Abstract:
Various methods and apparatuses provide unicast channel data acquisition, such as antenna information, from MBMS subframes. A method of operating a wireless communications network infrastructure entity is disclosed comprising transmitting a subframe ( 300 ) comprising a unicast symbol ( 301 ) in a first predetermined symbol position within said subframe ( 300 ), said unicast symbol ( 301 ) comprising a first antenna reference information; defining said first antenna reference information as a second antenna reference information; and transmitting a second subframe comprising a second unicast symbol in a second predetermined symbol position within said second subframe, said second unicast symbol comprising said second antenna reference information.

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to communication networks providing Multimedia Broadcast Multicast Service (MBMS), and more particularly to methods and apparatuses for providing and receiving unicast channel information when MBMS is promulgated within a communication network coverage area. 
     BACKGROUND 
     Multimedia Broadcast Multicast Service (MBMS) is described in various standards such as the Third Generation Partnership Project (3GPP), Universal Mobile Telephone System (UMTS) standards. More recently, MBMS functionality has been specified in relation to orthogonal frequency division multiplexing (OFDM) physical layers, such as those exemplified by the IEEE 802.16e-2005 amendment or 3GPP Long Term Evolution (LTE) project (to which the term Evolved MBMS or EMBMS is sometimes applied). MBMS over OFDM and other selected physical layer structures (of which EMBMS is an example) enables a multicast approach referred to as multicast broadcast single frequency network (MBSFN). OFDM is particularly favorable for MBSFN operation when the OFDM symbols contain a cyclic prefix (CP) that can constructively recover multiple signal paths from one or more transmission sources. MBSFN techniques operate by simulcasting (i.e. transmitting on identical time-frequency network resources) from various cells (base transceiver stations or “enhanced node Bs”, eNB&#39;s), over a given geographic area. Such areas may be defined by the radio coverage area of a group of base transceiver stations or even smaller areas as defined by antenna coverage sectors of the base transceiver station. Sometimes the term single frequency network area (SFA) is defined as the set of cells participating in the simulcasting procedure. 
     In addition to receiving MBMS service (where the generic term MBMS is understood to include 3GPP LTE-specific EMBMS), a mobile station, also referred to as “User Equipment” or “UE,” may also engage in communication using dedicated channels or other via unicast signaling where the UE communicates with a specific base transceiver station. 
     Receiving unicast information by a UE is distinguishable from MBMS delivery reception, because the MBSFN is generally broadcast from one or more base transceiver stations (i.e. the SFA) such that a UE receives one composite desired signal from the base transceiver stations. Therefore, the individual base transceiver station transmissions are indistinguishable to the UE in the case of MBMS reception. Note that in the current context, while MBSFN transmission from multiple base stations is expected to be a typical operating mode, transmission from a single base station is specifically recognised as a practical case. 
     However a UE must have the ability to make measurements of individual base stations both for mobility and link adaptation purposes. In the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) core specifications base transceiver stations may be defined with up to 4 antenna ports. A set of associated reference symbols (RS, numbered RS  0 ,  1 ,  2 ,  3 ) may be defined and applied to Multiple Input, Multiple Output (MIMO) transmission of unicast downlink traffic. However, the UE is generally required to access only RS  0  and  1  for mobility measurement purposes, and may further need to access only RS  0  and  1  for reception of any unicast transmission from a specific cell delivered in coordinated fashion with the MBSFN transmission. 
     As specified by 3GPP, a downlink subframe formatted for MBMS transmission using the multicast broadcast single frequency network approach (MBSFN) over the Multicast Channel (MCH) is applied by time-division multiplexing (TDM) unicast and MBSFN subframes (note that here, the terms MCH and MBSFN subframe are used to indicate a subframe simulcast by one or most base stations). In at least one configuration, in any MBSFN subframe region associated with control data, only RS  0  and RS  1  will be present and transmitted in unicast mode by each base station, and any reference symbols transmitted over the MBSFN or MCH portion of the subframe may not be used as a phase reference for demodulating MIMO unicast transmissions or for demodulating any subsequent unicast subframe (i.e. a sub-frame dedicated to use by the so-called downlink shared channel (DL-SCH) in the case of 3GPP LTE). Nor may reference symbols transmitted over the MBSFN be used for other purposes related to the cell-specific unicast transmission, such as link adaption or MIMO transmission rank adaption. 
     Accordingly, given the adopted TDM approach to MBSFN structure in 3GPP LTE (and similar systems), and unicast allocated subframes, it would not be possible to maintain appropriate link adaptation metrics for all possible (up to 4) MIMO streams for a unicast subframe following a broadcast subframe due to the absence of RS  2  and  3  during a MBSFN-allocated (i.e. MCH-allocated) subframe. 
