The operation of a cellular network typically involves transmitting cell-specific reference signals for each logical antenna port of a cell, wherein each logical antenna port is mapped to a respective physical antenna or respective group of physical antennas .

Different cells of a cellular network may reguire different types of operation depending, for example, on whether they are operated via radio spectrum exclusive to the operator of the cellular network (licensed spectrum) or radio spectrum shared with others (unlicensed spectrum) . The inventors for the present application have identified the challenge of effectively assigning radio resources to cell-specific reference signal transmissions in cellular networks comprising such different cells.

There is hereby provided a method comprising: using a first set of transmission positions in a sub-frame structure to transmit cell-specific reference signals for one or more antenna ports for a first cell of a radio access network, wherein said same first set of transmission positions are also usable in the radio access network to transmit cell- specific reference signals for a greater number of antenna ports for a cell of the radio access network. According to one embodiment, the method further comprises using said same first set of transmission positions to transmit cell-specific reference signals for said greater number of antenna ports for a second cell. According to one embodiment, the second cell is a primary cell, and the first cell is a secondary cell associated with the primary cell.

According to one embodiment the second cell is operated via the same access point as the first cell or operated via a different access point to the first cell.

According to one embodiment, the method further comprises transmitting configuration information about said cell- specific reference signals for said first cell via said second cell. According to one embodiment, the method further comprises transmitting said cell-specific reference signals for said first cell periodically once per a number of sub-frames.

According to one embodiment, the method further comprises transmitting configuration information about said number of sub-frames .

According to one embodiment, the method further comprises transmitting synchronization signals and/or channel state information reference signals for said first cell in a second set of transmission positions in the same sub-frame, wherein said first and second sets of transmission positions together form a time-contiguous group of transmission positions.

According to one embodiment, said second set of transmission positions are also used in the radio access network to transmit synchronisation signals and/or channel state information reference signals in the same sub-frame as said cell-specific reference signals for a greater number of antenna ports for a cell. According to one embodiment, said first set of transmission positions are used in the radio access network exclusively for cell-specific reference signals.

According to one embodiment, the method further comprises operating said first cell via unlicensed spectrum. According to one embodiment, the method further comprises transmitting configuration information about said cell- specific reference signals for said first cell via another cell operated via licensed spectrum.

According to one embodiment, the method further comprises transmitting configuration information about one or more of said cell-specific reference signals, synchronisation signals and channel state information reference signals for said first cell via another cell operated via licensed spectrum. According to one embodiment, said first set of transmission positions consist of a subset of a set of transmission positions usable in the radio access network for transmitting cell-specific reference signals for said greater number of antenna ports for a cell of the radio access network.

There is also hereby provided a method comprising: detecting cell-specific reference signals for one or more antenna ports for a first cell of a radio access network from transmissions in a first set of transmission positions in a sub-frame structure, wherein said same first set of transmission positions are also usable in the radio access network to transmit cell-specific reference signals for a greater number of antenna ports for a cell of the radio access network.

According to one embodiment, the method further comprises detecting in said same first set of transmission positions cell-specific reference signals for said greater number of antenna ports for a second cell.

According to one embodiment, the second cell is a primary cell and the first cell is a secondary cell associated with the primary cell.

According to one embodiment, the first and second cells are operated via the same access point or are operated via different access points. According to one embodiment, the method further comprises receiving configuration information about said cell-specific reference signals for said first cell via said second cell.

According to one embodiment, the method further comprises performing and reporting radio resource management

measurements and/or reporting a discovered cell list, based at least partly on said detected cell-specific reference signals for said first cell.

According to one embodiment, the method further comprises using said detected cell-specific reference signals for said first cell for downlink channel estimation of the one or more channels associated with said one or more antenna ports for said first cell.

According to one embodiment, the method further comprises detecting synchronization signals and/or channel state information reference signals for said first cell in a second set of transmission positions in the same sub-frame, wherein said first and second sets of transmission positions together form a time-contiguous group of transmission positions. According to one embodiment, said second set of transmission positions are also used in the radio access network to transmit synchronisation signals and/or channel state information reference signals in the same sub-frame as said cell-specific reference signals for said greater number of antenna ports for a cell.

