WO2018050995A1

WO2018050995A1 - Method for coordinated multipoint transmission with gain increase constraint, corresponding program products and device

Info

Publication number: WO2018050995A1
Application number: PCT/FR2017/052404
Authority: WO
Inventors: Nivine ABBAS; Berna Sayrac
Original assignee: Orange
Priority date: 2016-09-15
Filing date: 2017-09-11
Publication date: 2018-03-22
Also published as: FR3056065A1

Abstract

The invention relates to a method (1) for downlink transmission between at least one cell (§ = {S₁, S₂,..., ^SK_max-1}) of an access network and a terminal. The terminal is served by a server cell of the access network and benefits from a throughput (r) in the absence of cooperation from another cell of the access network. The method determines (5) a throughput gain of cells that are candidates for the cooperation in order to determine (6) the cooperating cells (C).

Description

CoMP transmission method with gain increase constraint, corresponding program products and device

Field of the invention

In general, the present invention relates to the field of telecommunications. More specifically, the field of the invention is that of transmission in a cellular type access network with CoMP function.

Prior art

A cellular access network has a topology which is conventionally represented schematically in FIG. 1. The topology of the AN access network comprises CE cells, CEs, CEc. Each cell is associated with a base station BS geographically located in one site. The cells have a certain radio range schematized by a hexagon. A base station may be multi cellular when it is equipped with several transmit antennas as shown in Figure 2 or when using a multi sectoral antenna. The range of such a cell is shown schematically by the third of a hexagon in FIG.

A mobile terminal UE or terminal, sometimes called user equipment or user, can be served by a cell if the power it receives from this cell exceeds a certain threshold, the terminal is said to be covered by the cell or within the scope of the cell. When the terminal is covered by multiple cells, the strongest received signal generally determines the server cell.

By taking up the terminology associated with the representation of FIG. 1, a terminal near the center of a cell (i.e close to a base station) will generally benefit from a high received power level emitted by this cell.

A cell is called a waitress if the terminal has a communication established via this cell. The selection of a server cell according to LTE release 8 is based on the average power received from reference signals (RSRP: Signal Received Power Reference). The cell with the strongest RSRP power has the role of waiter cell. A terminal may receive signals from other cells that are not its server cell. These signals are called interfering, they scramble the signal coming from its server cell increasing the noise received by this terminal. The noise received by a terminal limits the rate at which this terminal can benefit and therefore has a negative impact on the capacity of the access network.

When the terminal is located in boundary or cell edge, the phenomenon of interference is particularly important. Indeed, the terminal is more capable of receiving signals of power levels close to each other from different cells than a terminal near the center of the cell. The quality of the signal received by the terminal transmitted by its server cell is degraded by the interference thus limiting the rate at which the terminal can benefit.

The work within 3GPP within the framework of the LTE-advanced standard has led to the determination of coordinated transmissions between points of an access network according to a CoMP (Coordinated Multipoint) technique. According to these CoMP transmissions, a coordination occurs between several points of a group of points (cluster) called coordination to eliminate the interference that can receive a particular terminal at the edge of the cell or to transform the interference received by the terminal into a signal useful for this terminal. A terminal served by the cooperation of two or more cells is defined as a CoMP terminal. In this document, the term cooperation is used in the same sense as coordination.

The coordination groups can be initially defined by the operator of the access network and / or they can be parameterized for example according to indicators sent by the mobile terminals (CQI, PMI, RI). Optionally, the operator defines subgroups within the groups. The several points called cooperating points or cooperating cells can belong to a subgroup of a coordination group. A terminal measures the signal received from its server cell and other cells that may be interfering with it. To limit the return of indicators from the terminal to its server cell and to other cells via the backhaul, the cooperating cells may be limited to a subgroup of the coordination group defined by the server cell.

Figure 3 illustrates an access network with several coordination groups, each group being identified by a gray level.

According to [1], a point is a set of geographically co-located antennas but the sectors of the same site correspond to different points. A point defines a single cell.

There are several different CoMP transmission schemes.

