Yihenew Dagne Beyene

Cloud-RAN Architecture for Indoor DAS

A cloud radio access network (Cloud-RAN) is a new cellular technology that brings baseband processing units for a set of base stations into a central server retaining only the radio front-ends at the cell sites. This new architecture opens up opportunities for algorithms that require centralized processing. However, efficient implementation of the algorithms presents a number of challenges the most critical being latency, fronthaul capacity, and resource control. In this paper, we propose a software-defined radio-based architecture that addresses these problems and can be implemented on a cloud of general purpose computing platforms. We also present the practical implementation of Cloud-RAN running on an off-the-shelf server to validate the flexibility of the architecture. The implementation is able to realize various cellular networks, including heterogeneous networks, distribute-antenna systems, and transmission schemes, such as transmit antenna selection and open-loop transmit diversity.

Compressive sensing for MTC in new LTE uplink multi-user random access channel

In LTE, establishing a connection requires a relatively complex handshaking procedure. Such an approach is suitable for a system serving only a few high activity users, but it becomes very cumbersome for machine to machine (M2M) traffic, where large amounts of low activity users intermittently transmit a small number of packets. To avoiding excessive signaling overhead, each packet has to facilitate user detection, channel estimation, and data decoding. Even in the case of limited network activity, users may transmit simultaneously, resulting in packet collisions. It has been shown that such traffic can be best served by a Compressive Sensing (CS) detector. However, most of the CS-based multi-user detection (CS-MUD) research deals with Code Division Multiple Access (CDMA) type systems. In this work we propose a CS-MUD algorithm that is designed for single carrier OFDM (SC-OFDM) systems and, as such, can be integrated into LTE uplink subframes. Each packet contains both a user identification code (ID) and data. The CS algorithm uses the ID not only for user detection, but also for channel estimation. We investigate random and structured ID code generation and report system performance in both cases.

NB-IoT Technology Overview and Experience from Cloud-RAN Implementation

The 3GPP has introduced a new narrowband radio technology called narrowband Internet of Things (NB-IoT) in Release 13. NB-IoT was designed to support very low power consumption and low-cost devices in extreme coverage conditions. NB-IoT operates in very small bandwidth and will provide connectivity to a large number of low-data-rate devices. This article highlights some of the key features introduced in NB-IoT and presents performance results from real-life experiments. The experiments were carried out using an early-standard-compliant prototype based on a software defined radio partial implementation of NB-IoT that runs on a desktop computer connected to the network. It is found that a cloud radio access network is a good candidate for NB-IoT implementation.

Creating secondary spectrum usage opportunity for D2D communication with interference cancellation

In this demonstration, we consider secondary spectrum usage by D2D users in cellular network. Secondary usage is allowed if primary users are not impacted. We show how a Base Station (BS) creates new D2D communication opportunities on uplink resources by enabling Interference Cancellation (IC). BS can apply single-state IC when the D2D signal is strong enough. This can occur when the D2D transmitter is located near to the BS or when the BS explicitly orders D2D transmitter to increase its transmission power. This demonstration shows that D2D user successfully streams a video without influencing the cellular users live video streaming on the same spectrum resource. The demonstration is based on software defined radio platform with Universal Software Radio Peripherals (USRP) as the hardware front-ends.

Random Access Scheme for Sporadic Users in 5G

Machine-type communication (MTC) devices usually have low-data rate, and in some applications, latency is critical. In long-term evolution, establishing a connection requires a relatively complex handshaking procedure. Such an approach is suitable for a system serving only a few high-activity users, but it becomes cumbersome for MTC traffic, where large amounts of low-activity users intermittently transmit a small number of packets. We propose a random access channel (RACH)-based scheme for the future 5G system that allows MTC users to send small packets within the random access burst. Device activity detection, channel estimation, authentication, and data decoding are performed simultaneously from the same access burst. In addition to the inherent reduction in energy consumption, the scheme eliminates the signaling overhead and creates more resources for data transmission. We have constructed an analytical framework for the detection performance of a multiple-input–multiple-output receiver in a frequency-selective channel. This has been validated with simulations, and results show that a one-shot correlate-and-cancel algorithm is sufficient for activity detection and channel estimation when the base station either employs multiple receive antennas or schedules multiple resource blocks for MTC RACH.

