Patent Application: US-17820208-A

Abstract:
the present invention relates to a method for reducing interference in a cellular radio communication network , said network comprising a plurality of base stations and a plurality of terminals to be scheduled on frequency and / or time resources . according to the present invention , the method comprises the steps of : determining interference parameters from at least two base stations ; reporting said interference parameters from said at least two base stations to an interference coordinator ; determining at said interference coordinator resources to be allocated to said plurality of terminals minimising an overall interference criterion ; reporting from said interference coordinator to said at least two base stations the resource allocated to the different terminals .

Description:
in recent years , orthogonal frequency division multiple access ( ofdma ) has become an attractive transmission technology , which is part of various emerging system standards for broadband cellular communications . examples include the 3gpp long term evolution ( lte ) and 802 . 16e wimax . in ofdma , mobile terminals are multiplexed in time and frequency . a major problem in these systems is the inter - cell interference , which is caused by neighboring cells when transmitting on the same time and frequency slots . this problem can be solved by using beamforming antennas and coordinating the transmissions among base stations . this is known as interference coordination . in this paper , we present a distributed algorithm for interference coordination , which enhances the cell edge performance with global information provided by a central coordinator . the signaling delay during the communication with the central coordinator can be on the order of seconds , while an additional local interference coordination in each base station ensures a high performance even in dynamic environments . this combination of global and local coordination enhances the overall spectral efficiency by 50 % compared to a reuse 3 system while maintaining the same cell edge performance . orthogonal frequency division multiple access ( ofdma ) is the basis for several emerging standards for wireless broadband communication . in particular , it is the underlying transmission technology for 802 . 16e ( wimax ) and the future 3gpp long term evolution ( lte ). in ofdma , the different users are multiplexed in time and frequency based on an underlying ofdm system . therefore , ofdma is basically a combination of frequency and time division multiple access ( fdma and tdma ). a major problem in these systems is the inter - cell interference , which is caused by neighboring cells when transmitting in the same frequency / time slots . this eventually leads to severe performance degradation or even connection loss . there exist several approaches to mitigate inter - cell interference . the most common approach is to employ a frequency reuse pattern and avoid the usage of the same frequency bands in adjacent cells . the disadvantage of this scheme is the waste of precious frequency resources . instead , it is desirable to reuse the whole available frequency spectrum in every cell . another possibility to lower inter - cell interference is to use beamforming antennas , which direct their transmission power towards the currently served mobile terminal . this minimizes interference towards other mobile terminals . last but not least , the transmissions in different cells can be coordinated to optimize the interference situation in all cells . this is referred to as interference coordination ( ifco ). all these approaches can be combined to optimize the system performance in a dynamic fashion [ 13 ]. in [ 11 ] we introduced an interference coordination algorithm which is based on an interference graph . this graph represents critical interference relations among mobile terminals . if two mobile terminals in different cells have a critical relation , they may not be served on the same frequency / time resource . this scheme requires a central omniscient entity which is capable to acquire the system state instantly and perform scheduling decisions in all cells on a per - frame basis . naturally , such a scheme is not implementable . however , it provides important information about the key performance parameters and also delivers an estimate of the upper performance bound . in [ 13 ], we limited this interference graph based scheme to the coordination of the cell sectors served by the same base station . this was combined with fractional frequency reuse ( ffr ), which applies a frequency reuse of 1 in the inner portions of the cell and a frequency reuse of 3 in the outer portions . this combination can achieve the same aggregate cell throughput as the global scheme from [ 11 ], but it falls short with respect to the throughput at the cell borders . this paper has two main contributions . first , we considerably enhance the global scheme of [ 11 ] by an optimized generation of the interference graph . second , based on these optimizations , we present a novel interference coordination scheme named coordinated fractional frequency reuse , which achieves a high aggregate and cell border performance at the same time . this scheme does not require an omniscient device which performs scheduling decisions every frame . instead , it relies on the communication of the base stations with a central entity on a much longer time scale , e . g ., on the order of seconds . therefore , the proposed scheme is well implementable in a real system . this paper is structured as follows . section 2 introduces the considered 802 . 