An Overview of the ROF Technology Bala Elamaran Supervisor: Prof. Xavier N. Fernando 1
Contents Overview of ROF Technology Typical Applications Advantages Issues and Signal Processing Solutions Nonlinearity Chromatic Dispersion Electrical and Optical Noise Ideas for Future Research Conclusions and Acknowledgements 2
ROF: A subset of Fi-Wi Technology Current wireless cannot transmit gigabit data. Fibers cannot be brought to the last mile costeffectively. Hybrid Fiber-Wireless (Fi-Wi) is the solution. Fi-Wi can be either base band over fiber, IF over fiber or RF over fiber. Base band and IF techniques are limited to single channel transmission. ROF based on RF transmission provides the best performance vs. cost. 3
ROF Architecture 4
ROF Architecture In an ROF system, fibers transmit the RF signal between central-base station (CBS) and low power Radio Access Point (RAP). The RAP then transmits the broadband RF signal to customer units through air. The RAPs only implement optical to RF conversion for the downlink and RF to optical conversion for the uplink. Signal format dependent functions such as modulation, equalization and error correction are implemented at the CBS and at the portable hence RAPs are cost effective and ROF is a viable technology. 5
Early Work Started with proposals in early 1990s. Boston (USA), New Castle (UK) subway systems to extend the coverage of traditional macro cells. Ortel was one of the early players in this area. Companies such as Motorola researched ROF in their labs but never practically implemented them. There was no need for ROF at that time, because the technologies for broadband transmission were not yet invented. 6
General Use Cases for ROF Technology Fixed ROF systems Wireless local area networks, Optical control of phased array antennas, Cable TV Intelligent vehicle highway systems Mobile ROF systems Disaster-hit areas Special events 7
FUTURE Project in China The FUTURE project of the Chinese Government is aimed at evaluating and verifying key technology proposals that meet the requirements for the Beyond Third Generation (B3G) mobile communications. A novel ROF cell structure based on having multiple antennas in an ROF cell has been introduced under FUTURE project to overcome the limitations of the current wireless cell architecture. Multiple antennas in a single ROF cell will allow multiple-input multiple-output (MIMO) transmission technology to be applied. 8
Sydney Olympics and ROF Technology BriteCell TM with 500 RAPs was installed for the Sydney 2000 Olympics. On the opening day of the Olympics, over 500,000 wireless calls were made from Olympic Park venues. Just during the first few minutes of the opening ceremony, over 175,000 calls were made by the 110,000 spectators in the stadium. This traffic could not have been handled without the BriteCell TM Fi-Wi system that supported both 900 MHz and 1800 MHz RF carriers. 9
Sub-Carrier Multiplexing ROF systems can transmit different wireless channels in a sub-carrier multiplexed (SCM) manner. The traditional cellular and PCS channels and wireless local area networks (WLANs) can all be sub-carrier multiplexed onto the same single-mode fiber for transmission. Fourth generation (4G) cell phones can have the broadband capability of WLANs when they are in a WLAN hotspot pico cell. WLAN devices which are currently restricted to hotspots such as internet café can have data communications at a reduced rate while roaming outside the hotspots to achieve uninterrupted connection while traveling. 10
ROF Cell Connection Topology The star topology is simple and is popular for ROF links because of their easy installation and maintenance and reliability. But they cannot be expanded quickly and cheaply. Bus topology schemes on the other hand can be expanded quite easily. A combined bus-star topology should be used for ROF links. 11
Single Fiber handling Multiple RAPs A single central base station can handle a large number of RAPs in this scenario. Each RAP can have a cell radius of up to few hundred meters. A single-mode fiber possesses virtually unlimited bandwidth compared to our current needs and it can be tapped into by modulating the SCM RF signals destined for different RAPs with optical carriers separated by a fixed wavelength. This technique is called wavelength-division multiplexing (WDM) and can be further classified as dense WDM or coarse WDM depending on wavelength spacing. WDM networks are favored because of the commercial availability of FBGs and optical circulators. 12
Other Techniques for Multiplexing Time Division Multiplexing Need fast photonic switches Different ROF cells must be Time Synchronous. Asynchronous TDM (similar to Ethernet) is not suitable for streaming video and other real-time applications. Code Division Multiplexing Direct optical switching CDMA (DOS-CDMA) It uses analog-type optical PAM scheme with on-off switching. Even this technique requires fast operation of the optical device for coding and also requires optical routing which are complex. Frequency Division Multiplexing Very efficient usage of the optical bandwidth But requires coherent devices as well as narrowband optical filters. 13
mm-wave Carrier Frequencies As the need for bandwidth increases into the realms of 1 Gb/s and higher bandwidths, the RF carrier frequencies of the cellular and IEEE 802.11 standards will be incapable of handling the capacity requirements. Higher carrier frequencies in the millimeter-wave band for broadband transmission have been analyzed. One of these broadband systems is called local multipoint distribution system (LMDS) and operates at 28 GHz. Other mm-wave frequencies such as 60 GHz where a large atmospheric attenuation of 17dB/km facilitates efficient spectrum reuse, has been touted as the mmwave carrier frequency of choice in experiments. 14
Advantages ROF has many advantages such as it exhibits transparency to modulation formats and thus the system is sustainable for the long term. Existing long-haul fiber network infrastructure can be adapted easily for ROF, which means reduced initial capital costs. Various wireless standards can be integrated using ROF technology and transmitted with a single fiber to the central base station. Wireless transmission in pico cells does not need much RF power resulting in increased battery lifetime. Reduced Regulatory and Safety concerns because of low power and small size of RAPs. 15
