Abstract:
In one embodiment, an MLD scheme for a wireless data communication system including steps (a)–(f). Step (a) includes calculating a first vector length for each of a plurality of complex vectors corresponding to a constellation having a plurality of combinations of possible data values. Step (b) includes selecting a subset of the combinations based on the first vector lengths calculated in step (a). Step (c) includes calculating, for a first transmitter, a second vector length for each complex vector corresponding to a combination in the subset. If there are one or more other transmitters, then, for each other transmitter, the method includes implementing steps (d) and (e). Step (d) includes reducing the subset based on the second vector lengths calculated in step (c). Step (e) includes calculating, for a current transmitter, a second vector length for each complex vector corresponding to a combination in the reduced subset. Step (f) includes generating the data based on the combination of possible data values that corresponds to the complex vector having the smallest second vector length in the subset.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   The instant application is a continuation application of U.S. application Ser. No. 09/848,882, filed May 4, 2001 now U.S. Pat. No. 6,968,013, assigned to the assignee of the instant invention, and the disclosure therein is hereby incorporated by reference into the instant application. 

   BACKGROUND OF THE INVENTION 
   The invention relates to a wireless radiofrequency data communication system comprising: a base-station comprising N first groups and a signal processing-unit comprising memory means and processing means, wherein each first group comprises a receiver-unit provided with a receiver and at least one antenna which is connected to the receiver-unit, wherein the signal processing-unit is connected with each of the first groups for processing receive-signals generated by each of the first groups, and M second groups for transmitting radiofrequency signals to the first groups, wherein each second group comprises a transmitter-unit provided with a transmitter and at least one antenna which is connected to the transmitter-unit, wherein the memory means of the signal processing-unit are provided with information about the transfer-functions of radiofrequency signals from each of the antennas of the second groups to each of the antennas of the first groups, and wherein the transmitters and receivers operate on essentially the same radiofrequency or radiofrequency-band. 
   Wireless radiofrequency data communication systems of this type are known and find their applications in a variety of fields. An example of such an application can be found in the domain of digital communication for electronic-mail. In this application, each personal computer can be provided with at least one second set so that the personal computer can be incorporated in an network. The base-station may in that case be connected with a server of the network. Further examples are given, for instance, in the domain of mobile telephony. In the case of mobile telephony, the base-station is a telephony exchange. In the majority of the applications, more than one second set wants to communicate with the base-station. This means that the second group transmits signals to this base-station and also receives signals from this base-station. Since it would not be acceptable if all second groups would have to wait for each other&#39;s communication to be finished, there is a need for simultaneous communication. Simultaneous communication allows more second groups to communicate at the same time with the base-station. A straightforward and common way of realising simultaneous communication is to assign different radiofrequencies to the respective second groups. In this way, all data signals can be separated easily by the first groups in the base-station by frequency-selective filters. Furthermore, the base-station can communicate with each second group at the specific radiofrequency that has been assigned to the second group. A transmitted radiofrequency signal contains the actual information to be transmitted to the receiver. This actual information has been modulated on the radiofrequency carrier-signal. Several techniques have been developed for modulating information on the carrier-signal like frequency-modulation, phase-modulation, amplitude-modulation, et cetera. 
   A radiofrequency signal that is transmitted by a second group travels from the antenna of the second group along so-called travel-paths to the antennas of the first groups. During the travelling, depending of the specific travel-path, the radiofrequency signal is attenuated and also a phase-distortion is incurred on the radiofrequency signal. The phase-distortion of the radiofrequency signal can be corrected by the signal processing-unit in the base-station on the basis of the information about the transfer-functions. This can be of special interest if information is modulated on the radio-frequency signal according to a phase-modulation technique. 
   SUMMARY OF THE INVENTION 
   In an embodiment of the invention, a detection system increases the communication capacity of the wireless communication system per frequency or frequency-band used by the system. The embodiment increases the data communication capacity from the second groups to the first groups by creating multiple separate simultaneous data communication channels. This embodiment provides a detection system comprising M simultaneous separated communication signals for which the number N of first groups may be less than, equal to, or greater than the number M of second groups. 
   The present invention provides a wireless radiofrequency data communication system which is characterised in that the signal processing-unit is arranged to process, in use, the receive-signals on the basis of a Maximum Likelihood Detection method, such that, for each second group of the second groups, an individual communication channel is formed with the base-station, wherein these communication channels are generated simultaneously and separately from each other. 
   In this manner, multiple communication channels are realised on the same frequency, or within the same frequency-band, based on the principle that the signals can be separated thanks to the different characteristics of the transfer-functions. Due to the Maximum Likelihood detection technique, this holds for the cases wherein the number N of first groups is greater than, equal to, or less than the number M of second groups. Hence, the data communication capacity per frequency or frequency-band is indeed increased. The geometry of the configuration of the communication system determines the transfer functions that belong to the different travel-paths between the antennas of the first groups and the second groups in the base-station. These transfer functions are expressed by complex numbers. Each complex number expresses the transfer function between one antenna of one of the first groups and one antenna of the second group. The amplitude of the complex number is the attenuation of the signal&#39;s strength and the phase of the complex number is the phase modulation incurred during transmission along the travel-path. Since the used frequencies are relatively high, the transfer functions depend largely on the configuration. 
   An embodiment according to the invention is characterised in that each transmitter comprises means for modulating an information signal on a radiofrequency signal according to the Quadrature Amplitude Modulation (QAM) method, wherein so-called QAM-symbols are transmitted, and that each receiver comprises means for demodulating information signals from a received radiofrequency signal. 
   The Quadrature Amplitude Modulation is an amplitude- and/or phase-modulation technique in which information in binary format is modulated on a radiofrequency signal. 
   In a favourable embodiment of the invention, the wireless communication system is characterised in that the signal processing-unit is arranged to calculate, in use, a detection signal x DET  according to
 