16e system and its simulation model . section 3 rehearses the global scheme from [ 11 ] and presents an optimized generation of the interference graph . subsequently , section 4 briefly introduces the concept of ffr and derives the proposed coordinated ffr scheme . finally , section 5 presents a thorough performance evaluation of the coordinated ffr scheme , and section 6 concludes the paper . we consider an 802 . 16e - system [ 10 ] with a total available system bandwidth of 10 mhz and a mac - frame - length of 5 ms . this results in a total number of 49 ofdm - symbols per mac - frame and 768 data subcarriers per ofdm - symbol . each mac - frame is subdivided into an uplink and a downlink subframe . both subframes are further divided into zones , allowing for different operational modes . in this paper , we focus on the adaptive modulation and coding ( amc ) zone in the downlink subframe . in particular , we consider the amc 2 × 3 mode , which defines subchannels of 16 data subcarriers by 3 ofdm - symbols . this is illustrated in the left part of fig1 . a subchannel corresponds to the resource assignment granularity for a particular mobile terminal . the amc zone can therefore be abstracted by the two - dimensional resource field shown in the right part of fig1 . we assume the amc zone to consist of 9 ofdm - symbols , corresponding to a total number of 48 · 3 available subchannels . adaptive modulation and coding was applied ranging from qpsk 1 / 2 up to 64qam 3 / 4 . this results in a theoretical maximum data rate of about 6 . 2 mbps within the amc zone . the burst profile management is based on the exponential average of the terminal &# 39 ; s sinr conditions with channel feedback delay of one mac - frame . we consider a hexagonal cell layout comprising 19 base stations at a distance of d bs = 1400 m with 120 ° cell sectors as shown in fig2 . the scenario is simulated with wrap - around , making all cells equal with no distinct center cell . all cells were assumed to be synchronized on a frame level . throughout our paper , we evaluate the shaded observation area when investigating the cell coverage , and the average of all cell sectors when considering throughput metrics . besides the total sector throughput , we evaluate the 5 % throughput quantile , which is a good indication for the achievable throughput in the cell border areas [ 3 ]. it is captured by measuring the average short - term throughput of each terminal within 4 - second periods and calculating the quantile over all measurements . all throughput measures were taken on the ip - layer , capturing all overhead caused by fragmentation , padding , and retransmissions . every base station has 3 transceivers , each serving one cell sector . the transceivers are equipped with linear array beamforming antennas with 4 elements and gain patterns according to [ 1 ]. they can be steered towards each terminal with an accuracy of 1 ° degree , and all terminals can be tracked ideally . the system model was implemented as a frame - level simulator using the event - driven simulation library ikr simlib [ 1 ] with all relevant mac protocols , such as arq and harq with chase combining . the path loss was modeled according to [ 8 ], terrain category b . slow fading was considered using a log - normal shadowing model with a standard deviation of 8 db . frame errors were modeled based on bler - curves obtained from physical layer simulations . each sector contains n mobile terminals moving at a velocity of υ = 30 km / h . the underlying mobility model is a random direction model with a mean free path length of 50 m and a maximum turning angle of 25 °. all mobile terminals are bound to their respective cell sector in order to avoid handovers . a greedy traffic source is transmitting data towards each terminal , i . e ., there is always data available to be transmitted for a terminal ( see also [ 11 ]). in [ 11 ], a scheme for global interference coordination in a cellular ofdma network was proposed . it is based on an interference graph whose nodes represent the mobile terminals , and whose edges represent critical interference relations in - between the terminals . terminals which are connected must not be served using the same set of resources . for each terminal , the interference from base stations within a certain diameter d ic of the serving base station is calculated . afterwards , the largest interferers are blocked from using the same set of resources by establishing a relation in the interference graph . this is done such that a desired minimum sir d s is achieved for each terminal . for a detailed description , please refer to [ 11 ]. the original scheme in [ 11 ] assumed a central omniscient device which is capable of acquiring the system state instantly and perform scheduling decisions on a per - frame basis . naturally , such a scheme is not implementable . however , it provides important information about the key performance parameters and also delivers an estimate of the upper performance bound . the two major configurable parameters of the interference graph based scheme are the desired minimum sir ds and the coordination diameter d ic . we refer to d ic = 0 as zero - tier coordination , indicating a coordination only among sectors of the same base station . alike , a one - tier coordination means a coordination of neighboring base station sites , i . e ., d ic = d bs , and