Typically, these systems use MIMO schemes of 2×2 and 4×4. The 8×8 scheme is more rare, used in some turbo or advanced modes. The principal behind the technology is quite simple: using more antennas means more possible signal paths (and associated space-time complexity) and better quality of service (QoS) and link efficiency in terms of data rates. These advantages are significant but they definitely have associated costs:
• Increased cost and complexity of the radio head designs (multiple RF paths)
• Increased cost, power consumption and complexity of the digital signal processing designs
Another big challenge with MIMO systems, imposed by basic physics, is the fact that antennas must be separated by a minimum distance of half a wavelength in order to avoid mutual coupling between them as well as avoid excessive correlation between signal envelopes. For example, antennas in today’s wireless 4G systems should be separated by around 8 cm – making it very hard to squeeze multiple antennas into a smart phone!
New wireless technology like millimeter wave carrier frequencies can address this challenge. By pushing up the radio signal up to around 60 GHz, for example, the minimum spacing between antenna elements is reduced to around 2.5 mm. With such spacing requirements, it’s easy to imagine portable devices with large-scale MIMO schemes.
Additionally, new research in “massive MIMO technology” has demonstrated that there are considerable gains (QoS/link efficiency) when using a very large number (greater than 40) of service antennas. These new techniques not only use many antennas but also use time division duplex operation, which drastically reduces resource allocation by allowing each active terminal to utilize all the time-frequency bins .
Massive MIMO technologies 
A next-generation wireless broadband standard, called WiGig (IEEE 802.11ad), aims to achieve a high QoS and efficient link data rate of up to 7 Gbps by using a 60 GHz unlicensed carrier and massive MIMO techniques.
Implementation of massive MIMO complex algorithms is now possible using Nutaq’s MicroTCA FPGA-based scalable software-defined radio (SDR) systems (TitanMIMO-4, PicoSDR, uDigitizer, PicoDigitizer), where an almost infinite RF channel count is possible. Researchers also benefit from the Nutaq model-based design integrated Simulink tools, where the simulation, programming and deployment of complex FPGA-based applications is accelerated and simplified. The transition from m-based code algorithm simulation files to deployable FPGA image code can be accomplished without the need to know FPGA and VHDL programming. Additionally, all data communication interfaces, RF programming modes, calibration and synchronization aspects are embedded and pre-tested with the solutions.