Method for allocation of transmission resources in a cooperative cellular network

TECHNICAL AREA

The present invention relates generally to the field of cellular telecommunications and more particularly those employing a cooperation strategy.

STATE OF THE ART

One of promising techniques recently investigated as part of the new radio-mobile telecommunications standards, such as WiMAX, 3GPP LTE (<i>3GPP Long Term Evolution)</i> to increase the coverage and throughput of traditional cellular networks is that of a cooperation strategy deployed at the base or at the mobile terminal stations.

Are examples of cellular networks implementing a cooperation strategy in the article by <nplcit id="ncit0001" npl-type="s"><text>S. Shamai et al. entitled "Cooperative multi-cell networks: impact of limited backhaul capacity and inter-user links" published in Proc. of the Joint Workshop on Coding and Communications, Austria, Oct. 14-16.2007</text></nplcit> and in the article by <nplcit id="ncit0002" npl-type="s"><text>Hoymann C. et al. entitled "Flexible OFDM-based Wireless Relay Networks," published in Proc. of 15th IST Mobile and Wireless Communications Summit, June 2006</text></nplcit>.

Cooperation at the level of the base stations, called MCP (<i>Multi-Cell Processing</i>) Returns to implement a distributed antenna system type (MIMO<i>Multiple Input Multiple Output</i>) With a coding / joint decoding of the signals transmitted / received by the different antennas. This cooperation strategy, however, has the drawback of overloading the capillary network (<i>backhaul network</i>) Connecting the base stations.

Cooperation in mobile terminals is reflected by the fact that some terminals act as relays for end terminals, either for uplink (<i>uplink</i>) Or downlink (<i>downlink</i>). It was also suggested that some dedicated fixed relay are installed in a cell for this role. We will use the later term "relay cooperation" to designate either one or the other of these techniques, the relay receiving in both cases a source of a signal (base station for the downlink and terminal for the uplink) and retransmitting to a destination (base station for the uplink and terminal for the downlink).

The relay may retransmit the signal from the source in a full-duplex or half-duplex mode. Conventionally, full-duplex mode, the relay is able to receive and transmit simultaneously on the same resource when in half-duplex mode the relay is successively in a reception phase and a reception phase on a given resource.

An example of cooperation with relays operating in half-duplex mode is described in the article <nplcit id="ncit0003" npl-type="s"><text>O. Simeone et al. entitled "Uplink throughput of TDMA cellular mobile systems with multicell processing and Amplify-and-forward cooperation between" published in IEEE Trans. on Wireless Communications, Vol. 6, No. 8, pages 2942-2951, August 2007</text></nplcit>.

The <figref idrefs="f0001">Fig. 1</figref> represents very schematically a cellular network in cooperation with half-duplex relay. Shows two adjacent cells 110 and 120.

In cell 110, a source <i>s</i>₁ (Here a mobile terminal) transmits a data stream to a recipient <i>d</i>₁ (Here the base station <i>B</i>₁). Relay<i>r</i>₁ (Here a mobile terminal) also receives the data stream resulting from <i>s</i>₁ and relays it to the recipient <i>d</i>₁. Relay<i>r</i>₁ thus cooperates with the transmission of data between <i>s</i>₁ and <i>d</i>₁. For example, if the channel<i>s</i>₁-<i>d</i>₁ is of poor quality, particularly because of the presence of an obstacle between <i>s</i>₁ and <i>d</i>₁, the canal <i>s</i>₁-<i>r</i>₁-<i>d</i>₁ can allow the bypass and to obtain a satisfactory link quality. The data stream can be relayed by several terminals to further increase the spatial diversity of the transmission paths. In addition, it can be relayed at a time (<i>single-hop</i>) Or in several consecutive times (<i>Multi-hop</i>).

Similarly, the cell 120 includes a source <i>s</i>₂ (Here the base station <i>B</i>₂) Which transmits a recipient of the data stream <i>d</i>₂ (Here a mobile terminal) both directly and through the relay <i>r</i>₂ (Here a mobile terminal).

The relay terminal <i>r</i>₁ receives data from the source terminal <i>s</i>₁ during a phase of listening and transmits them to the base station during a transmission phase. The target base station thus receives the same data via different paths, a first time in the transmission interval of the source terminal and a second time during the transmission interval of the relay terminal. The co -operation is the same within the cell 120, the only difference is that the source is here the base station<i>B</i>₂ and the destination mobile terminal.