 x   DET =arg over set  min(∥ r−Hx   p   SET ∥),  (I)
 
where arg over set  min(∥ . . . ∥) is a function which, according to (I), yields that vector x DET  out of a set X SET  of P vectors x p   SET  (p=1, . . . ,P) for which the length ∥r−Hx p   SET ∥ of the complex N-dimensional vector r−Hx p   SET  is minimal, wherein r is a complex N-dimensional vector [r l , . . . ,r i , . . . ,r N ] T  with r i  being the signal received by the i th  first group of the N first groups, H is a complex [N×M] matrix containing transfer-functions h im  (i=1, . . . ,N; m=1, . . . ,M), wherein h im  is the transfer-function for transmission from the m th  second group of the M second groups to the i th  first group of the N first groups, and where x p   SET  is the p th  complex M-dimensional vector [x p   SET, 1 , . . . , x p   SET, m , . . . ,x p   SET, M ] T  of the set X SET , wherein the vectors x p   SET  in the set X SET  contain possible combinations of values which can be assigned by the second groups to an information signal x, where x is a M-dimensional vector [x 1 , . . . ,x m , . . . ,x M ] T  with x m  being the information signal transmitted by the m th  second group of the M second groups to the first groups and where x m  is one individual communication signal. Equation (I) is based on the model
 
 r=Hx+n,   (II)
 
where n is a complex N-dimensional noise vector [n 1 , . . . , n i , . . . n N ] T  containing noise terms picked up during reception. The model (II) gives the relation between the transmitted signal x=[x 1 , . . . ,x m , . . . ,x M ] T  and the received signal r=[r 1 , . . . ,r i , . . . ,r N ] T  in the base-station. The detection method according to the invention tries to match a fixed number of possible combinations of values for x in the equation (I). These possible value combinations are the vectors x p   SET , (p=1, . . . , P), which all together constitute the set X SET . The detection signal x DET  is that vector x p   SET  in the set X SET  which minimizes the length of the vector r−Hx p   SET .
 
   In a further embodiment the wireless radiofrequency data communication system is characterised in that the processing unit is arranged to apply, in use, the following approximation (III) in the calculation of (I)
 
∥ r−Hx   p   SET ∥=Σ i=1, . . . ,N (∥Real([ r−Hx   p   SET ] i )∥+∥ Im ([ r−Hx   p   SET ] i )∥),  (III)
 
wherein Σ i=1, . . . ,N ( . . . ) is a summation over the index i from 1 to N over the argument (∥Real([r−Hx p   SET ] i )∥+∥Im([r−Hx p   SET ] i )∥), where ∥(.)∥ yields the absolute value of its input argument and where Real( . . . ) is a function which, in equation (III), yields the real part of its complex argument [r−Hx p   SET ] i , with [r−Hx p   SET ] i  being the i th  component of the complex N-dimensional vector r−Hx p   SET , and where Im( . . . ) is a function which, in equation (III), yields the imaginary part of its complex argument [r−Hx p   SET ] i .
 
   The expression (III) offers a valuable approximation and simplification for the evaluation of equation (I). 
   An embodiment of the invention which is of special interest is a wireless radiofrequency data communication system characterised in that the set X SET  comprises all possible combinations of values which can be assigned to the signal x by the second groups. 
   In a preferred embodiment of the invention the wireless radiofrequency data communication system is characterised in that the signal processing-unit is arranged to find, in use, the detection signal x DET  according to a Reduced Search Technique wherein a search-tree is passed through according to the following steps 1 to 7:
     Step 1: calculate the lengths of the complex vectors v corresponding to all combinations of possible values which can be assigned to [x 1 , . . . ,x L ], wherein v is given by
 
 v =( r−Σ   i=1, . . . ,L   h   i   *x   p   SET, i ),  (IV)
 
where Σ i=1, . . . ,L  ( . . . ) is a summation over the index i from 1 to L over the complex argument [h i *x p   SET, i ] and where h i  is the i th  column [h 1,i, . . . , h N,i ] T  of the matrix H;
   Step 2: select the K combinations of values for [x p   SET, 1, . . . , x p   SET, L ] corresponding to the K smallest lengths of v as well as the K vectors v itself and set m=L+1;   Step 3: calculate the lengths of the C*K new vectors v given by
 
