Abstract:
An object of the present invention is to provide a mechanism of measuring a reference signal that is less over head consuming and less time consuming. The object is achieved by a method for managing uplink channel estimation in a base station. The base station is comprised in a radio access network using Frequency Division Duplex (FDD). The base station is associated with a first cell of a number of cells within the radio access network. The base station is adapted to connect at least one mobile terminal, being situated in the first cell, to a network infrastructure. The method comprises the step of transmitting a redirected reference signal on an uplink band to the mobile terminal. The redirected reference signal is intended for uplink channel estimation.

Description:
TECHNICAL FIELD 
     The present invention relates to a method and arrangement for a base station in a radio access network using FDD, and a method and arrangement for a mobile terminal in a in a radio access network using FDD. In particular the present invention relates to uplink channel estimation. 
     BACKGROUND OF THE INVENTION 
     With the increasing availability of efficient and low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld, telephone-like communication terminal. The different modulation and transmission schemes each have advantages and disadvantages. In a wireless network, the covering area of an operator is divided into cells. A cell corresponds to the covering area of one or more base stations communicating with terminals within the cell. When the user of a terminal moves between cells, or the radio coverage areas associated with different base station sites, an ongoing call must be switched to a different radio coverage channel or cell. The base station transmits signals to the terminals within the cell in down link and these terminals transmit signals to the base station in uplink. 
     Duplex in wireless networks relates to how signals, transmitted in the downlink and uplink, are multiplexed. In Frequency Division Duplex (FDD) systems, downlink and uplink signals are transmitted in different frequency bands, separated by a duplex distance. The duplex distance is often larger than the downlink and uplink signal bandwidths, resulting in a pair of spectrum allocations with a gap in between. This spectrum allocation is often referred to as paired spectrum. In Time Division Duplex (TDD) systems downlink and uplink signals are transmitted at the same frequencies but at different instants in time. E.g. Wideband Code Division Multiple Access (WCDMA) and IEEE 802.16-based systems can operate in both FDD and TDD modes. 
     A benefit of TDD systems is that the channel is reciprocal, i.e. the channel is the same in both uplink and downlink. This means e.g. that a terminal that can estimate the downlink channel also knows the uplink channel. This can be used for e.g. accurate power control, link adaptation, and antenna weight selection. 
     In FDD systems, the downlink and uplink channels are different because of the frequency separation between downlink and uplink bands, which leads to different multi path fading. A problem is how to perform uplink channel estimation for FDD systems. One way of solving the problem is to use reference signals, also called pilots. Each of the terminals within the cell sends a known reference signal with known amplitude/s and phase reference/s to the base stations. The base stations perform channel estimation by measuring each of the received reference signals. Channel estimation may e.g. be done by measuring the amplitude(s) and phase(s) of the received reference signals and comparing the results with the known transmitted signal, i.e. comparing with the known amplitude and phase reference, and in that way the base station generates estimates of the uplink channels. The base stations then either report this result back to each of the terminals within the cell, and/or uses the result themselves to schedule the users in appropriate parts of the band, select power levels, modulation and coding schemes etc., and report the decisions to the terminals. The FDD modes of WCDMA and 802.16 use the above principle which principle for example is described in US2006018368. 
     A disadvantage of this approach is that it involves a significant overhead. Uplink pilots have to be transmitted by and reported back to all terminals within the cell. If the uplink pilots are used for channel dependent scheduling, link adaptation, or power control in the frequency domain, pilots must basically cover the whole uplink bandwidth, which causes a large overhead and is time consuming. Once the report arrives at the terminal, the channel might have changed. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a mechanism of channel estimation in a FDD system that is less over head consuming and less time consuming. 
     The basic principle of solving the problem in a FDD system is to use the uplink band, being intended for uplink data transfer, to send reference signals from the base station to the mobile terminals for performing the channel estimation of the uplink band. 
     According to a first aspect of the present invention, the object is achieved by a method for managing uplink channel estimation in a base station. The base station is comprised in a radio access network using Frequency Division Duplex (FDD). The base station is associated with a first cell of a number of cells within the radio access network. The base station is adapted to connect at least one mobile terminal, being situated in the first cell, to a network infrastructure. The method comprises the step of transmitting a redirected reference signal on an uplink band to the mobile terminal. The redirected reference signal being intended for uplink channel estimation. 
