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B. Kamali, and A. H. Aghvami, “Belief Propagation Decoding of Reed-Solomon Codes; a Bit-Level Soft Decision Decoding Algorithm,” IEEE Transactions on Broadcasting, Vol. 51, NO. 1, pp. 106-113, March 2005.
Abstract
In this article we propose the application of Belief Propagation (BP) algorithm as a novel bit-level soft decision decoding (SDD) technique for Reed-Solomon (RS) codes. A brief tutorial on Belief Propagation algorithm is presented. A central issue in the application of BP algorithm to decoding RS codes is the construction of a sparse parity check matrix for the binary image of the code. It is demonstrated that Vardy’s technique may be applied to find a sparse parity check matrix for RS codes. However, this technique is not applicable to all cases. The BP algorithm is applied to two test codes. In one case, simulation models show that the BP algorithm outperforms the hard decision Euclidean decoding by more than 2 dB of additional coding gain. The results with the second test code are not as promising.
Full Paper
B. A. Butcher, JR., and B. Kamali, ”Comparison of Duobinary and 4 –Level Continuous Phase Frequency Shift Keying Signals for Narrowband Land Mobile Radio Applications” IEEE Transactions on Broadcasting, Vol. 45, No. 2, pp. 177-186, June 1999.
Abstract
The bit-error-rate performance of partial-response duobinary continuous-phase frequency shift keying (CPFSK) and full-response 4-level CPFSK are compared under the condition of equal RF spectral occupancy and equal data rate, for both Gaussian and Rayleigh fading channels. Incoherent limiter-discriminator detection is assumed for all cases. The spectral efficiency requirements of the FCC, for the allocated 220-222 MHz band for land mobile communications, are used to establish the system specifications. It is shown that 4-level CPFSK system significantly outperforms the 3-level duobinary CPFSK. This is due to the fact that the receiver bandwidth may be lowered for 4FSK thereby higher frequency deviation can be achieved resulting in higher SNR at the receiver which outweighs the noise margin advantage of duobinary system. Simulation systems are designed to measure the performance for both static and Rayleigh-fading narrowband channels.
B. Kamali, "Development of an Undergraduate Structured Laboratory to Support Classical and New-Base Technology Experiments in Communications," IEEE Transactions on Education, Vol. 37, no. 1, pp. 97-100, February 1994.
Abstract
This paper describes an effective method of teaching a communications laboratory course that supports classical as well as new base technology experiments to undergraduate electrical engineering students. Primarily, experiments dealing with the design of building blocks of analog and digital communications systems are targeted, which provides an opportunity for the students to get familiar with various signal processing techniques applied in modern communications systems, to learn how to operate communication laboratory test equipment, and to obtain exposure to some hardware/ software design and implementation of these subsystem. The second level of experimentation involves small systems in which discrete or integrated devices are used to build small systems. Level three involves experimentation with a complete communications system, such as a fiber optics communications system or a simulated satellite link. Four sample experiments are described in some detail.
B. Kamali, R. A. Bennett, and D. C. Cox, “Understanding WiMAX; an IEEE-802.16 standard-based Wireless Technology,” IEEE potentials, Vol. 31, No. 5, pp 23-27, September/ October 2012.
Abstract
Worldwide Interoperability for Microwave Access, WiMAX, is an IEEE 802.16 standard-based broadband cellular wireless solution in which Orthogonal Frequency Division Multiple Access (OFDMA) is the method of sharing communication resources among large number of users. This article describes major components of WiMAX networks and highlights technologies incorporated in the development of this wireless system vis-à-vis Physical (PHY) layer and Media Access Control (MAC) sublayer protocols. It is attempted to provide a continuous overall picture of this extensive technology in a short tutorial approach.
B. Kamali “Some Applications of Wireless Communications in Intelligent Vehicle Highway Systems,” IEEE Aerospace & Electronics Systems Magazine, Vol. 11, No. 11, November 1996, was nominated for the Harry R. Mimno Paper award.
Abstract
Intelligent Vehicle Highway Systems (IVHS) have generated a new challenge for communications industries. This is particularly true for mobile communications, since most essential IVHS communications links require wireless and mobile transmission. The IVHS and its major components are described. A generic block diagram for an IVHS network is presented. This block diagram defines the communications requirements for the IVHS network. The complete characterization of communication between the vehicle and the IVHS infrastructure is determined by the system architecture, however, by its very nature, this link is required to be wireless, mobile, and for the most part interactive. A survey of wireless technologies that have been tested, or implemented, and/or considered suitable for IVHS applications are discussed. Examples of operational and/or under development IVHS projects that applies the corresponding wireless technology are also provided
B. Kamali, “Error Control Coding: Protecting Digital Information through the Course of Transmission and Storage,” IEEE Potentials Magazine, May 1995, pp. 15-19.
