The invention relates generally to mobile communications. More particularly, the invention relates to transmission of cell-specific reference symbols.

Background

Cell-specific reference symbols/signals (CRSs) may be transmitted in downlink in order to allow the receiving user equipment (UE) to estimate the channel, for example. However, the application of the CRSs in downlink may cause interference towards other cells.

Brief description of the invention

According to an aspect of the invention, there are provided methods as specified in claims 1 and 20.

According to an aspect of the invention, there are provided apparatuses as specified in claims 22, 41 , and 43.

According to an aspect of the invention, there is provided a computer pro- gram product as specified in claim 44.

According to an aspect of the invention, there is provided a computer- readable distribution medium carrying the above-mentioned computer program product.

According to an aspect of the invention, there is provided an apparatus comprising means for performing any of the embodiments as described in the appended claims.

Embodiments of the invention are defined in the dependent claims. List of drawings

In the following, the invention will be described in greater detail with refer- ence to the embodiments and the accompanying drawings, in which

Figure 1 presents a communication network to which the embodiments are applicable to; Figure 2 shows example transmission of cell-specific reference symbols in physical resource blocks;

Figure 3 shows relationship between operational channel bandwidths and configurations of physical resource blocks in the long-term evolution;

Figures 4 and 12 illustrate methods according to some embodiments;

Figures 5, 6A, 7A, 7B, 7C, 8, and 1 1 depict transmissions of the cell- specific reference symbols, according to some embodiments;

Figure 6B illustrates application of common reference signaling according to an embodiment;

Figures 9A and 9B show transmission and reception of channel quality indicators, according to some embodiments; and

Figure 10 illustrates an apparatus, according to an embodiment.

Description of embodiments

The following embodiments are exemplary. Although the specification may refer to "an", "one", or "some" embodiment(s) in several locations of the text, this does not necessarily mean that each reference is made to the same embodiment(s), or that a particular feature only applies to a single embodiment. Single features of different embodiments may also be combined to provide other embodiments.

Figure 1 shows an example communication network where the embodi- ments of the invention are applicable to. Radio communication networks, such as the Long Term Evolution (LTE) or the LTE-Advanced (LTE-A) of the 3 ^rd Generation Partnership Project (3GPP), are typically composed of at least one base station 100, 104 providing coverage to a corresponding cell 102, 106, respectively. Further, typically there may be at least one user equipment (UE) 108 (also called user terminal (UT), a terminal device or a cellular mobile station, for example) and optional network elements that provide the interconnection towards the core network. In general, the base station 100, 104 may be configured to provide communication services according to at least one of the following radio access technologies (RATs): Worldwide Interoperability for Microwave Access (WiMAX), Wireless Local Area Network (WLAN), Global System for Mobile communications (GSM, 2G), GSM EDGE radio access Network (GERAN), General Packet Radio Service (GRPS), Universal Mobile Telecommunication System (UMTS, 3G) based on basic wideband-code division multiple access (W- CDMA), high-speed packet access (HSPA), LTE, and/or LTE-A. The present embodiments are not, however, limited to these protocols. The base station 100, 104 may be node B (NB) as in the LTE, evolved node B (eNB) as in the LTE-A, a radio network controller (RNC) as in the UMTS, a base station controller (BSC) as in the GSM/GERAN, or any other apparatus capable of controlling radio communication and managing radio resources within a cell. How- ever, for the sake of simplicity, let us assume from now on that the base station 100, 104 is an eNB applying LTE or LTE-A. Let us further consider as shown in Figure 1 that the UE 108 is located in the cell 102 covered by the eNB 100.

In the LTE, a downlink reference/pilot symbol (RS) is a special signal that exists at a physical layer. The RSs are not for delivering any specific information but, e.g., the purposes of delivering, by the eNBs 100, 104 as shown with reference numerals 1 10 and 1 12, a reference point for the UE(s) 108 to measure the downlink (DL) power or to estimate a DL channel quality. For example, when the UE 108 tries to detect the DL power, the UE 108 may measure the power of the RS (such as a reference signal received power, RSRP, or a reference signal strength indicator, RSSI) and consider that as the downlink cell power. The DL channel quality may be indicated to the eNB 100 as a channel quality indicator (CQI), as shown with reference numeral 1 14.

