Method and apparatus for interference mitigation in wireless systems

This application relates to coordinated multi-point transmission (CoMP) and, more particularly, to a method to mitigate interference for user equipment having CoMP capability.

In conventional multi-cell systems, users at the cell edge suffer from high inter-cell interference and are not able to achieve high throughputs because of their low signal-to-interference plus noise ratio. FIG. 1 shows a wireless neighborhood 100 having multiple cellular regions or cells 40 , each cellular region being serviced by a base station 30 . Base stations 30 A- 30 G occupy the wireless neighborhood 100 (collectively, base stations 30 ). A mobile device or other user equipment (UE) 20 moving through the wireless neighborhood 100 is occasionally located at the edge of a cellular region 40 . Although the UE 20 is serviced by the base station 30 A, when at the edge of the center cellular region 40 , the UE 20 may experience interference from base stations located in several of the surrounding cellular regions, namely, from base stations 30 B, 30 C, 30 D, and 30 E.

User equipment located at the edge of the cell 40 thus may experience high inter-cell interference, which, in turn, causes low throughputs because of a low signal-to-interference plus noise ratio (SINR).

Thus, there is a continuing need for a method to overcome the shortcomings of the prior art.

The foregoing aspects and many of the attendant advantages of this document will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein like reference numerals refer to like parts throughout the various views, unless otherwise specified.

FIG. 1 is a diagram of a wireless neighborhood having cellular regions and base stations serving user equipment, according to the prior art;

FIG. 2 is a block diagram of an enhanced base station that employs an enhanced interference mitigation method, including enhanced coordinated multi-point transmission and enhanced fractional frequency reuse, according to some embodiments;

FIG. 3 is a diagram of a cluster formed from three cells, where the cluster is used by the enhanced interference mitigation method of FIG. 2 , according to some embodiments;

FIG. 4 is a diagram of a wireless neighborhood having multiple clusters used to illustrate inter-cluster interference which is mitigated by the enhanced interference mitigation method of FIG. 2 , according to some embodiments;

FIG. 5 is a diagram of the enhanced coordinated multi-point transmission used by the enhanced interference mitigation method of FIG. 2 , according to some embodiments;

FIG. 6 is a diagram showing operations between the base stations and a backhaul used by the enhanced interference mitigation method of FIG. 2 , according to some embodiments;

FIG. 7 is a diagram of a cluster employing coordinated multi-point transmission used by the enhanced interference mitigation method of FIG. 2 , according to some embodiments;

FIG. 8 is a diagram of the backhaul role in employing coordinated multi-point transmission used by the enhanced interference mitigation method of FIG. 2 , according to some embodiments;

FIG. 9 is a flow diagram illustrating the enhanced coordinated multi-point transmission used by the enhanced interference mitigation method of FIG. 2 , according to some embodiments;

FIG. 10 is a diagram of a cluster that is partitioned for performing enhanced fractional frequency reuse by the enhanced interference mitigation method of FIG. 2 , according to some embodiments; and

FIG. 11 is a flow diagram illustrating the enhanced fractional frequency reuse operations performed by the enhanced interference mitigation method of FIG. 2 , according to some embodiments.

In accordance with the embodiments described herein, a method is disclosed to eliminate inter-cluster interference of user equipment located at the edge of a cluster of cells. The method operates by employing fractional frequency reuse (FFR) principles on clusters or combinations of cells in a wireless neighborhood, in which base stations in the cells coordinate their operations in a scheme known as coordinated multi-point transmission (CoMP). By using the FFR principles to single out edge users of the cluster, the method mitigates interference and increases throughput for the edge users. This new scheme may be considered as an extension of conventional FFR to the cluster level.

In the following detailed description, reference is made to the accompanying drawings, which show by way of illustration specific embodiments in which the subject matter described herein may be practiced. However, it is to be understood that other embodiments will become apparent to those of ordinary skill in the art upon reading this disclosure. The following detailed description is, therefore, not to be construed in a limiting sense, as the scope of the subject matter is defined by the claims.

