in each transmission stream are further assigned in time to multiple carriers available at each antenna.

7. Method according to claim 5 or 6, characterized in that the pre-determined differential transformation rule is an differential orthogonal design.

8. Method according to claim 7, characterized in that the differential orthogonal design is set up iteratively as complex orthogonal matrix from a group comprising transmission bits, constellation points of a differential space time block code, and at least one previously transmitted orthogonal design.

9. Method according to claim 8, characterized in that the transmission stream elements are derived from elements of the differential orthogonal design and different transmission stream elements are assigned to different carriers or different time slots of a carrier.

10. Method according to claim 9, characterized in that the transmission stream elements are mapped continuously with respect to successive orthogonal designs onto adjacent carriers and/or time slots.

11. Method according to one of the claims 1 to 10, characterized in that the step of assigning transmission stream elements divides into the following sub-steps: selecting a carrier and time slot for each transmission stream element; and

mapping each transmission stream element to the selected carrier and time slot.

12. Method according to claim 11, characterized in that the step of selecting a carrier and time slot for a set of N transmission stream elements t =[t_N, ..., t is achievable through specification of a set of carrier indices

C = {α_x, ..., α_N),α_t el,...,Ν_s ; wherein

N_s is the number of available carriers and , is the carrier selected for the i - th transmission stream element, and through specification of a set of time slot indices

$ = {βχ, ■■ -, β_N),β_i z \,...,T , - wherein

T is the number of available time slots on each carrier, and β_t is the time slot selected for the i -th transmission stream element.

13. Method according to claim 12, characterized in that the step of assigning transmission stream elements to the selected carriers and time slots is achievable through a carrier vector c_; representing the carrier of the i-th transmission stream element t₍. as column vector according to

a time slot vector ,. representing the time slot of the i-th transmission stream element t₍. as row vector according to

a product of the transmission stream element t, and a dyad of carrier vector c_t and time slot vector s, according to

'/^•£/'£/^•

14. Method according to claim 13, characterized in that the final assignment result is achievable through:

ι=l

15. Method according to one of the claims 12 to 14, characterized in that the number of transmission stream elements N , the set of carrier indices C and/or the set of time slot indices S vary over time.

16. Method according to one of the claims 1 to 15, characterized in that the mapping is executed at each antenna in a same manner.

17. Method according to one of the claims 14 to 16, characterized in that the elements in each row of

D_τ are transmitted at related antennas in sequential order.

18. Method of achieving space-time-frequency diversity reception in a multiple-carrier communication system using multiple antennas, comprising the

SUΘUD •

receiving a plurality of transmission streams and related transmission stream elements at each of the multiple antennas, wherein the transmission stream elements have been assigned in frequency and time to the multiple carriers at a transmitter to achieve space-time- frequency transmit diversity; de-mapping transmission stream elements in frequency and time to a stream of output symbols as reverse operation to the assignment at the transmitter at each antenna, and

forwarding the stream of output symbols for subsequent processing.

19. Method of achieving differential space-time- frequency diversity reception in a multiple-carrier communication system using multiple antennas, comprising the steps:

receiving a plurality of transmission streams and related transmission stream elements at each of the multiple antennas, wherein the transmission stream elements have been assigned in frequency to the multiple carriers at a transmitter to achieve differential space -time- frequency transmit diversity;

de-mapping transmission stream elements in frequency to a stream of output symbols as reverse operation to the assignment at the transmitter at each antenna, and

forwarding the stream of output symbols for subsequent processing.

20. Method according to claim 19, characterized in that transmission stream elements have also been assigned in time to the multiple carriers at a transmitter to achieve differential space-time- frequency transmit diversity and that de-mapping of transmission stream elements is also achieved in time onto a stream of output symbols as reverse operation to the assignment at the transmitter.

21. Method according to claim 19 or 20, characterized in that the transmission stream elements are derived from elements of the differential orthogonal design.

22. Method according to claim 21, characterized in that the transmission stream elements are de-mapped continuously with respect to successive orthogonal designs onto adjacent carriers and time slots.

23. Method according to one of the claims 18 to 21, characterized in that the step of de-mapping of N transmission stream elements t_ =[t_N, ..., t_x ] is achievable through specification of a set of carrier indices

C = { _x , ..., _N}, _t e l,...,N_s ; wherein

N_j is the number of available carriers and _t is the carrier selected for transmission of the i- th transmission stream element, and through specification of a set of time slot indices

S = {β_x, ..., β_N},β_: el,...,7\- wherein

T is the number of available time slots on each carrier and β_{ is the time slot selected for transmission of the i - th transmission stream element .

24. Method according to claim 23, characterized in that the step of de-mapping transmission stream elements from different carriers and time slots onto output symbols y. is achievable through

arranging T transmission stream elements on each of the N_s carriers into a diversity reception matrix D_R at each antenna,

determining a carrier vector c, from set of carrier indices C to represent the carrier of the i-th transmission stream element t₍. as column vector according to

determining a time slot vector s_: from the set of time slot indices S to represent the time slot of the i- th transmission stream element t, as row vector according to

calculating a product of transposed carrier vector c- , the diversity reception matrix D_R , and the transposed time slot vector s, according to

T y_t = £ι D_r - _S .

