Introduction
Gigabit data transport and processing technologies are required to respond to present and future information distribution and high-speed Internet application needs. Moreover, for broadband communication channels to reach individual users in any geographical environment demands integration of network segments in such media as metallic wire, fiber, and radio frequency (RF) or free-space optical wireless (FSOW). Fiber optics technology has already matured to terabit-per-second data transport. However, for places lacking fiber infrastructure, wireless technologies (RF and/or FSOW) are emerging as the transport media of choice in response to the daily increased demand for broadband networking. On the other hand, the maximum communication channel speed/data rates, link availability, and performance are limited in the wireless domain (microwave and millimeter-wave in particular) by wireless range, propagation effects, atmospheric turbulence, and environmental factors. Typical bit rate for an RF wireless system is in the lower megabits-per-second range for mobile, and a few hundred megabits per second for fixed wireless links. In addition, even at these low data rates, the link error performance and service quality are many orders of magnitude below those of fiber optic transmission systems. In response to these needs, we propose and demonstrate several new broadband network architecture and interface technology solutions based on the combined and complementary aspects of RF/microwave/millimeter-wave as well as FSOW links for integrated network operation. The combined scheme and architecture has extended the fiber optic reach and bandwidth utilization closer to the end user and, more important, into the wireless domain.
System Description
The fixed broadband wireless access (BWA) system trial is a short-range cellular-based point-to-point and/or point-to-multipoint distribution system that resembles the traditional local multipoint distribution services (LMDS) network architecture to enable gigabit capacity and high-speed data link capabilities. The system utilizes microwave, millimeter-wave, and FSOW technologies for access and distribution. Special emphasis is given to wideband wireless local loop (W-WLL) applications, versatile service access, rapid system deployment, and dynamic network reconfiguration. A segmented functional subnetwork topology, shown in Fig. 1, is adapted as described below [1–2].
Short Range Micro/Picocell Architecture -- In contrast to the conventional LMDS standard size cell of 2–5 miles in diameter, we have selected micro/picocells of < 500 m in radius for high-density populated regions. Figure 2 illustrates cell options and scenarios for customers concentrated in small urban areas such as inner city environments, college campuses, business parks, multistory/high-rise buildings, or planned housing complexes and development in small communities [3]. An access point (AP) and a hub are established utilizing a remote antenna. Direct line-of-sight "illumination," say the multistory building faces and windows, is achieved by either rooftop or sidewall-mounted shower-type antennas. The campus and small community access can be provided by projecting the signal from antennas mounted on street lampposts or neighboring buildings, as shown in Fig. 2.
Hybrid Fiber Radio Backbone Interconnection -- Hybrid fiber radio (HFR), RF photonics, and radio on fiber technologies are adapted to interconnect the APs to the backbone fiber network [4–6]. The links are capable of transporting both high-speed digital and analog signals as well as multiple wireless services based on subcarrier modulation (SCM) and wavelength-division multiplexing (WDM) technologies. The possibility of using the existing embedded fibers to the curb and neighborhood as well as FSOW tandem links [7] permits broadband backbone network integration and combined services through a single shared infrastructure, leading to faster deployment and lower system cost for service providers.
Network Operation Center -- A consolidated network operation center (NOC) for end-to-end network management and control is implemented to relocate the conventional base station control and switching facilities into the NOC to perform the required switching, routing, and service-mixing-function operations. The integration and merging of multiband HFR, FSOW, and digital fiber optic technologies at the NOC with fixed BWA has provided flexible and unified network operation as well as the possibility of end-to-end network management and control. The consolidation will benefit through lower infrastructure complexity and cost, resulting in a more reliable and centralized database and operations.
Portable Broadband Wireless Bridge and Access Node
We have also developed the concept and realization of a portable wireless access node for a bidirectional ATM-based connection to reach a fixed broadband fiber network. The goal of this effort is to demonstrate the feasibility of a rapidly deployed access node and backbone interconnection to the NOC for application in specialized scenarios, such as military theatres, emergency response, and disaster relief operations. Two portable nodes could also serve as a point-to-point wireless bridge [8] to connect two or more isolated networks in places not served by fibers, as depicted in the lower left corner of Fig. 1.