Abstract
Error control coding (ECC) is a signal processing technique that protects digital information against transmission and storage errors. Codes that combat randomly distributed errors are called random-error correcting codes. Codes that cope with errors that are clustered together are known as burst-error correcting codes. When data is presented as a stream of binary numbers, bit-oriented codes may be applied. In contrast to this, data might be arranged as words, where each word includes a number of bits. Codes that operate on words are said to be word-oriented codes. The article is limited to linear bit-oriented block codes
B. Kamali, “Some Classical Applications of Coded Orthogonal Frequency Division Multiplexing”, International Journal on Intelligent Technologies and Engineering Systems (IJITES 2011), October 2011.
Abstract
Coded orthogonal frequency division multiplexing (COFDM) is a spectrally efficient and robust transmission technique in the presence of frequency selective fading and impulse noise. A survey of initial applications of COFDM, particularly in the wireless arena, is presented. Early on, COFDM was applied in ADSL technology for which ANSI issued a standard. The standard allows for up to seven channels for transport of synchronous transfer mode with a maximum rate 6.144 Mbps for delivery of multimedia signal. In Europe COFDM has been adopted for both digital audio and digital video broadcasting. Primary reasons for the selection of COFDM for these applications, in addition to robustness and spectral efficiency, are the possibility to use single frequency network and hierarchical transmission. In North America the IEEE has approved a standard for wireless local area networks; the IEEE802.11, in which COFDM signaling has been adopted for a 5 GHz standard that supports data rates up to 54 Mbps. Four different modulation schemes and a variable rate convolutional code are supported by this standard. The parameters of the selected modulation scheme and the code rate are all data rate dependent. Performance of channel coding schemes used in COFDM is briefly discussed.
B. Kamali, “Modulation and Coding for Cellular Digital Packet Data Networks”, Applied Microwave & Wireless, Vol. 9, No. 3, June 1997.
Abstract
The structure of a digital cellular packet data (CDPD) network is briefly explained with the aid of a general block diagram. The air-interface represents the most error-prone portion of the CDPD network. To protect user's data, CDPD employs Reed-Solomon (RS) codes. RS codes and their single burst error correcting capabilities are described. A parameter that measures how efficiently the random error correcting capability of an RS code is used to correct a single burst of errors is introduced. Tables of single burst error correcting capabilities of RS codes constructed over GF(64) and GF(256) are given. It is shown that in order to provide signal robustness against 2.2 millisecond of mean fade-duration-time, a (63,47) RS code is required. The modulation scheme used in CDPD networks is required to be both bandwidth and power efficient. It is shown that GMSK with BT=0.5 satisfies all the CDPD network requirements
Recent NASA (GRC) Aeronautical Research Related Publications
R. J. Kerczewski, B. Kamali, et al, “Considerations for Improving the Capacity and Performance of AeroMACS,” accepted for publication and presentation at IEEE Aerospace Conference, to be held in Big Ski Montana, March 2014.
Extended Abstract
The Aeronautical Mobile Airport Communications System (AeroMACS) has progressed from concept through prototype development and testing, standards development and finally initial deployment for fixed applications. RTCA Special Committee 223 has completed the development of Minimum Operational performance Standards (MOPS) and has recently released a revision of the AeroMACS Profile. The International Civil Aviation Organization’s’ Aeronautical Communications Panel has established Working Group S (Surface Communications) which has initiated the development of international Standards and Recommended Practices (SARPS). The first operational AeroMACS deployment is scheduled for San Francisco International Airport (SFO) in early 2014.
With AeroMACS as an approved aviation communications system rapidly gathering momentum, and given the long time frames for development and deployment of even incrementally changed aviation systems, the point at which AeroMACS begins to become operationally congested due to expanding use coupled with various system constraints may be looming on the horizon – or at least it is prudent to consider ways to improve and extend AeroMACS capacity and performance. The key constraints on AeroMACS capacity are spectrum limitation (AeroMACS operates over 11 5 MHz channels in the 5091-5150 MHz band and potentially in the 5000-5030 MHz band) and transmit power limitations (AeroMACS must coexist with, and is constrained from interfering with, mobile satellite feeder links operating in the same band). In addition, the initial deployment of AeroMACS at SFO may need to use many of the 11 available channels for point-to-point fixed service applications, which may significantly limit mobile aircraft applications that will emerge in the future. To address the long term future of AeroMACS, activities in research, testing and demonstration of AeroMACS technologies continue at the AeroMACS test Bed at the NASA Glenn Research Center and Cleveland Hopkins International Airport. This paper will provide a description of these activities focusing on the following areas:
The analysis of the aggregate interference of AeroMACS installations with mobile satellite feeder links in the 5091-5150 MHz bands provides the basis for AeroMACS transmit power limitations. Improvements in the fidelity of the analysis and the assessment of techniques to reduce interference are intended to reduce the transmit power constraint, leading to AeroMACS capacity increases. One example of interference reduction is to place AeroMACS base stations at higher locations, such as air traffic control towers, such that the downward pointing angle of the AeroMACS antennas results in reduced interference at the satellite feeder link receivers.
The AeroMACS Test Bed is being prepared for testing of AeroMACS equipment being developed by additional vendors. These vendors will measure the performance of their planned AeroMACS offerings to assess interoperability and identify performance improvements. These tests will also help to validate current standards and AeroMACS Profile elements and potentially identify other improvements.
