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This study proposed and experimentally demonstrated a cost-efficient scheme that can deliver 60 GHz millimeter-wave (mm-wave) multi-gigabit wireless services over 125 km long-reach passive optical networks (PONs) without any dispersion compensation. By introducing a remote local exchange (LE) stage with robust signal regeneration and all-optical upconversion functionalities, the proposed long-reach optical-wireless access network can easily accommodate over 128 users with 2.5 Gb/s shared bandwidth as well as shifting the capital expenditure of multiple hybrid optical network units (ONUs) toward single LE headend. Experimental verification shows that the power penalties for wireless and wired services are 1.8 dB and 0.4 dB at 10-9 BER after 125 km optical fiber transmission.
©2009 Optical Society of America
1. Introduction
The delivery of 60 GHz millimeter-wave (mm-wave) wireless services over passive optical networks (PONs), a seamless integration of optical wired and wireless access networks with large bandwidth, high flexibility, and high connectivity, has attracted lots of attention due to the emerging last-mile, last-meter applications such as multi-channel HDTV distribution and interactive multimedia services for home and office networking in the near future1–3. However, the major issue of such hybrid optical-wireless access network is limited by the fiber transmission distance of 60 GHz mm-wave signals, which is mainly due to the fading and time-shifting effects induced by fiber chromatic dispersion1, and thus makes it merely suitable for integrating with those conventional PONs with coverage around tens of kilometer in radius. Such short-reach access system will lead to higher cost in terms of network deployment and management, and is expected to be upgraded urgently toward a next-generation access scenario with longer reach and less number of central offices (COs) in the metro area, which is highly expected to simplify the entire network infrastructure by consolidating both metro and access networks4, 5. Basically, delivering mm-wave and baseband signals at different wavelengths and then mixing them remotely, so called remote local oscillator delivery schemes, is regarded as effective alternatives to mitigate the dispersion effect in hybrid mm-wave optical-wireless links6–8, and is a promising deployment strategy for long-reach access networks. These schemes, however, usually require costly mm-wave-band electrical mixing at each hybrid optical network unit (ONU) for the remote signal upconversion. Therefore, in order to extend the reach of mm-wave wireless signals over PONs while simultaneously achieving cost reduction of all hybrid ONUs, a novel long-reach hybrid PON architecture is designed and experimentally demonstrated for delivering multi-gigabit wired and wireless services to 128 ONUs using remote signal regeneration and upconversion techniques. By introducing a local exchange (LE) headend with a remote optical upconverter shared by all hybrid ONUs, network reach can be extended up to 125 km, which is, to the best of our knowledge, the longest fiber transmission distance of a 2.5 Gb/s baseband data carried by 60 GHz mm-wave without requiring dispersion compensation. In addition, the proposed long-reach PON architecture with 60 GHz mm-wave service overlay can coexist with all legacy ONUs, and is potential for future upgrade toward 10 Gb/s optical-wireless systems.
2. Long-reach mm-wave optical-wireless access network
Figure 1 illustrates the conceptual diagram of proposed long-reach and conventional short-reach 60 GHz mm-wave hybrid optical-wireless access networks. Unlike the short-reach access sending upconverted mm-wave signals at single λS from an optical line terminal (OLTS) in central office (CO), the basic idea behind the proposed long-reach scheme is to deliver 2.5 Gb/s baseband signal and 60 GHz mm-wave carrier independently at different wavelength λL1 and λL2 from OLTL and then mix them at a LE with a remote optical upconverter, which is commonly utilized as an extended and managed network stage in optical access network4, 5. Thus, before arriving at LE headend, both uncorrelated lightwaves can suffer from the least dispersion-induced penalties over long-distance fiber transmission. After the LE, the output 60 GHz mm-wave at λL2 will carry 2.5 Gb/s data and then be delivered through remote node (RN) to hybrid ONUs or legacy ONUs for wireless and wired access. In addition, since one single optical upconverter is arranged at the LE to serve multiple hybrid ONUs, the structure as well as the expense of these hybrid ONUs can be further simplified and reduced by shifting their local upconversion functionality toward the LE.