A first so-called joint processing scheme (JP) covers the so-called transmission transmission (JT) transmissions, dynamic point selection (DPS) transmissions and so-called dynamic point blanking (DPB) transmissions. JT transmissions and DPS transmissions are described by [2] and illustrated by Figures 4a and 4b of [2]. According to the transmissions JT at least two cooperating cells, a server cell and another cooperating cell transmit simultaneously to serve the same user. According to DPS transmissions, a single cell among the cells of a coordination group transmits at a given time. On the other hand, dynamic switching occurs between the server cell to which the user is associated at a given instant and a so-called cooperating cell among the cells of the coordination group. Switching occurs to the coordination group cell that has the best radio link. According to the DPB transmissions, at least one cooperating cell of the coordination group is silenced while a server cell serves a user to limit the interference that affects the user.

A second scheme illustrated in Figure 4c of [2] is called coordinated scheduling / beamforming (CS / CB). According to this scheme, the data for a user is transmitted by a single cell among the cells of a coordination group. The scheduling decisions and the beam are coordinated between the cells of the coordination group to control the generated interference.

The invention originates in the context of JT transmissions according to a downlink CoMP mechanism. The transmission pattern of the JT transmissions requires the determination of at least one cooperating cell in addition to the server cell among the cells identified by their membership in a coordination group (cluster). However, the invention can be applied to DPB transmissions.

According to [3], the choice of the waiter cell defines the coordination group since a cell belongs to one and only one coordination group. According to the authors, when the difference between the average received power (RSRP) of a neighboring cell P _c and the average power received from the server cell P _s is less than a certain fixed threshold ΔΡ predefined in advance (ex 3dB, 12dB), the neighboring cell is defined as a cooperating cell for the terminal served by this server cell:

\ P _S - P _C \ <AP (1).

The subgroup of cooperating cells for the terminal comprises the cells whose average power received satisfies the relation (1). The subgroup of cooperating cells vis-à-vis the terminal is thus updated over time based on the power measurements made by the terminal. The implementation of the relation (1) requires overcoming the difficulty associated with determining the optimal value of the threshold ΔΡ which depends on several factors: the scenario and the level of interference in the network (for example the system with the formation of beamforming with which the level of interference is low by construction, system without channel formation for which the level of interference can be high), homogeneous network, HetNet network where small cells are deployed), the transmission scheme, the determination of the coordination groups, etc.

However, JT transmissions, and more generally CoMP type JT or DPB transmissions have the disadvantage of excessive consumption of radio resources due to the fact that several radio resources (of different cells) are dedicated to the same user. As a result, if the throughput gain of terminals consuming multiple resources is not large enough to compensate for the losses due to this excessive consumption of resources, performance is degraded. The ability of the system to handle all traffic, especially when the network is heavily loaded, is then impaired. This can degrade the stability condition of the system.

Main features of the invention

The subject of the invention is a downlink transmission method between at least one cell of an access network and a terminal, the terminal served by a cell of the so-called server access network benefiting from a bit rate r the lack of cooperation of other cells of the access network during transmission. The method comprises:

initializing an index j to zero, a rate r ₀ 'at the rate r, a set C of cells cooperating with the empty set,

for each cell called current cell of index i = [1, ..., K _max -1] among K _max - 1 cells of the access network said to be candidates for cooperation with the server cell during transmission to the terminal, taken by decreasing value of power received by the terminal, β ^ι being a gain at the rate of the terminal provided by the cooperation of the cell of index i and β \ being a determined constraint relative to the gain β ^ι , if the difference in flow rate relative to the flow rate r between the flow rate r [obtained with the cooperation of all the candidate cells up to the current cell of index i and the flow rate rj obtained with the cooperation of the cells included in the set C, j being the highest index of the cells included in the set C, is greater than the sum of the constraints then include all the cells

of indices k = [j + 1, ..., i] in the set C.

Thus, the transmission method determines a set C of cooperating cells from cells of the access network to cooperate during transmission between the server cell and the terminal. According to a first mode, this cooperation can be manifested by a simultaneous transmission with the server cell to the terminal in the case of a transmission according to a JT scheme. According to another mode, this cooperation can be manifested by a silence imposed on the cooperating cells (extinction of the cells) during the transmission between the server cell and the terminal in the case of a transmission according to a DPB scheme.