Investigations of future enhancements to the AeroMACS Profile to enable increased capacity and performance are on-going, focused on analysis of the use of multi-hop relaysbased on the IEEE 802.16j standard. Future testing of multi-hop relay performance in the AeroMACS Test Bed is being proposed to enable potential Profile enhancements to be assessed. The AeroMACS Profile is based on WiMAX, an IEEE 802.16 standard-based broadband cellular wireless solution in which multiple access technique is Scalable OFDMA. In discussing the progress of multi-hop relay analysis, we will also briefly review the highlights of WiMAX technology focusing on key physical layer signal processing techniques and MAC layer architectural characterization. We then describe the progress in 802.16j multihop relay analysis and the potential for achieving a flexible and cost effective radio extension for AeroMACS with virtually no increase in power requirements. We then demonstrate how the overall AeroMACS capacity may be enhanced through the concept of multihop gain.
B. Kamali, R. J. Kerczewski, “IEEE 802.16J Multihop Relays for AeroMACS Networks and the Concept of Multihop Gain,” Proceedings of IEEE ICNS2013, April 2013.
Abstract
The potential benefits and challenges of applications of IEEE 802.16j-based relays in AeroMACS networks are discussed at the outset. Perhaps the most important advantage of application of multihop relays in AeroMACS networks is the flexible and cost effective radio range extension that it may allow for airport areas shadowed by large constructions and natural obstacles with virtually no increase in the required network power levels. With respect to PHY layer RSs may be classified as Transparent Relays (TRS) and Non-Transparent Relays (NTRS). While a TRS essentially functions as a repeater and bears no logical connection to the subscriber station (SS), a NTRS operates as a “mini base station (BS)” and is physically and logically connected to the SSs that it serves. Regarding MAC sublayer functionalities, RSs may operate in centralized or distributed modes. Distributed mode means that the RS is capable of scheduling network resources in coordination with multihop relay base station (MR-BS); otherwise the RS is in centralized mode. The RS can be in distributed or centralized mode with respect to security arrangements as well. The NTRS relays may further be divided into two categories; time-division transmit and receive relays (TTR) and simultaneous transmit and receive (STR) relays; both of which are supported by IEEE 802.16j standard. The TTR relay communicates with its subordinate and superordinate nodes using the same radio channel. The employment of relays in an AeroMACS network requires no alteration in the subscriber system. The key concept of “multihop gain”, which explains how the application of multihop relay enables performance enhancement in AeroMACS networks, is introduced. Under a reasonable set of assumptions and using a simple analysis, multihop gain is quantified in the form of an equation that provides a raw measure of this gain in Decibel.
B. Kamali, R. A. Bennett, and D. C. Cox, “Understanding WiMAX; an IEEE-802.16 standard-based Wireless Technology,” IEEE potentials, Vol. 31, No. 5, pp 23-27, September/ October 2012.
B. Kamali, J. D. Wilson, R J. Kerczewski, “Application of Multihop Relay for Performance Enhancement of AeroMACS Networks,” Proceedings of IEEE ICNS2012, April 2012. This article was republished as a NASA Technical Memorandum (NASA/TM-2012-217658) in August 2012
Abstract
A new transmission technology, based on IEEE 802.16-2009 (WiMAX), is currently being developed for airport surface communications. A C-band spectrum allocation at 5091-5150 MHz has been created by ITU to carry this application. The proposed technology, known as AeroMACS, will be used to support fixed and mobile ground to ground applications and services. This article proposes and demonstrates that IEEE 802.16j-amendment-based WiMAX is most feasible for AeroMACS applications. This amendment introduces multihop relay as an optional deployment that may be used to provide additional coverage and/ or enhance the capacity of the network. Particular airport surface radio coverage situations for which IEEE 802.16-2009-WiMAX provides resolutions that are inefficient, costly, or excessively power consuming are discussed. In all these cases, it is argued that 16j technology offers a much better alternative. A major concern about deployment of AeroMACS is interference to co-allocated applications such as the Mobile Satellite Service (MSS) feeder link. Our initial simulation results suggest that no additional interference to MSS feeder link is caused by deployment of IEEE 802.16j-based AeroMACS.
B. Kamali and R. J. Kerczewski, “On Selection of Proper IEEE 802.16-Based Standard for Aeronautical Mobile Airport Surface Communications (AeroMACS) Application,” Proceedings of IEEE ICNS2011, May 2011
Abstract
A new aviation-specific transmission technology based on the WiMAX (Worldwide Interoperability for Microwave Access) IEEE 802.16e-based standard; over a newly available C-band allocation (5091-5150 MHz), has been recently recommended for the airport surface wireless communications network now known as Aeronautical Mobile Airport Communications System (AeroMACS). The proposed standards will be used to support fixed and mobile ground to ground applications and services. It has been established that no technical obstacle exists that would prevent the application of WiMAX networks to AeroMACS. In this article WiMAX networks and some of their salient features are briefly reviewed. The challenges of broadband radio communications through airport surface channels are discussed. A major concern about deployment of AeroMACS over the 5091-5150 MHz band is interference to co-allocated applications such as the Mobile Satellite Service (MSS) feeder link. This limits the power levels that are allowed for AeroMACS networks. We propose an investigation into the feasibility of the application of IEEE 802.16j Amendment (relay-based multi-hop network) to AeroMACS. The potential benefits of multihop relay configuration for AeroMACS networks are identified. Perhaps the most relevant benefit of the multihop relay configuration to AeroMACS is the flexible and cost effective radio range extension that it provides for airport areas shadowed by large constructions and natural obstacles without raising the required network power levels.
