Steffen Malkowsky

A flexible 100-antenna testbed for Massive MIMO

Massive multiple-input multiple-output (MIMO) is one of the main candidates to be included in the fifth generation (5G) cellular systems. For further system development it is desirable to have real-time testbeds showing possibilities and limitations of the technology. In this paper we describe the Lund University Massive MIMO testbed - LuMaMi. It is a flexible testbed where the base station operates with up to 100 coherent radio-frequency transceiver chains based on software radio technology. Orthogonal Frequency Division Multiplex (OFDM) based signaling is used for each of the 10 simultaneous users served in the 20 MHz bandwidth. Real time MIMO precoding and decoding is distributed across 50 Xilinx Kintex-7 FPGAs with PCI-Express interconnects. The unique features of this system are: (i) high throughput processing of 384 Gbps of real time baseband data in both the transmit and receive directions, (ii) low-latency architecture with channel estimate to precoder turnaround of less than 500 micro seconds, and (iii) a flexible extension up to 128 antennas. We detail the design goals of the testbed, discuss the signaling and system architecture, and show initial measured results for a uplink Massive MIMO over-the-air transmission from four single-antenna UEs to 100 BS antennas.

Reciprocity Calibration for Massive MIMO: Proposal, Modeling, and Validation

This paper presents a mutual coupling-based calibration method for time-division-duplex massive MIMO systems, which enables downlink precoding based on uplink channel estimates. The entire calibration procedure is carried out solely at the base station (BS) side by sounding all BS antenna pairs. An expectation-maximization (EM) algorithm is derived, which processes the measured channels in order to estimate calibration coefficients. The EM algorithm outperforms the current state-of-the-art narrow-band calibration schemes in a mean squared error and sum-rate capacity sense. Like its predecessors, the EM algorithm is general in the sense that it is not only suitable to calibrate a co-located massive MIMO BS, but also very suitable for calibrating multiple BSs in distributed MIMO systems. The proposed method is validated with experimental evidence obtained from a massive MIMO testbed. In addition, we address the estimated narrow-band calibration coefficients as a stochastic process across frequency, and study the subspace of this process based on measurement data. With the insights of this study, we propose an estimator which exploits the structure of the process in order to reduce the calibration error across frequency. A model for the calibration error is also proposed based on the asymptotic properties of the estimator, and is validated with measurement results.

The World’s First Real-Time Testbed for Massive MIMO: Design, Implementation, and Validation

This paper sets up a framework for designing a massive multiple-input multiple-output (MIMO) testbed by investigating hardware (HW) and system-level requirements, such as processing complexity, duplexing mode, and frame structure. Taking these into account, a generic system and processing partitioning is proposed, which allows flexible scaling and processing distribution onto a multitude of physically separated devices. Based on the given HW constraints such as maximum number of links and maximum throughput for peer-to-peer interconnections combined with processing capabilities, the framework allows to evaluate modular HW components. To verify our design approach, we present the Lund University Massive MIMO testbed, which constitutes the first reconfigurable real-time HW platform for prototyping massive MIMO. Utilizing up to 100 base station antennas and more than 50 field programmable gate array, up to 12 user equipment are served on the same time/frequency resource using an LTE-like orthogonal frequency division multiplexing time-division duplex-based transmission scheme. Proof-of-concept tests with this system show that massive MIMO can simultaneously serve a multitude of users in a static indoor and static outdoor environment utilizing the same time/frequency resource.

Performance characterization of a real-time massive MIMO system with LOS mobile channels

The first measured results for massive multiple-input, multiple-output (MIMO) performance in a line-of-sight scenario with moderate mobility are presented, with eight users served in real time using a 100-antenna base station 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 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 real-time bit-error rates for both the UL and DL without power control are presented. For a 100-antenna system, it is found that the channel state information update rate requirement may increase by seven times when compared with an eight-antenna system, whilst the power control update rate could be decreased by at least five times relative to a single antenna system.