The eNB 100, 104 may then carry out radio resource allocation at least partly on the basis of the indicated CQIs. Resources are allocated in physical re- source blocks (PRBs). Two PRBs 200 and 202 are shown in Figure 2. For the sake of simplicity Figure 2 assumes the LTE as the radio access technology. Each PRB 200, 202 may consist of 12 subcarriers in a frequency domain, each subcarrier covering 15 kHz bandwidth. In a time domain, each PRB 200, 202 may consist of 6 or 7 symbols (such as orthogonal frequency division multiplexing, OFDM, symbols), depending on whether a long or a short cyclic prefix is used. Thus, there may be 12 x 6/7 resource elements (RE) in one PRB 200, 202. In the time domain, one PRB 200, 202 lasts for 0,5 ms which equals to one slot. Thus, two PRBs 202, 202 equal to one sub-frame 204 having duration of 1 ms in the time domain. Typically, in the LTE, the resources are allocated in a granularity of sub-frames. The allocations of the PRBs 200, 202 to UE(s) 108 may be indicated in a physical downlink control channel (PDCCH).

Further, some resource elements of each PRB 200, 202 are reserved for the transmission of reference signals which are distributed evenly in the time and frequency domains, as shown in Figure 2 with left-leaning diagonal lines, in order to enable efficient and reliable channel estimation by the UE(s) 108. Thus, the RSs are car- ried by multiples of predefined REs in each PRB 200, 202 and the location of the RSs in the PRB 200, 202 may be specifically determined by an antenna configuration in the LTE. For example, in the LTE, the RSs may be transmitted during the first and fifth symbols of each slot (i.e. each PRB 200, 202) when the short cyclic prefix is used and during the first and fourth symbols when the long CP is used (as shown in Figure 2). Further, the reference symbols may be transmitted in every sixth RE in frequency domain, i.e. in every sixth subcarrier, as shown, in order to provide sufficient information for frequency domain channel estimation. The UE 108 receiving the RSs may determine the channel response for these REs on the basis of the received RSs. Interpolation may then be used to estimate the channel response on the remaining REs within the resource blocks 200, 202.

There may be two different types of reference signal: cell-specific reference signal (CRS), as depicted in Figure 2 with the left-leaning diagonal lines, and UE-specific reference signals. The UE-specific RSs may be transmitted within the PRBs allocated only to the specific UE. On the contrary, the CRSs may currently in LTE be broadcasted in every PRB 200, 202, thus spanning across all of an operational DL channel bandwidth of the corresponding cell 102, 106. In the LTE, there are different operational DL channel bandwidths available and the selection of which to use may depend on the traffic demand, traffic status, load of the cell, for example. Within each channel bandwidth, a certain number of PRBs may be used. Table 1 lists availa- ble operational channel bandwidths (BW) of the LTE.

Table 1 : Channel BW versus PRB configuration in the LTE

It should be noted that other channel bandwidths are available in theory, but the set of available channel BWs mentioned in Tablel are used in practice in the LTE. Figure 3 further displays how the operational channel BW 300, a PRB configuration 302 comprising a number of PRBs available for transmission, and actually applied set of allocated PRBs 304, 306 correspond to each other. Figure 3 assumes that the operational channel BW is 3 MHZ thus allowing 15 PRBs within the operational chan- nel BW 300. As shown, depending on the load of the cell, not all of the available PRBs of/within the operational channel BW 300 are used for transmitting control signaling and/or user data. Only the sets of PRBs 304 and 306 are actually applied and allocated to carry user data and/or control signaling to the UE(s) 108. In case of full load of the cell (e.g. many UEs in the cell), each of the available PRBs, such as each of the 15 PRBs within the 3 MHz operational channel BW, may be used.

As indicated above, currently in the LTE downlink, the CRSs are transmitted for the whole operational channel BW 302 regardless of whether all PRBs are used or not, i.e. regardless of the load of the cell. That is, each physical resource block shown in Figure 3 comprises four CRSs even though there may not be any user data or control signaling transmitted to the UE(s) 108 in some of the PRBs. These CRSs spanning the whole BW 300 have been found to introduce DL interference to the cells 102, 106, such as to other eNBs or to UEs in the cell 102 and in the neigh- boring cells. The interference may affect negatively in the DL throughput, for example.

Therefore, as shown in Figures 4 and 5, it is proposed that the eNB 100 applies, in step 400, an operational channel BW 300 to be used in data transmission, wherein the operational channel BW 300 is selected from a set of predetermined available operational channel BWs, such as indicated in Table 1. The predetermined available BWs may depend on the applied radio access technology, such as the LTE/LTE-A. The applied operational channel BW 300 may then correspond to a specific PRB configuration 302. Such selection may thus set a limit for the available number of PRBs for transmissions. For example, in Figure 5, there are 15 PRBs within the operational channel bandwidth 300, as determined by the PRB configuration of the LTE, assuming that the BE 300 is 3 MHz.

