Patent Publication Number: US-2012046064-A1

Title: Method and Arrangement in a Wireless Telecommunications System

Description:
TECHNICAL FIELD 
     The present invention relates to a method and an arrangement in a base station and a method and an arrangement in a user equipment. In particular, it relates to deriving a power headroom for a component carrier and assisting in deriving a power headroom for a component carrier. 
     BACKGROUND 
     In a typical cellular radio system, also referred to as a wireless communication system, wireless terminals, also known as mobile terminals and/or User Equipments (UEs) communicate via a Radio Access Network (RAN) to one or more core networks. The wireless terminals can be mobile stations or user equipment units such as mobile telephones also known as “cellular” telephones, and laptops with wireless capability, e.g., mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network. 
     The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a Radio Base Station (RBS), which in some networks is also called “eNB”, “NodeB” or “B node” and which in this document also is referred to as a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. The base stations communicate over the air interface operating on radio frequencies with the user equipment units within range of the base stations. 
     In some versions of the radio access network, several base stations are typically connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC). The radio network controller, also sometimes termed a Base Station Controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks. 
     The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies. 
     The LTE Release 8 (Rel-8) standard has recently been standardized, supporting bandwidths up to 20 MHz. However, in order to meet the upcoming International Mobile Telecommunication (IMT)-Advanced requirements, 3GPP has initiated work on LTE-Advanced. One of the parts of LTE-Advanced is to support bandwidths larger than 20 MHz. One important requirement on LTE-Advanced is to assure backward compatibility with LTE Rel-8. This should also include spectrum compatibility. That would imply that an LTE-Advanced carrier, wider than 20 MHz, should appear as a number of LTE carriers to an LTE Rel-8 user equipment. Each such carrier may be referred to as a component carrier. In particular for early LTE-Advanced deployments it can be expected that there will be a smaller number of LTE-Advanced-capable user equipments compared to many LTE legacy user equipments. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy user equipments, i.e. that it is possible to implement carriers where legacy user equipments can be scheduled in all parts of the wideband LTE-Advanced carrier. The straightforward way to obtain this would be by means of carrier aggregation. Carrier aggregation implies that an LTE-Advanced user equipment can receive multiple component carriers, where the component carriers have, or at least the possibility to have, the same structure as a Rel-8 carrier. 
     The number of aggregated component carriers as well as the bandwidth of the individual component carrier may be different for Uplink (UL) and Downlink (DL). A symmetric configuration refers to the case where the number of component carriers in DL and UL is the same whereas an asymmetric configuration refers to the case that the number of component carriers is different. It is important to note that the number of component carriers configured in a cell may be different from the number of component carriers seen by a user equipment: A user equipment may for example support more DL component carriers than UL component carriers, even though the cell is configured with the same number of UL and DL component carriers. 
     Uplink Power Control in LTE 
     The set of layer 1 functions in the base station includes power control and link adaptation. Layer 1 is also referred to as the Physical Layer. The Physical Layer comprises the basic hardware transmission technologies of a network. 
     The power control mechanism aims to keep the received Signal-to-Noise Ratio SNR, or Signal-to-Noise and Interference Ratio SINR if interference is accounted for, at a targeted value SNR target. Uplink power control is used both on the Physical Uplink Shared CHannel (PUSCH) and on the Physical Uplink Control CHannel (PUCCH). In both cases, a parameterized open loop combined with a closed loop mechanism is used. The base station uses the Physical Downlink Control Channel (PDCCH) to transmit Transmit Power Control (TPC) command scrambled using TPC-PUSCH-RNTI and TPC-PUCCH-RNTI respectively. (Radio Network Temporary Identifier (RNTI)) 
     The uplink link adaptation consists in the selection of modulation and channel coding, which is controlled by the network. The base station measures the uplink channel quality and orders the UE to use a specific Modulation and Coding Scheme (MCS) based on this. Other parameters may also be taken into account, such as UE power headroom (PH), scheduled bandwidth, buffer content and acceptable delay. The link adaptation function determines the transmission parameters (MCS), allocated bandwidth, and possibly MIMO related parameters based on an estimated SNR, or SINR if interference is estimated. 
     To perform these functions, the base station needs knowledge of the uplink gain of the UE to the base station. To achieve this knowledge, the base station must know the received power from the UE as well as the transmit power of the UE. Knowledge of the former can be obtained by measuring on the uplink transmission, however the UE transmit power is known only if the UE reports the transmit power to the base station. 
     Power Headroom Reporting 
     In LTE Rel-8, the UE measures the power headroom. The power headroom is a measure of the difference between the configured UE maximum power (Pmax) and the UE transmit power, in dB, which is calculated based on the nominal received power per resource block used on PUSCH, the number of scheduled resource blocks and the estimated pathloss. The value calculated is tied to the subframe in which the transmission of the report is performed. In LTE and LTE-A the time is divided into frames of 10 ms and each frame is divided into 10 subframes of length 1 ms. Scheduling is based on subframes, i.e. a smallest resources a terminal can get assigned is 1 subframe in time. 
