Abstract:
A method for controlling a heterogeneous wireless network ( 100 ) is proposed. The heterogeneous wireless network comprises at least one first base station ( 115 ) which manages communication of a plurality of first user equipments ( 125 M) positioned in at least one first cell ( 105 ) and at least one second base station ( 120 ) which manages communication of a plurality of second user equipments ( 125   p ) positioned in at least one second cell ( 110 ). Moreover, in the heterogeneous network ( 100 ), for each first user equipment ( 125 M) the at least one first base station ( 115 ) provides a respective first allocation information set over a respective first allocation time interval. Similarly, the at least one second base station ( 120 ) provides for each second user equipment ( 125   p ) a respective second allocation information set over a respective second allocation time interval. Furthermore, each first allocation information set and each second allocation information set define transmission resources for communications of each first user equipment ( 125 M) and of each second user equipment ( 125 M), respectively. In the heterogeneous wireless network ( 100 ), the at least one second cell ( 110 ) is at least partially comprised in the first cell ( 105 ). The method comprises the following steps. Said at least one first base station determines ( 305 ) a set of probably interfering first user equipments ( 125   M ) with at least one of the plurality of second user equipments ( 125   p ). Said step of determining the set of probably interfering first user equipments ( 125   M ) comprises the following steps. Comparing each second allocation information set with each first allocation information set having the first allocation time interval at least partially overlapped with the respective second allocation time interval, and identifying as a probably interfering first user equipment ( 125   M ) each first user equipment ( 125   M ) for which a correspondence is verified in said comparing allocation information sets. The method further comprises the following steps. Said at least one second base station ( 120 ) defines a detection time window in order to analyze univocal identifiers transmitted by probably interfering first user equipments ( 125   M ) in said set. The method comprises identifying ( 315 ) a subset of effectively interfering first user equipments ( 125   M ) with the plurality of second user equipments ( 125   p ). Said at least one first base station ( 115 ) modifies ( 325 ) said transmission resources for at least one first user equipment ( 125   M ) comprised in the subset of effectively interfering first user equipments ( 125   M ) in order to mitigate interferences.

Description:
BACKGROUND 
     Field 
     The present invention relates to communication systems. More particularly, the present invention relates to the field of wireless or mobile telecommunication networks. Even more particularly, the present invention relates to high speed transmission networks, such as wireless telecommunication networks belonging to the so called fourth generation (4G), including networks based upon Long Term Evolution (LTE) and/or Long Term Evolution Advanced (LTE-A) standards. 
     Overview of the Related Art 
     High-speed transmission networks, or simply networks, are able to simultaneously provide different services (e.g., voice and/or video real-time communications, data transfer, Web browsing, broadcasting, etc.) to a given number of user equipments (UEs, such as mobile phones, personal digital assistants, tablets, personal computers etc.). 
     Networks generally include a plurality of base stations, each of which manages communications for a given number of UEs. In the following, without any limitation, the term “eNode B” (eNB) (which is specifically used in LTE/LTE-A system) will be used as synonym of “base station”. In general, one eNode B may control one or more cells. 
     Generally, networks comprise cells of various size and are therefore identified as a heterogeneous networks, or HetNets. Particularly, each cell may be categorized as a macro-cell, a pico-cell, or other types of cell according to the size of the covered geographic area. A macro-cell is a relatively large geographic area (e.g., an area having a radius in the order of the kilometers such as one or more city blocks) and the associated eNB—which is usually denoted as MeNB—allows unrestricted access to UEs therein. A pico-cell is a relatively small geographic area (e.g., with a radius in the order of hundred of meters such as a large building) and the associated eNB—which is usually denoted as PeNB—may both allow a restricted or unrestricted access to UEs. 
     Furthermore, in a HetNet, the MeNB are deployed in a regular way forming a substantially continuous overall coverage area for the network; conversely, PeNB and/or other types of eNB are deployed in a quite random fashion. Therefore, very often occurs that inside a macro-cell one or more pico-cells and/or other types of cells are deployed. In this way, one or more pico-cells and/or other cell types result superimposed with a macro-cell. 
     In operation, each UE establishes a communication with a cell via downlink and uplink channels for accessing the abovementioned services (i.e., the UE is connected to the cell). The downlink refers to the communication link from the eNB to the UE, and the uplink refers to the communication link from the UE to the eNB. 
     In order to achieve the required high transmission speeds, networks utilizes orthogonal frequency division multiplexing (OFDM) for downlink communications. Conversely, single-carrier frequency division multiplexing (SC-FDM) is used for uplink communications since the high Peak-to-Average Power Ratio (PAPR) property of OFDM makes the same less favorable for uplink communication. OFDM and SC-FDM partition the system bandwidth into multiple orthogonal sub-carriers. The spacing between adjacent sub-carriers may be fixed, and the total number of sub-carriers may be dependent on the system bandwidth. The system bandwidth may also be partitioned into sub-bands, where one sub-band is formed by a certain number of adjacent sub-carriers. 
     Nonetheless, a downlink communication may experience interference due to concurrent transmissions performed by neighboring cells. Conversely, an uplink communication may cause interference to concurrent transmissions performed by other UEs communicating with the neighboring cells. Such interferences degrade performance on both the downlink and uplink. 
     Networks usually implement frequency diversity techniques for communicating. In case of wideband communication systems (such as in LTE and LTE-A systems), frequency diversity allows a signal to be spread over the frequency domain, resulting in a higher resistance to frequency selective fading, natural interference and noise. For example, SC-FDMA may spread information through all the available sub-carriers, so in case of loss of partial information on one (or even more) sub-carriers does not necessarily lead to lose the information modulated in the communication. 