In step 402, the eNB 100 may then allocate, within the applied operational channel BW 300, resource blocks for DL data transmission to at least one UE 108. The allocated resource blocks may be dedicated for DL transmission to the at least one UE 108. In an embodiment, the allocated PRBs comprise at least one PRB but less than the number of PRBs of the applied operational channel BW 300. That is, in case of Figure 5, the allocated PRBs may comprise any number of PRBs between 1 and 14. This may denote that there is partial/fractional load in the cell 102 to which the eNB 100 provides coverage to. That is, the cell 102 is not empty nor is it fully loaded. In case of empty cell 102, there would no reason to allocate any PRBs for dedicated DL data transmission and in case of full load, the whole channel bandwidth 300 would be allocated for DL data transmission. The selection of which PRBs to allocate for the DL data transmission may depend at least partly on the acquired channel quality information of the channel BW 300, for example. In the example case shown in Figure 5, there are 6 PRBs allocated for DL data transmission to the at least one UE 108 (=two PRBs in an allocated set marked with reference numeral 500 and four PRBs in an allocated set marked with reference numeral 502). Thereafter, in an embodiment, the eNB 100 may transmit the DL user-specific data (user data or control data) to the at least one UE 108 by using the allocated PRBs.

In step 404, the eNB 100 may transmit the CRSs in the DL in a subset of those REs of the applied operational channel BW 300 which are reserved for the cell- specific reference symbol transmissions, the subset comprising at least the reserved resource elements in the allocated PRBs. As said earlier, some specific REs of each PRB 200, 202 are reserved for the transmission of reference signals, as shown in Figure 2 with the left-leaning diagonal lines. The number and location of the REs re- served for the CRS transmission may depend on the applied system configuration. Such REs reserved for the CRS transmission (according to the applied system configuration) may be called reserved REs.

In an embodiment, the transmission of CRSs, according to the proposal, is shown with left-leaning diagonal lines on the corresponding PRBs in Figure 5. The CRSs are transmitted together with any DL user-specific data to the at least one UE 108 in the allocated PRBs 500, 502. It should be noted that the eNB 100 does not transmit the CRSs in each of the 15 PRBs of Figure 5, but only in a subset of PRBs. The subset of PRBs comprises at least the allocated PRBs but not each of the PRBs available within the applied operational channel BW 300. Thus, the CRSs are trans- mitted in the subset of reserved REs of the applied operational channel BW 300. In this embodiment, the reserved REs of the PRBs marked with white color are not used for the CRS transmission. Thus, the interference caused by the CRSs may be reduced significantly as each and every PRB does not carry the CRSs, as shown with white PRBs. By producing less interference, the channel coding may be simplified which releases radio resources. It may be appreciated also that the hardware of the eNB 100, 104 may be configured according to actual (wide) BW 300, but transmission of the CRSs takes place only in fraction (subset) of the band 300.

The UE(s) 108 may need the CRSs to be present only in those PRBs which carry DL user data dedicated to the at least one UE 108 in order to be able es- timate and decode that part of the channel BW 300 which carries the user data. As known, the UE(s) 108 may further use the CRSs in deriving CQIs to be reported back to the eNB 100. However, the eNB 100 may neither need any CQIs which refer to those parts of the channel BW 300 which are not used for DL data allocation. Especially in partial load cells, the eNB 100 need not CQIs from the whole BW 300 as DL allocations (e.g. locations of allocated PRBs within the applied channel BW 300) may not require any optimization at partial loads.

In an embodiment, the eNB 100 may transmit the CRSs within the applied operational channel BW 300 only in the allocated PRBs. Thus, the subset comprises only the reserved REs of those PRBs which carry user data dedicated to the at least one UE 108. This may significantly reduce the DL interference, as explained above. However, in another embodiment, CRSs are transmitted also in other, specific PRBs, as will be described later.

