In a wireless cellular communication system, one or multiple Downlink (DL) Common Reference Signals (CRSs) may be used for channel measurements, or for coherent demodulation and channel measurements, for a mobile terminal in a given cell. A mobile terminal is also denoted as a User Equipment (UE) in some wireless communication systems. Each CRS defines a so-called antenna port in a given cell and a common way to implement antenna ports is to associate an antenna port with a physical transmit antenna.
The Reference Signals (RSs) of different antenna ports should be orthogonal to each other to allow interference-free identification of corresponding propagation channel coefficients, i.e. the propagation channel from each transmit antenna to each receive antenna. The RSs are usually cell-specific to minimize interference between RSs in different cells. Without loss of generality, antenna ports are defined by CRS or cell-specific CRS throughout this document. Cell specific CRS implies that they are used by multiple UEs in a cell to measure the channel from each antenna port. Antenna ports may also be user specific, which means that they are used for measurements and/or demodulation by a specific single UE.
The CRSs are transmitted on exclusively reserved resources of a cell, such as time and frequency Resource Elements (RE), codes, etc. Data is not transmitted on these reserved resources to avoid interference with RSs, which would hamper the estimation of the channel propagation coefficients from that antenna port.
In order to be able to use DL CRSs properly and to perform standard communication with a base station, such as a eNB, the number of antenna ports used for DL channel measurements and/or DL transmission is very important information that a UE needs to know. After a UE obtain information about the number of antenna ports used in a cell, the UE will know which transmission mode is used for each physical channel, and which resources that are used for data transmission and which that are used for DL CRS. This is important information to avoid that received data is punctured by CRSs, since if a UE is not aware of all CRSs in radio resources, it will assume reception of data on those resources where there actually is a CRS transmission, and this will degrade the performance of data reception due to the interference from the CRSs.
Furthermore, knowing the number of CRSs is also important to measure multiple channels using CRSs and to detect physical channels, etc. In the Long Term Evolution (LTE) Release-8 (Rel-8) standard, the information about number of antenna ports is embedded in the signal transmitted on a Physical Broadcast Channel (PBCH). After successful cell search procedure, the UE will obtain time and frequency synchronization with a cell, as well as the cell Identity (ID) of the cell; and then the UE begins to detect the PBCH to obtain cell-specific information and the number of antenna ports.
In the LTE Rel-8 standard, three types of cell-specific CRSs are supported defining: one, two and four antenna ports (3GPP TS 36.211 v8.4.0). The number of antenna ports in a cell decides the maximum number of Multiple Input Multiple Output (MIMO) transmission layers supported by a eNB in said cell. For instance, if there are four antenna ports in a cell, up to four MIMO layers transmission can be supported by the eNB. The information about the number of antenna ports is embedded into the signal transmitted on the PBCH by using different Cyclic Redundancy Check (CRC) masks to indicate the number of antenna ports. As a UE has no prior information about the number of used antenna ports in a cell, i.e. the used CRC mask of the transport block of the PBCH, the UE has to make blind detection of that information, which means that it has to check all possible CRC masks and select the mask that is the most probable conditioned on the received PBCH signal.
The operation of embedding information about the number of antenna ports into PBCH at transmitter and the corresponding blind detection of PBCH at receiver for a LTE Rel-8 system will be described in the following.
Firstly, at the transmitter, the entire transport block bits of PBCH a0, a1, . . . aA-1 is used to calculate the CRC parity bits p0, p1, . . . pL-1, where A is the size of the transport block (i.e. the number of information bits) and L is the number of CRC parity bits which is set to 16 in the LTE Rel-8 standard. Secondly, according to the antenna port configuration of the cell, the CRC parity bits are scrambled by a sequence having length 16, x0n, x1n, . . . x15n corresponding to a number of antenna ports n, where n=1, 2 or 4. After scrambling, the masked CRC parity bits will be c0, c1, . . . c15, where ci=(pi+xin) mod 2, i=0, 1, . . . , 15. The mapping relation between the three scrambling sequences and the number of antenna ports according to the LTE standard is shown in Table 1 below.
TABLE 1CRC mask sequences for PBCH in LTE Rel-8Number ofantenna portsCRC mask sequences1<0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0>2<1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1>4<0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1>
Then, the masked CRC parity bits are attached to the tail of the transport block bits of the PBCH to obtain the information bits to be transmitted as a0, a1, . . . , aA-1, c0, c1, . . . c15.
Finally, a set of operations including channel coding, rate matching, modulation and resources mapping are performed on the information bits a0, a1, . . . , aA-1, c0, c1, . . . c15.
