APPARATUS AND METHOD FOR ORTHOGONAL COVER CODE (OCC) GENERATION, AND APPARATUS AND METHOD FOR OCC MAPPING FIELD OF THE INVENTION [0001] The present invention relates to transmission technology in the wireless communication system, and in particular to an orthogonal cover code generation apparatus and method and an orthogonal cover code mapping apparatus and method in a wireless communication system such as an LTE/LTE-A system. BACKGROUND OF THE INVENTION [0002] The next-generation wireless communication system LTE-A (Long Term Evolution-Advanced) of 3 GPP requires providing a peak rate of 1Gps and a peak spectrum efficiency of 30bps/Hz in the downlink. This brings challenge to the transmission scheme in the physical layer of the system. A multi-antenna MIMO (Multiple Input Multiple Output) system is able to support parallel data flow sending thereby greatly increasing the system throughput. Typically, the independent forward error correction encoding is firstly performed on the parallel data flow in the multi-antenna transmission, and then the encoded code words are mapped into the corresponding data transmission layer. In one transmission, the number of all the layers supported by the system is also referred to as a Rank of this transmission. The process of transforming data in each layer into data on each physical antenna is referred to as a pre-encoding process for a signal. LTE-A Rel-10 supports a pre-encoding technology with maximum Rank of 8. [0003] The sending terminal should transmit pilot sequences used for channel estimation, namely demodulation reference signals (DMRSs), for the receiving terminal to perform MIMO decoding and related demodulation. The design of DMRSs should satisfy that DMRSs corresponding to each data transmission layer are mutually orthogonal, i.e. ensure that there is no interference between equivalent channels of pre-encoded channels of respective sending antennas. In a Rel-10 system, DMRSs corresponding to each data transmission layer are distinguished in the manner of frequency division multiplexing (FDM) and/or code division multiplexing (CDM). The code division multiplexing is implemented by spreading sequences whose correlation is ideal with orthogonal cover code sequences. The orthogonal cover code sequences usually employ Walsh Code sequences or Discrete Flourier Transform sequences. [0004] If the orthogonal cover code sequences are mapped in the time domain, i.e. spread in the time domain, it is usually assumed that the channels in the physical resources corresponding to the cover code sequences are identical. Assuming that a spreading factor of a spreading sequence is M, the channel response of the M OFDM symbols are considered to be identical. This assumption is true in the low speed environment. However, with the increasing moving speed of a mobile station, variations of the channel response of the M OFDM symbols increase and the orthogonality of the spreading codes are destroyed, leading to mutual interference between respective data transmission layers and thus reducing the accuracy of the channel estimation. [0005] Moreover, in the Rel-10 system, DMRSs are subjected to the same pre-encoding process as that for data and are mapped onto each sending antenna. The pre-encoding process performs linear superposition on the DMRSs corresponding to each of the code division multiplexed data transmission layers. If the DMRSs corresponding to the M data transmission layers are superposed in the same direction, a signal with amplitude of M is gotten; and if the DMRSs corresponding to the M data transmission layers are superposed in the opposite direction, they are mutually canceled out and a signal with amplitude of 0 is gotten. If such power imbalance of each of the sending antennas occurs in the entire frequency bandwidth, the efficiency of the transmission power may be reduced apparently. [0006] The reference documents of the present invention are listed in the following, which are incorporated herein by reference as if they are described in detail in the present description. [0007] 1. [Patent Document 1]: Ishii Hiroyuki, Higuchi Kenichi, Base station apparatus, user apparatus and method used in mobile communication system (US 20100034077 A1); [0008] 2. [Patent Document 2]: Hooli Kari, Pajukoski Ka, et al., Method, apparatuses, system and related computer product for resource allocation (WO 2009056464 A1); [0009] 3. [Patent Document 3]: Kim Hak Seong, Yun Young Woo, et al., Method of transmitting scheduling reference signal (US 20100008333 Al); [0010] 4. [Patent Document 4]: Che Xiangguang, Guo Chunyan, et al., Variable transmission structure for reference signals in uplink messages (WO 2009022293 A2); [0011] 5. [Patent Document 5]: Cho Joon-young, Zhang Jianzhong, et al., Apparatus and method for allocating code resource to uplink ACK/NACK channels in a cellular wireless communication system (US 2009046646 Al); [0012] 6. [Patent Document 6]: Yang Yunsong, Kwon Younghoon, System and method for adaptively controlling feedback information (US 20090209264 Al); and [0013] 7. [Patent Document 7]: Pajukoski Kari P, Tiirola Esa, Providing improved scheduling request signaling with ACK/NACK or CQI (US 20090100917). SUMMARY OF THE INVENTION [0014] Hereinafter, a brief summarization about the present invention is given, so as to provide basic understanding of some aspects of the present invention. However, it should be understood that this summarization is not an exhaustive summarization about the present invention. It does not intend to be used to either determine a key or important part of the present invention or define the scope of the present invention. Its object is only to give some concepts about the present invention in a simplified form and hereby acts as a preamble of more detailed descriptions which will be presented later. [0015] In view of the above mentioned situation in the prior art, the object of the present invention is to provide an orthogonal cover code generation apparatus and method and an orthogonal cover code mapping apparatus and method, which may solve one or more of the problems in the prior art. [0016] In order to achieve the above mentioned object, according to one aspect of the present invention, there is provided an orthogonal cover code generation apparatus, including: a first orthogonal cover code sequence group generation means for generating a first group of orthogonal cover code sequences C1 represented by a matrix of [Cn, 1(1), Cn, 1(2),...Cn, 1(M)], which satisfy that any adjacent truncated sub cover code sequences [C2j-1, 1(2m-1), C2j-1, 1(2m)] and [C2j-1, 1(2m-1), C2j-1, 1(2m)] are also mutually orthogonal, wherein n is an index of N orthogonal cover code sequences included in the first group of orthogonal cover code sequences, M is a spreading factor of the orthogonal cover code sequence as a spreading sequence, N2, the DMRSs occupy extra 12 REs for transmitting the DMRSs of the third layer and the fourth layer. The pilots of the third layer and the fourth layer occupy the same PRB and they are distinguished by an orthogonal cover code of a length of 2. If the data flow is >4, the number of the REs occupied by the DMRSs dose not change and is still 24. Each data flow may be distinguished in the manner of the code division multiplexing (CDM) and/or the frequency division multiplexing (FDM). One of the feasible multiplexing manners is shown in Figure 4. The first, second, fifth and seventh layers are multiplexed in the manner of CDM and are distinguished by an orthogonal cover code of a length of 4. The time-frequency resources occupied are represented by the dark grids in the figure, which are referred to as CDM group 1 for short. The third, fourth, sixth and eighth layers are multiplexed in the manner of CDM and are distinguished by an orthogonal cover code of a length of 4. The time-frequency resources occupied are represented by the grids with twills in the figure, which are referred to as CDM group 2 for short. Moreover, the first, second, fifth and seventh layers and the third, fourth, sixth and eighth layers are multiplexed in the manner of FDM. [0064] Figure 5 shows a schematic view of mapping the four groups of orthogonal cover code sequences generated according to the present invention into the downlink DMRS resources in the Rel-10 system. It can be seen from the figure that the orthogonal cover code sequences are spread in the time domain. That is to say, the DMRSs corresponding to the same sub-carrier on the sixth, seventh, thirteenth and fourteenth OFDM symbols form a spreading code of a length of 4. For the time-frequency resource corresponding to CDM group 1, the generated four groups of orthogonal cover code sequences are mapped sequentially in turn in the order of C1, C2, C3 and C4, so as to guarantee that all the orthogonal cover code sequences are included as much as possible in the entire frequency band corresponding to CDM group 1. For the time-frequency resource corresponding to CDM group 2, the generated four groups of orthogonal cover code sequences are mapped sequentially in turn in the order of C4, C3, C2 and C1, so as to guarantee that all the orthogonal cover code sequences are included as much as possible in the entire frequency band corresponding to CDM group 2. The corresponding DMRS resources in each PRB, including CDM group 1 and CDM group 2, all in turn include all the four groups of orthogonal cover code sequences. For example, in the first PRB, all the four groups of orthogonal cover code sequences are included in the (k)th, (k+l)th, (k+5)th and (k+6)th sub-carriers. Therefore, the effect of randomizing pilot sequences is achieved and the peak power of the sending signal is effectively reduced. [0065] Figure 6 shows a schematic view of power distribution of mapping the pre-encoded four groups of orthogonal cover code sequences generated according to the present invention onto a first sending antenna. It can be seen from the figure that if all the row vectors in the pre-encoding matrix are 1, after the column vectors matrixes of the 4 groups of orthogonal cover code sequences C1-C4 are respectively multiplied by the row vectors of the pre-encoding matrix and the products are respectively added, on the (k)th sub-carrier, corresponding DMRSs of the first, second, eighth and ninth OFDM symbols are respectively 4, 0, 0 and 0; on the (k+1)th sub-carrier, corresponding DMRSs of the first, second, eighth and ninth OFDM symbols are respectively 0, 0, 4 and 0; on the (k+5)th sub-carrier, corresponding DMRSs of the first, second, eighth and ninth OFDM symbols are respectively 0, 0, 0 and 4; and on the (k+6)th sub-carrier, corresponding DMRSs of the first, second, eighth and ninth OFDM symbols are respectively 0, 4, 0 and 0. It is not difficult to see that the power of the DMRSs is uniformly distributed on the four OFDM symbols, so as to avoid the problem of imbalanced power. [0066] Figure 7 shows a schematic view of the orthogonality in time-frequency two-dimensions according to the mapping method of the present invention. The orthogonal cover code sequences are spread in the time domain, and the four pilot symbols in each sub-frame respectively correspond to four column vectors of the generated orthogonal cover code sequences. If the length of spreading is 2, the orthogonal cover code sequences mapped in this way also guarantee that the sequences corresponding to two pilot symbols in each sub-frame are orthogonal. Moreover, the sequences corresponding to adjacent four sub-carriers in each pilot symbol also satisfy the orthogonality of a length of 4 in the frequency domain. Furthermore, on two adjacent sub-carriers within a same CDM group, the corresponding DMRSs of adjacent two OFDM symbols also form a spreading code of a length of 4, i.e. the orthogonality is provided in the time-frequency two dimensions. For example, for CDM group 1, on the (k+l)th and (k+6)th sub-carriers, corresponding DMRSs of the first and second OFDM symbols also form mutually orthogonal spreading codes of a length of 4. [0067] Although, in the above, the orthogonal cover code generation method and orthogonal cover code mapping method according to embodiments of the present invention are described in detail in conjunction with the accompanying drawings, the skilled in the art should understand that the flow charts shown in Figures 1 and 3 are only exemplary, and the flow of the methods shown in Figures 1 and 3 may be correspondingly modified according to practical applications and specific requirements. For example, the performing order of some steps in the methods shown in Figures 1 and 3 may be adjusted or some processing steps may be omitted or added as required. [0068] The orthogonal cover code generation apparatus and orthogonal cover code mapping apparatus according to embodiments of the present invention are to be described in conjunction with the accompanying drawings as follows. [0069] Figure 8 shows a structural block diagram of an orthogonal cover code generation apparatus 800 according to an embodiment of the present invention, where only the parts that are closely associated with the present invention are shown for the sake of simplicity and clarity. In the orthogonal cover code generation apparatus 800, the orthogonal cover code generation method described above with reference to Figure 1 can be performed. [0070] As shown in Figure 8, the orthogonal cover code generation apparatus 800 may include a first orthogonal cover code sequence group generation means 810, a second orthogonal cover code sequence group generation means 820, a third orthogonal cover code sequence group generation means 830 and a fourth orthogonal cover code sequence group generation means 840. [0071] In the orthogonal cover code generation apparatus 800, the first orthogonal cover code sequence group generation means 810 may be used for generating a first group of orthogonal cover code sequences C1 represented by a matrix of [Cn,1(1), Cn, 1(2),.. .Cn, 1(M)], which satisfy that any adjacent truncated sub cover code sequences [C2j-1, 1(2m-1), C2j-1, 1(2m)] and [C2j-1, 1(2m-1), C2j-1, 1(2m)] are also mutually orthogonal, where n is an index of N orthogonal cover code sequences included in the first group of orthogonal cover code sequences, M is a spreading factor of the orthogonal cover code sequence as a spreading sequence, N