Efficient and low latency handover algorithms are required to exploit the massive MIMO. Algorithm should account the availability of antennas, beams, various types of cells (femto, pico, etc.), modulation schemes, bandwidth, and consider the mobile vehicle speed. Handover should not be a constraint to achieve 10 times improvement in capacity with massive MIMO.
Is handover is implemented along with massive MIMO ?
I think that the handover mechanism can be implemented in similar ways as in contemporary systems. If something needs to be changed, it will probably lead to simplification of the algorithms since the massive MIMO base stations can handle very many users and will seldom be overloaded.
But this is only my guess. I don't think that is has been analyzed much, so it might be a good research problem.
I wrote a paper called "Massive MIMO and Small Cells: Improving Energy Efficiency by Optimal Soft-Cell Coordination" (http://arxiv.org/pdf/1304.0553) some years ago. This paper touches upon the base station allocation problem, but did not discuss how to implement the handover mechanism in practice.
Another interesting question (one of many I expect to see on Massive MIMO--need a new acronym for that, btw. MMIMO? M-MIMO?). If you imagine the expected level of control over the channel condition that the massive MIMO base station has, the question of when it becomes necessary/viable to do a handover to another (presumably massive MIMO) base station becomes quite fuzzy. The continuous adaptation that the base station is doing to maintain the channel is constantly altering the "good" vs. "bad" condition.
On the other hand, if we consider the SNR constraints of MIMO, vs. typical handover scenarios, the mobile will likely not actually be in a MIMO condition, but rather in a transmit diversity (e.g. TM2 TXD) condition, since the base stations will need to be far enough apart to minimize interference. I'm not exactly sure what the fallback condition looks like on massive MIMO, but I'd expect it to support transmit diversity with beam forming in order to maintain the same mobile density regardless of the condition. Thus, the handover scenario would likely occur when the mobile sees the opportunity to move from TM2 on one base station to MIMO operation on another.
Try doing a search for "millimeter wave handover", as the term is almost synonymous with massive mimo.
Perhaps you can elaborate on the ways that you think that handover in massive MIMO will be different from handover in, say, LTE?
Small cell size is required for Pilot contamination reduction.
I don't think that small cell sizes will help much towards pilot contamination since it depends on the ratio between the strength of the desired signal and the interfering signals, which both increase by making the cells smaller. Fractional pilot reuse is a more effective way to reduce pilot contamination.
There exist handover (Handoff) for 4G and 5G. It is divided into two types, vertical and horizonal in hetregeneous networks. It is under research subject for researchers.
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