I prefer the firstone...

ok, but I want to know the open problems in these topics, do you know any paper or idea for these subjects?

thanks

I think in the first step it is better to categorize each of them (massive MIMO and mmWave) to some subcategories and then obtain a general understanding of each.

Then in the next step you can go deeper on the topic which is more interesting for you or you have more proficiency on it. I can suggest the followings for massive MIMO:

1. Precoding and Detection

2. Pilot Contamination

3. Energy Efficiency, Spectral Efficiency, Resource Allocation

4. Security Issues

5. Its combination with:

5.1. HetNet

5.2. Energy Harvesting

5.3. mmWave

5.4. D2D 

6. TDD reciprocity and calibration or FDD concepts and solutions

7. Architecture Issues: Distributed, Centralized, Cooperation ..

8. Performance Analysis, Hardware and Implementation Issues

Also check the events dedicated to Massive MIMO or mmWave, Like last year workshop on massive MIMO at GLOBECOM

If you are interested in massive MIMO, you can find a list of open problems in Part 4 of my tutorial from the ICC 2015 conference:

http://www.commsys.isy.liu.se/~ebjornson/presentation_ICCtutorial.pdf

I want to keep Performance enhancement of massive mimo systems as the topic for my PhD thesis.Energy Efficiency, Spectral Efficiency, Resource Allocation will be   the focus area of my work.Can anyone help me in identifying the base paper?