Cost-effective optical transponders for deployed metropolitan area networks

High-Speed, High-Performance DQPSK Optical Links with Reduced Complexity VDFE Equalizers

“…Recently, DQPSK has been revisited together with other direct detection techniques for cost efficiency [13] or nonlinearity mitigation [14]. Today, processing the signal electronically at symbol rate speeds is very typical in coherent optical transponders, however, in applications restricted by cost, direct detection is revisited [15][16][17] and DQPSK is a very important candidate [2]. Let I 1 (n) ∈ {0, 1} and I 2 (n) ∈ {0, 1} represent the encoded transmitted sequences of the I and Q channels, which are encoded and modulated using a standard DQPSK transmitter setup ( Figure 1).…”

“…The feasibility of the implementation of those methods on FPGA (Field-Programmable Gate Array) circuitry has recently been demonstrated [2,12,20], for rates up to 40 Gb/s. Due to the joint constructive/destructive processing, four signals are available as input to the equalizer.…”

“…As a consequence, the complexity of the corresponding balanced DQPSK equalizers are reduced to about half of that required for the joint processing case, however, at the expense of a significant deterioration in the attained performance [2,21]. The mathematical formulae that describes the VDFE can be expressed in a linear regression form as u l (n) = θ T l ϕ M (n), l = 1, 2 where ϕ M (n) is a vector that depends on the input signals and on the recovered bit streams, θ l , l = 1, 2 represents the vectors that carries the equalizer coefficients for the in-phase and the quadrature part of the equalizer, with T denoting the matrix transpose.…”

“…Transporting traffic at high bit rates, such as 100 Gbit/s, may become challenging as various impairments manifest themselves [1]. Combating those impairments is cumbersome and power consuming, hence, the preferred solution choices are not obvious in network segments that are sensitive to cost and power consumption, like in metropolitan area networks [2] and inter-data center networks [3]. Typically, these segments are subject to unpredictable traffic changes and all distortions vary in a nondeterministic way.…”

“…Chromatic dispersion (CD) is the prevailing linear impairment that causes intersymbol interference (ISI) [2]. Distortion compensation techniques have been made readily available with optical and electronic means [5][6][7].…”