Portrait of Dr Philippos Asimakopoulos

Dr Philippos Asimakopoulos

Research Associate

About

  • Ethernet fibre transport in cloud-radio access networks
  • Functional virtualisation in fibre-distributed radio access networks
  • 5G Harmonized Research and Trials for Service Evolution between EU and China (5G-DRIVE)

Publications

Article

  • Gomes, N. et al. (2018). Boosting 5G Through Ethernet: How Evolved Fronthaul Can Take Next-Generation Mobile to the Next Level. IEEE Vehicular Technology Magazine [Online] 13:74-84. Available at: https://doi.org/10.1109/MVT.2017.2782358.
    Current approaches to the fronthaul for centralized- or Cloud-Radio Access Networks (C-RANs) need to be revised to meet the requirements of next-generation mobile networks. There are two major challenges: first, fronthaul signals need to be transported over public fixed access networks, such as passive optical networks (PONs), typically sharing them with other services; second, higher data rates must be catered for due to larger radio bandwidths and greater use of multi-antenna techniques, such as massive MIMO. Using Ethernet as a new transport protocol for the fronthaul allows statistical multiplexing and enables convergence between fixed and mobile services. This new approach more easily benefits from common developments being made for service level agreements, functional virtualization and software-defined networking. Higher data rates will be supported by the move to new, and possibly flexible, functional split points inside the radio access network (RAN) protocol stack of the processing located in the central and distributed units, as is being investigated by a number of bodies. However, there are technical challenges with regard to latency and packet delay variation. This article summarizes the benefits of an Ethernet-based fronthaul for the next generation of mobile networks, its main challenges and how these may be overcome.
  • Assimakopoulos, P. et al. (2018). A Converged Evolved Ethernet Fronthaul for the 5G Era. IEEE Journal on Selected Areas in Communications [Online] 36. Available at: http://doi.org/10.1109/JSAC.2018.2874148.
    We assess the performance of two distinct functional splits based on latency/latency variation and mapping efficiency, both individually and in unison. By considering hardware-offloading possibilities for a low-layer split (especially a pre-resource mapper split) using an Option-6 software-based LTE split as an example, we show how data rate, Ethernet frame size and in general, traffic generation characteristics will be very important aspects in the design of the future Ethernet mapping function. Then, an integrated Ethernet fronthaul with legacy and new/evolved split functionality, operating at 100 Gb/s link rate is presented with state-of-the art sub-100 ns latency variation for a timing-protocol flow. This is achieved through the application of a gap-filling aggregator, used for the first time in such a mobile fronthaul application.
  • Assimakopoulos, P., Al-Hares, M. and Gomes, N. (2016). Switched Ethernet Fronthaul Architecture for Cloud-Radio Access Networks. Journal of Optical Communications and Networking [Online] 8:B135-B146. Available at: https://doi.org/10.1364/JOCN.8.00B135.
    A fronthaul design for current and future mobile networks based on the transport of sampled radio signals from/to base station baseband processing units (BBUs) to/from remote radio heads (RRHs), is presented. The design is a pure-Ethernet switched architecture that uses virtual local area network (VLAN) identifiers for the RRHs and flow identifiers for the antenna ports, and is compatible with current standardization definitions. A comprehensive analysis for the limits of the Ethernet fronthaul in terms of the total number of antennas that can be supported is carried out, based on the latency imposed by the Ethernet network. The analysis assumes the transportation of control and management (C&M) and timing information (based on the precision-time protocol, PTP) but is valid for other types of background traffic (for example, that generated by the implementation of different longterm evolution (LTE) functional subdivisions, in a fronthaul with mixed processing). A low-cost testbed using “smart SFP” in-line probes is presented and
    used to obtain measurements from an Ethernet fronthaul, transporting mixed traffic. The measurements show how background traffic affects hybrid-automatic repeat request (HARQ)
    retransmissions, and are used to validate the analysis. The effects of contention of PTP packets is discussed and a simple solution to overcome the effects of contention is proposed.
  • Assimakopoulos, P. et al. (2013). Statistical Distribution of EVM Measurements for Direct-Modulation Radio-Over-Fiber Links Transporting OFDM Signals. IEEE Transactions on Microwave Theory and Techniques [Online] 61:1709-1717. Available at: http://dx.doi.org/10.1109/TMTT.2013.2247615.
    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.