 v=v   old   −h   m   *x   p   SET, m ,  (V)
 
where v old  is one of the K vectors v resulting from the preceding step and where h m  is the m th  column of H;
   Step 4: select those K combinations of values for [x p   SET, 1 , . . . , x p   SET, m ] that correspond to the K smallest lengths of v as well as the K vectors v itself and set m=m old +1, where m old  is m from the preceding step;   Step 5: if m&lt;M then go to Step 3, else go to step 6;   Step 6: calculate the lengths of the C*K new vectors v given by
 
 v=v   old   −h   M   *x   p   SET, M ,  (VI)
   Step 7: the detection signal x DET  is determined as that combination of values x DET =[x p   SET, 1 , . . . , x p   SET, M ] which corresponds to the vector v with the smallest length,
 
wherein K and L are predetermined fixed integer values which control the size P of the set X SET  and wherein the constellation size C of the system is the number of values x p   SET, m  which can be assigned by one of the second groups to one component x m  (m=1, . . . ,M) of x and where v old  is one of the K vectors v resulting from Step 3, the calculated detection signal x DET  is the combination of values x p   SET  corresponding to the smallest vector v. In this manner the requisite number of calculations for obtaining the detection signal x DET  is known on beforehand, since the number P of vectors in the set X SET  is pre-determined by selecting values for K and L.
   

   An alternative embodiment according to the invention is characterised in that the signal processing-unit is arranged to find, in use, the detection signal x DET  according to a Reduced Search Technique wherein a search-tree is passed through according to the following steps 1 to 7:
     Step 1: calculate the values of the lengths of the C vectors v according to the C possible values x p   SET, 1 
 
 v =( r−h   1   *x   p   SET, 1 ),  (VII)
 
wherein h 1  is the first column of H;
   Step 2: select those combinations of values for x p   SET, 1  for which the lengths of v are smaller than T, as well as the corresponding vectors v and set m=2;   Step 3: calculate the lengths of the new vectors v given by
 
 v=v   old   −h   m   *x   p   SET, m ,  (VIII)
 
wherein v old  is one of the vectors v resulting from the preceding step and where h m  is the m th  column of H, and adjust the threshold T;
   Step 4: select those combinations of values for [x p   SET, 1 , . . . , x p   SET, m ] for which v is smaller than T, discard the other combinations and set m=m old +1, where m old  is m from the preceding step;   Step 5: if m&lt;M then go to Step 3, else go to step 6,   Step 6: calculate the lengths of the new vectors v given by
 
 v=v   old   −h   M   *x   p   SET, M ,  (IX)
   Step 7: the detection signal x DET  is determined as that combination of values x DET =[x p   SET, 1 , . . . , x p   SET, M ] which corresponds to the vector v with the smallest length, wherein T is a predetermined fixed threshold value which controls the size P of the set X SET  and wherein the constellation size C of the system is the number of values x p   SET, m  which can be assigned by one of the second groups to one component x m  (m=1, . . . ,M) of x, and wherein v old  is one of the vectors v resulting from step 3, the calculated detection signal x DET  is the combination of values x p   SET  corresponding to the smallest vector v. In this embodiment the number of requisite calculations for obtaining the detection signal x DET  is not known beforehand since the number of vectors v which are selected in the respective steps 2 and 4 may vary from step-to-step, depending on the adjusted value for the threshold T and the noise n in the signal r.   

   A further embodiment of the wireless radiofrequency data communication system according to the invention is characterised in that the signal processing-unit is arranged to find, in use, the detection signal x DET  according to a Reduced Search Technique which also comprises the following steps:
     Step A1: calculate an inner product z of the vector r with the u th  column h u  of the matrix H, where u is an integer 1≦u≦M, according to:
 
 z=h   *   u   r ,  (X)
 
where h *   u  is the complex conjugated and transposed of h u ;
   Step A2: calculate C M−1  terms Interf corresponding to all possible value combinations which can be assigned to [x 1 , . . . ,x u−1 ,x u+1 , . . . ,x M ], wherein the terms Interf are defined according to:
 
Interf=Σ (i=1, . . . ,M^i≠u)   x   i *( h   *   u   *h   i ),  (XI)
 
wherein Σ (i=1, . . . ,M^i≠u)  is a summation over the index i from 1 to M with the exception of the integer u;
   Step A3: estimate, on the basis of the equations (X), (XI) and z′ according to:
 
 z ′=Interf+ x   u *( h   *   u   *h   u ),  (XII)
 
where z′ is an approximation of z, the value for x u  corresponding to each of the value combinations [x 1 , . . . ,x u−1 ,x u+1 , . . . ,x M ], and constitute a test set X SET  comprising C M−1  vectors x p   SET , wherein each vector x p   SET  represents a value combination
 
[x 1 , . . . ,x u−1 ,x u+1 , . . . ,x M ];
   Step A4: determine the detection signal X DET  according to equation (I), wherein the test set is defined with the C M−1  vectors x p   SET  from the preceding step.   