Fig. 1. Network architecture of the proposed long-reach and conventional short-reach 60 GHz mm-wave hybrid optical-wireless access networks.
3. Proof-of-concept experimental setup
Figure 2 shows the proof-of-concept experimental setup for the proposed long-reach 60 GHz mm-wave hybrid optical-wireless access network employing remote optical upconverter at LE. At the CO, a baseband signal was generated by directly modulating an optical transmitter (Tx) at 1555.2 nm using 2.5 Gb/s pseudo random bit sequence (PRBS) data sequence with a word length of 231-1, while a 60 GHz optical mm-wave carrier was created by externally modulating a laser diode (LD) at 1547.9 nm using a LiNbO3 phase modulator (PM) driven by 30 GHz sinusoidal clock, which came from the combination of a frequency multiplier (FM) and a 7.5 GHz clock source. After that, a 50/25 GHz optical interleaver is utilized to couple the two lightwaves as well as to suppress the carrier of the optical mm-wave. After transmitted over 100 km standard single-mode fiber (SSMF), two lightwaves were fed into an LE for remote signal regeneration and upconversion. The LE consists of two optical circulators (OCs) and three in-line EDFAs for bidirectional amplification, and a remote optical upconverter module, which consists of a thin-film filter (TFF), a 2.5 Gb/s optical receiver (Rx) and a 2.5 GHz intensity modulator (IM). After fed into the TFF with low insert loss of 0.5 dB, two lightwave channels were separated; the 60 GHz optical mm-wave proceeded to pass through the IM, while the 2.5 Gb/s baseband signal was detected and regenerated by an optical receiver, and then was utilized to drive the IM directly to realize remote all-optical upconversion. The proposed remote optical upconverter is cost-effective since only low-frequency devices are sufficient to handle with 60 GHz signal upconversion. After transmission of another 25 km SSMF, 2.5 Gb/s data on 60 GHz optical mm-wave at 1547.9 nm were received for different applications. For wired users, a compact 2.5 Gb/s transceiver (TRx) with avalanche photodiode (APD) was used to retrieve baseband data while automatically rejecting the undesired high frequency bands. For wireless subscribers, after pre-amplification, the mm-wave downstream signal was direct detected by a 60 GHz PIN photodiode following by an electrical amplifier (EA) to boost the signal before sending to an antenna. The pre-amplifier is utilized to compensate for the relative low sensitivity of the 60 GHz PIN photodiode. For simplicity, we assume that there is no signal degradation in the wireless transmission link between hybrid ONUs and subscribers, and connect EA directly with a mixer at subscriber part to evaluate the performance of downconverted signal. In addition, 15 GHz clock source and an FM are used to generate 60 GHz clock for signal downconversion. Note that the 2.5 Gb/s TRx with a queuing functional block is responsible for sending both wired and downconverted wireless upstream (US) signals. Using time-division multiple access (TMDA) is one of the practical upstream solutions, which should be well defined at the MAC layer for hybrid optical-wireless networks in the near future9, and we will not address that in this research work.
Fig. 2. Proof-of-concept experimental setup for the proposed long-reach 60 GHz mm-wave hybrid optical-wireless access network with remote upconverter.
