SPACE TIME CODING

TECHNICAL FIELD

The present invention generally relates to data communication and in particular such communication utilizing space time coding.

BACKGROUND

A main striving force in the development of wireless and cellular communication networks and systems is to provide, apart from many other aspects, increased coverage or support of higher data rate, or a combination of both. Further, the cost aspect of building and maintaining the system has been of great importance and is expected to become even more so in the future. Until recently the main topology of wireless communication systems has been fairly unchanged, including the three existing generations of cellular networks. The topology of existing wireless communication systems is characterized by the cellular architecture with the fixed radio base stations and the mobile stations as the only transmitting and receiving entities in the networks typically involved in a communication session.

One way to introduce diversity in the received signal is to exploit the spatial diversity offered when multiple antennas are used at the transmitter with the possibility of using one or more antennas at the receiver. The use of multiple antennas offers significant diversity and multiplexing gains relative to single antenna systems. Multiple-Input Multiple-Output (MIMO) wireless systems can thus improve the link reliability and the spectral efficiency relative to Single-Input Single-Output (SISO) systems.

In MIMO systems a diversity coding technique denoted Space-Time Coding (STC) is traditionally employed. STC involves the transmission of multiple redundant copies of data, involving cleverly designed operations on the signal such as conjugation and negation, to efficiently compensate for fading and thermal noise in the hope that some of them may arrive at the receiver in a good enough state to allow reliable decoding.

Today the maximum code rate that can be achieved in STC MISO or STC MIMO systems is when using two transmit antenna together with Alamouti's coding [I]. Alamouti coding has the following STC coding matrix:

C = ^s ι ^S 2

_* *

~ ^S 2

where S ₁ and s ₂ are two data symbols and * denotes complex conjugate. A two-transmit antenna Alamouti system is the only STC system that today can achieve a code rate of one as other STC coding techniques and/or higher antenna number significantly reduces the achievable code rate. As is known in the art, the code rate is defined as the number of transmitted independent information elements, such as modulated symbols, divided by the number of transmit instances. In Alamouti diversity, two symbols are transmitted over two instances and hence gives a code rate one.

Another shortcoming of the prior art STC MIMO systems is that the receiver has to use Maximum Likelihood (ML) detection. Such ML detection is technically feasible in theory, but is practically infeasible in particular for thin clients having limited processing capability and power supply.

SUMMARY

There is therefore a need for a STC technique that can overcome the low STC code rates and other drawbacks of the prior art techniques.

It is a general object of the present invention to provide an efficient data communication in wireless communication networks.

It is another object of the invention to provide space time coding and decoding for usage in connection with such data communication.

These and other objects are met by the invention as defined by the accompanying patent claims.

Briefly, the present invention involves data communication between a transmitting communication unit and a receiving communication unit in a wireless network. The invention provides a novel form of space time encoding and decoding providing a selected balance of spatial multiplexing gain and diversity gain, which may be adopted based on the varying radio conditions of the network.

The transmitting unit has access to data elements, such as symbols, to be transmitted to the receiving unit. These two units together constitute a so- called MIMO system in that both units have access to a respective multi- antenna system employed for transmitting and receiving coded data elements, respectively. The invention can though also be applied to MISO systems, where the transmitting but not receiving unit has access to a multi- antenna system.

The transmitting unit combine space time encodes a first set of multiple data elements into a combined space time coded data element. Correspondingly, a set of at least one data element is space time coded into a space time coded data element. These space time coded data elements are mapped to respective transmitting antennas and transmission slots according to a space time coder matrix. Thus, the combined space time coded data element is provided to a first transmitting antenna of the multi- antenna system for transmission at a selected transmission slot. The combined space time coded data element is provided to a second transmitting antenna of the antenna system for transmission at the selected transmission slot.

As a consequence of this combine space time coding, N space time coded elements are transmitted at the selected over t time slots to achieve a coding

N rate of — > 1 . The invention therefore achieves code rates significantly higher

than the highest prior art STC code through usage of the combine space time

coding/ decoding of data elements. Thus, code rates over one are feasible according to the invention.

By selecting the data elements to be combine space time coded respective space time coded separately and selecting the antenna and time slot mapping of the (combined) space time coded data elements, different levels of, and hence trade off between, spatial multiplexing and diversity gains can be achieved by the transmitting unit. The present invention therefore provides a flexible data communication that can efficiently trade off between the degree of spatial multiplexing and the degree of diversity.

The receiving communication unit receives the combined space time coded data element and the space time coded data element by the receiving antennas of its multi-antenna system. The combined space time coded data element is space time decoded at least partly based on information of a previously space time decoded data element received by the multi- antenna system, preferably, from the transmitting unit. In this space time decoding of received coded data elements and due to the combine space time coding of at least some data elements, correctly received data elements can be used to increase the diversity gain of other data elements and /or can be used to obtain multiplexing gain and still obtain code rates over one.

The present invention is directed towards data transmission and receiving methods in the wireless communication network in addition to the transmitting and receiving communication units.

