WIRELESS COMMUNICATION SYSTEM, BASE STATION,TERMINAL AND METHOD OF GENERATINGCODEBOOK TECHNICAL FIELD The present disclosure generally relates to the field of communication, and especially to a method, a base station and a terminal for generating codebook as well as a wireless communication system comprising the base station and the terminal using the generated codebook. BACKGROUND ART The multi-antenna technology is an important component of the fourth generation wireless communication system. An important application in the multi-antenna technology is closed-loop downlink pre-coding. In this application, when signals are sent to a client by multiple antennas located in a base station, the client measures a downlink transmission channel from the base station to the client, and suggests a corresponding pre-coding matrix to the base station according to the channel condition. This suggestion is transmitted to the base station via a feedback link. Subsequently, in the downlink transmission, the base station may adopt directly the suggested pre-coding matrix to pre-code the signal to be sent to the client. In the above procedure for closed-loop downlink pre-coding, an important issue is how to feed back the pre-coding matrix suggested by the client. The means which is most popular and possibly most practical is a manner of codebook based feedback. In such a feedback manner, a codebook containing multiple pre-coding matrices is first defined and is applied at both the base station and the client. In this way, when suggesting a certain pre-coding matrix in the codebook to the base station according to the channel condition, the client can feed back the index corresponding to the pre-coding matrix to the base station. For instance, a codebook containing four pre-coding matrices, each of which is indexed respectively as {00}, {01}, {10} and {11}, is known at both the base station and the client. If the client suggests using the first pre-coding matrix, the bits {00} are fed back to the base station. A feedback manner based on the pre-coding matrix index can achieve improvement in both reducing the feedback overhead and assuring a high downlink throughput. A basic codebook design is based on feedback of a pre-coding matrix for a single antenna array. However, in the current LTE-A (Long Term Evolution-Advanced) standardization, a client may receive signals from multiple antenna arrays located in multiple base stations, for example, in downlink CoMP (Coordinate Multiple Points) as shown in Fig. 1. In such a case, a suggested pre-coding matrix needs to be fed back for each antenna array, and thus multiple pre-coding matrices need to be fed back for the multiple antenna arrays. In addition, in order to enable signals from the multiple antenna arrays be combined coherently at the client, the phase offsets between the pre-coding matrices need to be fed back to the base station as well. With respect to the feedback manner of the pre-coding matrices under the above downlink CoMP environment, for simplification, as an example, assuming that the base station 1 and the base station 2 are involved in the downlink transmission, and each of the base stations uses an antenna array containing two antennas for transmitting signals (as shown Fig. 2), the description of a currently known feedback manner is made in the following. For the downlink CoMP environment as shown in Fig.2, a manner of direct feedback at present is to use a two-antenna codebook defined in LTE standard Rel-8 1 1 1 1 version for both the base station 1 and the base station 2, i.e. i . - J - j 1 There are four pre-coding matrices and included in the _-JJ two-antenna codebook. Therefore, when the client performs feedback, two bits are used to feed back PMI1 (Pre-coding Matrix Index) suggested to be employed for the antenna array located in the base station 1, and two bits are used to feed back PMI2 suggested to be employed for the antenna array located in the base station 2 as well. Furthermore, one bit is additionally required to feed back the phase offset (-1 or 1) between the PMI1 and the PMI2 to the base station. In the above feedback manner, five bits in total are used for feedback. In some cases, however, people may expect to reduce the feedback overhead. In such cases, two possible feedback manners are known as follows. The first possible manner is substantially the same as the above five-bit feedback manner, and the difference is that the one bit for feeding back the phase offset between the PMI1 and the PMI2 is removed, that is, the phase offset is not fed back. In this way, only four bits in total are needed for feedback. However, since the phase offset is not fed back, the signals transmitted from the antenna arrays of the base stations 1 and 2 may not be combined coherently at the client, thus resulting in the reduction of the system throughput. Another possible manner is to reduce the number of bits for a pre-coding matrix index (for example, PMI2) from two bits to one bit, and meanwhile