In the so-called Long-Term Evolution (LTE) systems currently under development by members of the 3rd-Generation Partnership Project (3GPP), downlink transmissions are specified according to an Orthogonal Frequency-Division Multiple Access (OFDMA) scheme. Hence, the available physical resources in the downlink are divided into a time-frequency grid. Generally speaking, the time dimension of the downlink physical resource allocated to a particular base station (an Evolved Node B, or eNodeB, in 3GPP terminology) is divided into subframes of one millisecond each; each subframe includes a number of OFDM symbols. For a normal cyclic prefix length, suitable for use in environments where multipath dispersion is not expected to be extremely severe, a subframe consists of fourteen OFDM symbols. A subframe consists of twelve OFDM symbols if an extended cyclic prefix is used. In the frequency domain, the physical resources allocated to a given eNodeB are divided into adjacent OFDM subcarriers, at a spacing of fifteen kilohertz, with the precise number of subcarriers varying according to the allocated system bandwidth. For purposes of scheduling resources (i.e., allocating resources for use by a given mobile station), the downlink time-frequency resources are referenced in units called “resource blocks” (RBs); each resource block spans twelve adjacent subcarriers and one-half of one subframe. The term “resource block pair” refers to two consecutive resource blocks, i.e., occupying an entire one-millisecond subframe.
The smallest element of the LTE time-frequency grid, i.e., one subcarrier of one OFDM symbol, is called a resource element. There are several different types of resource elements, including resource elements used as reference signals (RS) as well as resource elements for carrying data symbols (e.g., coded information carrying symbols). The reference signals enable channel estimation, which can be used for coherent demodulation of the received signals and may also be used for various measurements. Each reference signal defines a so-called antenna port—since a specific RS is used for each port, a given antenna port is visible to mobile stations (user equipment, or UEs, in 3GPP terminology) as a separate channel. However, an antenna port is a logical entity that may or may not correspond to a single physical antenna. Thus, when an antenna port corresponds to multiple physical antennas, the same reference signal is transmitted from all of the physical antennas.
Cell-specific reference signals (also known as common reference signals) as well as UE-specific reference signals (user-equipment-specific reference signals, also known as dedicated reference signals) are supported in the current LTE specifications. At a given eNodeB, either 1, 2, or 4 cell-specific reference signals may be configured. However, only one UE-specific reference signal is available under the current specifications.
FIG. 1 illustrates a portion of the LTE time-frequency grid for the cases of 1, 2, and 4 cell-specific antenna ports (which may correspond, for example, to eNodeB's using 1, 2, and 4 transmit antennas, respectively). More particularly, FIG. 1 illustrates a resource block pair, i.e., twelve subcarriers over a single subframe, for each antenna port. The structure illustrated in FIG. 1 is generally repeated over the entire system bandwidth.
In FIG. 1, reference symbols 110 are high-lighted in the illustration of the resource block pair for the case of antenna port 1. Other reference symbols, for the additional antenna ports, are shaded but not high-lighted in each of the various grids. Thus, as can be seen, the reference signals for the different antenna ports are carried in OFDM symbols 0, 4, 7, and 11 (i.e., the first and fifth symbols of each of the two slots in the subframe), for one and two antenna ports. The four-port case includes additional reference symbols in OFDM symbols 1 and 8 as well.
At any given eNodeB, the actual resource grid may look slightly different from what is illustrated in FIG. 1 in that the reference-signal pattern may be shifted in frequency by an integer number of subcarriers. The specific shift depends on the cell identifier (ID); the number of unique shifts available depends on the number of cell-specific antenna ports that are configured. A close examination of FIG. 1 will reveal that there are six shifts yielding unique reference symbol patterns in the case of one cell-specific antenna port. Configurations for two and four cell-specific antenna ports will each support three different shifts, as there is, in these cases, a 3-sub-carrier frequency shift between reference symbols of different antenna ports.
Such frequency shifts serve at least two purposes. First, they enable more effective power boosting of resource elements used for reference signals, since these resource elements for adjacent cells are less likely to collide. Secondly, for purposes of channel-quality measurements, shifting allows the inter-cell interference to be measured for the reference-signal resource elements. Since the so-obtained interference is a mixture of reference-signal interference and data interference from other cells, such measurements thus take into account the load of interfering cells, at least to some extent.
As previously mentioned, UE-specific reference signals are also supported in the current LTE specifications. The pattern for a UE-specific reference is illustrated in FIG. 2, which also illustrates additional details of the layout of a resource block pair. As seen in FIG. 2, a resource-block pair comprises a twelve-subcarrier-by-fourteen-symbol grid of resource elements 220 (for the case of a normal length cyclic prefix), or two resource blocks together occupying a subframe 210. The subframe 210 in turn comprises an even-numbered slot 212 and an odd-numbered slot 214. The first one, two, three, or four symbols of the subframe are used for a control-channel region 240 (which may carry one or multiple Physical Downlink Control Channels, or PDCCHs); the resource block illustrated in FIG. 2 is configured with two symbols dedicated to the control-channel region 240. UE-specific reference symbols 230 are also illustrated in FIG. 2; these reference symbols appear in OFDM symbols 3, 6, 9, and 12. The UE-specific reference signal effectively defines a fifth antenna port.