     Unfortunately, each different type of cell usually determines a corresponding transmission power level for the downlink, which may exacerbate interference issues between neighboring cells. In detail, macro-cells usually impose the highest downlink transmit power (e.g., 20 W) in the wireless telecommunication network to the connected UEs, granting each downlink communication to reach any UE in any point in the whole macro-cell. Conversely, pico-cells, and/or other types of small cells impose a lower downlink transmission power (e.g., down to 1 W), since their coverage areas are smaller than the ones of the macro-cells. Consequently, downlink communications with low transmission power may suffer severe interferences from having downlink communications with higher transmission power. 
     Similarly, the interference problem also arises in the uplink as a consequence of the different coverage areas of macro and pico-cells, and is exacerbated by the fact that a power control system may increases an uplink transmission power trying to overcome such interference potentially provoking further interferences between uplink transmissions of the same cell, which may lead to severe interfering scenarios. For example, considering a macro-cell enclosing a pico-cell, the uplink communications between UE connected to the pico-cell and the pico-cell itself are likely to suffer severe interferences from uplink transmission performed by UE connected to the macro-cell close to the pico-cell. 
     Moreover, power control procedures provided in the uplink for adjusting the transmission power levels of UEs cannot be directly used to avoid such interference problems. Indeed, the power control system of a cell is effective only on UEs connected to the same cell while any further UEs connected to another cell and causing interferences cannot be power controlled by the cell that is victim of the interference. 
     In the art, solutions have been proposed in order to reduce the interference arising in HetNet, devising methods for controlling the transmission power of the eNB and of UE connected to such eNB as disclosed in the paper 3GPP R3-121299, “Analysis of Solutions for Mitigation of UL Interference in CB-ICIC”. TSG-RAN WG3 #76 Prague, Czech Republic, 21-25 May 2012. 
     Moreover the International patent application No. WO 2011/150296 discloses methods and an apparatus for uplink radio link monitoring in a Long Term Evolution system with enhanced inter-cell interference coordination. Various options are presented in an effort to transmit a sounding reference signal of a UE device served by an eNB in the HetNet, avoiding both interference from uplink transmissions from other UE being served by neighboring eNBs and collisions with the UE own channel quality information or physical uplink shared channel. 
     Furthermore, the International patent application No. WO 2012/048174 discloses systems and methods for managing inter-cell interference coordination actions for time-domain partitioned cells. In certain aspects, time-domain partitioning is accounted for by an eNB in determining whether to send frequency-based inter-cell interference information (e.g., uplink overload indicator) to neighboring eNB(s) and/or responsive actions to take in response for receiving frequency-based inter-cell interference information (e.g., uplink overload indicator, high interference indicator, and/or relative narrowband transmission power). 
     Finally, the International patent application No. WO 2012/024454 discloses an apparatus and a method for controlling inter-cell interference comprising detecting and measuring uplink interference; and reporting the level of the uplink interference to an inter cell interference coordination server using a backhaul link. The apparatus or method may include receiving a measured uplink interference level through a first backhaul link, determining a transmit power level based on the measured uplink interference level, and sending through a second backhaul link the transmit power level for reconfiguring either a UE or a Femto eNode B. 
     SUMMARY 
     The Applicant has found that the known solutions mentioned above fail in ensuring satisfactory interference immunity in the uplink communications. 
     Therefore, the Applicant has coped with the problem of devising a satisfactory solution able to suppress, at least partly, interferences arising between high transmission power UE connected an eNB and low transmission power UE connected to another eNB. 
     Particularly, one aspect of the present invention proposes a method for controlling a heterogeneous wireless network. The heterogeneous wireless network comprises at least one first base station which manages communication of a plurality of first user equipments positioned in at least one first cell and at least one second base station which manages communication of a plurality of second user equipments positioned in at least one second cell. Moreover, in the heterogeneous network, for each first user equipment the at least one first base station provides a respective first allocation information set over a respective first allocation time interval. Similarly, the at least one second base station provides for each second user equipment a respective second allocation information set over a respective second allocation time interval. Furthermore, each first allocation information set and each second allocation information set define transmission resources for communications of each first user equipment and each second user equipment, respectively. In the heterogeneous wireless network the at least one second cell is at least partially comprised in the first cell. The method comprises the following steps. Said at least one first base station determines a set of probably interfering first user equipments with at least one of the plurality of second user equipments. Said step of determining the set of probably interfering first user equipments comprises the following steps. Comparing each second allocation information set with each first allocation information set having the first allocation time interval at least partially overlapped with the respective second allocation time interval, and identifying as a probably interfering first user equipment each first user equipment for which a correspondence is verified in said comparing allocation information sets. The method further comprises the following steps. Said at least one second base station defines a detection time window in order to analyze univocal identifiers transmitted by probably interfering first user equipments in said set. The method comprises identifying a subset of effectively interfering first user equipments with the plurality of second user equipments. Said at least one first base station modifies said transmission resources for at least one first user equipment comprised in the subset of effectively interfering first user equipments in order to mitigate interferences. 
     Preferred features of the present invention are set in the dependent claims. 