B. Kamali, “An Overview of VHF Civil Radio Network and the Resolution of Spectrum Depletion,” Proceedings of IEEE ICNS 2010, May 2010.
Abstract
The spectral capacity of VHF aeronautical radio is rapidly reaching saturation in the United States and in Europe. A key functional objective for any future civil aviation communications technology is providing relief to the current spectral congestion. This article provides a review on the existing national and global aeronautical radio networks. The spectrum depletion problem is described. Recent developments in narrowband and wideband digital communications for potential applications in civil aviation are discussed. Comments are made on critical issues that must be considered when selecting a communication technology for aeronautics which addresses the spectral congestion problem and simultaneously supports transformation to a long term network centric solution. Recommendations are made on some possible communication technologies that may bring about short-term resolution to VHF spectrum depletion without requiring major changes in ground infrastructure or on-board avionics.
B. Kamali and A. Patel, "Single Sideband Technologies for Optimization of VHF Aerospace Communication", Proceedings IEEE ICNS2007, May 2007.
Abstract
The highest priority in air travel operation is justifiably placed on flight safety which is critically linked to the availability of reliable communications and navigation systems. The current aeronautical VHF spectrum dedicated to civil aviation communications in the US and the rest of the world isl9 MHz wide. With the 25-kHz commercial AM technology, this band supports 760 radio channels. In the United States, owing to the rise in the number of airplanes in both commercial and general aviation sectors, spectral congestion in the aeronautical VHF band is rapidly becoming a predicament. This article proposes single side band as an alternative technology that enables efficient use of the available spectrum while requiring minimal change in the infrastructure. It is shown that SSB in conjunction the Weaver modulation method presents an economically feasible solution to spectral congestion in VHF aeronautics, with which the system capacity can be increased by a factor of 3, 4, 5, 6, and even 7 (X-factor). Hardware costs increments for implementing either the analog or digital Weaver method are modest among schemes with different X-factors. This implies that it is possible to increase the present capacity of 760 to up to 5320 voice channels; which meets the challenge of required number of civil aviation radio channels for at least two decades.
B. Kamali “Relay-Fortified WiMAX Networks for Aeronautical Mobile Airport Communications System (AeroMACS)” Final Research report to NASA AIAA, August 2013
Abstract
Aeronautical Mobile Airport Communications System (AeroMACS) is a new network-centric aviation transmission technology which is based on IEEE 802.16-2009 standard. AeroMACS will be implemented to support fixed and mobile ground to ground applications and services. This report attempts to demonstrate that the IEEE 802.16j-based WiMAX is most feasible for AeroMACS applications. The principal argument in favor of application of IEEE 802.16j technology is the flexible and cost effective extension of radio coverage that is afforded by relay fortified WiMAX networks, with virtually no increase in the power requirements. By flexible radio outreach extension, we mean adding relays to the network as needed and as the airport is further developed. Some general remarks are made on WiMAX technology at the outset. Next, AeroMACS is described as a WiMAX-based wireless network. The IEEE 802.16j-based WiMAX technology and multihop relay systems are defined and classified. The modes of operation supported by IEEE 802.16j amendment; i.e., transparent and non-transparent relays are discussed in some detail. Non-transparent relay stations are further classified into Time Division Transmit Receive (TTR) and Simultaneous Transmit and Receive (STR) relay. The question of latency in IEEE 802.16j-based AeroMACS networks is addressed. Some advantages and challenges of using multihop relays in AeroMACS are described. Finally, a strong case is made for inclusion of multihop relays in AeroMACS networks is made and some concluding remarks are ventured.
B. Kamali, “IEEE 802.16j-Based WiMAX Technology for Aeronautical Mobile Airport Communications System (AeroMACS); Description, Benefits, and Challenges,” Final Report to NASA Glenn Research Center (GRC), Division of Communications, Instrumentation, and Control, August 2013.
Executive summary
Aeronautical Mobile Airport Communications System (AeroMACS) is a new network-centric aviation transmission technology which is based on IEEE 802.16-2009 standard (WiMAX). AeroMACS is being developed over a newly available C-band spectrum (5091-5150 MHz), and is envisioned to provide coverage in all areas of an airport surface for the next generation of air transportation systems. AeroMACS will be implemented to support fixed and mobile ground to ground applications and services. At the moment, a version of AeroMACS is in place at San Francisco airport to be tested for airport surface surveillance purposes and is planned to be up and running early next year. This report continues on further development of the core idea presented in the previous report (August 2011), as such the central theme of this write-up is making a strong case for and demonstrating that IEEE 802.16j-based WiMAX (relay-based multi-hop network) is most feasible for AeroMACS applications. Generally speaking, in all broadband cellular relay-augmented standards, the main idea is to complement the base stations (BS) with less complex, less costly, and easier-to-install relay stations instead of adding new BSs to the network.