one bit is still used to feed back the phase offset. Specifically, the two-antenna codebook defined in 1 1 1 1 the LTE standard Rel-8 version, that is l , is still used for the base - j - j . station 1, while a two-antenna codebook containing two pre-coding matrices l is used for the base station 2 . In this way, when the client is feeding back, two bits are still required to feed back the PMIl for the antenna array located in the base station 1, while only one bit is required to feed back the PMI2 for the antenna array located in the base station 2 . In addition, one bit is used to feed back the phase offset between the PMIl and the PMI2 to the base stations. Thereby, the feedback overhead is reduced to four bits. However, the method results in reduction of the spatial granularity of the codebook of the PMI2, and may also cause reduction of the throughput. Thus, it has become an issue of the art how to reduce the feedback overhead while keeping the precision of feedback for a multi-antenna array. SUMMARY OF THE DISCLOSURE In order to address the above issues, the present disclosure provides a method of generating codebook in a wireless communication system, a wireless communication system, a base station, and a wireless terminal by using the method. In the present disclosure, in order to show relationships between respective elements of codebook more distinctly, the respective elements of codebook may be also represented by their corresponding phases, and enable the amplitudes of all elements to be "1". According to one aspect of the present disclosure, there is provided a method of generating codebook in a wireless communication system with a plurality of antenna arrays, comprising steps of: providing a basic codebook which cotains a plurality of pre-coding matrices; and assigning phase offsets to certain pre-coding matrices in the basic codebook to form a codebook with phase offset. According to another aspect of the present disclosure, there is provided a wireless communication system by using a plurality of codebooks for communication, the wireless communication system including a plurality of base stations and a terminal performing downlink CoMP (Coordinate Multiple Points) transmission, each of the plurality of base stations being configured with one antenna array, and a plurality of antenna arrays configured in the plurality of base stations respectively corresponding to the plurality of codebooks. The base station comprises: a feedback information reception unit for receiving information fed back from the terminal; a codebook conservation unit for forming a codebook with phase offset by assigning phase offsets to certain pre-coding matrices in a basic codebook, or for storing the basic codebook and the codebook with phase offset in advance; and a sender unit for pre-coding data to be transmitted, and sending the pre-coded data to the terminal via the antenna array configured in the base station. The terminal comprises: a codebook conservation unit for forming a codebook with phase offset by assigning phase offsets to certain pre-coding matrices in a basic codebook, or for storing the basic codebook and the codebook with phase offset in advance; and an information feedback unit for feeding back indexes identifying the pre-coding matrices in the basic codebook and the codebook with phase offset to the plurality of antenna arrays in the plurality of base stations. According to a further aspect of the present disclosure, there is provided a base station performing downlink CoMP transmission, comprising: a feedback information reception unit for receiving information fed back from a terminal; a codebook conservation unit for forming a codebook with phase offset by assigning phase offsets to certain pre-coding matrices in a basic codebook, or for storing the basic codebook and the codebook with phase offset in advance; and a sender unit for pre-coding data to be transmitted, and sending the pre-coded data to the terminal via the antenna array of the base station. In the above base station and the wireless communication system according to the present disclosure, the antenna arrays in each of base stations performing downlink CoMP transmission contain the same number of antennas, and a first antenna array located in a first base station uses the basic codebook, while other antenna arrays located in other base stations use the codebook with phase offset. In the above base station and the wireless communication system according to the present disclosure, each of the antenna arrays contains two antennas, and the basic codebook is a two-antenna codebook defined in the LTE standard Rel-8 version. In the above base station and the wireless communication system according to the present disclosure, each of the antenna arrays contains four antennas, and the basic codebook is a four-antenna codebook defined in the LTE standard Rel-8 version. In the above base station and the wireless communication system according to the present disclosure, a