Method and apparatus for reducing the peak power of data sequences

Abstract: This application concerns the reduction of the peak power of data sequences, particularly for use in a QAM radio relay system. The peak power reduction leads to fewer problems with non-linear distortion, whether caused by the channel, or by the transmitter power amplifier. The power reduction is achieved by using a shaping code, which replaces sequences with high power, by sequences with lower power.

Description: Background of the invention The present invention relates to a method and system of transmitting and receiving digital signals over a transmission link, comprising the steps of: in transmission, encoding and modulating (II) the input digital signal obtaining an output digital signal in the form of digital sequencies, filtering (IV) it for providing a modulated signal to be transmitted over a non-linear channel (V, VI) subject to non-linearities and disturbancies; in reception, filtering (VII) the received signal from the non-linear channel, and decoding and demodulating (IX) the filtered received signal to obtain a reconstructed digital signal. State of the art The present digital transmission systems try to obtain high spectral efficiencies through gradually more complex modulation formats. The higher spectral efficiency is counterbalanced by the need of increasing the transmitted power to obtain a prefixed BER (Bit Error Rate: number of wrong bits to total number of bits ratio) value at the receiver. The power delived by the trasmitter generally is limited by the final power apmlifier, which has a greatly nonlinear behaviour. Therefore a serious problem arises with regard to the best exploitation of the nonlinear part of the input-output characteristic of the "channel", including in the latter the final amplifier of the transmitter. At present the problem is faced in one of the following ways (see e.g. the papers of G. Karam, H. Sari, "Analysis of predistortion, equalization and ISI cancellation techniques in digital radio systems with nonlinear transmit amplifier", IEEE Transaction on Communications, vol. 37, n. 12, Dec. 1989): 1) data predistortion: attempts are made to modify the constellation used for driving the nonlinear amplifier through a signal such as to obtain the desired constellation at its output; 2) analog signal predistortion: a nonlinear circuit having a characteristic opposite to the one of the above-defined "channel", is inserted in the path of the analog signal; 3) channel equalization and nonlinear cancellation of the ISI: the receiving equalizer tries to cancel the interferences connected with nonlinearity from the present signal sample (through a suitable nonlinear combination of pre- and post-cursors); 4) use of "circular" constellations so as to reduce the ratio between the peak power and the average power of the unfiltered signal. All the above solutions, under special circumstances, can provide unsatisfactory performances. In particular the first three are not very efficient in the presence of "hard limiter" characteristic of the transmission final amplifier; the last one gives anyway rise to slight gains which can be not sufficient in the case of a reception filter with very narrow bandwidth. From for example EP-A-0383632; or IEEE Journal on Sel. Ares in Comm., vol. 7, no.6, August 1989, pages 941-958, Forney: "Multidimensional Constellations - part II: Voronoi Constellations"; or Trans. IEICE vol. E71, no. 6, June 1988, Tokyo, pages 591-598, Nakamura et al.: "A New 90Mbps 68 APSK Modem With Honeycomb Constellation For Digital Radio Relay Systems", methods are known for minimizing the average power of the transmitted signal, possibly using circular two-dimensional constellations. These methods however do not address one of the real problems of the transmission channel, that is the non-linearity of the transmission final amplifier. Summary of the invention It is an object of the invention to provide a base-band system which - other conditions being unchanged - reduces the peak power of the filtered signal, i.e. at the input of the nonlinear channel defined above. It has been found, inter alia, that such a reduction is to the advantage of radio relay system links, e.g. allowing the use of smaller antennas or transmission over longer path sections. The outstanding features of the invention are set forth in the claims while the various aspects and advantages of the invention will become more apparent from the following not limiting description. The basic idea of the invention is based upon the possibility (other conditions such as minimum distance between transmitted points, average transmitted power, etc.. being unchanged) of avoiding transmission of sequences having an associated high peak power of the filtered signal, , replacing them with more suitable ones (i.e. with a lower peak power of the filtered signal). The possibility of carrying out this replacement is given by the increasing of the dimension of the alphabet of the transmitted points. In reception the unwanted sequences, suppressed in transmission, are reconstructed in their original form. By reducing in this way the peak power of the filtered signal it is possible to exploit in a much more efficient manner the nonlinear characteristic of the above-defined "channel". Such reduction allows to minimize the effects of the transmission channel nonlinearity, including possible nonlinearities of the transmission amplifier. Detailed description. Fig. 1 illustrates the schematic block diagram of a generic digital