The principal argument in favor of application of IEEE 802.16j technology is the flexible and cost effective extension of radio coverage that is afforded by relay fortified WiMAX networks, with virtually no increase in the power requirements and virtually no increase in interference levels to co-allocated applications. By flexible radio outreach extension, we mean adding relays to the network as needed and as the airport is further developed. A basic IEEE 802.16j-based WiMAX cellular network can be rolled out for an airport, and as the network expands relays can be added to meet the new coverage and transmission requirements. The second major argument in favor of IEEE 802.16j technology for AeroMACS is the ease with which throughput and capacity may be increased anywhere and at any time; temporary or permanent, on the airport surface.
At the outset some general remarks on WiMAX technology, with a focus on key features of this technology, frame structure, and its architecture are offered. Next, Aeronautical Mobile Airport Communications System (AeroMACS) is described as a WiMAX-based wireless network. This is undertaken by referencing the most recent AeroMACS profile. The IEEE 802.16j-based WiMAX technology and multihop relay systems are defined and classified. The two major modes of operation supported by IEEE 802.16j amendment; i.e., transparent and non-transparent modes are discussed in some detail. The benefits of employing multihop relays are listed. First and foremost among these benefits is that the IEEE 802.16j-defined relay deployment strategy presents a flexible, cost-effective, low-complexity, and easy-to-install-infrastructure alternative for network radio outreach extension in a variety of situations. Frame structure for double- hop relay augmented WiMAX networks is also explored. Non-transparent relay stations are further classified into Time Division Transmit Receive (TTR) and Simultaneous Transmit and Receive (STR) relay. The frame structure for double-hop TTR and STR relays are examined. The selection of relay type may affect a number of network parameters. Various relay types, i.e. amplify and forward (AF), decode and forward (DF), and demodulate and forward (D&F) relays are studied. The question of latency in IEEE 802.16j-based AeroMACS networks is addressed. Some advantages and challenges of using multihop relays in AeroMACS are described. Comments are also made on the matter of relay implementations that can operate over C-band. Finally, some concluding remarks are ventured, which includes several important remarks on points of reference for design of relay-fortified AeroMACS networks.
B. Kamali “Multihop Relay Technology for Aeronautical Mobile Airport Communication Systems (AeroMACS),” Final research report to NASA AIAA, August 2011.
Abstract
A new aviation-specific transmission technology based on IEEE 802.16-2009 standard is currently being developed for Aeronautical Mobile Airport Communications System (AeroMACS). The proposed technology will be used to support fixed and mobile ground to ground applications and services. Owing to practical shortfalls arising from early implementation of IEEE 802.16e-based WiMAX networks, the need for a new amendment to the standard was recognized early on. The IEEE802.16j, which allows for relay station (RS) to be used as an extension to the base station (BS), represents such an amendment. The central idea of this research work is to demonstrate that IEEE 802.16j-amendment-based WiMAX is most feasible for AeroMACS applications. First and foremost among benefits of IEEE 802.16j technology is the cost-effective, low-complexity, and easy-to-install-infrastructure alternative for wireless network radio outreach extension in a variety of situations. Secondly, the relays can provide capacity improvement and throughput enhancement in areas which are not sufficiently covered by the associated BSs. The examination of the most recent AeroMACS Profile draft reveals that there are no additional technical challenges in application of IEEE 802.16j-based WiMAX to AeroMACS. The application of multihop relay configuration to AeroMACS enables a reduction in path loss which can be viewed as a link budget “gain”. This “multihop gain” is then quantified with a simple analysis. The multihop gain can then be translated into performance improvement of various forms for AeroMACS. Particular radio coverage situations in airport surface for which IEEE 802.16-2009-WiMAX system either fails to offer a viable solution, or the resolution it provides is inefficient, costly, or excessively power consuming, are listed. In all these cases, it is argued that 16j technology offers a much better alternative. A major concern about deployment of AeroMACS over the 5091-5150 MHz band is interference to co-allocated applications such as the Mobile Satellite Service (MSS) feeder link. At NASA Glenn Research Center the software program Visualyse Professional is being utilized to estimate the AeroMACS interference power levels into MSS. To compare interference effect of IEEE 802.16-2009-based AeroMACS with that of IEEE-802.16j-based network we have conducted several simulation runs for various scenarios, using Visualyse Professional. The initial simulation results suggest that no additional interference to MSS feeder link is caused by deployment of IEEE 802.16j-based AeroMACS.
B. Kamali, “IEEE 802.16j-Based WiMAX Technology for Aeronautical Mobile Airport Communication System (AeroMACS)” Final Report to NASA GRC, Division of Instrumentation Communications and Control, August 2011.