SHORT DESCRIPTION OF THE DRAWINGS

The invention together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:

Fig. 1 is a flow diagram of a data transmitting method according to an embodiment of the present invention;

Fig. 2 is a flow diagram illustrating the combine STC encoding step of Fig. 1 in more detail according to an embodiment of the present invention;

Fig. 3 is a flow diagram illustrating additional steps of the data transmitting method of Fig. 1 ;

Fig. 4 is a flow diagram of a data receiving method according to an embodiment of the present invention;

Fig. 5 is a flow diagram illustrating an additional step of the data receiving method of Fig. 4;

Figs. 6A to 6C illustrate flow diagrams of a particular implementation embodiment of the data receiving method;

Fig. 7 is a diagram comparing the sum capacity achievable according to an embodiment of the present invention with the sum capacity achievable according to prior art arrangements;

Fig. 8 is a schematic overview of a wireless communication network according to the invention;

Fig. 9 is a schematic block diagram of a transmitting communication unit according to an embodiment of the present invention;

Fig. 10 is a schematic block diagram illustrating an embodiment of the STC encoder of Fig. 9;

Fig. 11 is a schematic block diagram of a receiving communication unit according to an embodiment of the present invention; and

Fig. 12 is a schematic block diagram illustrating an embodiment of the STC decoder of Fig. 11.

DETAILED DESCRIPTION

Throughout the drawings, the same reference characters will be used for corresponding or similar elements.

The present invention generally relates to data communication in a wireless communication system and in particular such data communication involving space time encoding/ decoding in connection with a multi-antenna transmitter.

The traditionally data transmission approaches in multi-antenna systems and in particular MIMO systems have been to either provide spatial multiplexing or high diversity coding. In the former approach, a data stream is split into multiple lower data rate streams, where each stream is transmitted from a different transmit antenna in the same frequency channel. Ideally, the receiver can separate these streams, creating parallel channels for free. This allows increasing the channel capacity. The latter approach transmit multiple, redundant copies of a data stream to the receiver in the hope that at least some of them is received in good enough state to allow reliable decoding.

The present invention provides a novel form of space time encoding that basically can be seen as a trade-off between spatial multiplexing and diversity coding. This allows increasing the transmission and STC code rate substantially as compared to the prior art transmit diversity schemes, such as in Alamouti coding MIMO systems.

The invention utilizes the fact that the space time encoding can use a combined STC encoding of multiple data elements, such as symbols, into a combined or composite STC data element. This combined data element is then transmitted according to a STC scheme at a selected time slot and by a

selected antenna of the multi-antenna system. This means that with a multi- antenna system a coding rate of more than one can be achieved by

TV transmitting N individual data elements over t time slots, where — > 1 .

Fig. 1 is a flow diagram illustrating a data transmission method of the invention implementable in a wireless communication network, such a mobile or cellular communication network. The method starts in step Sl , where a transmitting communication unit comprises data to be wirelessly transmitted over the network to a receiving communication unit. The invention can be used regardless of the actual type of data to communicate. Thus, the data can be generated at the transmitting unit or has previously been received by the unit and is to be forwarded to the receiving unit.

Step S 1 combine space time encodes a first set of multiple data elements to form a so-called combined space time coded data element. As was mentioned in the foregoing, the data elements are typically modulated symbols. However, the invention is actually not limited thereto but can also be operated with other data entities. For instance, it is possible to perform the combine space time encoding of step Sl at the bit-level, i.e. data bits, instead of at the symbol-level. In the following, the invention is discussed further in connection with operating on data symbols. This will, though, also cover other form of data entities, such as data bits, that can be used according to the invention.

The combine STC coding of step Sl processes (encodes) multiple, i.e. at least two, preferably two, symbols (data elements) into a single combined STC coded symbol. This form of combine STC coding uses two (or more) symbols S ₁ , s ₂ and codes them to the combined symbol x = /(S ₁₉ S ₂ ) . The function /(K ) denotes any processing of the symbols that allows, during decoding, retrieval of one of _?, , s ₂ given the other of S ₁ , s ₂ and x , i.e. S ₁ = g(s ₂ ,x) or s ₂ = g(s _} ,x) . Preferred such operations allowing combine STC coding is described further herein. The operation and function /(K ) can of course also operate on more

than two symbols. Generally, P symbols s _v s ₂ ,K , s _p can be combine STC coded to a combined symbol x and each of the P symbols can be retrieved during decoding given the combined symbol x and the remaining P - I symbols.

A next step S2 space time encodes a second set of at least one symbol (data element) to form a space time coded data element. This step S2 can be conducted according traditional STC coding, in which a single input symbol is encoded to a single coded symbol. An alternative embodiment of step S2 co-processes multiple symbols similar to step Sl to thereby combine STC encode these multiple symbols into a single combined symbol.

The STC encoding of steps Sl and S2 may be performed in series as indicated by Fig. 1 or in the opposite order to what is illustrated in Fig. 1. Alternatively, the encoding can be performed in parallel or at least partly overlapping in time.

The encoded symbols from steps Sl and S2 are provided from the encoding facility of the transmitting unit and mapped to respective antenna elements of the multi-antenna system for transmission in step S3. This step S3 involves transmitting the combined STC coded symbol from step S 1 by a first transmitting antenna of the multi- antenna system at an assigned transmission slot, such as time slot. The STC coded symbol from step S2 is correspondingly transmitted at the assigned transmission slot but by a second transmitting antenna of the multi-antenna system.

