In known MIMO systems, there is a desire to exploit the multi-path capabilities of the system to enhance the system capacity. One way to exploit the multi-path capabilities of a MIMO system is to incorporate multiple antennas or multi-band antennas at both the transmitter and receiver. That is, a transmitter sends multiple beams from multiple transmit antennas, and the beams are received by multiple receive antennas at a receiver.
It is desirable for the beams sent from the transmit antennas in a MIMO system to be wide. Accordingly, it has been necessary for known MIMO systems to include antennas or multi-band antennas spaced at a predetermined distance apart from one another. Such separation between the antennas prevents interference between the beams and prevents band-to-band coupling between beams from antennas operating at different frequencies.
However, due to space and size constraints, it may be desirable to place antennas of a MIMO system in close proximity to one another. For example, a base for the antennas may be of a limited size. In such a situation, it would be desirable to maintain the wide beam of the antennas while still preventing interference and band-to-band coupling between the antenna beams.
Known antennas placed within close proximity to one another in a MIMO system present several disadvantages. First, mutual surface radiation from the antennas can couple with each other. Additionally, when the antennas are elevated above a large ground reflector, a small antenna base can defocus the reflection of the main beam radiation. Finally, the low isolation between antennas can introduce signal interference.
Accordingly, there is a continuing, ongoing need for an antenna that can be used in connection with a MIMO system and placed within close proximity to a second antenna. Preferably, such an antenna is a high isolation multi-band monopole antenna.