Cellular radiotelephone signal which enables synchronisation at a frame of a supplementary channel by means of symbol numbering

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DESCRIPTION

Cellular radiotelephony signal that allows frame-level synchronization of a supplementary channel by numbering the symbols.

1. Scope of the invention

The scope of the invention is cellular radiotelephony. More precisely, the invention relates to the transmission of data, in particular at high speeds, in a radiotelephony system.

Known radiotelephony systems, such as GSM, are especially dedicated to voice communications. They implement a channel comprising two symmetrical paths: a downward path (from a land base station to a mobile station) and an upward path (from the mobile station to the base station).

Systems under development are also based on such a structure. Thus, the UMTS standard defined by ETSI provides for a symmetrical distribution between the descending and ascending routes.

It has also been proposed to complete the radiotelephony system by adding to the main channel at least one supplementary channel, only downstream and dedicated to the transmission of high-speed data, such as files transmitted over the Internet network.

Within the framework of the present invention, it is assumed that the radiotelephony system is of the type comprising a symmetric bidirectional main channel and at least one supplementary channel as mentioned above.

It is also assumed that the supplementary channel implements a multicarrier technique that ensures a distribution of the data in space time / frequency, and has a structure in predefined entities each comprising a predetermined number of symbols. By predefined entities, we understand frames or subframes.

More precisely, the invention relates to a technique that allows, in a terminal of a radiotelephony system such as the one mentioned above, a synchronization at the level of the entity considered (frame or subframe) of the supplementary channel.

The invention applies especially, but not exclusively, to the frame or subframe level synchronization of an OFDM HS-DPA supplementary link (supplementary channel) associated with a UMTS main link (two-way symmetric main channel).

2. Prior Art Solutions

For the sake of simplicity, the drawbacks of the prior art with the following particular application are presented below: synchronization at the subframe level of an OFDM HS-DPA supplementary link associated with a UMTS main link. It is clear, however, that this reasoning can be transferred to other radiotelephony systems comprising a symmetric two-way main channel and at least one supplementary channel. This reasoning can also be carried over to a frame-level synchronization of the supplementary link.

It is recalled that the supplementary link HS-DPA (for "High-Speed ​​Downlink Packet Access") is a downlink with high transmission speed associated with the main UMTS link. This is with a view to increasing the downstream transmission speed in order to offer services that need high speed (multimedia, video streaming, etc.).

As illustrated in FIG. 1, the UMTS main link has a structure organized in N, N + 1 frames, each comprising 15 slots (time slots) S1 to S15. As illustrated in FIG. 2, the HS-DPA supplementary link has a structure organized in N, N + 1 frames, each comprising up to 5 subframes SF1 to SF5. On the other hand, each slot or subframe comprises a set of symbols, and each symbol comprises a set of signal units (chips).

Two technical solutions have been proposed for the physical layer of the HS-DPA supplementary link:

- a spectrum spreading system according to the UMTS system;

- a system based on an OFDM multicarrier modulation.

With the first solution, a supplementary UMTS HS-DPA link is obtained which is inherent in the UMTS system. This, therefore, can benefit from all the techniques already used by the UMTS main link, such as channel estimation, power and clock control and, especially, the synchronization that is done with the CPICH signal specified in the standard. UMTS.

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With the second solution, a supplementary OFDM HS-DPA link is obtained that uses a different modulation from that used in the UMTS system (spread spectrum, CDMA). Because of this, not all the techniques used in the UMTS system can be used. Thus, specific techniques must be used in order to fulfill the same functionalities. However, some adaptation to the context can facilitate the establishment and maintenance of the OFDM link communication.

An OFDM subframe of the OFDM HS-DPA supplementary link and a UMTS subframe of the UMTS HS-DP supplementary link have the same duration (viz. 2 ms).

Time synchronization is one of the key elements in establishing a communication. This synchronization is cut into several "layers", due to the nature of the radio-mobile circular communication system that defines the two entities: subframe and frame (see figure 2). Thus, for the HS-DPA supplementary link, this synchronization is divided into several stages:

- chip level synchronization, which consists of finding the position of the symbols (and, therefore, of the chips included in these symbols) according to the clock used;

- synchronization at the subframe level, which consists of finding the beginning of the subframes;

- frame level synchronization, which consists of finding the beginning of each frame.

