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Resource Documents

Tutorial Paper

  1. Orthogonal Frequency Division Multiple Access : Is it the Multiple Access System of the Future?
    Srikanth S., Kumaran V. , Manikandan C., Murugesapandian.

    There is significant interest worldwide in the development of technologies for broadband cellular wireless (BCW) systems. One of the key technologies which is becoming the de-facto technology for use in BCW systems is the orthogonal frequency division multiple access (OFDMA) scheme. In this tutorial article, we discuss the reasons for the popularity of OFDMA and outline some of the important concepts which are used in OFDMA as applied to BCW systems. We shall use the IEEE 802.16 based WiMAX standards for highlighting some of the significant ideas in the practical use of OFDMA systems.

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Technical Reports

  1. A report on the “Design and Implementation of an Open Source Software/Tools based Wi-Fi security test bed”

    Securing the Wi-Fi wireless communication networks is very important, as it is highly susceptible to attacks and is challenging. To enable security in WiFi, one has to appropriately configure the various elements of the network with suitable software / tools. We have set up an open source software / tools based Wi-Fi security test bed. The various security mechanisms based on IEEE 802.1x and EAP methods such as EAP­TTLS, PEAP, TLS, MD5 and WPA PSK were installed and configured on RADIUS server, wireless client and Access Point (AP). Atheros AP and client cards were used in our setup. The open source software used includes FreeRADIUS, WPA-Supplicant, MADWIFI and Wireshark. The results of our experiments, captured on the wired and wireless side, are presented.

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Phd Thesis

  1. PERFORMANCE ANALYSIS OF RADIO RESOURCE MANAGEMENT SCHEMES IN WIRELESS DATA NETWORKS - R. JAYAPARVATHY

    In this thesis, we present a performance analysis of next generation cellular and adhoc wireless networks. We first consider a cellular system with orthogonal frequency division multiple access (OFDMA).  We study the performance of distributed dynamic packet assignment (DPA) schemes in OFDMA based cellular systems with data traffic, in which each user requires a block of OFDM sub-carriers for transmission. Most approaches in the literature that study the performance of wireless data networks do not take into account the interference conditions on the OFDM sub-carriers. In this thesis, we derive the expressions for the mean delay and the average system throughput both on the uplink (mobile-to-base station link) as well as on the downlink (base station-to-mobile link). Our analysis takes into account the interference conditions on the OFDM sub-carriers in addition to the traffic conditions. We model each cell as a buffer of infinite size. The state of the buffer as seen by newly arriving data traffic in a cell is modeled by a two-tuple of non-negative integers, (m, k), where m is representative of the traffic load and k represents the number of sub-carriers that cannot be used by newly arriving data traffic due to violation of interference constraints. We model the state space of the two-tuples (m, k) as a continuous time Markov chain (CTMC), and solve the CTMC to obtain the mean delay and the average system throughput. We first carry out our analysis for two dimensional and linear cellular systems on the uplink and then extend our analysis to two dimensional and linear cellular systems on the downlink. We illustrate the accuracy of our analysis by comparison with simulations. We compare the performance of DPA in OFDMA based cellular systems with that of circuit switched dynamic channel assignment (DCA) and show that DPA results in better resource utilization and  performs better than DCA both on the uplink as well as on the downlink, both for two dimensional as well as for linear cellular systems. Our analysis is applicable for fixed broadband wireless access (FBWA) systems like the IEEE 802.16d and can be used to evaluate the capacity of IEEE 802.16 WMAN.