Cellular networks using a relay cooperation raise new problems in terms of inter and intra-cellular interference. In conventional cellular networks, it is known to allocate separate transmission resources (e.g. frequency) to adjacent cells and reuse these resources in a predetermined pattern (<i>frequency reuse pattern</i>) When the cells are separated. Thus, for a given number of transmission resources, it reduces the level of inter-cell interference, especially critical to cell periphery.

It is possible to use such a resource allocation scheme in a cellular network in cooperation by half-duplex relay. However, since the number of connections is substantially higher than in a conventional cellular network, this allocation strategy would intensive transmission resources, as explained below.

The <figref idrefs="f0002">FIGS. 2A and 2B</figref> schematically illustrate a method for transmitting resource allocation in a cellular network respectively without and with a cooperation by half-duplex relay.

Is shown on the abscissa the transmission time intervals, hereinafter simply referred to as transmitting intervals for the different terminals and the ordinate another transmission resource, for example for an OFDM system frequency intervals (<i>frequency chunks</i>) δ<i>f</i>₁ and δ<i>f</i>₂.

In the absence of cooperation, cf. <figref idrefs="f0002">Fig. 2A</figref>, (the relays <i>r</i>₁ and <i>r</i>₂ are absent or inactive), sources <i>s</i>₁ and <i>s</i>₂ transmit their data flow during transmission intervals <i>T</i>₁ and <i>T</i>₂ by modulating the subcarriers belonging to the intervals δ respectively<i>f</i>₁ and δ<i>f</i>₂. There was<i>s</i>₁(<i>T</i>₁) and <i>s</i>₂(<i>T</i>₂) The respective data transmitted by <i>s</i>₁ and <i>s</i>₂ during intervals <i>T</i>₁ and <i>T</i>₂.

In the presence of cooperation, cf. <figref idrefs="f0002">Fig. 2B</figref>, the sources <i>s</i>₁ and <i>s</i>₂ transmit as before their stream of data during the transmission interval <i>T</i>₁ in the intervals δ<i>f</i>₁ and δ<i>f</i>₂. The half-duplex relay<i>r</i>₁ and <i>r</i>₂ are listening phase, respectively on δ<i>f</i>₁ and δ<i>f</i>₂ during the interval <i>T</i>₁ and retransmit the data received during the interval <i>T</i>₂, For example using the same frequency intervals δ<i>f</i>₁ and δ<i>f</i>₂. There was<i>r</i>₁(<i>T</i>₁) and <i>r</i>₂(<i>T</i>₁) The data sent by <i>r</i>₁ and <i>r</i>₂ during the interval <i>T</i>₁.

The <figref idrefs="f0002">Fig. 2C</figref> illustrates a resource allocation method when only one of two adjacent cells, by 110, using a strategy of cooperation.

The allocation for the transmission interval <i>T</i>₁ is identical to that illustrated in <figref idrefs="f0002">Fig. 2A or 2B</figref>. In contrast during the transmission interval<i>T</i>₂, relay <i>r</i>₁ relays the received data <i>s</i>₁ using the frequency interval δ<i>f</i>₁ and <i>s</i>₂ continues to transmit its data using the frequency interval δ<i>f</i>₂.

It is understood that for transmitting the same amount of data, ensure that two times more transmission resources in the cooperation of the configuration <figref idrefs="f0002">Fig. 2B</figref> that in the configuration without the cooperation of <figref idrefs="f0002">Fig. 2A</figref>. Similarly you have to use one and half times more resources in joint cooperation of the configuration of the<figref idrefs="f0002">Fig. 2C</figref> that in the configuration without the cooperation of <figref idrefs="f0002">Fig. 2A</figref>.

The purpose of the present invention is therefore to provide a method of resource allocation for cellular network cooperation by half-duplex relay that ensures a low level of inter-cell interference without mobilizing provided a significant amount of transmission resources .

PRESENTATION OF THE INVENTION

The present invention is defined by a method for allocating resources in a cellular network comprising at least two adjacent cells, a first cell comprising a first source, a first relay and a first receiver and a second cell comprising at least a second source and a second recipient. A first transmission resource is allocated to the first relay during a first transmission interval on which it retransmits to the first recipient of the previously received data from the first source during at least one previous transmission interval and said first resource is also allocated to said second source during said first transmission interval.

In a particular embodiment, the second cell includes a second relay and a second transmission resource allocated to it during a second interval of transmission where it retransmits the second recipient of the previously received data from the second source for at least one gap previous transmission, said second resource is also allocated to said first source during said second transmission interval.

Said first and second transmission intervals can be chosen to be identical or distinct.

