MXPA98010841A - Near-optimal low-complexity decoding of space-time codes for fixed wireless applications - Google Patents

Abstract

a receiver arrangement where signals received at a plurality of antennas are each multiplied by a respective constant and then summed prior to being applied to a maximum likelihood detector. The respective constants, lambda j, where j is an index designating a particular receiver antenna, are derived from a processor that determines the largest eigenvalue of the matrix LAMBDA A( LAMBDA *), where LAMBDA is a vector containing the values lambda j, and A is a matrix containing elements alpha ij, which is the transfer function between the i transmitter antenna to the j receiver antenna. The alpha ij terms are determined in the receiver in conventional ways.

Description

DECODING OF LOW COMPLEXITY, ALMOST OPTIMUM, OF SPACE-TEMPORARY CODES FOR FIXED, WIRELESS APPLICATIONS BACKGROUND OF THE INVENTION This invention relates to wireless systems and, more particularly, to systems that have more than one antenna in the receiver and in the transmitter. Physical constraints as well as narrow bandwidth, co-channel interference, adjacent channel interference, loss of propagation and multipath fading limit the capacity of cellular systems. There are severe damages, which resemble the wireless channel to a narrow tube that prevents the flow of data. However, the interest in providing high-speed wireless data services is increasing rapidly. Current cellular standards such as the IS-136 can only provide data rates up to 9.6 kbps, using 30 kHz narrow band channels. To provide broadband services, such as multiple media, video conferencing, voice and data simultaneously, etc., it is desirable to have data rates in the 64-144 kbps range. Transmission schemes for multiple antenna systems can be part of a solution to the problem of low data rates currently REF. 29134 available. Such schemes were proposed for the first time in the Wittneben articles, and by Seshadri and Winters, where the problem was dealt with in the context of the signaling process. An arrangement of the prior art having a single transmitting antenna and multiple receiving antennas is shown in Figure 1. Each of the receiving antennas receives the transmitted signal via a slightly different channel, where each channel is characterized by the function of transfer aa. Using a method known as "Maximum Ratio Combination", a detection method of the prior art contemplates multiplying each received signal that has been influenced by ai by the complex conjugate signal,. *, Added, and then processed. In the co-pending application entitled "Method and Apparatus for the Transmission of Data Using Spatial-Temporal Codes and Multiple Transmission Antennas ", presented in May, 1997, which has the Serial No. 08 / 847,635, and granted to the beneficiary of this invention, a coding perspective was adopted for propose spatial-temporal coding using multiple transmitting and receiving antennas Spatial-time coding integrates channel coding, modulation and multiple transmit antennas to achieve higher data rates, while simultaneously providing a diversity that combats The addition of channel coding can be shown to provide significant gains over the schemes of Wittneben and Seshadn and Winters In the co-pending application, space-time codes were designed for transmission using 2-4 transmitting antennas. well in lightly fading environments variables (such as in internal transmission media). The codes have user bandwidth efficiencies of up to 4 bits / sec / Hz which are approximately 3-4 times the efficiency of current systems. In fact, it can be shown that the codes designed are optimal in terms of the transaction between the diversity of advantages, transmission speed, decoding complexity and constellation size. It can be shown that when the number of antennas is increased, the gain is increased in such a way that it is not likely that a multi-element antenna will be tuned to, say, a particular address. Unfortunately, however, when maximum likelihood detection is employed in the receiver, the complexity of the decoding is increased when the number of transmitting and receiving antennas increases. Obviously it would be advantageous to provide a slightly suboptimal detection method that substantially reduces the computational complexity of the receiver.

Brief Description of the Invention Such a method is achieved with a receiver arrangement wherein the signals received in a plurality of antennas are each multiplied by a respective constant and then summed before being applied to a maximum likelihood detector. The respective constants,? J, where j is an index that designates a particular receiving antenna, are derived from a processor that determines the largest characteristic value of the matrix? A (? *) T, where? is a vector that contains the values?:, and A is a matrix that contains a2J elements, which is the transfer function between the transmitting antenna itself to the receiving antenna. The terms a13 are determined in the receiver in a conventional manner.

Brief Description of the Drawings Figure 1 represents a block diagram of the Maximum Ratio Combination detection; and Figure 2 depicts a block diagram of an array including a transmitter having a plurality of antennas, and a receiver having a plurality of antennas coupled to an efficient detection structure.