25. Method according to one of the claims 18, 21, or 22, characterized in that the stream of output symbols at each antenna is forwarded to a combiner for subsequent estimation of the transmitted symbols in a turbo detection step.

26. Method according to claim 25, characterized in that the step of turbo detection divides into the following sub-steps:

deriving log-likelihood ratios of code bits from receiver combiner outputs, de-interleaving log-likelihood ratios of code bits,

decoding the de-interleaved log-likelihood ratios for generation of a-posteriori log- likelihood ratios of code bits, wherein

log-likelihood ratios on code bits are subtracted in a bit wise manner from related a - posteriori log-likelihood ratios of code bits for generation of extrinsic log-likelihood information, and

the extrinsic log-likelihood information is provided as a priori log-likelihood information for the de-mapping step after interleaving thereof .

27. Method according to claim 26, characterized in that it further comprises the step of subtracting the extrinsic log-likelihood information from new log- likelihood values generated by the de-mapping step.

28. Method according to claim 26 or 27, characterized in that the mapping of a-posteriori log-likelihood ratios of code bits on symbols of the modulation scheme relies on a mapping scheme where constellation points of a modulation scheme with minimum Euklidean distance differ in as many bits as possible.

29. Apparatus for achieving space-time-frequency transmit diversity in a multiple-carrier communication system, comprising:

multiple antennas,

a transformation unit adapted to transform a sequence of transmission symbols into a plurality of transmission streams for supply to the multiple antennas according to a predetermined transformation rule,

an assignment unit adapted to assign transmission stream elements in each transmission stream in frequency and time to multiple carriers available at each antenna.

30. Apparatus according to claim 29, characterized in that the transformation unit is adapted to use an orthogonal design as pre-determined transformation rule .

31. Apparatus according to claim 30, characterized in that the orthogonal design is set up in the transformation unit as complex orthogonal matrix from a group of symbols comprising transmission symbols, complex conjugates of transmission symbols, and linear combinations of transmission symbols and complex conjugates of transmission symbols .

32. Apparatus according to claim 31, characterized in that the transformation unit is adapted to derive transmission stream elements from elements of the orthogonal design and that the transformation unit is adapted to assign different transmission stream elements to different carriers and different time slots of a carrier.

33. Apparatus for achieving differential space-time- frequency transmit diversity in a multiple-carrier communication system, comprising:

multiple antennas,

a transformation unit adapted to transform a sequence of transmission bits into a plurality of transmission streams for supply to the multiple antennas according to a pre-determined differential transformation rule;

an assignment unit adapted to assign ^* transmission stream elements in each transmission stream in frequency to multiple carriers available at each antenna.

34. Apparatus according to claim 31, characterized in that the transmission unit is further adapted to assign transmission stream elements in each transmission stream in time to multiple carriers available at each antenna.

35. Apparatus according to claim 33 or 34, characterized in that the pre-determined differential transformation rule for the transformation unit is implemented as an differential orthogonal design.

36. Apparatus according to claim 35, characterized in that the differential orthogonal design is set up iteratively in the transformation unit as from a group comprising transmission bits, constellation points of a differential space time block code, and at least one previously transmitted orthogonal design.

37. Apparatus according to claim 36, characterized in that the assignment unit is adapted to assign transmission stream elements continuously with respect to successive orthogonal designs onto adjacent carriers and time slots.

38. Apparatus according to one of the claims 29 to 37, characterized in that the assignment unit comprises : a selection unit adapted to select a carrier and time slot for each transmission stream element; and

- a mapping unit adapted to map each transmission stream element to the selected carrier and time slot.

39. Apparatus according to claim 38, characterized in that the selection unit is adapted to select a carrier and time slot for a set of N transmission stream elements _ t =[t_N, ..., t is operable through specification of a set of carrier indices

C = {α_x, ..., α_N},α_i e L...,N_s ; wherein

N_s is the number of available carriers and α is the carrier to be selected for the i -th transmission stream element, and through specification of a set of time slot indices

S = {β_x, ..., β_N},βi el,...,r ; wherein

T is the number of available time slots on each carrier, and β is the time slot to be selected for the i -th transmission stream element.

0. Apparatus according to claim 39, characterized in that the mapping unit adapted to map transmission stream elements to the selected carriers and time slots is operable through

a storage unit adapted to store a carrier vector c_;. representing the carrier of the i - th transmission stream element t,- as column vector according to

a storage unit adapted to store a time slot vector

S_; representing the time slot of the i-th transmission stream element t_;. as row vector according to

a multiplication unit adapted to generate a product of the transmission stream element t₍. and a dyad of carrier vector c_; and time slot vector s_t according to

41. Apparatus according to claim 40, characterized in that the final assignment result is achievable through operation of a summation unit adapted to determine :

^•£

42. Apparatus according to one of the claims 29 to 31, characterized in that the number of transmission stream elements N , the set of carrier indices C and/or the set of time slot indices S vary over time .