According to one embodiment, the method further comprises using said synchronisation signals for cell detection, and using said channel state information reference signals for radio resource management measurements and channel state information measurements.

According to one embodiment, said first cell is operated via unlicensed spectrum.

According to one embodiment, the method further comprises receiving configuration information about said cell-specific reference signals for said first cell via another cell operated via licensed spectrum.

According to one embodiment, the method further comprises receiving configuration information about one or more of said cell-specific reference signals, synchronisation signals and channel state information reference signals for said first cell via another cell operated via licensed spectrum.

According to one embodiment, said first set of transmission positions are used in the radio access network exclusively for cell reference signals.

According to one embodiment, said first set of transmission positions consist of a subset of a set of transmission positions usable in the radio access network for transmitting cell-specific reference signals for said greater number of antenna ports for a cell.

There is also hereby provided an apparatus comprising: a processor and memory including computer program code, wherein the memory and computer program code are configured to, with the processor, cause the apparatus to: use a first set of transmission positions in a sub-frame structure to transmit cell-specific reference signals for one or more antenna ports for a first cell of a radio access network, wherein said same first set of transmission positions are also usable in the radio access network to transmit cell-specific reference signals for a greater number of antenna ports for a cell of the radio access network. According to one embodiment, the memory and computer program code are further configured to, with the processor, cause the apparatus to use same first set of transmission positions to transmit cell-specific reference signals for said greater number of antenna ports for a second cell. According to one embodiment, the second cell is a primary cell, and the first cell is a secondary cell associated with the primary cell. According to one embodiment, the second cell is operated via the same access point as the first cell or operated via a different access point to the first cell.

According to one embodiment, the memory and computer program code are further configured to, with the processor, cause the apparatus to transmit configuration information about said cell-specific reference signals for said first cell via said second cell.

According to one embodiment, the memory and computer program code are further configured to, with the processor, cause the apparatus to transmit said cell-specific reference signals for said first cell periodically once per a number of sub- frames .

According to one embodiment, the memory and computer program code are further configured to, with the processor, cause the apparatus to transmit configuration information about said number of sub-frames.

According to one embodiment, the memory and computer program code are further configured to, with the processor, cause the apparatus to transmit synchronization signals and/or channel state information reference signals for said first cell in a second set of transmission positions in the same sub-frame, wherein said first and second sets of transmission positions together form a time-contiguous group of transmission positions .

According to one embodiment, said second set of transmission positions are also used in the radio access network to transmit synchronisation signals and/or channel state information reference signals in the same sub-frame as said cell-specific reference signals for a greater number of antenna ports for a cell.

According to one embodiment, said first set of transmission positions are used in the radio access network exclusively for cell-specific reference signals.

According to one embodiment, the memory and computer program code are further configured to, with the processor, cause the apparatus to operate said first cell via unlicensed spectrum.

According to one embodiment, the memory and computer program code are further configured to, with the processor, cause the apparatus to transmit configuration information about said cell-specific reference signals for said first cell via another cell operated via licensed spectrum.

According to one embodiment, the memory and computer program code are further configured to, with the processor, cause the apparatus to transmit configuration information about one or more of said cell-specific reference signals, synchronisation signals and channel state information reference signals for said first cell via another cell operated via licensed spe

According to one embodiment, said first set of transmission positions consist of a subset of a set of transmission positions usable in the radio access network for transmitting cell-specific reference signals for said greater number of antenna ports for a cell of the radio access network.

There is also hereby provided an apparatus comprising: a processor and memory including computer program code, wherein the memory and computer program code are configured to, with the processor, cause the apparatus to: detect cell-specific reference signals for one or more antenna ports for a first cell of a radio access network from transmissions in a first set of transmission positions in a sub-frame structure, wherein said same first set of transmission positions are also usable in the radio access network to transmit cell- specific reference signals for a greater number of antenna ports for a cell of the radio access network.