     Another aspect of the present invention proposes a heterogeneous wireless network adapted to implement said method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These, and others, features and advantages of the solution according to the present invention will be better understood by reading the following detailed description of an embodiment thereof, provided merely by way of non-limitative example, to be read in conjunction with the connected drawings, wherein: 
         FIG. 1  is a schematic representation of a portion of a heterogeneous network with a macro-cell enclosing a pico-cell; 
         FIG. 2  is a schematic representation of a structure of a frame used in uplink communications; 
         FIG. 3  is a flowchart schematically illustrating actions performed by evolved Nodes B in order to suppress interferences in a heterogeneous network according to an embodiment of the present invention; 
         FIG. 4A  is a schematic timing diagram of symbols transmitted in the heterogeneous network according to an embodiment of the present invention; and 
         FIG. 4B  is a schematic timing diagram of symbols transmitted in the heterogeneous network according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to the figures,  FIG. 1  is a schematic representation of a portion of a heterogeneous network, or HetNet  100 , with a macro-cell  105  enclosing a pico-cell  110 . 
     In detail, a Macro evolved Node B, or MeNB  115 , provides radio coverage inside the macro-cell  105 , whereas a Pico evolved Node B, or PeNB  120 , provides radio coverage inside the pico-cell  110 . In the example at issue, the pico-cell  110  is completely enclosed by the macro-cell  105 . Therefore, inside the pico-cell  110  transmissions provided to/from the MeNB  115  are superimposed with transmissions provided to/from the PeNB  120 . 
     In such a scenario, user equipments, or UEs, connected to the MeNB  115 , referred to as MUE  125   M  hereinafter, may happen to be positioned inside the pico-cell  110  also. Moreover transmissions performed by MUEs  125   M  inside the pico-cell  110  or close to the same may potentially interfere with transmissions between UEs connected to the PeNB  120 —referred to as PUE  125   P  hereinafter—and the PeNB  120 . 
     In greater detail, MUEs  125   M  may generate a high interference towards the PeNB  120  due to high power level of the uplink transmission required to compensate the path loss (i.e., energy lost during transmission propagation) and reach the MeNB  115  with a sufficient power level to be correctly received. Indeed, uplink transmissions of the MUEs  125   M  reach the PeNB  120  with relatively high power level. Particularly, the uplink transmissions of the MUEs  125   M  closer to the PeNB  120  than to the MeNB  115  may even be received at the PeNB  120  with higher power levels than those received at the MeNB  115  due to transmission path difference between the MeNB  115  and the PeNB  120 , since the energy lost by the MUEs  125   M  uplink transmission before reaching PeNB  120  is smaller than the energy lost before reaching the MeNB  115 . 
     In the solution according to an embodiment of the present invention, the HetNet  100  is adapted to nullify, or at least reduce, an interference at the PeNB  120 , due to uplink transmissions of the MUEs  125   M , thus increasing the quality and throughput experienced by communications between the PUEs  125   P  and the PeNB  120  (uplink communications). Such advantages are obtained thanks to information exchanged between the MeNB  115  and the PeNB  120 —and vice-versa the MeNB  115  and the PeNB  120 —and suitable scheduling decisions taken at the MeNB  115  (as will be described in detail in the following). 
     Referring to  FIG. 2  a typical structure of a frame  200  used in uplink transmission will now be described. 
     The generic uplink frame  200  has a standard duration Tf (e.g., Tf=10 ms). Such frame comprises ten sub-frames  210  also known as TTI (Transmission Time Interval). Generally, each sub-frame  210  has a duration Tsf (e.g., Tsf=1 ms) and, in its turn, comprises two time slots  215 , each of which with a same duration (e.g., 0.5 ms). Therefore, each frame  200  may also be considered as a structure comprising 20 time slots  215 . 
     Each time slot  215  is configured for containing a plurality of symbols  220  (i.e., electromagnetic pulses representing an integer number of bits to be transmitted). The number N of symbols in one time slot  215  is determined by a length of Cyclic Prefixes (CP)  225  associated to each symbol  220  in the time slot  215  (as will be described in the following). For example, a normal CP  225  determines seven symbols  220  (N=7) transmitted per time slot  215 , while an extended CP  225  determines six symbols  220  (N=6) transmitted per time slot  215  since the extended CP  225  has a greater size with respect to the normal CP  225 . 
     In detail, UEs  125   M  and  125   P  convert data to be transmitted into a sequence of modulated sub-carriers. Initially, data are modulated in order to form a sequence of modulated complex symbols. Such modulated complex symbols carriers are converted into N parallel data streams and a Discrete Fourier Transform (DFT) is performed on such data streams obtaining N DFT symbols, which are spread by the DFT operation over the available sub-carriers. Furthermore, the DFT symbols may be mapped to one of M (orthogonal) sub-carriers obtaining a set of complex symbols in the frequency domain. The complex symbols in the frequency domain may be transformed in time-domain symbols by means of an Inverse DFT (IDFT). After that, the CP is added to each symbol. The CP prefixes symbols with a copy of an end portion of the same symbol. Therefore, the CP facilitates the elimination the Inter Symbol Interference (ISI), and the repetition of the symbol end portion allows linear convolution to be modeled as circular convolution, which grants simpler frequency-domain processing. 