transmission system (blocks I, II, IV, V, VI, VII, IX, X) in which blocks III and VIII, subject of this invention, are inserted. In particular, fig. 1 shows:- a DATA SOURCE (ref. I) which outputs the digital sequence to be transmitted; an ENCODER/MODULATOR block (ref. II) which receives at the input the digital sequence to be transmitted and carries out the standard encoding operations designed for BER reduction (block, convolutional, Trellis Code Modulation, etc.., type encoding) and modulation operations, providing at its output one of the points of the constellation to be transmitted; a block PRE (ref. III), subject of the invention along with block VIII, which eliminates from the transmission the unwanted sequences in terms of peak power of the filtered signal, i.e. of the signal at the output of block IV described below; a transmission filter HTx (f) (ref. IV) which provides at its output the analog signal to be transmitted; a NONLINEAR DISTORTION block (ref. V) representing an unwanted nonlinear distortion on the signal path. It can be due to the nonlinear characteristic of the final amplifier of the transmitter (as it happens e.g. in microwave links) or, more generally, to a nonlinear behaviour of the transmission channel; a transmission channel (ref. VI) identified as "LINEAR CHANNEL", which outputs a signal consisting of the signal at its input added to and/or combined with disturbances of various kind; a reception filter HRx (f) (ref. VII) which receives the signal from the transmission channel and carries out a suitable filtering; a block PRD (ref. VIII), subject of the invention along with block III, which reconstitutes the signal in its original form containing the unwanted sequences suppressed in transmission by block III; a DECODER/DEMODULATOR block (ref. IX) which receives the outgoing signal from block VIII demodulates it and carries out the above-mentioned standard decoding operations, providing the user with the numeric sequence subject of the transmission; the USER (ref. X) which receives the numeric sequence. In an advantageous and therefore preferred embodiment, blocks PRE (III) and PRD (VIII) in accordance with the invention are realized in the form of digital encoders. As an example, fig. 2 shows a block diagram illustrating how it is possible to realize the block PRE in case of a radio relay system transmission using a quadrature amplitude modulation (QAM). Let M be the points of the two-dimensional constellation to be transmitted in the conventional case (hereinafter "standard2 constellation) and MR be the redundance points necessary for the encoding (carried out in PRE) subject of the invention; the resulting constellation is composed of (M+MR) points (hereinafter "expanded" constellation). Typically: 1< (M+MR)/M < 1.2. In fig. 2 there is shown the prefered embodiment of PRE; it includes: a delay element T (ref. XIII) which receives the last two-dimensional element of the block of N two-dimensional symbols outgoing from block XII and outputs it with a delay equal to one channel symbol interval. Such output will be indicated as "state" of the machine in the following; a mapping element identified as "(M+MR) MAP" (ref. XI) which receives at its input a block of N symbols of the "standard" costellation and provides (M+MR) blocks of N two-dimensional symbols of the "expanded" constellation. Each output block is relative to a particular "state" of the system and represents the best sequence to be transmitted (in the presence of that particular "state" of the machine) in terms of peak power of the filtered signal. a multiplexer "MUX" (ref. XII) having (M+MR) inputs and one output which, on the basis of the "state" at the output of block XIII selects (among the M+MR inputs) the suitable block of N symbols to be provided at the output. According to one particular aspect - even if not limiting - of the invention, the calculation of the "best sequence in terms of peak power of the filtered signal" is arranged as follows. Let hTX(t) be the pulse response of the transmission filter IV of fig. 1, T the symbol time, d (k) (k=1,2 ..., (M+MR)) the "state" of the system, Ci = (ci, ci+1, ..., cN-1) the generic block of N two-dimensional symbols. The "weight" w of block Ci = (ci, ci+1, ..., cN-1) given the state d(k), can be defined as the quantity: then assuming that the best sequences Ci (in terms of peak power of the filtered signal) are those having a lower "weight" w(d (k) , Ci). The block PRD can be realized through a circuit quite similar to the one shown in fig. 2 for block PRE; its description in terms of block diagram (being within the reach of those skilled in the art, in the light of what has been set forth hereinbefore) will be omitted for conciseness' sake. Reference has been made to specific embodiments represented in figs. 1 and 2 for simplicity and illustrative clearness reasons; therefore it is evident that these are susceptible to those variations, modifications, replacements and the like which, being within the reach of those skilled in the art, naturally fall within the sphere of the invention. The following possible variants are here mentioned by way of an example: in equation (1) a "state" consisting of several two-dimensional symbols could be envisaged; blocks XI and XII of fig. 2 coluld be replaced by a combinatory algebra, thus transforming the structure of block PRE into a convolutional one. N could be taken great enough to be able to eliminate in Fig. 2 the reaction through block XIII thus transforming the structure of block PRE into a "block" structure.

