Interleaving in coded modulation for mobile radio

Abstract: Interleavers used in fading channel applications can be matched to a particular code being used. This matching can occur even in a particular transmission scheme such as TDMA. Matching both the interleaver and the code increases the separation between interdependent signal points and provides an improvement in error performance, without an increase in time delay over known interleaver design.

Description: This invention relates to apparatus for and methods of use in a communications system, and to apparatus and methods for interleaving interdependent signal points generated by encoding input data. For some communication channels, coded modulation methods have been found to provide a "coding gain" in signal power (compared to so-called "uncoded" modulation methods) with the result of improving the error performance of the system without requiring additional bandwidth. For example, trellis coded modulation has proven to be a practical power-efficient and bandwidth-efficient modulation method for "random-error" channels characterized by additive white Gaussian noise (AWGN). This method is now being widely used in commercial telephone-line modems and has resulted in an increase of line rates of those modems to as much as 19.2 Kbits/s. Those in the art are now investigating the applicability of trellis coded modulation methods to "burst-error" channels, such as, "fading" channels, i.e., channels in which the signal amplitude can become so weak that accurate recovery of the transmitted information is difficult. These fading channels can be found, for example, in the digital cellular mobile radio environment (hereafter referred to more simply as "mobile radio"). In applying a trellis coded modulation method to mobile radio, it is desirable that the code exhibit a property called "time diversity" to improve the error performance of the communication system. This time diversity is manifested by an interdependence between the signal points that are produced by the coded modulation method. For example, consider the case where two signal points are produced over a time interval and there is a time-diverse interdependence between these signal points. As a result of this interdependence, the input data represented by the two signal points may be able to be accurately recovered even if one of the transmitted signal points is lost. However, the improvement in the error performance due to this time-diverse interdependence is limited in the mobile radio environment because typically a burst error extends over a number of time adjacent signal points. Continuing the above example, if both time-diverse transmitted signal points were lost due to a burst error, the input data may not be able to be accurately recovered. As such, the interdependence of signal points alone cannot be relied on to permit accurate recovery of the original signal. Rather, in such environments, an interleaver is often used to separate the interdependent (time-diverse) points to reduce the effects of the fading channel and further improve the error immunity of the system. In accordance with known interleaver design, an interleaver collects, or frames, the signal points to be transmitted into a matrix, where the matrix is sequentially filled up a row at a time. After a predefined number of signal points have been framed, the interleaver is emptied by sequentially reading out the columns of the matrix for transmission. As a result, signal points in the same row of the matrix that were near each other in the original signal point flow are separated by a number of signal points equal to the number of rows in the matrix. Ideally, in design of the interleaver, the number of columns and rows would be picked such that interdependent signal points, after transmission, would be separated by more than the expected length of the error burst for the channel. However, this may not be practicable, for as the number of columns and rows are increased, so is the signal delay due to the framing of the signal points. As a result, there is a system constraint on the size of the interleaver in order to keep the signal delay within acceptable limits. On the other hand, constraining the size of the interleaver limits the separation of those time-diverse interdependent signal points and, as a result, the improvement in error performance due to the interleaver. FR-A-2 592 258 describes an interleaving approach in which interleaving is obtained by the following algorithm: x k = x o + kp modulo B · n , where x o is the starting row, B is the number of code words, n is the number of symbols in, or size of, each code word, p is a prime number of B · n, and k takes on the values of 0 to B · n - 1. IEEE International Conference on Communications, 22-25 June 1986, Toronto, CA, IEEE New York, vol. 3 of 3, p. 1607-1611, WAI-HUNG NG, 'Mitigating Ionospheric Scintillation Effects by Integrating Coding and Interleaving' discloses an interleaving approach for convolutional codes that a) regroups the coded symbol stream, where the number of groups is equal to the size of the codeword, and then b) individually interleaves each group. According to one aspect of this invention there is provided apparatus as claimed in claim 1. According to another aspect of this invention there is provided apparatus as claimed in claim 4. According to a further aspect of this invention there is provided a method as claimed in claim 10. According to yet another aspect of this invention there is provided a method as claimed in claim 13. As noted above, in applying