FR2828352A1 - OFDM diversity receiver uses delay and switching to single receiver chain - Google Patents

Abstract

An OFDM (Orthogonal Frequency Division Multiplexing) diversity receiver (4) has two antennas (1A, 1B) connected through mixers (11A, 11B) and switches (2, 12) with one path delayed (8) by a time equal to the last N bits of a burst to allow evaluation by estimators (5) of the two paths.

Description

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Reception device including means for qualitatively evaluating the signals received via different antennas and method allowing such evaluation
The invention relates to a diversity reception device including means for qualitatively evaluating the signals received via different antennas. More particularly, the invention relates to the transmission of digital signals in the form of packets and transmitted by bursts and in particular using OFDM symbols (orthogonal frequency division multiplexing) made up of elementary signals modulated in quadrature over a large number of so-called orthogonal carriers. .

 It also relates to a method allowing such an evaluation in particular for antenna selection purposes.

 In a system which has a plurality of antennas for diversity reception purposes, it is conventional to provide for the possibility of selecting one of them considered to be qualitatively superior on the basis of evaluations carried out on the basis a selection criterion determined, for at least some of the different antennas.

 It is known to carry out an evaluation of the frequency response per reception path and to use this evaluation to determine the resulting influence in terms of bit error rate for the reception device, this implying the availability of frequency response assessments which are highly reliable for the paths being compared. It is necessary that the reception device is simultaneously operable for the signal arriving by the reception path then in service and for at least one signal which arrives by another path and which is intended to be used for comparison purposes, in particular if this comparison must lead to confirm or modify a selection. This practically leads to two receiver modules being implemented in parallel. One of the modules must have means for receiving, processing and transmitting the useful signal arriving by one of the paths, this path being that which is then selected for the purpose of transmitting the useful signal, downstream, in view

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 of its operation. It must also have means enabling it to submit the signal it receives to quality evaluation processing.

The other module must have sufficient means to receive the signal arriving by a path other than the path selected for transmission purposes and to allow the signal it receives to be subjected to quality evaluation processing. Most known reception devices try to avoid duplication as much as possible and more particularly the duplication of complete receiver modules for economic reasons.

 It is known to provide means of transmission protocols for carrying out quality assessments for signals received by different paths at the same diversity reception device based on the preamble appearing respectively in the header. of each packet group that is transmitted as part of a burst. A first quality evaluation is made for a first path from the preamble of a group which has just been received by this path and a second evaluation is made in the same way for a second path from the preamble which follows for the group of packets received by the second path. The reception device can switch from one path to another, that is to say from one antenna to another, at pre-negotiated instants, if necessary, without this having any negative consequence. on the overall performance of the device.

 However, such a solution is not systematically applicable, since it requires having preambles organized to enable it to be implemented at the level of the groups of packets constituting the signals to be evaluated. It is then usually planned to carry out signal quality evaluations carried out successively at the level of a group of packets transmitted by a first path during a relative burst of a signal and at the level of a group of packets transmitted by a second path during the next burst, it being understood that it is essential to avoid any risk of deterioration or loss of the device output signal due to the evaluation operations.

 A systematically applicable solution is therefore proposed by the present invention in the context of a reception device having a plurality of antennas usable for picking up digital signals grouped in the form of packets transmitted by bursts. This

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 device comprising at least a first reception path established from a first antenna and a second reception path established from a second antenna which lead to a common processing path with which these reception paths can be selectively connected. The processing path includes signal processing and evaluation means and these means make it possible in particular to evaluate the signal quality per path.

 According to a characteristic of the invention, the device comprises means for delaying the signal received by a reception path relative to that received by another by an amount corresponding to the time necessary to transmit to the processing means an evaluation structure , of determined size, corresponding to the last N of the bits transmitted during a burst on a path. N is a positive integer, smaller than the minimum number of bits transmitted by burst. This number N is chosen to allow the processing means to carry out a qualitative evaluation for a reception path with respect to another, from estimation structures which are provided by these two reception paths and which follow each other due to the delay imposed on a received signal compared to the other.

 According to a characteristic of the invention, the device comprises means for delaying the transmitted signal which are located upstream of a first switch to which the reception paths of the device terminate so as to be able to be selectively connected to the signal processing means.

