We have proposed and experimentally demonstrated a 60-GHz bidirectional radio-over-fiber system with downstream orthogonal frequency division multiplexing address (OFDMA) and wavelength reuse upstream single-carrier frequency division multiple address (SC-FDMA). In the downstream, a 3-dB optical coupler is used for two-carrier injection-locking a distributed feedback (DFB) laser in order to realize the single-sideband modulation. In the upstream, the weakly modulated one of the two downstream carriers is filtered out for wavelength reuse. Transmission of 9.65-Gb/s 16-QAM downstream OFDMA on 60-GHz carrier and 5-Gb/s QPSK upstream SC-FDMA (2.5 Gb/s for each user) are both successfully demonstrated over 53-km standard single mode fiber without chromatic dispersion compensation. The crosstalk between the downstream OFDMA and the upstream SC-FDMA can be neglected.
©2010 Optical Society of America
The rapid growing demand for high speed wireless access has promoted the development of various new kinds of communication systems that offer an enhanced bandwidth and mobility. Millimeter-wave (mm-wave) up to 60-GHz which offers a several-gigahertz license-free bandwidth has been considered as a promising candidate to support multi-gigabits wireless communications . However, the transmission loss of 60-GHz mm-wave in the air is very high and the distribution over the coax is also very expensive. Thus, radio-over-fiber (ROF) technology, the integration of wireless and optical communications, is a powerful solution for distribution of future high-speed mm-wave wireless access network [2–6]. It can make the wireless access network more flexible by using a fiber to connect the central office (CO) with remote base stations (BS), which can simplify the BS’s structure and move most of its functions, such as signal processing, to the CO.
In order to support several users simultaneously and satisfy them with their own unique demands, the 60-GHz ROF systems must be able to handle wireless signals which accommodate emerging services and applications. The orthogonal frequency division multiplexing address (OFDMA) technique, which can achieve high bit rates and supports multiple services over different orthogonal subcarriers, is very suitable for this wireless downstream scenario. However, due to its high peak-to-average ratio (PAPR), the OFDMA technique requires highly linear power amplifiers to avoid excessive inter-modulation distortion, which is not cost-effective for upstream. A modified form of OFDMA, referred to as the single-carrier frequency division multiple address (SC-FDMA), is investigated for the uplink multiple access scheme in the “long-term evolution (LTE)” of cellular systems by the Third Generation Partnership Project (3GPP) [7,8]. Due to its inherent single carrier transmission characteristics, it has lower PAPR under similar throughput performance and overall complexity compared with OFDMA. So it is a good choice to introduce the SC-FDMA as the upstream technique in the 60-GHz ROF systems.
Due to its high spectral efficiency, high tolerance against various fiber dispersion effects, extreme flexibility on multiple services access and dynamic bandwidth allocation, the OFDM technology has become more and more popular in optical high-capacity long-haul communication and next-generation passive optical network [9–11]. The mm-wave ROF system with OFDM modulation format is also regarded as a potential solution to support future wireless access [12–18]. However, in those bidirectional mm-wave OFDM-ROF systems with centralized source [15–18], the upstream performance of analog format such as the subcarrier SC-FDMA signal has not been evaluated. Meanwhile, in the mm-wave OFDM-ROF systems, optical single-sideband (SSB) modulation can be used to mitigate the received electric power fading due to the fiber chromatic dispersion and the inter-subcarrier interference after the photodiode [19–27]. The authors have introduced a new approach of SSB modulation in optical heterodyning mm-wave ROF systems based on an injection-locked semiconductor laser [28–31]. The requirement of the slave laser’s modulation frequency response needs not to be as high as it is in [25,27]. However, in our former SSB scheme, an optical circulator is used for injection locking, which is expensive for the real network deployment [28–31]. Moreover, how to integrate the injection-locking scheme with bidirectional OFDM-ROF system has not been studied. In this paper, we propose a new bidirectional 60-GHz mm-wave ROF system with SSB modulation of downstream OFDMA based on a two-carrier-injected distributed feedback (DFB) laser and wavelength reuse upstream SC-FDMA. The optical circular in the former downstream injection-locking setup is replaced by a 3-dB coupler, which reduces the total deployment cost. In the upstream, the scheme of centralized light source, which simplify the overall network architecture, is realized because the weakly-modulated one of the two downstream carriers is filtered out as the upstream light source. Transmission of 9.65-Gb/s 16-QAM OFDMA signals on a 60-GHz carrier for the downstream and 5-Gb/s QPSK SC-FDMA signals (2.5 Gb/s for each user) for the upstream are both successfully realized over 53-km standard single mode fiber (SSMF) using the proposed system design. The cross talk between the upstream SC-FDMA and the downstream OFDMA can be neglected, which means the bandwidth resource can be fully reused in our proposed bidirectional architecture.
2. Proposed system architecture
Figure 1 illustrates the proposed bidirectional ROF system with downstream OFDMA and wavelength reuse upstream SC-FDMA. In the downlink connection, the two optical phase-coherent carriers with 60-GHz spacing generated by external modulation are injected into the slave DFB laser through a 3-dB optical coupler. Only one carrier is tuned to lock the slave laser. When the slave laser is directly modulated by the immediate-frequency (IF) band downstream OFDMA signals, the injection-locked carrier is strongly modulated, while the unlocked carrier experiences a much weaker modulation. After SSMF transmission, the downstream is divided into two parts. One is sent to a high-speed photodiode (PD) to generate the downstream OFDMA mm-wave signal by optical-electrical (O/E) conversion. The other part is used to filter out the weakly modulated carrier for the optical upstream modulation by an optical band-pass filter (OBPF). At the user side, the OFDMA mm-wave signals are down-converted and demodulated to baseband for further digital signal processing (DSP). In the uplink connection, the received upstream SC-FDMA mm-wave signals from different users are combined and down-converted to IF band by an electrical mixer or envelope detection at the base station. They are used to drive a Mach-Zehnder modulator (MZM) to generate the upstream optical signals for transmission.
As shown in Fig. 2 , the OFDMA technology is a combination of time division multiple address (TMDA) and OFDM, in which the orthogonal subcarriers can be dynamically assigned to different services in different time slots. For the downstream data traffic, the CO encapsulates the services for each user into the given subcarriers and time slots according to the frequency/time domain schedule. In this architecture, the pre-assigned sub-channels, which contain one or more subcarriers, become transparent media for delivery of the various multiple services in packet- or circuit-switching. For example, the dedicated subcarriers, red and blue slots in Fig. 2, can be reserved as the simultaneous pipes for different services respectively. At the user side, each user picks out his own data from the proper orthogonal subcarriers which have been allocated to him. For the upstream data traffic, each user maps the frequency components of the single-carrier data to the pre-assigned subcarriers, sets all the other subcarriers to zero, and completes the modulation to generate an SC-FDMA mm-wave signal. Since they transmit the subcarriers sequentially rather than in parallel, the envelope fluctuations in the transmitted waveform of SC-FDMA signals is reduced and the burden of linear amplification at the user is mitigated. Moreover, if the traffic is organized with SC-FDMA frames, each of which consists of multiple SC-FDMA slots or symbols, the resource allocation may also be two-dimensional in both frequency and time domains as shown in Fig. 2. In the CO, the combined SC-FDMA signals are demodulated together and the data from different users are separated according to their pre-allocated orthogonal frequencies.
We have experimentally demonstrated a SSB modulation scheme using an injection-locked Fabry-Perot Laser, which has a large injection-locking wavelength range and is relatively cheaper for network deployment . Its flexibility on data transmission has also validated using similar experiment setup in . So this SSB modulation scheme based on injection-locked semiconductor laser can be simply integrated with the future wavelength division multiplexing ROF passive optical network (WDM-ROF-PON) to support more users, which is illustrated in Fig. 3 .
3. Experiment setup
The experimental setup is shown in Fig. 4 . The optical carrier suppression modulation is used to generate two phase-coherent carriers. It is realized by driving a Mach-Zehnder modulator (MZM) biased at Vπ with a 30-GHz reference signal. The modulator output is sent to an optical notch filter to select the two first-order modulation sidebands and reject the carrier. An erbium-doped fiber amplifier (EDFA) is introduced to get the proper injection power. The two phase-correlated optical carriers with 60-GHz spacing and 8.2-dBm total power are injected into a DFB laser through a 3-dB optical coupler. A polarization controller (PC) is used to align the polarization state of input light with that of the DFB laser. Only one of the two phase-correlated carriers is tuned to lock the slave laser. The DFB laser has a threshold current of 14 mA and is biased at 27 mA. The optical injection locking is maintained by controlling its temperature at 25 °C. The 9.65-Gb/s 16-QAM centered at 2.5 GHz is generated from an arbitrary waveform generator (AWG) to directly modulate the slave laser. The modulated output is launched into 53-km SSMF and the loss is compensated by two EDFAs at the two ends. After transmission, the received optical signal is divided into two parts by a 3-dB optical coupler. One is detected by a photodiode with 3-dB bandwidth of 70 GHz. The received 60-GHz subcarrier OFDMA signal is down-converted for further evaluation using a mixer and a local oscillator (LO). The other part is fed into an optical band-pass filter and the weakly-modulated optical carrier is filtered out for the upstream modulation. A 2.5-GHz-centered 5-Gb/s QPSK SC-FDMA signal from the AWG is used to simulate the down-converted two end users’ information coming from the antenna. After upstream transmission over the same length SSMF as the downstream channel, a photodiode with 3-dB bandwidth of 10 GHz and a low noise amplifier are employed to detect the SC-FDMA signal.
Figures 5(a) and 5(b) show the transmitter and receiver off-line DSP block diagram of OFDMA and SC-FDMA we used in the experiments respectively. At the transmitter side, the OFDMA or SC-FDMA signal is generated and up-converted to 2.5 GHz by digital I-Q modulation in off-line processing. It is uploaded into a Tektronix AWG7122B. The waveforms produced by the AWG are continuously outputted at a sample rate of 10 GS/s. For the downstream OFDMA, the Fast Fourier Transform (FFT) size is 256, from which 210 channels are used for data transmission. The cyclic prefix (CP) size is 16 and two OFDM symbols are applied in channel estimation and equalization for each 32 OFDM symbols. Since the up-sample rate for 16QAM is 3, the bit rate is 9.65 Gb/s. For the upstream SC-FDMA, the FFT size is 256 and 204 subcarriers (102 for each user) are used for data transmission. The CP size is 16 and QPSK is used for constellation mapping. Since the SC-FDMA signal is generated with three times up-sampling and 4 symbols are applied for in channel estimation for the next 200 symbols, the total bit rate is 5 Gb/s (2.5 Gb/s for each user). At the receiver side, the down-converted OFDMA signal from 60-GHz mm-wave or upstream SC-FDMA signal are sampled with a real-time oscilloscope (Tektronix DPO72004B) at the sampling rate of 25 GS/s. An off-line program is employed to demodulate the OFDMA or SC-FDMA signals. The channel estimation and equalization in the off-line receiver are used to overcome both frequency response of various electrical components and fiber dispersion.
4. Results and discussion
Figure 6(a) compares the optical spectra of the locked DFB laser with and without downstream 16-QAM OFDMA modulation. The shorter-wavelength carrier at 1551.440 nm is used to lock the DFB laser. When the DFB laser is directly modulated, the injection-locked carrier is strongly modulated and its spectrum is broadened, while the unlocked carrier experiences a much weaker modulation. The locked slave laser can be recognized as a gain-clamped amplifier with its cavity mode red-shifted . The red-shifted cavity mode supplies an optical gain to the modulation sidebands. The center wavelength of the optical gain profile can be controlled by injection power and frequency detuning. Generally, small injection power and negative frequency detuning lead to a cavity mode close to the injection-locked carrier. But enough injection power and proper frequency detuning are needed to insure stable locking. The total injection power of 8.2 dBm and frequency detuning of −15.5 GHz (frequency difference between the injection-locked mode and the free-running slave DFB laser, Δf = f locked - f free-running) are chosen to make the red-shifted cavity mode close to the locked carrier and get stable locking state. Since the shorter wavelength carrier is closer to the cavity mode, its modulation sidebands can be resonantly amplified. However, the unlocked carrier, which is 60 GHz away from the locked one, is out of the gain range of the cavity mode and its modulation sidebands cannot be amplified. So the direct modulation of the slave laser introduces large modulation index difference between the two carriers and the SSB modulation is realized. The inset (i) in Fig. 6(a) is the down-converted electrical spectrum of downstream OFDMA signals and there is no fading effect influence induced by 53-km SSMF transmission. Figure 6(b) shows the optical spectra of upstream after the MZM with and without QPSK SC-FDMA modulation. The received electrical spectrum of the upstream from two users is also illustrated as inset (ii) in Fig. 6(b).
Since the PAPR of OFDMA signal is inherently high, the modulation current intensity of the injection-locked slave laser should be optimized to avoid the improper signal clipping. As shown in Fig. 7(a) , the received bit error rate (BER) performances of downstream OFDMA under different modulation index is measured by adjusting the modulation voltage amplitude on the slave laser. When the modulation intensity is below 1.2, the BER performance is getting better with increase of modulation intensity, because the received signal intensity is larger under bigger modulation amplitude. However, as the modulation intensity gets larger than 1.2, the signal clipping problem appears, and the received BER performance degrades. So a modulation intensity of 1.2 is chosen for further downstream BER evaluation. When the upstream SC-FDMA signal is applied to the MZM, the modulation intensity should also be optimized since the transfer function of MZM is nonlinear and periodic. We also tested the BER performance of upstream SC-FDMA under different modulation intensity to find the optimal point. As shown in Fig. 7(b), the BER performance below modulation intensity 1.5 is not good due to the small received signal. It is also getting worse with increasing modulation intensity beyond 1.5 because of the modulation nonlinearity and the periodic transfer function of MZM. So the optimal modulation intensity for the upstream SC-FDMA is chosen at 1.5.
The received BER performances of the downstream OFDMA and upstream SC-FDMA shown in Figs. 8(a) and 8(b) are calculated from the measured error vector magnitude (EVM) . The insets also show the equalized constellation diagrams of some cases. For the downstream, the measured BER of 16-QAM exhibits 1.3-dB power penalty at BER of 10−4 compared with the back-to-back (BTB) case. It is due to the residual dispersion of the non-ideal SSB modulation and the additional ASE noise introduced by EDFA at the receiver side. The receiver sensitivity in our downstream experiment is much larger compared to the former studies [16,27]. That is due to the lack of 60-GHz amplifier. For the upstream, the power penalties of the two users at BER of 10−4 can be neglected. Since the upstream and the downstream signals occupy the same frequency band, the BER performance of upstream after SSMF transmission with downstream off is also tested in the experiment to evaluate the interference between downstream and upstream. As shown in Fig. 8(b), the crosstalk between downstream and upstream can be neglected because of the small modualtion index of the unlocked downstream carrier.
We have proposed and experimentally demonstrated a 60-GHz bidirectional ROF system with downstream OFDMA and wavelength reuse upstream SC-FDMA. In the downstream, a 3-dB optical coupler is employed for two-carrier injection locking a DFB laser to realize the optical single-sideband modulation of OFDMA signals. Transmission of a 9.65-Gb/s 16-QAM OFDMA signal on 60-GHz carrier is successfully demonstrated over 53-km downlink SSMF without chromatic dispersion compensation. The power penalty is about 1.3 dB. In the upstream, the need for a new light source at the BS is avoided by filtering out the weakly-modulated carrier from the downstream SSB signals. The power penalty of QPSK upstream SC-FDMA signals (5-Gb/s total, 2.5-Gb/s for each user) after transmission over 53-km SSMF can be neglected. The crosstalk between the downstream OFDMA and the upstream SC-FDMA can also be neglected, which improves the spectral efficiency greatly.
The authors would like to thank the reviewers for their constructive suggestions that help improve the manuscript. The authors would also like to thank Dr. Guangyuan Li for the instructive discussion with him. This work is supported by the National Natural Science Foundation of China (NSFC) under Grant 60736003 and the National Basic Research Program (973 Program) (No. 2010CB328201 and 2010CB328202).
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