This means that during this single transmission slot coded data corresponding to at least three symbols (depending on the number of symbols co-encoded of the first set and the number of symbols (co-)encoded of the second set) are transmitted from the two antennas. The method then ends or returns to step Sl or S2 to provide new coded symbols to be transmitted at subsequent transmission slots of the multi-antenna system.

The concept of the invention described above and disclosed in Fig. 1 can of course be applied to a STC encoding and multi-antenna system utilizing more than two transmitting antenna. The basic idea is though that at least one transmitting antenna of the system transmits a combined STC coded symbol at the same transmission slot as at least one other antenna of the system transmits a STC coded symbol or a combined STC coded symbol at the communication unit.

In summary, the communication unit has access to a stream of symbols s _v s ₂ ,K that are STC encoded and mapped to transmit antennas according to the invention to form the coded matrix:

^C l l C _n K ^C \M

C = C ₂₂ K ^C 2M

M M O M cκ\ K ⁰ KM

where K is the codeword length of the space time encoder and the columns of the coded matrix C represents the M different transmitting antenna and c _υ is the coded symbol that is to be transmitted in time slot i from antenna j . According to the invention, at least one c _y is combine STC encoded using multiple modulated symbols.

Fig. 2 is a flow diagram illustrating a particular embodiment of the co- encoding multiple data symbols into a combined STC coded symbol. The method starts in step SlO, which involves XOR encoding or XOR bitwise encoding the at least two symbols into a combined symbol: x - s _x θ s ₂ . The combined symbol is then STC encoded according to any prior art encoding technique just as any other data symbol. The method then continues to step S2 of Fig. 1.

The combine coding that can be used according to the present invention is preferably a linear data combining. Examples of such linear methods include

summation over a predetermined Z> -bit Galois field, such as bitwise XOR- operations, and modulus-operations.

XOR bitwise encoding is a very suitable encoding method because of its simplicity. Other codes may also be used, such as an erasure code like Reed Solomon. With respect to the Reed Solomon type of coding, the same operation as the XOR operation between two data words is for instance possible if one selects a shortened RS code with k = 2 and with n = 3 codewords. The non-systematic codeword n - k = l is resent instead of the two bitwise XORed words. Other erasure codes or erasure code oriented encodings may instead be used in the combined STC coding.

Instead of operating on single bits, segments of b bits may be used for the combine encoding. A Galois field of 2 ^b may then be used, under which addition is the encoding operation. With this notation, the XOR operation is just an addition in the Galois field of 2 ¹ .

A further example of an encoding operation suitable to use is based on the modulus operation. Per signal constellation symbol encoding is considered in the following, and the procedure can be repeated for multiple consecutive constellation symbols. The modulus operation is in this example performed both for the real and imaginary part independently when handling complex numbers and utilize a definition of the modulus operation and the mathematical observation that:

((A + B)modL - B)modL = (A)moάL

Thus, a real valued signal B can be superimposed on a real valued signal A and allow undisturbed recovery of the signal A (as long as the signal A does not exceed the quantization level L ), while the amplitude and hence the power is limited of the (non-linearly encoded) composite signal.

The present Invention has the advantage of being able to adapt the STC coding to the current radio environment and system service. This means that the STC code of the invention can provide different levels of spatial multiplexing and diversity coding by the selection of which symbols to combine STC encode and to which transmitting antenna and transmission slot such a combined STC symbols is mapped. For instance, during high Signal-to-Noise-Ratio (SNR) conditions, increased spatial multiplexing can be employed to increase channel capacity. However, if the SNR reduces, more diversity coding of STC scheme can be utilized to combat the deterioration of the radio environment. The present invention can be used in connection with a dynamic adjustment of the STC coding scheme or mode based on, for instance, different collected or estimated quality parameters.

Such an embodiment is schematically illustrated in Fig. 3. The method starts in step S20, where a quality parameter is estimated at the communication unit or elsewhere and then reported to the communication unit. This quality parameter is representative of the radio quality of the communication link(s) employed for transmitting the (combined) STC symbols to the receiving unit. Performing signal measurements and determining quality parameters are well-known in the art and are therefore not further described herein. For instance, such quality-related signal measurements can be performed using pilot signals. The quality parameter can be channel estimates between the transmitting and receiving antennas of the MIMO system, in particular if the transmitting communication unit is a network node (base station) or if TDD-based communication is employed. Actually any quality parameter descriptive of the current radio situation can be employed and the invention is not limited to usage of channel estimates.

The method then continues to step S21 where the STC encoding mode is dynamically updated based on the estimated quality parameter. This updating can be performed by switching between different pre-defined encoding schemes providing different associated spatial multiplexing gains and diversity gains. For instance, a first STC mode provides high

multiplexing gain at the cost of a lower diversity gain. A second STC mode would instead provide a high diversity gain but with lower multiplexing gain. Generally, Q different STC schemes may be available with their respective multiplexing gain mg _t and diversity gain dg _t , i = 1,K , Q .

Each such STC mode then has its special coder matrix defining at what time slots and by which antennas the combined STC coded symbols and any regular STC coded symbols of an input stream should be transmitted. Furthermore, the different STC coded modes are adapted for usage at different particular radio environments and/or system services as reflected by the estimated quality parameter. The quality parameter from step S20 therefore allows identification of the STC mode that is most suitable in terms of providing a balance or trade off between multiplexing and diversity gain for the current situation. The method then continues to step Sl of Fig. 1, where the symbols of the stream are encoded according to the selected STC mode.