   In an embodiment of the wireless radiofrequency data communication system according to the invention the multiple N of first groups exceeds the multiple M of second groups. 
   In another embodiment of the wireless radiofrequency data communication system according to the invention the multiple N of first groups is less than the multiple M of second groups. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In still another embodiment of the wireless radiofrequency data communication system according to the invention the multiple N of first groups is equal to the multiple M of second groups. 
     In the accompanying drawings, in which certain modes of carrying out the present invention are shown for illustrative purposes: 
       FIG. 1  is a diagram schematically showing a wireless data communication system according to the invention; 
       FIG. 2  is a diagram illustrating the use of Quadrature Amplitude Modulation (QAM) symbols by the wireless data communication system of  FIG. 1 . 
       FIG. 3  is a diagram illustrating the use of Quadrature Amplitude Modulation (QAM) symbols by a wireless data communication system which contains three first groups and two second groups. 
       FIG. 4  is a diagram illustrating the use of Quadrature Amplitude Modulation (QAM) symbols by a wireless data communication system which contains one first group and two second groups. 
       FIG. 5  is a diagram showing a search tree for illustrating the principle of Reduced Search Techniques in an embodiment of a wireless data communication system according to the invention. 
   

   DETAILED DESCRIPTION 
   An embodiment of a wireless radiofrequency data communication system according to the invention is shown in  FIG. 1 . This wireless radiofrequency data communication system comprises a base-station  2  which comprises a signal processing-unit  4  and multiple first groups  6 . i  (i=1,2, . . . ,N). Each first group  6 . i  comprises a receiver-unit  10 . i  provided with a receiver, and at least one antenna  12 . i  which is connected to the receiver-unit  10 . i . In this example each first group  6 . i  comprises one antenna  12 . i  connected to the receiver-unit  10 . i . The signal processing-unit  4  is at one side connected to each of the first groups  6 . i , i=1,2, . . . ,N, and at another side connected to each of the coding/decoding-units  14 . m , m=1,2, . . . ,M. Furthermore, the system comprises multiple second groups  16 . m  (m=1,2, . . . ,M), wherein each second group  16 . m  comprises at least a transmitter-unit  18 . m  and at least one antenna  20 . m  which is connected to said transmit-unit  18 . m . Each of the second groups  16 . m  is connected with a coding/decoding-unit  22 . m , m=1,2, . . . ,M. 
   In the configuration of  FIG. 1  the second groups can belong to mobile communication stations. An example of this is the situation wherein notebooks are provided with at least one second group. In this situation the notebooks can communicate via the second groups to the base-station. The base-station can be an internet and/or an intranet server. In general the number M of second groups will vary during the operation of the base-station. 
   The second groups  16 . m  (m=1,2, . . . ,M) can simultaneously transmit radiofrequency signals on essentially the same radiofrequency or radiofrequency-band to the first groups  6 . i . The radiofrequency signal which is transmitted from the second group  16 . m  travels from this second group along travel-paths T im  (i=1,2, . . . ,N) to the respective first groups  6 . i  (i=1,2, . . . ,M). The travel-path T im  is the travel-path from the second group  16 . m  to the first group  6 . i . For each travel-path T i,m  a transfer-function can be defined which is expressed with a complex number h im . The amplitude of the h im  is the attenuation of the signal&#39;s strength and the phase of h im  is the phase modulation incurred during transmission along the travel-path. The processing-unit  4  comprises memory means provided with information about the transfer-functions of radiofrequency signals along said travel-paths. Furthermore the processing-unit  4  comprises processing means for processing receive-signals generated by each of the first groups. 
   In  FIG. 1  a radiofrequency transmission signal x RF   m  is generated by the second group  16 . m  and transmitted to the first groups  6 . i  (i=1, . . . ,N). The generation of this radiofrequency transmission signal is as follows. The input to the coding/decoding-unit  22 . m  is a so-called Quadrature Amplitude Modulated symbol (short: QAM-symbol). This QAM-symbol is expressed as a complex number (an example of such a complex number is: 1+2j, where the real part of the QAM-symbol is 1 and the complex part is 2). Said QAM-symbol is modulated by the coding/decoding-unit  22 . m  on a low-frequency signal, which yields x m . This data-signal x m  is the input of the transmit-unit  18 . m , which transmit-unit modulates x m  on a radiofrequency signal yielding the radiofrequency signal x RF   m . The radiofrequency signal x RF   m  is transmitted by the antenna  20 . m  to the first groups. 
   Each first group  6 . i , i=1, . . . ,N receives a radiofrequency signal r RF   i  in the antenna  12 . l . This signal r RF   i  is a result of the interference of all transmitted signals x RF   m ,m=1, . . . M. The signal r RF   i  is demodulated by the receiver-unit  10 . i  to a low-frequency receive-signal r i  which contains the modulated information of data-signal x m . The N receive-signals r i , i=1, . . . ,N are fed to the input of the signal processing-unit  4 . The signal processing unit  4  calculates M output signals x i,DET , i=1, . . . ,M. Each output signal x i,DET  is an estimation for the data-signal x i . The detection signal x i,DET  is inputted to the coding/decoding-unit  14 . i , i=1, . . . ,M, which unit generates the QAM-symbol c i,DET . This QAM-symbol is an estimation for the QAM-symbol c i . 
   The operation of the signal processing unit  4  will now be elucidated on the basis of a mathematical framework. The relation between the data-signals x m , m=1, . . . ,M and the receive-signals r i , i=1, . . . ,N is given with the following vector equation:
 