coded modulation methods which exhibit time diversity to communication channels that display burst-error characteristics, such as mobile radio, the prior art has recognized that application of an interleaver improves the error performance of the system. At the same time, however, I have recognized that the error performance can be yet further improved by matching the X-fold time diversity of the particular code being used with the particular reordering sequence of the interleaver. In particular, the code and interleaver are matched in preferred embodiments of the invention when for any pair of a set of interdependent signal points in the interleaved signal point stream, those interdependent signal points contributing to the X-fold time diversity of the code are minimally separated by D₂ > D₁, where D₁ is the minimum separation between such interdependent signal points. This further increases the error immunity of the system while not increasing the required size of the interleaver matrix. Thus the advantages of the interleaver matrix are achieved without introducing any additional delay. One way to adapt the interleaver to be matched to a particular given code is to form a transformed matrix from the elements of the interleaver matrix and then read out adjacent columns of the transformed matrix. With this approach, the distance between particular interdependent signal points that contribute to the X-fold diversity of the code can be increased over that which would be provided following the known interleaver approach. Moreover, there may be situations where interdependent signal points are not confined to a particular row, but are spread across adjacent rows. As a result, a reordering of the columns of the interleaver will not affect, nor separate, independent signal points in the same column. These interdependent signal points will now be susceptible to burst errors and to demodulator-induced noise. (In preferred embodiments a constant-amplitude type of signal constellation is used in order to account for the fast variations in signal amplitude that is the hallmark of mobile radio and some other fading channels. Moreover, due to the fast variation in carrier phase that occurs in such channels, use of a non-coherent differential detection method is preferred. Both of these criteria are advantageously satisfied by the use of M-point differential phase shift keying, or M-DPSK. However, the M-DPSK demodulation process introduces correlated noise samples between received adjacent signal points.) These concerns may be addressed by additionally reordering the interdependent signal points within a column. As a result, the separation of signal points occurs in such a way so as to advantageously eliminate the correlation between noise samples introduced by the M-DPSK demodulating process and further guard against burst errors. In addition, in known interleaver design approaches, no attempt has been made on separating interdependent signal points that are spread across adjacent frames. We have realized that the particular reordering method can be changed for each sequential frame so that interdependent signal points which are spread across adjacent frames will be advantageously separated. Finally, besides matching the interleaver to fit a code with X-fold diversity, the code can be matched to a particular given interleaver, e.g., the straightforward interleaver of the prior art. Therefore, a code with X-fold time diversity can be matched to the known interleaver by generating a sequence of interdependent signal points, where any pair of interdependent signal points which exhibit X-fold time diversity are not adjacent, in contradistinction to the codes exhibiting time diversity in the prior art. Brief Description of the Drawing In the drawing, FIG. 1 is a chart illustrating N signal points produced over N signaling intervals, each signal point taken from an illustrative two-dimensional M-Phase shift keying constellation; FIG. 2 depicts a coder which is used to implement an illustrative two-dimensional, 8-state, 8-PSK trellis code; FIG. 3 shows an 8-PSK constellation which forms the basis of a first illustrative trellis code disclosed herein; FIG. 4 shows the state transition matrix for the trellis coder of FIG. 2; FIG. 5 depicts two-fold time diversity for the illustrative two-dimensional, 8-state, 8-DPSK code produced by the trellis coder of FIG. 2; FIG. 6 illustrates two permissible signal point sequences, five signal points in length, for the illustrative trellis code from the trellis coder of FIG. 2; FIG. 7 is a block diagram of a data communications system which includes an interleaver embodying the invention; FIG. 8 is a block diagram of the interleaver of FIG. 7; FIG. 9 illustrates the operation of the interleaver shown in FIG. 8 with respect to a two-dimensional trellis code with two-fold time diversity and a decoding depth of eight; FIG. 10 illustrates the operation of the interleaver embodying the principles of the invention with respect to a two-dimensional trellis code with two-fold time diversity and a decoding depth of eight; FIG. 11 illustrates a TDMA frame with three users; FIG. 12 illustrates the operation of a two column interleaver embodying the invention using the TDMA frame of FIG. 11; and FIG. 13 illustrates the operation of a four column interleaver embodying the invention using the TDMA frame of FIG. 11. Detailed Description Before proceeding with an example of an interleaver matched to the time diversity of a trellis code, it will be useful at this time to provide some general background information