This disclosure relates to resource aggregation in wireless communications, and more particularly to cell aggregation.

A communication system can be seen as a facility that enables communication sessions between two or more nodes such as fixed or mobile devices capable of wireless communications, access nodes such as base stations, servers and so on. A communication system and compatible communicating entities typically operate in accordance with a given standard or specification which sets out what the various entities associated with the system are permitted to do and how that should be achieved. For example, the standards, specifications and related protocols can define the manner how various nodes shall communicate, how various aspects of the communications shall be implemented and how the nodes shall be configured.

Communications between nodes can be carried on wired or wireless carriers. Examples of wireless systems include public land mobile networks (PLMN) such as cellular networks, satellite based communication systems and different wireless local networks, for example wireless local area networks (WLAN). Wireless systems can be divided in coverage areas typically referred to as cells. A cell can be provided by a base station, there being various different types of base stations. A base station system may provide a plurality of cell. Different types of cells can provide different features. For example, cells can have different shapes, sizes and other characteristics. An example of cellular communication systems is an architecture based on standards by the 3rd Generation Partnership Project (3GPP). Recent development of the 3GPP architecture is the long-term evolution (LTE) of the Universal Mobile Telecommunications System (UMTS) radio-access technology. A further development of the LTE is often referred to as LTE-Advanced. The various development stages of the 3GPP LTE specifications are referred to as releases.

A user can access the communication system by means of an appropriate communication device. A communication device of a user is often referred to as User equipment (UE) or terminal. A communication device is provided with an appropriate signal receiving and transmitting arrangement for enabling communications with other parties. In wireless systems a communication device provides a transceiver station that can communicate over an air interface. The communication device may access carriers provided by nods such as base stations, other communications devices and so on, and transmit and/or receive communications on the carriers.

A node may communicate simultaneously on a plurality of carriers. Coordinated multipoint transmission (CoMP) is a technique where combined results of reception by a plurality of stations from a communication device or reception of a transmission based on signals transmitted from a plurality of sources can be utilised. CoMP can be provided for example in heterogeneous network scenarios where carrier aggregation or cell aggregation is employed. In such arrangement a centralised processing unit controlling the relevant cells is also provided. Cell aggregation utilizes features of carrier aggregation such that different carriers are sharing the same frequency band. Cell aggregation may cause interference between different carriers and/or cells. For example, interference may become an issue in the context of downlink demodulation reference symbols (DMRS) allocations, in particular in view of channel estimation. In cell aggregation interference among DMRS may become relative strong. Thus it would be desirable to avoid direct DMRS collisions.

According to an aspect, there is provided amethod for allocating resources for simultaneous transmission by aggregated cells, comprising allocating resources for demodulation reference symbols in a transmission pattern of a first cell of the aggregated cells, and allocating different resources for demodulation reference symbols in a transmission pattern of a second cell of the aggregated cells.

According to another aspect there is provided method for receiving at a device simultaneously from aggregated cells, comprising receiving demodulation reference symbols of a first cell of the aggregated cells in a resource allocated in accordance with a transmission pattern of the first cell, and receiving demodulation reference symbols of a second cell of the aggregated cells in a resource allocated in accordance with a transmission pattern of the second cell, wherein different resources are allocated for demodulation reference symbols in the transmission pattern of the first cell and the second cell.

According to an aspect there is provided an apparatus for allocating resources for simultaneous transmissions by aggregated at least first and second cell, the apparatus comprising at least one processor, and at least one memory including computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to allocate a different resource in a transmission pattern of the second cell for simultaneous transmission of demodulation reference symbols than is allocated for the first cell.

According to a further aspect there is provided an apparatus for handling simultaneous reception from aggregated cells, the apparatus comprising at least one processor, and at least one memory including computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to handle simultaneous reception of demodulation reference symbols from at least a first cell and a second cell of the aggregated cells, wherein different resources are allocated for demodulation reference symbols in the transmission patterns of the first cell and the second cell.

In accordance with a more detailed embodiment, the resources are located depending on the type of the cell. The first cell may comprise a primary cell and the second cell may comprise a secondary cell.