 r=Hx+n,   (XIII)
 
where r is a N-dimensional complex vector [r 1 , . . . ,r i , . . . ,r N ] T , where H is a complex [N×M]-matrix containing the transfer-functions h im , (i=1, . . . ,N; m=1, . . . ,M) and wherein x is a M-dimensional complex vector [x 1 , . . . ,x m , . . . ,x M ] T . The noise vector n is a N-dimensional complex vector [n 1 , . . . ,n i , . . . ,n N ] T , with n i  being the noise term picked up during reception of the signal r RF   i . The signal processing unit  4  calculates detection signals x DET =[x 1,DET , . . . ,x ,M,DET ] T  which are estimations for the data-signals x. These calculations are performed according to a Maximum Likelihood method. It has to be noted here that the detection signals are sometimes called Maximum Likelihood signals. One important aspect of the Maximum Likelihood approach is that use is made of the knowledge that the QAM-symbol c m , m=1, . . . ,M can only take on a limited number of values. This number is called the constellation size C of the second group. So, the data-signal x m  for a particular m can only take on C different values. This implies that the total number of possible different value combinations of the data-vector x is C M . All these C M  different value combinations together constitute the test set X SET , wherein the p th  element of the set is a value combination which is a complex M-dimensional vector x p   SET . This implies that the signal processing unit can calculate a detection signal by trying to match all elements of the set and choosing the best fit according to (XIII). In order to save processing time, it is also possible to match only the elements of a subset of the set. For the latter approach, several reduced search techniques are developed according to different embodiments of the invention. The detection signals that are found by matching elements of the set are called Maximum Likelihood signals.
 
   The signal processing-unit  4  can find the Maximum Likelihood detection signal in particular according to the following algorithm. This algorithm will be described in mathematical terms. The detection signals x DET  are calculated by the signal-processing-unit  4  according to
 
 x   DET =arg over set  min(∥ r−Hx   p   SET ∥),  (XIV)
 
where arg over set  min(∥ . . . ∥) is a function which, according to (XIV), yields that vector x DET  out of a set X SET  of P vectors x p   SET  (p=1, . . . ,P) for which the length ∥r−Hx p   SET ∥ of the complex N-dimensional vector r−Hx p   SET  is minimal and where x p   SET  is the p th  complex M-dimensional vector [x p   SET, 1 , . . . ,x p   SET, m ,. . . ,x p   SET, M ] T  of the set X SET , wherein the vectors x p   SET  in the set X SET  contain possible combinations of values which can be assigned by the second groups to an information signal x, where x m  is one individual communication signal. Since the data-signals x m , m=1, . . . ,M are transmitted simultaneously each detection signal x m,DET  results in the generation of one of M simultaneous data communication signals.
 
   The principle of a Maximum Likelihood Detection method will now be illustrated with the schematic of  FIG. 2 . This figure shows QAM-symbols c m , m=1, . . . ,M, which are the possible inputs to the coding/decoding units  22 . m . The QAM-symbol c 1  is marked with a *26.1 and equals (−1+j). Each diagram  24 . m  contains a QAM-symbol c m  which can take on 4 different values. These possible values are marked with 0  28 . m . The actual possible values for the QAM-symbols are (1+j), (−1+j), (1−j), and (−1−j). Thus here the constellation size C of the second groups is C=4. As explained previously, the QAM-symbols are the information that is modulated on the corresponding data signal x m . However, from a mathematical point of view, the QAM-symbols are identical to the signals x m . So, the value of the low-frequency signal x 1  in the example of  FIG. 2  is (−1+j) and the value of x M  is (1−j). The transmission and the subsequent reception of the QAM-symbols to the first groups  6 . i  takes place via radiofrequency carrier signals. The mathematical formulation of this transmission is a transformation of the vector x with the matrix H and an addition of noise n, see equation (XIII). This is illustrated with the transformation block  30  which contains M input-signals x m , m=1, . . . ,M, and N output-signals r i ,i=1, . . . ,N. The values of the output signals r i , i=1, . . . ,N are represented with * marks  34 . i  in the diagrams  32 . i . Each diagram  32 . i  contains one received component r i  of r. As a consequence of the time-varying nature of H and as a consequence of noise the values of r i  will generally vary, even if the QAM-symbols c m  would have continuously constant values. Now the signal processing unit  4  calculates the transformation of all possible value combinations in the test set (or a subset of the test set) of the QAM-symbols, in order to try these elements in (XIV), according to:
 
 r   p   SP   =Hx   p   SET   ,p= 1 , . . . ,P.   (XV)
 
The complete test set comprises P=C M  combinations (but, for a subset of the test set, P&lt;=C M ). Equation (XV) yields possible receive-signals in the situation of absence of noise. These signals are indicated with 0 marks  36 . i  in the diagrams  32 . i . Finally, the Maximum Likelihood solution is that value combination x p   SET  in the test set, which corresponds to the 0 marks of its components, for which the value
 
∥ r−r   p   SP ∥,  (XVI)
 
is minimal. In the case in which the vectors r and r p   SP  comprise only one component, the Maximum Likelihood solution is found with the 0 in diagram  32 . 1  which is closest to the *.
 