According to one embodiment, the memory and computer program code are further configured to, with the processor, cause the apparatus to: detect in said same first set of transmission positions cell-specific reference signals for said greater number of antenna ports for a second cell. According to one embodiment, the second cell is a primary cell and the first cell is a secondary cell associated with the primary cell.

According to one embodiment, the first and second cells are operated via the same access point or are operated via different access points.

According to one embodiment, the memory and computer program code are further configured to, with the processor, cause the apparatus to receive configuration information about said cell-specific reference signals for said first cell via said second cell.

According to one embodiment, the memory and computer program code are further configured to, with the processor, cause the apparatus to perform and report radio resource management measurements and/or reporting a discovered cell list, based at least partly on said detected cell-specific reference signals for said first cell.

According to one embodiment, the memory and computer program code are further configured to, with the processor, cause the apparatus to use said detected cell-specific reference signals for said first cell for downlink channel estimation of the one or more channels associated with said one or more antenna ports for said first cell. According to one embodiment, the memory and computer program code are further configured to, with the processor, cause the apparatus to detect synchronization signals and/or channel state information reference signals for said first cell in a second set of transmission positions in the same sub-frame, wherein said first and second sets of transmission positions together form a time-contiguous group of transmission positions .

According to one embodiment, said second set of transmission positions are also used in the radio access network to transmit synchronisation signals and/or channel state information reference signals in the same sub-frame as said cell-specific reference signals for said greater number of antenna ports for a cell. According to one embodiment, the memory and computer program code are further configured to, with the processor, cause the apparatus to use said synchronisation signals for cell detection, and use said channel state information reference signals for radio resource management measurements and channel state information measurements.

According to one embodiment, said first cell is operated via unlicensed spectrum.

According to one embodiment, the memory and computer program code are further configured to, with the processor, cause the apparatus to receive configuration information about said cell-specific reference signals for said first cell via another cell operated via licensed spectrum.

According to one embodiment, the memory and computer program code are further configured to, with the processor, cause the apparatus to receive configuration information about one or more of said cell-specific reference signals, synchronisation signals and channel state information reference signals for said first cell via another cell operated via licensed spectrum.

According to one embodiment, said first set of transmission positions are used in the radio access network exclusively for cell reference signals.

According to one embodiment, said first set of transmission positions consist of a subset of a set of transmission positions usable in the radio access network for transmitting cell-specific reference signals for said greater number of antenna ports for a cell.

There is also hereby provided a computer program product comprising program code means which when loaded into a computer controls the computer to: use a first set of transmission positions in a sub-frame structure to transmit cell-specific reference signals for one or more antenna ports for a first cell of a radio access network, wherein said same first set of transmission positions are also usable in the radio access network to transmit cell-specific reference signals for a greater number of antenna ports for a cell of the radio access network. There is also hereby provided a computer program product comprising program code means which when loaded into a computer controls the computer to: detect cell-specific reference signals for one or more antenna ports for a first cell of a radio access network from transmissions in a first set of transmission positions in a sub-frame structure, wherein said same first set of transmission positions are also usable in the radio access network to transmit cell- specific reference signals for a greater number of antenna ports for a cell of the radio access network. Embodiments are described hereunder, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows an example of part of a cellular network, including an access point operating one cell via licensed spectrum and one cell via unlicensed spectrum; Figure 2 shows an example of apparatus for use at the access points of Figure 1;

Figure 3 shows an example of apparatus for use at the communication device of Figure 1; Figure 4 shows an example of transmission positions for cell- specific reference signals for four antenna ports for a licensed spectrum cell;

Figure 5 shows one example of transmission positions for cell-specific reference signals for two antenna ports for an unlicensed spectrum cell;

Figure 6 shows one example of transmission positions for cell-specific reference signals for one antenna port for an unlicensed spectrum cell; Figure 7 shows an example of transmission positions for cell- specific reference signals for two antenna ports,

synchronisation signals, and channel state information reference signals for an unlicensed spectrum cell;

Figure 8 shows an example of operations at an access point according to one embodiment;

Figure 9 shows an example of operations at a communication device according to one embodiment; and

Figure 10 shows another example of transmission positions for cell-specific reference signals for one antenna port for an unlicensed spectrum cell. Figure 1 schematically shows an example of part of a cellular radio access network. The following description is for the example of a Long Term Evolution (LTE) radio access network, but the same technigue is also applicable to other kinds of radio access networks.