     The symbol is then ready to be transmitted. For example, let us consider communication between a generic MUE  125   M  and the MeNB  115  (although the same consideration applies in communications between a generic PUE  125   P  and the PeNB  120 ). Firstly, the MUE  125   M  sends to the MeNB  115  a special signal denoted as Preamble Random Access Channel (PRACH) to request access to transmission channel (i.e., physical transmission medium for the communication) and wait for a response from the MeNB  115 . The MeNB  115  schedules the uplink resources (i.e., component carriers, transmission bandwidth, sub-frames and sub-carriers etc.) by assigning selected time/frequency resources (also referred to as resource blocks in the art) to the MUE  125   M  and informing the latter about transmission formats to use (by means of downlink transmission denoted as scheduling grant). Subsequently, the MUE  125   M  is able to transmit symbols through the Physical Uplink Shared Channel (PUSCH) and Physical Uplink Control Channel (PUCCH). In detail, the PUSCH is used for the transmission of general data, while the PUCCH is used for the transmission of control information (e.g., Channel Quality Indicator—CQI—reports and ACK/NACK information related to data received in the downlink). 
     In addition, the HetNet  100  may comprise further features for enhancing high speed communication. For example, let us consider the HetNet  100  being adapted to operate according to 3GPP LTE and/or LTE-A standards. In this case, the HetNet  100  (as described in 3GPP LTE Release 8/9) may be able to support different channel bandwidths (e.g., 1.4, 3, 5, 10, 15 and 20 MHz bandwidths) for the serving carrier by scaling the size of the DFT operation used in the OFDM technique. This allows a large flexibility in system deployment considering the different spectrum availabilities in different regions and countries (i.e., the HetNet  100  may operate according to different regional communication standards with no, or minimal, adjustment). 
     Furthermore, the HetNet  100  may be able to scale the system bandwidth above a predetermined bandwidth size (e.g., 20 MHz), thanks to Carrier Aggregation feature (as disclosed in 3GPP LTE Release 10). In particular, by means of Carrier Aggregation, system bandwidths above a predetermined bandwidth size may be supported by simultaneously aggregating a corresponding number of carriers. For example, a system bandwidth of 100 MHz may be supported by the aggregation of five carriers of 20 MHz each (each of such aggregated carrier is usually denoted in the art as component carrier). The component carriers may be located on adjacent or non-adjacent frequency bands. 
     Turning now to  FIG. 3 , it is a flowchart schematically illustrating actions performed by eNBs  115  and  120  in order to suppress interferences in the respective cells  105  and  110  of the HetNet  100  according to an embodiment of the present invention. 
     On a periodical basis or, alternatively, at randomly chosen time instants, the MeNB  115  determines ( 305 ) which MUEs  125   M  are potentially source of interference (denoted as probably interfering MUEs  125   M  hereinafter) for the uplink communications between PUEs  125   P  and PeNB  120 . 
     Then, the MeNB  115  reports ( 310 ) to the PeNB  120  a list of the probably interfering MUEs  125   M . 
     The PeNB  120  performs interference measurements ( 315 ) on the probably interfering MUEs  125   M  (comprised in the list received from the MeNB  115 ) and detects which probably interfering MUEs  125   M  are effectively interfering MUEs  125   M . 
     Subsequently, the PeNB  120  signals back ( 320 ) to the MeNB  115  an updated list comprising the effectively interfering MUEs  125   M . 
     Based on the updated list of the MUEs  125   M  received from the PeNB  120 , the MeNB  115  modifies ( 325 ) scheduled resources (i.e., component carriers, transmission bandwidth, sub-frames and sub-carriers etc.) for the listed effectively interfering MUEs  125   M . By performing the abovementioned actions it is possible to suppress, or at least mitigate, the interferences caused by MUEs  125   M  transmissions against uplink transmissions between the PUEs  125   P  and PeNB  120 , even if the PeNB  120  has no control over the transmission power level of the interfering MUEs  125   M  (as will be herein described in detail). 
     Advantageously, the MeNB  115  and the PeNB  120  are able to communicate together by means of a dedicated communication interface; for example, an X2 interface (in LTE/LTE-A standard). The X2 interface may be established between an eNB and its neighbor eNBs in order to exchange information when needed. Typically, the establishment of an X2 interface between two eNBs  115  and  120  is used to exchange traffic- or interference-related information. 
     Alternatively, in other embodiments of the present invention, the MeNB  115  and PeNB  120  may be controlled by network management units (e.g., a Network Controller, not shown in the figures) and all the actions for suppressing, or at least reducing, transmission interferences are performed by such central unit, which is adapted to communicate with the eNBs  115  and  120  of the HetNet  100 ; thus, without the need of establishing direct communications between MeNB  115  and PeNB  120 . 
     In the following, each action performed in the HetNet  100  for suppressing, or at least reducing, interferences in uplink transmissions will be further analyzed in order to fully appreciate advantages associated with embodiments of the present invention. 
     The MeNB  115  has various options for determining which MUEs  125   M  are possible interferers for the PUEs  125   P -PeNB  120  transmissions. A first option for determining probably interfering MUEs  125   M  is to use measurements reported to the MeNB  115  by the MUEs  125   M , while communicating with the same MeNB  115 . 
     Advantageously, although not limitatively, reported measurements are contained in control signals (e.g., Radio Resource Control—RRC) transmitted by the MUEs  125   M  toward the MeNB  115  already used in the HetNet  100  for network managing purposes. 
     In detail, such reported measurements comprise a list of eNBs neighboring each MUE  125   M . Thus, the MeNB  115  is able to determine which MUE  125   M  is a probably interfering MUE  125   M  by verifying the presence of the PeNB  120  among the neighboring eNB listed in the reported measurements. In addition or alternatively, in the case of absence of the PeNB  120  in the neighboring list of MUEs  125   M , the MeNB  115  is still able to determine if any of such MUE  125   M  is a probably interfering MUE  125   M  by verifying the presence in such neighboring list of one or more other eNBs (not shown in the drawings) of the HetNet  100  that are known to be geographically deployed close to the PeNB  120 ; for example, recognizing other eNBs positioned at a distance lower than a proximity threshold. 
