Over the last few years, Multiple-Input/Multiple-Output (MIMO) technology has been regarded as a key leverage that wireless service providers can exploit for boosting mobile network capacity. The basic theory of MIMO relies on the usage of multiple antenna elements at both the radio link ends in order to fulfill the transmission and reception of parallel data streams. This is generally referred as “spatial multiplexing gain.”
From a performance perspective, MIMOtechnology finds a perfect field of application in in-building solutions such as distributed antenna systems (DAS). In particular, several field measurements have demonstrated that capacity improvements versus single antenna schemes are quite substantial. The other side of the coin is the equipment and installation costs associated to in-building MIMO DAS deployments, which now have to support multiple RF branches. In practice, a 2x2 MIMO DAS requires double the infrastructure in terms of antennas, cabling and active equipment.
CommScope’s Distributed Coverage and Capacity Solutions team has been a pioneer in investigating alternative and more cost-effective solutions for MIMO DAS designs such as Interleaved MIMO. In a typical in-building design, the user is often close to two or more antennas at a time. This allows for interlacing MIMO paths to these antennas and actually implementing spatially separated MIMO branches.
It has been demonstrated that performance comparable with traditional “co-located MIMO” schemes can be achieved as long as the signal levels between the two paths do not exceed a specific limit. In this manner, the total bill of material of MIMO DAS projects could be reduced leading to a good cost-performance tradeoff.
LTE specs currently have a gap for Interleaved MIMO in the sense that there are no defined conformance tests for mobile terminals in order to support this alternative MIMO scheme. CommScope has actively promoted Interleaved MIMOwithin 3GPPas an effective solution to address the cost concerns of wireless service providers.