CoMP METHOD

The present invention generally relates to multi-cell coordinated multipoint transmission termed or CoMP in a cellular network.

In order to improve the performance and quality of experience of mobile users in a cellular network such as a LTE (Long Term Evolution) network defined in UMTS (Universal Mobile Telecommunications System) standard, when the mobiles users access and share high-bandwidth services, it is known ti use a multi-cell coordinated multipoint transmission or CoMP method.

Such a CoMP method relies on the use of the MIMO (Multiple Input Multiple Output) technique that allows reducing interference in the cellular network and increase efficiency by coordinating the transmission of radio signals between the different nodes of the cellular network.

Different transmission schemes are considered when running CoMP method, one of them is called dynamic cell selection or DCS. The DCS transmission scheme is a Joint Processing method.

In the DCS method, a resource block is transmitted to a mobile device attached to a first access node of the LTE network belonging to a group of coordinated cells by a second access node of the LTE network covering a cell belonging to the same group of coordinated cells as the first access node.

In this method, the access node transmitting the resource block to the mobile device is chosen as being the one having a transmission channel established with the mobile device presenting the smaller large-scale fading. Once the second access node has been chosen; the others access node covering cells belonging to the same group of coordinated cells are muted in order to mitigate interference during the transmission of the resource block.

Although the DCS method simplifies the coordination between cells in a LTE network by reducing the signaling and data exchange between the access nodes, such a method deteriorates the transmission performance of the mobile devices located at the center of a cell. This is due to the fact that some access nodes are muted.

Furthermore, in this solution the choice of the access nodes to be muted is empiric.

Hence there is a need to develop a method for mitigating interference in a LTE network using CoMP that can improve the existing situation.

To this end, the present method relates to a method for determining at least one antenna generating interference on a transmission channel established between a mobile device and a first access node, the antenna belonging to another access node, the method being run by the mobile device and comprising the steps of:

- sensing a radio signal emitted by the antenna,

- estimating a parameter representing a large-scale fading of the sensed radio signal,

- if the parameter representing a large-scale fading ration is lower than a adaptive threshold, sending the parameter representing a large- scale fading and an identification of the antenna to the first access node.

The invention also concerns a method for mitigating interference generated by at least one antenna on a transmission channel established between a mobile device and a first access node, the antenna belonging to another access node, the method being run by the first access node and comprising the steps of:

- determining whether the mobile device suffers interference generated by the antenna, if so:

- emitting, to at least the other access node, a request for muting the antenna, the request for muting comprising an identification of the antenna.

Such a solution allows direct coordination between at least two cells belonging to a communication network, each cell being covered by an access node, in a distributed way, i.e. without the use of a control node.

Once an access node determines that a mobile device attached to it suffers interference, the access node ask the other access nodes of the network to mute at least one of their antennas in order to mitigate interference generated by these antennas on the transmission channel established between the access node and the mobile device attached to it.

According to another characteristic of the method for mitigating interference of the invention, the method further comprises the steps of:

- receiving an acknowledgment message from the other access node comprising a parameter indicating that the antenna is muted,

- scheduling the mobile device for a data transmission through the transmission channel.

The access node schedules the mobile device only when all the interfering antennas are muted.

According to another characteristic of the method for mitigating interference of the invention, the method further comprises the steps of: - receiving a request for muting at least one antenna belonging to the first access node, the request for muting comprising an identification of the antenna belonging to the first access node,

- comparing a first randomly generated sequence number associated with the first access node with a second randomly generated sequence number associated with the second access node,

- if the first randomly generated sequence number is larger than the second randomly generated sequence number, scheduling the mobile device for a data transmission through the transmission channel.

This solution allows avoiding a conflict between two access nodes having resource block to transmit to a mobile device.

According to another characteristic of the method for mitigating interference of the invention, if the first randomly generated sequence number is smaller than the second randomly generated sequence number, the first access node mutes the antenna identified in the request for muting.

