Abstract:
Encoded digital symbols are transmitted via a first pair of antennas and at least one second pair of antennas. The sets of symbols used for the transmission via the second pair of antennas are re-ordered temporally into subsets of symbols with respect to the symbols used for the first pair of antennas. For the first pair of antennas, there is used a signal subjected to encoding with a code-division-multiple-access code and subjected to spreading with a spreading code, and, likewise, for the second pair or pairs of antennas there are used signals subjected to encoding with respective code-division-multiple-access code and subjected to spreading with a respective spreading code. At least one between the respective code-division-multiple-access code and the respective spreading code used for the transmission via the second pair of antennas is different from the code-division-multiple-access code and from the spreading code used for the transmission via the first pair of antennas. The solution can be extended to the use of a plurality of second pairs of antennas in transmission and/or to the use of a plurality of antennas in reception.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   The present application is a continuation of and claims the benefit under 35 U.S.C. § 120 to U.S. patent application Ser. No. 10/910,955, entitled “METHOD FOR TRANSMITTING SIGNALS USING ANTENNA DIVERSITY, FOR INSTANCE IN MOBILE COMMUNICATION SYSTEMS, TRANSMITTER, RECEIVER AND COMPUTER PROGRAM PRODUCT THEREFOR,” filed Aug. 3, 2004 now U.S. Pat. No. 7,460,581, assigned to the same assignee as the present application, and incorporated herein by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present disclosure relates generally to techniques for signal transmission with antenna diversity and has been developed with particular but not exclusive attention paid to the possible application in the framework of telecommunications systems based upon the CDMA/3GPP (Code-division Multiple Access/Third Generation Partnership Project) standard in its various versions, for example. 
   Reference to this possible application must not, however, be interpreted as in any way limiting the scope of the invention. 
   2. Description of the Related Art 
   In order to increase the performance of the aforesaid telecommunication systems, there have been proposed various transmission schemes: in this connection, the 3GPP standard has defined both open-loop techniques, referred to, respectively, as STTD and TSTD, and closed-loop solutions, based upon beam-forming techniques. 
   In order to improve the performance of the system, the 3GPP standard contemplates the use of techniques based upon the use of two transmitting antennas set at the base stations (BTS) in combination with strategies for encoding the data transmitted by them. 
   Recourse to the principle of antenna diversity in transmission, and, in particular, to the approach referred to as space-time coding (STC) with a number of transmitting antennas greater than two and increasingly complex encodings, draw on the pioneering results reported by G. J. Foschini et al. in Bell Labs Tech. J., Autumn 1996, and in the works of Telatar, “Capacity of multiantenna Gaussian channels” AT&amp;T Bell Labs, Tech. Rep., June 1995 and once again of Foschini and Gans in Wireless Personal Comm., March 1998. 
   The above studies have demonstrated that the spectral efficiency of a device can be considerably increased by adopting diversity techniques, not only in reception, but also in transmission. Space-time coding (STC) techniques are able to exploit the characteristics of multiple-reflection transmission environments to distinguish independent signallings transmitted simultaneously in the same frequency band. These techniques prove very effective in environments (such as, precisely, the environment of mobile communication networks), in which the main problem to be faced is that of multipath fading. 
   In particular, Space-Time Transmit Diversity (STTD) techniques, to which reference has already been made previously, is a type of space-time coding that enables improvement of the performance in terms of error probability by maintaining unvaried the transmission rate by means of a pair of antennas in transmission and a corresponding encoding of the data flow sent to them. In view of its simplicity, this solution has been introduced in the 3G standard in the implementation stage. 
   The essential characteristics of this solution adopted by the 3GPP/UMTS standard may be inferred from the diagram of  FIG. 1 . This scheme for data encoding, which is applicable in the cellular-communication environment in so far as it functions also with just one antenna in reception, basically envisages that the sequence of the input bits (b 0 , b 1 , b 2 , b 3 ) is transmitted unaltered via a first antenna A and is, instead, subjected to a combined action of shuffling and of complementing that is such as to bring the sequence of four bits referred to previously to be sent for transmission via the second antenna in the form of the modified sequence (b 2 , b 3 , b 0 , b 1 ). 
   From the point of view of QPSK coding and its representation in complex notation, this operation on the bits is mapped in a conjugation if the second bit (LSB) of the pair is complemented or in a conjugation with phase reversal (i.e., multiplication by −1) in the case where it is the first bit (MSB) of the pair that is complemented. 
   To complete the picture of the currently available solutions, it is also possible to cite the technique known as BLAST (Bell Labs Layered Space-Time), which contemplates the use of more than one antenna both in transmission and in reception. With this technique, spectral efficiencies higher than 30 bits/sec/Hz have been obtained, which cannot be obtained with conventional detection schemes, in environments that are not very noisy or not noisy at all and affected by multiple reflections. 
   Also a solution known as V-BLAST (Vertical BLAST) can be cited, which is substantially based upon a scheme that is simplified as compared to the BLAST technique, such as not to require codings between the flows transmitted and such as to enable, albeit with a presumably lower complexity, a performance in terms of spectral efficiency that is comparable with that of the BLAST technique. 
   At the moment, there are being studied techniques that envisage further improvement of the performance of the system by increasing the number of antennas in transmission and by partially modifying encoding, albeit by maintaining the compatibility with respect to the preceding versions of the 3GPP/UMTS standard—Release 1999. 
   For example, in the document RP020130 (now TR25.869) entitled “Tx diversity solutions for multipath antennas” presented at the TSG-RAN Meeting No. 15 held on Mar. 5-8, 2002, there is proposed the solution represented in  FIG. 2 . 
   This is, in practice, a scheme that contemplates the presence of four antennas or, more precisely, four pseudo-antennas designated, respectively, by A a , A b , A c  and A d . By adopting said scheme, the input signal x(t) is subjected, in a block designated by S, to the STTD-Rel. &#39;99 coding procedure for each pair of antennas. This procedure uses the technique also known as Alamouti space-time block coding for generating two distinct signals x 1  and x 2 , which are to be subjected first to a multiplication by respective factors χ and ξ in two multipliers in view of the supply to the antennas A a  and A c . The same signals are once again subjected to a multiplication by two factors e jφ  and e jψ , respectively, (in practice, a phase rotation is performed) in view of the supply to the antennas A b  and A d . 
   In practice, the pseudo-antennas in question are defined, respectively, as:
 