     According to a second aspect of the present invention, the object is achieved by an arrangement in a base station being comprised in a radio access network using Frequency Division Duplex (FDD). The base station is associated with a first cell of a number of cells within the radio access network. The base station is adapted to connect at least one mobile terminal, being situated in the first cell, to a network infrastructure. The base station arrangement comprises a transmitter for transmitting a redirected reference signal on an uplink band to the terminal, said redirected reference signal being intended for uplink channel estimation. 
     According to a third aspect of the present invention, the object is achieved by a method for managing uplink channel estimation in a mobile terminal. The mobile terminal is comprised in a radio access network using Frequency Division Duplex (FDD). The radio access network comprises a number of cells. The mobile terminal is situated in a first cell of said number of cells and the mobile terminal is connectable to a network infrastructure via a base station associated with the first cell. The method comprises the step of receiving a redirected reference signal on an uplink band from the base station. The redirected reference signal is intended for uplink channel estimation. 
     According to a fourth aspect of the present invention, the object is achieved by an arrangement in a mobile terminal being comprised in a radio access network using Frequency Division Duplex (FDD). The radio access network comprises a number of cells and the mobile terminal is situated in a first cell of said number of cells. The mobile terminal is connectable to a network infrastructure via a base station associated with the first cell. The mobile station arrangement comprises a receiver for receiving a redirected reference signal, on the uplink band, from the base station. The redirected reference signal is intended for uplink channel estimation. 
     Since the redirected reference signal is transmitted from the base station to the mobile terminal on the uplink band, there is no longer any need for each of the mobile terminals to transmit any reference signals on the uplink band to the base station that compares each of the received redirected reference signals with the known redirected reference signal and reports the result back to the terminals. Instead the mobile terminals are capable to do the channel estimation themselves which makes the present solution less over head consuming and less time consuming. 
     An advantage with the present invention is that the channel estimation is improved. Since the channel estimation can be performed more efficiently (using less overhead), more room for data transmission and increased capacity is provided. 
     A further advantage with the present invention is that the improved channel estimation leads to increased quality, capacity and coverage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram illustrating a radio access network. 
         FIG. 2  is a schematic block diagram illustrating an uplink band and a downlink band of an FDD system. 
         FIG. 3  is a schematic block diagram illustrating an uplink band of an FDD system 
         FIG. 4  is a flow chart illustrating a method in a base station. 
         FIG. 5  is a schematic block diagram illustrating an arrangement in base station. 
         FIG. 6  is a flow chart illustrating a method in a mobile terminal. 
         FIG. 7  is a schematic block diagram illustrating an arrangement in a mobile terminal. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention is defined as a method and an arrangement which may be put into practice in the embodiments described below. 
       FIG. 1  depicts a radio access network  100  comprising a number of cells and which radio access network  100  uses technologies such as e.g. WCDMA, Enhanced UMTS Terrestrial Radio Access (E-UTRA) (Universal Mobile Telecommunications System (UMTS)), 3G Long Term Evolution (3GLTE), Super 3G or Wireless World Initiative New Radio (WINNER). The radio access network  100  uses FDD technology. 
     The radio access network  100  further comprises a base station  114  associated with a first cell  112  of said number of cells. The base station  114  is connectable to an infrastructure network  130  such as e.g. the Public Switched Telephone Network (PSTN), an IP network or a radio access network. 
     One or more mobile terminals  120 ,  122  and  124  are located under radio coverage of the radio access network  100  and are connectable to the infrastructure network  130 , by means of the base station  114  associated with the cell  112  in which the mobile terminals  120 ,  122  and  124  currently are positioned. The mobile terminals  120 ,  122  and  124  may be personal digital assistants (PDA), laptop computers or any type of devices capable of communicating via radio resources. The frequency band that is designated for uplink data transfer, i.e. for data transfer from the mobile terminals  120 ,  122  and  124  to the base station  114 , is from now on called the uplink band and correspondingly, the frequency band that is designated for downlink data transfer, i.e. for data transfer from the base station  114  to the mobile terminals  120 ,  122  and  124 , is from now on called the downlink band. 
