OFDM is a modulation format that can be advantageously used in access
systems as it provides increased spectral efficiency, enhanced equalization
capabilities and flexible bandwidth allocation compared to single-carrier
modulation schemes. While baseband transmission is rather tolerant to the
nonlinear behavior of the transmission channel, OFDM requires a highly
linear transmission channel. One of the main contributors for channel
nonlinearity is chirp-GVD interaction, which leads to subcarrier-to-subcarrier intermixing interferences (SSII) after detection.
SSII can be mitigated through equalization of the received signal by
nonlinear filtering . One type of nonlinear filter is the Volterra
filter. In order to train filter coefficients, a known training sequence
is sent by the transmitter; the Volterra filter coefficients are then
trained using the LMS algorithm to reduce the error at the output of
the filter .
This application example demonstrates SSII mitigation by means of a
Volterra filter. The laser is directly modulated using DMT (real-valued
OFDM), while direct detection is performed at the receiver. The 9-Gbaud
signal is composed of 32 subcarriers, each modulated with 16QAM, and
includes 12,5% cyclic prefix resulting in a net signal rate of 31,5
Gbit/s. As fiber length is 40 km, modulation index is set at 25%
guaranteeing enough optical power at the receiver.
Three runs of the setup are performed: in the first run the Volterra
filter is off, in the second run it adapts its coefficients, and in the
final run the filter is active. For each run, a different transmitted
signal is used.
shows the error signal at the output of the filter.
shows transmitted and received spectra for the final run with and
without filter. The SSII can be observed in the 10 GHz-17 GHz range.
The BER of all subcarriers are reported in
for the cases with and without the filter.
show the constellation of 32 subcarriers before and after the filter.
Keywords: OFDM, access networks, Volterra, Equalization, Direct Modulation, Direct detection, chirp-GVD interaction
Similar demonstrations are available in VPItransmissionMaker Optical Systems and on the VPIphotonics Forum.
 W. Yan, B. Liu, L. Li, Z. Tao, T. Takahara, and J. C. Rasmussen, 'Nonlinear Distortion and DSP-based Compensation in Metro and Access Networks using Discrete Multi-tone' in ECOC (2012), paper Mo.1.B.2.
 N. André, K. Habel, H. Louchet, A. Richter, 'Equalization Techniques for High-speed OFDM-based Access Systems Using Direct Modulation and Direct Detection' in ICTON (2013)