 A   a   =A   1   +A   2 ,
 
 A   b   =A   3   +A   4 ,
 
 A   c   =A   1   −A   2 , and
 
 A   d   =A   3   −A   4 ,
 
in the case where a balancing of power is required between the transmitting antennas; otherwise, we have:
 
A a =A 1 ,
 
A b =A 2 ,
 
A c =A 3 , and
 
A d =A 4 ,
 
where A 1 , A 2 , A 3  and A 4  are the physical antennas.
 
   The diagram represented in  FIG. 2  uses the Alamouti technique, which is based upon the concept of transmitting the first branch with diversity according to the STTD scheme (s 1 , S 2 , . . . ) via a first antenna (A 1 ) and a replica subjected to phase rotation via the second antenna (A 2 ). The second branch with STTD diversity is transmitted in a similar way via the antennas A 3  and A 4 . 
   Once again,  FIG. 3  illustrates schematically a technique referred to as “phase hopping”, which contemplates a phase rotation between the antennas and between the symbols according to a given sequence of values (by maintaining the phase constant for at least two consecutive symbols). 
   In particular, the phase patterns proposed for the pseudo-antenna  2  and for the pseudo-antenna  4  are respectively: {0, 135, 270, 45, 180, 315, 90, 225} and {180, 315, 90, 225, 0, 135, 270, 45}, i.e., φ=ψ+π. Of course, the values indicated in braces refer to angles expressed in degrees. 
   BRIEF SUMMARY OF THE INVENTION 
   One embodiment of the present invention provides an innovative solution for a diversity transmission scheme, which can be applied, for example, in a 3GPP UMTS system with more than two antennas, whilst maintaining, however, a complete compatibility with the currently standardized STTD scheme and, in general, with the transmission schemes that envisage using just two antennas in transmission. 
   An embodiment of the invention also regards a corresponding transmitter, a corresponding receiver and also a computer product directly loadable into the memory of at least a digital computer and comprises software code portions for performing the steps of a method according to the invention when the computer product is run on a computer. 
   An idea underlying an embodiment of the solution described herein contemplates inserting a further degree of freedom in the four-antenna system, separating the two pairs of antennas. 
   This can be obtained using for each of the two pairs of antennas:
         a different CDMA code—for example, a different OVSF (Orthogonal Variable Spreading Factor) code or equivalent, such as a different Walsh-Hadamard (WH) code—and the same scrambling code; or else   the same CDMA code, but with a different scrambling code.       