The synchronization of the UMTS HS-DPA supplementary link can be done relatively easily. Indeed, as indicated above, since the UMTS HS-DPA supplementary link is closely linked to the UMTS system, its synchronization can be directly supported by that of the UMTS main link. Thus, the initial synchronization at the chip level of the UMTS HS-DPA supplementary link can be done by a temporal autocorrelation in a specific synchronization signal (PSCH) provided in UMTS. Once the synchronization at the chip level has been acquired, the synchronization at the subframe level of the UMTS HS-DPA supplementary link can be done by searching for the beginning of the UMTS slots (knowing that each subframe contains a predetermined number of UMTS slots, for example 3). This search is done with the help of the PSCH signal. This signal is in the form of a packet of 256 identical chips emitted at the beginning of each slot. Finally, the frame synchronization of the UMTS HS-DPA supplementary link is carried out using the SSCH (Secondary Synchronization Channel) signal, which has the same shape as the PSCH signal with the exception that the transmitted 256-chip packets are modulated by known information. Figure 3 illustrates the order of the different synchronization stages of the UMTS HS-DPA supplementary link, at the chip, slot and frame level respectively.

In contrast, the synchronization of the OFDM HS-DPA supplementary link is currently more difficult to perform, because the latter, unlike the UMTS HS-DPA supplementary link, is not closely linked to the UMTS system.

According to the current technique, the chip-level synchronization of the OFDM HS-DPA supplementary link can be obtained using a guard interval representing a part of the OFDM symbol (the last part). This chip-level synchronization is obtained by a simple classical autocorrelation in the received OFDM HS-DPA signal.

Such a synchronization technique is described in particular in the document “Kombination einer OFDM-basierten Luftschnittstelle mit einem UMTS-basierten Ansatz mittels Anpassung der zeitlichen Laenge eines OFDM-Multiblocks an die zeitliche Laenge eines UMTS-Time Slot, SIEMENS TEKINK vol . 20032 ".

But once this synchronization at the chip level is obtained, it is impossible to know the beginning of the subframes and frames, because the OFDM signal specified in the HS-DPA frame does not contain the PSCH and SSCH signals necessary for synchronizations at the subframe and plot.

According to the current technique, the synchronization at the subframe level of the supplementary OFDM HS-DPA link is specific to the OFDM system and is based on the insertion of specific signals (in the form of OFDM symbols) for synchronization at the subframe level.

The main drawback of the current technique is that the aforementioned specific synchronization signals increase the load of the supplementary OFDM HS-DPA link, or for a fixed load, they decrease the useful transmission speed.

3. Objectives of the invention

The object of the invention is in particular to alleviate this main drawback of the current technique.

More precisely, one of the objectives of the present invention is to provide a new technique that makes it possible to facilitate synchronization at the level of the considered entity (frame or subframe) of the supplementary channel of a cellular radiotelephony system (especially an OFDM HS supplementary link -DPA associated with a UMTS main link).

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The invention also aims to provide a technique of this type that limits the addition of specific signals for synchronization at the level of the entity considered, and makes it possible to preserve the structure initially defined for the supplementary channel.

Another object of the invention is to provide such a technique that is simple to implement and inexpensive.

Still another object of the invention is to provide such a technique which, in the case where it allows synchronization at the sub-frame level of the supplementary channel, makes the frame level synchronization of the supplementary channel equally easy.

Four. Essential characteristics of the invention

These different objectives, as well as others that will become apparent in the following, are achieved according to the invention with the aid of a cellular radiotelephony signal, of the type comprising:

- a symmetrical bidirectional main channel, comprising a main upstream and a main downstream path, which especially ensure the transmission at low or medium speed of data and signaling and control information;

- at least one supplementary channel assigned only to the downstream direction, which especially ensures high-speed data transmission, putting into practice a multi-carrier technique that ensures a distribution of the data in space time / frequency, and which presents a structure in entities predefined each comprising a predetermined number of symbols.

According to the invention, at least one symbol of each entity of the supplementary channel comprises a position identifier, which makes it possible to identify the position of said symbol in said entity, so that a terminal can read the position identifier of said symbol and from this deduce the start of the next entity, thus obtaining a synchronization at the level of the entity considered of the supplementary channel.