Conference or workshop item

  • Assimakopoulos, P., Birring, G. and Gomes, N. (2018). Effects of Contention and Delay in a Switched Ethernet Evolved Fronthaul for Future Cloud-RAN Applications. in: 2017 European Conference on Optical Communication (ECOC). IEEE, pp. 1-3. Available at: http://doi.org/10.1109/ECOC.2017.8346013.
    A Switched-Ethernet fronthaul transporting data generated by a Long-Term Evolution software base station with a MAC/PHY functional split is presented. Contention effects arising from the Ethernet fronthaul and the effects of priority-based scheduling are characterised
  • Gomes, N. and Assimakopoulos, P. (2018). Optical Fronthaul Options for Meeting 5G Requirements. in: International Conference on Transparent Optical Networks. IEEE. Available at: http://dx.doi.org/10.1109/ICTON.2018.8473762.
    New functional splits for the 5G Radio Access Network have been identified so that fronthaul will no longer need to transport sampled time-domain waveforms. However, the different functional split points place differing demands on the fronthaul transport, while also posing different constraints to 5G techniques, such as massive MIMO. According to these conflicting demands, it is likely that in many cases, more than one split point may be needed in the same radio access network.
  • Elbers, J. et al. (2018). Next-Generation Optical Fronthaul in the iCirrus Project. in: Optical Fiber Communication Conference 2018. IEEE, p. M3D.1. Available at: http://dx.doi.org/10.1364/OFC.2018.M3D.1.
    We discuss next-generation fronthaul solutions for 5G and legacy radio access networks. Architectures, findings and experimental results from recent lab and field trial activities are reported.
  • Assimakopoulos, P. et al. (2017). Ethernet-based fronthauling for cloud-radio access networks. in: 19th International Conference on Transparent Optical Networks (ICTON), 2017. IEEE. Available at: https://doi.org/10.1109/ICTON.2017.8025034.
    Standardization bodies such as IEEE and 3GPP, as well as other interest groups, are in the process of defining and standardizing different functional subdivisions within mobile network base stations, primarily to reduce the data rate requirements imposed on the transport architecture by 4th and 5th generation mobile systems. Ethernet is considered the leading candidate for the transport architecture as it brings benefits from structural and operational convergence in what is now termed the “xhaul” (x signifying front, mid or back). This paper reports on the performance of different functional subdivisions over a bridged Ethernet network and presents some of the main issues that occur when internetworking at the Ethernet transport level. In particular, it examines Priority-based Scheduling within an Ethernet fronthaul, which is also a subject of active standardization by IEEE.
  • Al-Hares, M. et al. (2017). Scheduling in an Ethernet Fronthaul Network. in: 2017 European Conference on Networks and Communications (EuCNC). IEEE. Available at: https://doi.org/10.1109/EuCNC.2017.7980783.
    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.
  • Al-Hares, M. et al. (2017). Traditional queuing regimes and time-aware shaping performance comparison in an Ethernet fronthaul network. in: 19th International Conference on Transparent Optical Networks (ICTON), 2017. IEEE. Available at: https://doi.org/10.1109/ICTON.2017.8025067.
    This paper compares the performance of traditional priority-based queuing regimes with a time-aware shaping scheduler in an Ethernet fronthaul. Different use-cases are considered for the high and low priority traffic generation, which are made to represent precision-time protocol traffic and traffic originating from different LTE functional subdivisions (function splits) respectively. It is shown that that the relative performance characteristics of the three scheduling regimes depend strongly on the utilization of the fronthaul links where contention is taking place and on the traffic generation characteristics of the different traffic sources.
  • Gomes, N. et al. (2017). Concepts and requirements for the Ethernet-based evolved fronthaul. in: 2017 IEEE Photonics Society Summer Topical Meeting. IEEE, pp. 41-42. Available at: https://doi.org/10.1109/PHOSST.2017.8012641.
    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.
  • Al-Hares, M. et al. (2017). Modeling Time Aware Shaping in an Ethernet Fronthaul. in: GLOBECOM 2017 - 2017 IEEE Global Communications Conference. IEEE, pp. 1-6. Available at: https://doi.org/10.1109/GLOCOM.2017.8254714.
    An Opnet model of a time-aware shaper (TAS) based on the IEEE 802.1Qbv standard is presented. The TAS model is assumed to be the scheduling entity in an Ethernet-based fronthaul network, comprising of Ethernet switches. The fronthaul transports different traffic flow types as envisioned in next generation Radio Access Networks (RANs), including those for a timing protocol (based on the precision time protocol) and those from the implementation of different RAN functional subdivisions. The performance of the TAS is compared to that of a strict priority regime and is quantified through the frame delay variation of the high priority traffic when this contends with lower priority traffic. The results show that with the TAS implementation, contention effects can be overcome and frame delay variation (frame jitter) can be removed. Timing instability in the significant events of the scheduler is considered and a solution to overcome this issue is proposed.
  • Birring, G., Assimakopoulos, P. and Gomes, N. (2017). An Ethernet-Based Fronthaul Implementation with MAC/PHY Split LTE Processing. in: GLOBECOM 2017 - 2017 IEEE Global Communications Conference. IEEE, pp. 1-6. Available at: http://doi.org/10.1109/GLOCOM.2017.8254712.
