Conference Paper

Linear Pre-Coding Performance in Measured Very-Large MIMO Channels

DOI: 10.1109/VETECF.2011.6093291 Conference: Proceedings of the 74th IEEE Vehicular Technology Conference, VTC Fall 2011, 5-8 September 2011, San Francisco, CA, USA
Source: DBLP

ABSTRACT

Wireless communication using very-large multiple-input multiple-output (MIMO) antennas is a new research field, where base stations are equipped with a very large number of antennas as compared to previously considered systems. In theory, as the number of antennas increases, propagation properties that were random before start to become deterministic. Theoretical investigations with independent identically distributed (i.i.d.) complex Gaussian (Rayleigh fading) channels and unlimited number of antennas have been done, but in practice we need to know what benefits we can get from very large, but limited, number of antenna elements in realistic propagation environments. In this study we evaluate properties of measured residential-area channels, where the base station is equipped with 128 antenna ports. An important property to consider is the orthogonality between channels to different users, since this property tells us how advanced multi-user MIMO (MU-MIMO) pre-coding schemes we need in the downlink. We show that orthogonality improves with increasing number of antennas, but for two single-antenna users there is very little improvement beyond 20 antennas. We also evaluate sum-rate performance for two linear pre-coding schemes, zero-forcing (ZF) and minimum mean squared error (MMSE), as a function of the number of base station antennas. Already at 20 base station antennas these linear pre-coding schemes reach 98% of the optimal dirty-paper coding (DPC) capacity for the measured channels.

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    • "Analytical spectral efficiency approximations are derived in [9] with several linear precoders and detectors for non-cooperative multi-cell massive MISO systems using time-division duplexing (TDD) operation. There are also several other studies that deal with the performance of massive MIMO/MISO systems with linear precoders [10]–[13]. In MU massive MISO systems, the computational complexity increases with large numbers of antennas and users. "
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    ABSTRACT: In this paper, we propose trellis coded quantization (TCQ) based limited feedback techniques for massive multiple-input single-output (MISO) frequency division duplexing (FDD) systems in temporally and spatially correlated channels. We exploit the correlation present in the channel to effectively quantize channel direction information (CDI). For multiuser (MU) systems with matched-filter (MF) precoding, we show that the number of feedback bits required by the random vector quantization (RVQ) codebook to match even a small fraction of the perfect CDI signal-to-interference-plus-noise ratio (SINR) performance is large. With such large numbers of bits, the exhaustive search required by conventional codebook approaches make them infeasible for massive MISO systems. Motivated by this, we propose a differential TCQ scheme for temporally correlated channels that transforms the source constellation at each stage in a trellis using 2D translation and scaling techniques. We derive a scaling parameter for the source constellation as a function of the temporal correlation and the number of BS antennas. We also propose a TCQ based limited feedback scheme for spatially correlated channels where the channel is quantized directly without performing decorrelation at the receiver. Simulation results show that the proposed TCQ schemes outperform the existing noncoherent TCQ (NTCQ) schemes, by improving the spectral efficiency and beamforming gain of the system. The proposed differential TCQ also reduces the feedback overhead of the system compared to the differential NTCQ method.
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    • "Massive MIMO [1]–[5] is an emerging technology in wireless access. By using a large number (tens to hundreds) of antennas at the base station, and serving many users in the same time-frequency resource, massive MIMO can improve the spectral and transmit-energy efficiency of conventional MIMO by orders of magnitude [6]–[9], and simple signal processing schemes are expected to achieve near-optimal performance [10]–[12]. The basic premise of massive MIMO is that, as confirmed by several experiments [13]–[16], the propagation channel has a large number of spatial degrees of freedom. "
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    ABSTRACT: Massive MIMO can greatly increase both spectral and transmit-energy efficiency. This is achieved by allowing the number of antennas and RF chains to grow very large. However, the challenges include high system complexity and hardware energy consumption. Here we investigate the possibilities to reduce the required number of RF chains, by performing antenna selection. While this approach is not a very effective strategy for theoretical independent Rayleigh fading channels, a substantial reduction in the number of RF chains can be achieved for real massive MIMO channels, without significant performance loss. We evaluate antenna selection performance on measured channels at 2.6 GHz, using a linear and a cylindrical array, both having 128 elements. Sum-rate maximization is used as the criterion for antenna selection. A selection scheme based on convex optimization is nearly optimal and used as a benchmark. The achieved sum-rate is compared with that of a very simple scheme that selects the antennas with the highest received power. The power-based scheme gives performance close to the convex optimization scheme, for the measured channels. This observation indicates a potential for significant reductions of massive MIMO implementation complexity, by reducing the number of RF chains and performing antenna selection using simple algorithms.
    Full-text · Article · Jul 2015 · IEEE Transactions on Communications
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    • "Upon increasing the number of antennas at the eNB, the channel correlation decreases, and the measured sumrates approach their theoretical limits. When the eNB employs 20 antennas, about 98% of the sum-rate of the ideal DPC scheme is achieved for a pair of single-antenna aided UEs by the ZF or RZF TPCs [29]. • Effects of the propagation environment [28]: Considering realistic environments, both a 128-antenna cylindrical and a linear array are employed at the eNB [28]. "
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