     Moreover, it is possible to use the HetNet  100  planning information (i.e., a description of eNBs  115  and  120  comprised in the HetNet  100  featuring their geographical position, peak and average transmission power etc.) together with measurements reported by the MUEs  125   M  for simplify the determining of probably interfering MUEs  125   M . Particularly, information regarding distance between PeNB  120  and MeNB  115  and their coverage area (i.e., the extension of the pico-cell  110  and macro-cell  105 , respectively) may be effectively exploited as an additional discriminating criterion for selecting probably interfering MUEs  125   M . For example, the MeNB  115  may be instructed to firstly verify the presence of the PeNB  120 , which is the closest eNB in the HetNet  100 , in the measures reported by the MUEs  125   M . 
     In addition, in an embodiment of the present invention, the MeNB  115  is able to perform a coarse determination of which MUE  125   M  is a probably interfering MUE  125   M  by exploiting a synchronization information, such as a timing advance signal, jointly with such planning information. The timing advance is used for synchronizing uplink transmissions of the MUEs  125   M  with the MeNB  115 . Particularly, the timing advance is proportional to a distance between the MUE  125   M  and the MeNB  115 . Therefore, it is possible to identify as probably interfering MUEs  125   M  each MUE  125   M  which is at a distance lower than a threshold distance from the PeNB  120  (known from the planning information). 
     It should be noted that other positioning system may provide a more precise identification of the probably interfering MUEs  125 M. For example, by using a Global Positioning Signal (which may be provided by each MUEs  125   M ) or positioning information (detected by a positioning server, not shown in the figures, which may be provided in the MeNB  115 ), it is possible to precisely know the position of the MUEs  125   M  and, therefore, verify which MUE  125   M  is a probably interfering MUE  125   M  at a distance from the PeNB  120  lower than the threshold distance (which position is known from the planning information). 
     Moreover, in order to efficiently determine probably interfering MUEs  125   M  it is possible to exploit an allocation information set, such as a scheduling grant, generated by the PeNB  120  for each PUE  125   P  which requests bandwidth for performing a communication. The scheduling grant is a downlink message used for the allocation of transmission resources both in downlink and in uplink. In detail each scheduling grant comprises different types of information: bandwidth allocation information (in terms of number and frequency position of the assigned resource blocks), information regarding the transmission scheme (e.g., number of simultaneously transmitted streams), Modulation/Coding Scheme (MCS) and power control commands. Ultimately, the transmission scheme information and MCS determine the asymptotic spectrum efficiency of the transmission (i.e., the number of communication bits per second that can be transmitted per Hertz of bandwidth) whereas the power control command determines the transmit power of the UE. As known, in high transmission speed HetNet  100 , each eNB  115 ,  120  performs scheduling functions signaling the information listed above to the respective served UEs  125   M  and  125   P . Particularly, the scheduling grant generated by the PeNB  120  contains the information to be used by each PUE  125   P  for their communication with the PeNB  120 . Therefore, if the PeNB  120  periodically transmits such scheduling grants also to the MeNB  115  together with corresponding time allocation information such as time stamps, the MeNB  115  is able to identify the probably interfering MUEs  125   M . 
     In detail, MUEs  125   M  having bandwidth allocation information (e.g., in terms of sub-frames and sub-carriers) similar to the bandwidth allocation information of the PUE  125   P  could be identified as probably interfering MUEs  125   M  if respective allocation time intervals—starting from the time instant indicated by the respective time stamps and extending substantially for a time duration corresponding to the transmission of the scheduling grant (for example, equal to a TTI, i.e. 1 ms)—are overlapped (at least in part). In other words, each MUE  125   M  is identified as a probably interfering MUE  125   M  if it has similar allocated resources to any PUE  125   P  during similar time intervals. 
     A further method for determining probably interfering MUEs  125   M , according to an embodiment of the present invention, provides that the MeNB  115  detects variations in the MCS provided by PeNB  120  to the PUEs  125   P  over subsequent scheduling decisions (i.e., sequences of scheduling grants provided over a observation period). This information, together with the resources allocated to the PUEs  125   P  (as defined by the respective scheduling grant), may be used for identifying as probably interfering MUEs  125   M  each MUEs  125   M  allocated over resource blocks that are totally or partially corresponding to the resource blocks allocated to one of the PUEs  125   P  and characterized by partially or totally overlapped transmission time intervals if is detected a MCS reduction for such one of the PUEs  125   P . The MCS reduction implies a spectrum efficiency degradation, which is due to a more robust modulation scheme and/or a lower coding rate provided by the PeNB  120  in order to enhance a protection of the communication of the PUEs  125   P  against the interference generated by the MUEs  125   M . 
     Moreover, the sequence of power control commands comprised in the scheduling grants (sent by PeNB  120  to the PUEs  125   P ) can be used by the MeNB  115  in order to identify probably interfering MUEs  125   M . For example, an increment in the transmit power of the PUE  125   P , eventually combined with the allocation information and/or with the MCS reduction (described above), may be used for identifying the probably interfering MUEs  125   M . In detail, the increment in the transmit power of the PUEs  125   P  above a power threshold give advice that uplink communications of the PUEs  125   P  are subjected to interferences. 
