Currently, fiber-to-the-premise (FTTP) technologies have been entering into the real network to unlock the bandwidth potential of fiber in optical access networks [1]. On the other hand, broadband wireless access (BWA) technologies like WiFi/WiMAX/3G are very popular because they are more convenient, scalable and flexible for roaming connections [2]. The convergence of mobile wireless networks and high-bandwidth optical fiber has the potential to increase capacity and mobility as well as decreasing overall costs in the access network. Thus, radio-over-fiber (ROF) based optical-wireless networks have emerged as an affordable alternative solution in environments such as conference centers, airports and hotels - and ultimately to homes and small offices [3]. Furthermore, to reduce the total system cost of the central office (CO) and base station (BS) infrastructure, WDM-PON has been recognized as a candidate for the integration with ROF systems providing Internet access users at larger than 1-Gbit/s bandwidth [4]. Thus the new combined system results in better scalability, reduced cost, and less installation and maintenance. Recently, we have reported several all-optical schemes for mixing or up-converting of millimeter-wave (mm-wave) signals [5]
[6]
[7]
[8]
[9]. Such schemes are crucial in order to lower the system cost while maintaining high performance in the WDM-PON/ROF systems. We here present an access network design that is capable of simultaneously delivering access services at baseband and mm-wave wireless over optical fiber. In this paper, we also present our experimental testbed results for a single network access platform. This platform delivers realtime video and data applications by using external intensity or phase modulation scheme for optical mm-wave carrier generation and upconversion. In addition, the transmission performance of optical-wireless data signals over 40-km fiber and 4.0-m indoor air space at bandwidths up to 2.5 Gb/s are investigated and a wireless link power budget is analyzed.
SECTION II.
Rof Based Optical-wireless Testbed Desgin
Fig. 1 illustrates the architecture for providing super-broadband wireless and wired services. The metro networks send the data to the CO, where the multi-channel mm-wave carriers are generated by means of unique external intensity or phase modulation methods that we have developed. These upconversion techniques possess many advantages that allow data to be transmitted over wired and wireless medium in a single platform [3]
[10]. First, the generation of the mm-wave carrier and the upconversion of the original data channel are performed simultaneously in the optical domain. Second, as a result of this process, two identical data signals are generated concurrently: one at the electrical baseband and another at the RF-carrier frequency. Next, the up-converted signals are multiplexed before they are transmitted over fiber to the remote node (RN) where an arrayed waveguide grating (AWG) is used to route the signals to the BS. At the BS, the signal is divided by a power splitter into two branches.
The first part is passed through a high-speed receiver and then electrically amplified using a narrow-band RF amplifier before being broadcasted by an antenna as a wireless signal. The other part is sent directly to a wall-mounted optical port via fiber access, and a user can utilize a simple, low-cost receiver to detect the baseband data signal by filtering out the high frequency mm-wave signal.
SECTION III.
Experiemtnal Setup for OCS Based MM-WAVE Generation
The experimental testbed setup is shown in Fig. 2. DFB laser array is turned on and modulated by a LiNbO3 Mach-Zehnder modulator (LN-MZM) driven by 2.5-Gb/s electrical signal with a pseudorandom binary sequence (PRBS) word length of
231−1
. The modulated signals are transmitted over 40-km single mode fiber (SMF-28) for de-correlation before they are up-converted using the OCS technique. Upconversion using OCS is realized by using a dual-arm LN-MZM biased at
Vπ
and driven by two complementary 17.5-GHz clocks. This results in a carrier suppression ratio of larger than 25 dB, and the separation frequency of the two optical sidebands of 35 GHz (Ka-band). The up-converted mm-waves are then amplified by an EDFA to obtain 12-dBm power before transmission over 25-km SMF-28 to the BS.
At the BS, one optical mm-wave channel is selected via optical filter and then divided into two branches. One part, used to generate the wired link, is passed through a low-speed avalanche photodiode (APD) that has a 3dB bandwidth of 2 GHz. Because the bandwidth is limited at 2 GHz, the high frequency RF component is filtered out. In order to produce the wireless signal, the second branch is pre-amplified by an EDFA with a small-signal gain of 30 dB. It is then filtered by a tunable optical filter (TOF) before it is detected by a PIN PD with a 3-dB bandwidth of 60 GHz. The converted electrical signal is boosted by a RF electrical amplifier (EA) with a bandwidth of 10 GHz centered at 40 GHz before it is broadcasted through a double ridge guide antenna with a gain of 19.2-dBi. After wireless propagation, the signal is received by another identical mm-wave antenna. A 35-GHz electrical LO signal is generated using a 1:4 frequency multiplier, and it is used to down-convert the electrical mm-wave signal.
The receiver sensitivities and eye diagrams for 2.5 Gb/s at different air distances are shown in Fig. 3. The power penalty after transmission over 25-km SMF is less than 1.5 dB. The receiver sensitivity degrades quickly when the wireless signals are transmitted beyond approximately10-m because the power is dispersed over air space. Signal degradation via multiple reflections from the wall is a also key factor that limits the maximum transmission distance for a 2.5-Gb/s wireless signal in our testbed environment (an office building hallway). We then set the data rate to 1.25 Gb/s, the BER curve and eye diagram is measured and shown in Fig. 4 after 25-km fiber transmission. It is observed that the receiver sensitivity is increased by about 1.5 dB compared to the 2.5 Gb/s measurement.
Using this scheme, video transmission of a 270-Mbit/s uncompressed SDTV signal was implemented, as shown in Fig. 5. At the CO, an A/D (analog-to-digital) converter is used to convert an analog video signal from a DVD player into 270Mbit/s (SMPTE 259M SDTV signal) serial digital interface (SDI) signal. This signal is used to drive an LN-MZM in order to modulate a continuous light (CW) from a tunable laser (TL). At customer end, the signal is split into two parts for wireless and wired signal delivery as previously described. The wired part, after O-E conversion, is converted from the SDI format to analog video signals with a D/A (digital-to-analog) converter for display on one TV. For the wireless part, the down-converted signal is similarly processed and displayed on another TV. The received videos on the two TVs are clear, noise-free and stable.
SECTION IV.
Experimental Setup for Pahse Modulation Based MM-WAVE generation
Fig. 1 depicts the experimental setup for the dual-service optical-wireless system by using optical phase modulator (PM) and fiber Bragg grating (FBG). At the CO, a tunable CW laser at 1549.1 nm is modulated by a LiNbO3 PM driven by a 15-GHz RF sinusoidal wave with amplitude of 6 V (the half-wave voltage of the PM is 11 V). The optical spectrum after phase modulation is shown in Fig. 7(i). It is seen that the power ratio between first- and second-order sidebands is larger than 15 dB. Then an FBG is used to suppress the optical carrier and convert the modulated lightwave to optical mm-wave, which is shown as inset (ii) in Fig. 7. The FBG has a 3-dB reflection bandwidth of 0.2 nm and a reflection ratio larger than 50dB at the reflection peak wavelength. It is seen that the suppression ratio for the carrier is larger than 35 dB. Next, the optical mm-wave is modulated by a LN-MZM driven by PRBS
231−1
. The corresponding optical spectrum and eye diagram are shown as inset (i) and (ii) in Fig. 6. The optical mm-wave is then amplified by an EDFA to13 dBm before transmission over SMF-28.
At the BS, the optical mm-wave is divided into two parts. The wired part is sent to a low-speed APD that has a 3-dB bandwidth of 2 GHz. In the case of the wireless part, the signal is pre-amplified by an EDFA and filtered by a tunable optical filter to suppress ASE noise, then detected by a PIN PD with a 3-dB bandwidth of 60 GHz. The converted electrical signal is boosted by an EA with a bandwidth of 10 GHz centered at 40 GHz, before it is broadcasted through a double ridge guide antenna with a gain of 19.2 dBi at frequency range of 18 to 40 GHz. After wireless transmission, the signal is received by another identical mm-wave antenna and down-converted through a mixer and tunable delay (TD) line without the need of an LO signal.
The receiver sensitivities and eye diagrams at different data rates and SMF distances are shown in Fig. 8. The wireless transmission distance is 4 m to emulate coverage of a single room. The power fluctuations after 40-km SMF propagation in the optical eye diagrams arise from the chromatic dispersion. The power penalty of all four data rates is less than 2 dB after 40-km transmission. We observe that the 250-Mb/s data signal has a 5-dB improvement in receiver sensitivity through the same SMF and air transmission distance compared to 2.5-Gb/s signal. The eye diagrams and BER after 25-km fiber transmission for wired and wireless data are shown in Fig. 9 and Fig. 10, respectively.
It is observed that 3.5-dB improvement of receiver sensitivity at a BER of
10−10
can be obtained when the data rate is decreased from 2.5
Gb/s
to 1.0
Gb/s
. This scheme can also implement 270-Mb/s uncompressed SDTV signals. The received videos on two TVs are again very clear, noise-free and stable. In comparison, the phase modulation based scheme is more stable due to the removal of control circuit, however, it needs an optical notch filter to suppress the optical carrier.
The wireless link power budget is also analyzed for a 35-GHz (8.57 mm) system over a 10.2-m wireless link based on the Friis transmission equation, which is given as follows:
PR=PT+GT+GR−20log4πRλ(1)
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P_{R}=P_{T}+G_{T}+G_{R}-20\log{4\pi R\over \lambda}
\eqno{\hbox{(1)}}
The parameters are shown in Fig. 11. In our testbed, the transmitted power
PT
is 15 dBm, the antenna gain
GT
is 19.2 dBi. At position
A1
, it is assumed that the emitted power is
PT
if there is no loss from the passive antenna. The equation (1) gives the received power
PR
as -30.1 dBm. The noise is dominated by the LNA noise figure (5 dB) based on Friis' formula (we do not consider the mixer phase noise and return loss here). Therefore, the signal to noise ratio (SNR) in this situation is 37.9 dB. The real SNR should be smaller than this if we include the multi-path distortion and phase noise from other components.
We have designed and demonstrated a super-broadband access testbed to deliver both wired and wireless services simultaneously via fiber and air. An order of magnitude larger access bandwidth than current state-of-the-art Wi-Fi systems can be provided. Realtime video and data applications including 270-Mb/s uncompressed SDTV signal and a 2.5-Gb/s data channel are successfully transmitted over a fiber-air link by using optical upconversion technologies in a testbed environment. Our novel techniques for efficiently generating and multiplexing mm-wave signals do not require expensive high frequency electronic equipment. Moreover, baseband and mm-wave signals can be easily separated and redistributed at the users' premises as demonstrated in our testbed. As a result, super-broadband ROF and WDM-PON systems are integrated into a single, practical access platform to provide future-proof information services to both stationary and mobile users of the future.