     Power headroom reports (PHR) may be transmitted together with data as MAC control elements. Transmission of a PHR is triggered when the path loss measured by the UE has changed by more than a certain value since the last transmission of a PHR (unless the prohibit timer is running). It can also be transmitted periodically, if configured by the network. 
     PHR triggers have been specified to minimize the overhead of the transmission, so that reports are sent by the UE to the base station only when necessary. 
     Carrier aggregation is a new technology component introduced in LTE-Advanced. So far wireless systems did not apply carrier aggregation but were either traditional Frequency Division Duplex (FDD) systems or Time Division Duplex (TDD) systems. 
     A problem is that a transmission system typically only had one UL transmitter and thus only PHR for this single UL transmitter was required. With carrier aggregation however the radio base station needs PHR of all component carriers. 
     SUMMARY 
     It is therefore an object of the invention to provide a mechanism for deriving the power headroom for a component carrier. 
     According to a first aspect of the invention, the object is achieved by a method in a base station for deriving a power headroom for a first component carrier. The base station is a radio base station and is comprised in a wireless communication system. The base station is configured to use carrier aggregation comprising a first component carrier and a second component carrier. The base station receives a power headroom report from a user equipment. The power headroom report comprises power headroom information for the second component carrier. The base station also establishes the pathloss relationship between the first component carrier and the second component carrier. The base station then derives the power headroom for the first component carrier based on the received power headroom information and the established pathloss relationship between the first component carrier  1  and the second component carrier. 
     According to a second aspect of the invention, the object is achieved by a method in a user equipment for assisting in deriving a power headroom for a first component carrier of the user equipment. The user equipment is served by a base station comprised in a wireless communication system. The user equipment is configured to use carrier aggregation comprising a first component carrier and a second component carrier. The user equipment transmits a power headroom report to the base station. The power headroom report comprises power headroom information for the second component carrier but not power headroom information for the first component carrier. This enables the base station to derive the power headroom for the first component carrier based on the transmitted power headroom information and an established pathloss relationship between the first component carrier and the second component carrier. 
     According to a third aspect of the invention, the object is achieved by a base station for deriving a power headroom for a first component carrier. The base station is a radio base station is comprised in a wireless communication system. The base station is configured to use carrier aggregation comprising a first component carrier and a second component carrier. The base station comprises a receiving unit configured to receive a power headroom report from a user equipment, which power headroom report comprises power headroom information for the second component carrier, and an establishing unit configured to establish the pathloss relationship between the first component carrier and the second component carrier. The base station further comprises a deriving unit configured to derive the power headroom for the first component carrier based on the received power headroom information and the established pathloss relationship between the first component carrier and the second component carrier. 
     According to a fourth aspect of the invention, the object is achieved by a user equipment for assisting in deriving a power headroom for a first component carrier  1  of the user equipment. The user equipment is served by a base station comprised in a wireless communication system. The user equipment is configured to use carrier aggregation comprising a first component carrier and a second component carrier. The user equipment comprises a transmitting unit configured to transmit a power headroom report to the base station. The power headroom report comprises power headroom information for the second component carrier but not power headroom information for the first component carrier. 
     Since, the user equipment sends a power headroom report for the second component carrier but not the first component carrier, and since the base station can establish the pathloss relationship between the first component carrier and the second component carrier, the base station can derive the power headroom for the first component carrier as well, based on the received power headroom information and the established pathloss relationship. The user equipment therefore requires to report power headroom only for one, or a few component carriers, but not all. The component carriers not being reported power headroom, can be derived by the base station. 
     An advantage with the invention is that because only the power headroom for one or few component carriers is reported, the proposed present solution reduces reporting overhead. 
     A further advantage with the invention is that the user equipment only needs to measure or calculate power headroom for one or few component carriers, rather than all of them. Reducing the number of carriers for which power headroom needs to be determined and reported simplifies user equipment implementations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described in more detail with reference to attached drawings illustrating exemplary embodiments of the invention and in which: 
         FIG. 1  is a schematic block diagram illustrating embodiments of a wireless communication network. 
         FIG. 2  is a schematic block diagram illustrating carrier aggregation. 
         FIG. 3  is a combined schematic block diagram and flowchart depicting embodiments of a method. 
         FIG. 4  is a table depicting impact of carrier frequency and pathloss exponent γ on pathloss. 
         FIG. 5  is a schematic block diagram illustrating embodiments of a user equipment. 
         FIG. 6  is a schematic block diagram illustrating embodiments of a base station. 
     