Executive summary
A new aviation-specific transmission technology based on IEEE 802.16-2009 standard (WiMAX) is currently being developed for Aeronautical Mobile Airport Communications System (AeroMACS) over a newly available C-band spectrum (5091-5150 MHz). The proposed technology will be used to support fixed and mobile ground to ground applications and services. Owing to practical shortfalls arising from early implementation of IEEE 802.16e-based WiMAX networks, the need for some modification and amendment to the standard was recognized early on. In particular, initial field trial of mobile WiMAX products has shown that IEEE802.16e systems provide poor Quality-of-Service (QoS) around WiMAX cell boundaries, for indoor users, as well as in areas severely shadowed by manmade structures and natural obstacles. For instance, even with application of advanced signal processing techniques such as OFDM, MIMO, and AMC ; the projected data rates require SINR levels at the front end of the receivers that are difficult to obtain at the WiMAX cell boundaries or in shadowed areas . To address this issue IEEE802.16j Multi-hop Relay (MR) Task Group has been working to define a new relay station (RS) that can be used as an extension to the base station (BS) and relay traffic between the BS and the subscriber station (SS).
The central idea in this report is proposing and demonstrating that IEEE 802.16j-amendment-based WiMAX (relay-based multi-hop network) is most feasible for AeroMACS applications. The report begins with some introductory remarks on IEEE 802.16j amendment. This amendment introduces multihop relay (MR) as an optional deployment that may be used to provide additional coverage or provide improved performance-enhanced capacity in an access network. IEEE 802.16j relays are then defined, classified, and their modes of operations is discussed in some detail. The benefits of employing multihop relays are listed. First and foremost among these benefits is that the IEEE 802.16j-defined relay deployment strategy presents a cost-effective, low-complexity, and easy-to-install-infrastructure alternative for wireless network radio outreach extension in a variety of situations. Secondly, the relays can provide capacity improvement and throughput enhancement in areas which are not sufficiently covered by the associated BSs. In the majority of usage models, however, relays are deployed to satisfy a combination of the two objectives just mentioned. A list of key usage model applications for relay augmented cellular networks is then provided. Among these usage cases are; radio coverage extension, the concept of “coverage hole filling”, cooperative relaying, and temporary coverage. The most recent AeroMACS Profile draft is briefly discussed and it is then argued that, other than those issues related to the application of IEEE 802.16j technology to any scenario, are no additional technical challenges in application of IEEE 802.16j-based WiMAX to AeroMACS. The application of multihop relay configuration to AeroMACS enables a reduction in path loss which can be viewed as a link budget “gain”. This “multihop gain” is then quantified with a simple analysis. The multihop gain can then be translated into performance improvement of various forms for AeroMACS. Particular radio coverage situations in airport surface for which IEEE 802.16-2009-WiMAX system either fails to offer a viable solution, or the resolution it provides is inefficient, costly, or excessively power consuming, are listed. In all these cases, it is argued that 16j technology offers a much better alternative. A major concern about deployment of AeroMACS over the 5091-5150 MHz band is interference to co-allocated applications such as the Mobile Satellite Service (MSS) feeder link. At NASA Glenn Research Center the software program Visualyse Professional is being utilized to estimate the AeroMACS interference power levels into MSS. To compare interference effect of IEEE 802.16-2009-based AeroMACS with that of IEEE-802.16j-based network we have conducted several simulation runs for various scenarios, using Visualyse Professional. The initial simulation results suggest that no additional interference to MSS feeder link is caused by deployment of IEEE 802.16j-based AeroMACS.
B. Kamali, “Feasibility of IEEE 802.16j-Based Mobile Multihop Relay WiMAX Networks for AeroMACS Applications,” Final research report to NASA GRC, Division of Instrumentation Communications and Control, August 2010
Executive summary
In 2007 a new spectrum over the C-band (MLS Extension band 5091-5150 MHz) became available for aeronautical mobile route services. A joint study of US FAA and EUROCONTROL determined that no single communication technology can satisfy all physical, operational, and functional requirements of various aeronautical transmission domains. It was recommended that a new aviation-specific transmission technology based on the IEEE 802.16e standard; over the newly available C-band, be developed for Aeronautical Mobile Airport Surface Communications (AeroMACS). The proposed standards will be used to support fixed and mobile ground to ground applications and services. This document briefly reviews the IEEE 802.16 standards. WiMAX technologies and their salient features are discussed next. The challenges of broadband radio communications through airport surface channels are listed. Most of these signal degrading factors exist in the majority of terrestrial cellular systems; however, some are more pronounced for the large airport surface channel. The super dynamic nature of the airport environment renders the channel afflicted with severe multipath fading; often characterized as “worse than Rayleigh”. Doppler spread, owing to higher speeds on the part of aircrafts, is much broader for this channel than it is for ordinary terrestrial cellular channels. Noise in the form of AWG is also of concern; as the large bandwidth raises the noise floor for the system. EUROCONTROL, in coordination with US FAA, has recently investigated the feasibility of the use of WiMAX Forum Mobile System Profile Released 1 for AeroMACS applications. According to a document published by EUROCONTROL [15]; no technical obstacles have been found which would make it impossible to apply this technology for AeroMACS. A major concern about development of AeroMACS over MLS Extension band is interference to co-allocated applications such as MSS. This limits the power levels that are allowed for AeroMACS networks. With this power restriction, the developed AeroMACS network for an airport may leave certain severely shadowed areas with no coverage or very weak linkage. To afford robust communications throughout the airport surface, while limiting the power levels, as well as providing a host of other benefits, this document recommends an investigation into the feasibility of the application of IEEE 802.16j Amendment, to AeroMACS. The potential benefits of multihop relay configuration for AeroMACS networks are discussed in some details. Perhaps the most relevant benefit of the multihop relay configuration to AeroMACS is the flexible and cost effective radio range extension that it puts forward for areas shadowed by large constructions and natural obstacles without raising the required network power levels. In preparation for assessment on feasibility of application of multihop relay WiMAX technology for AeroMACS networks, several tracks of required future studies are ventured and recommended at the conclusion.