On the Performance of Narrow-Band Internet of Things (NB-IoT)

Narrow-Band Internet of Things (NB-IoT) is 3GPPs cellular technology designed for narrow-band and Low-Power Wide Area Network (LPWN). NB-IoT provides deep indoor coverage for thousands of low-data-rate and low-powered devices. Coverage enhancement for devices in weak coverage condition is enabled by having signal repetitions over extended period of time in order to boost the signal quality. Performance gain from repetitions of signals is limited by channel estimation quality. In this paper we analyze impact of channel coherence time on the uplink coverage. In addition to analytical performance bound and simulation results, we also present coverage performance results from practical measurements using a NB-IoT prototype. It is reported how the NB-IoT coverage improvement is limited by the channel estimation quality and coherence time of the channel.

Spectrum sharing in D2D enabled HetNet

This demonstration presents dynamic spectrum use by an overlay Device-to-Device (D2D) connection in a Heterogeneous Network (HetNet) environment. D2D devices can reuse the uplink resources of the cellular network. In a HetNet, the cellular users should be protected from excessive interference from the D2D connections. This is especially so for small cells as D2D transmitters may come arbitrarily close to the small cell base stations. We demonstrate how the cellular traffic can be protected by prohibiting the D2D reuse close to the cellular base stations by measuring the signal strength at the D2D devices. Our measurement results show that in the case of small cells, fast changes in the signal strength proposes challenges to the decision making algorithm. A demonstration with a software defined radio TD-LTE platform is shown.

Spectrum sharing for MTC devices in LTE

In this paper, we present practical realization of spectrum sharing scheme for machine-type communications in LTE. We demonstrate that the proposed scheme supports dynamic spectrum access for large set of low-data rate transmitters. Unlike classical random-access type communications, the new scheme avoids the need for signaling procedures that are associated with channel reservation. Moreover, compressive-sensing based algorithm is able to identify transmitting nodes and simultaneously decode multi-user data. The proposed algorithm has been implemented on software-defined radio based TD-LTE testbed.

Angular Domain Data-Assisted Channel Estimation for Pilot Decontamination in Massive MIMO

Massive Multiple-Input-Multiple-Output (M-MIMO) system is a promising technology that offers to mobile networks substantial increase in throughput. In Time-Division Duplexing (TDD), the uplink training allows a Base Station (BS) to acquire Channel State Information (CSI) for both uplink reception and downlink transmission. This is essential for M-MIMO systems where downlink training pilots would consume large portion of the bandwidth. In densely populated areas, pilot symbols are reused among neighboring cells. Pilot contamination is the fundamental bottleneck on the performance of M-MIMO systems. Pilot contamination effect in antenna arrays can be mitigated by treating the channel estimation problem in angular domain where channel sparsity can be exploited. In this paper, we introduce a codebook that projects the channel into orthogonal beams and apply Minimum Mean-Squared Error (MMSE) criterion to estimate the channel. We also propose data-aided channel covariance matrix estimation algorithm for angular domain MMSE channel estimator by exploiting properties of linear antenna array. The algorithm is based on simple linear operations and no matrix inversion is involved. Numerical results show that the algorithm performs well in mitigating pilot contamination where the desired channel and other interfering channels span overlapping angle-of-arrivals.

Transmission strategies in downlink of multi-user multi-cell distributed antenna systems

The use of multiple-input multiple-output (MIMO) systems has been a central part of cellular systems for achieving higher capacity. Distributed antenna system (DAS) is a MIMO system with geographically distributed antennas that provide high coverage and capacity due to the spatial macro diversity. In a multi-user scenario, DAS provides better spatial separation among users. An optimal transmission strategy in a multiuser multi-cell DAS is unknown. We analyze and evaluate downlink capacity of three multiplexing strategies: orthogonal transmission, joint precoding, and space division multiplexing (SDM). Based on the evaluation we propose the most appropriate strategy. The sum-rate has been measured in different scenarios. Simulation results show that DAS has superior performance compared to co-located antennas systems (CAS). It is particularly shown that DAS with simple SDM can provide better capacity than joint transmission in addition to reduced complexity and less feedback overhead. This happens when the number of users increases and remote radio units have single antenna.