1. Field of the Invention
The present invention relates to a wireless communication apparatus having a plurality of antennas and a method of selecting an antenna thereof.
2. Description of the Related Arts
In recent years, wireless LANs using the IEEE 802.11 series of wireless communication standards have become popular and technologies to realize higher data transmission speed have been developed. One particular wireless communication technique for realizing wide band communication and increasing the efficiency of frequency utilization, MIMO (Multiple-Input Multiple-Output), is now attracting attention; it is being standardized and is scheduled to be adopted in IEEE 802.11n specification.
In MIMO communication, both transmitting and receiving sides have a plurality of antennas, and using a plurality of paths (transmission paths) that each path is unique, the transmitting side transmits a plurality of data simultaneously multiplexed on the same frequency (a technique known as space-division multiplexing).
As stated above, data transmission using space-division multiplexing via a plurality of antennas can realize an increase of transmission rate without an increase in frequency band usage.
When information is repeatedly transmitted via a plurality of unique transmission paths, reliability may be improved without an increase in the data transmission rate.
Here data transmitted from a plurality of antennas reaches the receiving side via different respective transmission channels.
In MIMO communications, to obtain a high transmission characteristic, correlation among a plurality of transmission channels must be low. One of the factors preventing low correlation among a plurality of transmission channels is coupling among a plurality of antennas. Here, the degree (amount) of coupling is a value representing what portion of a signal transmitted from an antenna A is absorbed by another antenna B.
At present, wireless LAN products executing MIMO communication are appearing; in most of them, the mounting area of their antennas, particularly in products such as access points, is relatively large.
An example in which a plurality of antennas are mounted in a wireless communication apparatus having a relatively large mounting area, such as access point of a wireless LAN, will now be explained.
FIG. 1 shows an example in which a plurality of antennas is mounted in an access point of a wireless LAN. In FIG. 1, Reference numeral 100 represents an access point, 101 through 103 are dipole antennas used in MIMO communication. In order to minimize the degree of coupling among respective antennas, the mounting distance between antennas 101 through 103 is set to be longer than one half of one wavelength. As stated above, as the distance between antennas increases, the degree of coupling between antennas decreases; by keeping the mounting distance between antennas to be about one half of one wavelength, the correlation between transmission channels can be treated to be zero under multiple path data transmission.
FIG. 2 shows an example in which a plurality of antennas is mounted in a wireless card module. In case of a wireless card module connected to a notebook-type personal computer, antennas are mounted in a relatively small area compared with the case of antennas mounted on an access point.
In FIG. 2, reference numeral 200 represents a notebook-type personal computer; 201 represents a wireless card module; 202 and 203 represent print antenna patterns mounted on a wireless substrate by pattern printing.
The size of the substrate of a wireless card module 201 is too small to mount a plurality of antennas while maintaining a mounting distance between antennas 202 and 203 of about half of one wavelength. So, in order to minimize the degree of coupling between antennas, they are mounted such that their polarization surfaces are perpendicular to each other.
When constructed in this way, because the polarization surfaces of horizontal and vertical polarization are perpendicular to each other, even though the mounting distance is less than a half of one wavelength, it is possible to keep correlation between transmission channels small under conditions of large channel fading.
The layout method of a plurality of antennas according to the above prior art example is limited to cases where sufficient open space can be maintained in the peripheral area of the plurality of antennas so as not to affect the antenna characteristic (e.g., the input-output reflection characteristic, radiation characteristic).
Because the peripheral area is open space, the mounting distance can be sufficiently maintained and polarization surfaces can be selected to be perpendicular to each other, thereby minimizing the degree of coupling between antennas.
However, when a plurality of antennas is mounted in a small wireless apparatus, because sufficient free space cannot be maintained near the antennas due to the existence of plastic or metal material near them, it is difficult to apply the mounting method according to the prior art example described above.
FIG. 3 shows an example in which a plurality of antennas is mounted in a small wireless mobile terminal. As shown in FIG. 3, reference numeral 300 represents the enclosure of the small wireless mobile terminal; 301 through 306 represent small antennas such as a chip antenna and in this example, six antennas are mounted.
As shown in FIG. 3, when a plurality of antennas are mounted in the small wireless mobile terminal, a protruding antenna such as a rod antenna cannot be mounted due to design requirements or mechanical strength limitations. Accordingly, it is necessary to mount a plurality of antennas inside the apparatus, and therefore it is also necessary to mount the antennas close to the metal or plastic material in the small wireless mobile terminal.
If metal or plastic material exists near antenna, it is difficult to realize good antenna characteristics.
For example, it is possible that the directionality of the antenna is in a specific direction due to the metal material near the antenna or that the resonant frequency of the signal is made to deviate from that intended due to the plastic material.
Further, the orthogonal nature of the perpendicular surface cannot be maintained due to the metal material existing nearby.
Therefore, when a plurality of antennas is mounted in the small wireless mobile terminal, because the mounting distance between antennas cannot be kept longer than one half of a wavelength due to the small mounting enclosure, the degree of coupling between adjacent antennas becomes large.
Because MIMO communication executes communication via a plurality of antennas, if the degree of coupling between antennas is large, the basic conditions necessary for realizing optimum MIMO communication, that is, no correlation between respective streams, cannot be maintained.
Accordingly, it is difficult to realize the separation processing of respective streams in the receiving side causing a deterioration in BER and a loss of the high speed and reliability characteristics of MIMO.
Further, because the transmission power is lowered due to the coupling between transmission antennas, there are several disadvantages to these conditions such as a shortened communication distance or an increase in power consumption if the communication distance is maintained.