For each candidate cell, for example ordered in a list, taken by decreasing power level, the method examines its contribution to the cooperation rate of the cooperating cells relative to the rate obtained with the waiter cell alone i.e. without cooperation. If the contribution on the cooperation rate is greater than a sum of constraints relating to the cells whose index goes from that of the current cell to that plus one of the last selected cell included in the set then the method selects all the cells up to the current cell to be cooperating cells and includes them in the set. Each constraint reflects the increase in gain at the cooperation rate that the cooperation of a cell must bring to be selected and be included in the set of cooperating cells.

Considering a symmetric topology of the access network, each gain increase β ¹ i = [1, ..., K _max - 1] must be 100%. Each of the constraints βίρ is then determined with a value greater than or equal to 100%. The current cell may not bring sufficient gain alone. However, it can be selected when examining the next current cell if its cooperation with this cell brings a gain of at least 200%. Such a situation may occur when the neighboring cells of the server cell cause interference to the signal received by the terminal of levels comparable to each other. The cooperation of only one of the two cells does not bring sufficient gain, on the other hand the cooperation of the two cells can bring a sufficient significant gain. The terminal can then be served simultaneously by the cooperation of the selected cells and the server cell. The terminal benefiting from cooperating cells is called CoMP.

In the case of an access network forming a non-symmetric network topology, the gain increase constraint value βίρ depends on the cell i, the stress values β ^ may not all be identical to each other.

According to one embodiment, the cells of the access network are grouped in different groups, the membership of the server cell to one of the groups identifies this group as grouping the K _max - 1 candidate cells.

In this mode, groups (clusters) are identified, for example by the operator of the access network. Thus, when a terminal is served by a cell belonging to one of the groups, the cells that can be selected to cooperate with this waiter cell are at most in the same group. This mode may have the advantage of limiting the return of indicators between the terminal and the cell waitresses and limit exchanges between waiter cells and candidates.

According to one embodiment, the increase constraint values of

gain of the candidate cells are all identical to each other.

This mode is more particularly adapted to a set of candidate cells considered as forming a symmetrical network topology.

According to one embodiment, the method further comprises the reception by the terminal of the value of gain increase constraint of each cell.

candidate for the index i.

This mode is particularly suitable for an implementation of the method by the terminal. Each candidate cell makes measurements to determine the value of a traffic increase factor at the edge of its cell. The cell then determines a value of the gain increase constraint that is greater than the increase factor

measured over a given period and transmits it to the terminal that receives it. This transmission can occur via a backhaul linking the different cells including the waiter cell. The waiter cell then transmits the constraints to the terminal.

According to one embodiment, the method further comprises the reception by the server cell of indicators on the power values received by the terminal from the candidate cells of index i as well as on a residual interference value. The indicators are therefore transmitted by the terminal to its waiter cell.

This embodiment is more particularly adapted to an implementation of the method by the terminal server cell.

According to one embodiment, the method further comprises the reception by the server cell of the gain enhancement constraint value β of each candidate cell of index i. Each candidate cell of index i thus transmits to the server cell the value β of gain increase constraint.

Each candidate cell makes measurements to determine the value of a traffic increase factor at the edge of its cell. The candidate cell then determines a value of the gain increase constraint that is greater than the factor

determined over a period of time and forwarded it to the waiter cell. The invention furthermore relates to a mobile terminal served in a downlink by a cell of a so-called waitress access network and which benefits from a bit rate r in the absence of cooperation from another cell of the access network. during transmission between the cell and the terminal. The terminal includes:

means for initializing an index j to zero, a rate r ₀ 'at the rate r, of a set C of cells cooperating with the empty set,

a suitable calculation means such that, for each cell called current cell of index i = [1,..., K _max -1] among K _max - 1 cells of the access network said to be candidates for cooperation with the server cell during transmission to the terminal, taken by decreasing value of power received by the terminal, β ^ι being a gain in the flow rate of the terminal provided by the cooperation of the cell of index i and being a determined constraint relative to the gain β ^ι , if the difference in flow rate relative to the flow r between the flow rate r [obtained with the cooperation of all the candidate cells up to the current cell of index i and the flow rate rj obtained with the cooperation of the cells included in the set C, j being the highest index of the cells included in set C, is greater than the sum of the constraints

then include all the cells of indices k = [j + 1, ..., i] in the set C.

