Flexible Duplexing for Maximum Downlink Rate in Multi-Tier MIMO Networks

Abstract

In this paper, we propose an algorithm to maximize downlink rate performance in the context of multiple-input multiple-output (MIMO) Heterogeneous Networks (HetNets). Specifically, we evaluate the benefits of flexible duplexing, a promising strategy that consists in combining uplink and downlink cells within the same channel use. In order to handle intercell interference, we rely on the interference alignment (IA) technique, taking into account the impact of the channel estimation errors on the inter-cell interference leakage. Determining the best uplink/downlink configuration is a combinatorial problem, and therefore we consider several approaches to reduce the computational demands of the problem. First, we use a statistical characterization for the average rates achieved by IA in order to avoid the calculation of alignment solutions for all possible settings in the network. Additionally, we propose two hierarchical switching (HS) strategies so that only a subset among the total number of combinations is explored. As a performance baseline, we include in the comparison the conventional time division duplex (TDD) approach and the well-known minimum mean square error (MMSE) decoder. The obtained results show that downlink rates achieved by implementing flexible duplexing and applying inter-cell IA significantly outperform conventional TDD transmissions. Finally, the proposed hierarchical schemes are shown to obtain almost the same rates as exhaustive search with much lower computational cost.

Experimental evaluation of flexible duplexing in multi-tier MIMO networks

“…• While perfect channel knowledge is assumed in theory, channel estimation errors arise in practice. The impact of the resulting misalignment is taken into account for the theoretical studies in [9,26], as well as in the experiments discussed in [27,28].…”

“…Also, the aforementioned work takes into account traffic asymmetries for a better characterization of 5G mobile communications. Such asymmetries are included as well in [9], where following the same line as in [4,5], flexible duplexing is combined with IA for optimal downlink rate. In addition, the hierarchical switching (HS) scheme, which checks only a subset of possible uplink/downlink configurations, is proposed in order to reduce the computational cost of UL/DL selection.…”

“…In this model, a partial bandwidth overlap is allowed among uplink and downlink frequency-division duplex (FDD) slots, and the data rate is optimized subject to power constraints, so as to determine user scheduling as well. Finally, a power minimization algorithm is proposed in [13] to optimize the total transmit power for a given UL/DL combination, and then combined with the HS approach in [9] to select the best duplexing configuration.…”

Power Minimization in Multi-tier Networks with Flexible Duplexing

“…Once the Max-SINR filters are calculated, the sequence is repeated for the original SINR constraints with the new filters. We also propose a hierarchical switching (HS) scheme that reduces the computational cost of UL/DL selection significantly [10]. By means of Monte Carlo simulations, we show that the proposed technique outperforms other existing strategies in a 2-tier HetNet.…”

“…Several studies on the benefits of flexible duplexing have been recently carried out, relying on different figures of merit. Most of the existing works assume that a fixed UL/DL combination is given, and present an analysis in terms of multiplexing gain [4][5][6], or in terms of throughput [7][8][9][10][11]. A frame structure for interference mitigation can be found in [12], and flexible duplexing is further proposed as a candidate for evaluation in real deployments in [13].…”