The version in the Kent Academic Repository may differ from the final published version. Users are advised to check http://kar.kent.ac.uk for the status of the paper. Users should always cite the published version of record.

A flexible subcarrier multiplexing system combining analog transport with digital domain processing is presented. By making use of bandpass sampling and applying a systematic mapping of signals into available Nyquist zones, the multiplexing system is able to present multiple signals at the same intermediate frequency at the remote site. This simplifies the processing required for multiple antenna systems. We further propose the use of trackand-hold amplifiers at the remote site. These elements are used to extend the mapping to a mapping hierarchy, offering flexibility in frequency placement of signals and relaxation of analog-to-digital converter bandwidth and sampling rate constraints. The system allows the transport of different numerologies in a number of next generation radio access network scenarios. Experimental results for large signal multiplexes with both generic and 5th-generation mobile numerologies show error-vector magnitude performance well within specifications, validating the proposed system. Simulation results from a system model matched to these experimental results provide performance predictions for larger signal multiplexes and larger bandwidths. Index Terms-Digital signal processing, massive-MIMO (mMIMO), millimeter wave (mmW), mobile fronthaul, radioover-fiber, subcarrier multiplexing (SCM). I. INTRODUCTIONT HE establishment of heterogeneous networking and multiantenna techniques as norms for next generation radio access networks (RANs) means that new network architectures will need to be employed that can seamlessly accommodate such features. These architectures will have to meet differing requirements in terms of data rate, latency constraints, system complexity and cost.Analog transport within the fronthaul, as an alternative to digital transport based either on centralized processing with the Common Public Radio Interface (CPRI) [1], or on alternative functional decompositions [2], [3], can provide high spectral

The version in the Kent Academic Repository may differ from the final published version. Users are advised to check http://kar.kent.ac.uk for the status of the paper. Users should always cite the published version of record.

Abstract-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.

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-Þb e rl i n ki su s e di ne x p e r i m e n t sa sa ne xample 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. Index Terms-Error vector magnitude (EVM), orthogonal frequency division multiplexing (OFDM), radio-over-Þber (RoF).Philippos Assimakopoulos received the B.Eng. degree in electronic engineering from the University of Bath, Bath, U.K., in 2003, and the M.Sc. degree in broadband and mobile communication networks and Ph.D. degree in electronic engineering from the University of Kent, Canterbury, U. K., in 2007 and 2012, respectively. He is currently with the Broadband and Wireless Communications Group, University of Kent. His research interests include low-cost microwave RoF networks for indoor and outdoor applications.