Characterization of Broadband Wireless Communication Systems
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Characterization of Broadband Wireless Communication Systems |
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Index of Contents
Channel Propagation Modeling for LMDS
Evaluation of Candidate MAC Layer Protocols for LMDS
Past Projects:
Web browsing and access to the Internet by small business and residential users has been growing at a fast pace over the past few years. This trend is expected to continue for at least the next few years. These users are no longer content with low speed modem connections to the Internet. These modems provide transmission rates up to at best 56 Kbits/sec, which is clearly not adequate for web pages with multimedia content. It makes much sense to provide cable TV, phone, fax, Internet access, and other data services to small business and residential users through a single, unified broadband access mechanism.
Our emphasis in this project is on a system called Local Multipoint Distribution Service (LMDS). LMDS is a solution for the so-called "last mile problem". It is intended for providing broadband wireless access to business and residential users. It can be a viable alternative to competing technologies such as cable-modem and xDSL in areas where expensive, in-ground cabling is not available. The Federal Communications Commission (FCC) allocated and auctioned one gigahertz of frequency bandwidth in the 28-31 GHz frequency band in February 1998. Since there were no standards for LMDS available at the time, the IEEE
802 LAN/MAN Standards Committee created the IEEE 802.16 Working Group on Broadband Wireless Access Standards in March 1999. Its mission is "to develop standards and recommended practices to support the development and deployment of fixed broadband wireless access systems." In the short time since its inception, the IEEE 802.16 has made significant progress in developing an industry-consensus standard for LMDS and certain other related systems. The development of such a standard is essential for driving down the cost of Customer Premise Equipment (CPE) so that LMDS may be able to compete with other technologies. From a business point of view, market forecasters predict that there will be a significant LMDS market worldwide in the next few years.
The National Wireless Electronic Systems Testbed (N-WEST) at NIST is a good information resource with uptodate information on LMDS market and standardization.
Channel Propagation Modeling for LMDS
PIs: Wei Zhang
Other Contributors: Nader Moayeri
In the 28-31 GHz frequency band partially designated for LMDS, transmitted signals are attenuated by rain, other forms of precipitation, and foliage in trees. In the absence of these factors, it makes a big difference whether there is a line-of-sight (LOS) propagation path between the transmitter and receiver antennas or not. Yet another major factor is the nonlinear effects in millimeter wave amplifiers and other devices. Therefore, it is important to have an accurate model for the communication channel encountered in LMDS. We have worked closely with the IEEE 802.16 Coexistence Task Group as well as the IEEE 802.16 Physical Layer (PHY) Task Group. Our work in this area includes some of the publications listed below, which have been mostly presented at IEEE 802.16 meetings.
Evaluation of Candidate MAC Layer Protocols for LMDS
PIs: Hoon Choi
Other Contributors: Nader Moayeri and Cyril Clavelin
Given that the users in a LMDS system have to share a common wireless channel, the standard for LMDS has to specify a Medium Access Control (MAC) layer protocol to govern how the users demanding various services requiring different bit rates and qualities of service should share this common medium.
This project is presently at the formation stage. The IEEE 802.16 Medium Access Control (MAC) Task Group has urged NIST to participate in evaluation of MAC layer protocol proposals under consideration by the Task Group. Eleven proposals had initially been submitted to the Task Group. Through a remarkable harmonization and consensus-building process, the number of proposals dropped to only two. The Task Group is about to define the process by which these remaining two contenders should be evaluated and compared. This involves building reference models for the two MAC proposals and specifying test scenarios and measurements to be made.
Past Projects:
PIs: Jan-Erik Hakegard
Other Contributors: Nader Moayeri and Hamid Gharavi
Terrestrial wireless point-to-multipoint (PMP) broadband communication systems transmitting on Ka-band (~ 30 GHz) are in the US referred to as LMDS (Local Multipoint Distribution Service) systems. LMDS systems have been distributing analog TV signals in several countries around the world since the beginning of the 1990s. The US LMDS auction in 1998 triggered efforts to develop a new generation of LMDS systems to compete with wireline technologies like fiber, coaxial cable and xDSL. New LMDS systems must be able to handle a multitude of services, each with its distinct data rate and Quality of Service (QoS) requirement. Typical traffic types include voice, video, data services (T1/T3), TCP/IP traffic, and even full ATM switched services with both permanent and virtual circuits. For LMDS to compete with other technologies, the cost of equipment, and especially Customer Premise Equipment (CPE), is a crucial factor, as Ka-band technology is still relatively expensive. The modem design must reflect this cost constraint, minimizing the cost of the radio frequency (RF) units.
In this project we looked at LMDS architecture and system design. Specifically, we looked at the coding and modulation schemes specified by DAVIC and ETSI for use in LMDS, as well as an alternative scheme based on pragmatic trellis coded modulation (PTCM) and amplitude phase modulation (APM). The major channel impairments, including propagation attenuation, nonlinear effects of the high power amplifier (HPA), phase noise, and different kinds of interference, were studied and approaches to combat them proposed.
Wei Zhang and Nader Moayeri, "Classification of Statistical Channel Models for Local Multipoint Distribution Service Using Antenna Height and Directivity," Document Number 802.16.1pc-00/07, presented at Session #5, IEEE 802.16 Meeting, Richardson, TX, January 10-14, 2000.
Wei Zhang and Nader Moayeri, "Use of Various Raindrop Size Distributions for Different Geographical Locations in Calculating the Rain Specific Attenuation," Document Number IEEE 802.16cc-99/41, presented at Session # 5, IEEE 802.16 Meeting, Richardson, TX, January 10-14, 2000.
Wei Zhang and Nader Moayeri, "Formulation of Multiple Diffraction by Buildings and Trees for Propagation Prediction," Document Number IEEE 802.16cc-99/28, presented at Session # 4, IEEE 802.16 Meeting, Kauai, HI, November 7-12, 1999.
Wei Zhang and Nader Moayeri, "Power-Law Parameters of Rain Specific Attenuation," Document Number IEEE 802.16cc-99/23, presented at Session # 4, IEEE 802.16 Meeting, Kauai, HI, November 7-12, 1999.
Jan-Erik Hakegard, "Considerations of Modem Design for LMDS Systems," NIST internal publication, May 1999.
Last updated on February 24, 2000.