     The base station  114  comprises a transmitter for transmitting information to the mobile terminals  120 ,  122  and  124 , both in the uplink band and in the downlink band. The base station  114  transmits a known redirected reference signal in the uplink band to the mobile terminals  120 ,  122  and  124 , currently being positioned in the cell  112  that is associated to the base station  114 . The redirected reference signal is to be used for estimating the uplink channel quality, and is therefore sent in the uplink band. Instead of that each of the mobile terminals  120 ,  122  and  124  transmit the redirected reference signal on the uplink band to the base station  114  that compares each of the received redirected reference signals with the known redirected reference signal and reports the result back to the mobile terminals, the base station  114  transmits the redirected reference signal in the uplink band which is received by all mobile terminals  120 ,  122  and  124 . This is advantageous since it is the mobile terminals  120 ,  122  and  124  that in the first case require the information provided by comparing the known redirected reference signal with the received redirected reference signal, and the mobile terminals  120 ,  122  and  124  are capable to do the channel estimation themselves which in this way decreases the amount of transmissions. Reporting back to the base station  114  is only necessary in some cases e.g. for channel quality dependent scheduling (se later on). It is given that the channel is reciprocal. The known redirected reference signal has known amplitude(s) and a known phase reference(s) and is compared with the amplitude(s) and phase reference(s) of the received redirected reference signal in a conventional way. 
     The basic downlink and uplink structure is depicted in  FIG. 2  wherein the longitudinal axis shows time and the lateral axis shows frequency. Arrows pointing downwards indicate transmittal from the base station  114  to the mobile terminals  120 ,  122  and  124 , and arrows pointing upwards indicate transmittal from the mobile terminals  120 ,  122  and  124  to the base station  114 . The downlink band is referred to as  200  and the uplink band is referred to as  210 . Since the different mobile terminals  120 ,  122  and  124  may transmit data in different parts of the uplink frequency band, redirected reference signals  220  should preferably be time multiplexed and preferably cover the entire uplink frequency band to enable channel estimation covering the entire uplink band. Frequency multiplexing of the redirected reference signals  220  is possible, but is less efficient as a relatively large frequency guard band would be required between simultaneous uplink and downlink transmissions. 
     The time multiplexing is preferably performed in the uplink band  210  such that redirected reference signal  220  is transmitted in one time frame and uplink data  230  is transmitted from the mobile terminals  120 ,  122  and  124  to the base station  114  in another time frame. It is preferable if the redirected reference signal  220  is surrounded by some guard period  240 , e.g. 60-70 μs, to obtain enough time for switching from downlink transmission to uplink transmission. Therefore it is preferable to not transmit redirected reference signals too often. The redirected reference signals  220  should be transmitted frequently enough to follow fast channel variations. However, the more frequently the redirected reference signal  220  are transmitted, the larger the overhead. Therefore, adapting the redirected reference signal  220  frequency in accordance with the channel coherence time may be a useful means to limit the overhead. Channel coherence time, however, is (typically) different for different mobile terminals depending on how fast they move and it may hence be necessary to select a frequency that is sufficiently high for a majority of the user population and simply accept that mobile terminals  120 ,  122  and  124  experiencing very fast channel variations, e.g. since these are fast moving, may not be able to utilize the redirected reference signal. Coherence time may be estimated as Tc =c/(f*v), where c is the speed of light, f is carrier frequency and v is the speed of the mobile terminal. E.g. if f=2GHz and v=1m/s, Tc=3e8/(2e9*1)=0.15s. The redirected reference signal  220  may for example be transmitted once every 5-30 ms frame, preferably once every 10 ms frame. It is also possible to transmit multiple redirected reference signals per frame. Note that interference is not reciprocal, uplink interference is however known by the base station  114  and the base station may signal (broadcast) this information to the terminals  120 ,  122  and  124 . The downlink data  250  is transmitted from the base station  114  to the mobile terminals  120 ,  122  and  124  in the downlink band  200  in conventional way. 
