The first measured results for massive multipleinput, multiple-output (MIMO) performance in a line-of-sight (LOS) scenario with moderate mobility are presented, with eight users served in real-time using a 100-antenna base Station (BS) at 3.7 GHz. When such a large number of channels dynamically change, the inherent propagation and processing delay has a critical relationship with the rate of change, as the use of outdated channel information can result in severe detection and precoding inaccuracies. For the downlink (DL) in particular, a time division duplex (TDD) configuration synonymous with massive MIMO deployments could mean only the uplink (UL) is usable in extreme cases. Therefore, it is of great interest to investigate the impact of mobility on massive MIMO performance and consider ways to combat the potential limitations. In a mobile scenario with moving cars and pedestrians, the massive MIMO channel is sampled across many points in space to build a picture of the overall user orthogonality, and the impact of both azimuth and elevation array configurations are considered. Temporal analysis is also conducted for vehicles moving up to 29 km/h and realtime bit error rates (BERs) for both the UL and DL without power control are presented. For a 100-antenna system, it is found that the channel state information (CSI) update rate requirement may increase by 7 times when compared to an 8-antenna system, whilst the power control update rate could be decreased by at least 5 times relative to a single antenna system.
This paper presents preliminary results for a novel 128-antenna massive Multiple-Input, Multiple-Output (MIMO) testbed developed through Bristol Is Open in collaboration with National Instruments and Lund University. We believe that the results presented here validate the adoption of massive MIMO as a key enabling technology for 5G and pave the way for further pragmatic research by the massive MIMO community. The testbed operates in real-time with a Long-Term Evolution (LTE)-like PHY in Time Division Duplex (TDD) mode and supports up to 24 spatial streams, providing an excellent basis for comparison with existing standards and complimentary testbeds. Through line-of-sight (LOS) measurements at 3.51 GHz in an indoor atrium environment with 12 user clients, an uncoded system sum-rate of 1.59 Gbps was achieved in real-time using a single 20 MHz LTE band, equating to 79.4 bits/s/Hz. In a subsequent indoor trial, 22 user clients were successfully served, which would equate to 145.6 bits/s/Hz using the same frame schedule. To the best of the author's knowledge, these are the highest spectral efficiencies achieved for any wireless system to date.
Abstract-Massive Multiple-Input, Multiple-Output (MIMO) has rapidly gained popularity as a technology crucial to the capacity advances required for 5G wireless systems. Since its theoretical conception six years ago, research activity has grown exponentially, and there is now a developing industrial interest to commercialise the technology. For this to happen effectively, we believe it is crucial that further pragmatic research is conducted with a view to establish how reality differs from theoretical ideals. This paper presents an overview of the massive MIMO research activities occurring within the Communication Systems & Networks Group at the University of Bristol centred around our 128-antenna real-time testbed, which has been developed through the Bristol Is Open (BIO) programmable city initiative in collaboration with National Instruments (NI) and Lund University. Through recent preliminary trials, we achieved a world first spectral efficiency of 79.4 bits/s/Hz, and subsequently demonstrated that this could be increased to 145.6 bits/s/Hz. We provide a summary of this work here along with some of our ongoing research directions such as large-scale array wave-front analysis, optimised power control and localisation techniques.
This paper evaluates the impact of spatially multiplexing more users within a massive multiple-input, multipleoutput (MIMO) system. Recent works on massive MIMO show that there is a peak value for sum spectrum efficiency (SE) achieved by serving a certain number of users. It was shown that until the sum SE reached its peak value, the maximum sum SE is achieved by serving all users simultaneously. These results were based on perfect channel state information (CSI), Shannon capacity calculations or using a very large number of antennas at the base station (BS). As opposed to the aforementioned results, we show that the maximum sum SE with practical number of antennas could be achieved by decreasing the number of users before the sum SE reached its peak value. This is shown by calculating the sum SE based on the Error Vector Magnitude (EVM) performance. Sum SE with zero-forcing (ZF) and matched filtering (MF) is presented for uplink (UL) data transmission in a single-cell. Channel matrices formed from both independent and identically distributed (IID) Rayleigh samples and measured data from a real massive MIMO system were used for performance evaluation. The impact of adding more users is also demonstrated by real constellations captured from an indoor massive MIMO trial.
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