QoS Constrained Pilot Allocation Scheme for Massive MIMO Systems

Ant colony‐based strategy to mitigate the pilot contamination problem in multicell massive multiple‐input multiple‐output systems

“…A set of antennas are deployed in each BS, while each UE is deployed with a single antenna. By adopting the independent channel model, 10,12 the channel vector that links a UE k of cell l to the BS of cell j can be expressed as where refers to the small‐scale‐fading (SSF) vector, which is frequency‐dependent and statistically independent for the K users, and it is characterized with a complex Gaussian distribution of zero‐mean vector and an identity covariance matrix , that is, . On the other hand, the large‐scale fading (LSF) coefficient is denoted and it is, typically, frequency independent.…”

“…A set of M ≫ K antennas are deployed in each BS, while each UE is deployed with a single antenna. By adopting the independent channel model, 10,12 the channel vector g j,kl ∈  M×1 that links a UE k of cell l to the BS of cell j can be expressed as…”

“…To pave the way to the upcoming next five generation, it was necessary to eliminate the PC problem from the mMIMO technology. Accordingly, several strategies have been proposed to deal with this issue 6‐19 . The harmful influence of the PC problem was analyzed in the work of Reference 7, where the pilot sequences are designed for each UE based on its specific requirements.…”

“…This through assigning OPSs to the UEs of low SINR; hence, the overall system throughput is significantly enhanced. A greedy algorithm is provided in the work of Reference 10, which aims to boost the per‐cell SE, unfortunately, at the expense of some UEs of low performance. In Reference 11, the UEs within cells are assigned the available OPSs based on their SINRs, where edge UEs of the adjacent cells, which are supposed to have a low SINRs, are assigned with OPSs, whereas the center UEs of all cells are compelled to reuse the same set of the OPSs; hence, the impact of the PC can be significantly reduced.…”

“…Accordingly, several strategies have been proposed to deal with this issue. [6][7][8][9][10][11][12][13][14][15][16][17][18][19] The harmful influence of the PC problem was analyzed in the work of Reference 7, where the pilot sequences are designed for each UE based on its specific requirements. These requirements were assumed to be available at the supported BSs; however, it was shown that this approach has some loopholes such as the problem of the nonzero cross-correlation between different pilot sequences, which can be exploited by an active attacker to reduce the capacity achieved by the UEs.…”