According to a first exemplary embodiment, are allocated to the first and second sources first and second frequency intervals and respectively, during each transmission interval is allocated to the first relay the second frequency interval to rebroadcast the data that it has received from the first source to the first frequency range during the preceding transmission interval.

According to a second exemplary embodiment, are respectively allocated to the first and second sources first and second frequency intervals during a current transmission interval and this allocation is permutated during the next transmission interval. the first relay is also allocated during the current transmission interval the second frequency range to rebroadcast the data it received from the first source during the preceding transmission interval on the second frequency interval, and is allocated to the first relay during the next transmission interval, the first frequency interval to retransmit the data are of the first source that has received during the current transmission interval on the first frequency interval.

According to a third embodiment, the second transmission interval follows the first transmission interval, and is allocated to the first and second sources respectively first and second frequency intervals during the first transmission interval, the second frequency interval being also allocated to the first relay during the first transmission interval to rebroadcast the data received from the first source during the previous transmission interval, the first frequency range being also allocated to the second relay in the second transmission interval to rebroadcast the data received from said second source during the first transmission interval.

According to a fourth exemplary embodiment, are respectively allocated to the first and second sources first and second frequency intervals. During the first and second relay transmission interval receive data from first and second sources respectively, and during the interval following transmission, is allocated to the first and second relays respectively the first interval and the second frequency interval to retransmit y the data respectively received from the first and second sources.

According to a fifth embodiment, are respectively allocated to the first and second sources first and second frequency intervals during a current transmission interval and this allocation is permutated during the next transmission interval. During the current transmission interval is allocated to the first relay the second frequency range to rebroadcast the data it received from the first source to the second frequency range during the previous transmission interval and the second relay the second frequency interval to rebroadcast the data it received from the second source to the first frequency range during the previous transmission interval. During the next transmission interval is allocated to the first relay the first frequency interval to retransmit the data are of the first source it has received during the current transmission interval on the first frequency interval and the second relay second frequency interval to rebroadcast the data from the second source he received during the current transmission interval on the second frequency interval.

According to a sixth embodiment, in each transmission interval, respectively allocated to the first and second sources first and second frequency intervals, and during that same interval, is allocated to the first relay the second frequency interval for there retransmit the data it received from the first source of the first frequency range during the previous transmission interval and the second relay, the second frequency range to rebroadcast the data it has received the second source of the second frequency range during the preceding transmission interval.

BRIEF DESCRIPTION OF DRAWINGS

Other features and advantages of the invention emerge on reading a preferred embodiment of the invention with reference to the accompanying drawings: <ul><li>The <figref idrefs="f0001">Fig. 1</figref> schematically shows a cellular network in cooperation relay known from the state of the art;</li><li>The <figref idrefs="f0002">FIGS. 2A and 2B</figref> respectively represent a resource allocation with and without cooperation relay in two adjacent cells;</li><li>The <figref idrefs="f0002">Fig. 2C</figref> represents an allocation of resources in a relay cooperation configuration in only one of two adjacent cells;</li><li>The <figref idrefs="f0003">Fig. 3A and 3B</figref> schematically illustrate two examples of resource allocation according to the invention for a cellular network having a relay cooperation in only one of the adjacent cells;</li><li>The <figref idrefs="f0004">FIGS. 3C to 3E</figref> schematically illustrate examples of resource allocation according to the invention for a cellular network having a relay cooperation in two adjacent cells.</li></ul>

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

again considering a cellular network of cooperative in which the cooperation strategy uses relays operating in half-duplex mode. And each cell may comprise a set of sources, a set of recipients and a set of relay, the latter may be empty if no relay cooperation is envisaged in the cell in question. When a cell served by a base station, is itself divided into sectors or microcells and that resource allocation is provided in each sector / microcell, we should, without loss of generality, the term "cell "then applies to the basic entity in which is performed the allocation of resources.

Generally, the cellular network uses the transmission resources may be transmission intervals (see TDMA), frequencies (cf. FDMA), frequency intervals (see OFDMA), orthogonal codes ( cf. CDMA) or combinations of such resources.

In a preferred embodiment relating to an OFDMA system, resources are selected as the transmission intervals (<i>timeslots</i>) And frequency intervals, specifically sub-carrier groups (<i>frequency chunks</i>) Of an OFDM multiplex.

An initial idea behind the invention is that the relay cooperation strategy is only effective to the extent that the channel between the source and the relay is only slightly interfered. Indeed, if that channel is interfered, then the decoded broadcast signal (type of relay<i>decode and forward</i>) Or amplified and then retransmitted signal (type of relay <i>amplify and forward</i>) May be of poor quality. It follows that the signal to noise ratio at the receiver level is less than in the absence of cooperation.