    A testbed implementation for an Ethernet fronthaul transporting signals arising from a long-term evolution (LTE) functional subdivision (“split”) at the media-access control (MAC)/physical layer (PHY) interface is presented. Based on open LTE base station software, the testbed demonstrates significant data rate reductions compared to current fronthaul implementations that rely on In-phase and Quadrature radio sample transportation and data rates that scale with cell load. All generated traffic flows are clearly distinguishable using appropriate packet headers. A selection of test cases and their corresponding results are presented to demonstrate the operation of the fronthaul and the performance of individual flows in terms of data rates and overheads.
  • Al-Hares, M. et al. (2016). The effect of different queuing regimes on a Switched Ethernet fronthaul. in: International Conference of Transparent Optical Networks. IEEE, pp. 1-4. Available at: http://doi.org/10.1109/ICTON.2016.7550324.
    This paper investigates the effects of different queuing regimes on the mean and standard deviation of the frame inter-arrival delay of a LTE traffic stream under the presence of background Ethernet traffic. The background traffic is used to represent traffic that would be generated by different functional subdivisions in the physical layer of traditional LTE base station. In this work, a Switched Ethernet architecture is used as the fronthaul section of a Cloud Radio Access Network (C-RAN). Contention in this fronthaul becomes an important issue since different traffic streams originating from different functional subdivisions with different quality of service specifications will be transmitted over the same physical links. Trunk ports then, will have to handle the queuing management and prioritization. Handling the traffic with different queuing regimes will reflect on the latency and latency variations of the LTE traffic.
  • Assimakopoulos, P. et al. (2016). Statistical distribution of packet inter-arrival rates in an Ethernet fronthaul. in: IEEE International Conference on Communications. IEEE, pp. 140-144. Available at: http://doi.org/10.1109/ICCW.2016.7503778.
    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.
  • Gomes, N. et al. (2016). The new flexible mobile fronthaul: Digital or analog, or both? in: International Conference on Transparent Optical Networks. IEEE, pp. 1-4. Available at: http://doi.org/10.1109/ICTON.2016.7550322.
    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.
  • Aighobahi, A., Assimakopoulos, P. and Gomes, N. (2015). Experimental analysis of single and multiple antenna units in uplink of radio-over-fiber distributed antenna system. in: Microwave Photonics (MWP), 2015 International Topical Meeting on. pp. 1-4. Available at: http://doi.org/10.1109/MWP.2015.7356668.
    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.
  • Gomes, N. et al. (2014). Fiber link design considerations for cloud-Radio Access Networks. in: IEEE International Conference on Communications ICC 2014. pp. 382-387. Available at: http://dx.doi.org/10.1109/ICCW.2014.6881227.
    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.
  • Assimakopoulos, P., Santamaria, R. and Gomes, N. (2013). EVM and SER performance of OFDM signals with different IFFT sizes under nonlinear distortion. in: Communications Workshops (ICC), 2013 IEEE International Conference. pp. 848-852. Available at: http://dx.doi.org/10.1109/ICCW.2013.6649352.
    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.
  • Assimakopoulos, P., Nkansah, A. and Gomes, N. (2013). Use of companding to reduce isolation requirements in the remote antenna unit of an OFDM radio over fiber link. in: Communications Workshops (ICC), 2013 IEEE International Conference. pp. 833-837. Available at: http://dx.doi.org/10.1109/ICCW.2013.6649349.
    We demonstrate the use of companding in a directly modulated radio-over-fiber link with the aim of reducing the isolation requirements in the remote antenna unit. The performance of the system is quantified through error vector magnitude measurements.
  • Assimakopoulos, P. et al. (2011). Multi-channel signal transmission through radio over fiber architecture. in: GLOBECOM Workshops (GC Wkshps), 2011 IEEE. IEEE, pp. 152-156. Available at: http://dx.doi.org/10.1109/GLOCOMW.2011.6162426.
    A laser model validated through measured performance metrics is designed and used for simulating the performance of a multi-channel 4th generation mobile/wireless radio over fiber transmission system. Predictions indicate that the architecture provides adequate performance in terms of EVM, for different IFFT sizes and modulation levels of up to 256 QAM (at least), and a combined raw data rate of up to 3.2 Gbps.
  • Assimakopoulos, P. et al. (2011). Comparison between Remote Antenna Units with Detachable Antennas and Photonic Active Integrated Antennas for Indoor Applications. in: XXX URSI General Assembly.
  • Wake, D. et al. (2010). Design and Performance of Radio Over Fibre Links for Next Generation Wireless Systems using Distributed Antennas. in: ICT Future Network & Mobile Summit, Florence, Italy.
  • Vieira, L. et al. (2009). Performance of 802.11g Signals over a Multimode Fibre-Fed Distrbuted Antenna System incorporating Optical Splitting. in: European Workshop on Photonic Solutions for Wireless, Access and In-House Networks.
  • Assimakopoulos, P. et al. (2009). Modelling of a DFB Laser at Low Bias Directly Modulated with an OFDM Signal for RoF Applications. in: Microwave Photonics 2009, MWP'09 International Topical Meeting. IEEE.
    A DFB laser biased at a low bias point is modeled using a modified AM/AM Rapp model. Distortion effects are simulated and compared to the analytical predictions made by Bussgang's theorem for soft limiter nonlinearity.
  • Assimakopoulos, P., Nkansah, A. and Gomes, N. (2008). Use of Commercial Access Point Employing Spatial Diversity with Switched Combining in a Distributed Antenna Network. in: 6th ISIS Workshop of FTTH, Wireless Communications and their Interaction.
  • Assimakopoulos, P., Nkansah, A. and Gomes, N. (2008). Use of a Commercial Access Point Employing Spatial Diversity in a Distributed Antenna Network with Different Fiber Lengths. in: IEEE Intl. Topical Meeting on Microwave Photonics, MWP2008. pp. 189-192. Available at: http://dx.doi.org/10.1109/MWP.2008.4666668.
    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.