4. Results and discussions
Figure 3(a) shows the measured optical spectra of the lightwave with 30 GHz double sideband (DSB) modulation before and after passing through an optical interleaver. After wavelength tuning and 50 GHz spectrum interleaving, only two first-order sidebands with exactly 60 GHz spacing was allowed to pass through the even port of the optical IL with low insertion loss. Thus, optical mm-wave with repetition frequency of 60 GHz was generated with carrier suppression ratio over 25 dB. Figure 3(b) illustrates the optical spectra of two independent downstream channels delivering 2.5 Gb/s baseband signal and 60 GHz optical mm-wave, respectively, and the corresponding passbands of the thin-film filter. Since the optical mm-wave was transported all-optically from end to end as a carrier, it is important to evaluate whether it can resist the effect of chromatic dispersion over long distance of fiber transmission. Thus, Figure 4(a) and (b) shows the measured result of 60 GHz optical mm-wave waveforms before and after 125 km transmission, respectively. Although the 60 GHz optical mm-wave did not carry data and was delivered independently with the baseband signal, one can still observe minor but negligible phase-decorrelation induced amplitude fluctuation of the detected 60 GHz clock after 125 km transmission, which is mainly contributed by the differential propagation delay of two 60 GHz spaced side bands10. In addition, Figure 4(c)–(f) shows the measured eye diagrams of the 2.5 Gb/s signal on 60 GHz optical mm-wave at different transmission distances by using conventional short-reach and proposed long-reach schemes, respectively. Although significant inter-symbol interference (ISI) can be observed in both schemes when signals were transmitted over additional 25 km after the signal upconversion, the proposed scheme with remote optical upconverter exhibited its high tolerance to chromatic dispersion in the 100-km fiber link between CO and LE. Figure 5 demonstrates that using the proposed remote signal regeneration and upconversion scheme, a baseband signal up to 10 Gb/s can be successfully upconverted to 60 GHz mm-wave band at an LE headend which is 50 km away from the CO. Note that at this moment the remote upconverter should be upgraded to 10 Gb/s accordingly. Since the theoretical fiber transmission distance for such mm-wave signal is only 12.5 km, the proposed scheme is promising for delivering mm-wave wireless services over future 10 Gb/s long-reach PON systems.
Fig. 3. Optical spectra of (a) the lightwave with 30 GHz DSB modulation before and after an OL and (b) two independent downstream channels carrying 2.5 Gb/s baseband signal and 60 GHz optical mm-wave, respectively.
Fig. 4. Waveforms of 60 GHz optical mm-wave before and after 125 km transmission, and eye diagrams of 2.5 Gb/s signals carried by 60 GHz optical mm-wave measured at different transmission distances by using conventional and proposed approaches, respectively.
Fig. 5. Measured eye disgram of an upconverted 10 Gb/s baseband signal on 60 GHz band at an LE which is 50 km away from the CO.
Figure 6(a) and (b) show the bit error rate (BER) performance and corresponding eye diagrams for wireless and wired services, respectively. For wireless services, after 125 km transmission, the receiver sensitivity and power penalty with pre-amplification are -28 dB and 1.8 dB at 10-9 BER, respectively. For wired services, the receiver sensitivity and power penalty without pre-amplification are -24.2 dB and 0.4 dB at 10-9 BER, respectively. Note that the back-to-back case in the proposed scheme is defined at the LE where the mm-wave signal is observed before being transmitted over another 25 km to multiple hybrid ONUs. After 125 km transmission, the downconverted eye diagram in Fig. 6(a) has obvious distortion due to the nature of fiber dispersion. In addition, since the output optical power at LE is about 5 dBm, the estimated power budgets for wireless and wired services are 33 dB and 29.2 dB at BER of 10-9, respectively. Assume that the proposed long-reach mm-wave hybrid optical-wireless access network is operated in time-division multiplexing (TDM) manner and uses an N-way optical power splitter as the RN, it will be able to accommodate at least 128 users with a shared bandwidth of 2.5 Gb/s (20 Mb/s per user) or it can provide about 78 Mb/s guaranteed bandwidth for 32 users to meet the requirement of future HD video-centric Internet services. Moreover, it is also scalable to multiple channels at the cost of increased modulators and receivers at LE, but will save more on numerous simplified hybrid ONUs.
5. Conclusions
We have proposed and demonstrated a novel hybrid optical-wireless access network testbed, which can deliver multi-gigabit data on 60 GHz optical mm-wave over 125 km long-reach passive optical network without any dispersion compensation. Robust signal regeneration and upconversion are utilized at remote local exchange to boost and broadcast downstream mm-wave signals as well as to minimize the capital expenditure of many hybrid ONUs. Such hybrid integration of optical-wireless system has great potentials to provide high-bandwidth video-centric services for both wireless and wired users in a simplified end-to-end architecture designed for the convergence of broadband optical-wireless access and metropolitan networks.
Fig. 6. BER performance and corresponding eye diagrams for (a) wireless and (b) wired services, respectively.
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