 According to a characteristic of a variant of the invention, the device comprises means for delaying the transmitted signal which are located downstream of a second switch through which the reception paths pass to reach the output switch, this second switch enabling to pass, on command, each reception path to the first switch, either directly or by means of means provided for delaying the transmitted signal.

 According to a characteristic of a variant of the invention, the means for delaying the transmitted signal are of the delay line type and act on the signals received, when these signals are in analog form.

 According to a characteristic of a variant of the invention, the means for delaying the transmitted signal are of the bit memory type and act on the signals received, when these signals are in digital form.

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 According to a characteristic of a variant of the invention, the device comprises signal processing and evaluation means which have the capacity to provide an evaluation of signal quality per path, from the estimation structures contained in the signals obtained in the signal processing means at the output of an equalizer which receives the signals successively transmitted to the common path by reception paths.

 According to a characteristic of a variant of the invention, the device comprises signal processing means which have the capacity to provide an evaluation of signal quality per path, from the estimation structures contained in the signals obtained in the signal processing means at the output of a decoder placed downstream of an equalizer which receives the signals successively transmitted to the common path by reception paths.

 According to a characteristic of a variant of the invention, the device comprises switching means by means of which the reception paths can be selected according to the evaluation results which are supplied by the signal processing means.

 The invention also proposes a method allowing a qualitative estimation of the signals respectively received by different antennas of a reception device, as envisaged above. This method provides for the possibility of delaying the signal received by a reception path, with respect to that received by another, under the conditions which have just been defined.

 According to a characteristic of a preferred variant of the method, the evaluation structure chosen corresponds to a positive integer number of OFDM symbols located at the end of the packet.

 The invention, its characteristics and its advantages are explained in the description which follows in conjunction with the figures mentioned below.

 FIG. 1 presents a simplified diagram relating to a diversity reception device in which an evaluation of the quality of the received signals is provided, according to the state of the art.

 FIG. 2 presents a block diagram of a diversity reception device having quality evaluation means, according to a first embodiment of the invention.

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 FIG. 3 presents a diagram of a diversity reception device having quality evaluation means, according to a variant implementation of the invention.

 The reception device illustrated in FIG. 1 is assumed to be a known device comprising at least two separate reception paths which are here symbolized by antennas 1A and 1 B. These antennas are those by which they are capable of being received the signals intended for the device. The signal received via an antenna is transmitted to a switch 2 which has the function of switching, to a single output 0, one of the signals which are respectively received by a different antenna. These transmissions are carried out through various reception paths which are not developed here insofar as, known in themselves, they have no direct connection with the object of the present invention. As indicated above, the signals considered here are more particularly digital signals grouped in the form of packets which are transmitted by bursts. In the following description, it is considered that these signals consist of OFDM symbols.

 A signal processing assembly 4 is placed at the output of switch 2, so as to take into account the signal received by a determined path. The transmission is selectively ensured by the switch 2, via its output O. It has means making it possible to demodulate the received signal and to equalize it, before ensuring its decoding, and its purpose is to restore the initial binary signal , used to generate the transmitted signal, with maximum fidelity from the received signal. The decoding is for example carried out using a Viterbi decoder. The processing is intended to allow exploitation, here assumed to be external, of the processed signal appearing at the output SE of the assembly 4.

 It is known to associate evaluation means 5 with the processing unit 4 in order to know the quality level of the received signal which appears at the output of the switch 2. Such knowledge is commonly used for selection purposes. path. It is possible to establish an evaluation for a first channel within the framework of a first burst and for a second path within the framework of the following burst. The evaluations can be based one on the result of the treatment carried out by the treatment unit 4 at the level of the

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 preamble of the packet group transmitted via the first path during a burst and at the level of the preamble of the packet group transmitted via a second path during the next burst.

 However, as indicated above, this is not always possible and the present invention therefore proposes a more general solution which is notably applied in the reception devices illustrated in FIGS. 2 and 3.