     Therefore, a means is required to enable observation of, and link adaptation in consideration of, the 3 rd  and 4 th  antennas (antennas numbered  2  and  3 ) while receiving MCH-allocated subframes or at least to minimize the impact of not observing antennas  2  and  3  while receiving MBSFN prior to re-commencement of non-MBSFN transmissions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating an exemplary MBSFN OFDM subframe containing one unicast OFDM control symbol and eleven OFDM MBSFN or MCH-related symbols with the single OFDM control symbol containing reference symbols (RS) for antenna  1  and antenna  2 . 
         FIG. 2  is a diagram illustrating an exemplary OFDM unicast subframe wherein various symbols of the subframe may contain antenna information. 
         FIG. 3  is a block diagram illustrating an exemplary OFDM subframe having a single control symbol wherein the antenna information contained by the control symbol may be redefined on subsequent subframes. 
         FIG. 4  is a block diagram illustrating an exemplary OFDM subframe having a first control symbol and wherein a second symbol carrying a unicast transmission is transmitted and which contains information for antenna  3  or antenna  3  and  4 . 
         FIG. 5  is a block diagram illustrating an exemplary OFDM subframe wherein the first unicast subframe after an MBSFN subframe contains information for antenna  3  or antennas  3  and  4 . 
         FIG. 6  is a block diagram of a base transceiver station or eNB in accordance with various embodiments. 
         FIG. 7  is a block diagram of a mobile station or user equipment (UE) in accordance with various embodiments. 
         FIG. 8  is a flow chart illustrating a method of operation of an eNB in accordance with the embodiment illustrated in  FIG. 3 . 
         FIG. 9  is a flow chart illustrating a method of operation of an eNB in accordance with the embodiment illustrated in  FIG. 4 . 
         FIG. 10  is a flow chart illustrating a method of operation of an eNB in accordance with the embodiment illustrated in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     Methods and apparatuses for providing unicast information during promulgation of MBMS frames are provided herein. 
     As previously discussed, a mobile station (“user equipment” or “UE”) must have the ability to make measurements of individual base stations, that is receive unicast information, both for mobility support and link adaptation. The present disclosure provides various solutions such that a UE may obtain such information while receiving MBMS data. 
     Turning now to the drawings wherein like numerals represent like components,  FIG. 1  illustrates an MBSFN subframe as defined by the 3GPP Radio Access Network (RAN) working group. In the MBSFN subframe  100  either the first OFDM symbol  101  (symbol number  0 ) or both the first OFDM symbol  101  and the second OFDM symbol  103  (symbol number  1 ) may contain unicast control information. More generally, a larger number of symbols could contain unicast control information. 
     As illustrated by  FIG. 1 , if only the first symbol  101  is required for sending the unicast control channel (referred to as the PDCCH in 3GPP LTE), then the first symbol  101  will not contain reference symbols (RS&#39;s) for any 3 rd  and 4 th  base transceiver station or “E-UTRAN NodeB” (eNB) antenna ports present. More particularly, RS 2  and RS 3  could not be transmitted using the RS locations defined in 3GPP TS 36.211, 3 GPP Technical Specification for Physical Channels and Modulation , Section 5.6.1.2, “Physical resource mapping” (March 2007) which is incorporated by reference herein. 
     For example a UE should be able to make Channel Quality Indicator (CQI) measurements and, more generally, rank adaptation and pre-coding vector optimisation for the various antennas during MBSFN subframe transmission. In a purely unicast subframe, as illustrated by  FIG. 2 , the OFDM subframe  200  may provide reference symbols for the various antennas at various suitable symbol location numbers. Thus, some solution is needed to provide reference symbols and enable CQI measurements for all antennas during an MBSFN subframe. 
     A first embodiment is illustrated by  FIG. 3 . In  FIG. 3  an MBSFN subframe  300  contains only a single unicast symbol  301 . Thus in the embodiment illustrated by  FIG. 3 , antenna rotation, or more generally re-mapping, is applied in subsequent MCH subframes. As illustrated in  FIG. 3 , the definition of antenna  0  (or  0  and  1 ) is re-mapped between all 4 antennas. This method is a non-invasive method to permit observation of all 4 eNB antennas. 
     Therefore, in sequence, MCH-allocated subframes (or group of subframes), antennas  0  through  3  are defined such that each antenna is designated as “antenna  1 ” for at least 1 transmission of the subframe, each antenna is designated as antenna  2  for other transmissions and so on. For example, if the eNB supports 4 antennas, the available eNB antennas are re-mapped in the unicast region, that is, the unicast symbol  301 , of the MCH-allocated subframe  300  according to an a priori known re-mapping sequence. It is to be understood that the mapping may be onto 1 or 2 reference symbol locations depending upon the sub-frame configuration. Rotation or swapping may also be permitted (i.e. in antenna port order, 12, 23, 34, 41 or 12, 34, 12, 34, etc.) for the embodiment of  FIG. 3 . The embodiment of  FIG. 3  can restrict a UE channel estimator to operate within an MBSFN subframe, but permits observation of all eNB antennas prior to re-commencement of unicast transmission. 