    
    
     DETAILED DESCRIPTION 
     Briefly describe the present solution involves a base station using knowledge of how path loss changes from one component carrier frequency to another component carrier frequency. Based on this knowledge and a power headroom report for one component carrier received from a user equipment, the base station derives the power headroom for the other second component carrier(s) as well. The user equipment therefore reports power headroom only for one, or a few UL component carriers, but not all. 
     More broadly, for cases where a base station or other wireless communication network entity requires knowledge of the transmit power headroom of a remote user equipment, for each of a number of component carriers, the base station derives the transmit power headroom for a first carrier, based on power headroom information received for a second carrier, and the path loss relationship between the first and second carriers. 
     For example, for two uplink component carriers, a user equipment reports transmit power headroom for one of the carriers, and the base station derives the transmit power headroom for the other component carrier, based on known or estimated path loss characteristics for the two component carriers. If multiple component carriers are involved, and power headroom is reported for more than one of them, all of the power headroom reports may be used to improve the accuracy of derived estimates of power headroom for those component carriers for which power headroom was not reported. 
       FIG. 1  depicts a wireless communications system  100 . The wireless communications system  100  such as an LTE Advanced communications system using LTE Advanced technology, WCDMA-HSPA with dual carrier, IEEE 802.16m or any other wireless communications system configured to use multiple UL transmitters. Therefore, even though the invention is outlined in the context of LTE-Advanced the methods is also applicable to other wireless communications systems with multiple UL transmitters. 
     The wireless communications system  100  comprises a base station  110  serving a first cell  115 . The base station  110  is a radio base station such as an eNB, a Radio Base Station (RBS) or any other network unit capable to communicate over a radio carrier with user equipments being present in the first cell. 
     A user equipment  120  being present within the first cell  115 , is served by the base station  110 , and is therefore capable of communicating with the first network node  110  over a radio carrier  125 . The user equipment  120  may be a terminal, e.g. a mobile terminal or a wireless terminal, a mobile phone, a computer such as e.g. a laptop, a Personal Digital Assistant (PDA), or any other radio network unit capable to communicate with a base station over a radio carrier. The user equipment  120  may be a legacy user equipment. A legacy user equipment is a user equipment of the same technology family but of an earlier release, e.g. LTE Rel-8 is a legacy technology with respect to LTE Rel-10 (LTE-A). 
     Carrier Aggregation 
     To assure an efficient use of a wideband carrier also for legacy user equipments, the base station  110  uses carrier aggregation. Carrier aggregation implies that the wideband carrier is divided into component carriers. In some embodiments, the component carriers have, or at least have the possibility to have, the same structure as a LTE Rel-8 carrier. In this way, e.g. legacy user equipments can be scheduled in a component carrier in all parts of the wideband carrier. 
       FIG. 2  depicts an example of carrier aggregation wherein the wideband carrier  125  comprises an aggregated bandwidth of 100 MHz, being divided into five component carriers, the first component carrier  1 , the second component carrier  2 , and a number of third component carriers  3 ,  4  . . . n, in this example each has a bandwidth of 20M Hz. 
     The present solution relating to a method in the base station  110  for deriving a power headroom for the first component carrier  1  according to some embodiments will no be described with reference to the combined signalling diagram and flowchart depicted in  FIG. 3 . The base station  110  is configured to use carrier aggregation comprising the first component carrier  1  and the second component carrier  2 . In some embodiments, the carrier aggregation further comprises a number of third component carriers  3 ,  4  . . . n as mentioned above. The method comprises the following steps, which steps may as well be carried out in another suitable order than described below: 
     Step  301   
     The user equipment  120  transmits a power headroom report to the base station  110 . The power headroom report comprises power headroom information for the second component carrier  2  but not any power headroom information for the first component carrier  1 . This enables the base station  110  to derive the power headroom for the first component carrier  1  based on the transmitted power headroom information and an established pathless relationship between the first component carrier  1  and the second component carrier  2 . 
     In some embodiments the carrier aggregation further comprises a number of third component carriers  3 ,  4  . . . n. In these embodiments, this step of transmitting  301 , further comprises transmitting a power headroom report to the base station  110  comprising power headroom information for at least one of the respective third component carriers  3 ,  4  . . . n. This enables the base station  110  to derive the power headroom for the first component carrier  1  further based on the established pathloss relationship between the first component carrier  1  and the at least one respective reported third component carriers  3 ,  4  . . . n. 
     The component carriers  2 ,  3 ,  4 , . . . n that the user equipment  120  has reported power headroom for, are also referred to as “k” in this document, where k can be any of 2, 3, 4, . . . n. 
     The power headroom report may trigged by an event such as e.g. if the pathloss exceeds a predetermined threshold value. 
     The transmission of the power headroom report may in some embodiments be performed periodically. 
     Seeing this step from the base station  110  perspective, the base station  110  receives the power headroom report from the user equipment  120 , which power headroom report comprises power headroom information for the second component carrier  2 . The power headroom report is referred to as PHR in  FIG. 3 . 
     In some embodiments the power headroom report received from the user equipment  120  further comprises power headroom information for at least one of the respective third component carriers  3 ,  4 , . . . n. 
     In the most general embodiment of the present solution, the base station  110  receives a power headroom report for the second component carrier  2  from the user equipment  120 . The base station  110  requires power headroom also for the first component carrier  1 , to perform link adaptation and power control on all component carriers. The power headroom for the first component carrier  1  is not reported from the user equipment  120  in any embodiment or example in the present solution. However, in some embodiments power headroom reports may be received by the base station  110  from the user equipment  120  also for the third component carriers  3 ,  4 , . . . n. The base station  110  may then derive (in steps below) power headroom for the first component carrier  1  from any of the second component carrier  2 , and/or the third component carriers  3 ,  4 , . . . n, when not being reported from the user equipment  120 , by using a received power headroom report from the user equipment  110  regarding any one or more of the second component carrier  2  and/or the third component carriers  3 ,  4 , . . . n. 
     The user equipment  110  may send the power headroom report for the second component carrier upon request from the base station  110 . The reporting may be performed in different ways such as e.g. via a trigger, for example if pathloss changes too much, the user equipment  120  automatically reports power headroom. Other ways are the user equipment  120  may be configured to periodically report power headroom, or the base station  110  explicitly requests a power headroom report from the user equipment  120 . 
     The power headroom in the received report may be calculated by the user equipment  110  using equation ( 1 ) below according to the following example. The power headroom is defined as the difference between configured maximum transmit power and the estimated power for PUSCH transmission, expressed in dB. 
       PH( i )= P   MAX −{10 log 10 ( M   PUSCH ( i ))+ P   O     —     PUSCH +α·PL+Δ TF ( i )+ f ( i )}  [1]
 