first antenna array in a first base station of the multiple base stations performing downlink CoMP transmission contains two antennas, while other antenna arrays in other base stations all contain four antennas. The basic codebook is a four-antenna codebook defined in the LTE standard Rel-8 version, and the codebook with phase offset is used for the other antenna arrays, while a two-antenna codebook defined in the LTE standard Rel-8 version is used for the first antenna array. In the above base station and the wireless communication system according to the present disclosure, a first antenna array in a first base station of the multiple base stations performing downlink CoMP transmission contains four antennas, while other antenna arrays in other base stations each contain two antennas. The basic codebook is a two-antenna codebook defined in the LTE standard Rel-8 version, and wherein the codebook with phase offset is used for the other antenna arrays, while a four-antenna codebook defined in the LTE standard Rel-8 version is used for the first antenna array. In the above base station and the wireless communication system according to the present disclosure, the basic codebook is a Discret Fourier Transform based three-bit four-antenna codebook: and the codebook with phase offset is In the above base station according to the present disclosure, the basic codebook is a four-antenna codebook defined in the LTE standard Rel-8 version: and the codebook which phase offset is 0 0 0 0 180 180 180 180 0 0 0 0 180 180 180 180 0 90 180 -90 -135 -45 45 135 0 90 180 -90 180 180 0 0 0 180 0 180 -90 90 -90 90 180 0 180 0 180 0 180 0 0 -90 180 90 -45 -135 135 45 180 90 0 -90 0 180 180 0 According to a further aspect of the present disclosure, there is provided a wireless communication system by using a codebook for communication, the wireless communication system including a base station and a terminal, the base station being configured with two antenna arrays. The base station comprises: a feedback information reception unit for receiving information fed back from the terminal; a codebook conservation unit for forming a codebook with phase offset by assigning phase offsets to certain pre-coding matrices in a basic codebook, or for storing a codebook obtained by combination of the basic codebook and the codebook with phase offset in advance, wherein the two antenna arrays correpond to the basic codebook and the codebook with phase offset respectively; and a sender unit for pre-coding data to be transmitted, and sending the pre-coded data to the terminal via the antenna arrays. The terminal comprises: a codebook conservation unit for forming a codebook with phase offset by assigning phase offsets to certain pre-coding matrices in a basic codebook, or for storing the basic codebook and the codebook with phase offset in advance; and an information feedback unit for feeding back indexes identifying the pre-coding matrices in the basic codebook and the codebook with phase offset to the antenna arrays in the base station. According to a further aspect of the present disclosure, there is provided a base station by using a codebook for communication, the base station being configured with two antenna arrays, the base station comprising: a feedback information reception unit for receiving information fed back from a terminal; a codebook conservation unit for forming a codebook with phase offset by assigning phase offsets to certain pre-coding matrices in a basic codebook, or for storing a codebook obtained by combination of the basic codebook and the codebook with phase offset in advance, wherein the two antenna arrays correspond to the basic codebook and the codebook with phase offset respectively; and a sender unit for pre-coding data to be transmitted, and sending the pre-coded data to the terminal via the antenna arrays. In the above base station according to the present disclosure, the basic codebook is a Discret Fourier Transform based three-bit four-antenna codebook: and the codebook with phase offset is In the above base station according to the present disclosure, the basic codebook is a four-antenna codebook defined in the LTE standard Rel-8 version: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 90 180 -90 45 135 -135 -45 0 90 180 -90 0 0 180 180 0 180 0 180 90 -90 90 -90 180 0 180 0 0 180 0 180 0 -90 180 90 135 45 -45 -135 180 90 0 -90 180 0 0 180 and the codebook which phase offset is 0 0 0 0 180 180 180 180 0 0 0 0 180 180 180 180 0 90 180 -90 -135 -45 45 135 0 90 180 -90 180 180 0 0 0 180 0 180 -90 90 -90 90 180 0 180 0 180 0 180 0 0 -90 180 90 -45 -135 135 45 180 90 0 -90 0 180 180 0 According to a further aspect of the present disclosure, there is provided a wireless terminal, comprising: a codebook conservation unit for forming a codebook with phase offset by assigning phase offsets to certain pre-coding matrices in a basic codebook, or for storing the basic codebook and the codebook with phase offset in advance; and an information feedback