FIG. 2 is a block diagram of a base station 30 employing the enhanced interference mitigation method 400 , according to some embodiments. The base station 30 is equipped, at a minimum, with a processor 82 and a memory 84 , such that the enhanced interference mitigation method 400 may be executed thereon. The enhanced interference mitigation method 400 enhances and combines two known principles, coordinated multi-point transmission and fractional frequency reuse, in a novel manner, to mitigate interference for user equipment located at the edge of a cluster. To illustrate the enhanced interference mitigation method 400 , the principles of enhanced coordinated multi-point transmission 200 and enhanced fractional frequency reuse 300 , as well as how those techniques are combined, are described in more detail below. In some embodiments, the enhanced interference mitigation method 400 is a software program that is loaded into the memory 84 of the base station 30 and executed by the processor 82 thereon. In other embodiments, the enhanced interference mitigation method 400 is a combination of software and hardware components, in which the software portion is executed by the processor 82 .

The wireless neighborhood 100 of FIG. 1 , introduced in the background section, illustrates the principle of inter-cell interference, in which the user equipment 20 experiences interference from base stations outside the relevant cell, that is, base stations that are not the user equipment's home base station. To remove inter-cell interference and increase the capacity of the network, network multiple-input-multiple-output (MIMO), also known as coordinated multi-point transmission (CoMP), has been recently proposed. Under CoMP, a limited number of neighboring cells form a cluster (CoMP set), and the base stations in the newly formed cluster perform joint coordination or pre-coding to serve users in the cluster by exchanging information through a backhaul.

FIG. 3 is a block diagram of a cluster or CoMP set 50 , according to some embodiments. The cluster 50 consists of three cells 40 A, 40 B, and 40 C, with supporting enhanced node base stations (eNBs) 30 A, 30 B, and 30 C, respectively. As used herein, an enhanced node base station is a base station that supports CoMP. According to CoMP, user equipment 20 (e.g., the mobile device) entering into the cluster 50 , will be serviced by joint coordination of each of the base stations 30 A, 30 B, and 30 C. Although the user equipment 20 is jointly serviced by the three eNBs, one of the eNBs remains the designated “home” base station. The home eNB is generally the base station that is closest to the user equipment (e.g., the one having the lowest path loss from the user equipment).

By using CoMP, interference for the users in the interior of the cluster 50 may be completely removed. However, just as with the cells 40 in the previous example ( FIG. 1 ), users on the edge of the cluster 50 are likely to experience interference caused by one or more base stations in the neighboring clusters (inter-cluster interference).

This is illustrated in the wireless neighborhood 100 of FIG. 4 , in which neighboring cells 40 form clusters 50 of size three. Three neighboring cells 40 A, 40 B, and 40 C form cluster 50 A (blue), neighboring cells 40 D, 40 E, and 40 F form cluster 50 B (rose), and neighboring cells 40 G, 40 H, and 40 J form cluster 50 C (lilac). The UE 20 is located at the edge of the cluster 50 B. The eNBs 30 D, 30 E, and 30 F in the cluster 50 B cooperate and transmit a useful signal to the UE 20 (black arrows) while the eNBs 30 C, 30 G, 30 K, and 30 M in the neighboring clusters transmit interfering signals to the UE 20 (red arrows) and are thus likely to cause inter-cluster interference to the mobile device.

Under enhanced CoMP 200 , the UE 20 estimates its channel direction to different eNBs by measuring reference signals 90 transmitted from different eNBs in the cluster 50 and feeds back the measured information to its “home” eNB by using the appropriate pre-coding matrix indicator (PMI). Moreover, the UE 20 measures the channel quality from different eNBs by estimating the SINR and reports the appropriate CQI to its “home” eNB.

The enhanced CoMP operations 200 are illustrated in FIG. 5 , according to some embodiments. Using cluster 50 B from FIG. 4 as an example, the UE 20 receives reference signals 90 D, 90 E, and 90 F from eNBs 30 D, 30 E, and 30 F, respectively. The UE 20 uses these reference signals to estimate its channel direction to different eNBs, then sends the measured information to the home eNB using the PMI, shown in FIG. 5 as PMI calculation 70 . Further, the UE 20 measures the channel quality from these same eNBs 30 D, 30 E, 30 F, and sends the measured CQI to the home eNB, shown in FIG. 5 as CQI calculation 80 . In the example of FIG. 5 , the eNB 30 F is the home eNB.

