Abstract
Flexible Pulse Amplitude Modulation (FlexPAM) consists of the use of non-standard PAM formats, such as PAM-5 or PAM-6, to increase the granularity of the link capacity for a given baud rate. In this paper, we demonstrate FlexPAM systems as a scalable solution for future data center interconnects, introduce a symbol-to-bit mapping that minimizes the effect of intensity noise in the performance of non-power of two number of levels formats, and present experimental and simulation results of an unamplified FlexPAM transmission system over 1 km of single-mode fiber. 20 GBaud transmission was successfully demonstrated experimentally for modulation formats ranging from OOK up to PAM-16 using an integrated InP-Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> laser. In addition, a simulation to closely replicate the experiment was used to determine the system requirements to achieve transmission rates of up to 200 Gbps on a single wavelength, at a baud rate of 53.5 Gbaud with component bandwidth limitations of 30 GHz.
Highlights
T HE increasing popularity of online services like streaming platforms, cloud storage, cloud computing, or video call services is driving an unprecedented rise in the bandwidth requirements of data centers (DC)
PAM-16 reaches below forward error correction (FEC) performance at received optical power (ROP) of −2 dBm, resulting in a receiver sensitivity penalty of 15 dB compared to on-off keying (OOK)
Even though lower order modulation formats are less affected by the signal-to-noise ratio (SNR) of the system, it can be seen that PAM-7 performs to PAM-8, and PAM-14 worse than PAM-16
Summary
T HE increasing popularity of online services like streaming platforms, cloud storage, cloud computing, or video call services is driving an unprecedented rise in the bandwidth requirements of data centers (DC). The transition towards a WDM implementation, in combination with some degree of optical switching (implemented using optical filters and multiplexers) [5]–[7], can result in a drastic variation in path loss present in individual server-to-server links in the DC This in turn leads to different achievable link data rates, which can be accommodated by operating at a constant baud rate with different modulation formats. With traditional PAM-M formats, where M is a power of two, the SNR requirements grow sharply as the number of levels increases from 4 to 8 and 16, limiting potential capacity increases This constraint can be alleviated by the use of probabilistic shaping (PS) of high-order traditional PAM-M formats [8], [9], which can potentially give enhanced capacity and performance at the cost of additional computational cost. The use of FlexPAM enables finer granularity in the modulation format that gives the maximum throughput for a specific system SNR (which may change due to temperature variations that alter the laser output power and receiver noise, and different optical paths as optical switching is implemented in future DC architectures) at the very modest computational cost of a lookup table
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