     It should be noted that the just described expedients for determining the probably interfering MUEs  125   M  are only examples of how such determination may be carried out, and thus they should not be considered as limiting in any way. 
     Once the MeNB  115  has determined the probably interfering MUEs  125   M , the MeNB  115  transmits to the PeNB  120  allocation information (time and/or frequency information) for identifying such probably interfering MUEs  125   M . For example, the MeNB  115  may transmits to the PeNB  120  a set of information related to the DeModulation Reference Signal (DMRS), each associated with a respective probably interfering MUE  125   M . As it is known, DMRS is a univocal identifier for a transmission and, thus, also for the MUE  125   M  which performed such transmission. 
     Considering now  FIG. 4A , which is a schematic timing diagram of symbols transmitted in the HetNet  100 , respectively, further features of the HetNet  100  according to an embodiment of the present invention will be disclosed. 
     For the sake of clarity, let us consider that the MeNB  115  and PeNB  120  are perfectly synchronized in downlink (condition attainable in the HetNet  100  by exchanging downlink synchronization signals between the MeNB  115  and the PeNB  120 ). In other words, a (macro) downlink symbol  405   M  of the MeNB  115  and a (pico) downlink symbol  405   P  of the PeNB  120  are transmitted at a same time instant T 0  that denotes the start of a downlink frame (for both MeNB  115  and PeNB  120 ). 
     Even in such circumstances, a (macro) uplink symbol  410   M  transmitted by the MUE  125   M  and an (pico) uplink symbol  410   P  transmitted by the PUE  125   P  to the PeNB  120  are not synchronized (i.e., their respective arrival time instants at the PeNB  120  are different). This is due to different propagation times (in their turns depending on different physical distances between each UE  125   M  and  125   P  and the eNB  115  and  120 ) and to the timing advance imposed by each eNB  115   e    120  to all the respectively served UEs  125   M  e  125   P . In detail, the timing advance is used for synchronizing symbols uplink transmissions (by anticipating, or delaying, an uplink transmission starting time) for each UE  125   M , 125   P , in order to receive all the uplink symbols  410   M  and  410   P  aligned at the serving eNB  115  and  120  receiver. Moreover, such alignment condition guarantees the orthogonality of the uplink-received signals thanks to the SC-FDMA signal properties. 
     The downlink symbol  405   M  is received at the intended MUE  125   M  after a propagation time Tp M , while the downlink symbol  405   P  is received at an intended PUE  125   P  after a propagation time Tp P . Similarly, the uplink symbol  410   M  is received at the MeNB  115  after the propagation time Tp M , and the uplink symbol  410   P  is received at the PeNB  120  after the propagation time Tp P . Moreover, the uplink symbol  410   M  is received also at the PeNB  115  after the propagation time Tp MP . 
     Advantageously, in order to help the PeNB  120  identifying the interfering MUEs  125   M , the MeNB  115  may communicate to the PeNB  120  the timing advance associated with each probably interfering MUE  125   M . Particularly, the MeNB  115  defines a respective timing advance for each MUE  125   M  coping with a propagation time (i.e., a delay due to the distance between the MUE  125   M  and the MeNB  115 ) of the MUE  125   M  transmissions. The timing advance synchronizes symbols transmissions from the MUE  125   M  with symbols reception of the MeNB  115  as a function of their distance. 
     In detail, thanks to the communicated timing advance, the PeNB  120  is then able to adjust a detection time window—the detection time window being a time period in which the uplink symbols  410   M  is expected to be received at the PeNB  120 —to correctly receive the DMRS associated to the probably interfering MUEs  125   M . In general, the timing advance may be used for facilitating detection and identification of each probably interfering MUEs  125   M  in order to improve the time accuracy in the DMRS detection by the PeNB  120 . 
     Particularly, a time instant t 1  that represents the start of the detection time window used by the PeNB  120  for detecting the presence of the DMRS may be calculated as follows:
 
 t   1   =T   0   −TA   MeNB  
 
where TA MeNB  is a time shift (e.g., a delay) corresponding to the timing advances imposed by the MeNB  115  to the MUEs  125   M  and signaled (through the X2 interface) also to the PeNB  120 .
 
     Taking into account that more than one MUE  125   M  may be analyzed by the PeNB  120  in a same TTI, the delay TA MeNB  is chosen as
 
 TA   MeNB =max{ TA   MeNB   1   ,TA   MeNB   2   , . . . ,TA   MeNB   K }
 
where K is the number of probably interfering MUEs  125   M  analyzed for DMRS detection in the same TTI.
 
     The length Δ of the detection time window used by the PeNB  120  for detecting effectively interfering MUEs  125   M  should be instead determined as
 
Δ=max{ Tp   MP   1   ,Tp   MP   2   , . . . ,Tp   MP   K }
 
where Tp MP   K  is the propagation delay associated with a symbol transmitted by the Kth MUE  125   M  (and intended to be received by the MeNB  115 ) received at the PeNB  120  (and probably interfering with PUEs  125   P  uplink transmissions). However, the quantity Tp MP   K  is not known to the PeNB  120 , since there is no direct interaction between the PeNB  120  and the MUEs  125   M .