on trellis codes. In general coded modulation repetitively operates on input data over N signaling intervals to produce a set of N interdependent signal points, one signal point in each signaling interval. The signal points are interdependent in that they are chosen together as a function of the particular input data values. Such a code is "2N-dimensional" since each of the N signal points is taken from a two-dimensional (2D) constellation. This is illustrated in FIG. 1, where each signal point (P i ) (i = 1,2...) is delivered during a respective signaling interval and represents a point from the predetermined 2D constellation--illustratively shown in FIG. 1 as a phase shift keying (PSK) constellation having eight signal points. In a trellis code, the value of a particular signal point P i is not only dependent on the current input data in a particular signaling interval, but also on prior input data from previous signaling intervals. This is shown in FIG. 2 in the context of an illustrative trellis coder where the output is a particular signal point from the signal constellation of FIG. 3 based on the value of the binary 3-tuple (Y2 n Y1 n , Y0 n ), and where the value of Y0 n is dependent on data from previous signaling intervals as a result of the storage elements 401, 402 and 403. The outputs from these storage elements at a time (n) identifies the state of the trellis coder at time (n) which is the value of the binary 3-tuple (W1 n W2 n W3 n ). In this example, the storage elements are updated every signaling interval (T) and as the input data I1 n and I₂n vary over time, the trellis coder will traverse various states. From this a state transition matrix can be constructed as illustrated in FIG. 4. The trellis coder of FIG. 2 generates a two-dimensional 8-PSK trellis code which has 8-states. In the state transition matrix of FIG. 4, for this illustrative trellis coder, the current state is listed along the column, the next state is listed across the row, and an element of the matrix is the signal point value associated with a transition from the current state to the next state. The blank elements of the matrix represent disallowed state transitions. For example, if currently in state (0,0,0) and the next state is also state (0,0,0), a signal point with the value of "0" is output, or if the current state is (1,0,0) and the next state is (0,1,1), the value of the signal point output is "2". In addition, this particular illustrative trellis code also exhibits the property of time diversity which will now be briefly described. Assume that the initial state W1, W2, W3, of the trellis coder of FIG. 2, at time n=1, is zero. It is easy to see from FIG. 4, that any pair of permissible infinite sequences of signal points (P₁,P₂,P₃ ,..., P ∞ ) originated from the same initial state 0 differ in at least two signal point positions. Two such sequences that differ in only the first and second signal points are shown in FIG. 5. The above statement is valid no matter what is the initial state. This shows that the trellis code of FIG. 2 has two-fold time diversity. It can be further verified from FIG. 4 that for each pair of permissible sequences of signal points that differ in only two signal points positions, the two signal point positions are always next to each other and the squared Euclidean distance between these two sequences is always 6, assuming that the radius of the 8-PSK constellation is one. The decoding depth D of a trellis code is defined as follows. The trellis decoder makes a decision on a received signal point P n (See FIG. 4) only after the point P n+D-1 is received. For a trellis code all signal points are interdependent, but for practical purposes only the sequence of any successive D signal points will be referred to as being interdependent as long as the decoding depth D is not less than the minimum decoding depth. The minimum decoding depth (MD) of the trellis code of FIG. 2 is determined as follows. It is chosen such that 1)the two-fold time diversity of the code is preserved in that any pair of permissible finite sequences of MD signal points long, originating from the same initial state and having different first signal points, differ in at least two signal point positions; and 2) the squared Euclidean distance between any two such permissible finite sequences of signal points differing in only two signal point positions remains at six. From FIG. 4 again, the minimum decoding depth of this trellis code can be determined to be six. This can be seen from FIG. 6. FIG. 6 shows two permissible finite sequences of five signal points long, originating from the same initial state 0 and having different first signal points 0 and 4. The two sequences differ in only two signal point positions. However, the squared Euclidean distance between the two sequences is only 4.58, smaller than 6. This figure shows that a decoding depth of five, for this illustrative trellis code, does not meet the minimum decoding depth requirement. On the other hand, a decoding depth of six or more, will ensure that any pair of permissible sequences of six or more signal points long, originating from the same initial state and having different first signal points, differ in at least two signal point positions, and if they differ in exactly two signal point positions, the squared Euclidean distance between them remains at 6. As an example, for the permissible all-zero sequence (0, 0, 0, 0, 0, 0) originating from state 0, the only other permissible sequence, originating from the same state, having different first signal points, and