Since 1990s, the wireless communication industry has witnessed explosive growth. With more and more mobile voice, data, and video services, higher bandwidths are required for the wireless communication systems. Available frequency band resources, however, are gradually in short supply. Thus, how to increase the frequency spectrum utilization becomes a critical issue in the wireless communication research.
Technologies that can be used to increase the frequency spectrum utilization include multiple access, signal detection, modulation, and channel coding. The Multiple Antennas System (MAS) technology has a huge potential of increasing the frequency spectrum utilization, and plays an increasingly important role in the wireless communication field.
The Smart Antenna (SA) is a kind of MAS, and is also called an Antenna Array System (AAS). The array element spacing of the SA is generally smaller than the coherent distance of the channel. By using the signal correlation between the antenna array elements, the beams can be formed, and high-gain narrow beams can be adaptively pointed to a mobile terminal in the communication. In addition, the null can be pointed to the interference direction.
FIG. 1 shows a structure of a typical SA. The SA array of a narrow-band signal includes M (>1) antenna array elements. m is one of the M antenna array elements. M receive antennas correspond to M receive channels, and M transmit channels correspond to M transmit antennas.
For a specific user, a Direction Of Arrival (DOA) estimating module estimates the DOA information of the user according to the receive signals on the M antenna array elements. An adaptive beam forming weight coefficient generator adjusts the weight vectors of the adaptive beam forming weight coefficient generator according to the DOA information of the specific user, and generates a weighted coefficient for each transmit channel. The weight coefficient tuner of each channel adjusts (by multiplication) a dedicated channel signal s (t) by using the weight coefficient of its own channel. Then, the weight coefficients w1, w2, . . . , wM of the M antenna array elements form a weight vector w, so that a pointing beam is formed for the specific user and follows up the user movement adaptively. The asterisk “*” in FIG. 1 represents a conjugation symbol.
Multiple Input Multiple Output (MIMO) is another kind of the MAS. Foschini theoretically proves the huge potential of the MIMO in increasing the frequency spectrum utilization. The channel capacity of the MIMO linearly increases along with the quantity of antennas (in positive proportion to the minimum number of antennas of the transceiver). The MIMO technology may also be considered to be a SA. The difference between the MIMO and the SA lies in the antenna array element spacing. Non-correlation should be kept between the MIMO system antennas.
In a cellular mobile communication system, the base station allocates a dedicated channel for each active user in a cell or sector to carry voice, data or video services. The base station based on multiple antennas can use technologies such as beam forming or pre-coding on the dedicated channel to transmit signals for the specific user and reduce the interference with other users.
In a real mobile communication system, a common channel is also required in the cell or sector to carry the common information that are needed by all mobile terminals, such as system information in the broadcast channel, reference signals in the synchronization channel, and pilot information, paging information and common control information in the Forward Access Channel (FACH). The common channel and the dedicated channel have different requirements on the coverage of the base station. The common channel requires that all the mobile terminals in the cell or sector should receive signals at the same time. Thus, the base station must have good full coverage performance.
There is an antenna array cell/sector coverage solution that the transmit time of the common channel signal is divided into timeslots; a group of weight vectors with complementary antenna patterns is selected, and complementary weight vectors are used in turn in the successive timeslots. Thus, the SA-based full cell/sector coverage is realized.
Although multiple complementary weight vectors are used in turn in the foresaid solution, the quantity of weight vectors is limited and the antenna pattern corresponding to each weight vector is fixed. Thus, the average values of antenna gains of multiple antenna patterns are not equal completely, but only in approximate direction, causing big differences in the Bit Error Rate (BER) in each direction. Therefore, the full coverage performance needs to be improved.
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