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We propose a modified localized carrier distribution scheme based on multi-tone generation to generate 60 GHz mm-wave for different wireless users and it improves the carrier utilization efficiency by 33.3%. The principle of multiple-user discrete Fourier transform spread optical orthogonal frequency-division multiplexing (DFT-S OFDM) Radio-over-fiber (RoF) system is presented. This multiple-user system is applicable to passive optical network (PON). Then we demonstrate a 8x4.65 Gb/s multiple-user DFT-S OFDM RoF-PON wireless access system over 40 km fiber link and 60 GHz wireless link using two localized carrier distribution scheme with different spectral efficiency. Compared to conventional OFDM, 2.3 dB reduction of receiver power using DFT-S OFDM modulation scheme and the calculated BER performance for 8 wireless users clearly demonstrates the feasibility of this spectrally efficient multiple-user RoF-PON scheme.
©2012 Optical Society of America
1. Introduction
A wide variety of video and multimedia services drive the demand of wireless access rate from today’s end users. Broadband and low loss capability of transmission over fiber link has led to an increasing interest in Radio-over-fiber (RoF) systems, which are related to the generation, processing and distribution of millimeter-wave signals. Users can benefit from large bandwidth and convenience in wireless in such kind of systems. With respect to wireless access networks, the 60 GHz access system with 7-GHz licensed-free band has been considered as a promising candidate to provide multi-Gbps. It’s also applicable to passive optical network (PON) scenario for the mobile access user [1–8]. The seamless integration of RoF and PON is a promising research direction, which conveys the data modulated RF signal on the fiber link so that the baseband data stream can be simultaneously delivered to mobile users. This could support a huge number of base stations (BSs) connected to a central station (CS) and meet the increasing demand of wireless access coverage [3,4]. However, the proposed multiple-user RoF systems have low spectral efficiency [5], or high cost because of the requirement of an individual light source for each WDM link [6]. On the other hand, it is demonstrated that a discrete Fourier transform spread optical orthogonal frequency division multiplexing (DFT-S OFDM) technique will become attractive for transmission in the next generation long-haul and access networks for its high spectral efficiency, the resistance to chromatic dispersion, and effective power-average-peak-ratio (PAPR) reduction, which utilizes additional DFT at the transmitter to reduce PAPR and improve systems’ tolerance to nonlinearity of fiber link and wireless link [9–11].
In this paper, we propose a modified localized carrier distribution scheme using multi-tone generation technique to generate 60 GHz mm-wave for different wireless users. Two schemes with different spectral efficiency are given. The principle of multiple-user DFT-S OFDM RoF-PON wireless access system is presented. Then we demonstrate a DFT-S OFDM multiple-user RoF-PON system over 40 km fiber link and 60 GHz wireless link using different localized carrier distribution schemes. The maximum total rate 8x4.65 Gb/s is obtained (4.65 Gb/s for each one of 8 wireless users). The DFT-S OFDM modulation scheme is compared with the conventional OFDM scheme. The BER performance of all channel for all wireless users are measured.
2. Principle
The generation and recovery schematic diagram of DFT-S OFDM is introduced in [9–11]. It introduces one more Discrete Fourier Transform (DFT) than conventional OFDM scheme. At first, the MxN payload is divided into M sets, and N points DFT is employed for each set. Thus the OFDM baseband consists of MxN carriers, and then the baseband signal would be mapped from frequency domain into time domain through MxN points IDFT (Inverse Discrete Fourier Transform), which is similar to the conventional OFDM scheme. Because of the subband-basis process before the IDFT in the transmitter, the possibility of high peak is reduced.