exhibiting two-fold time diversity is (4, 2, 0, 0, 0, 0). All other permissible sequences originating from the same state and having different first signal points differ from the all-zero sequence in at least three signal point positions. It will be appreciated that if a particular one signal point is lost due to fading, there is a significant likelihood that a time-adjacent signal point may also be lost. Therefore, the error immunity afforded by the built-in time diversity of the trellis code can be enhanced by time-separating the adjacent signal points via an interleaver so that it is less likely that the two points will fade concurrently. In accordance with known interleaver design, an interleaver collects, or frames, the signal points to be transmitted, into a matrix, where the matrix is sequentially filled up a row at a time. After a predefined number of signal points have been framed, the interleaver is emptied by sequentially reading out the columns of the matrix for transmission. As a result, any pair of interdependent signal points that were near each other in the original signal point flow are separated by a number of signal points equal to the number of rows (J) in the matrix. The size of a frame is represented by the dimensions of an interleaver. That is, the size of the row dimension is equal to the number of rows (J), and the size of the column dimension is equal to the number of columns (K). These dimensions can be optimized for the parameters (J) and (K) and ideally, in design of an interleaver, the number of rows (J) would be picked interdependent signal points, after transmission, would be separated by more than the expected length of an error burst for the channel. For example, in mobile radio, the effectiveness of an interleaver is maximized when the parameter J is greater than or equal to 1/4 of the carrier wavelength divided by the minimum vehicle speed of interest multiplied by the signaling rate. (This formula is based on the assumption that there is only a single user per mobile radio channel, as is the case when a frequency-division-multiplexing-access (FDMA) approach is used. The case where there is more than one user per channel, i.e., the so-called time-division-multiplexing-access (TDMA), will also be discussed below). On the other hand, the number of columns (K) is usually equal to the decoding depth of the trellis code. In particular applications, however, the dimensions of the interleaver may have to be less than optimum to limit the transmission (or framing) delay introduced by the interleaver. This may be necessary to ensure a desired level of data throughput or to avoid unnatural delays that may be otherwise introduced into a conversion. As a result, constraining the dimensions of the interleaver will involve tradeoffs in system design which can effect error performance. We have recognized that the interleaver design can be matched to the particular X-fold time diversity of the trellis coded modulation method being used in such a way that the separation interval (D₂) between interdependent time-diverse signal points that contribute to the X-fold diversity is increased over D₁, the minimal separation between any pair of interdependent signal points, which would be provided following the prior art interleaver approach. This further increases the error immunity of the system while not increasing the required size of the interleaver or the signal time delay associated with interleaver operation. In particular, I have realized that by re-arranging the order in which the columns of signal points are read out of the interleaver, the separation interval can be increased beyond the prior art J signaling intervals. An interleaver, embodying the invention is shown in relation to the rest of a communication system in FIG. 7. Input data on lead 11 is applied to an encoder/mapper 13 at a rate of m bits per T-second signaling interval. Over N signaling intervals of input data, the encoder/mapper 13 outputs N signal points on lead 16, where a particular signal point P i is taken from a two-dimensional (2D) M-PSK constellation such as is shown in FIG. 3 and is generated according to the particular trellis code being used. The N successive signal points output by encoder/mapper 13 on leads 16 are applied, in an original order, to interleaver 21 which reorders the signal points P i (as described more fully herein below). These reordered signal points Q i , output by the interleaver on lead 24, are applied to modulator 25 whose output, in turn, is applied to fading channel 30. In the receiver, demodulator 41 and deinterleaver 44 perform the inverse functions of modulator 25 and interleaver 21, respectively. Accordingly, the output of the latter, on leads 45, is the receiver, but channel-corrupted sequence of signal points, P i corresponding to the sequence of signal points appearing on leads 16 at the output of encoder/mapper 13. These are applied to decoder 51 which recovers and provides, on leads 53, the originally transmitted input data. The operation of the decoder is generally that of a "maximum likelihood detector", and will, for received signal points, make a decision as to the most likely value of the transmitted signal points. A block diagram of interleaver 21 is shown in FIG. 8. Each successively input signal point P i , on lead 16, is taken by input controller 210 and stored in memory element 211. The stored signal points maybe conceptualized as being in a matrix organization. As a result of the interleaver matrix organization, each successively input signal point is stored in a successive