Figure 1 only shows three eNodeBs (eNBs) 2, but a cellular network will typically comprise thousands of access points providing substantially continuous coverage over a wide geographical area. A cellular radio access network will also typically include other elements such as resource management entities etc., but these are not shown in Figure 1 for conciseness.

Figure 2 shows a schematic view of an example of user eguipment 8 that may be used for communicating with at least the eNBs 2 of Figure 1 via a wireless interface. The user eguipment (UE) 8 may be used for various tasks such as making and receiving phone calls, for receiving and sending data from and to a data network and for experiencing, for example, multimedia or other content. The UE 8 may be any device capable of at least sending or receiving radio signals to or from at least the eNBs 2 of Figure 2. Non-limiting examples include a mobile station (MS) , 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. The UE 8 may communicate via an appropriate radio interface arrangement of the UE 8. The interface arrangement may be provided for example by means of a radio part and associated antenna arrangement 205. The antenna arrangement may be arranged internally or externally to the UE 8, and may include a plurality of antennas capable of operating in a multi-layer transmission scheme.

The UE 8 may be provided with at least one data processing entity 203 and at least one memory or data storage entity 217 for use in tasks it is designed to perform. The data processor 203 and memory 217 may be provided on an appropriate circuit board and/or in chipsets.

The user may control the operation of the UE 8 by means of a suitable user interface such as key pad 201, voice commands, touch sensitive screen or pad, combinations thereof or the like. A display 215, a speaker and a microphone may also be provided. Furthermore, the UE 8 may comprise appropriate connectors (either wired or wireless) to other devices and/or for connecting external accessories, for example hands-free eguipment, thereto.

Figure 2 shows an example of a UE including a user interface, but the UE may also be a communication device without any user interface, such as a device that is designed for machine type communications (MTC) .

Figure 3 shows an example of apparatus for use at the eNBs 2 of Figure 1. The apparatus comprises a radio freguency antenna array 301 configured to receive and transmit radio freguency signals; radio freguency interface circuitry 303 configured to interface the radio freguency signals received and transmitted by the antenna array 301 and the data processor 306. The radio freguency interface circuitry 303 may also be known as a transceiver. The apparatus also comprises an interface 309 via which it can send and receive information to and from one or more other network nodes such as the core network and other eNBs 2. The data processor 306 is configured to process signals from the radio freguency interface circuitry 303, control the radio freguency interface circuitry 303 to generate suitable RF signals to communicate information to the UE 8 via the wireless communications link, and also to exchange information with other network nodes via the interface 309. The memory 307 is used for storing data, parameters and instructions for use by the data processor 306.

It would be appreciated that the apparatus shown in each of figures 2 and 3 described above may comprise further elements which are not directly involved with the embodiments of the invention described hereafter.

The following description adopts the example of an access point 2 operating at least one cell via unlicensed radio spectrum (i.e. radio spectrum for which the operator of the radio access network does not have exclusive use) . The use of unlicensed radio freguencies by the access point 2 may be subject to various conditions, such as: (i) (with a possible exception for short control signals) a regulatory Listen- Before-Talk (LBT) reguirement to first monitor the unlicensed radio freguencies for a short period of time to ensure the radio freguencies are not already occupied by some other transmission, such as a Wi-Fi transmission, and/or (ii) a reguirement to stop any transmissions via the unlicensed radio freguencies from time-to-time to monitor whether the unlicensed radio freguencies are still available for use by the RAN access point.