Position of resource elements for the demodulation reference symbols of one of the cells may be shifted such that different ports are allocated for use by different cells in the transmission of reference modulation symbols. The shifting may be automatic.

Allocation of different resources may comprise determining by the first cell demodulation reference symbol base sequences for the second cell. Cell identity of the second cell may be modified when determining the demodulation reference symbol base sequences. Orthogonal antenna ports may be flexibly allocated for the first cell and the second cell from a pool of antenna ports defined for demodulation reference signal transmission.

At least one resource element may be muted. Information about muting may be communicated from at least one of the cells by means of scrambling code or radio resource control signalling. Increased power may be used for transmission of demodulation reference symbols.

A node such as a base station or a mobile station can be configured to operate in accordance with the various embodiments.

A computer program comprising program code means adapted to perform the method may also be provided.

It should be appreciated that any feature of any aspect may be combined with any other feature of any other aspect.

Embodiments will now be described in further detail, by way of example only, with reference to the following examples and accompanying drawings, in which:

Figure 1 shows a schematic diagram of cell aggregation according to some embodiments;

Figure 2 shows a schematic diagrams of a mobile communication device according to some embodiments;

Figure 3 shows a control apparatus according to some embodiments; Figure 4 shows a schematic diagram of a control apparatus according to some embodiments;

Figure 5 shows a schematic flowchart according to an embodiment; and Figures 6 to 9 show transmission patterns for port allocations in accordance with certain embodiments.

In the following certain exemplifying embodiments are explained with reference to a wireless or mobile communication system serving mobile communication devices. Before explaining in detail the exemplifying embodiments, certain general principles of a wireless communication system, access systems thereof, mobile communication devices and cell aggregation are briefly explained with reference to Figures 1 to 4 to assist in understanding the described examples. A communication device 20 is typically provided wireless access via at least one base station or similar wireless transmitter and/or receiver node of an access system. In figure 1 two neighbouring and/or overlapping radio service areas 10 and 14 are shown being provided by base stations 11 and 15, respectively. The radio service areas are called herein as cells. It is noted that instead of two cells, any number of cells can be provided in a communication system. It is noted that the cell borders are schematically shown for illustration purposes only in Figure 1. It shall be understood that the sizes and shapes of the cells or other radio service areas may vary considerably from the similarly sized oval shapes of Figure 1. As shown, a base station site can provide more than one cell or sector. Each communication device and base station may have one or more radio channels open at the same time and may send signals to and/or receive signals from more than one source

Base stations are typically controlled by at least one appropriate controller apparatus so as to enable operation thereof and management of mobile communication devices in communication with the base stations. Figure 1 shows control apparatus 18 for control of the two base stations 1 and 15 and thus cells 10 and 14. The control apparatus can be interconnected with other control entities. The control apparatus can typically be provided with memory capacity and at least one data processor. The control apparatus and functions may be distributed between a plurality of control units. In some embodiments, each base station can comprise a control apparatus. In alternative embodiments, two or more base stations may share a control apparatus. In some embodiments at least a part of control apparatus may be respectively provided in each base station.

A non-limiting example of the recent developments in communication system architectures is the long-term evolution (LTE) of the Universal Mobile Telecommunications System (UMTS) that is being standardized by the 3rd Generation Partnership Project (3GPP). As explained above, further development of the LTE is referred to as LTE-Advanced. The LTE employs a mobile architecture known as the Evolved Universal Terrestrial Radio Access Network (E-UTRAN). Base stations or base station systems of such architectures are known as evolved or enhanced Node Bs (eNBs). An eNB may provide E-UTRAN features for cells such as user plane Radio Link Control/Medium Access Control/Physical layer protocols (RLC/MAC/PHY) and control plane Radio Resource Control (RRC) protocol terminations towards the communication devices. Other examples of radio access include those provided by base stations of systems that are based on technologies such as wireless local area network (WLAN) and/or WiMax (Worldwide Interoperability for Microwave Access). In Figure 1 these features are provided by base station system 18.

