FUTURE DIRECTIONS
7.4 Mobile WiMAX /Radio over Fiber for Broadband Internet Access in High-Speed Railway System
Delivering super broadband internet access with sufficient level of quality of service to the fast moving users such as high-speed train (~300 km/hr) passengers is an interesting and unresolved issue. Although, the current cellular and satellite technologies can provide limited services to the mobile users, these technologies can not be considered for fast moving train passengers due to their inherent limitations. A direct connection between a train passenger and the ground base station of the cellular network is not possible due to high penetration losses because of the Faraday cage characteristics of the
train. Also, there are tradeoffs among the speed of the train, available bandwidth, and handover issues. On the other hand, the satellite technology is not suitable for real time applications because of its inherent delay, limited bandwidth, and poor coverage in urban and hilly areas and tunnels. In this regards, WiMAX/Radio over fiber (ROF), an integration of wireless and optical systems, could be the powerful solutions for providing high bandwidth internet to fast moving train passengers. Recently, cellular trackside solutions based on RoF have been proposed in [79, 80], which discussed only the architectural aspects and networking perspective of distributing broadband services to the train. Here, three-layer architecture has been proposed to provide both external broadband internet services and internal on-demand entertainment services to the high-speed train passengers using WiMAX/Radio over fiber system.
7.4.1 Proposed Three-Layer ROF Based Transmission System
Figure 7.9 shows the system architecture of the proposed three-layer WiMAX/Radio over fiber based transmission system providing high quality broadband services to the high-speed train. In the first layer, the WiMAX/Radio signal carrying baseband data is distributed to various proxy base stations (PBS) from a central Railway WiMAX/RoF Distribution and Control Center (RDCC). The PBSs are located along the rail tracks and are connected to the RDCC using a ring-based fiber distribution network.
The RDCC performs all the expensive signal generation and processing for up-conversion of WiMAX/Radio signal, selection of wavelengths (λi) and corresponding intermediate/radio frequency (IFi/RFi) using dynamic optical layer handover policy before sending the downstream (DS) signal to the appropriate PBS. The upstream (US)
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signal from the train is down-converted and optically modulated at the PBS before sending it to the RDCC, where the baseband data is retrieved. The intermediate layer is the IF/RF wireless communication between the Proxy BS and the moving Train Access Point (TAP). The TAP is considered as the gateway to the train. It performs up-conversion and down-up-conversion of the IF/RF signals to and from the Proxy BS. Finally, the Intra-train RoF network provides both external broadband internet services and internal on-demand select services (such as Movie/Video on demand, interactive gaming, on-train conferences etc.) to the individual passenger using both wire line and wireless (WiFi) access to each carriage.
7.4.2 Experimental Configuration and Results for WiMAX/radio Over Fiber in High-Speed Train
Internet service Carriage 1 Carriage N
WiFi
CW Laser (centrally supplied) Duplexer
Internet service Carriage 1 Carriage N
WiFi
CW Laser (centrally supplied) Duplexer
Figure 7.9: Proposed three-layer ROF based transmission system.
The experimental setup of the proposed WiMAX/RoF based broadband services for high-speed train is shown in Figure 7.11. At the WiMAX/RoF distribution center, two 100 GHz spaced CW lightwaves at wavelength 1550.2 nm (λ1) and 1551.0 nm (λ2) are modulated using two LiNbO3 Mach-Zehnder modulator driven by two separate sinusoidal RF clock frequency of 5.5 GHz and 5.8 GHz, respectively. Figure 7.12(a) and 7.12(b) show the corresponding optical spectra as inset (i) and (ii) in Figure 7.11. After modulation, the generated optical mm-waves are combined, amplified (EDFA) and modulated by an intensity modulator (IM) driven at 100 Mb/s baseband data with pseudorandom bit sequence word length of 231-1. The output power of the EDFA is set to 6 dBm. Figure 7.11(c) shows the optical spectra of the combined signals after IM as inset (iii) in Figure 7.10. The optical mm-wave channel at wavelength λ1 and λ2 are transmitted over 20-km standard single-mode fiber (SMF-28) and separated by a
100-Figure 7.10: Experimental setup of the proposed WiMAX/radio over fiber for high-speed train. WiMAX / ROF Distribution Center
5.8GHz Radio
Omni-antenna
Intra-train Master Headend carriage access point WiMAX / ROF Distribution Center
5.8GHz Radio
Omni-antenna
Intra-train Master Headend carriage access point
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The optical spectrum after interleaver and route to PBS2 is shown in Figure 7.11(d) as inset (iv) in Figure 7.9. At PBS2, a commercial PIN receiver (O/E) is used to recover the 5.8 GHz mm-wave signal with 100-Mb/s baseband data, and boosted by using an electrical amplifier (EA) before broadcast by a commercial dish antenna. The 5.8 GHz ISM/UNII band solid parabolic dish wireless antenna has 32.5dBi of maximum gain and 50 of both horizontal and vertical beam width. At the intra-train master head-end, a
Figure 7.11: Optical spectrum. (a) λ1 with 5.5 GHz RF clock as inset (i), (b) λ2 with 5.8 GHz RF clock as inset (ii), (c) combined signals after IM as inset (iii), (d) after interleaver as inset (iv), (e) re-modulated signal after 400m SSMF as inset (v) in Figure 7.10.