column of a particular row, up to the number of columns of the matrix, and than along successive rows of the matrix, up to the number of rows of the matrix. FIG. 9 illustrates the operation of the interleaver on the illustrative trellis code (with two-fold time diversity between successive signal points) of FIG. 2. To illustrate the features of my invention, it is assumed that the decoding depth for this code for this code is chosen to be eight (In general, the decoding depth of a trellis code is chosen to be slightly greater than its minimum decoding depth.), and that the number of columns of the interleaver is eight. At a particular point in time, interleaver 21 will contain a frame of J x 8 signal points, with the first eight signal points (P₁,P₂,P₃,P₄,P₅,P₆,P₇,P₈) being stored in the first row, the second eight signal points (P₉,P₁₀,P₁₁,P₁₂,P₁₃,P₁₄,P₁₅,P₁₆) in the second row, etc., as illustrated in the matrix labeled "input matrix". The columns of the interleaver are read out in the order A, E, B, F, C, G, D, H. As a result, the time-diverse interdependent signal points that contribute to the two-fold diversity are now advantageously separated by 2J signal intervals instead of J signaling intervals as in the case of the prior art, as can be appreciated from the output sequence shown in FIG. 9. In addition, by reading out the columns of the matrix in the order: A, E, B, F, C, G, D, H, any sequence of eight successive signal points (which will be used by the decoder to make a decision on the first signal point of the sequence) are separated pairwise as far as possible (by at least J-2 signaling intervals In FIG. 9). (In the most straightforward type of implementation, the output controller 212 may wait until all of the signal points in the frame have been read in. In more efficient implementations, it may be possible for the interleaver to begin reading out signal points before all of the signal points in the frame have been read in, as long as enough signal points have been read in to assure a synchronous flow of signal points on the output lead 24). It will thus be appreciated that as a result of the above approach the interleaver has been matched to the two-fold diversity of the code. The inventive concept of matching the interleaver to the code as just described can additionally be characterized as a process by which the elements of the input matrix are reordered to create a "transformed" matrix, so that upon sequential transmission of adjacent columns of the transformed matrix, the time diverse signal points are separated by more than they would be following the prior art approached Such a transformed matrix is shown in FIG. 9. By separately reading out adjacent columns of the transformed matrix (as in the prior art), the aforementioned output sequence again results. In this illustrative example, the reordering was accomplished by performing a sequence of elementary transformations of the input matrix, e.g., by interchanging columns B and E. An elementary transformation is formed by interchanging either two rows or two columns. More generally, a code can be matched to the interleaver by performing any appropriate reordering of the elements of the input matrix, be it one or more elementary transformations (i.e., interchanging of rows and/or columns), or some other form of reordering, (e.g., mixing of rows and columns). Although this alternate view of matching the interleaver to the code will not be further discussed herein, it should be noted that it applies to the subsequent examples. Besides matching the interleaver operation to a particular trellis code with X-fold time diversity, a trellis code can be matched to the existing prior art interleaver by separating those adjacent X-fold time-diverse signal points. This can be illustrated by reconsidering the prior example where now the interdependent signal points are re-arranged by encoder 13 so that the time diversity is exhibited between the first and third signal point positions, or the second and fourth signal point positions, etc. The prior art interleaver will minimally separate any pair of interdependent signal points by D₁ (as before), and with the invention further minimally separate those interdependent time-diverse signal points of D₂, such that D₂ > D₁. In addition, FIG. 10 illustrates that by further re-arranging the rows, any correlation of demodulator induced noise is essentially removed. (In the M-DPSK demodulation process correlated noise samples are introduced between received adjacent signal points). In this example, the re-arrangement of the rows is accomplished by reading out the rows in the following order: 1,J2 + 1,2,J/2 + 2,...,J/2,J. Though the above further separates successive signal points from one another by at least 2J signaling intervals for the first frame, there is still a problem in the transmission of the succeeding frame since signal points P 8J and P 8J+1 will still be transmitted sequentially. Therefore, the interleaver operates in a state-dependent manner by identifying a state of "0" for the first frame and a state of "1" for the second frame, where the sequence of column and row transmission is changed between successive frames. In FIG. 10, the state of "0" is identified with the first frame, and the corresponding column and row transmission is already described. However for the second frame, a state of "1" is associated and the column and row transmission is now changed, with the columns being read out in a different order: E, A, F, B, G, C, H, D, and the rows being out in different order: J, J/2, ..., J/2 +2, 