     If only the first OFDM symbol  301  is assigned for PDCCH purposes in MBSFN sub-frame  300 , then TS 36.211 specifies RS locations for RS 0  and RS 1 . These locations may include hopping or shifting of RS locations within a symbol from one cell to the next, or from one subframe to the next. Thus the first embodiment method for making available observations of antenna ports  3  and  4  is to rotate, or re-map, the association of an RS with antenna ports in each MBSFN-allocated subframe. For example, in subframe n, antenna ports  0  and  1  are respectively associated with RS 0  and RS 1 , while in subframe n+1 antenna ports  2  and  3  are respectively associated with RS 0  and RS 1 , and so on. 
     In a second embodiment, which is illustrated by  FIG. 4 , reference symbols for antennas  2  and  3 , transmitted on a unicast basis by each base station participating in the MBSFN, are inserted into the MBSFN subframe in the position of an MCH or data or reference symbol. 
     In  FIG. 4 , a unicast control portion  401  contains reference symbols for antennas  0  and  1 . The unicast portion  401  may be followed by a series of MCH or MBSFN allocated OFDM symbols (symbols numbered  1  through  11 ). At least one of the MBSFN allocated OFDM symbols, for example symbol number  3   403  or symbol number  7   405  or symbol number  1   406 , will be replaced with a unicast OFDM symbol and will contain the additional unicast reference symbols for antennas  2  and  3 . 
     More specifically, if only the first OFDM symbol  401  in an MBSFN subframe  400  were initially allocated for PDCCH transmission, and therefore only RS 0  and RS 1  were initially available for processing at the UE, an additional unicast symbol containing RS 2  or RS 2  and RS 3  would be transmitted in the remaining portion of the MBSFN subframe, that is, into one of symbol numbers  1  through  11 , of the subframe  400 . Note that while transmission of either RS 2  or RS 2  and RS 3  is possible, further restrictions on the number of permitted antenna ports might restrict optionality to only transmitting RS 2  and RS 3 . 
     If a first OFDM symbol  401  and a second OFDM symbol in an MBSFN subframe were initially allocated for PDCCH transmission, only RS 0  and RS 1  may be made initially available for processing at the UE, even though locations for RS 2  and RS 3  are available. Reasons that only RS 0  and RS 1  may be initially made available include minimizing RS symbol overhead, or that the unicast control information itself does not gain a significant advantage from RS from e.g. 4 antennas. RS 2  and RS 3  may then be inserted into a MBSFN subframe that was initially configured for only RS 0  and RS 1 . 
     Furthermore, RS 2  and RS 3  may be inserted into the first unicast OFDM symbol that contains RS 0  and RS 1 , though the overhead if RS are spaced every 6 subcarriers would be quite high, with ⅔ of the symbol used for RS from 4 antennas. 
     In general, insertion may be accomplished via either replacing or puncturing. A multicast symbol may be replaced, or part of a multicast symbol may be punctured, with unicast RS information. A portion of the unicast symbol that is not initially used for RS from antennas  3  and  4  may be punctured. 
     Further in the various embodiments, the impact of such unicast RS replacement on the system may be minimized if MBSFN subframes are clustered together. 
     The method illustrated by  FIG. 4  can limit loss of spectral efficiency in MBSFN or MCH-allocated subframes due to unnecessary RS provisioning until it is required, that is, until just before the recommencement of MIMO-enabled unicast transmissions. This approach is most efficient if MBSFN or MCH subframes are clustered in one group or several groups in a specific time interval (which may be referred to as a resource allocation interval or superframe). In some embodiments, the last subframe of the group of MBSFN or MCH associated subframes may have RS from all 4 antennas. 
     Thus one embodiment illustrated by  FIG. 4 , requires unicast transmission of at least a second OFDM symbol in an MBSFN subframe or sequence of MBSFN subframes before the first subframe containing a MIMO-enabled unicast transmission. Specifically all cells, that is, all eNB&#39;s, participating in the MBSFN would need to transmit at least two unicast control symbols in the specified MBSFN subframes. 
     In some embodiments delivering such PDCCH allocations may be embedded in the semi-statically configured MBSFN subframe sequence, and incorporated by a centralised entity, which could be referred to as the Multicast Coordination Entity (MCE), in transport block dimensioning for delivery over each single frequency area (SFA, the area covered by an MBSFN). Further in the various embodiments, the impact of the additional unicast control overhead on the system operation may be minimized if MBSFN subframes are clustered together. 