In addition, in some embodiments, each UE 20 decides the actual size of its cooperation based on calculating the path losses from the eNBs 30 in the cluster 50 to the UE 20 , and comparing them with a predefined threshold value. Therefore, the actual “cooperating eNBs” (e.g., the number of eNBs that are used to perform enhanced CoMP 200 ) may be less than the number of eNBs in the cluster. As an example, where path losses exceed the predefined threshold value for eNB 30 D, eNB 30 E and eNB 30 F are the cooperating eNBs while eNB 30 D does not participate in the enhanced CoMP operations 200 .

FIG. 6 shows an example where three eNBs 30 A, 30 B, 30 C in a cluster 50 of having three cells 40 A, 40 B, 40 C jointly transmit respective signals 32 A, 32 B, 32 C to the UE 20 . In some embodiments, all of the information associated with the signal transmission, shown as channel information and data exchange 36 , is shared between all eNBs in the cluster 50 through a backhaul 34 , to be used for joint signal transmission to the UE 20 . The backhaul 34 is actually a link that connects the eNBs in a wireless neighborhood and is used to transfer data and channel information to the eNBs.

FIG. 7 is a diagram of the cluster 50 having three cells 40 A, 40 B, 40 C, each cell having a respective eNB 30 A, 30 B, and 30 C and servicing a different UE 20 A, 20 B, 20 C, according to some embodiments. Recall from FIG. 5 that the UE 20 measures the reference signals transmitted from different eNBs (which is how the UE estimates its channel direction to the eNBs), and then feeds this information back to its home eNB using the PMI (PMI calculation 70 ). The UE 20 also measures the channel quality from the different eNBs by estimating the SINR (CQI calculation 80 ), and reports this to the home eNB as well. FIG. 7 shows PMI/CQI feedback 38 , the results of the PMI calculation 70 and the CQI calculation 80 being fed back by each UE 20 A, 20 B, 20 C to its respective home eNB 30 A, 30 B, and 30 C. Although FIG. 7 makes it appear as though the eNBs are communicating with one another directly, the channel information and data exchange 36 is fed between eNBs through the backhaul 34 (not shown). Channel information and data are shared among all eNBs of the cluster 50 .

FIG. 8 provides a more detailed view of the backhaul 34 first introduced in FIG. 6 , according to some embodiments. The cluster 50 includes three cells 40 G, 40 H, 40 J, in which three respective eNBs 30 G, 30 H, and 30 J reside, and six UEs 20 M, 20 N, 20 P, 20 Q, 20 R, 20 S, with each eNB communicating with the backhaul 34 . After the eNBs collect the information, shown as channel information and data exchange 36 , a scheduler and beamformer 42 in a central processor 86 connected to the backhaul 34 collects data 44 and channel information 46 and selects the UEs in the cluster 50 to be served. The scheduler and beamformer 42 further calculates the pre-coding matrices that will be used to cancel the interference between UEs in the cluster 50 . In some embodiments, the scheduler and beamformer uses zero-forcing beamforming (ZFBF) to calculate the pre-coding matrices. Following this calculation, the appropriate signal is sent to each eNBs 30 G, 30 H, 30 J, to be transmitted to the selected UEs. In FIG. 8 , the UEs 20 N, 20 Q, and 20 R are selected while UEs 20 M, 20 P, and 20 S are not selected during this analysis.

FIG. 9 is a flow diagram summarizing the above-described enhanced CoMP operations 200 , according to some embodiments. As depicted in FIG. 5 , the UE receives the reference signals from the respective eNBs in the cluster 50 (block 202 ). From these reference signals, the UE is able to estimate its channel direction to each eNB in the cluster (block 204 ). The UE then calculates the CQI (CQI calculation 80 ) and PMI (PMI calculation 70 ) corresponding to all cooperating eNBs (block 206 ), where the number of cooperating eNBs may be less than all of the eNBs in the cluster. The calculated CQI 80 and PMI 70 are then sent to the home eNB (block 208 ), but are also received by the scheduler and beamformer 42 located in the central processor 86 (see FIG. 8 ) (block 210 ). Finally, the scheduler and beamformer 42 selects the UE, forms the beams for transmission to the UE, and sends them through the backhaul to the eNBs, where they are then sent to the UE (block 212 ).