a two - tier coordination implies a global coordination of all cells in our scenario . fig3 plots the 5 % throughput quantile over the aggregate throughput per cell [ 13 ]. the zero - tier coordination , which can be implemented locally within a base station , achieves a significant improvement compared to the reuse 3 scenario with respect to the overall cell sector throughput , but falls short with respect to the throughput quantile , i . e ., with respect to the cell edge performance . as the number of coordinated tiers increases , the performance with respect to both the aggregate throughput and the throughput quantile increases . global interference coordination based on the interference graph is directly related to the graph coloring problem [ 12 ]. in the graph coloring problem , each node in a graph needs to be assigned one color such that no connected nodes are assigned the same color . if the colors correspond to non - overlapping resources on the air interface , then the solution of the graph coloring problem corresponds to the assignment of disjoint resources on the air interface to nodes which have a relation in the interference graph . in general , the graph coloring has to be recomputed whenever the interference graph changes , i . e ., every frame . let m be the chromatic number of the interference graph , i . e ., m is the number of colors which are required to solve the graph coloring problem . then , for each scheduling round , the air interface resources need to be divided into m disjoint partitions . each of these partitions is then assigned to a mobile terminal based on its assigned color . note that a solution of the problem requires at least n colors , where n is the number of mobile terminals in each cell sector . in general , m will be larger than n , making more disjoint resource partitions necessary than there are mobile terminals to serve . consequently , the resource utilization in a cell sector will drop and in general be ρ = n / m & lt ; 1 . graph coloring is an np - hard problem . various heuristics have been proposed to find near - optimal solutions . in this paper , we use the heuristic dsatur [ 7 ] and a tabu search technique [ 9 ] to obtain colorings . while dsatur is quite fast , tabu search obtains much better solutions , though at the cost of a highly increased computational complexity . in contrast to these heuristics for the classical coloring problem , the resource assignment heuristic from [ 11 ], which was also used to generate the results of section 3 . 2 , solves a variant of the graph coloring problem . it differs from the just described original graph coloring problem in that only a certain number n f & lt ; n of mobile terminals is served within each scheduling round . in other words , only n f colors are used , and only n f mobile terminals need to be assigned a color . this leads to a much better resource utilization ρ as compared to when using a full coloring as described in this section , and consequently to a better system performance . in both cases , the resource utilization ρ depends on the structure of the interference graph . in particular , it depends on the vertex degree of all nodes . for the full coloring as described above , a higher vertex degree will lead to a larger chromatic number m and a lower resource utilization ρ . likewise , the performance of the coloring heuristic from [ 11 ] will suffer from a larger vertex degree . the most important parameters having an impact on the vertex degree are the coordination diameter d ic and the minimum desired sir d s . if d ic or d s are increased , the vertex degree and hence the chromatic number m increases , leading to a lower resource utilization ρ . this is illustrated in fig4 , which plots the mean vertex degree of a node in the interference graph depending on the position of the corresponding mobile terminal . the figure illustrates the increase in the vertex degree for an increase of d ic or d s . in the following section , we will evaluate ways to construct the interference graph in such a way that the vertex degree and hence m is reduced , eventually leading to a better resource utilization . the results from [ 13 ], which were rehearsed in the previous sections , show that a one - tier or two - tier coordination with a fairly low desired sir d s achieves an excellent cell edge performance while falling short with respect to the aggregate throughput . on the other hand , a zero - tier coordination provides an increased aggregate performance while falling short with respect to the throughput quantile . both configurations feature an interference graph with a relatively small vertex degree compared to the optimum case of a two - tier coordination with d s = 10 db , as can be seen from fig4 . this suggests that a separate generation of zero - tier and one / two - tier interference graphs and a subsequent merging of both graphs will lead to a lower vertex degree as in the two - tier case while providing an optimized sir within the area . as discussed before , a lower vertex degree will lead to a lower chromatic number of the interference graph and hence potentially increase the system performance . merging of the two graphs is done simply by including an edge in the merged graph whenever one of the two original graphs contains an edge . the performance of a system with a global coordination based on the combination of interference graphs is plotted in fig5 for a combination of a zero - tier interference graph with a one - tier ( left ) and a two - tier ( right ) interference graph . we denote the one - tier graph as inner graph , which is generated for a desired minimum sir d s , i , and the one / two - tier graph as outer graph , generated for a desired minimum sir d s , o . fig5 shows a strong dependence of the system performance on d s , o . as we increase d s , o from − 5 db in the one - tier case , we first observe a performance increase at d s , o = 0 db , while it decreases for d s , o = 5 db . for d s , o = 10 db the performance significantly improves again , and finally decreases for d s , o = 15 db and larger values . the two separate maxima can be explained by the superposition of the curves in fig3 , which show maxima for different values of d s . this superposition is caused by the merging of the interference graphs . the combination of zero - tier and one - tier interference graphs ( fig5 left ) has two optimal configurations . for d s , o = 0 db , the cell border performance is maximized , while for d s , o = 10 db the aggregate throughput is maximized . in contrast , the combination of zero - tier and two - tier interference graphs ( fig5 right ) shows an optimal operating point for d s , o = 10 db which maximizes both the aggregate throughput and the cell border performance . this raises the spectral efficiency of the global scheme by more than one third to over 1 . 1 bit / hz and can be explained with the better control of the two - tier scheme over the interference in the cell border areas . fig6 plots the throughput depending on the mobile terminal &# 39 ; s position for the global two - tier coordination according to section 3 . 1 and [ 11 ]. fig7 plots the same metric for the optimized coordination mechanism with combined zero - tier and two - tier coordination . we can observe an increased throughput at the cell borders but also a substantial performance increase in the central parts of the cell sectors . in this section , we first give an overview over state - of - the - art fractional frequency reuse techniques . subsequently , we introduce the concept of coordinated fractional frequency reuse . a system with a frequency reuse factor of 1 achieves a high resource utilization of 100 %, while suffering from heavy inter - cell interference in the cell border areas . on the other hand , a frequency reuse 3 system achieves acceptable interference conditions at the cell border , but has a resource utilization of only ⅓ . one possibility to resolve this dilemma is fractional frequency reuse ( ffr ). with ffr , a frequency reuse of one is applied in areas close to the base station , and a higher reuse factor in areas closer to the cell border . this idea was proposed for gsm networks ( see for example [ 6 ]) and has consequently been adopted in the wimax forum [ 2 ], but also in the course of the 3gpp long term evolution ( lte ) standardization , e . g ., in [ 5 ] and [ 4 ]. fig8 schematically illustrates the division of air interface resources . basically , there exist two options . [ 14 ] proposed that the reuse 1 and reuse 3 areas be on disjoint frequency bands ( fig8 bottom ), while [ 5 ] and [ 4 ] use the full set of available resources in the reuse 1 areas and one third of the same resources in the reuse 3 areas ( fig8 top ). in the remainder of this paper , we will base our work on the latter option . we will refer to mobile terminals in the reuse 1 area as reuse 1 terminals , and to mobile terminals in the reuse 3 area as reuse 3 terminals . proposes to combine ffr with a coordination of the transmissions among the sectors of one base station site . this lowers the interference between sectors belonging to the same base station . however , it is not possible to lower the interference caused by other base stations , which is why such a system still suffers from a rather low throughput at the cell border to neighboring base stations ( see [ 13 ] for an extensive performance evaluation ). in the following section , we will develop an ifco scheme which overcomes this problem , and which eventually could be implemented under realistic signaling delays . while the aggregate throughput in an ffr system can be increased by performing an additional local interference coordination among the sectors served by one base station ( zero - tier coordination ), the interference from other base stations cannot be controlled . coordinated fractional frequency reuse overcomes this drawback by introducing a generalized frequency reuse pattern for mobile terminals at the cell border which is coordinated by a central entity . this concept is illustrated in fig9 . every cell communicates all data which is necessary to generate an interference graph as described in section 3 to the central ffr coordinator . this includes measured interference and pathloss components for all mobile terminals . the coordinator then performs a complete graph coloring as detailed in section 3 . 