 The reception device, presented in FIG. 2, is assumed to be a device comprising at least two distinct reception paths which are respectively symbolized by antennas referenced 1A and 1B, as in FIG. 1. These antennas are once again assumed to be those through which the signals intended for the device can be received. As before, the signals received via the antennas are separately transmitted to a switch 2 making it possible to selectively switch them to a single output. The signals envisaged here are digital signals which are grouped in packets and transmitted by bursts, as indicated above, these are for example signals appearing in the form of OFDM symbols. Each burst arriving by a path is assumed to correspond to a group D of useful data which is generally preceded by a preamble P. A group is assumed to consist of one or more OFDM symbols, in the example envisaged here. As is known, each of these symbols, here referenced S, is itself made up of a plurality of symbols of smaller size and for example of 16.64, 128 small symbols or more.

 According to the invention, it is planned to make a quality evaluation from an evaluation structure, of determined or possibly determinable size, corresponding to an OFDM symbol or possibly to several. According to the invention, this evaluation structure corresponds to the N last bits received for a burst arriving via a determined path and preferably to the N bits which correspond to the last symbol transmitted during a burst, in the example, as envisioned above. The number N is chosen so as to allow a quality evaluation to be carried out under good conditions, it can therefore vary according to the evaluation method chosen. The evaluation structure present at the end of the salvo can be identified by different means,

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 in particular by an indication associated with the package or symbol which precedes it. The selection of a determined number N of bits received at the end of the burst, as a quality evaluation structure, practically always makes a quality evaluation possible as soon as this number N is chosen to be less than the number of bits transmitted. as part of a salvo.

 The reception device according to the invention, as illustrated in FIG. 2, comprises a signal processing assembly 4 corresponding to that envisaged in the device presented in FIG. 1. This assembly includes in particular equalization means provided to allow compensate for any level imbalances affecting a signal received via a reception path, taking into account the frequency response observed at reception, via this path. An equalization between the levels for the different frequencies used for a determined signal is carried out by the equalization means which tend to have a stabilized action at the end of the burst, there therefore where the quality evaluation is preferably carried out with a reception according to the invention. It is therefore possible to consider that the signal obtained at the end of the burst corresponds exactly to the signal which was supplied to the device initially responsible for ensuring its emission.

 To avoid equipping a reception device with two identical subsets of quality evaluation operating according to the same selection criterion and simultaneously working each for a different reception path, provision is made to use a means making it possible to delay the received signal by one of the paths relative to the signal received by another of these paths. Such a delay means is symbolized by a circuit 8 which is shown inserted in the reception path including the antenna 1 B in FIG. 2, where only two paths are shown. This circuit 8 is supposed to have the object of delaying the signal received by a reception path with respect to that received by the other by a time sufficient to allow each of the signals to be taken into account separately, one after the other, by the processing unit 4.

 The delay imposed on the delayed signal preferably corresponds to the time necessary for the transmission of the evaluation structure to be taken into account at the level of the received signal and therefore to an OFDM symbol, in the example considered above, or even to several OFDM symbols, if the evaluation structure includes several.

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 In the case where the evaluation is carried out on the last symbol S of a burst, the duration of the delay chosen therefore corresponds to a symbol time S. The symbol S received by the reception path comprising the antenna 1A such that presented in FIG. 2, is then used as an evaluation reference for the symbol S which is received by the path including the antenna 1B and which is delayed by the circuit 8. The evaluation can be carried out under conditions and with material elements, in particular evaluation means 5 corresponding to those mentioned in connection with FIG. 1, this being detailed below.

 The means constituting circuit 8, through which a signal is delayed, should ideally be located furthest upstream on the reception path where they must act and therefore practically at the antenna outlet, to limit the quantity requirements. equipment. However, as is known, for technical and / or economic reasons and more particularly because of the high frequencies provided for the transmission of signals which are for example of the order of 5 GHz, it is more acceptable to involve the means of delay at a path level where the frequencies are lower and for example at a level of an intermediate frequency stage of a reception path.

 A reception device, according to the invention, taking account of the remarks appearing above, is illustrated in FIG. 3. It is assumed to comprise at least two reception paths each comprising an antenna 1A, 1B, an input amplifier stage 10A, 10B and a transposition stage 11A, 11 B at an intermediate frequency in series.

 Each reception path is capable of being connected, to a signal processing assembly 4, via a switch 2, in order to be able to supply the output signal SE, with means and under conditions which correspond to those defined previously in connection with FIG. 2, except as regards the linking of the delay means here symbolized by a circuit 8.