In the case of one antenna port, the modulation symbols are directly mapped to the reserved resources on antenna port 0; in the case of two antenna ports, a transmit diversity scheme known as Space Frequency Block Coding (SFBC) is performed on the modulation symbols, and the output of the SFBC is mapped to the reserved resources on antenna port 0 and 1, respectively; in the case of four antenna ports, SFBC combined with Frequency Switching Transmit Diversity (FSTD) is performed on the modulation symbols, and the output of SFBC+FSTD is mapped to the reserved resources on antenna port 0, 1, 2 and 3, respectively. It should be observed from the above description that the information about the number of antenna ports is implicitly embedded into the PBCH.
At the receiver, the corresponding inverse operations to find the number of antenna ports are done by a UE who is accessing to the cell. As the UE knows that there are three hypothesises possible regarding the number of antenna ports (i.e. one, two or four antenna ports) the UE performs blind detection of the PBCH, which is illustrated in FIG. 1. In the procedure of blind detection, SFBC or SFBC+FSTD decoding, demodulation, channel decoding and CRC detection are all standard operations, so the details of them will not be further described, but the operation of removing the CRC mask will be explained in the following disclosure.
Assuming that the output of channel decoding is â0, â1, . . . , âA-1, ĉ0, ĉ1, . . . ĉ15, where the last 16 bits of information ĉ0, ĉ1, . . . ĉ15 are the CRC parity bits scrambled with a CRC mask corresponding to information about the number of antenna ports, as mentioned above. When blind detection of the PBCH is performed, the CRC parity bits are de-scrambled with an assumed CRC mask (i.e. removing the CRC mask) in the following way:{tilde over (c)}i=(ĉi+xin)mod 2where i=0, 1, . . . , 15;                n is the number of antenna ports,        xin is the defined CRC mask corresponding to n antenna ports        
If the assumed CRC mask is the same as the actual CRC mask used at transmitter, the above operation will completely remove the CRC mask embedded into CRC parity bits, and the probability of correct detection is increased.
As mentioned, in the LTE Rel-8 system up to four antenna ports can be supported on the DL. The LTE-Advanced (LTE-A) system of Release 10 (Rel-10) and beyond are supposed to be an extension of LTE system in which up to eight layers transmission (possibly even more layers for releases beyond Rel-10) will be supported to further increase system performance, such as peak data rate, cell average spectrum efficiency, etc (3GPP TR 36.814 v1.0.0). In order to support up to eight layers transmission more antenna ports than the antenna ports supported in LTE Rel-8 must be defined in a LTE-A communication system.
In addition, to fulfil LTE-A backwards compatibility requirement, it should still be possible for a LTE-A cell to also serve LTE UEs. In order to enable LTE UEs to operate in a LTE-A system, the antenna ports defined in LTE should also be supported in a LTE-A system, i.e. n number of LTE CRSs should also exist in a LTE-A system, where n=1, 2 or 4; and LTE UEs use LTE CRSs for coherent demodulation and channel measurement as in the LTE system, while LTE-A UEs may also use these LTE CRS for demodulation of the control channels, such as PBCH and Physical Downlink Control Channel (PDCCH).
Hence, in a LTE-A system there will be a number of LTE antenna ports used for transmitting LTE data/control and/or LTE-A control information; in addition, it is also possible to define a number of additional antenna ports used only for supporting LTE-A data transmission. For all antenna ports in a LTE-A system, the new defined additional antenna ports are denoted as LTE-A antenna ports, and the number of LTE-A antenna ports could be zero. Therefore, a communication system which can serve both LTE UEs and LTE-A UEs is needed. Also, since the number of LTE and LTE-A antenna ports may be different in a LTE-A cell, a question is how to signal the existence of LTE-A antenna ports to LTE-A UEs in way that is transparent to the reception of the number of LTE antenna ports.
It is thus clear that a LTE-A eNB may need to enable additional CRSs (antenna ports) for measurements and/or demodulation compared to the antenna ports defined by LTE CRSs (i.e. n=1, 2 or 4). Up to eight additional antenna ports (CRSs) may be needed for LTE-A UEs in Rel-10. These additional CRSs are denoted Channel State Information-RSs (CSI-RSs). It is thus another question how to signal the number of LTE-A antenna ports or the number of CSI-RS to LTE-A UEs in way that is transparent to LTE UEs.
Therefore, a signalling method which is backwards compatible to enable LTE UEs to obtain the number of LTE antenna ports, while the signalling of LTE-A antenna ports should be transparent to LTE UEs is needed in the art.