It may be worth noting that the operation channel BW 300 may comprise specific control parts 1 100A, 1 100B, and 1 100C, as shown in Figure 1 1. The REs of the control parts 1 100A, 1 100B, and 1 100C may be called control channel elements (CCEs). Some of the CCEs may be reserved for carrying the CRSs, as shown. The control part may carry, for example, a physical control format indicator channel (PCFICH), a physical hybrid-ARQ indicator channel (PHICH), and the PDCCH. The control parts 1 100A, 1 100B, and 1 100C user-specific control data or broadcast/multicast of control data to a plurality of UEs. The control parts 1 100A, 1 100B, and 1 100C may be located at the beginning of a sub-frame (consisting of two adjacent PRBs, as earlier explained). The space reserved for the control part may depend on the applied system configuration, such as on the LTE/LTE-A, Not each of the reserved control parts 1 100A, 1 100B, and 1 100C of the BW 300 need to be employed for transmission - the load and traffic status of the network may be considered first. However, for the sake of simplicity let us here assume that the control parts 1 100A, 1 100B, and 1 100C are transmitted to the downlink. In Figure 1 1 , the vertical and horizontal dotted lines represent the edges of the PRBs.

In an embodiment, the eNB 100 may additionally transmit CRSs on those of the reserved REs which locate within applied control parts 1 100A, 1 100B, and 1 100C of the operational channel BW 300. Thus, the CRSs may be additionally transmitted on those CCEs of the channel BW 300 which are reserved for the transmission of the CRSs. This is shown so that the REs in the control parts 1 100A, 1 100B, and 1 100C are marked with the left-leaning diagonal lines representing transmission of the CRSs. However, the white marked REs (typically carrying the CRSs in the LTE/LTE/A) need not be transmitted, thus reducing DL interference. Thus, even if a given PRB carries no user data to any UEs, the CRSs may be transmitted in the control parts 1 100A, 1 100B, and 1 100C of such white-marked PRBs. Such embodi- ment may allow for accurate detection/estimation of control parts 1 100A, 1 100B, and 1 100C without causing DL interference as much as in the prior art solutions.

In an embodiment, the eNB 100 may additionally transmit CRSs in at least one of those (reserved) REs which are reserved for CRS transmissions and which lo- cate within a predefined proximity of any of the control parts 1 100A, 1 100B, and 1 100C of the applied operational channel bandwidth 300. The predefined proximity may be a limit that is empirically derived or is based on mathematical modeling. The criterion for defining the proximity limit may to ensure that the receiving UEs 108 may reliably detect the whole control part, including the edges of the control part. In an embodiment, the predefined proximity may be, for example, three REs. Thus, each of the reserved REs locating within three REs from any of the control parts 1 100A, 1 100B, and 1 100C may be used for transmission as well. For the sake of simplicity, only REs 1 106A to 1 106D are illustrated in Figure 1 1 to carry CRSs. These REs are within the predefined proximity from the control part 1 100B. In an embodiment, the predefined proximity may correspond to those of the reserved REs which locate nearest (among all the reserved REs) to any of the control parts 1 100A, 1 100B, and 1 100C. Thus, in this embodiment, the reserved REs which are closest to any of the control parts 1 100A, 1 100B, 1 100C are additionally used for transmitting the CRSs. The measure of the closeness may be determined in the number of REs between a given control part and the reserved RE, for example. In an embodiment, the closeness is determined in the time domain. In an embodiment, the closeness is determined in both directions of the time domain from each control part 1 100A, 1 100B, 1 100C.

Figure 1 1 further shows that the PRBs 1 102A and 1 102B are allocated for user data transmission to UE #1 whereas the PRBs 1 104A and 1 104B are allocated for user data transmission to UE #2. As explained earlier and shown here as well, the REs of these PRBs 1 102A, 1 102B, 1 104A, and 1 104B carry CRSs and are transmitted to the UEs #1 and #2 to enable accurate channel estimation.

In an embodiment, as shown in Figure 5, the allocated PRBs comprise at least some non-adjacent PRBs. For example, in Figure 5, the allocated sets 500, 502 of PRBs are not adjacent to each other, but there are some PRBs in-between. This may allow for more flexibility in the DL data allocations. In another embodiment, however, the allocated PRBs are adjacent to each other, which may simplify the communication and improve channel estimation by the UE(s) 108.

In an embodiment, as shown in Figure 6A, the eNB 100 may additionally transmit CRSs in predetermined PRBs within the applied operational channel BW. In Figure 6A, there are 25 PRBs. Thus, a channel BW 300 of 5 MHz may be assumed here. The additional CRSs transmitted are shown with dashed PRBs and with a reference numeral 600 in Figure 6A. This may be beneficial as the predetermined PRBs may be arranged to comprise PRBs used for, e.g., timing and common signaling (broadcasting) to all UEs 108 in the cell and in other cells, also to the idle UEs. Thus, in an embodiment, the predetermined resource blocks 600 comprise resource blocks used for common signaling within the applied operational channel bandwidth.