The following detailed description relates to the example of cell-specific reference signals (CRS) as part of a discovery reference signal (DRS) set for an unlicensed spectrum cell for (i) radio resource management (RRM) measurements and cell-identification, and/or (ii) use at the beginning of a data transmission opportunity (TxOP) for fine time/freguency synchronisation to facilitate successful data channel decoding. The CRS for unlicensed spectrum are denoted here as eCRS .

The DRS for the unlicensed spectrum cell may, for example, be transmitted periodically once every predetermined number of sub-frames, such as once every 20-160ms.

As illustrated in Figure 1, in this example, a single access point 2 operates at least two cells: at least a primary cell 10 via licensed radio spectrum and at least one secondary cell 12 via unlicensed spectrum. The two cells are shown in Figure 1 to have coverage areas of substantially the same shape and size, but the coverage areas may have very

different shapes and/or sizes. Moreover, the at least two cells might be operated via different access points 2 and have different shape, cell center and size, when serving the UE 8 through: at least a primary cell 10 via licensed radio spectrum and at least one secondary cell 12 via unlicensed spectrum.

Radio resources for downlink transmissions via both the licensed and unlicensed spectrum cells (primary and secondary cells of the access point 2) are divided into physical resource blocks (PRBs) having a common size and structure, as shown by the time-freguency resource grids of Figures 4 to . For each PRB, the 12 horizontal rows indicate a set of 12 OFDM sub-carriers selected from the larger group of OFDM (orthogonal freguency division multiplexing) sub-carriers assigned to a cell; and the 14 vertical columns indicate the time resources assigned to the fourteen OFDM symbols that together make up a sub-frame. Each sguare within the resource grid indicates a unigue combination of OFDM carrier index and OFDM symbol index within the sub-frame structure. As mentioned above, downlink transmissions in both the licensed and unlicensed spectrum cells adopt this sub-frame structure, although the absolute freguencies of the OFDM carriers will be different between cells.

In this example, the transmission positions within the sub- frame resource grid for CRS for logical antenna ports #0 to #3 of the primary cell (licensed spectrum) are shown in Figure 4. These transmission positions follow the Rel-8 4-TX CRS pattern described in 3GPP TS 36.211. Logical antenna ports #0 to #3 are, for example, mapped to single different physical antenna elements of the access point 2. In one variation discussed further below, the primary cell itself may also transmit CRS for a number of antenna ports less than 4 (either the same number as the secondary cell or a

different number) , and may also use only a subset of the CRS transmission positions shown in Figure 4 (either the same subset as the secondary cell or a different subset), with one or more other cells of the same radio access network using the full CRS configuration of Figure 4 (i.e. using the same set of transmission positions to transmit CRS for four antenna ports) .

Figures 5 and 6 show two examples of sets of transmission positions for eCRS for one or two antenna ports of the secondary cell (unlicensed spectrum) access point, which eCRS form part of a set of discovery reference signals (DRS) additionally including primary and secondary synchronisation signals (SSS/PSS) and channel state information reference signals (CSI-RS), for which one example of transmission positions is shown in Figure 7 together with the eCRS transmission positions of Figure 5.

As can be seen from a comparison of Figure 4 with either of Figures 5 to 6, the eCRS for the antenna port(s) (e.g. one or two) of the unlicensed spectrum cell (secondary cell) occupy transmission positions in the sub-frame resource grid that are used in licensed spectrum cells for the CRS for a greater number (i.e. four) of antenna ports. In the example of Figure 5, transmission positions for CRS for antenna ports AP#0 and AP#2 for a licensed spectrum cell are mapped to transmission positions for eCRS for antenna port AP#0 for the unlicensed spectrum cell; and transmission positions for CRS for antenna ports AP#1 and AP#3 for the licensed spectrum cell are mapped to transmission positions for eCRS for antenna port AP#1 for an unlicensed spectrum cell. In the example of Figure 6, transmission positions for CRS for antenna ports AP#0 and AP#2 for a licensed spectrum cell are mapped to transmission positions for eCRS for antenna port AP#0 for the unlicensed spectrum cell; and there is only one antenna port for the unlicensed spectrum cell.