1550.5 1551.0
commercial omni-directional antenna is used to receive the broadcasted 5.8 GHz mm-wave signal. The HyperGain HG2458RD-TM is a triBand rubber duck wireless antenna with only 3-dBi gain. Inset figure of Figure 7.9 shows the received 5.8 GHz mm-wave with 100 Mb/s baseband data after the EA. A CW laser (λ3) at 1558.15 nm wavelength is directly modulated by the received 5.8 GHz mm-wave signal. Fig. 7.11(d) as inset (v) in Figure 7.10 shows the optical spectrum of the re-modulated signal. The re-modulated optical mm-wave signal is transmitted over 400 m of SMF-28 in side the train before received by another commercial receiver (O/E), perform down-conversion to recover 100 Mb/s baseband data. Figure 7.12(a) and (b) show the optical mm-wave before and after 20 km transmission at the railway distribution system as point (A) and (B) in Figure 7.10. Both the 5.8 GHz mm-wave and 100 Mb/s baseband data exhibit high extinction ratio. Again, Figure 7.12(c) and (d) show the corresponding optical eye before and after 400 m Intra-train RoF system as point (C) and (D) in Figure 7.10. The re-modulated 5.8 GHz mm-wave and 100 Mb/s baseband exhibit good extinction ratio. The BER
2ns/div
2ns/div 2ns/div2ns/div
(a) (point (A)) (b) (point (B))
2ns/div 2ns/div
(c) (point (D))
2ns/div 2ns/div
(d) (point (C)) Figure 7.12: Optical eye diagrams at different locations labeled in Figure 7.10.
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performances of the 100 Mb/s baseband data is shown in Figure 7.13 for both in the railway distribution system (20-km SMF-28) and in the train (400-m SMF-28). Inset of Figure 7.13 also shows the eye diagrams after transmissions. The error free transmission of 100 Mb/s baseband data over 5.8 GHz microwave is observed both in 20-km distribution line and 400-m Intra-train RoF system. However, there are around 2.0 dB of power penalty between the Proxy BS and the Intra-train master head-end. The power penalty could be due to signal quality degradation in the wireless transmission between the narrow beam width dish antenna and the omni-antenna.
7.4.3 Summary
Three-layer architecture to provide super broadband internet services in high-speed train systems using WiMAX/Radio signal over fiber is proposed and experimentally demonstrated. The experimental results show the error free transmission of 100 Mb/s baseband data carried by 5.8 GHz microwave WiMAX signal over 20 km distribution
-32 -30 -28 -26 -24 -22
Bit Error Rate After 20km (distribution )
2ns/div
Bit Error Rate After 20km (distribution )
2ns/div After 20km (distribution )
2ns/div
After 400m (Intra-train)
2ns/div After 400m (Intra-train)
2ns/div
Figure 7.13: BER measurements at railroad distribution RoF system (20-km SMF) and intra-train RoF (400-m SMF)
fiber and 400 m Intra-train fiber can be achieved with less than 2 dB power penalty. In the near future, upstream transmission in this proposed architecture will be demonstrated and also the whole configuration will be moved from immobile table to movable train to simulate the real system.
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CHAPTER 8 CONCLUSIONS
In this research, the main focus has been on the implementation and applications of novel architectures for reconfigurable broadband optical networks. To achieve this goal, in the first step, a re-circulating loop has been designed and set up to simulate long-distance transmission. Afterward, the research work further moved to metro network to solve the problem of fiber shortage and passive optical network to supply triple-play services.
The first study is long-distance transmission using a re-circulating loop in the networks with 32 ╳ 4 channels ROADM and dispersion-compensated interleaver pairs.
After that, the characteristics of the four-port interleaver had been explored and fully studied the bidirectional transmission by using unidirectional amplification scheme. Not only experimental demonstrated the straight-line, re-circulating loop and high bit rate transmission, but compared with the characteristics in different modulation formats and amplification techniques. Since the bit rate demand is much higher than in the past, PON technique is the effective solution for extensive bandwidth requirements of future services. The cost-effective bidirectional WDM-PON, select-cast WDM-PON and WDM-PON to provide triple-play services had been proposed and investigated.
8.1 Contributions
Primary contributions and experimental results of this dissertation are summarized here:
Novel three-port and four-port dispersion-free interleavers with temperature-compensated flat-top passband for bidirectional DWDM transmission systems had been successfully demonstrated.
In order to simulate long-distance transmission, one re-circulating loop has been setup.