     FIG. 3  gives a more detailed example of a wireless data communication system according to the invention, where the number of first groups N=3, the number of second groups M=2, and the constellation size C=2. In total, there are 4 possible value combinations which can be assigned to the data-signals x. In this example, the data-signal [ 26 . 1 ,  26 . 2 ] is x=[(0+j), (0+j)] T . Since the number of first groups is greater than the number of second groups, a robust communication system, fairly insensitive to noise, is realised. The possible values for the components of the data-signals x are schematically indicated with 0 marks  28 . 1  in the diagrams  24 . 1  and  24 . 2  in  FIG. 3 . The test set comprises all these possible value combinations (P=4), so the test set X SET  is given with:
   X   SET   ={x   1   SET   ; x   2   SET   ; x   3   SET   ; x   4   SET }, 
where:
   x   1   SET =[(0 +j ), (0 +j )] T ,   x   2   SET =[(0 −j ), (0 +j )] T ,   x   3   SET =[(0 +j ), (0 −j )] T ,   x   4   SET =[(0 −j ), (0 −j )] T ,  (XVII) 
wherein the notation X SET ={ . . . ; . . . } means that X SET  is the set of all vectors which are enumerated between the brackets {.}. In this case, the matrix H containing the transfer-functions is a complex [3×2]-matrix. The transformation of the elements of the test set X SET  (see (XVII)) according to (XV) results in 4 vectors r p   SP , each of which has 3 components. The values of the respective components of the vectors r p   SP  are depicted in the respective diagrams  32 . 1 ,  32 . 2 , and  32 . 2  of  FIG. 3  and marked with 0 marks  36 . i , i=1,2,3. The transformation of the data-signal x according to (XIII) yields the receive signal r. The respective components of this signal are indicated with * marks  34 . i  in the respective diagrams  32 . 1 ,  32 . 2 , and  32 . 2  of  FIG. 3 . Note that, despite the relation x=x 1   SET , the signal r is unequal to r 1   SP  as a consequence of the noise n. The Maximum Likelihood solution is found according to (XIV). Note that 2 simultaneous data communication channels are generated, which channels are the components of the vector x (the first channel is x 1  and the second channel is x 2 ).
 
     FIG. 4  gives an example of a wireless data communication system in which there are more second groups (M=2) than first groups (N=1). However, with a detection algorithm according to the invention, the signal processing-unit sometimes can recover the transmitted M-dimensional data-signal x. In this case, the data-signal [ 26 . 1 ,  26 . 2 ] is x=[(0+j), (0+j)] T . The constellation size C=2. So, there are 4 possible value combinations which can be assigned to the data-signals x. The possible values for the components of these data-signals are indicated with 0 marks  28 . 1  in the diagrams  24 . 1  and  24 . 2  of  FIG. 4 . The test set comprises all these possible value combinations (P=4), so the test set X SET  is given with (XVII). In this example, the matrix H containing the transfer-functions is a complex [1×2]-matrix:
   H=[h   11   h   12   ]=[h   1   h   2 ],  (XVIII) 
where the column h 1  is identical to the scalar h 11  and the column h 2  is identical to the scalar h 12 . The transformation of the elements of the test set X SET  (see (XVII)) according to (XV) results in 4 scalars r p   SP . The values of these 4 scalars r p   SP  are depicted in the diagram  32 . 1  of  FIG. 4  and marked with 0 marks  36 . 1 . The transformation of the data-signal x according to (XIII) yields the receive signal r. In this example, this signal is also a scalar which is indicated with a * mark  34 . 1  in the diagram  32 . 1  of  FIG. 4 . Note that, despite the relation x=x 1   SET , the signal r is unequal to r 1   SP  as a consequence of the noise n. The Maximum Likelihood solution is found according to (XIV) and is given with the 0 mark which is closest to the * mark  34 . 1 .
 
   The calculations of the signal processing unit based on the equation (XIV) can be approximated with the following expression in order to save processing time:
 
∥ r−Hx   p   SET ∥=Σ i=1, . . . ,N (∥Real([ r−Hx   p   SET ] i )∥+∥ Im ([ r−Hx   p   SET ] i )∥),  (XIX)
 
wherein Σ i=1, . . . ,N ( . . . ) is a summation over the index i from 1 to N over the argument (∥Real([r−Hx p   SET ] i )∥+∥Im([r−Hx p   SET ] i )∥), where ∥(.)∥ yields the absolute value of its input argument and where Real( . . . ) is a function which, in equation (XIX), yields the real part of its complex argument [r−Hx p   SET ] i , with [r−Hx p   SET ] i  being the i th  component of the complex N-dimensional vector r−Hx p   SET , and where Im( . . . ) is a function which, in equation (XIX), yields the imaginary part of its complex argument [r−Hx p   SET ] i .
 