unit for feeding back indexes identifying the pre-coding matrices in the basic codebook and the codebook with phase offset to at least one base station. In the above wireless terminal according to the present disclosure, the basic codebook is a two-antenna codebook defined in the LTE standard Rel-8 version. In the above wireless terminal according to the present disclosure, the basic codebook is a four-antenna codebook defined in the LTE standard Rel-8 version. In the above wireless terminal according to the present disclosure, the antenna arrays in each of base stations performing downlink CoMP transmission contain the same number of antennas, and a first antenna array located in a first base station uses the basic codebook, while other antenna arrays located in other base stations use the codebook with phase offset. In the above wireless terminal according to the present disclosure, each of the antenna arrays contains two antennas, and the basic codebook is a two-antenna codebook defined in the LTE standard Rel-8 version. In the above wireless terminal according to the present disclosure, each of the antenna arrays contains four antennas, and the basic codebook is a four-antenna codebook defined in the LTE standard Rel-8 version. In the above wireless terminal according to the present disclosure, a first antenna array in a first base station of the multiple base stations performing downlink CoMP transmission contains two antennas, while other antenna arrays in other base stations all contain four antennas. The basic codebook is a four-antenna codebook defined in the LTE standard Rel-8 version, and the codebook with phase offset is used for the other antenna arrays, while a two-antenna codebook defined in the LTE standard Rel-8 version is used for the first antenna array. In the above wireless terminal according to the present disclosure, a first antenna array in a first base station of the multiple base stations performing downlink CoMP transmission contains four antennas, while other antenna arrays in other base stations each contain two antennas. The basic codebook is a two-antenna codebook defined in the LTE standard Rel-8 version, and wherein the codebook with phase offset is used for the other antenna arrays, while a four-antenna codebook defined in the LTE standard Rel-8 version is used for the first antenna array. In the above wireless terminal according to the present disclosure, the basic codebook is a four-antenna codebook defined in the LTE standard Rel-8 version, wherein respective base stations of the base station side are all configured with one antenna array, and the information feedback unit feeds back an index identifying a pre-coding matrix in a two-antenna codebook defined in the LTE standard Rel-8 version to a first antenna array in a first base station of the base station side, and feeds back indexes identifying the pre-coding matrices in the codebook with phase offset to other antenna arrays in other base stations other than the first base station of the base station side. In the above wireless terminal according to the present disclosure, the basic codebook is a two-antenna codebook defined in the LTE standard Rel-8 version, wherein respective base stations of the base station side are all configured with one antenna array, and the information feedback unit feeds back an index identifying a pre-coding matrix in a four-antenna codebook defined in the LTE standard Rel-8 version to a first antenna array in a first base station of the base station side, and feeds back indexes identifying the pre-coding matrices in the codebook with phase offset to other antenna arrays in other base stations other than the first base station of the base station side. In the above wireless terminal according to the present disclosure, the basic codebook is a Discret Fourier Transform based three-bit four-antenna codebook: and the codebook with phase offset is In the above wireless terminal according to the present disclosure, the basic codebook is a four-antenna codebook defined in the LTE standard Rel-8 version: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 90 180 -90 45 135 -135 -45 0 90 180 -90 0 0 180 180 0 180 0 180 90 -90 90 -90 180 0 180 0 0 180 0 180 0 -90 180 90 135 45 -45 -135 180 90 0 -90 180 0 0 180 and the codebook which phase offset is 0 0 0 0 180 180 180 180 0 0 0 0 180 180 180 180 0 90 180 -90 -135 -45 45 135 0 90 180 -90 180 180 0 0 0 180 0 180 -90 90 -90 90 180 0 180 0 180 0 180 0 0 -90 180 90 -45 -135 135 45 180 90 0 -90 0 180 180 0 According to a further aspect of the present disclosure, there is provided a method of generating codebook in a wireless communication system with a plurality of antenna arrays, comprising steps of: providing a basic codebook which cotains two sets of base vectors, respective base vectors of each set of base vectors being orthogonal with each other; acquring vectors being orthogonal with at least one base vector in a second set of base vectors; making a vector whose correlation with one