The UE-specific reference signal is only associated with those resource-block pairs allocated for a particular Physical Downlink Shared Channel (PDSCH) transmission that relies on such reference signals (i.e., those transmissions that are mapped to antenna port 5). Thus, the reference symbols corresponding to a UE-specific reference signal are not necessarily transmitted in every subframe, or for all resource block pairs within one subframe. Unlike the cell-specific reference signals, precoding may be applied to UE-specific reference signals in the same manner as it may be applied to data-carrying resource elements. This makes such precoding effectively invisible to the mobile station, in the sense that any precoding will effectively be included in the channel estimates derived by means of the UE-specific reference signals. UE-specific reference signals thus provide enhanced flexibility in mapping a data transmission to different antenna configurations. In particular, the use of UE-specific reference signals facilitates the mapping of a particular downlink transmission to antennas spread out over different sites.
Data over the PDSCH is transmitted to a given mobile station using resource elements that correspond to the resource block pairs allocated to that mobile station for a given subframe. The particular resource block pairs involved in the transmission are dynamically selected and signaled to the mobile station as part of the resource-allocation content of the associated control channel, PDCCH, transmitted in the control-channel region of the subframe. As is apparent from FIGS. 1 and 2, some of the OFDM symbols outside of the control-channel region are used to carry reference symbols; hence, not all resource elements in that portion of the resource block pair can be used for PDSCH transmission. In other words, the mapping of PDSCH onto the resource grid is affected by the positions of the cell-specific reference symbols.
In a classical cellular deployment, the intended service area is covered by several cell sites at different geographical positions. Each site has one or more antennas servicing an area around the site. Often, a cell site is further subdivided into multiple sectors, where perhaps the most common case is to use three 120-degree-wide sectors. Such a scenario is illustrated in FIG. 3. Each sector forms a cell, and a base station associated with that cell is controlling and communicating with the mobile stations within that cell. In a conventional system, the scheduling and transmissions to mobile stations and reception from the mobile stations are to a large degree independent from one cell to another.
Differing simultaneous transmissions on the same frequencies in different cells close to each other will naturally interfere with each other and thus lower the quality of the reception of the different transmissions at a receiving mobile terminal. Interference is a major obstacle in cellular networks and is primarily controlled in conventional deployment scenarios by planning the network carefully, placing the sites at appropriate locations, tilting the antennas etc.
Performing independent scheduling between different cells has the advantages of being simple and requiring relatively modest communication capabilities between different sites. On the other hand, the cells affect each other in that signals originating from one cell are seen as interference in nearby cells. This indicates that there are potential benefits in coordinating the transmissions from nearby cells. In various cellular systems, separating transmissions in frequency and/or time between neighboring cell sites is commonly used to reduce interference. However, this separation has historically been statically configured. More recently, separation in the spatial domain, e.g., by means of advanced multi-antenna transmission schemes, has also been widely exploited, and coordination of neighboring transmissions in the time, frequency, and spatial domains has been proposed to mitigate interference. Such coordination has recently received substantial interest in both academic literature and standardization of new wireless technologies. In fact, so-called coordinated multi-point transmission (COMP), see 3GPP TR 36.814 v0.3.2 (R1-090929) is considered one of the key technology components for the upcoming release 10 of LTE (LTE-Advanced).
COMP may be classified into two separate but related technologies: coordinated scheduling and joint transmission, respectively. In the former case, the transmission to a given mobile station originates at a single cell site or sector at a time, while in the latter case multiple sites and/or sectors are simultaneously involved in the transmission. Thus, for example, several cell sites covering a group of cells, such as the group of seven circles inside the circle of FIG. 3, may coordinate their transmissions; a group of cells involved in such a coordination is here referred to as a COMP cluster.
Obviously, coordination between cell sites requires communication between the sites. This can take many forms and the requirements on data rates and latency for such inter-site communication are to a large extent dependent on the exact coordination scheme being used.
Apart from the potential problem of site-to-site communication capability, coordination exploiting time and frequency is easily achieved for OFDM systems like LTE by using the normal dynamic resource allocation feature, which selects the particular resource-block pairs for transmitting the PDSCH to a given mobile station in a given subframe. Spatial coordination, on the other hand, involves utilizing multiple antennas for the transmission; this can include transmission from antennas at geographically distinct cell sites. By modeling the signals as vector-valued signals and applying appropriate complex-valued matrix weights among the transmitting antennas, the transmission can be focused in the direction (in physical space or in a more abstract vector space) of the mobile station, while minimizing the interference to other mobile stations. This approach increases the signal-to-noise-plus-interference ratio (SINR) at the mobile station, and ultimately improves the overall performance of the system.
As previously indicated, the mapping of PDSCH onto resource elements in the LTE time-frequency grid may vary from one cell to another, even if the same resource blocks are used for the PDSCH. One reason is the use of different reference-signal frequency shifts for the cell-specific reference signals. Another reason is that the number of OFDM symbols used for control signaling can vary dynamically from 1 up to the 4 first OFDM symbols and may be different for neighbor cells. Hence, the particular serving cell to which a given mobile station is attached affects the mapping of PDSCH to resource elements in the time-frequency resource grid, as this mapping is intended to be compatible with how other resources such as the reference signals and PDCCH are allocated in that particular cell. This may create problems for coordinated multi-point transmission, where certain transmissions to a mobile station need to be performed from sites/sectors other than the serving (logical) cell, whether simultaneously or as part of a coordinated schedule.