   The signal processing-unit  4  finds the Maximum Likelihood detection signal according to (XIV). With this equation, the detection signal is found by trying to fit elements of the test set X SET . Since the test set comprises C M  elements, this can easily lead to a huge amount of processing time. To reduce the processing time, an embodiment of the invention provides an algorithm that can limit the search to a subset of X SET . A first algorithm, which provides an intelligent way of choosing a pre-determined subset of X SET , is given below and is called a Reduced Search Technique. In this Reduced Search technique, a search-tree is passed through according to the following steps 1 to 7:
     Step 1: calculate the lengths of the complex vectors v corresponding to all combinations of possible values which can be assigned to [x 1 , . . . ,x L ]. The vector v is given with
 
 v =( r−Σ   i=1, . . . ,L   h   i   *x   p   SET, i ),  (XX)
 
where Σ i=1, . . . ,L  ( . . . ) is a summation over the index i from 1 to L over the complex argument [h i *x p   SET, i ] and where h i  is the i th  column [h 1,i , . . . ,h N,i ] T  of the matrix H;
   Step 2: Select the K combinations of values for [x p   SET, 1 , . . . , x p   SET, L ] corresponding to the K smallest lengths of v as well as the K vectors v itself and set m=L+1;   Step 3: Calculate the lengths of the C*K new vectors v given by
 
 v=v   old   −h   m   *x   p   SET, m ,  (XXI)
 
where v old  is one of the K vectors v resulting from the preceding step and where h m  is the m th  column of H;
   Step 4: Select those K combinations of values for [x p   SET, 1 , . . . , x p   SET, m ] that correspond to the K smallest lengths of v as well as the K vectors v itself and set m=m old +1, where m old  is m from the preceding step;   Step 5: If m&lt;M then go to Step 3, else go to step 6;   Step 6: Calculate the lengths of the C*K new vectors v given by
 
 v=v   old   −h   M   *x   p   SET, M ,  (XXII)
   Step 7: the detection signal x DET  is determined as that combination of values x DET =[x p   SET, 1 , . . . , x p   SET, M ] which corresponds to the vector v with the smallest length,
 
wherein K and L are predetermined fixed integer parameters which control the size P of the set X SET  and where v old  is one of the K vectors v resulting from Step 3, the calculated detection signal x DET  is that element of the set X SET  for which the vector v is smallest. The principle of this reduced search technique is schematically illustrated in diagram  38  of  FIG. 5  (according to the example of  FIG. 4 ). In this example, the pre-determined parameter L=1 and K=4. Furthermore, the constellation size C=2, the number of second groups M=2, the number of first groups N=1, the matrix H is given with (XVIII), and the test set is defined with (XVII). The received vector r is a scalar r. The first stage  40  of the algorithm is to evaluate (XX). This results in principle in four lengths v 1 , v 2 , v 3 , and v 4  corresponding to four vectors v (note that in this example v and h 1  and h 2  are all scalars):
 
 v 1 =∥r−h   1   *x   1   SET, 1   ∥=∥r−h   1   *j∥ 
 
 v 2 =∥r−h   1   *x   2   SET, 1   ∥=∥r−h   1 *( −j )∥
 
 v 3 =∥r−h   1   *x   3   SET, 1   ∥=∥r−h   1   *j∥=v 1
 
 v 4 =∥r−h   1   *x   4   SET, 1   ∥=∥r−h   1 *( −j )∥= v 2.  (XXIII)
 
These values v 1  and v 2  are shown in  FIG. 5  in Step 1 of the algorithm  42 . According to Step 2 both values v 1  and v 2  are selected. Hereafter, in Step 3, and in Step 4, the lengths of 4 new vectors v are calculated. These 4 new lengths are:
 
 v 1 =∥r−h   1   *j−h   2   *j∥ 
 
 v 2 =∥r−h   1 *( −j ) −h   2   *j∥ 
 
 v 3 =∥r−h   1   *j−h   2 *( −j )∥
 
 v 4 =∥r−h   1 *( −j ) −h   2   *j∥   (XXIV)
   