3 . the result of the coloring process is a color index for every mobile terminal , which is transmitted back to the base stations . if n c is the number of colors in the coloring ( where n c ≦ m in general due to the sub - optimality of the graph coloring heuristics ), the amc - zone of a mac frame is then divided into n c parts , each corresponding to a reuse partition . this is illustrated in fig1 on the right side . the reuse 3 terminals at the cell border are then assigned to a partition depending on their color , while reuse 1 terminals in the inner cell areas can still utilize the full amc - zone . alternatively , in order to avoid a fragmentation of the amc - zone into very small partitions , the reuse partitions can be spread over several mac - frames , as it is illustrated in fig1 on the right side . in the following , the number of amc - zones ( i . e ., the number of mac - frames ) which are required will be called a virtual frame duration . in our example of fig1 , the amc - zone of a single frame is divided into n f = 4 partitions , and the virtual frame duration is ┌ n c / n f ┐=┌ n c / 4 ┐. in order for this scheme to be practical , the communication with the ffr coordinator needs to be limited . fig1 shows a signaling - time diagram of the communication with the ffr coordinator . the base stations report the data which is required to build the interference graph with an update period of t c , period . after a certain delay t c , delay , the coloring of the ffr coordinator arrives at the base stations and is then valid until the arrival of the next coloring . the delay t c , delay includes all signaling delays , the processing delay in the ffr coordinator , and also all necessary synchronization delays . coordinated ffr ensures a coordinated allocation of resources to reuse 3 terminals , i . e ., to mobile terminals at the cell border . this global coordination is done on a larger time - scale due to the signaling delays described above . in order to make the system more agile , we perform an additional local coordination in every base station ( zero - tier coordination ), which coordinates the transmission of all reuse 1 terminals . this local coordination can operate on up - to - date system state information of all cell sectors served by the respective base station . the procedure is summarized as follows . first , all reuse 3 terminals are reserved resources in their respective reuse partition ( cmp . fig1 ). second , in every frame , the regular zero - tier coordination scheme is applied in every base station , obeying all conflicts in the inner interference graph . this is repeated periodically . note that the scheduling and resource assignment is still done on a per - frame basis . if a reuse 3 terminal does not need to be scheduled in its reserved reuse partition , the idle resources can be used by reuse 1 terminals if allowed by the local interference coordination . this allows for a high resource utilization while at the same time ensuring good interference conditions for mobile terminals at the cell border . this section evaluates the performance of the proposed coordinated ffr . section 5 . 1 first investigates the performance of coordinated ffr under ideal signaling conditions . next , section 5 . 2 assesses the performance with realistic signaling delays . finally , section 5 . 3 shows the impact of the mobility scenario , and section 5 . 4 demonstrates the impact of the graph coloring heuristic . in this section , we will first evaluate the performance of coordinated ffr under ideal signaling conditions , i . e ., t c , delay = 0 , and t c , period is equal to the virtual frame duration . fig1 plots the 5 % throughput quantile over the aggregate throughput for different values of d s , o and d s , i . the left chart shows the performance if the outer coordination by the ffr coordinator is based on a one - tier coordination , whereas the right chart shows the results with an outer two - tier coordination . the coloring heuristic applied in the ffr coordinator was dsatur [ 7 ]. in general , the performance with a one - tier coordination is slightly better when it comes to the throughput quantile , and the two - tier coordination is slightly better when it comes to the aggregate throughput . when comparing these results to a system with global coordination as described in section 3 , the performance is significantly worse . however , we should mention once more that a globally coordinated system is not implementable and only serves as a performance reference . we therefore have to compare the performance of coordinated ffr to the reuse 3 system and to a system with classical ffr , which are both state - of - the - art . for d s , o = 0 db and d s , i = 20 db , the cell border performance of coordinated ffr is comparable to the cell border performance of the reuse 3 system , and significantly better than the cell border performance of the system with classical ffr . with respect to the aggregate throughput , we can achieve an improvement of about 45 - 55 % over the reuse 3 system at the same cell border performance . to look deeper into the difference between coordinated ffr with outer one - tier and outer two - tier coordination , fig1 plots the average number of colors n c over d s , o for both cases . in accordance with section 3 . 4 , the number of required colors n c increases as the coordination diameter increases . this leads to an increase of the virtual frame duration . as reuse 3 terminals are served only once in every virtual frame , the throughput for these terminals decreases significantly as d s , o and hence n c increases . on the other hand , this effect yields more available resources for the reuse 1 terminals , which accounts for the higher aggregate throughput in the two - tier case . this section considers the impact of the signaling delay and the update period on the system performance . fig1 plots the aggregate sector throughput ( left ) and the 5 % throughput quantile ( right ) for different update periods t c , period and signaling delays t c , delay . in general , the impact of t c , period and t c , delay influences the throughput quantile much