A number of the predetermined PRBs and a location of the predetermined PRBs may depend on the applied radio access technology configuration, such as the LTE/LTE-A. As shown in Figure 6B, the UEs 108 may then, by default, detect those predetermined PRBs marked with the reference numeral 600 within the channel BW 300 and determine, e.g., which eNB 100, 104 provides the strongest signal and, consequently, whether a handover is needed or not. In Figure 6B, it is assume that the UE 108 is currently operating under the eNB 100. This may be the reason why the transmission from the eNB 100 carries some DL user data (carrying CRSs) in addition to the PRBs 600 (also carrying CRSs). However, the transmission from the eNB 100 may apply only the predetermined PRBs (which carry CRSs from the eNB 104).

As said, in an embodiment, the UEs 108 detect the predetermined PRBs 600 by default from a predetermined location. However, in another embodiment, the eNB 100 may instruct the at least one UE 108 to measure DL radio signals from detectable cells exclusively from the predetermined PRBs 600 locating in the predetermined location. This may be beneficial, e.g., when another location within the channel BW 300 than the default location is used for the common transmission of the PRBs 600.

In an embodiment, the predetermined location comprises the middle point

602 of the applied operational channel BW 300. It may be, e.g., that according to the applied network specification, the preconfigured/default PRBs used for the common reference signaling locate around the middle point 602. Then it may be beneficial to transmit the additional CRSs (in addition to the CRSs transmitted in the allocated PRBs 500, 502 carrying DL user data) in those PRBs 600 which locate around the middle point 602.

Further, in an embodiment, as shown in Figure 6A, the predetermined number of PRBs comprises six middlemost PRBs 600 of the applied operational channel BW 300. Using such a default number of PRBs and such a default location may be beneficial when the applied network specification/configuration, such as the LTE/LTE-A specification, requires that the common reference signaling is to be transmitted in the six middlemost PRBs. As indicated in Table 1 , six PRBs correspond to channel BW of 1.4 Mhz. In an embodiment, the UE(s) 108 may perform and/or be instructed to perform neighbor measurements from the 1 .4 MHz band, despite of dy- namics in the CRS transmission. Thus, there may be an "allowedMeasBW" set to 6 PRBs. This may be preconfigured to the UE(s) 108 or the eNB 100 may indicate this to the UE(s) 108.

In an embodiment, as shown in Figure 7A, the eNB 100 may additionally transmit CRSs in at least one of those PRBs 700, 702 which are adjacent to at least one of the PRBs comprised in the allocated PRBs 500, 502. Whether or not to add CRSs to the adjacent PRBs 700, 702, may be configurable. In an embodiment, although not shown, the CRSs in the PRBs 600 (as in Figure 6A) may also be transmitted, e.g. if required by the network specifications. However, the addition of CRSs to the PRBs 600 is not necessary.

It may be advantageous to transmit CRSs also in the adjacent PRBs 700,

702 (i.e. outside the band of the allocated PRBs 500, 502) as then the UEs 108 may acquire more reference data for more reliably and error-freely detect the user data in the allocated PRBs 500, 502. Without the addition of CRSs to the adjacent PRBs 700, 702, the receiving UE(s) 108 may lose some data from the edges of the sets of allo- cated PRBs 500, 502. This may be because then the UE(s) 108 may not have any reference point for channel estimation outside PRBs allocated for DL user data transmission. However, by receiving CRSs also in the adjacent PRBs 700, 702, the UE(s) 108 may acquire such external reference point which may be used for interpolation, for example. Consequently, the UE 108 may more efficiently and reliably estimate the channel response throughout the bands corresponding to the sets of allocated PRBs 500, 502. This may be especially beneficial when high modulation and coding scheme is applied. In an embodiment, the adjacent PRBs 700, 702 comprise only those PRBs which are immediately adjacent to the allocated PRBs 500, 502.

In an embodiment, the adjacent PRBs 700 are adjacent in the frequency domain. These PRBs are shown with PRBs 700 having horizontal brick patterns. Additionally or alternatively, the adjacent PRBs 700 are adjacent in the time domain. These PRBs are shown with PRBs 702 having diagonal brick patterns (only one is shown in Figure 7A for the sake of simplicity).