     One embodiment of the present invention may be used in the radio interface framework WINNER. A schematic view over the basic uplink structure of the WINNER super-frame structure is depicted In  FIG. 3 , wherein the longitudinal axis shows time and the lateral axis shows frequency. The uplink band is referred to as  300  and the super-frame is referred to as  310 . Arrows pointing downwards indicate transmittal from the base station  114  to the mobile terminals  120 ,  122  and  124 , and arrows pointing upwards indicate transmittal from the mobile terminals  120 ,  122  and  124  to the base station  114 . The first part  320  of the super-frame  310  is designated to e.g. uplink synchronization and random access while the actual data transmission  330  resides in the latter part  340  of the super-frame  310 . The duration of the entire super-frame  310  is e.g. in the order of 5 ms. In this embodiment a resource is preferably reserved for the redirected reference signal  350  in the uplink band  300  in each super-frame  310  or less frequent if suitable. The redirected reference signal  350  resource may e.g. be reserved in the first part  320  of the super-frame  310  together with e.g. reserved resources for Random Access Channel (RACH)  360  and for uplink synchronisation  370 . The base station  114  accordingly uses the reserved redirected reference signal resource  350  to send the redirected reference signal to the mobile terminals  120 ,  122  and  124  in the uplink band. 
     In one embodiment of the present invention, the uplink band information estimated by the mobile terminals  120 ,  122  and  124  by comparing the known redirected reference signal with the received redirected reference signal is required by the base station  114  and the uplink band information is therefore reported back to the base station  114 . This may be performed by the mobile terminals  120 ,  122  and  124  reporting estimated uplink channel quality together with the downlink quality report. 
     One example when the uplink band information may be used for the base station  114  is when the base station  114  shall perform uplink quality based frequency domain scheduling in the time and possibly also in the frequency domain. This is e.g. usable within the WINNER technology mentioned above. 
     One example when the uplink band information may be used for the base station  114  is when the base station  114  shall perform uplink quality based frequency domain scheduling in the time and possible also in the frequency domain. This is e.g. usable within the WINNER technology mentioned above. 
     In this case the mobile terminals  120 ,  122  and  124  utilize the redirected reference signals received from the base station  114  for channel quality estimation. The mobile terminals  120 ,  122  and  124 , report the estimated channel quality back to the base station  114  that in turn uses the estimated channel quality as an input to the scheduling algorithm. The base station transmits uplink scheduling control messages to each of the mobile terminals  120 ,  122  and  124  (in the downlink) and each of the mobile terminals  120 ,  122  and  124  transmit data, using assigned resources based on the estimated channel quality reported to the base station  114 . 
     Compared to a conventional solution, in which mobile terminals send uplink reference signals that are used for channel quality estimation, the solution according to the invention may invoke a slightly higher delay. However, with many mobile terminals  120 ,  122  and  124  active in the uplink, all transmitting reference signals covering the entire uplink frequency band, the over head is however smaller which is an advantage that outweighs the disadvantage with said delay. 
     It should be noted that a set of ‘regular reference signals’ still may be transmitted from the terminal  120 ,  122  and  124  to the base station  114  to enable the base station to be able to demodulate the data. These regular reference signals need however only be transmitted together with data from the terminal  120 ,  122  and  124 , i.e. at time instants and frequency bands the terminal  120 ,  122  and  124  is transmitting. As these regular reference signals do not need to cover the entire frequency band, and do not need to be transmitted from terminals  120 ,  122  and  124  not transmitting data, they cause only limited overhead. The channel estimates based on these regular reference signals could also be used as input to the scheduler, possibly in combination with the channel estimates reported by the mobile terminals  120 ,  122  and  124 . 
     The present method steps performed by the base station  114  will now be described with reference to a flow chart depicted in  FIG. 4 .