Possible cells include those known as macro cells, pico cells and femto cells. For example, in LTE-Advanced the transmission/reception points or base stations can comprise wide area network nodes such as a macro eNode B (eNB) which may, for example, provide coverage for an entire cell or similar radio service area. Base station can also be provided by small or local radio service area network nodes, for example Home eNBs (HeNB), pico eNodeBs (pico-eNB), or femto nodes. Some applications can utilise radio remote heads (RRH) that are connected to for example an eNB. Cell areas can overlap, and thus a communication device in an area can listen to more than one base station. Smaller radio service areas can be located entirely or at least partially within a larger radio service area. A communication may thus communicate with more than one cell. In some embodiments LTE-Advanced network nodes can comprise a combination of wide area network nodes and small area network nodes deployed using the same frequency carriers (e.g. co-channel deployment).

Communications over wireless interfaces with a plurality of cells can utilise combining of the results of detecting a transmission from a communication device located within a plurality of cells or detecting a transmission based on signals transmitted from base stations of a plurality of cells. An example of this is coordinated multipoint transmission (CoMP). CoMP can be provided for example in heterogeneous network scenarios where there is a centralised processing unit. An example of such units is a single controlling macro eNB.

Carrier aggregation or cell aggregation can be used to increase performance. In carrier aggregation a plurality of component carriers on different frequencies are aggregated to increase bandwidth. In cell aggregation a plurality of cells can provide a plurality of carriers. Cell aggregation can be understood as an intra-frequency inter-site combination, a difference to carrier aggregation being that carrier aggregation is provided over multiple frequencies whereas cell aggregation can be provided on a single carrier resource, such as on a single frequency band. Therefore cell aggregation requires different capability from the communication device to that of carrier aggregation. For example, a cell aggregation capable radio may need to support one frequency only. Cell aggregation can utilize the current carrier aggregation features to support a flexible and powerful CoMP solution. In cell aggregation one of the cells may provide a primary cell (PCell) whereas the other cell or cells can provide at least one secondary cell (SCell).

It would be desirable to simplify the implementation and add flexibility of the aggregation schemes and/or interference control between different carriers and/or cells. Interference may be particularly problematic in view of reference symbols, for example demodulation reference symbols. In carrier aggregation of LTE Rel.10 the downlink demodulation reference symbols (DMRS) collision among carriers is not considered a problem because the different component carriers are on different frequencies. However, this may not be the case with cell aggregation where the carrier may be on the dame frequency band. Downlink demodulation reference symbols (DMRS) allocations are an example of an aspect of cell aggregation where interference problems may occur, for example for channel estimation.

Figure 2 illustrates orthogonality between different configurations of DMRS ports and scramble IDs. An example illustrating the problem of DMRS collision is now explained with reference to figure 2, assuming rank-1 stream is supported in each carrier in cell aggregation scenario and each cell uses antenna port 7 for DMRS. Rank in this context refers to a parameter which indicates the supported stream. Antenna port is . a logical port used for transmission of a physical channel or signal depends on the number of antenna ports configured for the physical channel or signal. Antenna port allocations can be communicated to the receiving device means of predefined positions in a transmission resource structure or pattern. Scrambling ID can be different or same between different cells. Because of different cell IDs the DMRS sequences between cells with ID1 and ID2 will not be exactly the same, although they may not be orthogonal either. An example of a current sequence generation formula that takes the cell ID into consideration is shown below:

Instead of full orthogonal, cells with different cell IDs can be considered as quasi-orthogonal at the user equipment side due to the orthogonality generated by the different DMRS sequences. This is illustrated by the dashed lines in Figure 2. This scheme has the same principle as DMRS allocation in multi-user (MU) mode whose orthogonality relies on different scram IDs and use of the same antenna port. Full orthogonality can be obtained by use of different ports (in code division multiplexing (CDM) manner) and same DMRS sequence, and thus orthogonality between ports 7 and 8 is schematically illustrated by solid arrows in Figure 2.

Any improvement from orthogonal DMRS would be desirable for cell aggregation since the user equipment 20 will potentially demodulate multiple independent physical downlink shared channels (PDSCHs) from the aggregated cells 10 and 4. This may require better channel estimation, which in turn would benefit from avoidance of direct DMRS collision in cell aggregation.

