Philippos Assimakopoulos

Fiber link design considerations for cloud-Radio Access Networks

Analog radio over fiber (RoF) links may offer advantages for cloud-Radio Access Networks in terms of component cost, but the behavior of the distortion with large numbers of subcarriers needs to be understood. In this paper, this is presented in terms of the variation between subcarriers. Memory polynomial predistortion is also shown to compensate for RoF and wireless path distortion. Whether for digitized or analog links, it is shown that appropriate framing structure parameters must be used to assure performance, especially of time-division duplex systems.

Statistical Distribution of EVM Measurements for Direct-Modulation Radio-Over-Fiber Links Transporting OFDM Signals

The effect of distortion on the error vector magnitude (EVM) performance of orthogonal frequency-division multiplexing (OFDM) signals with different numbers of subcarriers and the connection to the peak-to-average power ratio (PAPR) of such signals is investigated. A low-cost and low-complexity directly modulated radio-over-fiber link is used in experiments as an example of a link limited by distortion. Statistical distributions of the EVM over a large number of transmitted OFDM frames are gained from experimental measurements and analyses of idealized processes. The measurement results show that as the number of subcarriers is reduced, the distribution means are more affected by extreme values. This effect results in mean EVMs for signals with different numbers of subcarriers that are not dependent in the expected way on the statistical PAPR of the transmitted OFDM signals. Instead, it is shown that in regions of moderate distortion, the median of the EVM is more closely related to the statistical PAPR and to the required back-off for signals with different numbers of subcarriers.

Use of commercial Access Point employing spatial diversity in a Distributed Antenna Network with different fiber lengths

A distributed antenna system (DAS) with two remote antenna units (RAUs) fed by a single access point (AP) using different MMF lengths is demonstrated using vertical cavity surface emitting lasers (VCSELs) and PIN photodiodes. Direct intensity modulation and direct detection (IM-DD) were employed in the system. The AP employs spatial diversity. Throughput measurements of the distributed IEEE802.11g signal were carried out at different locations in an office and indicated that high throughputs could be maintained even when the fiber length difference between the two RAUs was 200 m. These results are compared with throughput measurements taken when only a single port of the AP is used to drive the RAUs, that is, without making use of the spatial diversity. The results show a significant improvement in performance with spatial diversity.

Statistical distribution of packet inter-arrival rates in an Ethernet fronthaul

This paper investigates the effects of background traffic streams in the packet inter-arrival rates of an LTE traffic stream, when these streams are transported over the same Ethernet fronthaul network. Contention of background traffic with LTE traffic can occur in a Cloud-RAN that is transporting traffic streams originating from Constant Bit-Rate (CBR) sources such as the Common Public Radio Interface (CPRI) and from other non-CBR sources originating from different LTE physical layer functional subdivisions. Packet inter-arrival statistics are important in such a network, as they can be used to estimate and/or predict buffer sizes in receiving network nodes. Buffer management will also be important for traffic streams originating from functional splits (such as direct LTE MAC transport block transportation) where user plane data and control primitives have to be time aligned at the receiving node.

The new flexible mobile fronthaul: Digital or analog, or both?

It has become apparent that current fronthaul technology cannot be simply extended to meet the projected demands of 5G and beyond mobile systems. This current technology, based on the transport of sampled radio waveforms, has been the preferred option, with analog radio over fiber reserved to relatively niche application scenarios. However, for future systems, it is recognised that different functional splits between the central location and the remote units are needed; sampled waveform transport is not scalable to these systems. We propose a flexible fronthaul, therefore, in which both digital and analog transport technologies can coexist. Using practical examples from our work, we describe where these technologies can be used in the future fronthaul.

EVM and SER performance of OFDM signals with different IFFT sizes under nonlinear distortion

Simulation results are presented showing the behavior of the error-vector-magnitude and symbol-error rate under distortion conditions for OFDM signals with different IFFT sizes and modulation level, and smoothness of the nonlinearity. While more generally applicable, the results presented consider practical design goals for radio-over-fiber (RoF) links which need to impart minimum performance degradation on the transported signals.

Scheduling in an Ethernet fronthaul network

This paper investigates and compares the performance of different scheduling techniques in an Ethernet fronthaul network in the presence of both time-sensitive/high priority and background traffic streams. A switched Ethernet architecture is used as the fronthaul section of a cloud radio access network (C-RAN) and a comparison of two scheduling schemes, strict priority scheduling and time-aware shaping, is carried out. The different streams are logically separated using virtual local area network identifiers and contend for the use of trunk links formed between aggregator/switch nodes. The scheduling schemes are applied in the access and trunk ports in the fronthaul, and need to handle the queue management and prioritization of the different streams. In such cases, contention-induced latency variation has to be characterized, especially when the fronthaul transports precision time protocol traffic, as it directly leads to errors in timestamping. OPNET models for strict priority and time-aware schedulers have been built and employed, and simulation results are used to compare the performance of the two scheduling schemes.

Experimental analysis of single and multiple antenna units in uplink of radio-over-fiber distributed antenna system

Increasing the number of antennas either at the transmitter or receiver has been shown to improve system reliability without occupying additional spectrum. In this paper, we experimentally investigate the error vector magnitude (EVM) of single and multiple remote antenna units (RAU) focusing on uplink transmission. We demonstrate that for 64-QAM modulation, the EVM requirement of 6.5% could be achieved with multiple separated RAUs in situations where a single RAU fails to meet this requirement. The EVM result was obtained as the transmitting device was placed at different locations in a typical office environment with OFDM signals gathered through the RAUs and brought back to a central unit for processing. The EVM results show that using multiple RAUs and an efficient signal combining technique, here, maximal ratio combining (MRC), the EVM performance could reduce by approximately 2% when the distance between the RAUs was 0.3m and further reduced by 4% and 6% when the inter-RAU distance was 2m and 4m, respectively, compared to a single RAU.

Comparison between remote Antenna Units with Detachable antennas and Photonic Active Integrated Antennas for indoor applications

Measurements are presented for the performance evaluation of two distinct low cost and low power Radio over Fiber (RoF) architectures for indoor applications. One is a low power consumption component based remote antenna system — with detachable antennas and the second is a system which features Photonic Active Integrated Antennas with lossless matching circuits.

Concepts and requirements for the Ethernet-based evolved fronthaul

The use of Ethernet in the fronthaul permits convergence and exploitation of statistical multiplexing gains of the new interfaces, but minimum latency and latency variation requirements may become challenging. The techniques proposed to meet these challenges are summarized.