Claims: Method of transmitting and receiving digital signals over a transmission link, comprising the steps of: in transmission, encoding and modulating (II) the input digital signal obtaining an output digital signal in the form of digital sequencies, filtering (IV) it for providing a modulated signal to be transmitted over a non-linear channel (V, VI) subject to non-linearities and disturbancies; in reception, filtering (VII) the received signal from the non-linear channel, and decoding and demodulating (IX) the filtered received signal to obtain a reconstructed digital signal; characterized in that it further comprises the steps of: in transmission, replacing (III) those of the said digital sequencies having an associated peak power of the signal to be transmitted higher than a given value, with new digital sequencies having an associated peak power of the signal to be transmitted lower than the given value; in reception, restoring (VIII) the said digital sequencies having an associated peak power of the signal to be transmitted higher than the given value before demodulating, so as to minimize the effects of said non-linearities and disturbancies on said reconstructed digital signal. Method of transmitting and receiving digital signals as in claim 1, characterized in that the transmission link is a digital radio relay system link and the modulation is a QAM quadrature amplitude modulation having a two-dimensional constellation of points (M) being said digital sequencies; and in that said replacing is carried out on said two-dimensional constellation of points. Method of transmitting and receiving digital signals as in claim 2, characterized in that said replacing comprises the following steps: enlarging the number of constellation points (M) by adding a number of redundance constellation points (MR), so obtaining an expanded constellation composed of (M+MR) points; calculating a weight function (w) of a pulse response h TX (t) of said transmission filtering (IV) extended to said expanded constellation of (M+MR) points; said digital sequencies having an associated peak power of the signal to be transmitted higher than a given value being the expanded constellation points having the weight function (w) higher than a given value. Method of transmitting and receiving digital signals as in claim 3, characterized in that said weight function (w) is: where T is a symbol time, d (k) [k=1,2 ..., (M+MR)] a "state" of the system, Ci = (ci, ci+1, ..., cN-1) a generic block of N two-dimensional constellation points. System for transmitting and receiving digital signals over a transmission link, comprising: in transmission, encoding and modulating means (II) of the input digital signal obtaining an output digital signal in the form of digital sequencies, first filtering means (IV) for providing a modulated signal to be transmitted over a non-linear channel (V, VI) subject to non-linearities and disturbancies; in reception, second filtering means (VII) of the received signal from the non-linear channel, and decoding and demodulating means (IX) of the filtered received signal obtaining a reconstructed digital signal; characterized in that it further comprises: in transmission, replacing means (III) of those of the said digital sequencies having an associated peak power of the signal to be transmitted higher than a given value, with new digital sequencies having an associated peak power of the signal to be transmitted lower than the given value; in reception, restoring means (VIII) of the said digital sequencies having an associated peak power of the signal to be transmitted higher than the given value, said restoring means being inserted before said demodulating means, so as to minimize the effects of said non-linearities and disturbancies on said reconstructed digital signal. System for transmitting and receiving digital signals as in claim 5, characterized in that the transmission link is a digital radio relay system link and the modulation is a QAM quadrature amplitude modulation having a two-dimensional constellation of points (M) being said digital sequencies; and in that said replacing is carried out on said two-dimensional constellation of points. System for transmitting and receiving digital signals as in claim 6, characterized in that said replacing means (III) comprise: delaying means (T) which receive the last two-dimensional element of the block of N outgoing two-dimensional symbols from a multiplexer (MUX) and outputs it with a delay equal to one channel symbol interval T; mapping means ((M+MR)MAP) which receive a block of N symbols of said constellation of points and provide (M+MR) output blocks of N two-dimensional symbols of an "expanded" constellation, each output block representing the best sequence to be transmitted in terms of peak power of the filtered signal at the output of said first filtering means (VII); multiplexing means (MUX) having (M+MR) inputs and one output which, on the basis of the "state" at the output of said delaying means (T) select among the M+MR blocks present at its input the block of N symbols to be provided at the output. System for transmitting and receiving digital signals as in claim 7, characterized in that said mapping means comprise means for calculating each output block representing the best sequence to be transmitted in terms of peak power, according to the following weight function (w): where T is a symbol time, d (k) [k=1,2 ..., (M+MR)] a "state" of the system, Ci = (ci, ci+1, ..., cN-1) a generic block of N two-dimensional constellation points.

Inventors: SANDRI ANDREA, SPALVIERI ARNALDO, SANDRI, ANDREA, SPALVIERI, ARNALDO
Applicants: CIT ALCATEL, ALCATEL
Application year: 1993
Main classification: H04L27
IPC codes: H04L27