   In addition, the encoding on the two new antennas is partially changed by inserting an interleaving operation on 4 symbols—in this case (more in general on M symbols), whilst on the first two antennas the coding of the Release &#39;99 standard is maintained to ensure compatibility in regard to systems that use the preceding versions of the standard. 
   In this connection, it is to be noted that the Release &#39;99 in question is in course of implementation, and the first services are at the moment served on limited areas by some operators. This enables a higher performance to be achieved both with respect to the current scheme and with respect to the scheme currently under discussion at the 3GPP, eliminating at the same time the need for implementing a phase-hopping technique on the antennas  2  and  4 , this being an operation which of course presupposes the need to have available corresponding circuits, of which it is, instead, possible to do without by adopting the technique described herein. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     One or more embodiments of the invention will now be described, purely by way of non-limiting example, with reference to the annexed drawings, in which: 
       FIGS. 1 to 3 , which regard the prior art, have already been described previously; 
       FIG. 4  is a block diagram illustrating an embodiment of the transmission technique described herein; and 
       FIG. 5  illustrates an embodiment of the corresponding reception technique. 
   

   DETAILED DESCRIPTION  
   Embodiments of a method for transmitting signals using antenna diversity, for instance in mobile communication systems, transmitter, receiver and computer program product therefor are described herein. In the following description, numerous specific details are given to provide a thorough understanding of embodiments. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention. 
   Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
   An embodiment of the solution provided herein refers to the case of the use of four transmission antennas designated respectively by Tx 1 , Tx 2  and Tx 3 , Tx 4 . 
   The solution described herein can, however, be extended also to a larger number of antennas. This can be obtained in a simple way both by varying the length of the interleaving on the additional pairs of antennas and by using another channelling/spreading code for these antennas, albeit maintaining unvaried the data rate. The implementation of such extensions on the basis of what is described herein constitutes for a person skilled in the art a design task such as not to require a further detailed description herein. 
   In this connection, it is once again to be noted that the aforesaid extensions do not in general entail an added burden in terms of hardware in so far as the generic base station of a third-generation mobile communication system (BTS 3G) must already be able to transmit all of the codes simultaneously. 
   In the diagram of  FIG. 4 , it is assumed that there is at input a flow of data S 1 , S 2 , S 3 , S 4  coming from a generic modulator a known type (M-PSK or M-QAM). 
   Said flow of data is sent to a block  10  capable of performing simultaneously a demultiplexing function (DMUX), together with a permutation function. 
   Basically, the module  10  splits the flow of input data between two lines designated, respectively, by  12  and  14 . 
   On the output line  12 , the data flow is sent without undergoing any variation, then to be transmitted to an STTD encoder  16  built in accordance with Release &#39;99 of the 3GPP standard. There are then provided subsequent spreading operations with a code c 1  implemented in blocks  181 ,  182  in view of forwarding to the antennas Tx 1  and Tx 2  after prior combination, in two adder nodes designated, respectively, by  201  and  202 , with the respective pilot flows, which are to be used by the receiver for channel estimation. 
   The portion of the transmitter associated to the output line  14  of the block  10  is structurally similar, in the sense that this too comprises an STTD encoder designated by  22 , with associated thereto at output two spreading modules  241 ,  242 , which are to generate signals with a correspondingly widened spectrum. These signals are then supplied to the antennas Tx 3  and Tx 4  after addition of the respective pilot flows in two nodes designated by  261  and  262 , respectively. 
   The basic differences between the two “channels” coming under the lines  12  and  14  are the following:
         whereas on the line  12  there is present the unaltered flow of data, just as it comes from the modulator at input to the block  10 , on the line  14  there is present a data flow in which each set of four symbols is temporally swapped (and consequently subjected to shuffling or interleaving) by subsets, typically in pairs, causing the sequence (S 1 , S 2 , S 3 , S 4 ) to become, at output from block  10 , the sequence (S 3 , S 4 , S 1 , S 2 ); and   the spreading operation performed in the blocks  241  and  242  uses a second code c 2 , different from the code c 1  used by the spreading blocks  181 ,  182 ; in other words, the two pairs of antennas Tx 1 , Tx 2 , on the one hand, and Tx 3 , Tx 4 , on the other hand, use different spreading codes, i.e., c 1  and c 2 , respectively.       