     P MAX  is the configured maximum transmit power in dBm, M PUSCH (i) is the number of allocated resource blocks. P O     —     PUSCH  is the nominal reception power per resource block at base station  110  in dBm. PL is the pathloss in dB and α controls the power control behavior, Δ TF (i) is a transport format dependent offset in dB. f(i) depends on the transmit power control. The index i is the subframe number and expresses the subframe-dependency. 
     Step  302   
     In this step the base station  110  establishes the pathloss relationship between the first component carrier  1  and the second component carrier  2 . 
     In some embodiments, this step further comprises establishing the pathloss relationship between the first component carrier  1  and each of the respective reported third component carriers  3 ,  4  . . . n. 
     The base station  110  may establish the pathloss relationship according to the following examples: 
     The pathloss in a component carrier in dB may be approximated as 
       PL(dB)=20lg( K )+γlg( f )+βlg( d )  [2]
 
     where f is the frequency in Hz, d the distance in m; and γ, β and K are parameters of the equation model. The parameter γ describes the frequency dependency whereas β describes the increase of the pathloss with distance. For free space propagation β is equal to 20 whereas for cellular systems β is often assumed to be between 30 and 40. Values for γ are summarized in the table depicted in  FIG. 4 , which table relates to impact of carrier frequency and pathloss exponent γ on pathloss, The parameter K describes the path loss at reference frequency (1 Hz) and reference distance (1 m). 
     The pathloss relationship between the first component carrier  1  and the second component carrier  2  may be represented by ΔPL=PL 2 −PL 1 , wherein PL 2  is the pathloss in the second component carrier  2  and wherein PL 1  is the pathloss in the first component carrier  1 . 
     In some embodiments wherein power headroom information for a number of third component carriers  3 ,  4 , . . . n are received, the pathloss relationship between the first component carrier  1  and each of the respective reported third component carriers  3 ,  4 , . . . n may be represented by respective ΔPL 13 =PL 3 −PL 1 , ΔPL 14 =PL 4 −PL 1 , and ΔPL 1n =PL n −PL 1 . PL 1  denotes the pathloss of the first component carrier  1 , PL 2  denotes the pathloss of the second component carrier  2 , and each of the PL 3, 4 . . . n  denotes the pathloss of the respective third component carriers  3 ,  4 , . . . n. In some embodiments, the base station  110  only requires to establish the pathloss difference ΔPL but not the absolute pathloss values. 
     Using model equation [2] the pathloss difference ΔPL 12  between the first component carrier  1  and the second component carrier  2  may be established by using model equation 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     where f k  is the frequency in Hz of component carrier k (k=1, 2, . . . , n), d k  the distance in meter of component carrier k (k=1, 2, . . . , n), γ k  describes the frequency dependency of component carrier k (k=1, 2, . . . , n), β k  describes the increase of the pathloss with distance of component carrier k (k=1, 2, . . . , n), and K k  is the pathloss at reference frequency and reference distance of the equation model of component carrier k (k=1, 2, . . . , n), [3]. 
     The subscript  1  denotes the first component carrier  1 , the subscript  2  denotes the second component carrier  2 , and the subscript  3, 4 . . . n  denotes the third component carriers  3 ,  4  . . . n. The parameters of the propagation models at the different carrier frequencies are typically known from the cell planning. Since d is not known at base station  110  this method requires the same β values across component carriers, i.e. the same β values for the first component carrier  1  and the second component carrier  2 . 
     In some embodiments, where β varies across component carrier frequencies, the example outlined above how to establish ΔPL cannot be used since d is unknown. 
     In some embodiments an alternative way that may be applied for varying β values is to use model equation [1] to estimate the pathloss at those frequencies where power headroom is reported. This is possible since in [1] all quantities besides the pathloss are known to the base station  110 . These one or multiple pathloss estimates may then be used to extrapolate or interpolate the pathloss to non-reported UL component carrier frequencies. Interpolation and extrapolation would be done using model equation [2], i.e. taking the logarithmic dependency of the path loss on the frequency into account. Once the pathloss is known at the non-reported UL frequencies, equation [1] may be used with the parameters for the non-reported UL component carrier frequencies, to calculate the power headroom for non-reported UL component carriers. 
     In some embodiments one or more pathloss PL k  for which power headroom are reported are estimated by using the equation: 
       PH k ( i )= P   MAX,k −{10 log 10 ( M   PUSCH,k ( i ))+ P   O     —     PUSCH,k +α k ·PL k +Δ TF,k ( i )+ f   k ( i )}  [1],
 
