Karl F. Nieman

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.

Time-domain compression of complex-baseband LTE signals for cloud radio access networks

Modern cellular networks such as Long-Term Evolution (LTE) transport complex-baseband samples between remote radio hardware and processing equipment. Common Public Radio Interface (CPRI) links are widely used in practice and enable flexible radio head deployments, distributed antenna systems, and advanced spatial processing such as coordinated multi-point (CoMP) transmission and reception. Current CPRI links already have insufficient capacity to support 20 MHz bandwidth LTE for a basestation with three sectors and four antennas per sector. By supporting eight antennas per sector and up to 5× system bandwidth, LTE-A will require substantial increases in CPRI capacity. In this work, we develop compression methods that exploit the temporal and spectral structure of LTE signals with the goal of achieving high compression with limited impact on end-to-end communication performance. Our contributions include (i) design of a low-complexity compression method for LTE and (ii) validation of this method using an LTE link-level simulation. Our method achieves up to 5× compression for uplink and downlink signals.

Cyclic spectral analysis of power line noise in the 3–200 kHz band

Narrowband OFDM Power Line Communication (NB-OFDM PLC) systems are a key component of current and future smart grids. NB-OFDM PLC systems enable next-generation smart metering, distributed control, and monitoring applications over existing power delivery infrastructure. It has been shown that the performance of these systems is severely limited by impulsive, non-Gaussian additive noise. A substantial component of this noise has time-periodic statistics (i.e. it is cyclostationary) synchronous to the AC mains cycle. In this work, we analyze the cyclic structure of power line noise observed in a G3 PLC system operating in the CENELEC 3-148.5 kHz band. Our contributions include: (i) the characterization of noise measurements in several urban usage environments, (ii) the development of a cyclic bit loading method for G3, and (iii) the quantification of its throughput gains over measured noise. Through this analysis, we confirm strong cyclostationarity in power lines and identify several sources of the cyclic noise.

Doppler estimation and correction for shallow underwater acoustic communications

Reliable mobile underwater acoustic communication systems must compensate for strong, time-varying Doppler effects. Many Doppler correction techniques rely on a single bulk correction to compensate first-order effects. In many cases, residual higher-order effects must be tracked and corrected using other methods. The contributions of this paper are evaluations of (1) signal-to-noise ratio (SNR) performance from three Doppler estimation and correction methods and (2) communication performance of Doppler correction with static vs. adaptive equalizers. The evaluations use our publicly available shallow water experimental dataset, which consists of 360 packet transmission samples (each 0.5s long) from a five-channel receiver array.

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.

Properties and Applications of Electrically Small Folded Ellipsoidal Helix Antenna

A comprehensive analysis of the radiation properties of an electrically small folded ellipsoidal helix antenna (EHA) is presented, showing its ability to self-resonate and impedance match without external components. Three antennas with different sizes and geometries have been designed to work at the 2.4-GHz ISM band and are realized using a selective laser sintering (SLS)-based fabrication process. The benefits of using this antenna for various size-restricted applications such as medical implants are also described.

Electrically small folded ellipsoidal helix antenna for medical implant applications

The design and fabrication of electrically small folded ellipsoidal helix antennas is presented for medical implant applications. With ellipsoidal aspect ratio as an additional variable, such antennas have improved radiation resistance tunability over spherical helix antennas while still providing high bandwidth (low Q) and radiation efficiency with small values of ka. A novel 3-D antenna fabrication procedure based on selective laser sintering (SLS) is utilized to rapidly tape-out the ellipsoidal helix antennas on medical implant packages.

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.

Wideband monopulse spatial filtering for large receiver arrays for reverberant underwater communication channels

Underwater acoustic communication (ACOMMS) is critical for many applications including marine science, oceanographic exploration, offshore surveying/drilling, and military uses. ACOMMS data rates are usually limited by multiple propagation paths with different time delays and Doppler characteristics. It is often difficult to coherently recombine all paths, especially in shallow water, leaving incoherent paths that interfere with the receiver. One way to suppress unwanted paths is with a directional receiving array. Indeed, many existing large, directional acoustic arrays could be used as ACOMMS receivers. In a number of these arrays, wideband monopulse outputs could be made available. These directional beam outputs, in monopulse pairs, can selectively suppress, or even null, offending multipath when combined with a simple scalar weight. Using an experimental system, we show how a relatively short equalizer, using as inputs the wideband monopulse beam outputs of a large array, can form the backbone of an ACOMMS system that performs effectively in a multipath-limited environment. Our contributions include (i) a multipath-Doppler channel model validated by experimental results, (ii) a receiver design that utilizes monopulse processing, and (iii) an analysis of its performance using simulated and experimental data.

FPGA implementation of a message-passing OFDM receiver for impulsive noise channels

Conventional orthogonal frequency division multiplexing (OFDM) communication systems are typically designed assuming additive white Gaussian noise and interference statistics. However, in many applications, such as Wi-Fi and powerline communications (PLC), impulsive statistics are often observed. Impulsive noise can degrade the signal-to-noise ratio (SNR) of all subcarriers and impair communication performance. In this work, we design and implement a real-time OFDM receiver with approximate message passing (AMP) to estimate and mitigate impulsive noise. The goal is to meet throughput and latency requirements while guaranteeing improved communication performance in impulsive noise. Our contributions include (i) modeling functional parallelism in an AMP OFDM receiver in synchronous dataflow, (ii) converting an AMP OFDM PLC receiver to using only fixed-point data and arithmetic, and (iii) mapping the receiver in fixed-point onto a Field Programmable Gate Array (FPGA) target using a high-level graphical synthesis tool. Our FPGA OFDM transceiver testbed achieves full streaming throughput at G3-PLC rates and recovers up to 8 dB SNR of impulsive noise over a wide SNR range.