In a first particular embodiment of the invention, said predefined entities are subframes of the supplementary channel.

In a second particular embodiment of the invention, said predefined entities are frames of the supplementary channel.

Preferably, each symbol of the supplementary channel comprises a position identifier.

The fact that all the symbols comprise a position identifier makes it possible to optimize the synchronization at the level of the considered entity (frame or subframe) of the supplementary channel.

It is clear, however, that the present invention also covers the case where only some (even a single) of the symbols of each entity comprise a position identifier.

In a first particular embodiment of the invention, said position identifier is inserted in frequency.

Advantageously, in the case in which said multicarrier technique uses a plurality of subcarrier frequencies distributed in a given frequency band, said position identifier comprises bits carried by subcarriers regrouped in the same predefined portion of said frequency band, with the order to be subjected to an almost constant channel.

Advantageously, said position identifier is encoded with differential encoding.

In a second particular embodiment of the invention, said position identifier is inserted in time.

Advantageously, the main channel and / or the supplementary channel ensure the transmission of information related to the range, within a frame of the supplementary channel structure, of a subframe whose start can be detected, so that it allows synchronization at the level frame of the supplementary channel detecting the start of the next frame, as a function of said synchronization at the subframe level and of said information relating to the range of said subframe.

Advantageously, the main channel and / or the supplementary channel also ensure the transmission of information relating to the transmission mode of the subframes of the supplementary channel, said synchronization at the frame level of the supplementary channel being a function, in addition, of the aforementioned information regarding the mode of transmission.

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In an advantageous embodiment of the invention, the main channel implements a spread spectrum access technique (CDMA) and is preferably a UMTS link. On the other hand, said supplementary channel implements, for example, a multicarrier technique based on OFDM modulation or IOTA modulation.

The principle and implementation of IOTA modulation are described in French patent application FR95 05455, incorporated herein by reference.

Advantageously, the main channel previously ensures the transmission of a notification inviting said terminal to carry out said synchronization at the level of the supplementary channel entity considered, with a view to tilting the main channel terminal towards the supplementary channel.

It should be noted that this notification procedure (which preferably uses the paging channel of the main channel), can be used to launch any type of synchronization procedure at the level of the considered entity (frame or subframe) of the supplementary channel and therefore equally the above-mentioned classical procedure (that is, based on specific synchronization signals transmitted on the supplementary channel).

Within the framework of the present invention, this notification therefore invites the terminal to detect and read a position identifier of a current symbol on the supplementary channel.

Preferably, said notification is transmitted on the paging channel included in said main channel.

The invention also relates to a method for synchronizing a supplementary channel associated with a symmetric bidirectional main channel; said symmetric bidirectional main channel comprising a main ascending path and a main descending path, which especially ensure the transmission at low or medium speed of data and of signaling and control information; said supplementary channel being assigned only to the downstream direction, ensuring especially high-speed data transmission, putting into practice a multi-carrier technique that ensures a distribution of the data in space time / frequency, and that presents a structure in predefined entities that they each comprise a predetermined number of symbols. According to the invention, the method comprises a stage of synchronization of the supplementary channel at the level of the entity considered, which in turn comprises the following stages:

- detection and reading of a position identifier of a current symbol in the supplementary channel, said position identifier allowing to identify the position of said symbol in the entity to which it belongs;

- deduction of the beginning of the next entity based on the position identifier of the current symbol.

The invention also relates to a terminal of a cellular radiotelephony system, of the type comprising means for transmitting a main upstream channel, means for receiving a main downstream path and means for receiving at least one supplementary channel; said main upstream pathway and said main downstream path forming a symmetrical bidirectional main channel that ensures especially the transmission at low or medium speed of data and of signaling and control information; the aforementioned supplementary channel being assigned to the downstream direction only, especially ensuring high-speed data transmission, putting into practice a multi-carrier technique that ensures a distribution of the data in space time / frequency, and that presents a structure in predefined entities that they each comprise a predetermined number of symbols. According to the invention, the terminal comprises means for synchronizing the supplementary channel at the level of the entity considered, these comprising:

- means for detecting and reading a position identifier of a current symbol in the supplementary channel, said position identifier allowing to identify the position of said symbol in the entity to which it belongs;

- means of deducing the start of the next entity as a function of the position identifier of the current symbol.