   These four lengths  44  are depicted in  FIG. 5 . From Step 5 of the algorithm, a jump is made to Step 6. Finally, in Step 6 the calculation of the Maximum Likelihood detection signal is finished. In this case, the detection signal equals the test vector x 4   SET : x DET =x 4   SET . In the algorithm stated above, the processing load of the signal processing-unit  4  is known beforehand and can be controlled with the parameters K and L. It is important to note that the detection signal comprises two components whereas the receive-signal r comprises only one component. So, in principle two separate simultaneous communication channels are formed. This example shows an important aspect of the nature of the detection algorithm according to certain embodiment of the invention, namely that the number of receivers may be less than the number of transmitters, and thus the number of receivers may also be less than the number of simultaneous communication channels. 
   In another embodiment of the wireless data communication system, the signal processing-unit  4  also finds the Maximum Likelihood detection signal according to (XIV). However, in this embodiment, the processing load of the signal processing-unit  4  is not known beforehand. The subset of the test set X SET  is controlled with a real value threshold parameter T. The algorithm of this Reduced Search Technique is given below and comprises seven steps:
     Step 1: Calculate the values of the lengths of the C vectors v according to the C possible values x p   SET, 1 
 
 v =( r−h   1   *x   p   SET, 1 ).  (XXV)
   Step 2: Select those combinations of values for x p   SET, 1  for which the lengths of v are smaller than T, as well as the corresponding vectors v and set m=2;   Step 3: Calculate the lengths of the new vectors v given by
 
 v=v   old   −h   m   * x   p   SET, m ,  (XXVI)
 
wherein v old  is one of the vectors v resulting from the preceding step, and adjust the threshold T;
   Step 4: Select those combinations of values for [x p   SET, 1 , . . . , [x p   SET, m ] for which v is smaller than T, discard the other combinations and set m=m old +1, where m old  is m from the preceding step;   Step 5: If m&lt;M then go to Step 3, else go to step 6,   Step 6: Calculate the lengths of the new vectors v given by
 
 v=v   old   −h   M   *x   p   SET, M ,  (XXVII)
   Step 7: the detection signal x DET  is determined as that combination of values x DET =[x p   SET, 1 , . . . , x p   SET, M ] which corresponds to the vector v with the smallest length, wherein v old  is one of the vectors v resulting from step 3. The principle of this algorithm can also be illustrated with  FIG. 5 . The relevant parameters in the example of  FIG. 5  are M=2, N=1, C=2. After the first Step, the 4 vectors v result, the lengths of these vectors are given with (XXIII). Also a pre-determined threshold value T 1    46 , which is applied in Step 2 is presented in the  FIG. 5 . As a result of the threshold T 1    46 , the branch ∥r−h 1 *j∥ (corresponding to v 1  and v 3  in (XXIII)) is discarded in the subsequent Steps. This means that only the lengths of the vectors v 2  and v 4  are calculated in Step 3 of the algorithm. Hereafter the threshold T 1    46  is adjusted to the new threshold value T 2    48  for the next Step 4. Next, the Steps 4 and 5 are performed, which includes the selection of the smallest vectors v, after which the algorithm jumps to Step 6. Step 6 selects the Maximum Likelihood detection signal between the two remaining test signals x 2   SET  and x 4   SET . The Maximum Likelihood solution appears to be x DET =x 4   SET .   

   The embodiments of the wireless data communication system comprising the Reduced Search Techniques as described hereinbefore can also be applied with a modification for providing a further reduction in processing time. According to this modification, a number of test vectors in the test set reduced by a factor of C have to be checked out in equation (I). In this modification, the signal processing-unit is arranged to find, in use, the detection signal x DET  according to a Reduced Search Technique which also comprises the following steps:
     Step A1: calculate an inner product z of the vector r with the u th  column h u  of the matrix H, where u is an integer 1≦u≦M, according to:
 
 z=h   *   u   r,   (XXVIII)
 
where h *   u  is the complex conjugated and transposed of h u ;
   Step A2: calculate C M−1  terms Interf corresponding to all possible value combinations which can be assigned to [x 1 , . . . ,x u−1 ,x u+1 , . . . ,x M ], wherein the terms Interf are defined according to:
 
Interf=Σ (i=1 , . . . ,M^i≠u)   x   i *( h   *   u   *h   i ),  (XXIX)
 
wherein Σ i=1, . . . ,M^i≠u)  is a summation over the index i from 1 to M with the exception of the integer u;
   Step A3: estimate, on the basis of the equations (XXVIII), (XXIX) and z′ according to:
 
 z ′=Interf+ x   u *( h   *   u   *h   u ),  (XXX)
 
where z′ is an approximation of z, the value for x u  corresponding to each of the value combinations [x 1 , . . . ,x u−1 ,x u+1 , . . . ,x M ], and constitute a test set X SET  comprising C M−1  vectors x p   SET , wherein each vector x p   SET  represents a value combination
 
[x 1 , . . . ,x u−1 ,x u ,x u+1 , . . . ,x M ];
   Step A4: determine the detection signal X DET  according to equation (A), wherein the test set is defined with the C M−1  vectors x p   SET  from the preceding step.
 
The approximation (XXX) is based on the assumption that the noise n can be neglected.
   

   In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiments. However, it should be noted that the invention can be practised otherwise than as specifically illustrated and described Without departing from its spirit or scope. For example, the wireless data communication system according to the invention can be used not only for simplex communication (from the second groups to the first groups) but also for duplex data communication (from the second groups to the first groups and vice versa). In the latter case the communication from the first groups to the second groups may or may not be arranged in such a way as to obtain a multiple of simultaneous data communication channels.