 The signal processing assembly 4 presented comprises a demodulator 13 with which an equalizer 14 is associated in series with a decoder 15. The equalizer 14 is possibly included in the demodulator 13, in particular when the latter is an OFDM demodulator, as considered here.

 Circuit 8 is for example constituted by a delay line positioned downstream of an intermediate frequency stage of the

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 paths to which this delay line can be selectively connected through an input switch 12. If the signals are converted into a digital train at the intermediate frequency, the delay means constituting the circuit 8 may otherwise be constituted by a memory allowing to store, in binary form, the elements constituting a signal received via a reception path, these elements corresponding for example to one or more symbols, as required. The output switch 2 makes it possible in particular to switch successively to the processing unit 4, a signal received from a reception path, via the input switch 12, and a signal which has been received from another reception path. and which is transmitted via the delay means. The conditions of this transmission via the delay means correspond to those which have been defined above in connection with FIG. 2, for the purposes of quality evaluation.

 The delay imposed by the delay circuit 8 is fixed as a function of the number of bits to which an evaluation structure corresponds. It therefore corresponds to the duration provided for a symbol S, if the number N of bits provided at the level of the evaluation structure corresponds to the number of bits provided for such a symbol.

 Switches 2 and 12 are chosen so as to switch in times which are negligible, their switching being triggered during the guard time provided between two successive symbols.

 The quality evaluation for the various reception paths specific to a diversity reception device, such as that shown diagrammatically in FIG. 3, can be obtained in the manner indicated below.

 A path quality evaluation is carried out at the level of the circuit 5 for, as the case may be, the or one reception path, said to be in evaluation, other than that which is then used to serve the decoder 13 so that it provides the output signal SE.

 It is assumed here, for purposes of explanation, that: the reception path including the antenna 1B is used to transmit to the signal processing unit 4 the signal which it receives through the antenna 1B, this transmission being effected by an undelayed link, via switches 12 and 2, duly switched;

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the quality evaluation to be carried out concerns the reception path including the antenna 1A which is then linked by the switch 12 with an input of the delay means of the circuit
8.

 The quality evaluation for the path under evaluation is triggered by the connection of an output of the delay means of the circuit 8 with the input of the signal processing assembly 4, via the output switch 2, duly switched, as soon as the evaluation structure of the burst being transmitted by the operating path has been received by the equalizer.

 The evaluation structure received at the end of the burst via the evaluation reception path is then transferred to the signal processing unit 4, via the output switch from the delay means of the circuit 8. In the case of the device as illustrated in FIG. 3, it therefore appears successively at the output of the equalizer 14, after demodulation, the equalized evaluation structure coming from the reception path in evaluation and that coming from the reception path in evaluation. The structure coming from the reception path in operation can then be used as a reference to assess the quality of the reception path under evaluation. In a first example of implementation of the invention, provision is made to make direct use, by the evaluation means 5, of the evaluation structures, coming from the equalizer 14 by the reception path in operation and by the evaluation reception path, to provide an evaluation E at the level of a subset 16 from a comparison carried out between two evaluation structures successively received each on a different path. This evaluation can be used to control the switch 12 so as to exploit the path which presents the best quality, to transmit the signal received to the decoder 15, via the output switch 2, the demodulator 13 and the equalizer 14, after the switch 2 has been properly switched for this purpose. A reliable evaluation can of course be obtained by a series of comparisons relating to pairs of two equalized evaluation structures each obtained from one of the two paths envisaged above. This solution has the advantage of being fast and inexpensive in hardware.

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 It is of course conceivable to carry out evaluations, as envisaged above, by repetition, when the number of reception paths is greater than two.

 It is also possible to obtain greater precision by determining the error rate obtained for a path in evaluation compared to a path in operation on the basis of an evaluation structure, as envisaged above.

 To this end, it is planned to use the signal obtained from an equalized evaluation structure obtained at the end of the burst at the output of the equalizer 7. It is then planned to use an encoder 18 connected at the output of the decoder 15 for reconstructing the binary signal used for transmitting the signal received by the path in operation, from this received signal. The reconstructed binary signal can then be exploited by hardware and / or software means symbolized by a module 17 to constitute a reference making it possible to calculate the binary error rate BER obtained from the evaluation structure taken into account for the path in evaluation. The error rate obtained can be exploited to produce a path selection signal intended for the switch 12, in order either to keep the path in operation, or to ensure the operation of the reception path to be used in its place.

