Olli Apilo

Review of Latest Advances in 3GPP Standardization: D2D Communication in 5G Systems and Its Energy Consumption Models

Device-to-device (D2D) communication is an essential part of the future fifth generation (5G) system that can be seen as a “network of networks,” consisting of multiple seamlessly-integrated radio access technologies (RATs). Public safety communications, autonomous driving, socially-aware networking, and infotainment services are example use cases of D2D technology. High data rate communications and use of several active air interfaces in the described network create energy consumption challenges for both base stations and the end user devices. In this paper, we review the status of 3rd Generation Partnership Project (3GPP) standardization, which is the most important standardization body for 5G systems. We define a set of application scenarios for D2D communications in 5G networks. We use the recent models of 3GPP long term evolution (LTE) and WiFi interfaces in analyzing the power consumption from both the infrastructure and user device perspectives. The results indicate that with the latest radio interfaces, the best option for energy saving is the minimization of active interfaces and sending the data with the best possible data rate. Multiple recommendations on how to exploit the results in future networks are given.

The Distribution of Link Distances in Distributed Multiple-Input Multiple-Output Cellular Systems

The probability density function (pdf) of the distance between randomly located user equipment (UE) and its nth closest base station (BS) is studied in this paper. The knowledge of these pdfs is essential in the analysis of cellular distributed multiple-input multiple-output (MIMO) systems where N BSs cooperate in transmission or reception. We show that earlier results on ordered distance distributions in regular point patterns can be applied to the analysis of distributed MIMO systems when the UE distribution is uniform and the BS locations form a regular lattice. We present previously unpublished pdfs of the distance between a cell edge UE, whose distance to the closest BS is at least r, and four closest BSs in the hexagonal cell topology. The pdfs are verified by simulated histograms. As an example on the application of the results, we show how the signal-to-noise ratio (SNR) gain from uplink cooperative reception increases as the transmitting UE moves further from the cell center. The results from this paper can be applied to the analysis of received and transmitted (in case of power adaptive transmission) power in distributed MIMO systems.

Power consumption trade-off between power amplifier OBO, DPD, and clipping and filtering

This paper investigates the trade-off between power amplifier (PA) nonlinearity, output power backoff (OBO), digital predistortion (DPD), and clipping and filtering (CF) in terms of energy consumption. The energy efficiency of a PA depends on the OBO of the signal and usually increases as the OBO decreases. Peak to average power ratio (PAPR) reduction methods like CF and PA linearization algorithms like DPD methods allow the use of a smaller OBO. Those algorithms show best results when used together. By studying the trade-off between the power consumed in the CF and DPD circuits versus having a larger OBO for a target bit-error rate (BER), we show that below a certain PA output power the power consumption is not reduced by decreasing the OBO. We also show that the increase in distortion noise when using DPD and CF might defeat the purpose of their use in terms of power consumption. Given the trend toward smaller cells like femtocells, requiring less output power, CF and DPD may not be required at all, leading to a simpler and power saving transmitter design.

Energy Efficiency of Power-Adaptive Spatial Diversity Methods

The energy efficiency of power-adaptive multipleinput multiple-output (MIMO) diversity methods is studied in this paper. The considered diversity methods are antenna selection (AS), maximum ratio transmission (MRT), and equal gain transmission (EGT) at the transmitter and maximum ratio combining (MRC) at the receiver. The transmitter energy efficiency is evaluated using the average power amplifier (PA) efficiency and the transceiver energy efficiency is evaluated using the bit error rate (BER) as a function of average PA input signal-to-noise ratio (SNR), which is a new metric. With the new metric, the effect of the PA efficiency is taken into account in the performance evaluation. The analytical results are verified by Monte Carlo simulations. It is shown that larger diversity in the spatial or frequency domain improves the average PA efficiency in a system with channel inversion. The BER results show that the performance improvement from channel inversion diminishes due to the nonideal PA efficiency. Even though MRT is the received SNR maximizing transmitter diversity method, EGT requires less PA input energy per bit when a PA with nonideal efficiency is used. These conclusions could not have been reached using the traditional SNR metrics that do not measure the PA input energy.