B. Kamali, “VHF Systems Optimization”, Final research report to NASA GRC, Division of Instrumentation Communications and Control, August 2005.
Abstract
The safety of air travel and air operations are critically linked to the availability of reliable aeronautical communications and navigation systems. The VHF radio spectrum allocated for aeronautical communications lies over 118-127 MHz. This provides 760 radio channels with 25 kHz of spectral spacing. The capacity of aeronautical radio channels is rapidly reaching saturation in the United States and in Europe. In that light the key functional objective for any future aeronautical communications systems is providing relief to the congested VHF aeronautical band by either substantially increasing the number of voice channels, or using the spectrum more efficiently, or a combination thereof. The objective of this project is to study the current aeronautical communications systems, operating over VHF band, and make recommendations on selection of future aeronautical communications technologies for the optimization of the VHF system, where the most important criterion of optimization is the maximization of spectral capacity. Three sets of technologies are recommended for future aeronautical communication systems which could provide dramatic capacity improvement. First, analog transmission technologies that provide a short term resolution; these technologies bring about a “quick fix” but are difficult to integrate into an all-digital network. Nevertheless, if implemented, they will postpone spectrum depletion for up to two decades in the US. Two technologies are recommended, European AM 8.33 kHz, and migration from DSB-SC to SSB-SC. Secondly, digital transmission technologies for mid-term resolution. In addition; these technologies are suitable for integration into some digital network architecture. These are essentially narrowband digital transmission technologies, in particular VDL Mode-3 and VDL Mode- E are recommended. Thirdly, technologies recommended for long term resolution. Alternative I; Mix Technologies for all Domains: Convert the 25-AM channels to 8.33-AM, then use an overlay digital technology, perhaps B-VHF, for data transmission, which further increases the spectral capacity and has additional benefits of integrating data and voice transmission. Alternative II; digital-only technologies for all domains: Convert 25-AM channels to VDL Mode-3, this provides integrated data-voice capability while quadruples the number of VHF radio channels. And use overlay digital technology (B-VHF) to further expand the capacity. This alternative, while enjoys all the advantages listed on the second part of alternative I, provides a higher capacity than alternative I, however, the entire on-board and ground avionics have to be redesigned and redeployed. Alternative III; digital-only technologies: Convert 25-AM channels to VDL Mode-3, and use the available VHF channels for en-route and terminal integrated voice/data communications. Use overlay digital technology (B-VHF) for en-rout and terminal communications only. Use the available C-band (5000-5150 MHz MLS band) with one of the following digital technologies for airport surface communications. IEEE 802-16; a technology that can support voice and data communications for fixed LOS and mobile NLOS scenarios, therefore suitable for surface domain communications, or apply Transform domain communication; a novel digital transmission technology that is suitable for surface communications. This alternative is an all-digital scheme that provides the highest possible spectral capacity for VHF aeronautical communications; however, the avionics become complicated and more expensive.
NASA (JPL) Space Communications Research Related Publications
B. Kamali, and A. H. Aghvami, “Belief Propagation Decoding of Reed-Solomon Codes; a Bit-Level Soft Decision Decoding Algorithm,” IEEE Transactions on Broadcasting, Vol. 51, NO. 1, pp. 106-113, March 2005.
Abstract
In this article we propose the application of Belief Propagation (BP) algorithm as a novel bit-level soft decision decoding (SDD) technique for Reed-Solomon (RS) codes. A brief tutorial on Belief Propagation algorithm is presented. A central issue in the application of BP algorithm to decoding RS codes is the construction of a sparse parity check matrix for the binary image of the code. It is demonstrated that Vardy’s technique may be applied to find a sparse parity check matrix for RS codes. However, this technique is not applicable to all cases. The BP algorithm is applied to two test codes. In one case, simulation models show that the BP algorithm outperforms the hard decision Euclidean decoding by more than 2 dB of additional coding gain. The results with the second test code are not as promising.
B. Kamali, “Generation of Binary Image Codes and Sparse Parity Check Matrices for Reed-Solomon Codes,” Proceedings of International Conference on Telecommunications (ICT 2005), May 2005, Cape Town, South Africa.