For the multiple-user RoF-PON wireless access systems, one of the key techniques is multi-tone generation. The beat carrier and signal carrier are chosen from these carriers and transmitted over fiber link simultaneously. A conventional scheme called interleaved distribution scheme introduced in [5], as shown in Fig. 1(a) .The adjacent two subcarriers are considered as the signal carrier and beat carrier and they would generate mm-wave signal after beating at the photodiode (PD). Because one beat carrier and two signal carriers which are located in its both sides could beat and generate the mm-wave of the same frequency, the utilization efficiency of carriers could be improved further. Therefore, we propose a modified carrier distribution scheme called localized carrier distribution scheme for multiple-user RoF-PON systems (as shown in Fig. 1(b) and (c)). The total generated subcarriers are divided into beat carrier sets and signal carrier sets. The product of the number of carriers per set and the carrier frequency spacing should equal the frequency of the desired mm-wave. So we present two localized carrier distribution schemes. The beat carrier (black) could be used twice and the two signal carriers with 60 GHz frequency spacing at the left and right side of the beat carrier could generate 60 GHz mm-wave after received by PD. The difference between these two schemes is the carrier frequency spacing and the number of carriers per set, which leads to different spectral efficiency. For interleaved carrier distribution scheme [5], it should be noted that although the frequency spacing is 50 GHz, the double sideband suppressed carrier (DSB-SC) modulation format is employed and 60 GHz mm-wave could be generated. The number of beat carriers is over-abundance. Because each beat carrier and the signal carrier at its left or right side could generate mm-wave, some of the beat carriers are useless. Furthermore, the frequency spacing is 50 GHz, which is larger than the scheme we proposed. Therefore, the modified scheme we proposed not only reduces the frequency spacing but also make full use of the generated carriers. Assuming that the total number of carriers is L, the number of carriers that could be used as signal carriers is 2L/3 for both localized schemes, while the number is L/2 for the interleaved carrier distribution scheme. Therefore, the scheme we propose could improve the carrier utilization efficiency by 33.3%. Also, the scheme introduced in Fig. 1(c) has higher spectral efficiency than the scheme presented in Fig. 1(b) for its narrower carrier frequency spacing.
Fig. 1 (a) Interleaved carrier distribution scheme [5], (b) localized carrier distribution scheme with 20 GHz carrier frequency spacing, (c) localized carrier distribution scheme with 15 GHz carrier frequency spacing.
Figure 2 shows the architecture of the proposed modified multiple-user DFT-S OFDM RoF-PON system. A multi-tone generator is employed to generate frequency-lock multi-tone. Then the optical signal carriers are separated from the generated multi-tone and demultiplexed for different base stations (BSs). Then the different channels are modulated by wireless data for delivery to BSs after multiplexing signal carriers and beat carriers. After transmission over fiber link, these channels are demultiplexed and the specified signal and beat carrier are picked up by the tunable optical filter (TOF) for each BS (only 3 BSs are shown in Fig. 2). It should be noted that the employed interleaver and TOF are replaced by the waveshaper (WSS) in our experiment for convenience. For the practical application, a fixed demultiplexer and filter could be designed to reduce the cost. These BSs are located in different place, thus the access coverage is increasing. At the BS, one signal carrier and one beat carrier with 60 GHz frequency spacing are beating at the PD, and then the DFT-S OFDM signal is conveyed by 60 GHz mm-wave and delivered through the antenna. In order to minimize the influence of fiber dispersion and nonlinearity distortion, a 4QAM DFT-S OFDM signal is employed for its efficient PAPR reduction [10,11]. Note that the architecture of the multiple-user DFT-S OFDM RoF-PON system is the same for different localized carrier distribution scheme.
Fig. 2 Schematic diagram of the multiple-user DFT-S OFDM RoF-PON wireless access system, ECL: external cavity laser, PM: phase modulator, EA: electronic amplifier, TOF: tunable optical filter.
