Nikhil Kundargi

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.

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.

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.

LabVIEW based platform for prototyping dense LTE networks in CROWD project

Next generation wireless networks (5G) have to cope with significant traffic increase due to high quality video transmission and cloud-based applications. Such requirements create the need for a revolutionary change in architecture rather than a series of local and incremental technology updates. A dense heterogeneous deployment of small cells such as pico/femto cells in addition to high power macro cells is foreseen as one of the potential solutions to achieve these requirements. While there is significant amount of research in this area that relies on simulations at PHY, MAC and higher layers, it is still necessary to validate the algorithms for next generation systems in a real-time testbed. However, the ever increasing complexity in all layers of current and future generations of cellular wireless systems has made an end-to-end demonstration of the network limited to industrial research labs or large academic institutions. In this paper, we show a LabVIEW1 based PXI platform in which LTE-like SISO OFDM PHY Layer is integrated with an open source protocol stack to prototype PHY/MAC cross layer algorithms within CROWD2 Software Defined Networking (SDN) framework as a solution to tame dense deployment of wireless networks.

LOS Throughput Measurements in Real-Time with a 128-Antenna Massive MIMO Testbed

This paper presents initial 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 12 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. To the best of the authors knowledge, this is the highest spectral efficiency achieved for any wireless system to date.

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.

A Framework for Inference Using Goodness of Fit Tests Based on Ensemble of Phi-Divergences

In this paper we study the inferential use of goodness of fit tests in a non-parametric setting. The utility of such tests will be demonstrated for the test case of spectrum sensing applications in cognitive radios. We provide the first comprehensive framework for decision fusion of an ensemble of goodness-of-fit testing procedures through an Ensemble Goodness-of-Fit test. Also, we introduce a generalized family of functionals and kernels called Φ-divergences which allow us to formulate goodness-of-fit tests that are parameterized by a single parameter. The performance of these tests is simulated under Gaussian and non-Gaussian noise in a MIMO setting. We show that under uncertainty in the noise statistics or non-Gaussianity in the noise, the performance of non-parametric tests in general, and phi-divergence based goodness-of-fit tests in particular, is significantly superior to that of the energy detector with reduced implementation complexity. In particular, the false alarm rates of our proposed tests is maintained at a fixed level over a wide variation in the channel noise distributions. Additionally, we describe a collaborative spatially separated version of the test for robust combining of tests in a distributed spectrum sensing setting and quantify the significant collaboration gains achieved.

Channel Idle Time Statistics Based Spectrum Accessing Strategies With CSMA Based Primary Networks

Channel statistics based secondary transmission strategy design has been studied intensively in the past. In this work, we consider the same problem but with a carrier sensing multiple access based primary networks, i.e., the primary users that would react to interference. In such primary networks, users always need to perform carrier sensing before accessing the channel. Such networks include 802.11 WLAN and 802.15 WPAN. The secondary transmission strategy design when the primary network uses carrier sensing based network access differs a lot from what has been considered in the literature. Here, we show through experiments with 802.11 networks that under secondary interference, the primary users can experience not only packet collisions but also transmission delay. Under long-term secondary interference, the primary users might even choose to transmit in another channel. This phenomenon is known as the channel capture effect. Next, with this in mind, we formulate a novel transmission strategy design problem that aims to maximize secondary transmission time with design constraints on the probability of packet collision or delay, as well as the probability that the primary user switches channel. Finally, we describe a software-defined radio device that we built to execute the designed transmission strategies on the 2.4 GHz band. The performance of both the primary and secondary users are studied. Results show that our consideration for the primary user reactions is necessary. In particular, the designed transmission strategy better protects the primary user transmission while maintaining secondary user performance.

WiMAC: Rapid Implementation Platform for User Definable MAC Protocols Through Separation

This demo presents WiMAC, a general-purpose wireless testbed for researchers to quickly prototype a wide variety of real-time MAC protocols for wireless networks. As the interface between the link layer and the physical layer, MAC protocols are often tightly coupled with the underlying physical layer, and need to have extremely small latencies. Implementing a new MAC requires a long time. In fact, very few MACs have ever been implemented, even though dozens of new MAC protocols have been proposed. To enable quick prototyping, we employ the mechanism vs. policy separation to decompose the functionality in the MAC layer and the PHY layer. Built on the separation framework, WiMAC achieves the independence of the software from the hardware, offering a high degree of function reuse and design flexibility. Hence, our platform not only supports easy cross-layer design but also allows protocol changes on the fly. Following the 802.11-like reference design, we demonstrate that deploying a new MAC protocol is quick and simple on the proposed platform through the implementation of the CSMA/CA and CHAIN protocols.

Demo: LabVIEW based framework for prototyping dense LTE networks

In the demo, we illustrate the integration aspects of LTE testbed using LabVIEW SISO OFDM Physical layer and open source NS-3 LTE LENA stack. We leverage the NS-3 LENA module, which allows researchers to conduct simulations by emulating large network of base-stations, mobile devices and core network. However, the validity of results produced from such simulations is limited due to the physical layer model and may not accurately reflect the true behavior of densely-deployed LTE networks. Hence, we implemented selected LTE physical channels in LabVIEW to facilitate more realistic emulation of the LTE testbed and integrate it with MAC/higher layers of LENA protocol stack. The main goal of the testbed is to be able to show the artifacts caused in interference limited environment and demonstrate the performance of Enhanced Inter-cell Interference Coordination (eICIC) algorithms proposed by researchers in 3GPP. This demo shows preliminary results and we plan to use this testbed to showcase different aspects of Software Defined Networking (SDN) framework which is used to fine-tune different aspects of dense deployments within EU FP7 CROWD research project.