 
     Nevertheless, an estimation of such propagation delays Tp MP   K  may be obtained by analyzing the timing advances sent by the PeNB  120  to the PUEs  125   P . Preferably, although not limitatively, a design of the detection time window could be based on the maximum value of the timing advances sent by the PeNB  120  to the PUEs  125   P  over a sufficiently long observation interval (i.e., an interval comprising a plurality of transmission frames). The length of this observation interval should be large enough in order to obtain a sufficient statistic that includes all the possible positions of the served PUEs, which substantially correspond to positions occupied by interfering MUEs  125   M  in the pico-cell  110  (i.e., the strongest interfering MUEs  125   M , which are closer to the PeNB  120  than interfering MUEs  125   M  outside the pico-cell  110 ). 
     The length Δ of the detection time window used by the PeNB  120  may be thus be approximated as
 
Δ=max{ Tp   PP   1   ,Tp   PP   2   , . . . ,Tp   PP   M }
 
where Tp PP   K  is a propagation delay after which a symbol transmitted by the Kth PUE  125   P  is received at the PeNB  120 .
 
     In order to minimize a computational effort, the search operation of the DMRS performed by the PeNB  120  can then be restricted over the time interval [t 1 ; t 1 +Δ], where the length Δ of the detection time window may be updated on a long-term basis by collecting the timing advance statistics at the PeNB  120 . 
     Alternatively the MeNB  115  may forward to the PeNB  120  a copy of configuration information comprised in a PRACH used by the probably interfering MUEs  125   M . Such configuration information may be used by the PeNB  120  for estimating the associated propagation delay Tp MP   K  and, thus, the length Δ of the detection time window. Particularly, the PRACH used in HetNet  100  contains configuration information necessary to synchronize the uplink transmission of UEs  125   M  and  125   P  with the respective eNB  115  and  120 . Indeed, thanks to the copy of configuration information comprised in the PRACH, the PeNB  120  may be able to substantially identifying communications from the MUEs  125   M  as performed by the MeNB  115 . 
     In a further embodiment according to the present invention, the PeNB  120  may directly estimate the propagation delay Tp MP   K  associated with the probably interfering MUEs  125   M , by jointly using copy of configuration information comprised in the PRACH (received from the MeNB  115 ) and the corresponding timing advance information of the probably interfering MUEs  125   M  signaled by the MeNB  115 . 
     In one embodiment of the present invention, the PeNB  120  is equipped with two different operative blocks adapted to perform a Fast Fourier Transform (FFT, not shown in the figures), denoted as FFT blocks hereinafter for the sake of brevity, in order to detect, at the same time, the DMRS of probably interfering MUEs  125   M  and of PUEs  125   P . In detail, a first FFT block is associated to a first detection time window and is adapted to detect uplink symbols  410   P  incoming from the PUEs  125   P , while a second FFT block is associated to a second detection time window and is adapted to detect uplink symbols  410   M  incoming from the MUEs  125   P . 
     In one alternative embodiment of the present invention, of which a schematic timing diagram of symbols transmitted in the HetNet  100  is shown in  FIG. 4B , in order to reduce the processing complexity, the downlink symbols transmitted by the MeNB  115  are advantageously time shifted, for example delayed, in a semi-static way (i.e., the delay being incremented over days, thus not compromising the MUEs  125   M  synchronization with the HetNet  100 ). In the example at issue, the transmission of the downlink symbols by the MeNB  115  is offset by a delay τ set equal to T 0 −TA MeNB . Such delay τ applied to the downlink symbols aligns the uplink symbols of the MUEs  125   M  and PUE  125   P  received at the PeNB  120 . This allows the detection of each DMRS of both the MUEs  125   M  and PUE  125   P  with a single FFT block provided in the PeNB  120 . 
     Preferably, although not limitatively, a Self-Organizing Networks (SON) algorithm may be provided in the HetNet  100  comprising the capability of automatically applying the delay τ to the MeNB  115  downlink frame in a semi-static way without any loss of synchronization between the MUEs  125   M  and the rest of the HetNet  100 . For example, the SON algorithm may be adapted to monitor downlink transmission timing in the cells  105 , 110  of the HetNet  100  in order to estimate a preferred value for the delay τ, and apply such delay τ daily to the downlink transmissions preferably during low-traffic periods (e.g., during nighttime) in the HetNet  100 . 
     Generally, more than one pico-cell (and more than one PeNB) may be deployed at least partly in the same macro-cell. In such case, it is not possible synchronizing symbols transmissions at each PeNB by shifting the transmissions of the MeNB, since each PeNB communication normally has a different propagation time, physical distances from respective PUEs and timing advances with respect to others PeNBs. On the contrary, it is possible to estimate a time advance τ P  (for example as previously described for the delay τ) for each PeNBs deployed at least partly in the same macro-cell. Then each time advance τ P  may be used to shift (anticipate) the downlink frame of the respective PeNB, thus aligning again the uplink symbols of the MUEs and PUEs at each PeNB. 
     It should be noted that a SON algorithm may be implemented also in a HetNet comprising a plurality of pico-cell deployed at least partly in the same macro-cell. The SON algorithm may be adapted to monitor downlink transmission timing in the cells, estimate the time advance τ P  corresponding to each PeNBs and apply such time advance τ P  periodically (e.g., daily) to the downlink transmissions preferably during low-traffic periods (e.g., during nighttime) in the HetNet  100 . 
     At this point, the PeNB  120  is able to identify which probably interfering MUEs  125   M  is an effectively cause of interferences measuring an indication of the transmission power of each probably interfering MUEs  125   M , thanks to the information received from the MeNB  115  (i.e., DMRS configuration parameters). For example, the PeNB  120  may directly measure the power of such interfering uplink transmission incoming from the probably interfering MUEs  125   M . 