A second idea behind the invention is to take advantage of the half-duplex operation of the relevant relay. Specifically when such relay is in the transmission phase on a given resource, for example a frequency band, it can not receive at the same time on this resource. This resource may then be allocated simultaneously to another source in a neighboring cell. Even this allocation would lead interference on the channel between the relay and the receiver, the interference will be less penalizing in terms of signal to noise ratio at the receiver than if it had occurred on the channel between the source and the relay.

The <figref idrefs="f0003">Fig. 3A</figref> shows an example of resource allocation according to the invention in a cellular network with cooperative relay, such as that shown in <figref idrefs="f0001">Fig. 1</figref>. It is assumed that only the cell 110 uses a cooperative relay, ie the relay<i>r</i>₂ is absent or inactive.

During the interval <i>T</i>₁, source <i>s</i>₁ transmits its data on resource δ<i>f</i>₁ and source <i>s</i>₂ transmits its data on resource δ<i>f</i>₂. In the same interval, the relay<i>r</i>₁ receives the data <i>s</i>₁ δ on the resource<i>f</i>₁ and transmits the resource δ<i>f</i>₂ those he has previously received during the previous interval (here <i>T</i>₀). As previously were appointed by<i>s_i</i>(<i>T_j</i>) The data emitted by the source <i>s</i>_i during the interval <i>T_j</i> and <i>r_i</i>(<i>T_j</i>) Data transmitted by the relay <i>r_i</i> during the interval <i>T_j</i>. In half-duplex mode, a relay is capable of receiving a first resource, eg δ<i>f_i</i> and transmit simultaneously on a second resource δ<i>f_j</i>≠ δ<i>f_i</i>. Note that the data transmitted by the relay are not necessarily in a form identical to that of the data it has received. This received data may for example be decoded then re-encoded.

During the interval <i>T</i>₂, Resource allocation is repeated identically and the relay <i>r</i>₁ transmits the resource δ<i>f</i>₂ the data it has received the resource δ<i>f</i>₁ during the interval <i>T</i>₁.

It is thus understood that the channel <i>s</i>₁-<i>r</i>₁ using δ<i>f</i>₁ never interfered with communication between <i>s</i>₂ and <i>d</i>₂ using δ<i>f</i>₂.

The <figref idrefs="f0003">Fig. 3B</figref> Another example of allocation according to the invention for cooperation same configuration as that of the <figref idrefs="f0003">Fig. 3A</figref>.

Unlike the previous allocation scheme, the allocation of resources δ<i>f</i>₁ and δ<i>f</i>₂ is reversed between the first and second intervals. More specifically, during the interval<i>T</i>₂, relay <i>r</i>₁ receives the data <i>s</i>₁ δ on the resource<i>f</i>₂ and transmits the resource δ<i>f</i>₁ the data it has received <i>s</i>₁ δ on the resource<i>f</i>₁ during the interval <i>T</i>₁.

And the relay <i>r</i>₁ receives and transmits alternately on one and the other resource to the difference of the diagram of <figref idrefs="f0003">Fig. 3A</figref>. However, as before, it is noted that the channel<i>s</i>₁-<i>r</i>₁ does not interfere with communication between <i>s</i>₂ and <i>d</i>₂.

The <figref idrefs="f0004">Fig. 3C</figref> illustrates an example of resource allocation according to the invention in a cellular network implementing a relay cooperation in two adjacent cells 110 and 120.

The allocation of resources for the transmission interval <i>T</i>₁ is identical to that shown in <figref idrefs="f0003">FIGS. 3A and 3B</figref>. In other words, the sources<i>s</i>₁ and <i>s</i>₂ transmit their data using the resources respectively δ<i>f</i>₁ and δ<i>f</i>₂And the relay <i>r</i>₁ retransmits on resource δ<i>f</i>₂ the data it has received <i>s</i>₁ during the interval <i>T</i>₀. However, unlike the previous allocation patterns, the relay<i>r</i>₂ passes during the interval <i>T</i>₂ δ on the resource<i>f</i>₁ the data it previously received in the previous interval δ on the resource<i>f</i>₂.

It will thus be understood that the channels <i>s</i>₁-<i>r</i>₁ and <i>s</i>₂-<i>r</i>₂ never interfered. However, cooperation is only effective once on two insofar as the relays are in a reception interval in two. In the illustrated case, the relay<i>r</i>₁ is not received during the interval <i>T</i>₂ but only during the interval <i>T</i>₁. Similarly, the relay<i>r</i>₂ is not received during the interval <i>T</i>₁ but only during the interval <i>T</i>₂.