In an embodiment, however, all of the reserved resource elements, which are reserved to carry the CRSs in the PRB 700, 702 according to the applied radio access technology (such as the LTE), are not used. Instead, the eNB 100 may transmit the CRSs only in a subset 704, 706 of the reserved resource elements of the adjacent PRBs 700, 702. In an embodiment, the subset may comprise the reserved REs locating within a half of each of the adjacent PRBs 700, 702. The halves 704, 706 may be determined in the same domain (time or frequency) as in which the adjacent PRB 700, 702 is adjacent to the allocated PRBs 500, 502. For example, in the LTE, as shown in Figure 2, each PRB 200, 202 carries four CRSs in predefined locations. However, in this embodiment, only the half 704, 706 which is closest to the allocated PRB 500, 502 is used for the CRS transmission. Thus, each adjacent PRB 700, 702 may carry two CRSs, instead of four. On one hand this may allow for reasonably efficient and reliable estimation of the channel throughout the band comprising the allocated PRBs 500, 502. On the other hand this may cause less interference than transmitting all the predefined four CRS in the adjacent PRBs 700, 702.

This is shown in more detail in Figure 7B from the frequency domain point of view. The dashed line 708 represent the edge of the first half 704 of the PRB 700. The resource elements 704 marked with left-leaning diagonal lines carry CRSs and are transmitted, while the resource elements marked with vertical lines do not carry CRSs. It should be noted, that typically according to the LTE, also the resource elements marked with the vertical lines would carry CRSs. However, the receiving UE(s) 108 may not need these CRSs for the estimation of the allocated PRBs.

Figure 7C shows the adjacent PRB 702 from the time domain point of view. The dashed line 710 represent the edge of the first half 706 of the PRB 702. The resource elements 706 marked with left-leaning diagonal lines carry CRSs and are transmitted, while the resource elements marked with vertical lines do not carry CRSs. Also here it may be noted, that typically according to the LTE, also the resource elements marked with the vertical lines would carry CRSs. However, the receiving UE(s) 108 may not need these CRSs for the estimation of the allocated PRBs. Thus, it may be better not to transmit the CRSs in these resource elements as they may cause unwanted DL interference.

In an embodiment, as shown in Figure 8, the eNB 100 may select the allocated PRBs (shown with the left-leaning diagonal lines and carrying the user data and CRSs) at least partly on the basis of the identifier (ID) of the cell 102, to which the eNB 100 provides coverage to, in order to avoid selecting same PRBs as neighboring eNBs. As each cell has its own ID and neighboring cells do not have the same IDs, the allocation of PRBs (carrying the user data and CRSs) may advantageously be or- thogonal compared to the neighboring eNB 104, as shown in Figure 8. Therefore, the eNB 100 providing coverage to the cell 102 (having a cell ID #1 ) does not allocate PRBs applying the same frequencies as the PRBs allocated by the eNB 104 providing coverage to the cell 106 (having a cell ID #2). That is, the allocations of the PRBs car- rying user data do not overlap. This may be beneficial in order to further reduce interference.

Further, in an embodiment, the eNB 100 may select the allocated PRBs to comprise PRBs which are distributed evenly across the applied operational channel BW 300. This may allow a fair usage of PRBs by each eNB 100, 104 so that each eNB 100, 104 may apply at least partly those portions of the available channel BW 300 which may be better than other portions from the signal quality point of view. In order to acquire knowledge of the channel quality information across the total channel BW 300, the eNB 100 may periodically transmit CRSs in each of the PRBs within the applied operational channel BW 300. As a result of doing so, the eNB 100 may obtain CQI values over, on each of the PRBs within the applied operational channel BW 300. The eNB 100 may then apply the CQI values in the PRB allocation to the UE(s) 108. The UE(s) 108 may be preconfigured with the timing of the periodic transmission of such "full-band" CRSs, or the eNB 100 may indicate an upcoming "full-band" CRS transmission to the UE(s) 108. In one embodiment, the UE 108 may continuously lis- ten to the whole BW 300 and when the UE 108 detects CRSs being transmitted across the whole BW 300, the UE(s) 108 may report the wide-band CQI, instead of a narrow-band CQI (called also sub band-CQI). In one embodiment, the UE 108 may continuously listen to the whole BW 300 and selectively report CQI values for sub bands with best quality CRSs being transmitted across the whole BW 300.

In an embodiment, the eNB 100 may indicate the UE(s) 108 in the cell 102 and in the neighboring cells that the eNB 100 applies the transmission of CRSs only in a subset of the channel BW 300. A possible indication may be one bit, for example. The indication may indicate that the CRS transmissions are performed only in the subset of the reserved REs of the operational channel BW 300.