   The corresponding reception and decoding system, illustrated in  FIG. 5 , contemplates the presence of a receiving antenna Rx, which is to receive, in a combined way, the signals coming from the transmission antennas Tx 1 , Tx 2 , Tx 3  and Tx 4 . The signals received present, of course, the typical alterations induced by propagation in the transmission channel C, namely, the addition of noise N and the presence, in the signals received by the various transmission antennas, of phenomena of multipath fading that act in different ways in regard to each signal (this fact being, precisely, at the basis of the operation of diversity techniques). 
   The signal coming from the receiving antenna Rx is sent to two matched filters  301  and  302 , which are to perform the de-spreading operation, eliminating the two spreading codes c 1  and c 2  introduced in the transmission stage. 
   The operation of the filters  301 ,  302  is based upon the formulae given in what follows, which have been developed just for the case of just one antenna in reception but can be extended (according to criteria that are evident to a person skilled in the art) to the case of more than one antenna in reception. 
   In particular, the signal received in four signalling intervals can be expressed in the following way:
 
 r   1   =S   1   h′   1   c   1   −S*   2   h′   2   c   1   +S   3   h′   3   c   2   −S   4   h′   4   c   2 
 
 r   2   =S   2   h′   1   c   1   −S*   1   h′   2   c   1   +S   4   h′   3   c   2   −S*   3   h′   4   c   2 
 
 r   3   =S   3   h″   1   c   1   −S*   4   h″   2   c   1   +S   1   h″   3   c   2   −S*   2   h″   4   c   2 
 
 r   4   =S   4   h″   1   c   1   −S*   3   h″   2   c   1   +S   2   h″   3   c   2   −S*   1   h″   4   c   2 
 
where:
         h′ 1  and h″ i  represent the channel coefficients, and   c 1  and c 2  are the two spreading codes used on the two pairs of antennas Tx 1 , Tx 2  and Tx 3 , Tx 4 , respectively.       

   In the example given above, there has been considered, for reasons of simplicity, just one path from the generic transmitting antenna to the receiving antenna, but the mathematical expression given above can be readily extended—as is evident for a person skilled in the art—to the case of propagation on multiple paths, in general on N different paths. 
   The channel coefficients h′ 1  and h″ i  are assumed as being more or less constant (or estimated to be such) on two symbol time intervals. The corresponding estimation, implemented according to what is proposed by the 3G standard in Release &#39;99 (but also already starting from Release 5), is performed according to known criteria in a channel-estimation block  32  that sends the corresponding coefficients to a linear receiver  34 , which is to supply at output the symbols received S 1 , S 2 , S 3 , S 4 . 
   It will, however, be appreciated that the solution described herein, as regards the channel-estimation function, is in no way tied down to the adoption of the specific technique described in the 3G standard. The solution described herein can in fact be used also together with other estimation methods. 
   After executing the de-spreading operation, in the first two symbol time intervals, there is obtained:
 
 S′   1   =h′*   1   r   11   +h′   2   r*   21 
 
 S′   2   =h′   2   r*   11   +h′*   1   +r   21 
 
 S′   3   =h′*   3   r   12   +h′   4   r*   22 
 
 S′   4   =−h′   4   r*   12   +h′*   3   +r   22 
 
whence we obtain the estimates S′ 1  S′ 2  S′ 3  S′ 4 .
 