In following, different examples of STC schemes adapted for providing different levels of multiplexing and diversity gains are presented for the case of a 2x2 MIMO system. Usage of such a MIMO system should merely be seen as an illustrative example and the invention is not limited to using two transmitting antennas and/or two receiving antennas.

In a dominant spatial multiplexing scenario, the encoder matrix of the STC mode can be given as:

C = ^S 2 S ₂

S ₂ ® ^S l ® s ₄ X _x X ₄

In the first time slot, the transmission is identical to the case of traditional horizontal spatial multiplexing. However, in the second transmission slot the signal transmitted from an antenna of the 2-antenna system is the combination of the previously encoded symbol of the other antenna with a

new symbol. In this embodiment, a first set of symbols s ₂ and s ₃ is combine STC encoded into the combined STC coded symbol represented as x _x = s ₂ ® s ₃ and transmitted at the same transmission slot as the combined coded symbol x ₄ = S ₁ θ s ₄ obtained by combine STC encoding a second set of symbols ^ ₁ and s ₄ . The two symbols S ₁ and s ₂ are also STC encoded separately for transmission in the first transmission slot. A code rate of two is achieved.

The same procedure can then be applied to further symbols of the data stream.

A 2x2 Alamouti scheme can provide a diversity order up to four but with a code rate equal to one. However, not all of this diversity gain might be useful. For instance, if the Signal-to-Interference-Ratio (SINR) for the link is already very high, not much can be gained by introducing more diversity. In clear contrast, it would be better to trade it off for a multiplexing gain. The following STC scheme provides such a trade off between diversity and multiplexing gain.

In this example the encoder matrix is given by:

S ₂

X ₂ X ₃

* *

- X, X ₂

The first and second symbols S ₁ and S ₂ are also STC encoded separately for transmission in the first transmission slot. This corresponds to a purely spatial multiplexing type transmission. In the second and third time slots, the scheme provides a pure diversity Alamouti type transmission. However, in clear contrast to the classical Alamouti matrix, the encoded symbols are combined STC encoded symbols x ₂ and x ₃ , where x ₂ = S ₁ θ s ₃ and p _i = s ₂ @ s ₄ .

In the encoder matrix, * denotes complex conjugate. A code rate of 4/3 is achieved in this illustrative STC scheme example.

Another STC scheme example that provides a trade-off between spatial and diversity gain is to use an encoder matrix according to:

S ₁ S _] ® S ₃ S ₁ X ₂

C =

S ₂ W s ₃ X ₁ S ₂

This scheme provides an antenna mapping such that each of the original symbols _?, , s ₂ , s ₃ is transmitted on two different antennas, thus maximizing the transmit diversity. X ₁ = s ₂ θ ^ ₃ and χ ₂ = S ₁ θ s ₃ represent the combine STC coded symbols. The scheme also provides a code rate of 3/2.

The schemes presented above should be seen as illustrative examples of STC schemes and STC encoder matrices that can be used in connection with the combine STC encoding of symbols of the invention. The person skilled in the art readily realizes how other schemes having different levels of spatial and diversity gain and code rates can be generated based on the above-described principles.

Fig. 4 is illustrates a flow diagram of a data receiving method implementable in a wireless communication network. This receiving method involves receiving a stream of STC encoded symbols as previously described in connection with the data transmission method.

The method starts in step S30 where a receiving communication unit receives STC coded symbols (data elements) transmitted by the multi- antenna system of the transmitting communication unit. The receiving unit preferably also comprises a multi-antenna system thereby having access to multiple receiving antennas to form a MIMO system. The multiple number of receiving antennas can be equal to or different from the multiple number of transmitting antennas. In the following the invention is described further

with a multi-antenna system at the receiving unit. However, the invention can also be used in connection with a MISO system, i.e. where a single receiving antenna is employed in the receiving communication unit.

Briefly, step S30 involves receiving a combined STC coded symbol by at least a first and a second receiving antenna of the multi-antenna system. This combined STC coded data element is transmitted by a first transmitting antenna of the transmitting multi-antenna system at a given transmission slot. Step S30 also comprises receiving a (combined or regular) STC coded symbol at the first and second receiving antennas. This symbol was transmitted by a second transmitting antenna of the transmitting antenna system in parallel with the combined STC coded symbol.

A next step S31 STC decodes the received combined STC coded symbol and the STC coded symbol to obtain STC decoded symbols. The decoding of the combined STC coded symbol is performed at least partly based on information of another symbol of the stream received by the receiving antennas and decoded at the communication unit. This other symbol can be the symbol sent in parallel with the combined symbol, a symbol sent at a previous transmission slot or even a symbol sent at a following transmission slot. Thus, decoding a combined STC coded symbol x using another decoded symbol s, gives s ₂ = g(x,s _i ) . For instance, XOR-based decoding gives S ₁ ® x = S ₁ θ (j, θ s ₂ ) = s ₂ . The method then ends or returns to step S30 for receiving further (combined) STC coded symbols of the data stream.