List of Figures

Other characteristics and advantages of the invention will emerge more clearly on reading the following description of embodiments, given as simple illustrative and non-limiting examples, and the appended drawings, among which:

FIG. 1 is a diagram illustrating the basic topology of a cellular access network according to the state of the art,

FIG. 2 is a diagram showing a basic topology of a cellular access network with multi-sector antenna base stations defining multi-cellular base stations according to the state of the art,

FIG. 3 is a diagram of an access network with several coordination groups, each group being identified by a gray level according to the state of the art,

FIG. 4 is a flowchart of a method according to the invention,

FIG. 5 is a model for modeling a cell by a queue according to the state of the art, FIG. 6 is a diagram illustrating a homogeneous access network with hexagonal topology and intra-site coordination,

FIG. 7 is a diagram illustrating, for the network of FIG. 6, a coordination zone 1.2 with terminals that require the coordination of two cells, FIG. 8 is a diagram illustrating, for the network of FIG. a coordination zone 1, 2 with terminals which require the coordination of two cells and a coordination zone a, b, c with terminals which require the coordination of three cells,

FIG. 9 is a diagram illustrating a homogeneous access network with a hexagonal topology and the coordination of a central cell with its neighbors, for example according to an inter-site coordination,

FIG. 10 illustrates for a cell the distribution of its radio resources without CoMP cooperation and with CoMP cooperation,

FIG. 11 cross-hatched the values that verify the inequality

C ²

υ≤-, with δΡ = 10 log ₁₀ ζ,

FIG. 12 represents the function F according to the 3GPP LTE standard for a SISO channel,

FIG. 13 is a diagram of a non-homogeneous access network, not perfectly symmetrical,

FIG. 14 is a curve illustrating the evolution in time of a K factor for increasing traffic at the edge of the cell,

Figure 15 is a diagram of a terminal according to the invention.

Description of particular embodiments

The context of the invention is that of an access network comprising several cells. These cells are grouped in different groups (cluster) as shown in FIG. 3. Each cell CE, CEs, CEc is associated with a base station BS of the access network AN. A base station can be associated with multiple cells. As illustrated in FIG. 1, UE terminals can be at the edge of cells CE, CEs, CEc.

The transmission method according to the invention relates to the downstream path. The transmission takes place between at least one cell of the access network AN and a terminal UE. The terminal is served by the CEs cell of the so-called waitress access network and benefits from a flow r in the absence of cooperation of another CEc cell, CE of the access network when of the transmission. The access network comprises a maximum number K _max of cooperating cells including the server cell for a given user.

A flowchart of the process is shown in FIG. 4. The method 1 according to the invention comprises an initialization 2 and a determination 3 of cooperating cells CEc by successively examining the impact of a cell on the cooperation.

A cell can be modeled by a queue where users arrive in a random process at the average arrival rate λ as shown in Figure 5. Users are served according to the PS (processor sharing) service discipline. they share the resources of the cell equitably. They are served at the average global service rate μ.

The cell load is defined as the ratio of the user arrival rate to the average overall service rate of the cell, ie:

To guarantee the stability of the cell, the arrival rate must be lower than the service rate:

The stability condition of the cell is then as follows:

The radio conditions in a cell are heterogeneous. Users can have different average rates of service depending on their radio conditions (evaluated by the average SINR which takes into account transmission losses and shadowing) i = 1, ... M. by μ _; the average rate of service associated with the radio condition i and by p _t the percentage of the users benefiting from a radio condition i. The charge of the cell can thus be expressed as follows:

The global average service rate μ is nothing other than the harmonic average of the different average service rates:

Consider the case where the cell is divided into two zones of different radio conditions: the cell center area and the cell edge area. The charge of the cell is thus:

Consider that the access network is homogeneous with a hexagonal topology so symmetrical.

For example, in the case of an intra-site cooperation where the cells of the same site cooperate with each other, the hatched area in FIG. 6 corresponds to the zone of cooperation. On this zone the transmissions to the users of zone 1 and zone 2 of FIG. 7 require the cooperation of several cells to be served. If we consider the right-hand IEC cell of FIG. 7, the users of zone 1 are associated with this IEC cell and require the cooperation of neighboring cells. Zone 2 users are associated with neighboring cells and require the cooperation of this IEC cell. These users in zone 2 create an additional load on the IEC cell since they are additional users for this IEC cell, unlike users located in zone 1 who are part of its own users.

