Large MIMO systems, often referred to as massive MIMO systems, can be defined as those that use tens or hundreds of antennas in the communication terminals.
Traditional MIMO systems may have two or four, some may even have eight antennas, but this has been the limit on early systems that have adopted MIMO.
The concept of massive MIMO or large MIMO systems is entering many areas of development as it is able to offer some distinct advantages.
Massive MIMO benefits
There are many advantages to using large MIMO technology. Using more antennas in a MIMO system creates more degrees of freedom in the spatial domain and therefore this enables greater improvement in performance to be achieved:
- Increasing data rate: The increase in the number of antennas allows for a greater number of paths to be used and hence a much greater level of data to be transferred within a given time.
- Increasing basic link signal to noise ratio: One of the basic advantages of the use of MIMO systems is that it can be used to improve the signal to noise ratio of the overall system. The use of large MIMO or massive MIMO enables this to be taken to a greater level. There is also an increase in hardening against intentional jamming as a result of the large diversity.
- Channel hardening: Increasing the number of antennas significantly to make a massive MIMO system means that the system becomes less sensitive to the actual entries of the channel matrix. In turn this has further advantages in the area of signal processing. It is necessary for linear detectors to perform matrix inversions and this can be done more easily within the processing as this capability increases with technology developments.
One of the key issues with any MIMO system is the placement of the antennas. For many systems using physically small units, the antenna placement can present some issues.
In order that the MIMO system is able to operate satisfactorily, the correlation between antennas must be small. As a rule of thumb, s spacing of λ/2 (where λ is the wavelength of the signal) is considered necessary to provide almost no correlation between the antennas.
In order to achieve this a variety of approaches can be taken.
- Use high frequencies: In order to be able to accommodate the higher antenna numbers required for large MIMO systems, the use of higher frequencies shortens the wavelength of the signals, thereby allowing the antenna spacing in terms of wavelengths to be accommodated within a given physical space. Many systems are considering the use of frequencies above 10 GHz, extending even as far as 60GHz and beyond and .
- Use volumetric rather than linear spacing: It is possible to use the three dimensions within an item to provide spacing within three dimensions rather than just two dimensions as in a linear fashion. Although many items, including mobile phones are often thin and therefore this approach may not be applicable, in some instances a cube will be able to accommodate more antennas by using spacing in three dimensions.
- Use of spatial modulation: The number of RF chains needed for a massive MIMO system can be reduced without compromising the spectral efficiency by using spatial modulation. Spatial modulation is a form of modulation that only requires the use of one transit chain for multiple antennas. Effectively it uses one antenna from an array at a time for transmission.
Spatial modulation adopts a simple but effective coding mechanism which establishes a one to one mapping between blocks of transmitted information bits and the spatial positions of the transmitter antenna in the overall antenna array.
Massive MIMO paths
One of the key requirements for a large MIMO system is that there is a rich diversity of signal paths between the transmitter and receiver. This is normally present within a typical indoor and most urban environments. Other environments where there are less paths will not be able to provide the same benefits with a MIMO let alone a large MIMO system as fewer paths will be available.
Inadequate spacing between the antennas will mean that they tend to correlate more and the gains of MIMO systems cannot be realised.
Another situation that can occur, even in a rich scattering environment is where all the paths pass through a pinch point. This can result in there being fewer independent spatial dimensions and accordingly the performance will be reduced.