was demonstrated using polarization division multiplexed PAM-4 signal [2]. 112 Gb/s short
reach transmission using single polarization PAM-4 signal was demonstrated using SiP
Mach-Zehnder modulator [3]. In our previous work, we have also successfully demonstrated a
112 Gb/s PAM-4 transmission system with commercial available 25G EML-TOSA and PIN-
ROSA [4].
Another alternative scheme that may provide good system performance and high data rate
using low-cost and bandwidth limited optical components is carrier-less amplitude and phase
modulation (CAP). 10 Gb/s multi-level CAP for short reach communications was
experimentally studied [5]. 40 Gb/s CAP-32 system was successfully demonstrated for low
cost data communication links [6]. Higher order CAP signals were also experimentally
demonstrated for short reach applications [7]. Using multi-band CAP, high-speed AWG and
EML, 102 Gb/s signal transmission over 15km have been realized [8]. Subsequently, 400
Gb/s O band transmission over 20 km and 40 km of SSMF using multi-band CAP signal has
been reported [9].
Discrete multi-tone (DMT) modulation also known as direct detection orthogonal
frequency domain modulation (DD-OFDM), is another attractive scheme for low-cost and
bandwidth limited short reach communication systems. DMT, as one kind of multi-carrier
modulation techniques [10–14], shows high spectrum efficiency, flexible multi-level coding,
and high tolerance to channel impairments. A 52.8 Gb/s DMT signal transmission over 20km
of SSMF has been achieved with a distributed feedback laser [12]. A single wavelength,
single polarization 101 Gb/s DMT signal generated with a 64 GSa/s AWG and a directly
modulated laser has been transmitted over 10km of SSMF [13]. More recently, 4x117 Gb/s
DMT signals have been successfully transmitted over 40 km of SSMF [14].
Although many researches and performance comparisons on PAM, CAP and DMT have
been reported [15–17], there were no reported full system level comparisons among these
modulation formats in 100 Gb/s short reach transmission systems. In this paper, we provide a
comprehensive comparison of three typical advanced modulation formats for short reach
transmission systems by using the same simulation model and experimental setup. They
include PAM-4, CAP-16 and DMT. We focus on realizing a bit rate of 112 Gb/s with
commercially available components using the modulation formats under study. The remainder
of this paper is organized as follows. Section 2 presents the digital signal processing required
different modulation formats at the transmitter and the receiver. In section 3, performance
comparison is carried out using the same simulation setup for different formats in terms of
received optical power, bandwidth of transmitter, relative intensity of noise (RIN) and thermal
noise of the receiver. Section 4 presents experimental setup for different modulation formats
to realize a data rate of 112 Gb/s and experimental results are presented and discussed. To the
best of author’s knowledge, for the first time, a single-band CAP-16 signal with a bit rate of
112 Gb/s on single wavelength and single polarization was experimentally demonstrated for
short reach transmission system. In section 5, computational complexity of DSP for different
modulation formats is studied. Section 6 concludes this paper.
2. Digital signal processing for different modulation formats
Arbitrary waveform generator (AWG) and analogue to digital convertor (ADC) are used in
the system, digital signal processing can be applied at both transmitter and receiver sides. In
this section, we describe the digital signal processing required for different modulation
formats at both ends of the transmission system.
2.1 DSP for PAM-4 signal
Figure 1 shows the DSP flow chart for PAM-4 signal at the transmitter and the receiver sides.
A 2
16
de Bruijn bit sequence was used for bit to symbol mapping and the generation of PAM-
4 signal. In order to achieve a bit rate of 112 Gb/s (100Gbit/s excluding Ethernet and FEC
overhead), a baud rate of 56GB/s is required for PAM-4. In the experimental setup, the
Received 17 Nov 2014; revised 21 Dec 2014; accepted 25 Dec 2014; published 20 Jan 2015
26 Jan 2015 | Vol. 23, No. 2 | DOI:10.1364/OE.23.001176 | OPTICS EXPRESS 1178