A possible mobile communication device for communication with a plurality of base stations will now be described in more detail with reference to Figure 3 showing a schematic, partially sectioned view of a communication device 20. Such a communication device is often referred to as user equipment (UE) or terminal. An appropriate mobile communication device may be provided by any device capable of sending radio signals to and/or receiving radio signals from multiple cells. Non-limiting examples include a mobile station (MS) such as a mobile phone or what is known as a 'smart phone', a portable computer provided with a wireless interface card or other wireless interface facility, personal data assistant (PDA) provided with wireless communication capabilities, or any combinations of these or the like. A mobile communication device may provide, for example, communication of data for carrying communications such as voice, electronic mail (email), text message, multimedia and so on. Users may thus be offered and provided numerous services via their communication devices. Non-limiting examples of these services include two-way or multi-way calls, data communication or multimedia services or simply an access to a data communications network system, such as the Internet. User may also be provided broadcast or multicast data. Non- limiting examples of the content include downloads, television and radio programs, videos, advertisements, various alerts and other information.

The mobile device may receive signals from a multiple of cells over an air interface 27 via appropriate apparatus for receiving and may transmit signals via appropriate apparatus for transmitting radio signals. In Figure 3 transceiver apparatus is designated schematically by block 26. The transceiver apparatus 26 may be provided for example by means of a radio part and associated antenna arrangement. The antenna arrangement may be arranged internally or externally to the mobile device.

A mobile communication device is also provided with at least one data processing entity 21 , at least one memory 22 and other possible components 23 for use in software and hardware aided execution of tasks it is designed to perform, including control of access to and communications with access systems and other communication devices. The data processing, storage and other relevant control apparatus can be provided on an appropriate circuit board and/or in chipsets. This feature is denoted by reference 24.

The user may control the operation of the mobile device by means of a suitable user interface such as key pad 25, voice commands, touch sensitive screen or pad, combinations thereof or the like. A display 28, a speaker and a microphone can be also provided. Furthermore, a mobile communication device may comprise appropriate connectors (either wired or wireless) to other devices and/or for connecting external accessories, for example hands-free equipment, thereto.

Figure 4 shows an example of a control apparatus for a communication system, for example to be coupled to and/or for controlling one or more stations providing cells. It is noted that in some embodiments each base station comprises a separate control apparatus that may communicate control data with each other. The control apparatus 30 can be arranged to provide control on communications in the service area of the system. The control apparatus 30 can be configured to provide control functions in association with cell aggregation on a single carrier resource by means of data processing facility in accordance with certain embodiments described below. For this purpose the control apparatus comprises at least one memory 31 , at least one data processing unit 32, 33 and an input/output interface 34. Via the interface the control apparatus can be coupled to a receiver and a transmitter of the base station. The control apparatus can be configured to execute an appropriate software code to provide the control functions. It shall be appreciated that similar component can be provided in a control apparatus provided elsewhere in the system for controlling reception of sufficient information for decoding of received information blocks.

A wireless communication device, such as a mobile station or a base station, can be provided with a Multiple Input / Multiple Output (MIMO) antenna system. MIMO arrangements as such are known. MIMO systems use multiple antennas at the transmitter and receiver along with advanced digital signal processing to improve link quality and capacity. The transceiver apparatus 26 of Figure 3 can provide a plurality of antenna ports. More data can be received and/or sent where there are more antennae elements.

The following describes certain exemplifying embodiments where cell aggregation is provided for a communication device. The described embodiments provide ways of supporting downlink DMRS collision handling for a device receiving simultaneously from a multiple of aggregated cells. Coordination of downlink DMRS allocation among aggregated cells to avoid collisions can be used to enhance DMRS based channel estimation. In the embodiments demodulation reference symbols (DMRS) by different cells can be treated differently depending on the type of the transmitting cell. For example, the treatment can be different depending on whether the transmitting cell is a primary cell or a secondary cell for the receiving device and/or if the transmitting cells is a macro, femto or pico cell. DMRS is typically user equipment specific. This enables different treatment of different cells for different receiving user equipments. A network entity such as eNB can control the overall allocations and provide centralized control. The operation can be controlled by a primary cell.