The invention still relates to a base station of a cellular radiotelephony system, of the type comprising means for receiving a main upstream path, means for transmitting a main downward path and means for transmitting at least one supplementary channel; said main upstream pathway and said main downstream path forming a symmetrical bidirectional main channel that ensures especially the transmission at low or medium speed of data and of signaling and control information; the supplementary channel being assigned only to the supplementary channel, ensuring especially high-speed data transmission, putting into practice a multi-carrier technique that ensures a distribution of the data in space time / frequency, and presenting a structure in predefined entities comprising, each a predetermined number of symbols. According to the invention, the base station comprises means for inserting a position identifier in at least one symbol of each entity of the supplementary channel, said position identifier allowing to identify the position of said symbol in said entity, in such a way that a terminal can read the position identifier

ES 2 345 205 T3 of said symbol and from this deduce the beginning of the next entity, thus obtaining a synchronization at the entity considered of the supplementary channel.

5. List of figures

Other characteristics and advantages of the invention will become apparent upon reading the following description of a preferred embodiment of the invention, given as an indicative and non-limiting example, and of the attached drawings, in which:

- figure 1 illustrates the known structure of a UMTS main link, organized in frames and slots;

FIG. 2 illustrates the known structure of an HS-DPA supplementary link (OFDM or UMTS), organized in frames and subframes;

FIG. 3 illustrates, within the framework of the known technique, the relationships between the three synchronizations of the UMTS HS-DPA supplementary link at the chip, slot and frame levels respectively;

FIG. 4 illustrates an OFDM HS-DPA supplementary link according to a particular embodiment of the invention, in which the symbols are numbered;

FIG. 5 illustrates, within the framework of the technique according to the invention, the relationships between the three synchronizations of the OFMD HS-DPA supplementary link, at the chip, subframe and frame levels respectively.

6. Description of an embodiment of the invention

In the description that follows, the case of a cellular radiotelephony signal comprising a UMTS main link (symmetric two-way main channel) and an OFDM HS-DPA supplementary link (supplementary channel) is considered by way of example.

The invention defines characteristics of this signal, which make it possible to facilitate synchronization at the subframe level, as well as at the chip and frame levels, of the supplementary OFDM HS-DPA link.

Figure 4 illustrates an OFDM HS-DPA supplementary link according to a particular embodiment of the invention, in which the symbols are numbered.

As already indicated above (see the reasoning in Figure 2), the HS-DPA OFDM supplementary link has a structure organized in N, N + 1 frames each comprising up to 5 SF1 to SF5 subframes of 2 ms. Each subframe SF1 to SF5 comprises a set of symbols. Each symbol comprises a set of signal units (chips).

In the example illustrated in FIG. 4, two successive subframes SF1, SF2 of the OFDM HS-DPA supplementary link have been represented. Each one comprises six symbols 41 to 46. Conventionally, each OFDM HS-DPA symbol is encoded in a plurality of subcarrier frequencies spread over a predetermined frequency band 47.

In the particular embodiment of the invention illustrated in figure 4, each OFDM HS-DPA symbol comprises a position identifier, which indicates its position within the subframe in which it is contained. The position identifier is here a number of a sequence of numbers ranging from 1 to 6, and that is repeated in each subframe.

More generally, it is recalled that within the framework of the present invention, the supplementary link has a structure in predefined entities (frames, subframes, or others) that each comprise a predetermined number of symbols, and the position identifier included in a given symbol indicates the position of this symbol within the entity in which it is contained.

The position of the number (or more generally of the position identifier) ​​in each symbol must be known in advance by the receiver (also called a terminal). Thus, when receiving the OFDM HSDPA supplementary link, the terminal can read the position identifier of the current symbol and from it deduce the start of the next subframe (see detailed description below, in connection with Figure 5). In this way, the terminal performs a synchronization at the subframe level of the supplementary link.

In the example illustrated in figure 4, the position identifier is inserted in frequency in the supplementary OFDM link HS-DPA. In other words, the position identifier of each symbol is added to the data of the source signal before the execution of the inverse Fourier transformation (TFDI), or other treatment, that ensures the passage of frequency in time. It is recalled that the multicarrier technique typically comprises the following steps: the source signal undergoes channel coding (if it is a digital signal) or is simply sampled (if it is an analog signal); then, the data is interlaced in frequency and / or in time, to form a series of symbols called OFDM symbols; these symbols are treated later, for example by the transform

ES 2 345 205 T3 inverse Fourier (TFDI), and undergo digital / analog conversion and RF transposition, before being broadcast.