For the IEC cell, bit / s edge traffic in zone 1 (edge)

increases by a factor K, the arrival rate at the border ie for the zone encompassing the zone 1 and the zone 2 then becomes

by several cells (two according to the illustration) benefit from a gain of flow β (gain of cooperation) which increases their rate of service which becomes

The new load of the cell becomes the following:

boy Wut

Given that the topology of the network is symmetrical and according to the number of cooperating cells considered in the case illustrated then K = 2.

To ensure that there is no degradation of the stability of the IEC cell, the new charge p 'of the IEC cell must be less than its load p in the absence of coordination:

p'≤p (9)

It is therefore necessary that:

β≥Κ (10) and therefore that:

β≥ 2 (11) In other words, a cell cooperates only when its cooperation brings at least an average gain of 100%. This implies a coordination gain constraint β _τ = 100%.

Inequality (11) gives the condition of stability in the absence of CoMP for a symmetric topology.

According to the illustration of Figure 8, there is a border zone a, b, c around the point common to the three hexagons for which three cells cooperate for the terminals that are present there. The users in the area referenced b and c create an additional load to the IEC cell since, although they are not associated with this cell, they benefit from the cooperation (ie transmission) of this cell. The load of the IEC cell in the border area can be expressed as:

is the percentage of users in the zone referenced 1 outside zone a,

ie users located on the edge that require the cooperation of two cells. is the service rate of these users when served without cooperation.

is the percentage of users in the border area a,, ie users

located on the border which requires the cooperation of three cells. is the service rate of these users when served without cooperation.

Border traffic in Zone 1 outside the zone increased by a factor of K ₂ .

However, these edge users served by two cells benefit from an average flow rate gain β ₂ . In addition, the edge traffic in zone a increases by a factor K ₃ .

However, these edge users served by three cells benefit from an average flow rate gain β ₃ . The edge load given by equation (12) thus becomes:

Taking into account the edge load given by equation (13), the new load of the cell then becomes:

The topology of the access network being symmetrical, then K ₂ = 2 and K ₃ = 3 in the illustrated case. The guarantee of a lack of degradation of stability requires that:

This condition is verified if:

The inequalities (16) give the condition of stability of the server cell in the presence of CoMP for a symmetrical topology.

In other words, a cell cooperates only when its cooperation brings at least an average gain of 100% throughput. Let β _{τ be} the coordination gain constraint then β _τ = 100%.

In the case illustrated in FIG. 9, the conditions (16) above remain true. According to an example shown in Figure 10, a cell can devote 3/5 of its radio resources to users of neighboring cells in a CoMP case, or 6/7 resources it allocated to its users edge (cell-edge) in the case without CoMP without losing its stability. The condition for maintaining stability is that the cell's commitment to CoMP cooperation with its neighboring cells improves at least seven times the throughput of its edge users.

With reference to FIG. 1, P _s is the power received by the terminal UE coming from the server cell CEs and P _c is the power received from the cooperating cell CEc. / is the total interference perceived by the user from all neighboring cells CE except for the cooperating cell CEc and N is the noise. The signal to interference plus noise ratio, SINR (Signal to noise and interference ratio), perceived by the user in the absence of cooperation is:

When the cell CEc cooperates with the waiter cell CEs, the SINR becomes:

To maintain the stability condition corresponding to the inequalities (16) in the case of cooperation, the new flow rate in the presence of cooperation must be at least twice as large as the original flow in the absence of cooperation, ie a relative flow gain due. at coordination of at least 100%.

The flow rate r of which the user can benefit when it is served without cooperation can be expressed in the form:

with F an increasing function which makes it possible to obtain the flow from the SINR.

In the case of a Gaussian channel, the classical F function is given by Shannon's ability:

where W is the bandwidth.

Given the expression (19) of the rate and the expression (20) of the function F, then an increase of at least 100% of the rate due to the gain provided by the coordination implies an increase of at least 100% of log ₂ (l + SINR):

By replacing SINR and SINR 'by their expression:

The shaded area of Figure 11 represents the solution to this inequation (27).

This hatched area illustrates the fact that there is not a single value of δΡ which makes it possible to guarantee that the cooperating cell brings a gain of 100%. This figure emphasizes that the increase in flow due to the gain provided by the coordination depends on the δΡ and in addition on the residual interference.