Figure 8 also shows the additional CRS transmissions on the adjacent

PRBs 700, 702 and on the predetermined location 600 (such as around the middle point 800 of the BW 300). As shown, both of the eNBs 100, 104 may co-locate some CRSs in the middlemost PRBs in order to provide common radio signal measurement location for all UEs, also to the idle UEs in the cells 102 and 106. In an embodiment, the adjacent PRBs (marked with horizontal brick patterns, for the sake of simplicity on- ly frequency domain is shown) may overlap between the cells 102 and 106. In another embodiment, the allocation of the PRBs for DL user data transmission takes into account the possible CRS transmissions on the adjacent PRBs, thus allocating the DL data PRBs so that the adjacent PRBs do not overlap with the DL allocations of any of the neighboring cell(s).

In an embodiment, as shown in Figure 12, the UE(s) 108 may indicate to the eNB 100 that the UE 108 is capable to operate with the reception of CRSs only in the subset of those REs of the applied operational channel BW 300 which are reserved for CRS transmissions, the subset comprising at least the reserved REs in the allocated PRBs, as shown in step 1200. A possible indication may be one bit, for example. Thereafter in step 1202, the UE 108 may receive the cell-specific reference symbols in the subset of the reserved resource elements of the operational channel BW 300.

In an embodiment, the eNB 100 may request the at least one UE 108 to report at least one narrow-band CQI which is based on a predefined PRBs, instead of the wide-band CQI based on the whole BW 300. In an embodiment, the number of the predefined PRBs is less than the number of PRBs available within the applied operational channel BW 300. Thus, the narrow band CQI is different that the wide-band CQI. The number of predefined PRBs may correspond to the smallest number of ad- jacent PRBs in which CRSs are transmitted in DL, such as 6 PRBs. Again it may be noted that the eNB 100 may not need constant information of wide-band CQIs because, in partial load of the cell, the eNB 100 may efficiently allocate DL user data even without up-to-date knowledge of the wide-band CQI. In an embodiment, the predefined PRBs correspond to the allocated PRBs.

In an embodiment, the UE(s) 108 may be instructed or preconfigured to report CQIs based on only PRBs which carry CRSs. In Figure 9A, the reported CQI is based on only the portion of the BW 300 which carries CRSs. In this case, the eNB 100 may approve and apply the received CQI. However, in an embodiment, the eNB 100 may disregard CQIs which are at least partly based on PRBs not comprising CRSs. In an embodiment, the eNB 100 may disregard CQIs which are totally based on DL PRBs not comprising CRSs. As shown in Figure 9B, the CQI is derived partly from a portion of the BW 300 which comprises PRBs (white part) not carrying CRSs. In this case, the eNB 100 receiving the CQI may disregard it and not apply it for DL allocation decisions or for modulation and coding scheme (MCS) selection. This may be advantageous so that the applied CQIs are all valid CQIs based on PRBs which carry CRSs. In one embodiment, the UEs 108 may themselves decide to determine only such CQIs which are fully based on PRBs carrying CRSs. Thus, the UEs 108 may restrain from transmitting CQIs which are at least partly based on PRBs not comprising CRSs.

In an embodiment, the granularity of CRS transmissions within the BW

300 is set the same as the sub-band of a sub-band based CQl reporting. In general, the granularity of CQl report can be divided into three levels: wideband CQIs, UE selected sub-band CQIs, and higher-layer configured sub-band CQIs. The wideband report may provide one CQl value for the entire downlink system bandwidth 300. In the UE selected sub-band CQl reports, the UE may divide the system bandwidth 300 into multiple sub-bands, select a set of preferred sub-bands (the best M sub-bands, for example,), and report one CQl value for the wideband and one differential CQl value for a set of sub-bands (assuming CQl reporting takes place over the selected M sub- bands). The higher-layer configured sub-band report may provide the highest granu- larity. In this case, the entire system bandwidth 300 may be divided into multiple sub- bands. The UE 108 may then report one wideband CQl value and multiple differential CQl values, one for each sub-band.

In an embodiment, the described embodiments are applicable also to multi-antenna solutions which may apply separate CRS transmissions from each antenna port. Thus, in this case, each antenna port may transmit CRSs only in the subset of the PRBs of the applied channel BW 300. Further each antenna port may transmit CRSs also in the predetermined location for the common signaling (as depicted in Figure 6A) and possibly also in the PRBs which are adjacent to the PRBs allocated for DL user data transmission from that antenna port.