Datasets / databases

  • Assimakopoulos, P., Al-Hares, M. and Gomes, N. (2017). Switched ethernet fronthaul architecture for cloud-radio access networks. [Excel file]. Available at: http://dx.doi.org/10.1364/JOCN.8.00B135.
    A fronthaul design for current and future mobile networks based on the transport of sampled radio signals from/to base station baseband processing units to/from remote radio heads (RRHs) is presented. The design is a pure-Ethernet switched architecture that uses virtual local area network identifiers for the RRHs and flow identifiers for the antenna ports and is compatible with current standardization definitions. A comprehensive analysis for the limits of the Ethernet fronthaul in terms of the total number of antennas that can be supported is carried out, based on the latency imposed by the Ethernet network. The analysis assumes the transportation of control and management and timing information [based on the precision-time protocol (PTP)] but is valid for other types of background traffic (for example, that generated by the implementation of different Long-Term Evolution functional subdivisions in a fronthaul with mixed processing). A low-cost test bed using "smart small factor pluggable" in-line probes is presented and used to obtain measurements from an Ethernet fronthaul, transporting mixed traffic. The measurements show how background traffic affects hybrid-automatic repeat request retransmissions and are used to validate the analysis. The effects of contention of PTP packets are discussed, and a simple solution to overcome the effects of contention is proposed.
  • Assimakopoulos, P. et al. (2016). Statistical Distribution of Packet Inter-Arrival Rates in an Ethernet Fronthaul. [Excel file]. Available at: http://dx.doi.org/10.1109/ICCW.2016.7503778.
    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.
  • Al-Hares, M. et al. (2016). The effect of different queuing regimes on a switched ethernet fronthaul. [Excel file]. Available at: http://dx.doi.org/10.1109/ICTON.2016.7550324.
    This paper investigates the effects of different queuing regimes on the mean and standard deviation of the frame inter-arrival delay of a LTE traffic stream under the presence of background Ethernet traffic. The background traffic is used to represent traffic that would be generated by different functional subdivisions in the physical layer of traditional LTE base station. In this work, a Switched Ethernet architecture is used as the fronthaul section of a Cloud Radio Access Network (C-RAN). Contention in this fronthaul becomes an important issue since different traffic streams originating from different functional subdivisions with different quality of service specifications will be transmitted over the same physical links. Trunk ports then, will have to handle the queuing management and prioritization. Handling the traffic with different queuing regimes will reflect on the latency and latency variations of the LTE traffic.
  • Gomes, N. et al. (2016). The new flexible mobile fronthaul: Digital or analog, or both? [datafile]. Available at: http://dx.doi.org/10.1109/ICTON.2016.7550322.
    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.