Detailed Description Figure 1 depicts a block diagram of a receiver according to the principles of this invention. This includes a transmitter 10 having a plurality n of transmit antennas 1, 2, 3, 4 and a receiver 20 having a plurality m of receiving antennas 21, 22, 23, 24. The signals received by the receiving antennas are multiplied by the elements 25, 26, 27 and 28, and added in the adder 30. More specifically, the signal received from the antenna j is multiplied by a value,? j, and added. The collection of factors? Can it be seen as a vector? The outputs of the receiving antennas are also applied to the processor 40 which, using conventional techniques, determines the transfer functions ai for i = 1, 2, 3, ..., ny = 1, 2, 3, ..., m. These transfer functions can be evaluated, for example, through the use of sequences of instructions that are sent by different transmitting antennas, one antenna at a time. The signals a in the processor 40 are applied to the processor 45 in Figure 1 where the multiplier signals?,, J = 1, 2, 3, ..., m are calculated. The processor 45 also evaluates a set of combined values of the transfer function? I f í = l, 2, 3, ..., n (which are described in more detail below). The signals? Of the processor 45 and the output signal of the adder 30 are applied to the detector 50, which detects the transmitted symbols according to the calculations described below. It is assumed that the symbols transmitted by the antennas of the transmitter 10 have been encoded in blocks of L time intervals, and that the fading is constant within a range. A password comprises all symbols transmitted within a range, and therefore corresponds to Cj Ci C¡ •. • Cj C? X? C¡ •. . C? C3 C3 C3,. . C3 •. . Cm Cm Cm •. . Cm, where the supra index designates the transmit antennas and the subscript designates the transmission time (or position within a range). From the point of view of a single antenna, for example, antenna 1, the signal that is received at antenna 1 in response to a transmitted symbol C t1 in time interval t is: R, = c, '(n l, + an? 2 + a "? I + ... + aim? M) - Cl? (when the noise is ignored). If each value of?, - is set to a * ij (where a * u is the complex conjugate of ctij) then the received signal would simply be *. - '!' Sh a. / -i producing a constitutive addition. Of course, the values of? they can not be fixed to equal a * :; and concurrently to equal the values of a * ^, where i? l; and hence the difficulty. When all the transmitting antennas are considered, then the received signal is .

According to the description of the present, the objective is maximi zar ^ I? ± | 2 because by doing this, the? = Rt signal contains much more information about cJ, i = l, 2,,. . . , n of what is possible. However, it can be easily demonstrated that if an A matrix is constructed such that =? (O, *) rO / 5 where O; = (a2i a-l2, a13 ... a,, then | , | 2 = ?? (? *) R.

The receiver, in this way, has to maximize? A (? *) R, subject to the restriction ||? | G = 1. The solution to this problem is to choose what? is the characteristic vector of A that corresponds to the maximum characteristic value of A. Consequently, the processor 45 develops the matrix A of the values of a ^, finds the characteristic values of A in a conventional manner, selects the maximum characteristic value of A, and create the vector? Once known, the processor 45 develops the signals ?? for 1 = 1, 2, 3, ..., n (where? L =? 3ai), and apply them to detector 50. Finally, the detector 50 minimizes the metric of all possible passwords in a conventional way. As can be seen, this method reduces the complexity of the decoding by almost a factor of m. Figure 1 describes separate multipliers to multiply the signals received by the multiplication factors? , and describes blocks of separate elements 30, 40, 45 and 50. It should be understood, however, that different modalities are also possible. For example, it is very conventional to incorporate all the aforementioned elements in a single processor for special purposes, or in a single processor controlled by a stored program (or a small number of processors). Other modifications and improvements can also be incorporated, without departing from the spirit and scope of the invention, which are defined by the following claims.

It is noted that in relation to this date, the best method known by the applicant to carry out the aforementioned invention, is the conventional one for the manufacture of the objects to which it relates. Having described the invention as above, property is claimed as contained in the following:

Claims (6)

1. A receiver, characterized in that it comprises: a plurality n of antennas, wherein n is greater than one; circuits for obtaining n signals transmitted from m antennas of a transmitter, where m is greater than one; and processing means for developing a sum signal corresponding to the addition of n signals that are each previously multiplied by a respective factor? J f where i is the integer index that specifies the factor?; multiply the signal received from the antenna i from the plurality of antennas, develop values for the transfer functions a ?, where i is an index that refers to the transmit antennas, and j is an index that refers to the antennas the transmitters, to develop the factors j of the transfer functions, and to detect the symbols transmitted by transmitting antennas included in the sum signal.

2. The receiver according to claim 1, characterized in that the factors? they relate to the transfer functions to ^.

3. The receiver according to claim 1, characterized in that the factors are components of a vector? where A is a characteristic value of? A (? *) T, and where A is a matrix containing the elements a ,,. The receiver according to claim 1, characterized in that the detection compares the sum signal with a signal corresponding to the symbols c1 possibly transmitted by the transmitting antenna i of transmitting antennas multiplied by the corresponding factors?,. 5. The receiver according to claim 4, characterized in that the corresponding factor? 2 is related to the factors? J f for j = l, 2.3,: n, and for a17. 6. The receiver according to claim 4, characterized in that the detection maximizes the metric, where Rt is the sum signal in the time interval t within a range having L time intervals, and c i is the symbol that could have been transmitted on the transmit antenna i in the time interval t.