2, J/2 + 1, 1. The third frame can return to the ordering of the first frame(state 0) and the process repeats itself. In this way successive signal points are still further separated, and a separation of at least 2J signaling intervals is maintained between successive signal points across the frames. Another example illustrates the concept of the invention in a TDMA environment. FIG. 11 is an illustrative TDMA implementation with three users, where each user is allocated one of the three time slots in a TDMA frame and it is assumed that two columns of an interleaver can be transmitted in each time slot. If the prior art interleaver and trellis code illustrated in FIG. 9 were used, it would take four TDMA frames to transmit all eight columns of the interleaver, and then the time-diverse signal points would be separated by J signal points in a TDMA frame. However, for system consideration it maybe advantageous that the number of interleaver columns be reduced to two as shown in FIG. 12, with one column being transmitted in each time slot. It can be seen from FIG. 12 that adjacent signal points are split between columns A and B. These columns are additionally separated in time by the TDMA frame format. However, it would be additionally advantageous to further separate any eight successive signal points (since the decoding depth is eight), and to essentially remove the correlation of demodulator induced noise. As a result, reordering the transmission of the rows, as shown in FIG. 12, further separates successive signal points and removes the correlation of demodulator induced noise. The reordering is accomplished by observing that any eight successive signal points are generally spread across four or five consecutive rows, as a result the rows are read out in the following order: 1, J/2 + 1, 5, J/2 + 5,..., J - 3, 2, J/2 + 2, etc. In this particular case, one or two interleaving states can be used. There may be only a marginal gain to using two interleaving states due to the additional separation afforded by the TDMA separation. A final example of an interleaver embodying the invention is shown in FIG. 13. Using the same TDMA framing as in FIG. 11, a four column interleaver is used with the same illustrative trellis code and a decoding depth of eight. Here again, the columns are additionally separated by the TDMA frames and these columns A, B, C, and D are transmitted successively with successive signal points always appearing in different columns. (Note: due to the time diversity between any two successive signal points, reordering the columns could be done, but the effect would be marginal, for example, if the columns were reordered to be A, C, B, D, P₁ and P₂ would be additionally separated but not P₂ and P₃). The rows are again reordered in the following order: 1, J/2 + 1, 3, J/2 + 3,..., J - 1, 2, J/2 + 2,..., J. As a result, any eight successive signal points are additionally separated. From the above examples, it will be appreciated that in the general case various re-arranging methods in either the row or column dimensions of the interleaver matrix, can be used--depending on the interdependence of the signal points--in order to further separate those interdependent signal points. In particular, those signal points that contribute to the X-fold diversity of the code are further separated by more than the number of rows (J) in the interleaver. Further, subsequent frames may have to change the reordering algorithm for the rows, and/or columns, to maintain separation between those interdependent signal points across successive frames. The foregoing merely illustrate the invention. For example, although the invention is described herein principally in terms of reading out different rows and columns of a two dimensional interleaver matrix, this same reordering can also be accomplished in the way data is input to the interleaver matrix, or in an interleaver matrix of any dimension. Further, matching the interleaver to the code can be accomplished by reordering, in any fashion, the elements of the interleaver, or input matrix, into a transformed matrix, such that, upon transmission of sequential columns, interdependent signal points are advantageously further separated. Also, besides matching the interleaver to the code, the code can be matched to the prior art interleaver by reordering, in any fashion, the sequence of interdependent signal points generated to the interleaver such that those time-diverse interdependent signal points are not adjacent. In addition, although the invention is described and defined herein with reference to, for example, an interleaver "matrix" and a transformed "matrix," it will be appreciated that this is but a convenient way of defining the various signal point interrelationships. Thus one may carry out the invention, in that the particular reordering of signal points taught herein is achieved, without necessarily physically storing those signals in a two-dimensional "matrix" configuration. Rather, particular memory addressing regimes or other ways of reordering the signal points which are the signal-processing equivalent of actually storing the signal points in, and reading them out from, various matrices should be understood as being encompassed by definitions of the invention which may include matrix terminology. Finally, though the interdependent signal points were generated by trellis coded modulation with built in time diversity, the same approach could be used for any coding method (e.g., block coding) where an interdependence of signal points exist, and an interleaver is used.