base vector of a first set of base vectors is the highest among the acquired vectors be a new base vector; generating other new base vectors; and replacing the base vectors with the new base vectors respectively to form a new basic codebook. According to a further another aspect of the present disclosure, there is provided a method of generating a codebook in a wireless communication system, the method comprising: a setting step of setting a basic codebook comprising a plurality of base vectors which are grouped into a plurality of sets, respective base vectors in each set of base vectors being orthogonal with each other; a computation step of computing a new base vector corresponding to one base vector in one set of the plurality of sets of base vectors, so that the new base vector equals to another base vector in other sets of base vectors other than the one set of base vectors, and repeating the computation step to acquire a plurality of new base vectors corresponding to the plurality of base vectors; and a generation step of generating a new basic codebook using the respective new base vectors. In the above method according to the present disclosure, the plurality of base vectors in the basic codebook are Wm , which are DFT vectors, and differences between directions of base vectors Wm and wm „ , +.n are n times of 11.25° in turn,? where m and n are integer numbers in following embodiments and claims, although they are also possible to be real numbers. In the above method according to the present disclosure, the obtained new base vectors are ' . ' is also DFT vectors but the directions of which are different from the corresponding base vectors Wherein w 'm = w m+n , for example, it is possible to havew' m = m+ , or w 'm = m+2 , or etc. In the above method according to the present disclosure, the form of a rank 2 pre-coding vector in the new basic codebook is and/or wherein used to precode the signals in different layers sent from a base station respectively, are used to precode the signals in different layers sent from the base station respectively. According to a further another aspect of the present disclosure, there is provided a base station for performing communication using a codebook, the base station comprising: a feedback information reception unit for receiving information fed back from a terminal; a codebook conservation unit for saving a basic codebook and/or a new basic codebook, the basic codebook comprising a plurality of base vectors which are grouped into a plurality of sets, respective base vectors in each set of base vectors being orthogonal with each other, the new basic codebook being obtained in a way of: computating a new base vector corresponding to one base vector in one set of the plurality of sets of base vectors, so that the new base vector equals to another base vector in other sets of base vectors other than the one set of base vectors, and repeating the above operation to acquire a plurality of new base vectors corresponding to the plurality of base vectors, and generating the new basic codebook using the respective new base vectors; and a sender unit for pre-coding data to be transmitted by using the basic codebook and/or the new basic codebook based on the information fed back from the terminal, and sending the pre-coded data to the terminal via an antenna array. In the above base station according to the present disclosure, the plurality of base vectors in the basic codebook are Wm , which are DFT vectors, and differences between directions of base vectors Wm and wm+ .n are n times of 11.25° in turn,? where m and n are integer numbers in following embodiments and claims, although they are also possible to be real numbers. In the above base station according to the present disclosure, the obtained new base vectors are 'm . 'm is also DFT vectors but the directions of which are different from the corresponding base vectors wm . Wherein w 'm = w m+n , for example, it is possible to havew' w m+ , r w w m+2 , r w w + etc. In the above base station according to the present disclosure, the form of a rank 2 pre-coding vector in the new basic codebook is and/or w . - w are used to precode the signals in different layers sent from a base station respectively, are used to precode the signals in different layers sent from the base station respectively. According to a further another aspect of the present disclosure, there is provided a terminal for performing communication using a codebook, the terminal comprising: a codebook conservation unit for saving a basic codebook and/or a new basic codebook, the basic codebook comprising a plurality of base vectors which are grouped into a plurality of sets, respective base vectors in each set of base vectors being orthogonal with each other, the new basic codebook being obtained in a way of: computating a new base vector corresponding to one base vector in one set of the plurality of sets of base vectors, so that the new base vector equals to another base vector in other