     for each of the one or more power headroom reported carrier k=2, 3, 4 . . . n, which quantities besides the pathloss PL are known by the base station and which quantities in the equation [1] are component carrier specific. 
     The one or more pathloss PL k  k=2, 3, 4 . . . n for which power headroom are reported comprises the pathloss PL 3, 4 . . . n , at the frequency of each of the respective third component carriers  3  and/or the pathloss PL 2  at the frequency of the second component carrier  2 . 
     In these embodiments the model equation 
       PL(dB)=20lg( K )+γlg( f )+βlg( d )  [2]
 
     may be used to extrapolate or interpolate the pathloss PL 1  of the first component carrier  1  frequencies from the estimated pathloss PL k , k=2, 3, 4 . . . n. In these embodiments the APL is established by ΔPL 1k =PL k −PL 1 , k=2, 3, 4 . . . n, using the established pathloss PL k , k=2, 3, 4 . . . n and the extrapolated or interpolated pathloss PL 1 . 
     In a specific embodiment, model equation [2] is used to calculate the distance d at one or all frequencies where power headroom is reported and thus pathloss is known, since path loss on the reported second carrier may easily be calculated based on the reported power headroom. Since d is the same for all carrier frequencies the multiple estimates may be averaged to improve accuracy. In this example d 2  is calculated for the second component carrier  2  by using model equation 
       PL 2 (dB)=20lg( K   2 )+γ 2 lg( f   2 )+β 2 lg( d   2 )  [2]
 
     also d 3  may be calculated for the third component carriers  3 ,  4 , . . . n in a similar way, with component carrier frequency and model parameters valid at the third component carriers frequency. 
     d 2 =d 1  and d 3 =d 1 , since d is the same for all carrier frequencies, or d 1  is calculated as the average of d 2  and d 3 , thus when d 1  is calculated, pathloss may be derived for the non-reported component carrier  1  by using the calculated d 1  and the equation model 
       PL 1 (dB)=20lg( K   1 )+γ 1 lg( f   1 )+β 1 lg( d   1 )  [2]
 
     where f 1  is here the frequency of the non-reported first component carrier  1  and the other model parameters (β 1 , K 1 , and γ 1 ) are the model parameters at the first component carrier frequency and may be the same or may be different for each component carrier. 
     Accordingly, this means that this specific embodiment may be performed such that the pathloss PL k , k=2, 3, 4, . . . , n, is estimated by using the equation: 
       PH k ( i )= P   MAX,k −{10 log 10 ( M   PUSCH,k ( i ))+ P   O     —     PUSCH,k +α k ·PL k +Δ TF,k ( i )+ f   k ( i )}  [1]
 
     for each respective reported power headroom carrier k=2, 3, 4 . . . n. The distance d k  to the user equipment  120  is calculated by using equation model 
       PL k (dB)=20lg( K   k )+γ k lg( f   k )+β k lg( d   (k) )  [2]
 
     for the frequencies of each of the at least one power headroom reported carriers k=2, 3, 4, . . . , n, [2]. 
     The pathloss PL 1  at the frequency of the first component carrier is estimated using each reported power headroom carrier k=2, 3, 4 . . . n, and by using each of the respective calculated d k , being equal to d 1 , and the equation model 
       PL 1 (dB)=20lg( K   1 )+γ 1 lg( f   1 )+β 1 lg( d   1 )  [2]
 
     at the frequencies of the first component carrier  1 . An average PL 1  may be derived across all estimated PL 1 . 
     If power headroom report is known for multiple component carriers, say on a second third component carrier ( 2 ) and a third component carrier ( 3 ), the obtained estimates for the distance d may be averaged prior using these distances to calculate the pathloss at the non-reported component carrier frequency. Alternatively the d values obtained from each reported component carrier may be used to obtain multiple estimates of the pathloss at the first component carrier frequency which are then averaged to obtain the final estimate. 
     According to another specific embodiment, the pathloss PL k , k=2, 3, 4, . . . , n, may be estimated by using the equation: 
       PH k ( i )= P   MAX,k −{10 log 10 ( M   PUSCH,k ( i ))+ P   O     —     PUSCH,k +α k ·PL k +Δ TF,k ( i )+ f   k ( i )}  [1],
 
     for each respective reported power headroom carrier (k=2, 3, 4 . . . n). The distance d k  to the user equipment  120  is calculated by using equation model 
       PL k (dB)=20lg( K   k )+γ k lg( f   k )+β k lg( d   k )  [2],
 
     for the frequencies of each of the at least one power headroom reported carriers k=2, 3, 4, . . . , n. In this embodiment, an average d 1  is derived across all calculated d k . The pathloss PL 1  at the frequency of the first component carrier is then estimated using the average d 1 , and the equation model 
       PL 1 (dB)=20lg( K   1 )+γ 1 lg( f   1 )+β 1 lg( d   1 )  [2]
 