       401 . The base station transmits a redirected reference signal  220 , 350  on the uplink band to the mobile terminal  120 ,  122 , and  124  which redirected reference signal  220 , 350  is intended for uplink channel estimation.     402 . When the mobile terminal ( 120 , 122 , 124 ) that knows the origin redirected reference signal, has performed the uplink channel estimation by estimating the received redirected reference signal and comparing the known redirected reference signal with the received redirected reference signal, the base station  114  may receive a report from the mobile terminal ( 120 , 122 , 124 ), which report comprises the uplink channel quality, estimated by the mobile terminal ( 120 , 122 , 124 ). The base station may at the same time receive a regular uplink reference signal from the mobile terminal  120 , 122 , 124 , and estimate the uplink channel quality, based on the received regular reference signal.     403 . The base station  114  may perform uplink quality based frequency domain scheduling based on the uplink channel quality comprised in the report and, also based on the estimated uplink channel quality, based on the received regular reference signal if such a regular signal is received. This may be performed by input the received channel quality to the scheduling algorithm for uplink quality based frequency domain scheduling in the time and/or in the frequency domain.     404 . The base station  114  then transmits an uplink control message, on a downlink channel to the mobile terminal  120 , 122 , 124 , based on the performed uplink quality based frequency domain scheduling.   

     To perform the present method steps, the base station  114  comprises an arrangement  500  as depicted in  FIG. 5 . The base station arrangement  500  comprises a transmitter  502  for transmitting the redirected reference signal  220 , 350  on an uplink band to the mobile terminal  120 , said redirected reference signal  220 , 350  being intended for uplink channel estimation. The transmitter  502  may be adapted to transmit the redirected reference signal  220 , 350  time multiplexed, and further to transmit the redirected reference signal  220 , 350  such that it covers the entire uplink frequency band. In one embodiment the transmitter further is adapted to transmit the redirected reference signal  220 , 350  surrounded by a guard period  240 , which guard period  240  for example is 60-70 μs. In another embodiment the transmitter  502  is adapted to transmit the redirected reference signal  220 , 350  frequency multiplexed. The transmitter  502  may be arranged to transmit the redirected reference signal  220 , 350  with a frequency that is adapted in accordance with the channel coherence time. The channel coherence time may be estimated as Tc=c/(f*v). The redirected reference signal  220 , 350  may be transmitted the e.g. in every 5-10 ms frame, preferably in every 10 ms frame. In one embodiment the transmitter  502  is arranged to transmit multiple redirected reference signals  220 , 350  per time frame. In a further embodiment the WINNER technology is used, in which embodiment the transmitter  502  is arranged to transmit the redirected reference signal  350  in the first part of a super-frame. 
     In a further embodiment the base station arrangement  500  comprises a receiver  504  arranged for receiving a report from the mobile terminal  120 , 122 , 124  that has received the redirected reference signal. The report comprises the uplink channel quality, estimated by the mobile terminal  120 , 122 , 124 , which mobile terminal  120 , 122 , 124  knows the origin redirected reference signal and estimates the received redirected reference signal and compares the known redirected reference signal with the received redirected reference signal. In one embodiment the base station arrangement  500  comprises means  506  for performing uplink quality based frequency domain scheduling based on the uplink channel quality comprised in the report. The uplink quality based frequency domain scheduling means  506  may be arranged to input the received channel quality to the scheduling algorithm for uplink quality based frequency domain scheduling in the time and/or in the frequency domain. In another embodiment is the receiver  504  further arranged to receive a regular uplink reference signal from the mobile terminal  120 , 122 , 124  and in this embodiment the base station arrangement  500  further comprises an estimation unit  508  for estimating the uplink channel quality, based on the received regular reference signal. In this case the uplink quality based frequency domain scheduling means  506  further is arranged to input channel quality, based on the received regular reference signal, to the scheduling algorithm for uplink quality based frequency domain scheduling in the time and/or in the frequency domain. The transmitter  502  may be arranged to transmit an uplink control message on a downlink channel to the mobile terminal  120 , 122 , 124  based on the performed uplink quality based frequency domain scheduling. 
