Abstract
We report on a fiber-optic transport system for next-generation wireless-communication
systems utilizing 4 × 4
multiple-input multiple-output (MIMO) orthogonal-frequency-division-multiple-access
(OFDMA) technology. Our system supports time-division-duplex (TDD)-based
wireless signals operating at 3.775 GHz. To accommodate the TDD-based MIMO
signals over a single strand of optical fiber, we utilize nine-channel coarse
wavelength-division-multiplexed optical channels: one for link delay measurement
and TDD control signal transmission, four for downlink, and the others for
uplink. The system first measures the propagation delay between the central
base station (CBS) and the remote antenna (RA) and sends the result to delay
modules to compensate for the delay added by the transmission link. This procedure
makes the CBS and RA emit the downstream signals simultaneously into the
air and, consequently, helps avoid the performance degradation caused by
the propagation delay of the radio-over-fiber system. The system then sends
the MIMO signals together with TDD control signals to the RA. Our experimental
demonstration is carried out with 1-Gb/s OFDMA signals having pilot, control,
and data channels. For downstream, the error vector magnitudes (EVMs) are
measured to be < -30
dB after a 3.9-km transmission over conventional single-mode fiber when the
antenna output power is set to be 24 dBm. For upstream, the best EVMs are
measured to be < -35
dB. Both the downstream and upstream performance is limited by the shot and
thermal noise of the receiver when the signal power is low, whereas nonlinear
distortions of electrical amplifiers start to degrade the system performance
as the signal power increases. We also measure the crosstalk between channels.
It is measured to be less than -42 dB for all channels, which is found to be caused by board-to-board
interference at the RA.
© 2007 IEEE
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