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Description
Upcoming wireless communication will be based on a combination of various
technologies such as WiFi, WiMax, UMTS, GSM or Bluetooth. This evolution implies more complexity at
the base stations, and thus, a higher cost for the network. In Radio-over-Fiber (RoF) systems,
this complexity is removed in a single central station and the transmission of the radio signals
between the central and base stations is realized with optical fibers. In this configuration,
the cost of the base stations (acting as passive antenna) can be drastically reduced. Another
advantage of RoF systems is the possible seamless integration of different wireless technologies
with CATV and broadband internet (e.g., Ethernet).
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Typical Results
The transmission of 3x (16QAM 100 Mb/s) channels over 100 km SSMF is simulated using the schematic
illustrated in
.
The three channels are multiplexed in the electrical domain and the resulting RF signal is used to
drive a Mach-Zehnder modulator (MZM).
In unidirectional transmissions, the system performances are limited by noise (from the laser,
the optical and electrical amplifiers and the photodetector) and by second- and third-order
intermodulation distortions (IMD2, IMD3) resulting from the nonlinear nature of the transmission
line.
In this example, the magnitude of the driving signal is varied: at low modulation index, the
transmission is limited by noise. For higher modulation index, the SNR is improved, but the transfer
function of the MZM becomes nonlinear and nonlinear distortions are generated. The signal spectra
for low and high modulation index are displayed in
.
The system BER is reported against the magnitude of the driving RF signal in
.
It is estimated using three methods: Gaussian approximation, PDF fitting [1] and the Monte Carlo method.
At low modulation powers, the optical noise dominates and the signal statistics is Gaussian. All
approaches give the same results. However, at high modulation powers, nonlinear distortions arise
and the Gaussian approach underestimates the BER.
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Further Information
Keywords: Radio-over-Fiber (RoF), Subcarrier Modulation (SCM), Quadrature Amplitude Modulation (QAM), Intermodulation distortions, Second-Order Spur (IMD2), Third-Order Spur (IMD3), Bit Error Rate (BER)
Similar demonstrations are available in VPItransmissionMaker Optical Systems and on the VPIphotonics Forum.
[1] G. Mouil Sil, H. Louchet, A. Richter, OFC/NFOEC 2007, paper JThA82, 2007.