An embodiment, as shown in Figure 10, provides an apparatus 1000 comprising a control circuitry (CTRL) 1002, such as at least one processor, and at least one memory 1004 including a computer program code (PROG), wherein the at least one memory 1004 and the computer program code (PROG), are configured, with the at least one processor 1002, to cause the apparatus 1000 to carry out any one of the described embodiments. The memory 1004 may be implemented using any suitable data storage technology, such as semiconductor based memory devices, flash memory, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory.

In an embodiment, the apparatus 1000 may be or be comprised in a base station (also called a base transceiver station, a Node B, a radio network controller, or an evolved Node B, for example). In an embodiment the apparatus 1000 is or is comprised in the eNB 100.

The apparatus 1000 may further comprise communication interface (TRX) 1006 comprising hardware and/or software for realizing communication connectivity according to one or more communication protocols. The apparatus 1000 may also comprise a user interface 1008 comprising, for example, at least one keypad, a microphone, a touch display, a display, a speaker, etc.

The control circuitry 1002 may comprise a physical resource block allocation circuitry 1010 for allocating PRBs in the DL according to any of the embodiments. In an embodiment, the circuitry 1010, performing operations of a packet scheduler, may allocate PDCCHs to minimum number of PRBs in order to reduce interference. In an embodiment, the circuitry 1010 may co-assign the PDCCHs and the PDSCHs to same PRBs so as to further reduce DL interference. The control circuitry 1002 may comprise a cell-specific reference symbol transmission circuitry 1012 for transmitting CRSs in the DL only in the subset of the PRBs available in the operational channel bandwidth, according to any of the embodiments.

As used in this application, the term 'circuitry' refers to all of the following: (a) hardware-only circuit implementations, such as implementations in only analog and/or digital circuitry, and (b) combinations of circuits and software (and/or firmware), such as (as applicable): (i) a combination of processor(s) or (ii) portions of processors/software including digital signal processor(s), software, and memory(ies) that work together to cause an apparatus to perform various functions, and (c) circuits, such as a microprocessor(s) or a portion of a microprocessor(s), that require software or firmware for operation, even if the software or firmware is not physically present. This definition of 'circuitry' applies to all uses of this term in this application. As a further example, as used in this application, the term 'circuitry' would also cover an implementation of merely a processor (or multiple processors) or a portion of a processor and its (or their) accompanying software and/or firmware. The term 'circuitry' would also cover, for example and if applicable to the particular element, a baseband integrated circuit or applications processor integrated circuit for a mobile phone or a similar integrated circuit in a server, a cellular network device, or another network device.

The techniques and methods described herein may be implemented by various means. For example, these techniques may be implemented in hardware (one or more devices), firmware (one or more devices), software (one or more modules), or combinations thereof. For a hardware implementation, the apparatus(es) of embodiments may be implemented within one or more application-specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), proc- essors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof. For firmware or software, the implementation can be carried out through modules of at least one chip set (e.g. procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in a memory unit and executed by proces- sors. The memory unit may be implemented within the processor or externally to the processor. In the latter case, it can be communicatively coupled to the processor via various means, as is known in the art. Additionally, the components of the systems described herein may be rearranged and/or complemented by additional components in order to facilitate the achievements of the various aspects, etc., described with re- gard thereto, and they are not limited to the precise configurations set forth in the given figures, as will be appreciated by one skilled in the art.

Embodiments as described may also be carried out in the form of a computer process defined by a computer program. The computer program may be in source code form, object code form, or in some intermediate form, and it may be stored in some sort of carrier, which may be any entity or device capable of carrying the program. For example, the computer program may be stored on a computer program distribution medium readable by a computer or a processor. The computer program medium may be, for example but not limited to, a record medium, computer memory, read-only memory, electrical carrier signal, telecommunications signal, and software distribution package, for example. Coding of software for carrying out the embodiments as shown and described is well within the scope of a person of ordinary skill in the art.

Even though the invention has been described above with reference to an example according to the accompanying drawings, it is clear that the invention is not restricted thereto but can be modified in several ways within the scope of the appended claims. Therefore, all words and expressions should be interpreted broadly and they are intended to illustrate, not to restrict, the embodiment. It will be obvious to a person skilled in the art that, as technology advances, the inventive concept can be implemented in various ways. Further, it is clear to a person skilled in the art that the described embodiments may, but are not required to, be combined with other embodiments in various ways.