3. Experimental setup and results
Figure 3 indicates the experimental setup of multiple-user RoF-PON system. For the multi-tone generator, a lightwave at 1550.2 nm from external cavity laser (ECL) with a linewidth less than 100 kHz is employed as a signal source. The phase modulator (PM, Vπ = 2.9V) is driven by 15 GHz RF signal. The first waveshaper (WSS1, Finisar WS-AA-4000S) is employed to separate the signal carriers and beat carriers. The transmitted DFT-S OFDM wireless data is generated off-line by MATLAB program and mapped to 4-QAM constellation. The DFT-S OFDM baseband signal is constructed with 256 subcarriers. It’s divided into 4 sub-bands, 4 subcarriers of each sub-band are used for phase estimation. A discrete multi-tone (DMT) technique [12] is utilized and the resulting output after IDFT is of real value. This leads to a much simpler cost-effective transmitter structure. For the conventional OFDM modulation scheme, the number of subcarriers in the baseband signal is also 256 and 16 subcarriers are chosen for phase estimation. An arbitrary waveform generator is used to produce RF signals at 5 GSample/s, and subsequently driving intensity modulator (IM) between the minimum and maximum transmission. The fiber launch power is set to 2.3 dBm.
Fig. 3 Experimental setup of the DFT-S OFDM multiple-user RoF-PON system, ECL: external cavity laser, PM: phase modulator, EA: electronic amplifier, IM: intensity modulator, AWG: arbitrary waveform generator, WSS: waveshaper.
Two localized carrier distribution schemes with different carrier frequency spacing and different number of carriers per set are employed for both conventional OFDM signal and DFT-S OFDM signal. Figure 4(a) and Fig. 5(a) shows the optical spectrum of multi-tone after phase modulator of two localized carrier distribution scheme respectively. It’s observed that the generation comb has not very good flatness. Because the beat carriers will not be injected into intensity modulator and have less insertion loss, the middle beat carriers are adjusted to have lower power than the signal carriers. This would ensure that the power deviation of signal carriers and beat carriers at the PD is not very large. The separated signal carriers after modulation by wireless data are shown in Fig. 4(b) and Fig. 5(b). Figure 4(c) and Fig. 5(c) indicate the spectrum after multiplexing all the subcarriers including signal carriers and beat carriers of two localized carrier distribution schemes respectively.
Fig. 4 The localized carrier distribution scheme with 20 GHz carrier frequency spacing, the spectrum (a) after phase modulator, (b) after wss1, (c) after 2:1 coupler, (d) after wss2, (e) calculated BER of DFT-S OFDM signal for all 6 wireless users after 40 km fiber link and 60 GHz wireless link.
Fig. 5 The localized carrier distribution scheme with 15 GHz carrier frequency spacing, the spectrum (a) after phase modulator, (b) after wss1, (c) after 2:1 coupler, (d) after wss2, (e) calculated BER of DFT-S OFDM signal for all 8 wireless users after 40 km fiber link and 60 GHz wireless link.
After 40 km fiber link, the second waveshaper (WSS2) is used as a multi-band pass filter and the specified signal carrier and beat carrier are filtered and beating at the PD to generate 60 GHz mm-wave for one wireless user, the optical spectrum is shown in Fig. 4(d) and Fig. 5(d) respectively. Then the 60 GHz mm-wave is amplified by a power amplifier (PA) with bandwidth of about 7 GHz centered at 60 GHz and broadcasted through a double-ridge guide rectangular horn antenna with a gain of 15 dBi, frequency range of 50-75 GHz. At the end user terminal, the broadcasted wireless signal is received by another 60 GHz horn antenna with 15 dBi gain at frequency range of 50-75 GHz. The broadcast distance between two antennas is 4 cm. The distance is not very long, but we would improve it in the future work. The received signal is amplified before mixing with 60 GHz RF clock using a V-band balanced mixer for direct signal down-converted, then amplified with an electronic amplifier again after a lowpass filter (LPF) [2]. The down-converted 4QAM DFT-S OFDM signal is then sampled by a high-speed oscilloscope at a sampling rate of 50 GSa/s and processed off-line with a MATLAB program. Each signal carrier and its corresponding beat carrier could be chosen for different wireless users, so the bit rate for single user is 4.65 Gb/s, taking into account the pilot cost.
For the offline processing, the cyclic prefix is removed after signal synchronization and digital bandpass filter. 256x2 points DFT is used to convert the signal in time domain into frequency domain. Then the baseband signal is separated, and 4 sub-band signals are processed individually. The processing of channel and phase noise estimation are the same as conventional OFDM for each sub-band, then a 64-point IDFT is used to recover the signal. For the conventional OFDM scheme, the offline process is similar to DFT-S OFDM, but there is no baseband separation and extra IDFT process. The captured and processed DFT-S OFDM block includes over 160 symbols. So the total bits for bit error counting are approximate to 105.