   After another two symbol time intervals there is likewise obtained:
 
 S′   1   =h″*   3   r   32   +h″   4   r*   42 
 
 S′   2   =−h″   4   r*   32   +h″*   3   +r   42 
 
 S′   3   =h″*   1   r   31   +h″   2   r*   41 
 
 S′   4   =h″   2   r*   31   +h″*   1   +r*   41 
 
whence we obtain the estimates S″ 1  S″ 2  S″ 3  S″ 4 , where
 
r 11 =r 1 c 1  r 12 =r 1  c 2 
 
r 21 =r 2 c 1  r 22 =r 2  c 2 
 
r 31 =r 3 c 1  r 32 =r 3 c 2 
 
r 41 =r 4 c 1  r 42 =r 4 c 2 
 
   Finally, the estimates of the four symbols received are extracted by summing the two sets of partial estimates according to the relations:
 
 {tilde over (S)}   1   =S′   1   +S″   1 
 
 {tilde over (S)}   2   =S′   2   +S″   2 
 
 {tilde over (S)}   3   =S′   3   +S″   3 
 
 {tilde over (S)}   4   =S′   4   +S″   4 
 
   The estimates in question constitute, precisely, the output signals indicated in the diagram of  FIG. 5  by S 1  S 2  S 3  S 4 . 
   The above convention has been adopted for reasons of simplicity, taking into account the fact that, clearly, the estimation of the output signals corresponds exactly to the input signals transmitted, i.e., the signals sent at input to the block  10  of  FIG. 4  in the case of ideal operation of the system. 
   It will be appreciated that the decoding technique just described can be readily extended to the case of M generic pairs of transmitting antennas with M&gt;2. Also in this case, the result can be obtained simply (and according to criteria that are evident for a person skilled in the art on the basis of the indications here provided) both by varying the interleaving length on the pairs of additional antennas, and by using another channelling/spreading code for them. 
   Also at the receiver end, it is possible to use a number of receiving antennas. Given that the reception system is linear, using the same method just described for each receiving antenna, the total estimate of the symbol will now be given by the sum of the various contributions of estimation supplied by each receiving antenna. 
   The tests conducted by the present applicant show that the adoption of an embodiment of the technique just described leads to considerable advantages in terms of performance. This applies as regards the performance in terms of bit-error rate (BER) and according to a direct comparison with the proposals currently under debate at the 3GPP (usually indicated by the post-fix “Rel5”). 
   With respect to the known solution referred to above, one embodiment of the technique described herein moreover enables elimination from the base station of the phase-rotation function and the corresponding circuit (both at a hardware level and at the level of software components) by maintaining, at the same time, the so-called full rate; in other words, space-time coding does not reduce the transmission data rate. The results of the comparisons to which reference has been made previously used the same channel-estimation system proposed by the standard. 
   Once again, the demodulation technique described herein is maintained in the linear form, involving just de-spreading on two or more codes, which can be rendered perfectly serial and in line with a possible adoption of multiple-code transmission for a single client already envisaged by the standard. 
   Of course, as already indicated previously, instead of using, as in the example embodiments illustrated herein, two different scrambling codes c 1 , c 2 , with a solution that is altogether equivalent it is possible to keep the same scrambling code for the two sets of spreading blocks  181 ,  182  and  241 ,  242  illustrated in  FIG. 2 , using, however, for the two channels corresponding to the lines  12  and  14 , at output from the module  10 , two different codes of the OVSF type (or of any other type that can be used in a CDMA scheme). 
   Consequently, without prejudice to the principle of the invention, the details of implementation and the embodiments may vary widely with respect to what is described and illustrated herein, without thereby departing from the scope of the present invention, as this is defined in the claims that follow. 
   All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety.