The pilot contamination problem is one of the major obstacles that limit the performance of time-division duplex (TDD) multi-cell massive multiple-input multiple-output (MIMO) systems. Pilot contamination results from the re-use of the same set of pilot sequences in the different cells of the system. In this paper, we compare between two different scenarios of pilot signals allocation with respect to the impact of pilot contamination. We derive lower bounds on the achievable rates and study the performance of both scenarios under different system settings. Our results show that although increasing the number of base
station (BS) antennas improves the system performance, it does not eliminate the effect of pilot contamination. Thus, when the pilot contamination is high, it should be countered by allocating more system resources for the training phase. This can be achieved by increasing the number of pilot sequences to guarantee an orthogonal pilot sequence for each user in the system. Further, we show that the pilot sequences allocation strategy also depends on the characteristics of the communication environment: a low mobility environment has better performance
when orthogonal sequences are allocated to all users while the opposite is true for a high mobility environment. Index Terms—LSAS, massive MIMO, multicell MIMO, pilot contamination, pilot allocation.