sets of base vectors other than the one set of base vectors, and repeating the above operation to acquire a plurality of new base vectors corresponding to the plurality of base vectors, and generating the new basic codebook using the respective new base vectors; and a information feedback unit for feeding back indexes identifying pre-coding matrixes in the basic codebook and/or the new basic codebook to a base station. In the above terminal according to the present disclosure, the plurality of base vectors in the basic codebook are Wm , which are DFT vectors, and differences between directions of base vectors Wm and m+n are n times of 11.25° in turn,? where m and n are integer numbers in following embodiments and claims, although they are also possible to be real numbers. In the above terminal according to the present disclosure, the obtained new base vectors are 'm . 'm is also DFT vectors but the directions of which are different from the corresponding base vectors wm . Wherein w 'm = w m+n , for example, it is possible to have w 'm = w +,l , or w 'm = m+2 ,? or w 'm = m+3 ,? etc. In the above terminal according to the present disclosure, the form of a rank 2 pre-coding vector in the new basic codebook is wherein precode the signals in different layers sent from in different layers sent from the base station respectively. By applying the method of generating a codebook with phase offset from a basic codebook provided by the present disclosure, and by using the generated codebook in a wireless communication system, base stations and terminals, the phase offsets between the antenna arrays may be fed back without increasing the size of the codebook, and the spatial granularity of the codebook is not reduced. Therefore, compared to the methods employed in the prior art, the present disclosure can effectively improve the precision of feedback for a multi-antenna array with the same feedback overhead. BRIEF DESCRIPTION OF THE DRAWINGS These and/or other aspects and advantages of the present disclosure will become more distinct and easier to be understand from a detailed description of embodiments of the present disclosure in combination with attached drawings below, in which: Fig.l shows an exemplary systematic framework for downlink CoMP transmission system; Fig. shows a schematic diagram for applying a codebook generation method of an embodiment of the present disclosure under an environment in which two base stations take part in a collaborative downlink transmission and each of the base stations employs an antenna array containing two antennas to perform signal transmission; Fig.3 shows a flow chart of a codebook generation method according to a first embodiment of the present disclosure; Fig.4 shows a schematic diagram for applying a codebook generation method of an embodiment of the present disclosure under an environment in which two base stations take part in a collaborative downlink transmission and each of the base stations employs an antenna array containing four antennas to perform signal transmission; Fig. 5 shows a schematic diagram for applying a codebook generation method of an embodiment of the present disclosure under an environment in which multiple base stations take part in a collaborative downlink transmission and each of the base stations employs an antenna array containing two antennas to perform signal transmission; Fig. 6 shows a configuration for eight dual-polarized antennas; Fig. 7 shows a structural block diagram of a base station using a codebook generated according to the codebook generation method of the first embodiment of the present disclosure to perform communication; Fig. 8 shows a structural block diagram of a wireless terminal using a codebook generated according to the codebook generation method of the first embodiment of the present disclosure to perform communication; Fig. 9 shows an exemplary system of a multi-user transmission system; Fig. 10 shows the grouped orthogonal property of base vectors in a DFT based three-bit four-antenna codebook; Fig. 11 shows the orthogonal property of the pre-coding vector corresponding to PMIO in a four-bit codebook for eight dual-polarized antennas according to a sixth embodiment; Fig. 12 shows the orthogonal property of the pre-coding vector corresponding to PMIO in a four-bit codebook for eight dual-polarized antennas according to an eighth embodiment; and Fig. 13 is a flow chart of a method of generating codebook in a wireless communication system of multiple antenna arrays according to another embodiment of the present disclosure. DESCRIPTION OF THE EMBODIMENTS Some specific embodiments of the present disclosure will be described in detail in combination with attached drawings below. If the detailed description of some related prior art may confuse the main points of the disclosure, the detailed description thereof will not be provided here. In respective embodiments, identical reference numerals are used to denote elements or units performing the same functions.