Free-Space Optical Wireless Access and High-Speed Backbone Reach Extension
This is an emerging advanced technology providing many new approaches and platforms for high-bandwidth wireless access and distribution networks. The technology, in combination with the millimeter-wave network topology, has created potential for increased capacity, and extended the fiber-based bandwidth and services to users via wireless. In our demonstrator, an FSOW point-to-point link is employed to complement and extend the NGI wireless access capabilities for true gigabit-per-second data transport. The combined and side-by-side millimeter-wave/
FSOW hybrid network topology shown in Fig. 1 provides direct performance comparison with the millimeter-wave links in various environmental conditions (e.g., multipath, rain fade) required for the design and implementation of high-reliability networks. Moreover, this topology ensures a higher degree of link availability when the millimeter-wave fails during the rain or the FSOW power budget falls below the specified threshold during foggy weather. It has been shown that the hybrid technology can increase the current millimeter-wave network capacity and high-speed data transport capabilities [7].
A Measurement-Based Channel Model -- To investigate millimeter-wave propagation issues, we use a high-resolution channel sounder at the 38 GHz LMDS band to model the channel based on the measurements and simulation results. The model addresses the performance limits for broadband point-to-multipoint wireless access in terms of data transport capability under realistic commercial deployment conditions. Based on the model, a broadband channel-adaptive radio modem is examined for dynamic selection of channel quality, channel switching, and bandwidth allocations [9]. Propagation characterization, modeling, and simulation was performed for a short-range BWA system to provide sight selection design rules and solutions for adaptive channel configuration and operation mechanisms. A set of comprehensive data processing tools has been developed that, in combination with the channel sounder, develops statistical models for the broadband millimeter-wave channels [9].
System Architecture Advantages
Compared to the traditional LMDS system [1], the system technology and heterogeneous network topology described above possess many technological and operational advantages:
- Increased coverage and user penetration percentage in each individual cell due to densely positioned users in the service area. This relaxes the tedious effort of cell frequency and polarization reuse planning.
- This in turn leads to a simpler design of overlapping cells for higher coverage and permits higher spectra efficiency utilization.
- The required AP hub and customer transmitting power (at millimeter wave) are immediately scaled down (15 dB minimum) due to the relatively short cell radius. The result is a low-power, low-cost system solution and less complex MMIC hardware design.
- A major reduction in system interference (adjacent channel and adjacent cell) comes from constraints and limitations imposed by the power amplifiers' nonlinearities in high-power systems due to spectral regrowth.
- As a result, possible reduction in the required radio channel spacing can be achieved, leading to increased system capacity due to higher spectra utilization and efficiency.
- The near short-range directly projected line-of-sight (LOS) propagation path becomes free from "major" multipath interference, intercell interference, and obstructions (buildings, moving objects, trees, and foliage). Consequently, the propagation path loss approaches that of square law, leading to a power-efficient system.
- An additional improvement in the system gain margin (7–10 dB) and link availability comes from the short LOS distance that removes the signal reception limitation due to excessive rain attenuation and system downtime experienced in higher-power, longer-range LMDS systems.
- The utilization of a hybrid millimeter-wave/FSOW network topology extends the broadband network reach without utilizing the radio spectrum. It can also provide high-capacity links, increased frequency reuse of millimeter-waves, and greatly enhanced network reliability and availability.
Implementation and Test Results
We have implemented experimental BWA links and an asynchronous transfer mode (ATM)-based networked testbed infrastructure for experimentation toward high-speed Internet applications and W-WLL performance evaluation.
The testbed comprises a single AP and three user nodes (two fixed and one portable), as shown in Fig. 3 and Fig. 4, operating in the 5.8/28/38 GHz bands. A side-by-side high-speed point-to-point FSOW link (Fig. 1), in parallel or tandem, was also implemented to extend the backbone fiber bandwidth to the AP operating up to 622 Mb/s rates [7]. On all the links, network demonstrations have been carried out for mixed services: broadcast 80-channel video and RF wireless data channels with speeds at 1.5, 25, 45, and 155 (OC-3) Mb/s rates in 4-, 16-, 32-, or 64-quadrature amplitude modulation (QAM) formats. The key issue in the topology described here is that the AP transmitter has low power (
–10 dBm at 38 GHz), with the subscriber return path transmitting power of –4 dBm, practical for mass deployments.
The implemented portable node of Fig. 4 is equipped with an OC-3 connection that occupies 50 MHz of bandwidth for 16-QAM modulation. The performance of the OC-3 portable node was also field-tested using a data stream supplied by either a bit error test set or an Internet advisor ATM analyzer. Error-free operation was achieved in a 20š sector of a 470-m microcell environment.
Figure 5 depicts the functional elements and interconnection in the ATM-based BWA and distribution network in the NOC. The ATM switch is programmed to combine and perform traffic distribution, mixed service integration, and dynamic user interconnection paths. The combined ATM wireless/fiber network operation as well as service integration have been evaluated and tested using an Internet advisor ATM analyzer [4, 8]. Error-free millimeter-wave/optical transmission and network operation were achieved for 155 Mb/s data channels switched between three users in cells up to 470 m in radius.
Figure 6 illustrates several examples of integrated HFR and RF photonics for wireless/fiber internetworking and interface options. The advantage of microwave and RF photonics [3, 4] is that it not only expands and merges broadband distribution and access, but it also incorporates "networked" functionality and control into the wireless links. The top figure indicates integration of several different wireless bands (e.g., PCS, NII, millimeter-wave, FSOP) into a single HFR using WDM technology. The system integration has also been demonstrated based on a single optical wavelength and synchronized multicarrier millimeter-wave radios with modular IF stages. The millimeter-wave subcarriers are selected with one-to-one fiber/wireless channel mapping to provide unified end-to-end network operation and continuity [5–6].
The lower left part of Fig. 6 depicts the role of HFR for multiple AP signal distribution, centralized control of individual antenna beam and phases, as well as frequency band selections. Here, the otherwise traditional "antenna remoting" function has been replaced by a multiple service access link with centralized network management and control.
The lower right part of Fig. 6 depicts yet another example, utilizing the HFR technology to distribute high-stability, low-phase-noise local oscillator (LO) and sync signals to the millimeter-wave up/downconverters in the APs and base terminals. The experimentally deployed LO distribution demonstrated lower harmonics and superior phase quality in millimeter-wave systems, as well as lowering electrical intermediate frequency (IF)/RF terminal design complexity, component counts, and overall cost compared to pure all-electrical solutions. A two-channel (12 and 16 GHz) photonic unit was demonstrated for evaluating the performance of a switched dual-band photonic link in distributing LO/sync signals [6, 7, 10]. The scheme provides the flexibility of frequency tuning, channel selection, and dynamic bandwidth allocations for wireless access systems.
Conclusion
We have introduced and demonstrated a short-range LOS LMDS-like millimeter-wave and FSOW architecture for a BWA system that possesses many technological and operational advantages. These include ease of installation and alignment, low radiation power, and, effectively, the link being free from major multipath, obstructions (trees, buildings, and moving objects), and adjacent cell interference. We have also presented several system architecture and implementation scenarios for a complementary millimeter-wave/FSOW system highly suitable for integration of a BWA network with the existing backbone fiber network. The proposed system architecture is suitable for deployment in a highly developed, densely populated, urban inner city environment where large-capacity broadband services are in great demand but lacking wired broadband access infrastructure.
Acknowledgments
This article is based on our previously published material from WAS 2000 organized by DELSON GROUP. The partial support of this work by DARPA is gratefully acknowledged. We also acknowledge the contribution of Virginia Tech MPRG in modeling and propagation measurement for this project.
References
[1] H. Izadpanah et al., "MM-Wave Wireless Access Technology For The Wideband Wireless Local Loop Applications," IEEE RAWCON '98, Colorado Springs, CO, Aug. 1998.
[2] H. Izadpanah, "LMDS: A Broadband Wireless Access Technology: An Overview," The 3rd IAA Annual Conf. Comp. and Commun., CUNY, Sept. 1998.
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[4] H. Izadpanah et al., "An mm-Wave Broadband Wireless Access Technology Demonstrator For the NGI Network Extension," Int'l. Conf. Broadband Wireless Access System, Dec. 2000, San Francisco, CA
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Biography
Hossein Izadpanah holds a B.Sc. in physics, an M.Sc. in microwave system engineering, and a Ph.D. in solid-state electronics. From 1969 to 1984 he was an associate professor at Shiraz University in Iran. From 1985 to 1997 he was at Bellcore working on multigigabit fiber optic transmission systems, OE functional circuits, and efficient RF circuits for wireless applications. Since April 1997 he has been a senior research scientist at HRL Laboratories working on broadband wireless access technology, HFR, and free-space optical wireless networks.