     Step  303   
     In this step the base station  110  derives the power headroom for the first component carrier  1  based on the received power headroom information and the established pathloss relationship between the first component carrier  1  and the second component carrier  2 . 
     In some embodiments this step of deriving  303  the power headroom for the first component carrier  1  further is based on the established pathloss relationship between the first component carrier  1  and each of the reported respective third component carriers  3 ,  4  . . . n. 
     This step may be performed as follows: 
     In this case, comprising multiple component carriers, potentially all of the parameters in equation ( 1 ) may be component carrier specific. According to the present solution, the power headroom difference between two component carriers may be expressed as: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           Δ 
                            
                           
                               
                           
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                             PH 
                              
                             
                               ( 
                               i 
                               ) 
                             
                           
                         
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                                 PH 
                                 
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                                   2 
                                   ) 
                                 
                               
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                                 ( 
                                 i 
                                 ) 
                               
                             
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                                   ) 
                                 
                               
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                                 ) 
                               
                             
                           
                           = 
                         
                       
                     
                   
                   
                     
                       
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                             { 
                             
                               
                                 P 
                                 MAX 
                                 
                                   ( 
                                   2 
                                   ) 
                                 
                               
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                                   ( 
                                   1 
                                   ) 
                                 
                               
                             
                             } 
                           
                           - 
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                          
                         
                           
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                                 10 
                                  
                                 
                                   
                                     log 
                                     10 
                                   
                                    
                                   
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                                         PUSCH 
                                         
                                           ( 
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                                           ) 
                                         
                                       
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                                         ( 
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                                         ) 
                                       
                                     
                                     ) 
                                   
                                 
                               
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                                     10 
                                   
                                    
                                   
                                     ( 
                                     
                                       
                                         M 
                                         PUSCH 
                                         
                                           ( 
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                                           ) 
                                         
                                       
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                                         ( 
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                         = 
                           
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                             { 
                             
                               
                                 
                                   P 
                                   CONF 
                                   
                                     ( 
                                     2 
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                                   ( 
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                                   ( 
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                          
                         
                           { 
                           
                             
                               
                                 α 
                                 
                                   ( 
                                   2 
                                   ) 
                                 
                               
                               · 
                               
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                                   ( 
                                   2 
                                   ) 
                                 
                               
                             
                             - 
                             
                               
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                                   ( 
                                   1 
                                   ) 
                                 
                               
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                   [ 
                   4 
                   ] 
                 
               
             
           
         
       
     
     The subscripts  1  and  2  denote first and second component carrier, respectively. Besides the pathloss, all parameters are signalled from the base station  110  to the user equipment  120  and are summarized in the quantities P CONF,1  and P CONF,2  for the first and second component carrier, respectively. P CONF,1  and P CONF,2  may easily be calculated at the base station  110 . Using 
       ΔPL=PL (2) −PL (1)  
 
     and substituting PL (1)  in equation [4], it is obtained for the difference in power headroom 
       ΔPH( i )={ P   (2)   CONF ( i )− P   (1)   CONF ( i )}−{α (2) ·PL (2) −α (1) ·(PL (2) −ΔPL)}  [5]
 
     If it furthermore is assumed the same α values for the two component carriers, equation [5] is simplified to 
       ΔPH( i )={ P   (2)   CONF ( i )− P   (1)   CONF ( i )}−α·ΔPL  [6]
 
     The base station  110  only requires to use the established pathloss difference ΔPL but not the absolute pathloss values. The expressions [5] and [6] may be evaluated at the base station  110  assuming that ΔPL can be predicted accurately enough. Once ΔPH(i) is known, the PH for the non-reported component carrier may be calculated as 
       PH 1 ( i )=PH 2 ( i )−ΔPH 1k ( i ).  [7]
 
     Hence, this step  303  of deriving the power headroom for the first component carrier  1  “PH 1 ” based on the received power headroom information and the established pathloss relationship between the first component carrier  1  and the second and/or third component carriers k=2, 3, 4 . . . n may be performed by using model equation 
       ΔPH k ( i )={ P   CONF,k ( i )− P   CONF,1 ( i )}−{α k ·PL k −α 1 ·(PL k −ΔPL 1k )}  [5]
 
     for any of the power headroom reported carriers ( 2 ,  3 ,  4  . . . or n), and deriving PH 1 (i) by using 
       PH 1 ( i )=PH k ( i )−ΔPH 1k ( i ), ( k= 2, 3, 4, . . . ,  n )  [7]
 