According to a variant not shown, the relay <i>r</i>₁ and <i>r</i>₂ are permanently in reception, respectively on the resources δ<i>f</i>₁ and δ<i>f</i>₂. During the interval<i>T</i>₁, relay <i>r</i>₁ transmits the resource δ<i>f</i>₂ the previously received data <i>s</i>₁ during intervals <i>T</i>_-1 and <i>T</i>₀. Similarly, during the interval<i>T</i>₂ relay <i>r</i>₂ transmits the resource δ<i>f</i>₁ previously received data during intervals of <i>s</i>₂ during intervals <i>T</i>₀ and <i>T</i>₁. Note, however, that during the interval<i>T</i>₁, the canal <i>s</i>₂-<i>r</i>₂ is interfered by retransmission <i>r</i>₁ and during the interval <i>T</i>₂ the canal <i>s</i>₁-<i>r</i>₁ is interfered by retransmission <i>r</i>₂. In addition, the data must be retransmitted by the relay at a rate two times higher than the flow rate of the sources.

The <figref idrefs="f0004">Fig. 3D</figref> shows another example of resource allocation according to the invention. The relay by cooperation of situation is the same as that envisaged in<figref idrefs="f0004">Fig. 3C</figref>.

During the first transmission interval <i>T</i>₁, source <i>s</i>₁ transmits data using the resource δ<i>f</i>₁ and source <i>s</i>₂ transmits data using the resource δ<i>f</i>₂. The relays<i>r</i>₁ and <i>r</i>₂ receive data respectively <i>s</i>₁ and <i>s</i>₂ while <i>T</i>₁ and retransmit them for <i>T</i>₂ δ on resources<i>f</i>₁ and δ<i>f</i>₂ respectively. Sources continue to transmit their data on the same resources in the second transmission interval.

And channels <i>s</i>₁-<i>r</i>₁ and <i>s</i>₂-<i>r</i>₂ are not interfered nor for <i>T</i>₁ or during <i>T</i>₂.

The <figref idrefs="f0004">Fig. 3E</figref> shows a final example of resource allocation according to the invention. The relay cooperation situation is still the same as that envisaged in<figref idrefs="f0004">Fig. 3C</figref>.

During the transmission interval <i>T</i>₁ the sources <i>s</i>₁ and <i>s</i>₂ transmit their data resources δ<i>f</i>₁ and δ<i>f</i>₂. Relay<i>r</i>₁ is in receipt of data <i>s</i>₁ δ on the resource<i>f</i>₁ and retransmits on resource δ<i>f</i>₂ those previously received from this source during the interval <i>T</i>₀. Similarly, the relay<i>r</i>₂ receives the data <i>s</i>₂ δ on the resource<i>f</i>₂ during this interval and transmits the resource δ<i>f</i>₁ those previously received therefrom during the interval <i>T</i>₀.

During the interval <i>T</i>₂ resource allocation is reversed for both sources <i>s</i>₁ and <i>s</i>₂ as for relay <i>r</i>₁ and <i>r</i>₂. In other words, the sources<i>s</i>₁ and <i>s</i>₂ transmit their data resources δ<i>f</i>₂ and δ<i>f</i>₁. Relay<i>r</i>₁ receives the data <i>s</i>₁ δ on the resource<i>f</i>₂ and retransmits on resource δ<i>f</i>₁ those received previously during the interval <i>T</i>₁. Similarly, the relay<i>r</i>₂ receives the data <i>s</i>₂ δ on the resource<i>f</i>₁ and retransmits on resource δ<i>f</i>₂ those received previously during the interval <i>T</i>₁.

During the interval <i>T</i>₁ or interval <i>T</i>₂ the canal <i>s</i>₁-<i>r</i>₁ is interfered by retransmission <i>r</i>₂ and channel <i>s</i>₂-<i>r</i>₂ is interfered by retransmission <i>r</i>₁.

According to a variant not shown of the example illustrated in <figref idrefs="f0004">Fig. 3E</figref>The allocation of resources during the second interval is chosen identical to the first interval.

In all the above examples, half-duplex mode it operates a relay belonging to a cell, by allocating to a source of a neighboring cell the resource used by the relay during the same transmission interval.

Although the present invention has been previously described in relation to two neighboring cells, the skilled person will understand that it may extend without difficulty to any number of such cells. Similarly, although the present invention has been illustrated using two transmission intervals and more generally of two transmission resources for two sources, those skilled in the art will understand that it applies generally to any number of transmission resources for a plurality of sources.