 Other variants of the invention are possible, in particular with regard to the evaluation of the bit error rate. For example, if the error correction coding is a Reed-Solomon type code, the decoding circuit calculates the number of errors to be corrected before correcting them. The decoding circuit can then directly supply the bit error rate.

Claims (9)

1 / Reception device having a plurality of antennas operable to receive digital signals grouped in the form of packets transmitted by bursts, said device comprising at least a first reception path (1 DA, 11A) established from a first antenna (1A) and a second reception path (10B, 11B) established from a second antenna (lob) which terminate in a common processing path (2,4) with which these reception paths can be selectively connected , said processing path comprising processing means (4) and signal quality evaluation (5) having the capacity to provide a signal quality evaluation per path, characterized in that it comprises means (8 ) to delay the signal received by a reception path compared to that received by another by an amount corresponding to the time necessary to transmit to the processing means an evaluation structure, of deter mined, corresponding to the last N of the bits transmitted during a burst on a path, N being a positive integer, smaller than the minimum number of bits transmitted per burst, which is chosen to allow the processing means to perform a qualitative evaluation for a reception path with respect to another, from estimation structures which are provided by the two reception paths and which follow each other due to the delay imposed on a signal received with respect to the other.  2 / Device according to claim 1, characterized in that it comprises means (8) for delaying the transmitted signal which are located upstream of a first switch (2) to which the reception paths of the device lead to be able to be selectively connected to the signal processing means.  3 / Device according to claim 2, characterized in that it comprises means for delaying the transmitted signal which are located downstream of a second switch (12) through which transit the reception paths to reach the first switch (2 ), this second switch (12) making it possible to pass, on command, each <Desc / Clms Page number 13>  reception path to the output switch, either directly or through means (8) provided for delaying the transmitted signal.  4 / Device according to one of claims 1 to 3, characterized in that the means for delaying the transmitted signal are of the delay line type and act on the signals received, when these signals are in analog form.  5 / Device according to one of claims 1 to 3, characterized in that the means for delaying the transmitted signal are of bit memory type and act on the signals received, when these signals are in digital form.  6 / Device according to one of claims 1 to 5, characterized in that it comprises signal processing and evaluation means which have the capacity to provide a signal quality evaluation by path, from estimation structures contained in the signals obtained in the signal processing means (4) at the output of an equalizer (14) which receives the signals successively transmitted to the common path by reception paths.  7 / Device according to one of claims 1 to 5, characterized in that it comprises means (5 ') for providing an evaluation of signal quality by path, from the estimation structures contained in the signals obtained in the signal processing means (4 ') at the output of a decoder (15) placed downstream of an equalizer (14) which receives the signals successively transmitted to the common path by reception paths.  8 / Device according to one of claims 1 to 7, characterized in that it comprises switching means (2,12) through which the reception paths can be selected according to the evaluation results which are provided by the signal processing means.  9 / Method allowing a qualitative estimation of the signals respectively received by different antennas (1A, 1B) of a communication device <Desc / Clms Page number 14>  reception, designed for digital signals put in the form of packets which are transmitted by bursts, when the device comprises an individual reception path (10A, 11A) by antenna and means for signal processing and evaluation (4 ′, 5 ') having the capacity to provide a quality evaluation according to a determined criterion for a reception path compared to another, characterized in that it provides the possibility of delaying the signal received by a reception path, with respect to that received by another, of an amount corresponding to the time necessary to transmit to the processing means an evaluation structure corresponding to the last N of the bits transmitted during a burst on a path, N being a smaller positive number that the number of bits transmitted during a burst, which is chosen to allow the processing means to carry out a qualitative evaluation for a reception path relative to another, from the estimation structures which are provided by these two reception paths and which follow each other due to the delay imposed on a signal received by one path with respect to that received by the other path.  101 Method according to claim 9, in which the evaluation structure chosen corresponds to a positive integer number of OFDM symbols transmitted last.