In accordance with an embodiment shown in Figure 5 cell aggregation is initiated at 40. Different resource element positions are provided at 42 for the demodulation reference symbols (DMRS) in resource patterns of a primary cell (Pcell) and a secondary cell (Scell). The two cells may then communicate accordingly the DMRS resource elements to the device and thereafter communicate accordingly at 44 in the downlink with the device.

In accordance with an embodiment the differentiation at 42 is provided by shifting resource elements positions in a resource pattern so that different antenna ports are allocated for different cells. The principle of resource element shifting is illustrated in Figure 6 showing resource allocation patterns, or physical resource blocks for a primary cell (PCell) and a secondary cell (SCell). More particularly, in the embodiment shifting can be used to manage DMRS in a frame or pattern for cell aggregation so that the DMRS resource elements are differently positioned for the different cells in the respective resource patterns. In this example each resource block consists of a pattern comprising twelve subcarriers and fourteen symbols, these providing 12 x 14 resource grid of resource elements (RE). Certain resource elements are defined for antenna ports, more particularly for ports 7 and 8, as will become evident from below. An example of standardized antenna port definitions can be found from 3GPP TS 36.211 , version 10.2.0 of June 2011.

In accordance with an example, when an user equipment receives transmission from a primary cell (PCell) and a secondary cell (SCell) the latter can use shifted DMRS positions, as shown by the pattern on right. Use of the time shifted positions may be configured as a default operation at a controller of the relevant cell such that the shifted positions are always automatically used in the pattern for a cell acting as a secondary cell. The arrangement may be such that the shifting is used as a default mode unless otherwise instructed. An advantage of shifting the positions of the resource elements for DMRS allocations is that all signalling is implicit and the currently existing control signalling can be used. As ports 7 and 8 are available in both cells, two orthogonal layers can be readily transmitted in both cells.

Power boosting can be applied to the transmission of the DMRS to improve the quality of the received DMRS.

In accordance with an embodiment, shown in Figure 7, muting of some resource elements (RE) may be provided in combination with shifting of DMRS resource elements. This may mean somewhat higher signalling overhead but on the other hand the muting can be used to achieve full orthogonality among four layers of DMRS in two cells. The additional signalling may be provided in various manners. According to one possibility scrambling ID is used to indicate whether muting is in use or not. For example, scrambling id = 0 can be used to indicate that ports 7 and 8 are without muting and scrambling id = 1 can be used to indicate that the muting is on. Muting may also be configured via Radio Resource Control (RRC) signalling.

In accordance with an embodiment controller of a PCell can determine the DMRS base sequences also for a SCell. The control can be provided on the physical layer. The control apparatus of a primary cell can determine how the other cells should transmit the DMRS. For example, port 7 in a first cell (e.g. PCell) and port 8 in a second cell (e.g. SCell) are orthogonal when they are transmitted from different cells. In most applications orthogonality is likely to be achievable at a lower cost than with DMRS shifting and muting since only twelve resource elements may need to be used. This can be provided, for example, using a cell ID shift in SCell transmission to provided DMRS orthogonality between the cells.

A difference to the resource element position shifting is that the DMRS sequence between the different cells is same but DMRS is kept orthogonal in the two cells by means of code division multiplexing (CDM). Code division multiplexing (CDM) orthogonality may not be as robust against the effect of different radio channels from different cells as what may be achievable by the shifting and muting embodiment of Figure 7, and thus the effective orthogonality seen at the user equipment might be worse in certain applications than what is achievable by the shifting and muting embodiment. Another difference is that whereas muting and shifting can be provided transparently to a user equipment and thus modifications thereof are not necessary, in a form of this embodiment the cell ID for the secondary cell is modified to make use of the same DMRS sequence with primary cell by CDM orthogonality and thus the user equipment may need to be able to process the cell ID shift to be able to determine the correct DMRS sequence.