Abstract
In order to apply bit-level soft decision decoding to Reed-Solomon (RS) codes, one needs to transform the original RS code into its binary image code. The initial step in most transformation techniques is to find a generator matrix for the binary image code of the original RS code. An efficient and direct method of obtaining such a matrix is introduced and it is shown that the technique is applicable to all RS codes with any linear mapping of the extension field to the binary field. Vardy’s technique for finding another generator matrix of the binary image code, which possesses certain structural properties, is briefly explained. An interesting and promising idea in soft decision decoding of RS codes is the application of the Belief Propagation (BP) decoding algorithm to the binary version of the code. The problem arises when it is attempted to find a sparse parity check matrix for the code. It is demonstrated that Vardy’s technique, which produces a sparse generator matrix, may be applied to find a sparse parity check matrix for some RS codes.
B. Kamali, “Coded Orthogonal Frequency Division Multiplexing: Signal Characterization and Applications,“ Final Study Report to Section 331 DSP Research Group NASA Jet Propulsion Laboratory, August 20, 2003
Abstract
Orthogonal frequency division multiplexing (OFDM) is a spectrally efficient and a robust transmission technique in the presence of many channel adversities such as frequency selective fading and impulse noise. Perhaps, the key motivation behind the widespread adoption of OFMD signaling in mobile communication is its ability to convert a wideband frequency selective fading channel into a series of approximately flat fading narrowband subchannels. In other words OFDM is effective in removing the frequency-dependent aspect of fading without requiring sophisticated equalization procedures; however, it does not eliminate the fading effect itself. Channel coding is the technique of choice for mitigating the effect of fading on the signal. It is shown that OFDM modulation and demodulation process may be realized by discrete Fourier transform. This implies that the bank of subcarrier oscillators and coherent demodulators that are needed in the implementation of conventional FDM systems are no longer required and are replaced with a computationally efficient fast Fourier transform algorithm. Moreover, an end to end digital/ software implementation for OFDM system is a possibility. It has also been shown that OFDM exhibit some optimality in performance over frequency selective fading channels. In this report basic principles of OFDM signaling are discussed at the outset, this leads to a brief passage on how to select OFDM parameters for a specific application. Subsequently, an overview of major applications of coded OFDM (COFDM) is presented. A successful application of COFDM was in ADSL technology for which ANSI issued a standard in 1995. The standard provides for up to seven channels for transport of synchronous transfer mode data whose rates may be programmed in any combination of 32 kbps. The maximum possible rate for delivery of multimedia signal to the user’s premises is 6.144 Mbps. In Europe COFDM has been adopted for both digital audio and digital video broadcasting. A major reason for the selection of COFDM is the possibility to use single frequency network. In a single frequency network the user receives several copies of the same signal from a number of transmitters with different delays. As long as the time difference between the arrivals of two signals is less than OFDM symbol guard interval, no ISI or ICI will occur. But more importantly, the presence of two or more time-shifted copies of the same signal provides a diversity advantage in the network. A unique feature of the terrestrial DVB (DVB-T) standard is the hierarchical transmission. The system allows for combination of different levels of QAM modulation and different code rate for the inner convolution/turbo code, which renders the system a tradeoff between bit rate and signal ruggedness. A functional block diagram of hierarchical /nonhierarchical DVB-T system is presented. In the US the IEEE approved a standard for wireless local area networks; the IEEE802.11, in which COFDM signaling was adopted for the new 5 GHz standard that supports data rates up to 54 Mbps. To accommodate various supported data rates, the standard allows for the use of four different modulation schemes and a variable rate convolutional code. The parameters of the selected modulation scheme and the code rate are all data rate dependent. A number of channel coding schemes have been applied and proposed for COFDM signaling. A brief section on performance of various channel coding schemes used in COFDM is provided.
B. Kamali, “Novel Techniques for Bit-Level Soft Decision Decoding of Reed- Solomon Codes” Final research report to data processing (coding) group of NASA Jet Propulsion Laboratory, August, 2002.
Abstract
Some new techniques for bit-level soft decision decoding of Reed-Solomon (RS) codes are discussed in this report. The initial step in the majority of these techniques is to find a generator matrix for the binary image code of the original code. An efficient and direct method of obtaining such a matrix is introduced and it is shown that the technique is applicable to all RS codes with any linear mapping of the extension field to the binary field. Vardy’s technique on finding another generator matrix of the binary image code, which possesses certain structural properties, is briefly explained. Liu’s turbo decoding of RS code based on Vardy’s decomposition and self concatenation is discussed. Although an efficient algorithm, it is shown that this technique is not suitable for the legacy RS code. An interesting and promising idea in soft decision decoding of RS codes is the application of Belief Propagation (BP) decoding algorithm to the binary version of the code. The problem arises when it is attempted to find a sparse parity check matrix for the code. It is demonstrated that Vardy’s technique, which produces a sparse generator matrix, may be applied to find a sparse parity check matrix for an RS code. It is shown that this technique is not applicable to all cases. The BP algorithm is applied to two test codes. In one case, simulation models shows that BP algorithm outperforms the hard decision Euclidean decoding by more than 2 dB of additional coding gain. The results with the second test code are not as promising.
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