     The mobile terminal which may be any of the mobile terminals  120 ,  122  and  124  is in this example referred to as mobile terminal  120 . The mobile terminal  120  is currently being positioned in the cell  112  associated to the base station  114 . The mobile terminal  120  comprises a receiver for receiving downlink information from the base station  114  in both the uplink band and in the downlink band. The mobile terminal  120  receives the redirected reference signal, sent from the base station  114 , in the uplink band. The mobile terminal  120  uses the redirected reference signal, sent in the uplink band for estimating the uplink channel quality. The mobile terminal  120  knowing the origin redirected reference signal, estimates the uplink channel by estimating the received redirected reference signal and comparing the known redirected reference signal with the received redirected reference signal. As mentioned above, the known redirected reference signal has known amplitude and a known phase reference and is compared with the amplitude(s) and phase(s) of the received signal in a conventional way. The estimated uplink channel quality may be used by the mobile terminal itself and/or be reported back to the base station  144  so that the base station  114  can utilize the information, the latter is described above. 
     The estimated channel quality from redirected reference signals sent from the base station  114  in the uplink frequency band can be used by the mobile terminal  120  for e.g. adaptive modulation and coding or power control. The mobile terminal  120  is aware of the uplink interference level, provided e.g. by means of downlink signalling. By means of the estimated channel quality, the mobile terminal  120  is on its own capable of determining which transmission format, such as modulation and coding scheme, which power to use in the next transmission. The advantage with this is that the downlink signalling is reduced and that the time between channel quality estimation and transmission is shortened. For RACH or contention-based data transmission, the estimated channel quality information provided by the mobile terminal  120  may further give useful indications of which power level and transmission format the mobile terminal  120  shall use in order to achieve reliable transmission without causing excessive interference in the network. To adapt these transmissions to the fast channel variations is otherwise difficult. 
     The method steps performed by the present mobile terminal  120  will now be described with reference to a flow chart depicted in  FIG. 6 .
       601 . The mobile terminal  120  receives a redirected reference signal  220 , 350  on an uplink band from the base station  114 , said redirected reference signal  220 , 350  being intended for uplink channel estimation.     602 . The mobile terminal  120  estimates the received redirected reference signal     603 . The mobile terminal  120  then performs the estimation of the uplink channel quality by comparing the known redirected reference signal with the estimated received redirected reference signal. This may performed by comparing the amplitude(s) and/or phase(s) of the origin redirected reference signal and received redirected reference signal.     604 . The mobile terminal  120  may transmit a report to the base station  114 , the report comprising the estimated uplink channel quality.     605 . The mobile terminal  120  determines adaptive modulation and/or coding scheme and/or which power to use, and/or antenna weights, based on the estimated uplink channel quality.   

     To perform the present method steps, the mobile terminal  120  comprises an arrangement  700  as depicted in  FIG. 7 . The mobile terminal arrangement  700  comprises a receiver  702  for receiving a redirected reference signal  220 , 350  on the uplink band, from the base station  114 , said redirected reference signal  220 , 350  being intended for uplink channel estimation. The mobile terminal knows the origin redirected reference signal transmitted by the base station  114  and the mobile terminal arrangement  700  comprises an estimation unit  704  for estimation of the uplink channel quality, the estimation unit  704  being arranged to estimate the received redirected reference signal, and further being arranged to compare the known redirected reference signal with the estimated received redirected reference signal. The estimation unit  704  may further be arranged to make the comparing based on the amplitude(s) and/or phase(s) of the origin redirected reference signal and received redirected reference signal. The mobile terminal arrangement  700  may further comprise a determination unit  706  for determining adaptive modulation and/or coding scheme and/or which power to use, and/or antenna weights, based on the estimated uplink channel quality. The mobile terminal arrangement  700  may further comprise a transmitter  708  arranged to transmit a report to the base station  114 , the report comprising the estimated uplink channel quality. 
     The present managing uplink channel estimation mechanism can be implemented through one or more processors, such as the processor  510  in the base station arrangement depicted in  FIG. 5  and the processor  710  in the mobile terminal arrangement depicted in  FIG. 7 , together with computer program code for performing the functions of the invention. 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 method when being loaded into the access point and/or 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 access point and/or user equipment remotely. 
     The present invention is not limited to the above-describe 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 appended claims.