Although by using enhanced CoMP 200 , interference for the UEs in the interior of the cluster 50 may be removed entirely, the UEs on the edge of the clusters may be adversely affected by interference caused by eNBs in neighboring clusters, known herein as inter-cluster interference. So, like the inter-cell interference experienced by the UE 20 in FIG. 1 , the UE 20 A in FIG. 4 experiences inter-cluster interference when the cells 40 are organized for CoMP.

To eliminate the inter-cluster interference, an inter-cluster interference mitigation (ICIM) method 400 is proposed, according to some embodiments. Under the ICIM method 400 , enhanced fractional frequency reuse (FFR) 300 is used in conjunction with enhanced CoMP 200 , as described above, to mitigate interference experienced by cluster edge UEs and to thereby increase the throughput of such cluster edge UEs. FIG. 10 is a diagram showing three clusters 50 A, 50 B, 50 C in which base stations within each cluster employ enhanced CoMP 200 , as described above, as well as enhanced FFR 300 , to coordinate operations to/from UEs traveling through the respective clusters. Employing the principles of fractional frequency reuse to clusters, each cluster 50 A, 50 B, 50 C is partitioned into a center region 62 (green) and an edge region, with cluster 50 A having a yellow edge region 64 A, cluster B having a blue edge region 64 B, and cluster 50 C having a red edge region 64 C. The depiction in FIG. 10 merely represents that the clusters are partitioned into center regions and edge regions, and is not intended to reflect the relative amounts of each. Therefore, the edge regions may be larger (thicker) or smaller (thinner) than is depicted in FIG. 10 .

Each cluster has a defined frequency band, also featured in FIG. 10 , such that UEs disposed at the center region 62 of any of the clusters utilize the frequency range 52 , while UEs disposed at the edge regions utilize smaller but dedicated regions of the frequency band. UEs at the edge region 64 A (cluster 50 A) utilize frequency range 54 , UEs at the edge region 64 B (cluster 50 B) utilize frequency range 56 , and UEs at the edge region 64 C (cluster 50 C) utilize frequency range 58 . What the frequency band depiction in FIG. 10 illustrates is that UEs at the center of the cluster, which are not as likely to experience inter-cluster interference as edge UEs, are able to share the frequency band 52 shown in green. On the other hand, the edge-located UEs, which are more likely to experience inter-cluster interference, are supplied with a dedicated portion of the frequency band 54 (yellow), 56 (blue), or 58 (red) so as to mitigate such interference. The UE located in the edge region 64 A of cluster 50 A only shares the frequency band 54 with other UEs located in the same edge region 64 A, but does not share the frequency band with a UEs located in the edge region 64 B of cluster 50 B, the latter UE instead using the frequency band 56 . In this way, the edge UEs are likely to transmit to and receive from their respective eNBs without interference.

FIG. 11 is a flow diagram showing operations performed by the enhanced FFR 300 of the ICIM method 400 ( FIG. 2 ), according to some embodiments. Initially, cells 40 are combined to form clusters 50 (block 302 ). In the examples herein, the clusters are of size three, with three cells making up the cluster, but the cluster size may be larger or smaller. The system resources, namely, the frequency band, are then divided into orthogonal resources suitable for performing FFR (block 304 ).

Next, the enhanced FFR 300 of the ICIM method 400 determines the location of the UE (block 306 ). If the UE is located in one of the edge regions 64 A, 64 B, or 64 C (the “yes” prong of block 306 ), the enhanced FFR method 300 allocates resources for the edge region UEs, namely the frequency band 54 , 56 , or 58 (block 310 ). Otherwise, the UE is located in a center region of one of the clusters 50 and the enhanced FFR 300 allocates resources accordingly, namely, the frequency band 52 (block 308 ). In either circumstance, the enhanced FFR 300 serves the UE using the above-described enhanced CoMP 200 ( FIG. 9 ), namely, by eNBs 30 residing in the cluster 50 coordinating their operations so as to serve the UE.

By combining enhanced CoMP 200 with enhanced FFR 300 , the ICIM method 400 ensures that the interference will be eliminated (or significantly reduced) for all the users in a cluster. For users in the interior of the cluster, the interference is mitigated due to the enhanced CoMP 200 , while for users at the edge of the cluster, the interference is mitigated due to the enhanced FFR 300 . Furthermore, the total throughput of the system is improved, in some embodiments.

While the application has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention.