Mobile computing: proliferation and chaos!

According to a recent report, US IP traffic could reach an annual total of one zetabyte, or one million million billion bytes by 2021 and more interestingly, the mobile device will be the primary connection tool to the Internet for most people in the world (PEW Internet and American Life Project). Although we are seeing a sharp increase in WLAN deployment (IEEE 802.11) as well as an advent of variety of wireless standards in recent years including WRAN (wireless regional area networks, IEEE 802.22), WPAN (wireless personal area networks, IEEE 802.15) and WiMAX (broadband wireless access, IEEE 802.16), they may not be sufficient to deal with the ever increasing demand on wireless connectivity. Challenges such as the limited spectrum, interoperability, scalability, and latency would easily turn the last-mile solution into last-mile headache unless measures are taken serioulsly. Security is of course is a big concern in many fronts.

This is a partial list of what we are doing at MCRL.

  • Hardware security attack (clock glitching) and Side channel analysis
  • Indoor localization
  • Energy efficient mobile hotspot and GPS tethering
  • Software radio and Zigbee steganography
  • Past Work
  • Work uncompleted
  • Hardware security attack (clock glitching) and Side channel analysis

    Security of electronic systems have emerged as a serious concern. With increasing connectivity and ease of access to these systems, the security vulnerability is increasing at an alarming pace. This project is intended to provide a holistic view on information and software security as well as hardware security. In particular, hardware security attacks such as hardware IP piracy, side-channel analysis, fault injection, and clock glitching are a growing concern to both semiconductor industry as well as end users. Our work is based on Altera FPGA prototyping systems and Quartus.

    We received NSF award (TUES Award #1245900, September 2013) for the work on security education. See ECE news, CSU news, and Plain Dealer news.

    Indoor localization

    Indoor localization based on RF fingerprints is not new but we found it is not straightforward in a large scale venue. We surveyed the RF scene at a large venue (505x237m2) and revealed challenges as well as proposed solutions (Mobiquitous, Tokyo, December 2013).

    The increasing popularity of 4G-capable mobile devices such as smartphones and tablets has brought a lot of attention and concern on the security and privacy of 4G communications. In comparison with the 2G and 3G mobile networks, 4G networks use adaptive modulation and coding to provide constant network access with much higher bandwidth to these mobile devices. We study security and privacy of 4G communications, fast indoor positioning in large-scale, chaotic venues, efficient modulation and coding for 4G mobile communications, characterizing the noise in wireless channels with statistical physics approaches, and secure medical communication systems. (National Science Foundation, MRI Award #1338105, September 2013)

    Read our recent papers in JNCA 2017, IEEE WCNC 2015, and at Mobiquitous 2013. (pdf) Details of NSF award is found from here.

    Energy efficient mobile hotspot and GPS tethering

    Mobile hotspot is useful to provide connectivity to Wifi-only devices. We designed energy efficient mobile hotspot and implemented with pseudo null and pseudo beacon frames in the context of virtual AP framework. (IEEE INFOCOM, Toronto, April 2014)

    Read our recent papers at IEEE WCNC 2015 and IEEE INFOCOM 2014. (pdf)

    Software radio and Zigbee steganography

    Security at the physical layer attracts attention due to its low complexity. We studied physical layer watermarking based on information redundancy in spread spectrum communication in Wifi and Zigbee protocols. This work is implemented using USRP/GNU Radio software radio platform. (IEEE MILCOM, San Diego, November 2013)

    Read our recent papers at IEEE MILCOM 2013 (pdf.)

    Past Work

    1. RF scene fingerprinting
    2. Anatomy of communications
    3. Modulation and bandwidth scaling and diversity
    4. Uncertainty and vagueness in mobile computing scenario
    5. Autonomous wireless network architecture (SINA)
    6. Cache invalidation in Infrastructure-based vehicular ad hoc networks (IVANETs)


    1. RF scene fingerprinting

    Geo-location is critically important in many mobile applications including mobile social network and u-health applications. The proliferation of WLAN infrastructures can be usefully utilized for indoor localization using WLAN fingerprints. Our work shows that identifying a room or a place is feasible as tens of accessible APs are accessbile in typical urban environments. We design and implement a cooperative indoor localization technique using Android-based mobile devices for mobile social network applications.

    Another important research agenda in this area is to identify mobile devices by characterizing their analog signals. While authentication and security measures at higher layers can be compromised by malicous attackers, their analog signals are not easily subjective to random manipulation. This research could help to offer secure u-health applications. We are using a combination of USRP and GNU Radio, which is identified as one of Key Technologies to Watch in 2008 by Bristol Systems. Click here for more information on USRP / GNU Radio. Also, see a class website. Read our recent papers at IEEE SmartE 2010 (pdf.)




    2. Anatomy of communications

    Mobile networks are easily vulnerable to stresses such as high traffic and high node speed as well as privacy and security attacks. As existing link and network layer solutions do not take these extreme conditions into consideration, it is critically important to know whether a mobile network is still a dependable subnet under such situations. The goal of this research is to seek novel methods to survive the stress while achieving a reasonable performance. We are exploring several measures including bandwidth subchanneling.

    We analyze the computational complexity of communication functions using GNU Radio SDR software to take communication and computation in a unified framework. Several optimizations are possible; i.e., rate-distance tradeoff turns into rate-distance-energy tradeoff. Currently we're profiling several modulation and coding schemes by using Oprofile profiling tool.

    This work was supported in part by National Science Foundation, Major Research Instrumentation (MRI) Program. The project is entitled "Performabiity in Mobile Wireless Networks."




    3. Modulation and bandwidth scaling and diversity

    We designed and experimented Multihop Opportunistic Transmission Protocol (MTOP) that significantly advances the communication efficiency in a mixture of devices with different bit rates. It is being integrated with bandwidth scaling and modulation diversity techniques to further improve the network performance. We also designed Path-centric Rate Adaptation (PRAM) algorithm, which is a cross-layer routing scheme. It is being tested using ns-2 simulation as well as Emulab emulation environment.

    This work was supported in part by National Science Foundation, Network Technology and Systems (NeTS) Program. The project is entitled "Exploring Data Access in Internet-based Wireless Mobile Networks" Read our papers at IEEE ICCCN 2009 (pdf) and IEEE INFOCOM 2010 (pdf.)




    4. Uncertainty and vagueness in mobile computing scenario

    Inspired by the Smart Dust project started in 1998, researchers have shown enormous interests in wireless sensor networks (sensornets) because of their long-term potential in many interesting pervasive applications. However, one of the challenges for sensornets to become a prevailing technology in the next decade comes from the complexity of managing the raw data distributed in a sensornet and turning it into information for decision-making purposes. Two major sources of the complexity are resource constraints and uncertainties. While the former (e.g., severe energy constraints and limited bandwidth) has attracted a lot of attention from the research community, little work has been done on the complexity due to ontological and epistemic uncertainties. Ontological uncertainty can emerge from the lack of specification of what kind of entities could exist and epistemic uncertainty emerges due to inadequate representation of knowledge that is often incomplete, imprecise, fragmentary, and ambiguous. The main goal of the proposed research is to develop a conceptual framework that deals with those uncertainties based on classical rough set theory (Wiki page) and to reconsider sensornet architectures and algorithms in a way to support as well as exploit the framework.

    For example, one can imagine a surveillance network for contaminant detection in water distribution system. A conventional detection problem asks for a tradeoff between the probability of detection and the false alarm rate because the consequence of a warning could be the stoppage of the infrastructure service which is quite a costly measure, the false alarm must be avoided as much as possible. However, a major challenge is that there are so many contaminants that a large array of (contaminant-specific) sensors might be required, while still leaving the system vulnerable to contaminants for which no effective sensor was available or not employed. This ontological uncertainty can be approached by monitoring more general water parameters to identify a signature of “normal” conditions, and flag anomalies. This would require a more significant sensornet component, and would involve a more substantive “multi-parameter filtering” problem - to estimate the state of the system and derive a conditional probability that the condition is normal or anomalous. Questions to how many sensors, where to sense, and how often to sense constitute epistemic uncertainty: the observation of coarser granularity offers less detail while the clumping of information into an aggregate form may prevent finer entities from being distinguished. While uncertainty in general affects a systems ability to perform with accuracy and precision, the impact of uncertainty in sensornets. We are also considering to apply the same idea in sensornet-based rehab applications.

    Read one of our recent papers published in IJMC (2009, pdf).




    5. Autonomous wireless network architecture (SINA)

    Extrapolating from the phenomenal success of social network services, it is not unrealistic to anticipate that mobile social networking would play an important role in the near future. However, since social activity would be inherently transient due to user mobility rendering the corresponding interaction spontaneous, challenges are to discover interaction opportunities spontaneously and to support interactions while taking the limited resource of users’ mobile devices and the scarce wireless bandwidth into consideration. To address these challenges, We design and implement autonomic networking architecture called SINA. It exploits contexts such as application requirements, user preferences, the availability of services and resources as well as topology of nodes to find an interaction opportunity. A minimal set of resources are used because (i) a network instance is created only when an interaction opportunity is found and (ii) it is organized and maintained in an application-aware manner.




    6. Cache invalidation in Infrastructure-based vehicular ad hoc networks (IVANETs)

    Wireless networks are increasingly popular as the last-mile solution for a ubiquitous communication infrastructure. This trend in combination with the growing interest in accessing vast amount of resources in Internet has driven the developments of hybrid wireless network architectures such as Internet-based mobile ad hoc networks (IMANETS) and Internet-based vehicular ad hoc networks (IVANETS). For instance, the backbone network of wireless mesh networks is typically architected as an IMANET. One of key features of these networks is multi-hopping, which extends the coverage of a wireless network without investing for an additional infrastructure. However, it is well-researched that these multi-hop wireless networks possess the scalability problem in the sense that per-client bandwidth is critically limited and decreasing as the network size grows. In order to improve the data accessibility, which refers to the capability of allowing mobile users to access desired data with high success rate, a prudent data caching scheme is required. In multi-hop wireless networks, a successful data caching scheme stores data not only in Internet gateways but also in individual client devices as they typically play as intermediate routers. Correspondingly, three key design issues are: which data to cache (limited storage space), how to invalidate cached data (invalidation message explosion), and how to implement and operate relaxed cache consistency model in IMANETS and IVANETS.

    Therefore, this project aims at developing algorithms and communication protocols that allow efficient and correct data caching in Internet-based wireless mobile networks. In this proposal, we investigate the following four major areas: (i) Cache management scheme for IMANETS: We investigate the problem of information search and access, and propose cache management mechanisms including a cache admission control and a cache replacement policy that improve the overall information accessibility; (ii) Cache invalidation strategies for IMANET: For providing data consistency in an IMANET, we propose several push and pull-based cache invalidation schemes, the design of which is not straightforward due to the complications caused by multi-hop message relay, operation cost model, and uncertainty in message delivery; (iii) Cache invalidation scheme for IVANETS: To access the Internet service and information on the wheel, we consider the integration of VANET with wireless infrastructure. Under this environment, cache management (which data to cache) is less of a problem because the storage space in a vehicle is not critically limited. On the other hand, high host mobility complicates the invalidation process. We propose a location-based cache invalidation scheme to reduce the negative impact of mobility, the cost of broadcast, and the query latency; and (vi) Cache consistency strategies for IVANETS: Strong cache consistency is not easily achievable or is too costly to achieve in IVANETS due to fast-changing network topologies. We propose weak cache consistency strategies that make a tradeoff between cache consistency and bandwidth requirements for ensuring the consistency.

    Read one of our recent papers published in IJMC (2009, pdf), ICCCN 2009 (pdf), and MoveNet 2008 (pdf). This research was supported in part by National Science Foundation (NSF) under the Grants CNS-0831853.

    Work uncompleted

    1. Dual-band migration
    2. Visible light communication
    3. Multi-hop Decode-and-Forward in Wireless Multihop Networks
    4. Multi-resolution Modulation
    5. Accelerating Wireless Communication via Pipelining
    6. On Synchronization for Cooperative Communication
    7. Structured Routing in Wireless Mesg networks


    1. Dual-band migration

    The great success of wireless LANs has evolved to using two frequency bands at 2.4GHz and 5GHz. For example, IEEE 802.11a, 802.11n, 802.11ac and 802.11ad are mean to use 5GHz band, which supports a wider band and more number of channels. Since the 2.4GHz band is crowded with not only conventional Wifi traffic but also Bluetooth and Zigbee traffic, many dual-band APs are designed to steer clients from 2.4GHz to 5GHz band as long as the clients are dual-band capable and reachable. However, a naive steering could cause load unbalanced across bands by blindly favoring 5GHz over 2.4GHz. Another problem is that the band steering is not dynamic, i.e., once an association with an AP and a band is determined, it does not change.

    This study is intended to propose a dynamic band steering mechanism, which balances the load across the dual bands and across multiple APs in the proximity as well as dynamically changes the association by use of Channel Switching Announcement (CSA), which is a standard feature of IEEE 802.11.




    2. Visible light communication


    3. Multi-hop Decode-and-Forward in Wireless Multihop Networks

    The field of wireless networking has received unprecedented attention from the research community during the last decade due to its great potential to create new horizons for communication beyond the Internet. Wireless LANs (WLANs) based on the IEEE 802.11 standard, called Wi-Fi hot spots, have become prevalent in public as well as residential areas. Numerous efforts, planned or unplanned, have been made to provide Internet connectivity over a larger geographical area, which is known as wireless mesh networks (WMNs). They could be used as a backbone to support mobile social networking (MSN) for exchanging information and multimedia data. Extrapolating from the phenomenal success of social network services such as Facebook and Twitter, it is not unrealistic to anticipate that MSN would play an important role in the near future. Vehicular ad hoc networks (VANETs) will become an important part of future ubiquitous communication infrastructure as they support the last-mile solution for drivers on the wheel. As ground transportation is considered one technological area in which change is long overdue, a break-through can be achieved through a well-designed VANET infrastructure. A common and unique characteristic of the above-mentioned emerging wireless networks is multi-hop communication.

    One clear disadvantage of high-rate communication is its vulnerability to noise/interference as well as to node mobility because it changes the signal strength, and thus, SINR. Recently, there has been active research on cooperative communication at the PHY layer, which refers to scenarios in which distributed radios interact with each other to jointly transmit information in wireless environments. In other words, cooperative communication exploits diversity offered by multiple users, known as multiuser or cooperative diversity. It dramatically improves bit error rate (BER), resulting in a more reliable transmission and a higher throughput.

    This research will propose multihop decode-and-forward (MDAF) mechanism, which is essentially a cooperative communication method and will improve the link reliability that is critical for communication at high rates. It exploits an inherent repetition of the same packet over multiple hops. In multihop communication, an intermediate node would receive transmission not only from its immediate predecessor but also from its other predecessors at least partially because frame header is always transmitted at the lowest rate. Then, it can soft-decision combine the two copies of the same frame to successfully decode the frame. This repetition-based cooperative communication refers to the scenarios where cooperating radios transmit in two different time slots, which avoids the synchronization problems otherwise.

    [1] S. Moh and C. Yu, A Cooperative Diversity-based Robust MAC Protocol in Wireless Ad Hoc Networks, IEEE Trans. Parallel and Distributed Systems, 2011.
    [2] G. J. Bradford and J. N. Laneman, An Experimental Framework for Evaluating Cooperative Diversity, Proc. Annual Conf. Information Sciences and Systems (CISS), 2009.
    [3] Korakis, T.; Knox, M.; Erkip, E.; Panwar, S., Cooperative network implementation using open-source platforms, IEEE Communications Magazine, Vol. 47, No. 2, Feb. 2009.
    [4] M. Knox and E. Erkip. Implementation of cooperative communications using software defined radios. Proc. Int’l Conf. Acoustics, Speech and Signal Processing (ICASSP), 2010.
    [5] Zhang, J., Jia, J., Zhang, Q., and Lo, E. M. Implementation and evaluation of cooperative communication schemes in software-defined radio testbed, Proc. IEEE INFOCOM, 2010.
    [6] Z. Lin, E. Erkip, and A. Stefanov, Cooperative Regions and Partner Choice in Coded Cooperative Systems, IEEE Trans. on Communications, Vol. 54, Issue 7, pp. 1323-1334, July 2006.
    [7] J. N. Laneman, Cooperative Diversity: Models, Algorithms, and Architectures, Cooperation in Wireless Networks: Principles and Applications, F. H. P. Fitzek and M. D. Katz (Eds.), Chapter 6, Springer, Netherlands, 2006.
    [8] A. Nosratinia, T. E. Hunter, and A. Hedayat, Cooperative Communication in Wireless Networks, IEEE Communication Magazine, Vol. 42, No. 10, pp. 74-80, Oct. 2004.




    4. Multi-resolution Modulation

    In [1], multiple sub-bands of OFDM are used to support multiple clients. A key contribution of [1] is that the best modulation and coding rate is selected for each sub-band based on SNR characteristics of the particular sub-band. Their observed that different subbands in the 802.11a spectrum show a difference in SNR of over 20dB, while that for individual subband is relatively stable for periods of upto 5 seconds.

    We explore the possibility of using the similar idea of transmitting a broadcast/multicast message to multiple receivers at different rates. For example, consider to broadcast an RREQ and a probe. To find out the link quality between nodes, every node must broadcast the same message at different rates. But if it can be transmitted at multiple rates all at once, it would be beneficial.

    In an adaptive channel coding framework, it is important to offer a hierarchy of resolutions of noise immunity to adapt to varying channel condition, which is referred to as multi-resolution channel coding [2]. It offers unequal error protection. Two decades-long study provide us systematic ways of embedding high-rate codes (lower noise immunity) in low-rate codes (high noise immunity). This property is useful for applications requiring compatibility of a single transmitted information stream with several channel capacities or receiver resolutions, as in broadcast or multicast where there is no feedback, i.e., where the transmitter is uninformed of the instantaneous channel capacity. For example, in a two-resolution case, both receiver resolutions have access to the coarse information layer, while the stronger receiver can additionally extract the embedded detail information layer. This idea can be extended to cover modulation and demodulation systems, for example, in defining embedded modulation schemes like QAM [2]. In [3], multi-resolution modulation was considered, where low- and high-resolution data is transmitted simultaneously. Critical information is conveyed in the low-resolution data and non-critical but beneficial information is conveyed in the high-resolution data. Similar work has been reported in the literature [4, 5], which discuss MR-QAM and MR-CPFSK.

    [1] Hariharan Rahul and Farinaz Edalat and Dina Katabi and Charles Sodini, Frequency-Aware Rate Adaptation and MAC Protocols, ACM MOBICOM 2009.
    [2] K. Ramchandran and M. Vetterli, Ch. 7 Multiresolution Joint Source-Channel Coding, Ed. H. V. Poor and G. W. Wornell, Wireless Communications: Signal Processing Perspectives, Prentice Hall, 1998.
    [3] Conner, K.F.; Baum, C.W., Multiresolution FSK/ASK signaling for frequency-hop communication systems, IEEE MILCOM 1997.
    [4] Ashutosh Saxena, Ajit K. Chaturvedi, A New Embedded Multiresolution Signaling Scheme for CPFSK, B. Tech. Research Thesis, IIT Kanpur, India, April 2004.
    [5] Yong Pei and James W. Modestino, Cross-Layer Design for Video Transmission over Wireless Rician Slow-Fading Channels Using an Adaptive Multiresolution Modulation and Coding Scheme, EURASIP Journal on Advances in Signal Processing, 2007.




    5.Accelerating Wireless Communication via Pipelining

    As WMNs become more popular and their scale and complexity continue to grow, they become increasingly vulnerable to problems such as end-to-end latency, bandwidth degradation, and radio interference. Among them, the issue of latency is particularly important, as the trend of Internet usage has shifted from short-lived applications such as web browsing and emails to long-lived, delay sensitive multimedia applications. Latency is also a critical issue in emergency-related VANET applications. It is important to offer sufficient bandwidth but the corresponding design schemes should not adversely impact the latency. Therefore, the goal of this proposal is to support low-latency applications in wireless multihop networks by addressing the following three issues.

    This research will develop and experiment a new physical layer capability, in which a packet is divided into smaller elements and pipeline-delivered over multiple hops at alternate channels like cut-through or wormhole switching in optical networks and parallel machines [1]. Technologies behind this new capability are borrowed from three recent set of studies - PHY-layer switching [2], [3], [4], [5], channel width adaptation [6] and multichannel scheduling [7]. Key contribution of this research in this regard is to integrate and materialize the ideas on a GNU Radio/USRP-based software radio testbed.

    [1] J. Kim, C. R. Das. Hypercube Communication Delay with Wormhole Routing. IEEE Trans. Computers, 43(7), 1994.
    [2] R. Ramanathan, F. Tchakountio. Channel Access over Path Segments for Ultra Low Latency MANETs. IEEE MILCOM, 2007.
    [3] R. Ramanathan. Challenges: A Radically New Architecture for Next Generation Mobile Ad Hoc Networks. ACM MoboCom, 2005.
    [4] R. McTasney, D. Grunwald, D. Sicker. Low-Latency Multichannel Wireless Mesh Networks. IEEE ICCCN, 2007.
    [5] R. Ramanathan, F. Tchakountio. Ultra Low-Latency MANETs. BBN Technical Memorandum TM-2023, 2006.
    [6] R. Chandra, R. Mahajan, T. Moscibroda, R. Raghavendra, P. Bahl, A Case for Adapting Channel Width in Wireless Networks, ACM Sigcomm, 2008.
    [7] G. Narlikar, G. Wilfong and L. Zhang, Designing Multihop Wireless Backhaul Networks with Delay Guarantees, IEEE INFOCOM, 2006.




    6. On Synchronization for Cooperative Communication

    In mobile wireless networks, signal fading (due to communication environment) and interference (due to other nodes) are two major obstacles in realizing their full potential in delivering signals. As for the latter, a node’s data transfer not only provides interference to other nodes depriving their opportunity of using the medium but also incurs energy wastage by rendering them to overhear. Therefore, a node is regarded as a greedy adversary to other nodes in its proximity as they compete with each other to grab the shared medium.

    An obviously better idea is to render the nodes not to compete but to cooperate, exploiting the broadcast nature of wireless communications. Recently, there has been active research on cooperative communication at the PHY layer [1]–[3], which has a potential to significantly enhance the communication reliability in interference-rich environment. It refers to scenarios in which distributed radios interact with each other to jointly transmit information in wireless environments [3]. In other words, cooperative communication exploits diversity offered by multiple users, known as multiuser or cooperative diversity. It dramatically improves bit error rate (BER), resulting in a more reliable transmission and a higher throughput.

    This paper considers the synchronization problem in the abovementioned cooperative communication scenarios. Many existing researches on cooperative communication assume perfect synchronization among cooperating radios but this is generally very difficult to achieve due to its distributed nature [7], [59]. Note that this is different from conventional synchronization problem where the receiver synchronizes to the transmitter. It is also important to distinguish it from the scenarios where cooperating radios transmit in two different time slots [8–10]. A recent study showed how the synchronization problems in this case, signal alignment and signal combination at the receiver, can be handled in practice [4]. Other studies address the problem of different signal strengths from cooperating radios when combining their signals [8]–[10]. They either place multiple transmitters equal-distanced from the receiver just for the purpose of experiments or apply the RSSI-based weighting when combining. Our goal in this paper is to study the synchronization problem when cooperating radios transmit simultaneously on the same channel. Both the transmitters’ timing and the propagation delay to the receiver must be within a tolerable range [11], [12]. We concentrated on the former issue because the difference in propagation delay is relatively smaller than the former, particularly in short-range communications [4].

    However, it is also a matter of fact that they rather focus on resolving the synchronization issues in the context of the platform of choice. For example, in GNU Radio/USRP platform, the RF front end, which is responsible for gain (power amplifier and low-noise amplifier) and frequency conversion, is implemented in hardware (USRP) but the electronics part, which is responsible for frequency synthesis, filtering, modulation, upconverting, etc. [13], [14], is implemented in GNU Radio software and runs on a host PC. USRP is connected to the PC via USB (USRP1) or Ethernet (USRP2). In other words, there is a non-negligible delay from the GNU Radio software to the host PC’s operating system, to device drivers, and to the RF front end on the USRP. [4] is concerned about the later part of the chain (the random queueing delay at the Ethernet connection in units of microsecond). [5] observes that there exists a larger delay and jitter at the earlier part of the chain (between GNU Radio and device drivers in units of hundreds of microseconds). Both of them, either larger or smaller delay, does harm in transmitting in symbol-level synchrony. It is similarly observed in [15].

    They suggested to use timestamp method [4]–[6]. In [4], a sender and two transmitters are synchronized with the same external clock for their USRP and their original and relayed transmissions are based on timestamp to realize the synchronous transmission. This is similarly approached in [5], [6]. The important missing component in previous studies is how to mandate cooperating radios to transmit at the same time beyond the context of existing platforms. In fact, a newer version of USRP, or called USRP2, holds a bigger FPGA (Xilinx Spartan 3-2000 FPGA) and a SD socket so that it is ready to function as a stand-alone radio device and thus, eliminating a major part of delay and jitter. Under this scenario, cooperating radios could be stay in synchrony based on MAC control. Earlier, we presented a MAC layer protocol, called cooperative diversity MAC (CD-MAC), that achieves the cooperative communication capability based on the MAC timing [16]. Unlike many previous studies, CD-MAC uses a single partner (relay); i.e., each transmitter monitors its neighbors and dynamically determines a single partner as the one that exhibits the best link quality. Whenever needed, it sends its signal together with its partner in a cooperative manner to improve the communication reliability. A key element of the CD-MAC is to synchronize the two radios based on MAC timing.

    [1] Z. Lin, E. Erkip, and A. Stefanov, “Cooperative Regions and Partner Choice in Coded Cooperative Systems”, IEEE Trans. on Communications, Vol. 54, Issue 7, pp. 1323-1334, July 2006.
    [2] J. N. Laneman, “Cooperative Diversity: Models, Algorithms, and Architectures,” Cooperation in Wireless Networks: Principles and Applications, F. H. P. Fitzek and M. D. Katz (Eds.), Chapter 6, Springer, Netherlands, 2006.
    [3] A. Nosratinia, T. E. Hunter, and A. Hedayat, “Cooperative Communication in Wireless Networks,” IEEE Communication Magazine, Vol. 42, No. 10, pp. 74-80, Oct. 2004.
    [4] Zhang, J., Jia, J., Zhang, Q., and Lo, E. M. Implementation and evaluation of cooperative communication schemes in software-defined radio testbed, Proc. IEEE INFOCOM, 2010.
    [5] G. Nychis, T. Hottelier, Z. Yang, S. Seshan, P. Steenkiste, Enabling MAC Protocol Implementations on Software-defined Radios, Proc. USENIX NSDI, 2009.
    [6] Y. J. Chang and M. A. Ingram, Cluster Transmission Time Synchronization for Cooperative Transmission using Software Defined Radio, Proc. IEEE ICC Workshop on Cooperative and Cognitive Mobile Networks (CoCoNet3), 2010
    [7] Li, X., Wu, Y., and Serpedin, E. Timing synchronization in decode-and-forward cooperative communication systems. IEEE Trans. Signal Processing, Vol. 57, No. 4, Apr. 2009.
    [8] G. J. Bradford and J. N. Laneman, An Experimental Framework for Evaluating Cooperative Diversity, Proc. Annual Conf. Information Sciences and Systems (CISS), 2009.
    [9] Korakis, T.; Knox, M.; Erkip, E.; Panwar, S., Cooperative network implementation using open-source platforms, IEEE Communications Magazine, Vol. 47, No. 2, Feb. 2009.
    [10] M. Knox and E. Erkip. Implementation of cooperative communications using software defined radios. Proc. Int’l Conf. Acoustics, Speech and Signal Processing (ICASSP), 2010.
    [11] P. Murphy, A. Sabharwal, and B. Aazhang, On Building a Cooperative Communication System: Testbed Implementation and First Results, EURASIP Journal on Wireless Communications and Networking, 2009.
    [12] Li, Xiaohua, Wireless Information Assurance and Cooperative Communications, Technical Report of SUNY, Binghamton, 2005.
    [13] V. Raghunathan, C. Schurgers, S. Park, and M.B. Srivastava, Energy-aware wireless microsensor networks, IEEE Signal Processing Magazine, vol. 19, pages 40-50, March 2002.
    [14] A. Y. Wang, S. H. Cho, C. G. Sodini, and A. P. Chandrakasan, Energy efficient modulation and MAC for asymmetric RF microsensor systems, Proc. ACM ISLPED, 2001.
    [15] T. Schmid, O. Sekkat, and M. B. Srivastava, An Experimental Study of Network Performance Impact of Increased Latency in Software Defined Radios, WinTECH 2007.
    [16] S. Moh and C. Yu, A Cooperative Diversity-based Robust MAC Protocol in Wireless Ad Hoc Networks, IEEE Trans. on Parallel and Distributed Systems (to appear).




    7. Structured Routing in Wireless Mesg networks

    A wireless mesh network must utilize equal-capacity nodes in the network and should be able to guarantee a minimum throughput for each pair of nodes. However, it is observed that achieve significantly different throughputs among the node pairs. This is partly due to interference and collisions at the MAC layer and partly due to non-uniform responsibility of packet forwarding. For example, in recent years, there has been active research in capacity analysis of wireless mesh networks, which is usually based on previous research in general ad hoc networks [1, 2, 3]. Kozat and Tassiulas analyzed the throughput capacity of random ad hoc networks with infrastructure support [4]. They have shown that per-node capacity decreases as the rate of Θ(W/log(N)) with infrastructure support, where W and N denote link bandwidth and the number of nodes, respectively, which is a significant improvement over the estimate from previous research [1], Θ(W/(Nlog(N))). Liu, et al. similarly have shown that the capacity of a MANET can be improved significantly by the introduction of n gateways where n is the number of nodes in the network [5].

    However, these studies assume that base stations are neither traffic source nor destination but participate only in relaying packets. This is exactly the reverse in wireless mesh networks, where most of traffic is coming or destined at base stations. In this case, nodes near the base stations would undertake more packet forwarding functionality and extend packet delay due to their long packet queue, which is in turn caused by equal medium sharing principle mandated by the underlying medium access control protocols such as IEEE 802.11 DCF. Jun and Sichitiu have proven that per-node capacity decreases as the rate of O(W/N), which is significantly lower than previous estimates [6]. They observed that bottleneck area, called bottleneck collision domain, is usually found near base stations due to the asymmetric nature of traffic pattern and it critically limits the network throughput. Wang and Ramanathan proposed to alleviate this problem by providing different medium access priority to different class of traffic [7].

    The purpose of this paper is to propose a unique solution suitable for multihop environment rather than one based on single-hop technology. New wireless communication facility known as variable beamwidth formation is considered to create opportunities toward this end. And, this paper attempts to provide an in-depth understanding of the network: What is the maximum achievable capacity of a mobile ad hoc network and how much can it be improved with new communication facilities such as directional antennas? This paper suggests a novel cross-layer optimization technique called Structured Routing with Directional Antennas (SRDA). The main idea of SRDA is to direct frames to distinictive directions in a time division manner using directional antennas and the corresponding MAC algorithm to effectively alleviate the adverse effect of collisions and interference.

    Directional antenna is costly but may be reasonable to be used in mesh networks. For example, when directional antennas are applied to IEEE 802.11 networks, a routing protocol needs to take into account the selection of directional antenna sectors. Directional antennas can reduce exposed nodes, but they also generate more hidden nodes. However, CSMA/CA has very low frequency spatial-reuse efficiency, which significantly limits the scalability of CSMA/CA-based multihop networks.

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