43. Apparatus according to one of the claims 29 to 42, characterized in that the assignment is executed at each antenna in a same manner.

44. Apparatus according to one of the claims 41 to 44, characterized in that the elements in each row of D_τ are transmitted at related antennas in sequential order.

45. Apparatus for achieving space-time-frequency diversity reception in a multiple-carrier communication system, comprising:

multiple antennas, a reception unit adapted to receive a plurality of transmission streams and related transmission stream elements at each of the multiple antennas, wherein the transmission stream elements have been assigned in frequency and time to the multiple carriers at a transmitter to achieve space-time-frequency transmit diversity;

a de-mapping unit adapted to de-map transmission stream elements in frequency and time to a stream of output symbols as reverse operation to the assignment at the transmitter at each antenna, and

an output unit adapted to forward the stream of output symbols for subsequent processing.

46. Apparatus for achieving differential space-time- frequency diversity reception in a multiple-carrier communication system, comprising:

multiple antennas,

a reception unit adapted to receive a plurality of transmission streams and related transmission stream elements at each of the multiple antennas, wherein the transmission stream elements have been assigned in frequency to the multiple carriers at a transmitter to achieve differential space-time-frequency transmit diversity;

a de-mapping unit adapted to de-map transmission stream elements in frequency to a stream of output symbols as reverse operation to the assignment at the transmitter at each antenna, and

- an output unit adapted to forward the stream of output symbols for subsequent processing.

47. Apparatus according to claim 46, characterized in that transmission stream elements have also been assigned in time to the multiple carriers at a transmitter to achieve differential space-time- frequency transmit diversity and that the de- mapping unit is adapted to de-map transmission stream elements also in time onto a stream of output symbols as reverse operation to the assignment at the transmitter.

48. Apparatus according to claim 47, characterized in that the de-mapping unit is adapted to de-map transmission stream elements from elements of the differential orthogonal design.

49. Apparatus according to claim 48, characterized in that the de-mapping unit is adapted to de-map the transmission stream elements are de-mapped continuously with respect to successive orthogonal designs onto adjacent carriers and time slots.

50. Apparatus according to one of the claims 45 to 49, characterized in that the de-mapping unit adapted to de-map N transmission stream elements _ t ..., t_x ] is operable through specification of a set of carrier indices

C = {α_x, ..., α_N},α_i e. \,...,N_s ; wherein

N, is the number of available carriers and α, is the carrier selected for transmission of the i -th transmission stream element, and through specification of a set of time slot indices

S = {β_x, ..., β_N},β_; e l,...,T ; wherein

T is the number of available time slots on each carrier and β_i is the time slot selected for transmission of the i - th transmission stream element .

51. Apparatus according to claim 50, characterized in that the de-mapping unit adapted to de-map transmission stream elements from different carriers and time slots onto output symbols y_t is operable through use of an arrangement unit adapted to arrange T transmission stream elements on each of the M carriers into a diversity reception matrix D_R at each antenna,

a storage adapted to store a carrier vector c_; from set of carrier indices C to represent the carrier of the i- th transmission stream element t,- as column vector according to

a storage unit adapted to store a time slot vector

S from the set of time slot indices S to represent the time slot of the i -th transmission stream element t_t as row vector according to

a processing unit adapted to calculate a product of transposed carrier vector c,^r , the diversity reception matrix D_R , and. the transposed time slot vector s according to

52. Apparatus according to one of the claims 46, 47, or 51, characterized in that it further comprises a combiner adapted to receive the stream of output symbols at each antenna, to combine related streams, and to forward its output to a turbo decoder.

53. Apparatus according to claim 52, characterized in that the turbo decoder comprises:

a de-mapper adapted to derive log-likelihood ratios of code bits from receiver combiner outputs,

- a de-interleaver adapted to de-interleave log- likelihood ratio of code bits,

a decoder adapted to decode the de-interleaved log-likelihood ratios for generation of a- posteriori log-likelihood ratios of code bits, wherein

a subtracter adapted to subtract log-likelihood ratios on code bits in a bit wise manner from related a-posteriori log-likelihood ratios of code bits for generation of extrinsic log- likelihood information, and

an interleaver adapted to provide the extrinsic log-likelihood information as a priori log- likelihood information for the de-mapping step after interleaving thereof .

54. Apparatus according to claim 53, characterized in that it further comprises a subtractor adapted to subtract the extrinsic log-likelihood information from new log-likelihood values generated through de-mapping.

55. Apparatus according to claim 53 or 54, characterized in that the de-modulator is adapted to use code bits on mapped symbols of the modulation scheme on the basis of a mapping scheme where constellation points of a modulation scheme with minimum Euklidean distance differ in as many bits as possible.

56. Computer program product directly loadable into the internal memory of a mobile communication unit, comprising software code portions for performing the steps of one of the claims 1 to 28, when the product is run on a processor of the mobile communication unit.