In another example (not shown) , transmission positions for CRS for all antenna ports AP#0 to AP#3 for a licensed spectrum cell are mapped to transmission positions for eCRS for one antenna port AP#0 for the unlicensed spectrum cell. In another example (not shown) , transmission positions for CRS for antenna ports AP#1 and AP#2 for a licensed spectrum cell are mapped to transmission positions for eCRS for one antenna port AP#1 for the unlicensed spectrum cell; and transmission positions for CRS for antenna ports AP#0 and AP#3 for a licensed spectrum cell are mapped to transmission positions for eCRS for another antenna port AP#0 for the unlicensed spectrum cell.

Figure 4 shows the example of a licensed spectrum cell including CRS transmission positions for four antenna ports in a sub-frame. However, a licensed spectrum cell could include CRS transmission positions for a different number of antenna ports in a sub-frame. Figures 5 to 7 show examples of an unlicensed spectrum cell including eCRS transmission positions for one or two eCRS antenna ports in a sub-frame using the CRS transmission positions of licensed spectrum cells having 4 CRS antenna ports. However, any combination of number of CRS antenna ports for the serving licensed spectrum PCell and number of eCRS antenna ports for the unlicensed spectrum SCell of Figure 5-7 are possible. As an example, it is possible to have a two antenna port PCell on licensed spectrum, using antenna port #0 and antenna port#l from Figure 4 according to TS 36.211 for CRS transmission, combined with a two antenna port SCell on unlicensed spectrum using the eCRS transmission positions shown in Figure 5 or 7 for eCRS transmission. In this case, the eCRS transmission positions for the unlicensed spectrum cell (SCell) are a subset of the CRS transmission positions of a 4 antenna port licensed spectrum cell, although the licensed spectrum PCell would only use 2 CRS antenna ports itself, i.e. only make CRS transmissions for 2 antenna ports, not the full 4 antenna ports. As another example, the licensed spectrum cell may make CRS transmissions for only two antenna ports using the transmission positions labelled CRS AP#0 and CRS AP#1 in Figure 4, and the CRS transmission positions for the two antenna ports AP#0 and AP#1 in Figure 4 could be mapped to eCRS transmission positions for one antenna port for the unlicensed spectrum cell, as shown in Figure 10. In Figure 10, the combination of eCRS, PSS/SSS and CSI-RS no longer occupy a time-contiguous set of OFDM symbols, but one or more extra control signals may be transmitted in at least OFDM symbol#8 to form a time-contiguous set of discovery reference signals .

In the examples shown in Figures 5, 6 and 7, the transmission positions used for eCRS in the unlicensed spectrum cell comprise a sub-set of the transmission positions used for CRS in a licensed spectrum cell (i.e. there is no mapping of the CRS transmission positions in the first two OFDM symbols in the sub-frame resource grid of Figure 4 to CRS transmission positions in the resource grids of Figures 5 to 7) .

Reducing the number of OFDM symbols that DRS for the

unlicensed spectrum occupy in a single sub-frame may be advantageous for enabling reduction of the number of sub- frames between DRS transmissions whilst meeting the

conditions for short control signals (SCS) exempt to LBT reguirements . However, the sub-frame structure for an unlicensed spectrum cell may involve full mapping of eCRS transmission positions, i.e. also including eCRS

transmissions for one or more of the relatively limited number of antenna ports for the unlicensed spectrum cell in the CRS transmission positions shown in OFDM symbols #0 and #1 in Figure 4. For example, this could be done selectively in times when the access point 2 determines that it has an opportunity to make data transmissions to the UE 8 via the unlicensed spectrum cell, e.g. after a LBT procedure. As shown in Figure 7, the discovery reference signals for the unlicensed spectrum cell may additionally include at least synchronisation signals (SSS/PSS) and/or channel state information reference signals (CSI-RS) . In the example shown in Figure 7, at least one of SSS/PSS and CSI-RS are