     If power headroom of more than one UL component carrier is reported, all reported power headroom may be used to improve accuracy of the power headroom to be derived of the non-reported UL component carriers. For example, model equations [5] or [6] and [7] applied to all UL component carriers with reported power headroom deliver multiple estimates for the power headroom of non-reported UL component carriers. These multiple estimates per UL component carrier may be averaged to improve accuracy. 
     E.g. in some embodiments power headroom information for more than one component carrier  2 ,  3 ,  4  . . . n is received. In these embodiments power headroom for the first component carrier  1  PH 1 (i) is derived for each received information, and an average PH 1 (i) is derived across all derived PH 1 (i). 
     Other algorithms to calculate the pathloss difference or pathloss are envisioned as well. 
     To perform the method steps above within the user equipment  120  for assisting in deriving a power headroom for a first component carrier ( 1 ), the user equipment  120  comprises an arrangement depicted in  FIG. 5 . As mentioned above, the user equipment  120  is served by the base station  110  comprised in the wireless communication system  100 . The user equipment  120  is configured to use carrier aggregation comprising the first component carrier  1  and the second component carrier  2 . 
     The user equipment  120  comprises a transmitting unit  510  configured to transmit a power headroom report to the base station  110 . The power headroom report comprises power headroom information for the second component carrier  2  but not power headroom information for the first component carrier  1 . 
     In some embodiments the carrier aggregation further comprises a number of third component carriers  3 ,  4  . . . n. In these embodiments the transmitting unit  510  is further configured to transmit a power headroom report to the base station  110  comprising power headroom information for at least one of the respective third component carriers  3 ,  4  . . . n. 
     The transmitting unit  510  may further be configured to transmit the power headroom report trigged by an event such as e.g. if the pathloss exceeds a predetermined threshold value. 
     The transmitting unit  510  may further be configured to transmit the power headroom report periodically. 
     To perform the method steps above within the base station  110  for deriving a power headroom for a first component carrier  1 , the base station  110  comprises an arrangement depicted in  FIG. 6 . As mentioned above, the base station  110  is a radio base station and is comprised in the wireless communication system  100 . The base station  110  is configured to use carrier aggregation comprising a first component carrier  1  and a second component carrier  2 . The carrier aggregation may further comprise a number of third component carriers  3 ,  4  . . . n. 
     The base station  110  comprises a receiving unit  610  configured to receive a power headroom report from a user equipment  120 . The power headroom report comprises power headroom information for the second component carrier  2 . 
     In some embodiments the receiving unit  610  is further configured to receive a power headroom report from the user equipment  120  comprising power headroom information for at least one of the respective third component carriers  3 ,  4  . . . n. 
     The base station  110  further comprises an establishing unit  620  configured to establish the pathloss relationship between the first component carrier  1  and the second component carrier  2 . 
     In some embodiments the establishing unit  620  further is configured to establish the pathloss relationship between the first component carrier  1  and each of the respective reported third component carriers  3 ,  4  . . . n. 
     The pathloss PL relationship between the first component carrier  1  and the second component carrier  2  may be represented by ΔPL 12 =PL 2 −PL 1 . The pathloss relationship between the first component carrier  1  and each of the respective reported third component carriers  3 ,  4  . . . n may be represented by respective ΔPL 13 =PL 3 −PL 1 , ΔPL 14 =PL 4 −PL 1 , and ΔPL 1n =PL n −PL 1 . PL 1  may denote the pathloss of the first component carrier  1 , PL 2  may denote the pathloss of the second component carrier  2 , and each of the PL 3, 4 . . . n  may denote the pathloss of the respective third component carriers  3 ,  4  . . . n. 
     The establishing unit  620  may further be configured to establish ΔPL by using 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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                               12 
                             
                           
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                               2 
                             
                             - 
                             
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                           == 
                             
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     where f k  is the frequency in Hz of component carrier k, k=1, 2, . . . , n, d k  the distance in meter of component carrier k, k=1, 2, . . . , n, γ k  describes the frequency dependency of pathloss of component carrier k, k=1, 2, . . . , n, β k  describes the increase of the pathloss with distance of component carrier k, k=1, 2, . . . , n, and K k  is the pathloss at reference frequency and reference distance of the equation model of component carrier k, k=1, 2, . . . , n, [3]. 
     The establishing unit  620  may further be configured to estimate one or more pathloss PL k  for which power headroom are reported by using the equation: 
       PH k ( i )= P   MAX,k −{10 log 10 ( M   PUSCH,k ( i ))+ P   O     —     PUSCH,k +α k ·PL k +Δ TF,k ( i )+ f   k ( i )}  [1]
 
     for each of the one or more power headroom reported carrier k=2, 3, 4 . . . n. The one or more pathloss PL k  k=2, 3, 4 . . . n for which power headroom are reported may comprise the pathloss PL 3, 4 . . . n , at the frequency of each of the respective third component carriers  3  and/or the pathloss PL 2  at the frequency of the second component carrier  2 . The quantities: PH k  is the power headroom reported by the user equipment  120 , P MAX  is the configured maximum transmit power in dBm, M PUSCH (i) is the number of allocated resource blocks, P O     —     PUSCH  is the configured reception power per resource block at base station  110  in dBm, α controls the power control behaviour, Δ TF (i) is a transport format dependent offset in dB, f(i) depends on the transmit power control the index i is the subframe number and expresses the subframe-dependency. The quantities in the equation [1] are component carrier specific, and which quantities besides the pathloss PL k  are known by the base station  110 .
 
The establishing unit  620  may further be configured to use equation
 
       PL(dB)=20lg( K )+γlg( f )+βlg( d )  [2]
 
     to extrapolate or interpolate the pathloss FL 1  of the first component carrier  1  frequencies from the estimated pathless PL k , k=2, 3, 4, . . . , n.
 