Issues The need for high dynamic range and the limited linearity of optical transmitters. The crosstalk between the sub-carrier multiplexed channels due to nonlinearity. The effects of chromatic dispersion on mm-wave ROF links. The cumulating electrical and optical noise of the ROF links. 16
Linearity Requirements Wireless links require wide dynamic range in the order of 80-90dB for transmission to account for wireless channel impairments. ROF links have a dynamic range that is 20-30dB less than required. The limited linearity is due to the nonlinear operation of the optical transmitter which can be described by Volterra series. This results in amplitude dependent amplitude (AM-AM) and phase (AM-PM) distortions. Since most signals transmitted through the ROF links are vector format signals with information both in amplitude and phase, these distortions present severe limitation on the performance of the systems. 17
Channel Estimation & Equalization ROF system can be modeled as a Weiner system because it consists of orthogonal linear dynamic function (wireless) and static nonlinear function (optical). Autocorrelation property of PN sequences can be used to characterize such systems and the wireless channel is estimated first. Optical channel is evaluated next using least squares polynomial method on a Vandermonde matrix consisting of the outputs of the linear wireless channels. The inverse of a Weiner system is a Hammerstein system, but decision feedback equalizers (DFE) perform better for ISI; So a Hammerstein type DFE is used instead. 18
Actual Vs. Estimated ROF Channel 1 Wiener System Identification Impulse Response 0.5 0 0.5 Actiual 1 0 5 10 15 20 25 30 35 Delay τ 1 Impulse Response 0.5 0 0.5 Estimated 20 symbols with spreading 31 1 0 5 10 15 20 25 30 35 Delay τ 19
CrossTalk between ROF Channels As more and more RF channels are modulated, the total RF power increases beyond the linear dynamic range of the optical transmitter. As the modulation of the optical carrier becomes nonlinear, higher-order harmonics of the SCM RF channels are generated and they disrupt the error-free transmission of each other. This is in addition to in-band nonlinear distortion. The receiver sensitivity versus adjacent channel spacing and power was measured and it can be seen from the Figure that for an ROF link there is sensitivity degradation even at high RF channel spacing. 20
SCM Adjacent Channel Interference 10 Performance of the Optical link with variable Interference Power 15 + + Interference Frequency = 2.1 GHz Minimum Detectable Power dbm 20 25 30 35 o o Interference Frequency = 1.86 GHz QPSK modulation @ 1 Msymbol/s Desired Carrier = 1.8 GHz With Optical Link Without the optical link 40 45 10 8 6 4 2 0 2 4 6 8 10 Interfering Power dbm 21
Chromatic Dispersion Caused by the variation in fiber material property with wavelength of light, which causes variation in the refractive index of the fiber. Single mode fibers have zero dispersion around 1310nm but unfortunately most of the installed fiber base is at 1550nm. To attain cost-effective ROF implementation we need to use the 1550nm fibers, where the dispersion of the fiber is 17ps/nm/km. For a 60GHz RF carrier frequency modulated on to the 1550nm carrier, the upper and lower sidebands are exactly out of phase at approximately 1 km resulting in complete loss of the signal. 22
Chromatic Dispersion Reduction Chromatic dispersion can be overcome if only one sideband is transmitted resulting in optical single side band (SSB) transmission. A self-homodyne coherent system based Technique. A dual-electrode mach-zehnder modulator (MZM) with its two RF inputs phase-shifted by 90 degrees An electro-absorption modulator (EAM) based technique which requires the laser light to be split into 2 paths phase-shifted by 90 degrees from one another. Fiber-Bragg-Grating based technique. 23
FBG-Based Optical SSB Generation FBG is fiber-compatible, low-loss and low-cost. FBG characteristic is generated by exposing fiber to UV light through a phase mask which generates the corrugated filter structure. FBG based filter implemented by Kitayama et. al and Novak et. al. A 14dB suppression of unwanted sideband was obtained. 24
Electrical and Optical Noise Impairments An ROF link will only be viable if the SNR of the RF signal in both electrical and optical domains is kept above a certain level that allows successful application of signal processing techniques. Quantum noise increases with the mean optical power Thermal noise depends on the receiver design. Relative intensity noise (RIN) also plays an important role in analog fiber optic links and is dependent on the modulation index. Wireless channel noise includes thermal noise and man-made noise including interference from other wireless networks. 25
Wireless and Optical Channel Noise The noise is added twice (at the optical and wireless receivers) as shown and the signal is weak in both these places. Hence, the overall SNR is the weighted sum of the two SNRs and will be smaller than the smallest SNR. 26
BER Performance of WCDMA vs. CDMA 27
Ideas for Research in ROF area Gigabit Signal Processing Azadet et. Al. IEEE J. Solid State Circuits March 2002. A/D conversion for Gigabit signal processing is impossible. Use parallel processing to overcome this. Multilevel Signal Processing Winters et. al. IEEE Trans. Comms. September 1990. Tradeoff between reduced chromatic dispersion (because of smaller BW) vs. reduced SNR. FPGA Implementation of nonlinear techniques developed by Dr. Fernando. FBG filter optimized with linear chirp for the wanted sideband. 28
Conclusions The ROF technology will become indispensable as the need for cheap, high-quality, standard-independent broadband wireless communications increase. Implementation of this technology is still in its infancy because broadband wireless standards are just emerging. The major issues with the ROF technology have to be solved so as to poise it to take the challenge when requested. Optical transmitter nonlinearity, chromatic dispersion and electrical and optical noise in the wireless and fiber channels are the three major limitations of the ROF system. This presentation has reviewed the current state-of-the-art in electrical and optical signal processing techniques to overcome these issues. 29
Acknowledgements Dr. Fernando for accepting me to work under him on ROF research. Roland Yuen for some of the figures. The ADROIT Group members. 30