transmitted on at least one sub-carrier in OFDM symbols #5, #6, #9 and #10 so that the transmission positions for the eCRS, SSS/PSS and CSI-RS within the sub-frame structure together form a time-contiguous group of transmission positions within the sub-frame structure; i.e. the DRS transmissions in the sub-frame structure extend contiguously across 8 OFDM symbols. This time-contiguity may be advantageous for preventing another user of the unlicensed spectrum starting a transmission before the RAN access point 2 has finished transmitting DRS in a sub-frame. If the DRS were not wholly time-contiguous within the sub-frame

structure, there is a risk that another user of the same unlicensed radio spectrum may detect a gap in the DRS transmissions in a sub-frame as an opportunity to start its own transmissions via the same unlicensed radio spectrum, which transmissions could hamper reception of the DRS for the unlicensed spectrum cell at UEs 8. Furthermore, a gap in the DRS transmissions within a sub-frame may, depending on the specific local regulations etc., force the access point 2 to perform a LBT procedure during the gap before continuing the DRS transmissions in the same sub-frame.

Figures 8 and 9 show one example of operations at the access point 2 and UE 8 in accordance with one embodiment based on Figures 4 and 7. The access point 2 operates the primary cell (licensed spectrum cell) including: transmitting CRS for 4 antenna ports in the transmission positions shown in Figure 4, and transmitting configuration information about eCRS, PSS/SSS and CSI-RS for the unlicensed spectrum secondary cell (STEP 800) . The access point 2 also transmits eCRS for two antenna ports, SSS/PSS and CSI-RS for the secondary cell (unlicensed spectrum cell) in the transmission positions shown in Figure 7 (STEP 802) . The UE 8 detects configuration information about eCRS, PSS/SSS and CSI-RS for the secondary cell from transmissions via the primary cell (STEP 900), and detects eCRS, PSS/SSS and CSI-RS for the secondary cell (STEP 902) . The UE 8 performs one or more of RRM measurements, cell detection and CSI measurements based on the eCRS,

PSS/SSS and CSI-RS for the secondary cell (STEP 904) . In one variation, the access point 2 may be equipped with only a smaller number of physical transmit antennas (e.g. two physical antennas) and may transmit CRS for e.g. 2 antenna ports (instead of 4 antenna ports) for the licensed spectrum cell in STEP 800, and use a mapping of the same antenna ports to only one antenna port for the unlicensed spectrum cell in STEP 802. Correspondingly, the UE 8 would then only receive CRS for e.g. two antenna ports for the licensed spectrum cell in STEP 900 and detect in the same transmission positions eCRS for only one antenna port of the unlicensed spectrum cell in STEP 902.

In another variation, both the licensed spectrum primary cell and the unlicensed spectrum secondary cell may transmit CRS and eCRS for an egual number of antenna ports less than the four antenna ports shown in Figure 4, and both primary and secondary cells may only use a subset of the CRS

transmissions shown in Figure 4. For example, both the licensed spectrum primary cell and unlicensed spectrum secondary cell may transmit CRS or eCRS for only one or two antenna ports. In such example, there may be CRS

transmissions for the primary cell only on CRS AP#0 and CRS AP#2 of Figure 4 (i.e. without CRS transmission on CRS AP#1 and CRS AP#3 of Figure 4), and mapping of CRS AP#0 and CRS AP#2 of Figure 4 to eCRS AP#0 for the secondary cell, and mapping of CRS AP#1 and CRS AP#3 of Figure 4 to eCRS AP#1 for the secondary cell. The same radio access network comprising these primary and secondary cells may also include other cells that use the same set of transmission positions to transmit CRS for more than two antenna ports, e.g. four antenna ports. In general, as the licensed spectrum primary cell and the unlicensed spectrum secondary cell may not be operated via the same access point 2, but via different access points, the number of physical transmit antennas (and number of logical antenna ports) at different access points may be different. More generally, CRS transmission positions of licensed band CRS antenna ports are mapped to eCRS transmission positions for unlicensed spectrum secondary cells .