Average efficiency of power amplifiers in power-controlled systems with multi-antenna diversity

We present a study of average efficiency of power amplifiers (PAs) in power-controlled systems with multi-antenna diversity. The efficiencies are analytically calculated using the transmitted power distributions. Several well-known multiple-input multiple-output (MIMO) diversity techniques, power control rules, PA classes, and channel models are considered. The considered diversity techniques are maximum ratio combining, selection combining, transmitting antenna selection, and maximum ratio transmission. Also the probability of an OFDM symbol clipping caused by PA saturation is calculated for the system under study. The analytical results are verified using Monte Carlo simulations. The study shows that the input signal distribution, diversity techniques, and the power control cutoff value have a significant effect on the average PA efficiency. The highest average PA efficiency is achieved when input signal has constant amplitude and selection combining is used. The presented results can be applied for an arbitrary PA e.g. when comparing the efficiency of different PAs.

Cell Splitting for Energy-Efficient Massive MIMO

In this paper, we propose a novel cell splitting approach for massive multiple-input multiple-output (MIMO) base stations to improve energy efficiency. The user equipments (UEs) in the cell are divided into two groups based on their distances to the base station. These two UE groups are scheduled at different time slots, which effectively splits a cell into inner and outer cells. The number of transmitting and receiving antennas together with the downlink and uplink transmission powers are adapted according to the number of cell edge and center UEs to maximize energy efficiency. We propose two algorithms to optimize the number of antennas and transmission powers. Cell splitting is able to reach energy efficiency (EE) gain of 11-41 % depending on the UE density when compared to a conventional load-adaptive massive MIMO system. The inevitable loss of cell edge UE rates can be controlled by setting a target UE rate, which also reduces the search space of the optimization algorithm.

Power efficiency, phase noise, and DC offset in constant envelope OFDM transceivers

This paper considers constant envelope orthogonal frequency-division multiplexing (CE-OFDM) that is an attractive candidate for the future Long Term Evolution over satellite and device-to-device communications in merging public safety and commercial networks. The constant envelope modulation allows power amplifier (PA) to operate near saturation levels thus maximizing power efficiency. Because the phase noise transforms just into an additive noise term after the phase detector, the CE-OFDM has significant advantage compared with phase noise sensitive OFDM. The transceiver power efficiency is evaluated by measuring the bit error rate as a function of average PA input signal-to-noise ratio so that the effects of the PA nonlinearities are taken into account in the performance evaluation. Our simulation results show that with a typical nonlinear satellite amplifier, the CE-OFDM has up to 6.0dB gain compared with the OFDM. This is beneficial especially in a channel having high attenuation. The consistent gains for the ideally linearized amplifier and common terrestrial three-way Doherty amplifiers are 2.2 and 2.5dB, respectively. The enhanced PA efficiency enables battery to be smaller or last longer in mobiles devices. For CE-OFDM implementation, a simple transmitter and receiver structure with novel direct current offset removal is presented by keeping in mind that the key factor for the rapid uptake of the future fifth generation systems is the maximization of technology commonalities with existing systems. Copyright

Unequal Power Amplifier Dimensioning for Adaptive Massive MIMO Base Stations

In this paper, we propose a novel power amplifier (PA) dimensioning method for massive multiple-input multiple-output (MIMO) systems whose number of transmitting antennas is adapted according to the number of user equipments (UEs) in the cell. The dimensioning method sets the maximum output powers of PAs unequally according to the pre-calculated average per-antenna transmission powers for different number of UEs. This allows PAs to operate at higher efficiency when the average per-antenna transmission powers vary due to the adaptive number of transmitting antennas. The performance of the method is evaluated in the symmetric multi-cellular scenario using a comprehensive power consumption model that considers both base station and UEs. When simple class-B PAs are used at the base station, unequal PA dimensioning reduces the PA power consumption up to 42 % when compared to the conventional equal PA dimensioning. This improves the total system energy efficiency. The benefits of the proposed unequal PA dimensioning are that no prior knowledge of the UE distribution is needed and good performance is achieved for all UE densities.