FORMATION OF A SINGLE SIDEBAND QUADRATURE PHASE SHIFT KEYING OF RADIO SIGNALS AT SUB-CARRIER FREQUENCIES IN A COHERENT OPTICAL COMMUNICATION SYSTEM

Abstract

The coherent optical communication technology allows to create a transmission system over
long distances with high bandwidth. Higher order modulation formats such as Quadrature Phase
Shift Keying (QPSK) provide high data rates in a simple, reliable and cost-effective manner. These
multi-bit modulation methods are transmitted as a symbol implementing simple in-phase and
quadrature (IQ) modulation formats. To increase throughput in coherent optical systems, optical
subcarrier multiplexing is used, when several radio signals are multiplexed on one optical carrier.
This multiplexing is limited by double sideband, resulting in reduced received subcarrier power
and degraded carrier-to-noise ratio. To accumulate these effects, many ways of implementing
optical single sideband (OSSB) are proposed. The paper proposes an analysis of a coherent optical
transmission system for generating single-sideband QPSK modulation using an optical I/Q
modulator and M-ary RF pulse shaping. The pseudo-random bit sequence generator generates a
10 Gb/s modulation baseband signal. Each bit sequence is converted into an M-ary sequence of
symbols from binary signals using phase shift keying (PSK). The M-ary pulse generator generates
a multi-level pulse shaping according to the sequence of M-ary input symbols. Electrical pulses
are applied to the electrodes of the electro-optical modulator according to the Mach–Zehnder
interferometer (MZM) scheme. The I/Q modulator operated at the quadrature operating point in
both the in-phase and quadrature branches, with an extinction ratio 60 dB and 3 V switching voltage.
The light wave is emitted by a CW laser with a wavelength of 1550 nm.