Andreas Wolfgang

Calculated and measured absorption cross sections of lossy objects in reverberation chamber

Reverberation chambers can be used to measure radiation efficiency of small antennas when these are located close to lossy objects. The lossy objects represent a heavy loading of the chamber. This loading is characterized by the mean absorption cross section of the lossy objects. This paper describes how this mean absorption cross section can be calculated from the scattered far field of an object by using the forward scattering theorem, or from a more laborious near-field evaluation. Results for lossy spheres and cylinders are calculated by using three different codes, based on spherical mode expansion, finite difference time domain techniques, and moment methods, respectively. The results for the cylinder are compared with measured levels in a reverberation chamber.

A Preliminary Study on Waveform Candidates for 5G Mobile Radio Communications above 6 GHz

This paper provides an overview and preliminary comparison of several multi-carrier and single-carrier waveforms that are potential candidates for future 5G mobile radio communications above 6 GHz. The waveforms are assessed primarily based on the established and known results as well as recent findings keeping in view the design requirements that are relevant to using frequencies above 6 GHz, especially the millimeter wave frequencies. The Key Performance Indicators and degrees of freedom in the design of different waveforms and their potential applications for mm-wave communications are discussed. Certain features that are particularly interesting for mm-wave communication and require further investigations are also highlighted. Furthermore, a common framework for synthesizing different waveform candidates has been developed. Finally, a preliminary qualitative comparison of different multi-carrier and single carrier waveforms has been derived.

Impact of Frequency Selective Channels on a Line-of-Sight MIMO Microwave Radio Link

Two line-of-sight (LoS) multiple-input multiple-output (MIMO) systems for point-to-point microwave radio links are simulated and analyzed. Both 2x2 spatially-separated, single-polarized and 4x4 spatially-separated, dual-polarized systems are considered in the presence of a frequency selective and time varying channel. Time dependence is introduced by moving the antennas during the simulation and thereby introducing the need for an adaptive space-time-equalizer (STE). Our results indicate that a time-varying frequency selective LoS MIMO channel puts higher requirements on the STE and introduces a higher SNR penalty compared to a single-input single-output (SISO) system.

Resource allocation and linear precoding for relay assisted multiuser MIMO systems

In this paper we investigate the use of infrastructure half-duplex amplify-and-forward (AF) relays in the downlink of a multi-user (MU) multiple-input multiple-output (MIMO) system, which uses linear precoding based on block-diagonalization. Owing to the half-duplex mode of the relay, the communication is divided into two time-slots. During the first time-slot the relay nodes are treated as an additional mobile station (MS), while during the second time-slot the base-station (BS) and the relays form a distributed, virtual antenna array. In contrast to many relay applications, the purpose of the proposed scheme is not to increase diversity, but rather to increase the system throughput at the borders of a communication cell or in areas with a high user density. The performance of the proposed architecture is investigated by examining the sum-capacity of the system as well as the distribution of the capacity over the cell-area.

Downlink performance and capacity of distributed antenna systems based on realistic channel model

Recent academic studies of distributed antenna system (DAS) using a frequency-flat fading channel model show that DAS outperforms standard systems, where the base-station (BS) antenna elements are centralized at one location. In this paper the performance of a DAS under time-varying frequency-selective fading based on a realistic channel model is investigated. Specifically, we show that if we shift the hexagonal cellular layout in the conventional system and use sectorized antennas instead of omnidirectional antennas at each BS, the performance in terms of outage probability and outage capacity will improve by a large extent without a need for additional BS towers. The results show that with the same total transmit power and bandwidth DAS can reduce the inter-cell interference (ICI) in a multicell environment and improve the outage capacity especially near the cell boundary.

Numerical FDTD simulations of a validation case for small antenna measurements in a reverberation chamber

For mobile phone antennas, the working environment varies in time. Their radiation characteristics are better described in terms of radiation efficiency than properties such as radiation pattern. This paper describes the validation of the results of antenna measurements in a reverberation chamber by numerical simulations with an FDTD code. We show that the return loss and radiation efficiency of an antenna can be measured with good accuracy in the reverberation chamber. It should also be pointed out that even the free space values of these parameters can also be measured with good accuracy in the reverberation chamber.

Phase Noise Mitigation in OFDM-Based Backhaul in the Presence of Channel Estimation and Synchronization Errors

In this paper, a carrier recovery scheme for phase noise mitigation (in the presence of imperfect synchronization and channel estimation) is proposed for orthogonal frequency division multiplexing (OFDM) based wireless backhaul. This scheme models the phase noise as a piece-wise linear function and compensate the phase noise within each OFDM symbol. It is shown that the carrier recovery scheme can be applied to both OFDM and orthogonal frequency-division multiple access (OFDMA). Since the proposed scheme does not require detailed knowledge of statistical properties of the phase noise, it can be easily applied to practical OFDM/OFDMA systems. Hence the proposed scheme is applicable to both point-to-point and point-to-multipoint backhauls.

Higher Order MIMO Outdoor-to-Indoor Measurements Using Repeaters

In this paper we present results from a outdoor-to-indoor MIMO measurement campaign where we study the effect of repeaters on singular values, spatial richness, capacity, and delay spread distributions. We present results from different repeater scenarios such as, e.g. different outdoor/indoor repeater deployments with single/dual polarized antennas. The measurements show that repeaters significantly enhance the received SNR while they increase the delay spread and decrease the spatial richness of the MIMO channel. However, the main conclusion is that a significant increase in channel capacity is attained when deploying repeaters mostly thanks to their ability to provide spatial multiplexing over the high-quality eigenmodes they may provide. The number of such eigenmodes is limited by the number of repeaters, deployment, and their antennas.

Phase Noise Effect on MIMO-OFDM Systems with Common and Independent Oscillators

The effects of oscillator phase noises (PNs) on multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems are studied. It is shown that PNs of common oscillators at the transmitter and at the receiver have the same influence on the performance of (single-stream) beamforming MIMO-OFDM systems, yet different influences on spatial multiplexing MIMO-OFDM systems with singular value decomposition (SVD) based precoding/decoding. When each antenna is equipped with an independent oscillator, the PNs at the transmitter and at the receiver have different influences on beamforming MIMO-OFDM systems as well as spatial multiplexing MIMO-OFDM systems. Specifically, the PN effect on the transmitter (receiver) can be alleviated by having more transmit (receive) antennas for the case of independent oscillators. It is found that the independent oscillator case outperforms the common oscillator case in terms of error vector magnitude (EVM).

CP-OFDM and UF-OFDM in the Presence of Phase Noises and Their Mitigations

In this paper, we study the performances of cyclic prefix orthogonal frequency division multiplexing (CP-OFDM) and universal filtered orthogonal frequency division multiplexing (UF-OFDM) in the presence of phase noises. A phase noise mitigation scheme was proposed for CP-OFDM in a previous work. In this work, we extend the scheme to UF-OFDM. Phase noise estimation parameters are studied extensively in order to improve the performance of the scheme with a respect of the performance-complexity trade-off. The proposed scheme does not require detailed knowledge of statistics of the phase noise and can effectively compensate the phase noise within each multicarrier symbol with reasonable complexity.