The position identifier included in each symbol is encoded, for example, with differential encoding, so that the receiver (terminal) can decode it without knowing the propagation channel. It will be meant, in effect, that the position identifier must be identified by the receiver before the latter knows the propagation channel (ie without equalization).

Furthermore, the bits of the position identifier are carried, for example, by subcarriers regrouped in the same predefined portion 48 of the frequency band 47, in order to be subjected to a quasi-constant channel. This recommendation is achievable because the amount of information required to encode the position identifier (number) is small.

According to a variant of the invention (not illustrated), the position identifier is inserted in time in the supplementary OFDM HS-DPA link. In other words, the position identifier of each symbol is added to the data of the source signal after the execution of the inverse Fourier transformation (TFDI) or other treatment, which ensures the passage of frequency in time.

Figure 5 illustrates, within the framework of the technique according to the invention, the relationships between the three synchronizations of the OFDM HS-DPA supplementary link, at the chip, subframe and frame levels respectively.

The terminal first performs a chip-level synchronization of the HS-DPA OFDM supplementary link, indicated by 51. This is obtained, for example, by performing an autocorrelation on the guard interval, which represents the last part of the OFDM symbol.

Then, the terminal performs the synchronization at the subframe level of the supplementary OFDM HS-DPA link, indicated by 52. ​​Indeed, once the synchronization at the chip level has been obtained, a demodulation of an OFDM HS-DPA symbol is possible (operation indicated by 521 ) because the start and end of the symbol are known. After demodulation of the OFDM symbol, the terminal determines the number (position identifier) ​​of this symbol in the subframe to which it belongs (operation indicated by 522). This number gives information about the beginning and end of the subframe and, therefore, at the end, allows determining the beginning of the next subframe (operation indicated by 523), that is, knowing the synchronization at the subframe level of the OFDM HS supplementary link -DPA.

Optionally, the cellular radiotelephony system according to the invention also allows the terminal to perform in a simple way the synchronization at the frame level of the supplementary OFDM HS-DPA link, indicated by 53, after the synchronization at the subframe level has been carried out. 52.

This assumes, for example, that the terminal knows:

- the range, within a frame of the OFDM HS-DPA supplementary link, of the subframe whose start is detected during the synchronization step 52 at the subframe level of the OFDM HS-DPA supplementary link;

- the transmission mode of the subframes of the OFDM HS-DPA supplementary link. Indeed, there may be several modes: transmission of all subframes, one frame every three, etc.

Indeed, from the aforementioned range, from the aforementioned transmission mode and from the synchronization at the subframe level, the terminal can detect the start of the next frame of the OFDM HS-DPA supplementary link.

The terminal's knowledge of the aforementioned range and mode of transmission is obtained, for example, by transmitting information on this range and this mode of transmission on the UMTS main link. It can also be envisaged that this information transmission is carried out by the supplementary OFDM HSDPA link, alone or in combination with the main UMTS link.

It is clear that other embodiments of the invention can be considered.

In particular, provision can be made for only a part of the symbols to comprise a position identifier.

It can also be envisaged that the UMTS main link previously ensures the transmission of a notification that invites the terminal to carry out the synchronization at the subframe level of the OFDM HS-DPA supplementary link, with a view to tilting the main UMTS terminal towards the supplementary link. OFDM HS-DPA.

For example, this notification is transmitted on the paging channel (PICH channel, for "Paging Indicator Channel") comprised in the main UMTS link. It is recalled that the PICH channel belongs to the UMTS system and is made up of 300 bits (b0, ... b299) transmitted during the period of a 10 ms frame. The length of the spreading sequence used is 256. The last 12 bits (b288, .. b299) are not used in the UMTS standard and are therefore free. In order to report the toggle to the OFDM HS-DPA supplementary link, these 12 bits are used, for example as follows:

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- 4 bits for notification of tipping towards the supplementary link (these bits may contain information related to the carrier frequency of the supplementary channel);

- 8 bits to indicate the type of service, the transmission speed, the exchange mode, etc.