     The embodiments of  FIG. 3  and  FIG. 4  may have inefficiencies if a single stream transmit diversity technique such as cyclic delay diversity is employed for MBSFN or MCH subframe transmission but not for unicast transmission (e.g. in order to minimize RS overhead for MCH) or for the final MCH subframe before unicast transmission. 
     For such cases,  FIG. 5  illustrates another embodiment in which reference symbols for at least the 3 rd  and 4 th  antenna are located in the first unicast subframe  503  after an MBSFN subframe  501 . The first unicast subframe  503  of the subsequent unicast subframes are required to contain reference symbols for antennas not observable in MCH subframes. 
     The next, that is, the second unicast subframe may then commence with standard operation such as transmitting data. Because this method places a constraint on the system, the various embodiments may ease this constraint by grouping MCH subframes into one contiguous block. It is to be understood that a similar grouping approach may also be employed to enhance the embodiments described with respect to  FIG. 3  and  FIG. 4 . 
     However, a disadvantage may occur in that if the contiguous MCH subframes form a long enough block, a gap in unicast traffic may become noticeable to the user (e.g. typically gaps of 100 ms or greater are noticeable for voice). Therefore an alternative embodiment may define bursts or clusters of MCH subframes which are of a maximum size in time. 
       FIG. 6  is a block diagram of a base transceiver station or eNB  600  in accordance with the various embodiments. A base transceiver station, or eNB may be alternatively referred to as a cell. The eNB  600  will comprise a transceiver or multiple transceivers  601  and comprises multiple antennas (not shown). The transceiver  601  is coupled to a processor or processors  603 . In the various embodiments the processor/s  603  comprise a unicast reference symbol  605  module for including antenna reference symbols into unicast and multicast subframes in accordance with the various embodiments herein described. 
     It is to be understood that  FIG. 6  is for illustrative purposes only and is for illustrating the module  605  in accordance with the present disclosure, and is not intended to be a complete schematic diagram of the various components and connections therebetween required for a base transceiver station/eNB. Therefore, an eNB will comprise various other components not shown in  FIG. 6  and still be within the scope of the present disclosure. 
       FIG. 7  is a block diagram illustrating components of a mobile station in accordance with the various embodiments. Mobile station  700  comprises at least user interfaces  705 , at least one processor  703 , a graphical display  707  and one or more transceivers  701  capable of receiving signals from multiple antennas. The processor or processors  703  will also comprise a symbol decoding module  709  such that the mobile station may detect antenna reference symbols and make CQI measurements in accordance with the various embodiments described herein. 
     It is to be understood that  FIG. 7  is for illustrative purposes only and is for illustrating the symbol decoding module  709  of a mobile station in accordance with the present disclosure, and is not intended to be a complete schematic diagram of the various components and connections therebetween required for a mobile station. Therefore, a mobile station will comprise various other components not shown in  FIG. 7  and still be within the scope of the present disclosure. 
       FIG. 8  is a flow chart illustrating a method of operation of an eNB in accordance with the embodiment illustrated in  FIG. 3 . In  801  reference symbols for a first antenna set, which may comprise one or more antennas, is defined by an eNB. In  803  a unicast symbol is transmitted on a first subframe including the reference symbols. In  805 , the reference symbols of the first antenna may be redefined at the same OFDM symbol number position of the first subframe for a second antenna or set of antennas in a next subframe, or at any other suitable OFDM symbol number position. In  807  a unicast symbol in the second subframe includes the second antenna or second antenna set reference symbols. Note that the reference symbol may not be explicitly provided but rather in some embodiments, only the time-frequency location of the reference symbol, in other words, unicast antenna reference information, may be provided such that the reference symbol or symbols may be obtained by the UE. 
       FIG. 9  illustrates a method of operation of an eNB corresponding to  FIG. 4 . Thus in  901  and  903  a unicast symbol is transmitted on a first subframe which includes reference symbols for a first antenna or first antenna set. In  905 , a multicast symbol position is defined for transmitting a second unicast symbol containing a second antenna or second antenna set reference symbols. The second unicast symbol, which may be a control symbol, is transmitted in the MBSFN subframe in  907 . 
       FIG. 10  illustrates a method of operation of an eNB in accordance with the embodiment illustrated in  FIG. 5 . Thus assuming a unicast subframe was transmitted after a given multicast subframe with antenna symbol information in  1001 , reference symbol information will be again transmitted on the next unicast subframe following 
     Thus various embodiments were disclosed for enabling measurements, such as CQI measurements, when a multi-antenna base transceiver station such as an eNB is configured to transmit a carrier frequency or frequency layer in a unicast/MBMS-mixed configuration. 
     While the preferred embodiments have been illustrated and described, it is to be understood that the disclosure is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.