The higher residual interference I + N, the greater the power received from the cooperating cell P _c must be close to the power P _s (δΡ smaller) to meet the guarantee of a gain of 100% flow.

Therefore, a CEc cell should not be co-operated if the UE user is located in an area that is heavily interfered with by one or more other cells THIS. In this case, it is preferable that all the interfering cells (including the CEc cell) cooperate or that none of them co-operate.

Maintaining the stability of the server cell and therefore the access network requires that a cooperating cell contributes to a flow rate increase of at least 100% in the case of a symmetrical topology.

Unlike the known techniques which define a cooperating cell based on the difference between the received powers and which require to predefine a threshold δΡ, the method according to the invention is based on the gain in flow to determine the cooperating cells.

^is the set of candidate cells to become cells

cooperating with the waiter cell during transmission for a given user. K _max is the maximum number of cooperating cells for a single user including his waiter cell.

(19) gives the expression of the flow rate r of which the user can benefit when it is served without cooperation.

The flow rate that the user can benefit when the first n cells of

'together

cooperate in the transmission towards the user has for expression:

The 3GPP LTE standard defines the F function according to the CQI quality indicator tables. This function has the form illustrated in Figure 12, for a SISO channel.

Generally the access network is not perfectly symmetrical, it may include shadowing and it can be heterogeneous as shown in Figure 13. The network heterogeneity may originate in a non-homogeneous distribution traffic, unsymmetrical topology, or channel variation over time. In that case :

K ₂ ≠ 2 and K ₃ ≠ 3 and then β _τ ≠ 100%.

Given the load (8) and the stability condition (10) of the server cell then the server cell and each candidate cell must make measurements to determine the correct value of the edge traffic increase factor K.

K can be determined based on the exponential time average whose rate is shown in Figure 14 with a time constant t _c . The temporal average at the instant ta for expression:

being the traffic (ie the amount of data downloaded in bits / s) associated with the

all CoMP users involving cell j in the transmission,

being the traffic associated with the CoMP users associated with the cell j (thus served by) and requesting the cooperation of the neighboring cells.

Similarly, the average coordination gain can be evaluated through a

exponential mean with a time constant t _c :

where is the average rate gain of CoMP users involving cell j

in the transmission.

A coordination gain constraint of cell j must be determined from

to ensure that:

(31)

is the constraint that must be put on the CoMP users of the cells

neighbors that need to be served by cell j, for example

being a given offset).

At each period T (of the order of a few seconds), each cell j updates the constraint based on the measurements made during this period. The values of

are exchanged between the candidate cells and the cooperating cells forming part of the same group. This exchange can take place via the X2 interface with reference to the infrastructure defined in the LTE standard, in particular in the specification document [4].

When the additional charge caused to a cell j exceeds the gain provided by its cooperation with its neighboring cells, the cell j increases the constraint

thus decreasing its commitments towards its neighboring cells. It is thus less involved in transmissions to users associated with other cells. On the other hand, in the case where the cooperation of this cell with other cells gives it a sufficiently large gain that compensates for the additional load that is caused to it, the cell can decrease the constraint to accept more CoMP users of

outside.

The course of the process according to a first embodiment of the invention illustrated by Figure 4 is the following. This first embodiment of the method according to the invention is expressed in pseudo language C in Appendix A.

During the initialization phase 2 of the method according to the invention, the index j is initialized to zero, the index i is initialized to one, the rate r ₀ 'is initialized to the rate r and the set C of cells cooperating is initialized to the empty set.

is a constraint determined to guarantee a gain β ^ι at the flow rate. ] ^is the set of

candidate cells to become cooperating cells with the server cell during transmission for a given user. K _max is a parameter that can be set in advance.

The method according to the invention takes, by decreasing power value received by the terminal, each 4 cell called current cell index i = [1, ..., K _max - 1] among the K _max - 1 candidate cells. So each candidate cell is taken from the cells of the set

j being the highest index of the cells included in the set C, if the flow difference relative to the flowrate r between the flow obtained with the cooperation of

all the candidate cells up to the current cell of index i and the rate rj obtained with the cooperation of all the cells included in the set C is greater than the sum of the constraints then the process includes all the

index cells in the set C and the value of the index j is

forced to the value of the index i.