The receiving structure of the invention can be made simple and does not require the very complex maximum likelihood detection as the prior art schemes. Generally and in more detail, the received signals from the multiple receiving antennas are processed. This signal processing preferably comprises estimating the power present in the received radio signal. A Received Signal Strength Indication (RSSI) or some other power descriptive parameter, such as channel estimates, may be determined. The processed output signal is digitized through an analog to digital conversion before

being down converted to baseband. After down conversion the transmitted signal from each transmit antenna are estimated at each receiver antenna as described further herein.

As is known in the art, transmissions through multi-path channels usually experience inter-symbol interference at the receiver. Such interference can be mitigated by an equalizer at the receiver to compensate for channel distortion. Such equalization can either be done before or during the signal estimation.

STC decoding is then performed on the output from the signal estimation to yield an estimation of the transmitted data symbols. The STC decoding is preferably performed at least partly based on the power/ channel parameter determined at the receiver.

The present invention can be used in connection with different symbol detection algorithms and is not limited to the complex maximum likelihood detection. In clear contrast, the invention can be used with more technically feasible detection algorithms that are practically implementable also in thin clients, such as mobile telephones. A preferred example of such a detection algorithm is successive interference cancellation. Successive interference cancellation can also be used in combination with symbol detection algorithms as minimum mean squared error estimation, linear minimum means squared error estimation and maximum likelihood. It is also possible to use any of these other detection algorithms instead of successive interference cancellation.

In the following, the present invention is described further in connection with a successive interference cancellation algorithm. This should, however, merely be seen as an illustrative example and the invention is not limited thereto but can use other detection algorithms including any of the above- mentioned ones.

Let y _tJ denote the received signal at time slot i by receiving antenna j .

Assuming a 2-antenna system, the received signal during two time slots is given by:

where is the channel matrix with h denoting the channel

(estimate) between transmitting antenna i and receiving antenna j , C is the

data matrix and ξ = is the noise and inter-cell interference with ξ denoting the noise and inter-cell interference at the i"' receiver antenna for the transmission phase j .

Fig. 5 illustrates an additional preferred step of the data receiving method. The method starts in step S40, where the channel matrix for the current MIMO system is estimated at the receiving communication unit. Such channel estimation is well-known in the art and is not discussed in detail herein. For instance, the channel state information can be estimated using power estimations of a pilot signal and/or a training sequence S . In such a case, the channel matrix can be estimated based on:

where r = HS + ξ .

The method then continues to step S30 where the coded symbols are received. The decoding of the symbols involves estimation of a respective signal quality parameter for the received (combined) STC coded symbols based on the channel matrix. These estimated quality parameters are further employed in the symbol detection of the decoding.

The decoding of the STC coded symbols furthermore preferably Involves the estimation of reception power of the multiple receiving antennas. This procedure is schematically illustrated in the decoding example of Figs. 6A to 6C. The method continues in Fig. 6A from step S30 of Fig. 4. Received powers from the transmitting antennas are estimated, such as RSSI or some other power descriptive parameter, in step S50 for a given time slot. These estimated power parameters are compared with each other in a next step S51. The purpose of this comparison is to identify the transmitting antenna that currently provides the highest received power at the receiving unit. The estimation and comparison of steps S50 and S51 may advantageous be used based on the channel estimates as power representing parameters. In such a case, step S51 involves checking whether max {h _{x x} ,h _X2 } > max (A ₂₁ , A ₂₂ ) in this illustrative example with a 2x2 MIMO system.

In the following example, the decoding is described further in connection with SIC detection using a 2x2 MIMO system, where the transmitting communication unit STC encoded and sent symbols according to the following coded matrix:

S ₁ S ₁ tfc> S ₃ S ₁ X ₂

S ₂ θ S ₃ S ₂

This should, however, merely be seen as an illustrative example and the invention can, as has been disclosed further herein, be applied to other forms of receiver structure than SIC detection, other MIMO configurations and with other STC schemes than the above-presented.

Continuing from step S51 , if the received power from the first transmitting antenna is more strongly experienced at the receiving communication unit than the received power from second transmitting antenna (max (A ₁₁ , A ₁₂ } > max (A ₂₁ , A ₂₂ )), the method continues to step S52. Step S52 involves detecting, during the first time slot, the received coded symbol S ₁ .

This symbol detection involves estimating a signal quality parameter for the symbol S ₁ :

where T _s denotes the SINR (signal quality parameter) for symbol s , p denotes half of the power available at the transmitter, h _y denotes the channel between transmitting antenna i and receiving antenna j and ξ _k denotes the noise term at antenna k .

The SIC detection continues by cancelling the detected symbol ^ ₁ from the received signals y _u , y _2] at the first and second receiving antennas in step S53. The other signal transmitted by the transmitting antenna system during the first time slot is then detected in step S54:

The method then continues to step S58 of Fig. 6B.

However, if the received power from the second transmitting antenna is instead stronger in the comparison of step S51 (max (A ₁₁ , A ₁₂ } < max (A ₂₁ , A ₂₂ )). The method continues to step S55, where the STC coded symbol transmitted by the second antenna during the first time slot is detected:

The detected symbol signal is cancelled from the received signals in step S56 and the symbol J ₁ transmitted by the first antenna during the first time slot is detected in step S57:

The method then continues to step S58 of Fig. 6B.

