4,661 research outputs found
Fractional Pilot Reuse in Massive MIMO Systems
Pilot contamination is known to be one of the main impairments for massive
MIMO multi-cell communications. Inspired by the concept of fractional frequency
reuse and by recent contributions on pilot reutilization among non-adjacent
cells, we propose a new pilot allocation scheme to mitigate this effect. The
key idea is to allow users in neighboring cells that are closest to their base
stations to reuse the same pilot sequences. Focusing on the uplink, we obtain
expressions for the overall spectral efficiency per cell for different linear
combining techniques at the base station and use them to obtain both the
optimal pilot reuse parameters and the optimal number of scheduled users.
Numerical results show a remarkable improvement in terms of spectral efficiency
with respect to the existing techniques.Comment: Paper presented at the IEEE ICC 2015 Workshop on 5G & Beyond -
Enabling Technologies and Application
Area spectral efficiency of soft-decision space–time–frequency shift-keying-aided slow-frequency-hopping multiple access
Slow-frequency-hopping multiple access (SFHMA) can provide inherent frequency diversity and beneficially randomize the effects of cochannel interference. It may also be advantageously combined with our novel space-time–frequency shift keying (STFSK) scheme. The proposed system’s area spectral efficiency is investigated in various cellular frequency reuse structures. Furthermore, it is compared to both classic Gaussian minimum shift keying (GMSK)-aided SFHMA and GMSK-assisted time- division/frequency-division multiple access (TD/FDMA). The more sophisticated third-generation wideband code-division multiple access (WCDMA) and the fourth-generation Long Term Evolution (LTE) systems were also included in our comparisons. We demonstrate that the area spectral efficiency of the STFSK-aided SFHMA system is higher than the GMSK-aided SFHMA and TD/FDMA systems, as well as WCDMA, but it is only 60% of the LTE system
Control and data channel resource allocation in OFDMA heterogeneous networks
This paper investigates the downlink resource allocation problem in Orthogonal Frequency Division Multiple Access (OFDMA) Heterogeneous Networks (HetNets) consisting of macro cells and small cells sharing the same frequency band. Dense deployment of small cells overlaid by a macro layer is considered to be one of the most promising solutions for providing hotspot coverage in future 5G networks. The focus is to devise an optimised policy for small cells’ access to the shared spectrum, in terms of their transmissions, in order to keep small cell served users sum data rate at high levels while ensuring that certain level of quality of service (QoS) for the macro cell users in the vicinity of small cells is provided. Both data and control channel constraints are considered, to ensure that not only the macro cell users’ data rate demands are met, but also a certain level of Bit Error Rate (BER) is ensured for the control channel information. Control channel reliability is especially important as it holds key information to successfully decode the data channel. The problem is addressed by our proposed linear binary integer programming heuristic algorithm which maximises the small cells utility while ensuring the macro users imposed constraints. To further reduce the computational complexity, we propose a progressive interference aware low complexity heuristic solution. Discussion is also presented for the implementation possibility of our proposed algorithms in a practical network. The performance of both the proposed algorithms is compared with the conventional Reuse-1 scheme under different fading conditions and small cell loads. Results show a negligible drop in small cell performance for our proposed schemes, as a trade-off for ensuring all macro users data rate demands, while Reuse-1 scheme can even lead up to 40 % outage when control region of the small cells in heavily loaded
Uplink Sounding Reference Signal Coordination to Combat Pilot Contamination in 5G Massive MIMO
To guarantee the success of massive multiple-input multiple-output (MIMO),
one of the main challenges to solve is the efficient management of pilot
contamination. Allocation of fully orthogonal pilot sequences across the
network would provide a solution to the problem, but the associated overhead
would make this approach infeasible in practical systems. Ongoing
fifth-generation (5G) standardisation activities are debating the amount of
resources to be dedicated to the transmission of pilot sequences, focussing on
uplink sounding reference signals (UL SRSs) design. In this paper, we
extensively evaluate the performance of various UL SRS allocation strategies in
practical deployments, shedding light on their strengths and weaknesses.
Furthermore, we introduce a novel UL SRS fractional reuse (FR) scheme, denoted
neighbour-aware FR (FR-NA). The proposed FR-NA generalizes the fixed reuse
paradigm, and entails a tradeoff between i) aggressively sharing some UL SRS
resources, and ii) protecting other UL SRS resources with the aim of relieving
neighbouring BSs from pilot contamination. Said features result in a cell
throughput improvement over both fixed reuse and state-of-the-art FR based on a
cell-centric perspective
Advanced Coordinated Beamforming for the Downlink of Future LTE Cellular Networks
Modern cellular networks in traditional frequency bands are notoriously
interference-limited especially in urban areas, where base stations are
deployed in close proximity to one another. The latest releases of Long Term
Evolution (LTE) incorporate features for coordinating downlink transmissions as
an efficient means of managing interference. Recent field trial results and
theoretical studies of the performance of joint transmission (JT) coordinated
multi-point (CoMP) schemes revealed, however, that their gains are not as high
as initially expected, despite the large coordination overhead. These schemes
are known to be very sensitive to defects in synchronization or information
exchange between coordinating bases stations as well as uncoordinated
interference. In this article, we review recent advanced coordinated
beamforming (CB) schemes as alternatives, requiring less overhead than JT CoMP
while achieving good performance in realistic conditions. By stipulating that,
in certain LTE scenarios of increasing interest, uncoordinated interference
constitutes a major factor in the performance of CoMP techniques at large, we
hereby assess the resilience of the state-of-the-art CB to uncoordinated
interference. We also describe how these techniques can leverage the latest
specifications of current cellular networks, and how they may perform when we
consider standardized feedback and coordination. This allows us to identify
some key roadblocks and research directions to address as LTE evolves towards
the future of mobile communications.Comment: 16 pages, 6 figures, accepted to IEEE Communications Magazin
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