Abstract-With the continuously increasing demand of cost effective, broadband wireless access, radio-over-fiber (RoF) starts to gain more and more momentum. Various techniques already exist, using analog (ARoF) or digitized (DRoF) radio signals over fiber. Each with their own advantages and disadvantages. By transmitting a sigma delta modulated signal over fiber (SDoF), a similar immunity to impairments as DRoF can be obtained while maintaining the low complexity of ARoF. This letter describes a detailed experimental comparison between ARoF and SDoF that quantifies the improvement in linearity and error vector magnitude (EVM) of SDoF over ARoF. The experiments were carried out using a 16QAM constellation with a baudrate from 20 to 125 Mbaud modulated on a central carrier frequency of 1 GHz. The sigma delta modulator (SDM) runs at 8 or 13.5 Gbps. A high speed VCSEL operating at 850 nm is used to transmit the signal over 200 m multimode fiber. The receiver amplifies the electrical signals and subsequently filters to recover the original RF-signal. Compared to ARoF, improvements exceeding 40 dB were measured on the third order intermodulation products when SDoF was employed, the EVM improves between 2.4 to 7.1 dB.

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A new high-speed delta-sigma modulator (DSM) topology is proposed by cascading a bit reduction process with a multi-stage noise shaping MASH-1-1 DSM. This process converts the two-bit output sequence of the MASH-1-1 DSM to a singlebit sequence, merely compromising the DSM noise-shaping performance. Furthermore, the high clock frequency requirements are significantly relaxed by using parallel processing. This DSM topology facilitates the design of e.g. wideband software defined radio (SDR) transmitters and delta-sigma radio-over-fiber transmitters. Experimental results of the FPGA implementation show that the proposed low-pass DSM can operate at 21 GS/s, providing 520 MHz baseband bandwidth with 42.76 dB signal-to-noiseand-distortion ratio (SNDR) or 1.1 GHz bandwidth with 32.04 dB SNDR (based on continuous wave measurements). An all-digital transmitter based on this topology can generate 218.75 MBd 256-QAM over 200 m OM4 multimode fiber in real-time, with 7-GS/s sampling rate and an error vector magnitude below 1.89%.

The fifth generation (5G) cellular network is expected to include the millimeter wave spectrum, to increase base station density, and to employ higher-order multiple-antenna technologies. The centralized radio access network architectures combined with radio-over-fiber (RoF) links can be the key enabler to improve fronthaul networks. The sigma-delta modulated signal over fiber (SDoF) architecture has been proposed as a solution leveraging the benefits of both digitized and analog RoF. This work proposes a novel distributed antenna system using sigma-delta modulated intermediate-frequency signal over fiber (SDIFoF) links. The system has an adequately good optical bitrate efficiency and high flexibility to switch between different carrier frequencies. The SDIFoF link transmits a signal centered at a 2.5 GHz intermediate frequency over a 100 m multi-mode fiber and the signal is up-converted to the radio frequency (24-29 GHz) at the remote radio unit. An average error vector magnitude (EVM) of 6.40 % (-23.88 dB) is achieved over different carrier frequencies when transmitting a 300 MHz-bandwidth 64-QAM OFDM signal. The system performance is demonstrated by a 2×1 multiple-input single-output system transmitting 160 MHzbandwidth 64-QAM OFDM signals centered at 25 GHz. Owing to transmit diversity, an average gain of 1.12 dB in EVM is observed. This work also evaluates the performance degradation caused by asynchronous phase noise between remote radio units. The performance shows that the proposed approach is a competitive solution for the 5G downlink fronthaul network for frequency bands above 24 GHz.

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All-digital radio-over-fiber (RoF) transmission has attracted a significant amount of interest in digital-centric systems or centralized networks because it greatly simplifies the front-end hardware by using digital processing. The sigma-delta modulator (SDM)-based all-digital RoF approach pushes the digital signal processing as far as possible into the transmit chain. We present a real-time 100-GS/s fourth-order single-bit SDM for all-digital RoF transmission in the high-frequency band without the aid of analog/optical up-conversion. This is the fastest sigma-delta modulator reported and this is also the first real-time demonstration of sigma-delta-modulated RoF in the frequency band above 24 GHz. 4.68 Gb/s (2.34 Gb/s) 64-QAM is transported over 10-km standard single-mode fiber in the C-band with 6.46% (4.73%) error vector magnitude and 3.13 Gb/s 256-QAM can be even received in an optical back-to-back configuration. The carrier frequency can be digitally tuned at run-time, covering a wide frequency range from 22.75 to 27.5 GHz. Besides, this high-speed sigma-delta modulator introduces less than 1 µs latency in the transmit chain. Its all-digital nature enables network virtualization, making the transmitter compatible with different existing standards. The prominent performance corroborates the strong competitiveness of this SDMbased RoF approach in high-frequency RoF 5G communication.

We demonstrate an optical transmitter consisting of a limiting SiGe BiCMOS driver co-designed and co-packaged with a silicon photonic segmented traveling-wave Mach-Zehnder modulator (MZM). The MZM is split into two traveling-wave segments to increase the bandwidth and to allow a 2-bit DAC functionality. Two limiting driver channels are used to drive these segments, allowing both NRZ and PAM4 signal generation in the optical domain. The voltage swing as well as the peaking of the driver output are tunable, hence the PAM4 signal levels can be tuned and possible bandwidth limitations of the MZM segments can be partially alleviated. Generation of 50 Gbaud and 53 Gbaud PAM4 yields a TDECQ of 2.8 and 3.8 dB with a power efficiency of 3.9 and 3.6 pJ/bit, respectively; this is the best reported efficiency for co-packaged silicon transmitters for short-reach datacenter interconnects at these data rates. With this work, we show the potential of limiting drivers and segmented traveling-wave modulators in 400G capable short-reach optical interconnects.