     Each probably interfering MUE  125   M  will be identified as an effectively interferer if the measured power level of the associated DMRS (i.e., uplink transmission power level) is greater than a predetermined threshold power level. Advantageously, such threshold power level may be set according to a maximum interferer power level which do not compromise uplink transmissions incoming from the PUEs  125   P . 
     Alternatively, the PeNB  120  may estimate the interference associated with each probably interfering MUEs  125   M  by utilizing the Signal to Interferer plus Noise Ratio (SINR). The SINR calculation is typically performed by the PeNB  120  in order to evaluate the best Modulation and Coding Scheme (MCS) for the PUEs  125   P . The MCS is selected in such a way to maximize the rate of successful symbols transmission (i.e., throughput) in the future transmission in order to allow the eNBs  115  and  120  for an efficient resource scheduling. 
     Advantageously, the PeNB  120  may identify effectively interfering MUEs  125   M , among the probably interfering MUEs  125   M , by verifying if a value of the SINR at the frequency (or frequencies) associated with the uplink transmission of each probably interfering MUEs  125   M  falls below a minimum SINR value which do not compromise uplink transmissions incoming from the PUEs  125   P . In this case the PeNB  120  assume that all the measured interference is generated by the interfering MUEs  125   M  signaled by the MeNB  115 . 
     It should be noted that using the SINR for determining the effectively interfering MUEs  125   M  requires less processing power than power measurements, as the SINR estimation is already performed by the PUE  120  for estimating the best MCS to be provided in the scheduling grant. Conversely, power measurements are specifically performed for identifying the effectively interfering MUEs  125   M . However, such SINR identification is sub-optimal compared to the power level measurement identification previously described. Indeed, the interference estimation based on the SINR measurement cannot always be univocally associated to one of the probably interfering MUEs  125   M  signaled by the MeNB  115 . For example, there can be other UEs (also not shown in the figures) served by other neighboring eNBs that contribute to the measured interference level superimposed with the probably interfering MUEs  125   M  signaled by the MeNB  115 —especially, if the PeNB  120  is at least partly comprised in more than one macro-cell (covered by other MeNB, not shown in the figures), close to the boundary of the enclosing macro-cell  105 , or close to other PeNBs. 
     Afterwards, the PeNB  120  is able to signal the effectively interfering MUEs  125   M  back to the MeNB  115  (again, through the X2 interface) with an updated list (for example, comprising DMRS configurations and/or measured power levels) identifying the effectively interfering MUEs  125   M . 
     In addition, it should be noted that the synchronization between UEs  125   M  and  125   P  and the HetNet  100  impacts the subsequent power measurements performed by the PeNB  120 ; in particular, the ability of the PeNB  120  to identify the DMRS of the probably interfering MUEs  125   M . Therefore, in order to relax synchronization requirements, the MeNB  115  may configure the extended CP for the symbols in the sub-frames to be analyzed by the PeNB  120 . In detail, each transmitted symbol has to reach the intended receiver with a maximum delay equal to its own CP, and, since the extended CP is larger than the normal CP, the former relaxes synchronization requirements. However, such reconfiguration affects the HetNet  100  efficiency since the number resources available in the MeNB  115  are reduced (since with the extended CP only 6 symbols per slot may be transmitted). 
     According to an embodiment of the present invention, in order to limit such loss in efficiency, another SON algorithm may be provided in the HetNet  100  to be used to reconfigure resources of the sub-frames depending on the ability of the PeNB  120  to identify the DMRS of the probably interfering MUEs  125   M . For example, the SON algorithm may be adapted to identify DMRS detection failures (i.e., when the PeNB  120  is not able to identify to which probably interfering MUE  125   M  belongs the incoming uplink symbols  410   M ) and to instruct to the MeNB  115  to switch from normal CP to extended CP for symbols transmitted through a corresponding sub-frame. Alternatively, the PeNB  120  may be adapted to directly provide information of DMRS detection failures to the SON algorithm, or to the MeNB  115 . 
     Finally, the MeNB  115  takes actions on the effectively interferer MUEs  125   M  in order to suppress, or at least to substantially reduce, the interferences brought by the latter to the uplink transmissions between PUEs  125   P  and the PeNB  120 . 
     Preferably, although not necessarily, the MeNB  115  changes the allocation in the transmission band of the effectively interfering MUEs  125   M . For example, the MeNB  115  may modify the scheduling of the resources for the effectively interfering MUEs  125   M , in order to associate the latter with a different component carrier sets or with different sub-carriers sets (possibly orthogonal with respect to the component carrier/sub-carriers sets associated to PUEs  125   P ). 
     It should be noted that, since the quality of communications is impacted by the orthogonality between sub-carriers of the MUEs  125   M  and of the PUEs  125   P , in one embodiment of the present invention, the MeNB  115  and PeNB  120  may provide disjoint DMRS to the MUEs  125   M  and PUEs  125   P . Advantageously, the disjoint DMRS are selected in order to grant orthogonality between the sub-carriers of the MUEs  125   M  and of the PUEs  125   P . In this way, it is possible to attain lower interferences between uplink communications performed in the cells  105  and  110  than the case in which DMRS are assigned randomly. In addition, also the DMRS detection performed by the PeNB  120  is facilitated thanks to the separation of MUEs  125   M  and PUEs  125   P  granted by the orthogonality thereof.