1024 CHINESE OPTICS LETTERS / Vol. 8, No. 11 / Novemb er 10, 2010
Long-reach 60-GHz radio-over-f iber system based on
turbo-coded OFDM
Zizheng Cao (ùùùfff¾¾¾), Jianjun Yu ({{{ïïï)
∗
, Qi Tang (/// jjj),
Guanjun Zeng (QQQ)))), and Lin Chen ( )
Key Lab oratory for Micro/Nano Opto-Electronic Devices of Ministry of Education and School of Computer and
Communication, Hunan University, Changsha 410082, China
∗
E-mail: yujianjun@hnu.cn
Received March 23, 2010
Turb o-coded 1.25-Gb/s orthogonal frequency-division multiplexing (OFDM) signals in 60-GHz radio-over-
fiber system are demonstrated. It can overcome impairments in fib ers and extend transmission distance.
Exp erimental results show that the transmission distance of turbo-coded OFDM signals at 1.25 Gb/s with
co ding (pure bit rate of 830 Mb/s) can be extended by over 30%.
OCIS co des: 060.0060, 060.1155.
doi: 10.3788/COL20100811.1024.
Long-reach optical access networks can increase band-
width of customer units while simultaneously reducing
the number of base stations
[1]
. Radio-over-fiber (RoF)
technique can reduce cost of wireless access
[1−21]
. Pre-
vious works based on long-reach optical access networks
focused on low frequency signals, such as Wi-Fi oper-
ating at 5.8 GHz
[1]
. High frequency signals over fiber
were constantly discussed in studies on short-reach less
than 30-km networks, especially in 60-GHz wireless ac-
cess networks with huge bandwidth availability of over
7-GHz unlicensed band
[6−10]
. Therefore, the study on 60-
GHz wireless signals over long-reach optical access net-
works has been proven interesting. Previous experimen-
tal results show that optical millimeter wave (mm-wave)
signals generated by optical carrier suppression (OCS)
manifested high-receiver sensitivity and largely reduced
the bandwidth of optical and electrical components
[13]
.
However, due to fiber dispersion, the transmission dis-
tance of optical mm-wave signals generated by OCS at 60
GHz has been limited. Theoretical analysis have shown
that the maximum transmission distance is less than 40
km for 1-Gb/s on/off keying signal carried by a 60-GHz
OCS mm-wave
[14]
. Using orthogonal frequency division
modulation in tandem with single-sideband modulation,
transmission distance limited by fiber dispersion was
largely increased
[9]
, but its scheme has been compli-
cated. Channel codes can be introduced to improve
transmission distance, but only few reports focused on
this topic. Turbo codes serving as channel codes have
been used to overcome the impairments in the simple
configuration of RoF systems through direct detection
(DD). The introduction of turbo codes has been known
to reduce spectral efficiency because of the number of
redundancy bits. In our experiment, we demonstrate
the transmission of 1.25-Gb/s turbo-coded orthogonal
frequency-division multiplexing (OFDM) signals with
double-sideband transmission over a 180-km single mode
fiber SMF-28 long-reach optical link. The maximum
transmission distance at bit error rate (BER) of 2×10
−3
is extended by over 30% by employing the turbo coding
technique.
Figure 1 shows the principle of the RoF architec-
ture with turbo codes and optical frequency quadru-
ple scheme. At the central station, laser was used to
generate continuous wave (CW) lightwaves. Then, we
employed an external modulator to realize an all-optical
up-conversion. A directional coupler (DC) biased Mach-
Zehnder modulator (MZM) at top peak power to gen-
erate optical mm-waves with four times radio frequency
(RF). Base-band data were then modulated on the opti-
cal mm-wave. A 50/100-GHz interleaver (IL) with one in-
put port and two output ports was employed to separate
the optical carrier and the second-order sidebands at the
central station. Two second-order sidebands were filtered
out in one output port, while the optical carrier was
filtered out in the other output port. After transmission,
the two peaks of the second-order sidebands exhibited im-
pulses. They generated mm-wave signals at the quadru-
ple repetitive frequency of the RF signal after detection
Fig. 1. Principle of turbo-coded OFDM signals transmission
over SMF-28 in 60-GHz RoF long-reach system. Tx: trans-
mitter; Rx: receiver; DFB: distributed feedback laser; SSMF:
standard SMF; LPF: low-pass filter; DAC: digital-to-analog
converter; ADC: analog-to-digital converter; AWG: arbitrary
waveform generator; LO: local oscillator; PRBS: pseudo ran-
dom binary sequence; QPSK: quadrature phase shift keying;
OA: optical amplifier; IM: intense modulator; CP: cycle pre-
fix; TDS: real time oscilloscop e.
1671-7694/2010/111024-04
c
° 2010 Chinese Optics Letters
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