Implementation of Low-Latency Signal Processing and Data Shuffling for TDD Massive MIMO Systems

Low latency signal processing and high throughput implementations are required in order to realize real-time TDD massive MIMO communications, especially in high mobility scenarios. One of the main challenges is that the up-link and down-link turnaround time has to be within the coherence time of the wireless channel to enable efficient use of reciprocity. This paper presents a hardware architecture and implementation of this critical signal processing path, including channel estimation, QRD-based MMSE decoder/precoder and distributed reciprocity calibration. Furthermore, we detail a switch-based router implementation to tackle the stringent throughput and latency requirements on the data shuffling network. The proposed architecture was verified on the LuMaMi testbed, based on the NI SDR platform. The implementation supports real-time TDD transmission in a 128 x 12 massive MIMO setup using 20 MHz channel bandwidth. The processing latency in the critical path is less than 0.15 ms, enabling reciprocity-based TDD massive MIMO for high-mobility scenarios.

Serving 22 Users in Real-Time with a 128-Antenna Massive MIMO Testbed

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 authors knowledge, these are the highest spectral efficiencies achieved for any wireless system to date.

Stress Test of Vehicular Communication Transceivers Using Software Defined Radio

Wireless vehicular communication is, in contrast to other terrestrial types of wireless communications, more dynamic in nature. Both the transmitter and the receiver are moving at high speeds relative to each other, which generates highly dynamic wireless channels. Such channels are characterized by short stationarity regions and large Doppler spreads. Modem manufacturers face a challenge when designing and implementing equipment for such environments. Similarly, for testing and evaluation real-life measurements with vehicles are required, which often is an expensive and slow process. This paper tackles this problem by proposing a method for stress testing transceivers based on the design and implementation of a real-time wireless channel emulator for wireless vehicular communications using a software defined radio (SDR). The emulator together with the proposed test methodology enable quick on-bench evaluation of wireless modems. In the paper we also apply the test on two different IEEE 802.11p modem implementations and characterize the packet error rate performance for different Doppler-delay combinations.

Temporal Analysis of Measured LOS Massive MIMO Channels with Mobility

The first measured results for massive multiple-input, multiple-output (MIMO) performance in a line-of-sight (LOS) scenario with moderate mobility are presented, with 8 users served by 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 correlation of the MIMO channel vector over time is inspected for vehicles moving up to 29km/h. 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.

A Simulation Framework for Multiple-Antenna Terminals in 5G Massive MIMO Systems

The recent interest in massive multiple in multiple out (MIMO) has spurred intensive work on massive MIMO channel modeling in the contemporary literature. However, current models fail to take the characteristics of terminal antennas into account. There is no massive MIMO channel model available that can be used for the evaluation of the influence of different antenna characteristics at the terminal side. In this paper, we provide a simulation framework that fills this gap. We evaluate the framework with antennas integrated into Sony Xperia handsets operating at 3.7 GHz as this spectrum is identified for the 5G new radio standard by 3rd Generation Partnership Project. The simulation results are compared with the measured terminal performance when communicating with the Lund University’s massive MIMO testbed under the same loading conditions. Expressions are derived for comparison of the gain obtained from different diversity schemes computed from measured far-field antenna patterns. We conclude that the simulation framework yields the results close to the measured ones and that the framework can be used for antenna evaluation for terminals in a practical precoded massive MIMO system.

Transmission Schemes for Multiple Antenna Terminals in Real Massive MIMO Systems

In massive MIMO performance evaluations it is often assumed that the terminal has a single antenna. The combination of multiple antennas in a terminal and massive MIMO precoding at the base station side can further improve overall system performance. We present measurement results for multi antenna terminals operating in different transmission schemes and how they perform under varying loading conditions. Gain expressions are derived that enable easy comparison between the transmission schemes. The evaluation is performed on realistic antennas integrated into Sony Xperia handsets tuned to 3.7~GHz and operated together with the Lund University massive MIMO (LuMaMi) test bed. It is concluded that the approach used in todays mobile systems, where up link and down link are addressed independently, will not provide the best performance. The performance can be improved by the selection of transmission schemes optimized for massive MIMO.