As mentioned above, the DRS for the unlicensed spectrum cell may be transmitted periodically once every predetermined number of sub-frames. The smallest period (i.e. smallest number of sub-frames between DRS sub-frames) at which the DRS for the unlicensed spectrum may be transmitted without falling outside the definition of SCS and therefore without first reguiring a LBT procedure may depend on the number of OFDM symbols that the DRS occupy in a sub-frame.

The DRS may also be transmitted in the first sub-frame of a transmission opportunity (TxOP) (after a successful LBT procedure) for transmitting data to a UE 8. As mentioned above, in this case, CRS for the unlicensed spectrum cell (i.e. eCRS) may additionally be transmitted in the

transmission positions used for transmitting CRS for the licensed spectrum cell in OFDM symbols #0 and #1.

Alternatively or additionally, the CRS for the unlicensed spectrum cell may also be transmitted in the first fractional sub-frame after the start of a downlink data transmission, provided that the DL data transmission starts in OFDM symbol #4 at latest.

One advantage of the above-described technigue is that it facilitates the multiplexing of the DRS and eCRS for the unlicensed spectrum cell with data and control channels (e.g. Physical Downlink Shared Channel (PDSCH) , Physical Downlink Control Channel (PDCCH) , Enhanced Physical Downlink Control Channel (EPDCCH) ) as well as other reference signals (such as the UE-specific reference signals used for data channel demodulation) via the same unlicensed spectrum cell in the event that there is a valid transmission opportunity (TxOP) for the unlicensed spectrum cell. In the same sub-frame that one UE 8 is receiving data transmissions via the unlicensed spectrum cell, other UEs 8 may still need DRS for performing one or more of RRM measurements, cell detection and CSI measurements .

In the examples shown in Figures 5 to 7, the eCRS for the unlicensed spectrum cell (and a subset of the CRS

transmission positions for a licensed spectrum cell) occupy OFDM symbols #4, #7, #8 and #11; but other examples may use different combinations of OFDM symbols within the sub-frame structure for both the CRS for the licensed spectrum cell and a subset of the CRS (that is, eCRS) for the unlicensed spectrum cell, whereby the transmission positions for DRS for the unlicensed spectrum cell can be delayed or advanced within the sub-frame structure by one or multiple OFDM symbols .

The above-described operations may require data processing in the various entities. The data processing may be provided by means of one or more data processors. Similarly various entities described in the above embodiments may be

implemented within a single or a plurality of data processing entities and/or data processors. Appropriately adapted computer program code product may be used for implementing the embodiments, when loaded to a computer. The program code product for providing the operation may be stored on and provided by means of a carrier medium such as a carrier disc, card or tape. A possibility is to download the program code product via a data network. Implementation may be provided with appropriate software in a server.

For example the embodiments of the invention may be

implemented as a chipset, in other words a series of

integrated circuits communicating among each other. The chipset may comprise microprocessors arranged to run code, application specific integrated circuits (ASICs), or

programmable digital signal processors for performing the operations described above. Embodiments of the invention may be practiced in various components such as integrated circuit modules. The design of integrated circuits is by and large a highly automated process. Complex and powerful software tools are available for converting a logic level design into a semiconductor circuit design ready to be etched and formed on a

semiconductor substrate.

Programs, such as those provided by Synopsys, Inc. of

Mountain View, California and Cadence Design, of San Jose, California automatically route conductors and locate

components on a semiconductor chip using well established rules of design as well as libraries of pre stored design modules. Once the design for a semiconductor circuit has been completed, the resultant design, in a standardized electronic format (e.g., Opus, GDSII, or the like) may be transmitted to a semiconductor fabrication facility or "fab" for

fabrication .

In addition to the modifications explicitly mentioned above, it will be evident to a person skilled in the art that various other modifications of the described embodiment may be made within the scope of the invention.