Massive MIMO and small cells: How to densify heterogeneous networks

Abstract

Abstract-We propose a time division duplex (TDD) based network architecture where a macrocell tier with a "massive" multiple-input multiple-output (MIMO) base station (BS) is overlaid with a dense tier of small cells (SCs). In this context, the TDD protocol and the resulting channel reciprocity have two compelling advantages. First, a large number of BS antennas can be deployed without incurring a prohibitive overhead for channel training. Second, the BS can estimate the interference covariance matrix from the SC tier which can be leveraged for downlink precoding. In particular, the BS designs its precoding vectors to transmit independent data streams to its users while being orthogonal to the subspace spanned by the strongest interference directions; thereby minimizing the sum interference imposed on the SCs. In other words, the BS "sacrifices" some of its antennas for interference cancellation while the TDD protocol allows for an implicit coordination across both tiers. Simulation results suggest that, given a sufficiently large number of BS antennas, the proposed scheme can significantly improve the sum-rate of the SC tier at the price of a small macro performance loss.

Degrees of freedom of uplink-downlink multiantenna cellular networks

“…Under various practical scenarios, potential benefits of reverse TDD have been analyzed in the context of coverage [46], area spectral efficiency [46], [47], throughput [48], [50], and so on.…”

Pricing based interference control in reversed time division duplex heterogeneous networks

“…To avoid the heavy signalling on training the channels between the tier-1 base stations (BSs) and the tier-2 BSs in conventional time division duplex (TDD) systems, reversed TDD protocol has been proposed in [3], where the tier-2 BSs perform downlink transmissions while the tier-1 users perform uplink transmissions. On the other hand, it is expensive or even infeasible to establish wired backhaul links from the tier-2 BSs (the smaller BSs) to the tier-1 BSs as cellular networks become more densified, so that wireless backhaul has to be considered in certain scenarios [4].…”

Energy efficient power control for the two-tier networks with small cells and massive MIMO

“…Although small cells and massive MIMO have been compared in [2], they are not necessarily rivals. Actually, a two-tier network architecture that incorporates the small cells and massive MIMO can attain the benefits of both technologies [3], [4]. In the two-tier network architecture, massive MIMO ensures full-area coverage, while small cells mainly serve indoor and outdoor hotspots.…”