Claims: Apparatus for use in a communication system CHARACTERIZED BY: means for encoding input data (13) to generate a sequence of interdependent signal points, the signal points having an original order and the encoding means causing the signal points to exhibit an X-fold time diversity, where X is an integer greater than one; and means for interleaving the interdependent signal points (21) to form a reordered sequence of signal points, where a) the X-fold diversity of the signal points caused by the encoding means and b) the reordering sequence of the interleaving means, are matched. The apparatus of claim 1 wherein the reordered sequence generated by the interleaving means is such that any pair of interdependent signal points are minimally separated by a distance D₁, and any pair of interdependent signal points which exhibit the X-fold time diversity are minimally separated by a distance D₂, with D₂ being greater than D₁. The apparatus of claim 1 wherein the encoding means produces a sequence of interdependent signal points in which any pair of the interdependent signal points which exhibit X-fold time diversity are separated. Apparatus for interleaving interdependent signal points generated by encoding input data, said apparatus comprising: means for storing (211) said signal points in an input matrix, each successively input signal point being stored in a successive column of a particular row, up to the number of columns of the matrix, and then along successive rows of the matrix, up to the number of rows of the matrix; and CHARACTERIZED BY: means for reading (212) out said signal points to form a reordered sequence of signal points where any pair of interdependent signal points are minimally separated by a distance D₁, and any pair of interdependent signal points which exhibit X-fold time diversity are minimally separated by a distance D₂, with D₂ being greater than D₁, and where X is a selected integer greater than one. The apparatus of claim 4 wherein the means of reading out forms a reordered sequence of signal points by reading out adjacent columns of a transformed matrix of signal points in which elements of the input matrix are reordered. The apparatus of claim 5 wherein the means of reading out reorders the elements of the input matrix by forming an elementary transformation of the input matrix. The apparatus of claim 5 wherein the reordering formed by the reading out means is such that adjacent signal points from a column of the input matrix are separated in the reordered sequence. The apparatus of claim 5 wherein the reading out means operates in a state-dependent manner and the re-ordering sequence for each state is different. The apparatus of claim 4 wherein each input signal point is taken from a two-dimensional M-PSK constellation, where M is a selected integer. A method for use in a communications system CHARACTERIZED BY: encoding input data to generate a sequence of interdependent signal points the signal points having an original order and the encoding means causing the signal points to exhibit an X-fold time diversity, where S is an integer greater than one; and interleaving interdependent signal points to form a reordered sequence of signal points, where a) the X-fold time diversity of the signal points caused by the encoding means and b) the reordering sequence of the interleaving means are matched. The method of claim 10, wherein the interleaving step further comprises the step of reordering the input signal points such that any pair of interdependent signal points are minimally separated by a distance D₁, and any pair of interdependent signal points which exhibit X-fold time diversity are minimally separated by a distance D₂, with D₂ being greater than D₁. The method of claim 10, wherein the encoding step further comprises the step of producing a sequence of interdependent signal points, where any pair of the interdependent signal points which exhibit X-fold time diversity are separated. A method for interleaving interdependent signal points, generated by encoding input data, comprising the steps of: storing said signal points in an input matrix, each successively input signal point being stored in a successive column of a particular row, up to the number of columns of the matrix, and then along successive rows of the matrix, up to the number of rows of the matrix; and CHARACTERIZED BY: reading out said signal points to form a reordered sequence of signal points where any pair of interdependent signal points are minimally separated by a distance D₁, and any pair of interdependent signal points which exhibit X-fold time diversity are minimally separated by a distance D₂ with D₂ being greater than D₁, and where X is a selected integer greater than one. The method of claim 13 wherein the reading out step further comprises the step of reading out adjacent columns of a transformed matrix of signal points in which elements of the input matrix are reordered. The method of claim 14 wherein the reading out step further comprises the step of reordering the elements of the input matrix by forming an elementary transformation of the input matrix. The method of claim 14 wherein the reading out step further comprises the step of reordering the signal points such that adjacent signal points from a column of the input matrix are separated in the reordered sequence. The method of claim 14 wherein the reading out step further comprises the step of operating in a state-dependent manner where the re-ordering sequence for each state is different. The method of claim 13 wherein each input signal point is taken from a two-dimensional M-PSK constellation, where M is a selected interger.

Inventors: WEI LEE-FANG, WEI, LEE-FANG
Applicants: AT & T CORP, AT&T CORP.
Application year: 1990
Main classification: H04L1
IPC codes: H04B14, H04L1, H04L27, H03M13