more than it does the aggregate throughput . this is expected , since the coloring information is used to coordinate mobiles at the cell border , while the aggregate performance is dominated by the local zero - tier coordination , which is not degraded by any signaling delays . an increase of the delay t c , delay leads to a worse performance compared to an identical increase of the update period t c , period , in particular for the throughput quantile . this is logical , since an increase of the update period implies outdated coloring information only for the later time points of the update period , while an increase of the signaling delay leads to outdated coloring information all the time . while the throughput quantile continuously decreases as t c , delay and t c , period increases , the aggregate throughput increases for certain larger values of t c , delay and t c , period . we first note that this increase in fig1 ( left ) is very small and well within the error bars of the graphs . however , regarding the results from previous studies on ffr [ 13 ], where a decrease of the throughput quantile typically leads to an increase of the aggregate performance , the observed slight throughput increase is plausible . fig5 . 3 plots the throughput depending on the terminal position . shown is the result for t c , delay = 1000 ms , t c , period = 2000 ms , and for a combined zero - tier / one - tier coordination with d s , o = 0 db and d s , i = 20 db . compared to fig6 and 7 , it shows a less distinct throughput increase in the cell center areas , which is typical for ffr systems [ 13 ]. the graph also shows a smooth and rather symmetric degradation of the system performance from the cell center to the cell edge , with large well - to - medium covered areas . all results presented so far have been obtained in a high mobility scenario with mobile terminals moving at a speed of υ = 30 km / h . when mobile terminals move at high speeds , the coloring information provided by the ffr coordinator becomes outdated relatively quickly . hence , the signaling delay t c , delay and the update period t c , period have a big impact on system performance . a major use case for 802 . 16e are slowly moving terminals , for example carried by pedestrians , or nomadic terminal usage , where users are stationary and occasionally relocate to a new position . for these scenarios it is directly obvious that the information provided by the ffr coordinator will become obsolete much slower . therefore , the performance of the system will be much less sensitive to larger values of t c , delay and t c , period . in the previous sections , the coloring heuristic dsatur [ 7 ] was applied . it is known that more sophisticated heuristics like tabu search [ 9 ] deliver results where the number of required colors n c is much closer to the chromatic number m of the graph . for our scenario , the coloring heuristic has only a minor impact . for the case of t c , delay = 1000 ms and t c , period = 2000 ms , the throughput performance with tabu search is only slightly better than the performance with dsatur . in particular , the 5 % throughput quantile remains almost unchanged , while the aggregate performance increases by about 1 %. the average number of colors n c decreases from 15 . 8 to 14 . 7 , which means that the average virtual frame duration remains identical at ┌ n c / 4 = 4 . since reuse 3 terminals are served only once every virtual frame duration , this points out why the throughput quantile does not increase . in this paper , we have first presented an optimized algorithm for global interference coordination in a cellular ofdma network . even though such an approach is not implementable since it requires almost instant communication among base stations , it delivers important performance values which can serve as an estimate for an upper performance bound of an interference coordinated system . in our example , the 802 . 16e system achieves an overall spectral efficiency of more than 1 . 1 bit / hz / s while maintaining an excellent cell edge throughput which is twice as large as in a classical reuse 3 system with beamforming antennas . in the second part of this paper , we proposed the concept of coordinated fractional frequency reuse , which bases the resource assignment in the outer areas of the cell on information from a central ffr coordinator . this scheme does not require a global omniscient device and can be implemented in a distributed way . the communication with the central coordinator can take place with realistic signaling delays on the order of seconds , while still maintaining a competitive system performance even in scenarios with a high terminal mobility . in particular , the cell edge performance of a classical system with fractional frequency reuse can greatly be improved to almost match the cell edge performance of a reuse 3 system . at the same time , the spectral efficiency reaches more than 0 . 72 bits / hz / s , which is a 50 % improvement over the reuse 3 system . finally , we argued that these values will be even better when moving to more static scenarios , since signaling delays will have less impact . mobile wimax — part 1 : a technical overview and performance evaluation . technical report , wimax forum , february 2006 . 3gpp ts 25 . 814 . physical layer aspects for evolved universal terrestrial radio access ( utra ) ( release 7 ). 3rd generation partnership project , june 2006 . 3gpp tsg ran wg1 # 42 r1 - 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