According to another characteristic of the method for mitigating interference of the invention, the method in order to determine whether the mobile device suffers interference generated by the antenna, the first access node further run the steps of:

- measuring a parameter representing a quality of experience index of the mobile device,

- comparing the parameter representing the quality of experience index with a predetermined threshold,

- if the parameter representing the quality of experience index is lower than the predetermined threshold, determining the identification of the antenna generating the interference. The invention relates also to a mobile device capable of determining at least one antenna generating interference on a transmission channel established between the mobile device and a first access node, the antenna belonging to another access node, the mobile device comprising means for:

- sensing a radio signal emitted by the antenna,

- estimating a parameter representing a large-scale fading of the sensed radio signal,

- if the parameter representing a large-scale fading is lower than a adpative threshold, sending the parameter representing a large-scale fading and an identification of the antenna to the first access node.

Another object of the invention is an access node capable of mitigating interference generated by at least one antenna on a transmission channel established between a mobile device and the access node, the antenna belonging to another access node, the first access node comprising means of:

- determining whether the mobile device suffers interference generated by the antenna, if so:

- emitting, to at least the other access node, a request for muting the antenna, the request for muting comprising an identification of the antenna.

The invention relates to a communication system comprising at least a first and a second access nose, the first access node being capable of mitigating interference generated by at least one antenna on a transmission channel established between a mobile device and the first access node, the antenna belonging to the second access node, the first access node comprising means of: - determining whether the mobile device suffers interference generated by the antenna, if so:

- emitting, to at least the second access node, a request for muting the antenna, the request for muting comprising an identification of the antenna,

and, the mobile device comprising means for:

- sensing a radio signal emitted by the antenna,

- estimating a parameter representing a large-scale fading of the sensed radio signal,

if the parameter representing a large-scale fading is lower than a adaptive threshold, sending the parameter representing a large-scale fading and an identification of the antenna to the first access node.

Finally, the invention concerns also two computer programs, in particular computer programs on or in an information medium or memory, suitable for implementing the three methods of the invention. These programs can use any programming language, and be in the form of source code, object code, or of code intermediate between source code and object code such as in a partially compiled form, or in any other desirable form for implementing the configuration method according to the invention.

The information medium may be any entity or device capable of storing the programs. For example, the medium can comprise a storage means, such as a ROM, for example a CD ROM or a microelectronic circuit ROM, or else a magnetic recording means, for example a diskette (floppy disk) or a hard disk.

Moreover, the information medium may be a transmissible medium such as an electrical or optical signal, which may be conveyed via an electrical or optical cable, by radio or by other means. The programs according to the invention may in particular be downloaded from a network of Internet type.

The present system and method are explained in further detail, and by way of example, with reference to the accompanying drawings wherein:

FIG. 1 represents a network N in which the methods of the invention are run,

FIG. 2 represents the different steps of a method for mitigating interference generated by access nodes on the transmission channel established between the mobile device and another access node ,

FIG. 3 shows in details a sub-step of the method of the invention run by an access node in order to determine if the transmission channel established between the mobile device and the access node suffers interference,

FIG. 4 represents the steps run by the mobile device in order to determine which antenna of the network is generating interference on the transmission channel established between the mobile device and the access node,

FIG. 5 represents an access node running the different steps of the method for mitigating interference generated by other access nodes on the transmission channel established between the mobile device and the access node,

FIG.6 represents a mobile device capable of determining which antenna of the network is generating interference on the transmission channel established between the mobile device and the access node: Figure 1 represents a network N in which the methods of the invention are run. The network N is for example a LTE network in which MIMO techniques can be used. The network N comprises a plurality of cells C1 , C2, C3. Each cell C1 , C2, C3 is covered by an access node, respectively, AN1 , AN2, AN3. Each access node AN1 , AN2, AN3, comprises at least one antenna for radio signal transmission. A mobile device UE, such as a mobile phone, a laptop or a tablet for example, is attached to the access node AN2 covering the cell C2. Hence, a transmission channel is established between the mobile device UE and the access node AN2.

Figure 2 represents the different steps of a method for mitigating interference generated by access nodes AN1 and AN3 on the transmission channel established between the mobile device UE and the access node AN2. This method for mitigating interference is run by the access node AN2.

The method for mitigating interference object of the invention is performed in a distributed way, where the access node AN2 initiates coordination with the other access nodes AN1 and AN2 without the control from a control node (not represented on figure 2).

In a first step E1 , the access node AN2 finds itself in a state called No Coordination. In a step E2, the access node AN2 determines if the transmission channel established between the mobile device UE and the access node AN2 suffers interference.

If the transmission channel established between the mobile device UE and the access node AN2 does not suffer interference, the access node AN2 transmits a radio signal to the mobile device UE. Indeed, when in the No Coordination state, the access node AN2 can only transmit a radio signal to a mobile device UE which does not suffer interference. If the transmission channel established between the mobile device UE and the access node AN2 does suffers interference, the access node AN2 send a request for muting at least one antenna belonging to the one of the other access nodes AN1 , AN3 in a step E3.The access node AN2 is then a state called waiting for coordination.

The request for muting at least one antenna comprises an identification of at least one antenna generating interference on the transmission channel.. In addition, the request for muting at least one antenna contains a sequence number Num randomly generated and stored in the access node AN2.

When in the state of waiting for coordination, the access node AN2 is not allowed to transmit a radio signal to mobile device UE for which the transmission channel established between the mobile device UE and the access node AN2 suffers interference.

In a step E4, access node AN2 initiates a timer. If prior to the timer expiration, the access node AN2 does not receive an answer to the request for muting at least one antenna from at least one of the access node AN1 , AN3, the access node AN2 restarts the timer and the access node AN2 remains in the waiting for coordination state. The duration of the timer is randomly selected upon each initiation of the method.

In a step E5, the access node AN2 receives an answer to its request for muting an antenna from another access node AN1 , AN3.

If the answer is a request for muting at least one antenna, the access node AN2 compares a sequence number Num1 comprised in the received request for muting with the sequence number Num randomly generated. If the sequence number Num is larger than the sequence number Num1 , the access node AN2 remains in the waiting for coordination state. If the sequence number Num is smaller than the sequence number Num1 , the access node AN2 switches to a state called In Coordination Slave. In a step E6, the access node AN2 mutes the antenna identified in the received request for muting at least one antenna. In a step E7, the access node AN2 sends an acknowledgment message from the other access node AN1 , AN3, comprising a parameter indicating that the identified antenna is muted.

If the sequence numbers Num and Num1 generated by both access nodes AN2 and AN1 or AN3 are the same, teach access node regenerates a sequence number Num and Num1 and sends these new sequence numbers Num and Num1 to each other.

If the answer received during step E5 is an acknowledgment message from the other access node AN1 , AN3,

Then in step E8, the access node AN2 schedules the UEs whose Interfering Antennas Set is declared by the "Coordination Request" message sent to the neighbours.

Figure 3 shows in details the sub-step of step E2 run by the access node AN2 in order to determine if the transmission channel established between the mobile device UE and the access node AN2 suffers interference. It also helps the access node AN2 to decide whether to start or end coordination with the other access nodes AN1 , AN3.

In a sub-step E20, the access node AN2 measures a quality of experience QoE index of the mobile device UE in the following way:

where and ^ is the QoE index and the expected throughput for quality of service QoS guaranteed of the mobile device UE, respectively. ^Ri indicates the average throughput assigned to the mobile device UE. I R is calculated as: where ^a is a constant between 1 and 0. & is actually deduced in an interval of ^ timeslots and based on the results of previous coordination. Thus, ^{Ri {n)} refers to the current value of average throughput, ^ (« - 1) denotes the value obtained in the calculation of the last interval and ^Ri represents the payload transmitted to the mobile device UE in the latest interval composed of P timeslots. Obviously, ⁱ depends on the quality of service QoS requirement of mobile device UE. For instance, the mobile device UE can set with the smaller value in the case of delay-sensitive service.

The quality of experience QoE index is regularly updated for each mobile device UE attached to the access node AN2.

During sub-step E21 , the access node AN2 chooses the mobile device

UE whose quality of experience is below a predetermined threshold ^η and filters out the mobile devices UE for which the transmission channel established between the mobile device UE and the access node AN2 does not suffer interference. Then the access node AN2 selects, in a sub-step E22, the mobile devices UE for which the transmission channel established between the mobile device UE and the access node AN2 suffers interference before running step E3.

Sub-step E20 is run regularly by the access node AN2. Then if the quality of experience QoE index of the mobile device is above the threshold V , the access node AN2 can end the coordination with the other access nodes AN1 , AN3 in order to mitigate interference. Such a method targets at making the decision of whether to initiate cell coordination with the consideration of throughput fairness between the different mobiles devices UE.

Figure 4 represents the steps run by the mobile device UE in order to determine which antenna of the network N is generating interference on the transmission channel established between the mobile device UE and the access node AN2.

During a step G1 , two parameters u _t and t ₀ are set. Parameter u _t is set as: u _t =T where τ refers to a group of antennas belonging the coordinated cells C1 , C2 and C3. Parameter t ₀ is set as t ₀ = argmin/ _{i t} , where ^ll * represents the large-scale fading of k-th antenna in the set of antennas τ that is sensed by the mobile device UE.

During a step G2, the mobile device UE senses radio signals emitted by the different antennas in the set of antennas τ . This gives i _lJk , where i identifies an antenna among the set ^Ti of antennas belonging to the access node AN2, and k identifies an antenna in the set of antennas τ .

y i ² .

In a step G3, the mobile device UE estimates a parameter

y i ² representing a large-scale fading of the sensed radio signals.

In a step G4, the mobile device UE determines if the parameter y i ² .

representing a large-scale fading ratio is lower than a y i ² threshold + i , where \u,\ represents the number of antennas of

the set of antennas u _: If the parameter representing a large-scale fading ratio is lower than the adaptive threshold 1- , +i , the mobile device UE

Σ -1 calculates a new value of t ₀ and then a new value of ur. t ₀ _ _new = arg min z and u _i . _new = u _i -{t ₀ _ _n respectively, in a step G5.

The mobile device UE runs steps G2 to G5 as long as the parameter y i ² .

representing a large-scale fading ratio is lower than the y ί threshold + i , each time using the new values of t ₀ and u _t

\u\ -\ calculated during step G5.

When the parameter representing a large-scale fading ratio is larger than the threshold 1 - + i ,the mobile device UE

\u\ -\ determines the set of antennas /. generating interference on the transmission channel established between the access node AN2 and the mobile device UE, during a step G6. /. =u _i -T _i ]u _i , where c/. is the last value for which the parameter ^{}eUi 1,1} ₂ representing a large-scale

fading ratio is lower than the threshold 1 - + 1

\u\ -\

After obtaining the set z _; of antennas generating interference on the transmission channel established between the access node AN2 and the mobile device UE, the mobile device UE send it, in a step G7, to the access node AN2 for the coordination. Normally, /. just needs to list the index of the antennas included in the set. Since /. is deduced from the estimation of large-scale fading, the mobile device UE is only required to inform it to the access node AN2 after detecting the change in the previously reported one.

Figure 5 represents the access node AN2, AN1 , AN3, running the different steps of the method for mitigating interference generated by access nodes AN1 and AN3 on the transmission channel established between the mobile device UE and the access node AN1 .

In the invention, the access node AN2 initiates coordination with the other access nodes AN1 and AN2 without the control from a control node.

The access node AN2 comprises means 10 for determining if the transmission channel established between the mobile device UE and the access node AN2 suffers interference.

If the transmission channel established between the mobile device UE and the access node AN2 does not suffer interference, the access node AN2 transmits a radio signal to the mobile device UE through transmission means 1 1 connected to the determination means 10.

If the transmission channel established between the mobile device UE and the access node AN2 does suffers interference, the access node AN2 send a request for muting at least one antenna belonging to the one of the other access nodes AN1 , AN3 through transmission means 12 connected to the determination means 10.The access node AN2 is then a state called waiting for coordination.

The request for muting at least one antenna comprises an identification of at least one antenna generating interference on the transmission channel.. In addition, the request for muting at least one antenna contains a sequence number Num randomly generated and stored in data base DB of the access node AN2 connected to the transmission means 12.

The access node AN2 comprises a timer Tim. If prior to the timer expiration, the access node AN2 does not receive, through receiving means 13 connected to the timer Tim, an answer to the request for muting at least one antenna from at least one of the access node AN1 , AN3, the access node AN2 restarts the timer Tim and the access node AN2 remains in the waiting for coordination state.

If the received answer is a request for muting at least one antenna, the access node AN2 compares a sequence number Num1 comprised in the received request for muting with the sequence number Num randomly generated, through comparison means 14 connected to the receiving means 13.

The access node AN2 comprises means for muting 15 the antenna identified in the received request for muting at least one antenna. The means for muting 15 are connected the comparison means 14.

The access node AN2 also comprises means for measuring 16 a quality of experience QoE index of the mobile device UE. Those measuring means 16 are connected to the determination means 10.

Connected to measuring means 16, the access node AN2 comprises means for choosing 17 the mobile device UE whose quality of experience is below a predetermined threshold ^η and filters out the mobile devices UE for which the transmission channel established between the mobile device UE and the access node AN2 does not suffer interference. The access node AN2 comprises, connected to choosing means 17, selecting means 18 for selecting the mobile devices UE for which the transmission channel established between the mobile device UE and the access node AN2 suffers interference. Figure 6 represents a mobile device capable of determining which antenna of the network N is generating interference on the transmission channel established between the mobile device UE and the access node AN2.

The mobile device UE comprises means DBI for storing two parameters c/. and t ₀ . Parameter c/. is set as: υ, =τ where τ refers to a group of antennas belonging the coordinated cells C1 , C2 and C3. Parameter t ₀ is set as t ₀ = argmin/ _{i t} , where represents the large-scale keU _l

fading of k-th antenna in the set of antennas τ that is sensed by the mobile device UE.

The mobile device UE comprises means 20for sensing radio signals emitted by the different antennas in the set of antennas τ .

Connected to the sensing means 20, the mobile device UE comprises means 21 for estimating a parameter representing a large-

scale fading of the sensed radio signals. The estimating means 21 are also connected to the storing means DB1 .

The mobile device UE comprises means 22 for determining if the parameter enting a large-scale fading ratio is lower than

a threshol where \u,\ represents the number of antennas

of the set of antennas u The determining means 22 are connected to the estimating means 21 and to the storing means DB1 .

The mobile device UE comprises means 23 for calculating a new value of r ₀ and then a new value of u _t : t ₀ _ _new = ar _g min /, _t and u _i _ _new =u _i -{t ₀ _ _n keU, respectively, when the parameter e-scale

fading ratio is lower than the ad . Those

calculating means 23 are connected to the determining means 22.

The mobile device UE comprises means 24 for determining the set of antennas /. generating interference on the transmission channel established between the access node AN2 and the mobile device UE,

∑ i ² - when the parameter ^{}eUi 1,1} ₂ representing a large-scale fading ratio is

"i,k larger than the threshold + i . Those determining means 24 are

connected to the determining means 22.

After obtaining the set z _; of antennas generating interference on the transmission channel established between the access node AN2 and the mobile device UE, the mobile device UE send it, through transmitting means 25 connected to the determining means 24, to the access node AN2 for the coordination. Normally, /. just needs to list the index of the antennas included in the set. Since /. is deduced from the estimation of large-scale fading, the mobile device UE is only required to inform it to the access node AN2 after detecting the change in the previously reported one.