The establishing unit  620  may further be configured to establish ΔPL is by ΔPL 1k =PL k −PL 1 , k=2, 3, 4, . . . , n, using the established pathloss PL k , k=2, 3, 4, . . . , n and the extrapolated or interpolated pathloss PL 1 .
 
     In some embodiments, the establishing unit  620  is further configured to estimate the pathloss PL k , k=2, 3, 4, . . . , n, by using the equation: 
       PH k ( i )= P   MAX,k −{10 log 10 ( M   PUSCH,k ( i ))+ P   O     —     PUSCH,k +α k ·PL k +Δ TF,k ( i )+ f   k ( i )}  [1],
 
     for each respective reported power headroom carrier k=2, 3, 4 . . . n. In these embodiments, the establishing unit  620  may further be configured to calculate the distance d k  to the user equipment  120  by using equation model 
       PL k (dB)=20lg( K   k )+γ k lg( f   k )+β k lg( d   (k) )  [2]
 
     for the frequencies of each of the at least one power headroom reported carriers k=2, 3, 4, . . . , n, [2]. In these embodiments, the establishing unit  620  further is configured to estimate the pathless PL 1  at the frequency of the first component carrier for each reported power headroom carrier k=2, 3, 4, . . . , n, by using each of the respective calculated d k , being equal to d 1 , and the equation model 
       PL 1 (dB)=20lg( K   1 )+γ 1 lg( f   1 )+β 1 lg( d   1 )  [2]
 
     at the frequencies of the first component carrier  1 . The establishing unit  620  in these embodiments may further be configured to derive an average PL 1  across all estimated PL 1 . 
     In some other embodiments, the establishing unit  620  further is configured to estimate the pathloss PL k , k=2, 3, 4, . . . , n, by using the equation: 
       PH k ( i )= P   MAX,k −{10 log 10 ( M   PUSCH,k ( i ))+ P   O     —     PUSCH,k +α k ·PL k +Δ TF,k ( i )+ f   k ( i )}  [1],
 
     for each respective reported power headroom carrier k=2, 3, 4 . . . n, In these embodiments, the establishing unit  620  is further configured to calculate the distance d k  to the user equipment  120  by using equation model 
       PL k (dB)=20lg( K   k )+γ k lg( f   k )+β k lg( d   k )  [2],
 
     for the frequencies of each of the at least one power headroom reported carriers k=2, 3, 4, . . . , n, [2], wherein an average distance d 1  is derived across all calculated distance d k . In these embodiments, the establishing unit  620  further is configured to estimate the pathloss PL 1  at the frequency of the first component carrier by using the average distance d 1 , and the equation model 
       PL 1 (dB)=20lg( K   1 )+γ 1 lg( f   1 )+β 1 lg( d   1 )  [2].
 
     The base station  110  further comprises a deriving unit  630  configured to derive the power headroom for the first component carrier  1  based on the received power headroom information and the established pathloss relationship between the first component carrier  1  and the second component carrier  2 . 
     In some embodiments, the deriving unit  630  further is configured to derive the power headroom for the first component carrier  1  based on the established pathloss relationship between the first component carrier  1  and each of the reported respective third component carriers  3 ,  4  . . . n. 
     The deriving unit  630  may further be configured to derive the power headroom for the first component carrier  1  “PH 1 (i)” based on the received power headroom information and the established pathloss relationship between the first component carrier  1  and the second and/or third component carriers k=2, 3, 4 . . . n, by using model equation 
       ΔPH k ( i )={ P   CONF,k ( i )− P   CONF,1 ( i )}−{α k ·PL k −α 1 (PL k −ΔPL 1k )}  [5]
 
     for any of the power headroom reported carriers  2 ,  3 ,  4  . . . or n, and deriving PH 1 (i) by using 
       PH 1 ( i )=PH k ( i )−ΔPH 1k ( i ),  [7]
 
     k=2, 3, 4, . . . , n. 
     In some embodiments wherein receiving unit  610  further is configured to receive power headroom information for more than one component carrier  2 ,  3 ,  4  . . . n, the deriving unit  630  may further be configured to derive the power headroom for the first component carrier  1  PH 1 (i) for each received information, and to derive an average PH 1 (i) across all derived PH 1 (i). 
     The present mechanism for deriving a power headroom for the first component carrier  1 , may be implemented through one or more processors, such as a processor  640  in the base station  110  depicted in  FIG. 6  or such as a processor  520  in user equipment  120  depicted in  FIG. 5 , together with computer program code for performing the functions of the present solution. The program code mentioned above may also be provided as a computer program product, for instance in the form of a data carrier carrying computer program code for performing the present solution when being loaded into the base station  110  or into the user equipment. One such carrier may be in the form of a CD ROM disc. It is however feasible with other data carriers such as a memory stick. The computer program code can furthermore be provided as pure program code on a server and downloaded to the base station  110  or to the user equipment  120 . 
     When using the word “comprise” or “comprising” it shall be interpreted as non-limiting, i.e. meaning “consist at least of”. 
     The present invention is not limited to the above described preferred embodiments. Various alternatives, modifications and equivalents may be used. Therefore, the above embodiments should not be taken as limiting the scope of the invention, which is defined by the appending claims.