Step S58 estimates the best antenna branch for the second transmit slot, preferably based on the estimated received powers (channel estimates) of the first and second transmitting antennas for the second transmit slot. If the first transmitting antenna is determined to provide strongest received power at the receiving communication unit in step S59, the method continues to step S60. This step S60 detects the symbol x, transmitted by the first antenna during the second transmit slot:

In this example, the noise terms are assumed to be constant during the first and second transmit slots. The detected signal is cancelled from the received signals y _u , y ₂₂ in step S61 and the signal s ₂ sent by the second antenna during this time slot is detected in step S62:

The method continues to step S66 of Fig. 6C.

However, if the received power from the second antenna instead was stronger in the comparison of step S59, the method continues to step S63, which detects the symbol from the second antenna at the second time slot:

The detected symbol signal is cancelled from the received signals in step S64 and the symbol X ₁ transmitted by the first antenna during the second time slot is detected in step S65:

The method then continues to step S66 of Fig. 6C.

Following the SINR evaluation of the transmitted coded symbols, the modulated symbols will be estimated by the STC decoder of the receiving communication unit.

In a first embodiment of the STC decoding of the invention, the decoder investigates whether the STC coded symbols s _x , s ₂ (in other words the symbols that were not combined STC coded) were received with sufficient power by the communication unit in steps S66, S67 and S72. This investigation is preferably performed based on the SINR values estimated for the detected symbols according to the method disclosed in Figs. 6A and 6B using SIC detection or some other signal/ symbol detection algorithm.

For instance, step S66 investigates whether the first symbol S ₁ was detected at sufficient power to enable efficient decoding. This investigation is preferably conducted by comparing the SINR estimated for the symbol with a minimum quality threshold T . If the SINR value exceeds the threshold, the

method continues to step S67, where the SINR value for the second symbol s ₂ is likewise compared to a minimum quality threshold, typically the same as was used in step S66.

If both symbols were received with sufficient power, both the received combined STC coded symbols x, , x ₂ can be used to obtain the third symbol S ₃ . The equivalent SINR for the three symbols will then be given as follows:

r - γ r ^" = γ.

+ r

Thus, in this case both the combined STC coded symbols can be used for increasing the diversity gain and decoding the third symbol, thereby significantly increasing the decoding probability for that symbol. All the data symbols transmitted by the two transmitting antennas during the two time slots are thereby decoded at the receiving communication unit.

If the first symbol but not the second symbol was received with sufficient power, the method continues from step S67 to step S70. In this case we use the combined STC coded symbol X ₁ to increase the diversity gain of the symbol s ₂ not received with sufficient power. The other combined STC coded symbol x ₂ is used to obtain multiplexing gain by decoding ^ ₃ :

r. = γ

If however, the second but not the first symbol was received with sufficient power, the method continues from step S72 to step S73. In this case, we use the combined STC coded symbol x ₂ to increase the diversity gain of S ₁ and

the other combined STC coded symbol x, to obtain multiplexing gain by- decoding S ₃ :

r' = γ + γ

The scenario where neither symbol s, nor s ₂ was received with sufficient power would render the code with a code rate equal to one as the prior art techniques in step S77, which is not described in more detail.

The comparison of the SINR values of the first and second symbols with the threshold of steps S66, S67 and S72 can be performed in any order. Thus, first the first symbol can be investigated followed by comparing the SINR of the second symbol as illustrated in the figure. In an alternative approach, SINR of the second symbol is first compared to the threshold followed by the investigation for the first symbol. The comparisons may alternatively be performed in parallel.

In an alternative approach the SINR-threshold comparisons of steps S66, S67 and S72 may be omitted. In such a case, the equivalent SINR values are calculated for the three different cases described above and disclosed in steps S68, S70 and S73. A respective sum capacity parameter is calculated for each of the three cases in steps S69, S71, S74, such as:

c _sum = iog ₂ (i + r; ) + iog ₂ (i + r. ) + iog ₂ (i + r.

A next step S75 compares the three sum capacities C ₁ , C ₂ and C ₃ and the largest sum capacity C _f is identified. The decoding algorithm selects, in this embodiment, the decoding scenario that gives the highest sum-capacity in step S76.

The embodiment that merely employs SINR-threshold comparisons to select one of the available decoding scenarios may be less computationally expensive as only the calculations according to one of the steps S68, S70 and S73 are performed. The other embodiment may be more computationally expensive but may give a better result as an "exhaustive" search among the different scenarios is performed to identify the currently most suitable scenario. It is anticipated by the present invention that other selection criteria than the sum capacity may be used. Furthermore, the two embodiments may actually be combined.

One of the STC schemes of the invention using combined STC coded symbols

S ₁ S ₁ θ S ₃ with the coding matrix of C = was evaluated and compared to S ₂ θ S ₃ s ₂ a 2x2 Alamouti system in a multi-cell network. The transmit antennas were assumed to be uncorrelated. Fig. 7 illustrates the sum capacity of the STC scheme of the invention (empirical Cumulative Distribution Function, CDF) as opposed to that of a 2x2 Alamouti scheme with Maximum Ratio Combining (MRC) at the receiver. The average sum capacity of the Alamouti scheme is 3.8524 b/s/Hz, whereas that of the invention is 5.7623 b/s/Hz. The invention therefore yields an average sum-capacity gain of 50 %.