Firstly, we have tested all channels and the calculated BER of DFT-S OFDM signal for each wireless user after 40 km fiber link and 60 GHz wireless link is shown in Fig. 4(e) and Fig. 5(e) for both localized carrier distribution schemes. The inserted constellations in Fig. 4(e) and Fig. 5(e) show that each channel is clearly recovered. Moreover, for the scheme introduced in Fig. 1(b), the BER for 6 channels are all below 1x10−3, while the 2nd, 7th, 8th channel of the scheme with 15 GHz carrier spacing introduced in Fig. 1(c) is beyond 1x10−3, but still below 3.8x10−3 (7% FEC threshold). Because the latter scheme has narrower carrier frequency spacing (15 GHz), so the impact from adjacent channel is larger. On the other hand, for the nonperfect filter effect of WSS, there are some leakage when choosing signal and beat carrier, and this leakage is more serious for the scheme with narrower frequency spacing (as shown in Fig. 4(d) and Fig. 5(d)).
Then we measured the BER performance versus received optical power of the 8th channel which is the worst in the scheme with 15 GHz frequency spacing, using DFT-S OFDM and conventional OFDM modulation respectively. We also present the BER performance of one chosen channel from the system based on the interleaved carrier distribution scheme. It is observed that the proposed localized distribution scheme, indicated by the black line in Fig. 6 , has about 2 dB penalty at BER of 10−3 than the case that employing the interleaved scheme. Because the proposed scheme has only 15 GHz frequency spacing and the number of carriers is much more within a fixed frequency band than the interleaved scheme, the effect of XPM is obvious which leads to more serious inter-channel interference. Furthermore, the proposed scheme has narrower frequency spacing, so the leakage caused by the WSS when choosing the target carriers is more apparent. The BER performance of the localized scheme without wireless transmission link is also presented. The DFT-S OFDM scheme has about 0.9 dB improvement of receiver power at BER of 1x10−3 than the conventional OFDM scheme when the transmission link is only 40km SSMF. After wireless link, about 2.3 dB reduction of receiver power at BER of 1x10−3 denotes that the PAPR reduction of the DFT-S OFDM scheme could improve the performance of the RoF-PON system further. Due to smaller PAPR when DFT-S OFDM modulation is employed, the transmitter works at the linear region and has better receiver sensitivity without wireless link. When the wireless transmission is employed, it would cause more serious distortion when the PAPR is high. Therefore, DFT-S OFDM modulation scheme could also improve the performance of wireless transmission.
Fig. 6 BER performance versus received optical power of the RoF-PON system with different modulation formats and carrier distribution schemes.
4. Conclusion
In this paper, we propose a modified localized carrier distribution scheme based on multi-tone generation technique to generate 60 GHz mm-wave for multiple wireless users and it improves the carrier utilization efficiency by 33.3%. The DFT-S OFDM technique is employed and the data for multiple wireless users are transmitted over the fiber link simultaneously. Two localized carrier distribution scheme with different spectral efficiency are given. Then we demonstrate a DFT-S OFDM multiple-user RoF-PON wireless access system over 40 km fiber link and wireless link. The maximum total rate 8x4.56 Gb/s is achieved. Compared to the conventional OFDM scheme, 2.3 dB reduction of receiver power at BER of 1x10−3 by employing DFT-S OFDM modulation scheme shows that it could improve the performance of wireless transmission. The calculated BER for 8 wireless users clearly demonstrates the feasibility of this spectrally efficient multiple-user RoF-PON system.
Acknowledgments
This work was partially supported by the NHTRDP (973) of China (Grant No. 2010CB328300), and NNSF of China (No. 61107064, No. 61177071, No. 600837004), Doctoral Fund of Ministry of Education, Pujiang Fund and Shuguang fund.
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