Before moving on to the next cell of the set S, increment 7 by one index i.

The previous mode considers that the cells of the set § are classified in order of decreasing power since the index i is incremented. It is immediate for a person skilled in the art to adapt the process to a classification in ascending order. It is likewise immediate to adapt the method to replace the index management i of step 7 by a test on the power level to determine the next cell to be considered in step 4.

According to a second embodiment, the topology of the candidate cells is considered to be symmetrical. In this case, the constraints β \ are all identical to β _τ which must be at least 100% to ensure the stability of the server cell expressed in the form of inequation (15). The method according to this mode is expressed in pseudo language C in Annex A. It is identical to the method according to the first mode except that the inequality

Thus, a cell next to a server cell cooperates with the transmission to a user only if its cooperation increases the average throughput of the user by a factor of at least two.

The gain provided by the cooperation of a cell depends on the level of interference. For example, a user may be strongly interfered with by two neighboring cells. The cooperation of one of the two cells does not bring a gain of at least 100%. However, the cooperation of the two cells can bring a gain of at least 200%, which satisfies the requirement of stability required. Such a scenario can occur when neighboring cells cause similar or similar levels of interference. In this case, there is no gain to cooperate only one of the two cells in the transmission. Either the two cells cooperate with the waiter cell, or none of them. In the example where K _max = 3, the method according to the invention verifies if s- brings at least a gain of β _τ = 100%. If this is the case, s ₁ is defined as a cooperating cell for the user. The method then verifies whether s ₂ brings at least β _τ = 100% additional gain. In the case where s ₁ does not contribute to a gain of flow β _τ = 100%, the method verifies whether the cooperation of s ₁ and s ₂ can provide 2β _τ = 200% of flow gain. In the case where the latter condition is not verified, neither of the two cells is retained as a cooperating cell.

FIG. 15 schematizes a mobile terminal according to the invention. The UE terminal is served in a downlink by a cell of a so-called waitress access network. The terminal benefits from a flow r in the absence of cooperation of other cell of the access network during the transmission between the cell and the terminal. The terminal comprises an emission-receiving interface E / R known in itself, a memory MEM comprising a buffer memory, a suitable calculation means equipped for example with a microprocessor μ and controlled by the computer program Pg stored on a support of information such as a memory zone for implementing the transmission method according to one embodiment of the invention.

At initialization, the code instructions of the computer program Pg are for example loaded into the memory MEM before being executed by the processor of the processing unit μΡ. The microprocessor of the processing unit μΡ implements the steps of the transmission method described above, according to the instructions of the computer program Pg.

Thus, the microprocessor initializes a subscript j to zero, a rate r _Q 'at the rate r, and a set C of cells cooperating with the empty set.

The means μΡ of computation considers each cell called current cell of index i = [1, ..., K _max - 1] among K _max - 1 cells of the access network said to be candidates for cooperation with the waiter cell during the transmission to the terminal, taken by decreasing value of power received by the terminal, β ^ι being a gain at the rate of the terminal provided by the cooperation of the cell of index i and β being a determined constraint relating to the gain β ^ι . If the difference in flow rate relative to the flow rate r between the flow rate r obtained with the cooperation of all the candidate cells up to the current cell of index i and the flow rate r obtained with the cooperation of the cells included in the set C, j being the highest index of the cells included in the set C, is greater than the sum of the constraints then the means of

calculation includes all cells of indices k = [f + 1, ..., i] in set C.

In the document terms are used according to the Anglo-Saxon terminology used in the field.

References cited in the document:

[1]: TR 36.819 vi l.1.0 (2011-12) published by the 3GPP

[2]: "Coordinated multipoint transmission / reception techniques for LTE-advanced" by Mamoru Sawahashi, Yoshihisa Kishiyama, Akihito Morimoto, Daisuke Nishikawa and Motohiro Tanno, NTT Docomo, IEEE Wireless Communications, June 2010

[3]: Stefan Brueck, Lu Zhao, Jochen Giese and Awais Amin, "Centralized scheduling for joint transmission coordinated multi-point in LTE-advanced", Qualcomm CDMA technologies, IEEE 2010 International ITG Workshop on smart antennas

[4]: 3GPP TS 36300 vl3.4.0 (2016-06) Annex A

Conducted procedure of the pseudo-language method C according to a first embodiment:

Input data: a set of candidate cells

Output data: a set

cooperating cells

Conducted procedure of the pseudo-language method C according to a second embodiment:

Input data: a set of candidate cells

Output data: a set

cooperating cells

Claims

CLAIMS 1. Method (1) for downlink transmission between at least one cell (CE, CEs, CEc) of an access network (AN) and a terminal (UE), the terminal served by a cell (CEs) the so-called server access network having a rate r in the absence of cooperation of other cells of the access network during transmission, characterized in that the method comprises:

initialize (2) an index j to zero, a rate r ₀ 'at the rate r, a set C of cells cooperating with the empty set,

for each cell (4) called current cell of index i = [1, -, K _max - 1] among K _max - 1 cells of the access network said to be candidates for cooperation with the server cell during transmission to the terminal, taken by decreasing value of power received by the terminal, β ^ι being a gain at the rate of the terminal provided by the cooperation of the cell of index i and being a constraint determined

relative to the gain β ^ι , if (5) the flow difference with respect to the flow r between the flow rate obtained with the cooperation of all the candidate cells up to the current cell of index i and the flow obtained with

the cooperation of cells included in the set

j being the highest index of the cells included in the set

is greater than the sum of the constraints then include (6) all

the cells of indices k = [j + 1, ..., i] in the set C.

2. Method (1) according to claim 1, wherein the cells of the access network are grouped in different groups, the membership of the server cell to one of the groups identifies this group as grouping the K _max - 1 candidate cells. .

3. Method (1) according to one of claims 1 and 2, wherein the gain increase stress values of the candidate cells are all identical.

between them.

4. Method (1) according to one of claims 1 to 2, further comprising the reception by the terminal of the gain increase constraint value of each candidate cell index i.

5. Method (1) according to one of claims 1 to 3, further comprising the transmission of the terminal (UE) to its server cell (CEs) of indicators on the power values received from the candidate cells of index i and only on a residual interference value.

6. Method (1) according to the preceding claim, further comprising transmitting each candidate cell index i to the server cell the value of

gain increase constraint.

7. Method (1) according to one of claims 1 to 3, further comprising the reception by the server cell (CEs) of indicators on the power values received by the terminal (UE) from the index candidate cells. i as well as on a residual interference value.

8. Method (1) according to the preceding claim, further comprising the reception by the server cell of the value of gain increase constraint of each

Candidate cell with index i.

9. Mobile terminal (UE) served in a downlink by a cell (CEs) of an access network (AN) called a waitress, the terminal benefiting from a flow r in the absence of cooperation from another cell (CE , CEc) of the access network during transmission between the server cell and the terminal, characterized in that the terminal comprises: means (μΡ) for initializing an index j to zero, a flow rate r, of a

set C of cells cooperating with the empty set,

means (μΡ) of adapted calculation such that, for each cell called current cell of index i = [1, ..., K _max - 1] among K _max - 1 cells of the access network said to be candidates for cooperation with the server cell during transmission to the terminal, taken by decreasing value of power received by the terminal, β ^ι being a gain in the rate of the terminal benefit provided by the cooperation of the cell of index i and β \ being a determined constraint relative to the gain β ^ι , if the flow difference relative to the flow r between the flow obtained with the

cooperation of all the candidate cells up to the current cell of index i and the rate rj obtained with the cooperation of the cells included in the set

being the highest index of the cells included in set C, is larger that the sum of the constraints then include all the

cells of indices k = [j + 1, ..., i] in the set

10. Computer program on an information carrier, said program comprising program instructions adapted to the implementation of a transmission method according to any one of claims 1 to 5, when said program is loaded and executed in a terminal intended to implement the transmission method.

A computer program on an information medium, said program comprising program instructions adapted to the implementation of a transmission method according to any one of claims 1 to 3, 7 and 8, when said program is loaded and executed in a server cell for implementing the transmission method.

12. Information carrier comprising program instructions adapted to the implementation of a transmission method according to any one of claims 1 to 5, when said program is loaded and executed in a terminal intended to implement the transmission method.

13. Information carrier comprising program instructions adapted to the implementation of a transmission method according to any one of claims 1 to 3, 7 and 8, when said program is loaded and executed in a server cell intended to implement the transmission method.