Fig. 8 is a schematic overview of a wireless communication network 1 according to the present invention comprising a transmitting communication unit 100 and a receiving communication unit 200 together forming a so- called MIMO system. The communication units 100, 200 can represent any unit, terminal or node in a communication network 1 having access to a multi- antenna system 110, 220 and being capable to wirelessly communicate with another unit, terminal or node in the network 1. Illustrative examples of such units 100, 200 include base stations and other stationary or mobile network nodes. Further examples include user terminals, such as mobile telephones and terminals, laptops, computers and PDAs with communication equipment.

The figure also schematically indicates the channels It ₁₁ -Ii ₂₂ between the individual transmitting antennas 112, 114 and the receiving antennas 212, 214.

Fig. 9 is a schematic block diagram of a transmitting communication unit 100 according to an embodiment. The unit 100 comprises a transmit buffer 120 containing data to be transmitted to one or more other communication units in the network. This data can be previously received data originating from another communication unit. In such a case, the received data is entered in a receive buffer 130 of the unit 100 and is transferred, optionally following further processing to the transmit buffer 120. In an alternative approach, the receive and transmit buffers 120, 130 co-exist in the same memory facility and the individual data elements therein in are simply flagged as either being received or to be transmitted.

In addition, the data to be transmitted can instead or also come from an application 140 present in the unit 100.

In either case, the data elements (symbols) are fetched from the transmit buffer 120 and are forwarded to a connected STC encoder 150 according to the invention. This STC encoder 150 is arranged for space time encoding sets of data elements into STC coded data elements as previously described herein. Thus, the STC encoder 150 encodes a first set of multiple data elements into a combined STC coded data element. The encoder 150 also encodes a second set of at least one data element into a STC coded data element (in the case of one data element in the second set) or a combined STC coded data element (in the case of multiple data elements in the second set).

The encoded data elements are forwarded from the encoder to a connected antenna mapping unit 160. This mapping unit 160 is also connected to the transmitting antennas 114 of the multi-antenna system 110 in the communication unit 100. The mapping unit 160 is arranged for providing

the combined STC coded data element from the STC encoder 150 to the first transmitting antenna 112 for transmission at a given transmission slot. The unit 160 also provides the (combined) STC coded data element from the encoder 150 to second transmitting antenna 114 for transmission at the given time slot.

This antenna mapping unit 160 is preferably implemented for mapping input STC coded data elements to correct transmitting antenna 112, 114 based on information from the STC encoder 150. This information corresponds to the particular STC scheme employed by the STC encoder 150 when space time encoding the input data elements. In a particular embodiment, the information could the be coder matrix of the STC scheme, an indication allowing information of the particular coder matrix or some other information descriptive of the antenna units and transmit slots by which STC encoded data elements should be transmitted.

The communication unit 100 preferably also comprises a link quality unit 170. This quality unit 170 provides a quality parameter representative of the communication quality of a communication link between the communication unit 100 and the destined receiving unit. The unit 170 can perform quality estimations based on signals received from the external unit. Alternatively, the antenna system 110 receives the quality estimates from an external unit, such as the receiving communication unit. In such a case, this received parameter is employed by the quality unit 170.

The units 140 to 170 of the communication unit 100 may be implemented in hardware, software or a combination of hardware and software. The units 110 to 170 may all be implemented in the communication unit 100. A distributed implementation is also possible, in particular for a network situated communication unit 100. In such a case, the units 110 to 170 may be distributed among multiple inter-connected network nodes.

Fig. 10 is a schematic block diagram of a possible implementation of the STC encoder 150 of Fig. 9. The encoder 150 comprises an XOR encoder 152 arranged for performing XOR-based combination of at least two input data elements to form a combined data element. This XOR operation is preferably an XOR bitwise encoding of the multiple data elements. In an alternative approach, another combining algorithm or function as previously described can be used by the STC encoder 150.

In a preferred embodiment, the STC encoder 150 has access to multiple STC schemes defining the particular antenna and transmit slot mapping for the data elements and in addition identifying the data elements that should be combine space time encoded according to the invention. As was mentioned above, these STC schemes provide different spatial multiplexing gain and diversity gain levels, and also different code rates. This means that the STC schemes are adapted to usage at different network conditions and /or in connection with different system services. The encoder 150, thus, preferably comprises a mode selector 154 arranged for selecting between these available STC schemes or modes. The selector 154 preferably performs this mode or scheme selection based on input information. This input information typically comprises the quality parameter provided by the link quality unit of Fig. 9. In such a case, the mode selector 154 can select the particular STC mode that provides the most "optimal" mixture of multiplexing and diversity gains for the current radio condition as assessed from the quality parameter. Other selection input information could be data indicating the current system service employed by the communication unit.

The STC encoder 150 then STC encodes input data elements from an input stream or indeed multiple such streams according to the selected STC scheme/mode. For instance, the STC encoder 150 can operate according to any of the previously described STC schemes and coder matrices.

The units 152 and 154 of the STC encoder 150 may be implemented in hardware, software or a combination of hardware and software. The units

152 and 154 may all be implemented in the STC encoder 150. A distributed implementation is also possible with at least one of the units 152 and 154 implemented elsewhere in the transmitting communication unit.