According to an embodiment flexible DMRS port allocation is provided on the physical layer. Orthogonal antenna ports for the first cell and the second cell can be flexibly / dynamically allocated from a pool of antenna ports defined for demodulation reference signal transmission. Any candidate port or combination of DMRS antenna ports may be allocated without limitation. To illustrate this embodiment, assume a system where ports 7 and 8 only can be allocated. If the total rank for a user equipment is two or less and ports 7, 8, 9 and 10 only can be allocated if the total rank for a user equipment is four or less. Flexible DMRS port allocation can be supported assuming that user equipments in cell aggregation always have only one codeword active per cell, as then there are three bits available in the grant to select which port to use in the cell. By allowing flexible allocation of all eight orthogonal DMRS ports (for example, as defined in LTE Release 10), same effect can be achieved as by the shifting described above. For example as shown in Figure 8, a user equipment can be allocated port 7 and port 9 in PCell and SCell, respectively. Orthogonality is provided since port 7 and port 9 are defined in different positions.

The allocation can be communicated from the primary cell to the secondary cell via an appropriate interface, for example such as Common Public Radio Interface (CPRI) or X2. User equipment can be provided information about the ports to receive for example via physical downlink control channel (PDCCH) or radio resource control (RRC) signalling. Power boosting and/or data muting can also be used to further improve the efficiency, if necessary. This method can also be extended to higher rank transmission, e.g., allocating ports 7, 8, 11 , 12 in a primary cell, and allocating ports 9, 10, 13, 14 in a secondary cell. By means of this DMRS port allocations in case of high order transmission, such as rank-4 transmission for each cell, may also be provided with increased flexibility.

Flexible port allocation can also be used to extend the above described embodiment where primary cell determined the sequences also for the secondary cell to higher rank transmission, i.e. for transmissions higher than rank-1. For example, as shown in Figure 9, when there is rank 2 transmission in each cell it is possible to allocate port 7 and 9 to PCell and port 8 and 10 to SCell.

Differentiation between DMRS from different cells can be extended on multi-user multiple input multiple output (MU-MIMO) arrangements. Providing orthogonality for two or more receiving devices can also be provided when multi-user rank is high, for example four in each cell.

The required data processing apparatus and functions of a base station apparatus, a communication device and any other appropriate apparatus may be provided by means of one or more data processors. The described functions at each end may be provided by separate processors or by an integrated processor. The data processors may be of any type suitable to the local technical environment, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASIC), gate level circuits and processors based on multi core processor architecture, as non limiting examples. The data processing may be distributed across several data processing modules. A data processor may be provided by means of, for example, at least one chip. Appropriate memory capacity can also be provided in the relevant devices. The memory or memories may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. In general, the various embodiments may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. Some aspects of the invention may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device, although the invention is not limited thereto. While various aspects of the invention may be illustrated and described as block diagrams, flow charts, or using some other pictorial representation, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof. The software may be stored on such physical media as memory chips, or memory blocks implemented within the processor, magnetic media such as hard disk or floppy disks, and optical media such as for example DVD and the data variants thereof, CD.

Coordination of DMRS allocation can provide benefits for example in view of channel estimation and gain for the performance. Communication devises supporting cell aggregation the above embodiments handling of DMRS collision can benefit from gain in performance. Certain coordinated multipoint schemes may become simple to implement and/or flexibility may be provided to support a wider set of such schemes

It is noted that whilst embodiments have been described in relation to LTE-Advanced, similar principles can be applied to any other communication system or indeed to further developments with LTE. For example, the invention is applicable for cell aggregation even when CoMP is not supported, for example to multi-flow in standards such as High Speed Packet Access (HSPA). Also, instead of carriers provided by a base station a carrier may be provided by a communication device such as a mobile user equipment. For example, this may be the case in application where no fixed equipment provided but a communication system is provided by means of a plurality of user equipment, for example in adhoc networks. Therefore, although certain embodiments were described above by way of example with reference to certain exemplifying architectures for wireless networks, technologies and standards, embodiments may be applied to any other suitable forms of communication systems than those illustrated and described herein.

The foregoing description has provided by way of exemplary and non- limiting examples a full and informative description of the exemplary embodiment of this invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention as defined in the appended claims. Indeed there is a further embodiment comprising a combination of one or more of any of the other embodiments previously discussed.