Dear Shah Mahdi Hasan,

Massive connectivity: is a challenge that we want to overcome by three major possible solutions:

1. Efficient utilization of available spectrum available till 4G or LTE: These works would address sophisticated MAC protocols those which increase spectral efficiency by effectively utilizing what is available (like frequency selectivity of the channel, etc.): Reducing the losses that we incur by adding cyclic prefix (CP) or other guard bands

2. NOMA (non orthogonal multiple access): Better utilization of existing spectrum by possible intended overlapping of two or more users data to the same frequency in power domain or code domain and use of complex successive interference cancellation (SIC) receiver to separate the overlapped data. Huge gains in spectral efficiency and fairness with increased complexity.

3: mmWave: New frequencies (Not till used for usual wireless) poses hardware challenges, size and power consumption

There are other things also such as Massive MIMO and small cells, the compatibility of which should again be tested for mmWave frequencies.

Specifically for uplink, synchronization is an important issue and lots of works focuses on how under imperfect synchronization, we can still maintain an error free decoding. Other category of work discusses how to maintain extreme synchronization by accurate estimation of offset (time or frequency).

Thanks and Regards,

Vinay Trivedi

IIT Patna

Dear Vinay K. Trivedi , thanks a lot for your incisive answer! Waiting for others' insightful inputs.