Fig. 11 is a schematic block diagram of a receiving communication unit 200 according to an embodiment. The unit 200 comprises a multi-antenna system 210 comprising at least first 212 and second 214 receiving antennas. This antenna system 210 is arranged for receiving, from a transmitting communication unit having a transmitting multi- antenna system, a combined STC coded data element transmitted at a given time slot by a first transmit antenna and a (combined) STC coded data element transmitted by a second transmit antenna at the given time slot.

In an alternative embodiment, the receiving communication unit 200 only comprises a single receiving antenna 212, basically resulting in a MISO system with the multi-antenna transmitting communication unit. In the following, the receiving unit 200 is described further in connection with a multi-antenna unit 200. The invention is, though, not limited there to but can be applied to single-antenna units.

The signals received by the receiving antennas 212, 214 are forwarded to a codeword or signal quality estimator 220. This estimator 220 estimates the transmitted signals from each transmit antenna at each receive antenna 212, 214. The estimator 220 preferably estimates a respective signal quality parameter for the received (combined) STC coded data elements, such as respective SINR values. In a typical implementation, the estimator 220 comprises or is connected to a channel estimator 240. This channel estimator 240 is implemented for estimating a channel matrix descriptive of the individual communication channels (see Fig. 8) between the transmitting and receiving antennas 212, 214 of the MIMO system. The estimator 240 preferably performs this channel estimation based on signals received from the transmit antennas, such as based on pilot signals.

The channel estimates are forwarded from the channel estimator 240 to the signal quality estimator 220. There the estimates are employed by the quality estimator 220 in calculating the SINR values for the received signals as previously described.

The channel estimator 240 or another dedicated unit in the communication unit 200 preferably also estimates a respective received power from the transmit antennas. This received power corresponds to the channel estimates if all transmit antennas transmitted with equal power. Otherwise the channel estimates can be used as received power estimates by providing antenna weights based on power division among the transmit antennas.

The determined power parameters are forwarded to a power or channel comparator 250 of the communication unit 200. The comparator 250 compares the estimated power/ channel parameters for the purpose of deciding the transmit antenna contributing to the largest received power at the communication unit 200 at a given transmit slot.

This power comparison is employed by the quality estimator 220 for the purpose of determining the SINR values for the received STC coded data elements. In a preferred embodiment, the quality estimator 220 uses the result from the power/ channel comparison in a successive interference cancellation procedure.

The estimated SINR values for the received data elements are fed to a STC decoder that is arranged for space time decoding the data elements. According to the present invention, the decoding of a combined STC coded data element is performed at least partly based on information (SINR value) of another space time decoded data element received by the multi-antenna system 210. This decoded data element can be a data element transmitted at the same time slot as the combined STC coded data element, at a previous time slot or even a following time slot.

The STC decoder 230 uses the input SINR values (or some other signal quality estimates) from the estimator 220 and estimates the received data elements based on determined equivalent SINR values. These equivalent values are calculated based on the input SINR values and based on a particular decoding mode adopted by the STC decoder 230, which is discussed further herein.

The units 220 to 260 of the communication unit 200 may be implemented in hardware, software or a combination of hardware and software. The units 210 to 260 may all be implemented in the communication unit 200. A distributed implementation is also possible, in particular for a network situated communication unit 200. In such a case, the units 210 to 260 may be distributed among multiple inter-connected network nodes.

Fig. 12 is a schematic block diagram of a STC decoder 230 implementable in the communication unit 200 of Fig. 11. STC decoder 230 comprises a quality determiner 232 arranged for calculating the equivalent SINR values based on the input SINR values from the signal quality estimator of Fig. 11. This calculation is dependent on the particular STC scheme and based the particular reception mode as has been previously described.

A mode selector 234 is arranged for selecting a particular decoding mode depending on whether one or multiple of the data elements were received with high enough power. The mode selector 234 preferably compares the respective SINR values for those data elements that were encoded and transmitted as single STC coded data element with minimum power thresholds. This comparison determines which of the data elements that can be correctly decoded separately or which requires further diversity gain from a combined STC coded data element that is a combination of the given data element and at least one other data element.

The quality determiner 232 uses information of the particular decoding mode from the selector 234 and calculates the equivalent SINR values for all the

received data elements using the multiplexing/ diversity gain dictated by the decoding mode. In such a case, the quality determiner 232 and mode selector 234 can operate as previously described in connection with Figs. 6A to 6C.

In an alternatively embodiment, the quality determiner 232 calculates equivalent SINR values according to all of the available decoding modes, or at least a portion thereof. A capacity determiner 236 calculates a respective capacity parameter, such as sum capacity, for each mode based on the equivalent SINR values determined for the particular modes. The mode selector 234 then selects which decoding mode that should be employed for the received data elements based on these capacity parameters, basically by selecting the mode that maximizes the sum capacity.

The units 232 to 236 of the STC decoder 230 may be implemented in hardware, software or a combination of hardware and software. The units 232 to 236 may all be implemented in the STC decoder 230. A distributed implementation is also possible with at least one of the units 232 to 236 implemented elsewhere in the transmitting communication unit.

It will be understood by a person skilled in the art that various modifications and changes may be made to the present invention without departure from the scope thereof, which is defined by the appended claims.

REFERENCES

[1] Alamouti, "A simple transmit diversity technique for wireless communications", IEEE Journal on Selected Areas in Communication, 16(8): 1451-1458, October 1998