    In the second part of the thesis, we evaluate the performance of a distributed medium access control mechanism (MAC) in wireless adhoc networks. We consider IEEE 802.11 based wireless local area networks (WLAN) operating in the adhoc mode using the distributed co-ordination function (DCF). Most analytical models in the literature that evaluate the DCF performance, consider an always on traffic model and do not take into account, the limit on the maximum number of packet retransmissions, as well as the effect of freezing the back off counter due to channel capture by other stations. We consider an on-off traffic model and present a stochastic reward nets (SRN) based analysis to evaluate the mean delay and the average system throughput of the IEEE 802.11 DCF. Our analysis takes into account all the DCF operations including the freezing of the backoff counter due to channel capture by the other stations and the maximum retry limit for a packet. The places and transitions in the SRN representation model the sequence of operations involved in the IEEE 802.11 DCF MAC protocol, and we take into account the freezing of the backoff counter due to channel capture by the other stations and the dropping of packets due to the limit on the maximum number of retransmissions of a packet, by providing suitable guard functions in the SRN. The SRN formulation not only provides the probability of retransmission of a packet due to collisions (and hence, the average system throughput), but also provides the mean delay suffered by the first packet (i.e., the packet at the head of line (HOL)) at every station. To compute the mean delay of the subsequent packets at a station, we model each station as an M/G/1 queue, with the mean service time to be the mean delay suffered by the HOL packet. We provide the analytical results for WLAN systems operating at data rates of 2, 11 and 54 Mbps, and validate our analysis by comparison with simulations. We observe that wireless channels with larger data rates result in better mean delay performance. Also, channels with higher data rates support larger packet sizes, i.e., provide better effective system goodput, for the same arrival rate.

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MS (By Research) Thesis

  1. MAC enhancements to support quality of service in wireless networks - Rajesh S

    High raw data rates at physical layer have become possible in wireless communication. But for multimedia (voice, video, games) applications to use these, it is necessary to design medium access control (MAC) schemes that can support quality of service (QoS). In this thesis, we propose MAC enhancements to improve the performance of wireless networks. Specifically, we address the MAC issues in wireless ad hoc networks and wireless local area networks (WLANs) to support traffic with QoS requirements.We have assumed the use of carrier sense multiple access (CSMA) based schemes in this work. The wireless channel is a shared medium and hence it is necessary to have efficient MAC schemes. In wireless ad hoc networks, the MAC is usually distributed, whereas, in WLAN, the MAC could be distributed or centralized based on topology and requirement. Support for QoS involves catering to two types of traffic:

    • Traffic that has priority assigned per packet (based on type of data in the packet or the link end-points involved). This requires the MAC to differentiate (prioritize) and give the appropriate ratio of the available bandwidth to the traffic.
    • Traffic streams that must be guaranteed certain bandwidth, delay and other requirements (put together as QoS requirements) irrespective of other traffic in the system

    Traffic streams with QoS guarantees are supported in the centralized MAC scheme only.

    Contributions made in this thesis are:

    • Wireless ad hoc networks with directional antenna at nodes are considered and we propose a MAC protocol that uses the directional antennas used by the nodes. We show that there is improvement in throughput and delay performance due to this.
    • We then consider WLANs with centralized MAC and propose an enhanced MAC to improve the delay and throughput performance of the system so that the QoS requirements can be met.

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  2. Bandwidth Estimation and Analysis of Multi-hop Ad hoc Networks - K.Vijayalakshmi

    Medium access control protocols and its effect on capacity are important aspects in communication system design for any shared medium like wireless. In multi-hop ad hoc networks that use a distributed and contention based channel access mechanism such as those specified in the IEEE 802.11 standard, the capacity of individual links are not known. Existing methods in literature attempt to solve this using measurement based approaches. This thesis proposes graph theoretical and
    real-time approaches to estimate the capacities of individual links in a multi-hop ad hoc network and uses analytical modeling to derive node throughputs and successful transmission probabilities of individual nodes in multi-hop ad hoc networks. 

    In a contention based system based on the IEEE 802.11 standard, the capacity of links depends on the time available for the links to be active which in turn depends on the probability of nodes’ transmission. While the link capacity can be expressed by the number of times it gets activated, it can be more accurately expressed in probabilistic terms. This is due to the randomness introduced in the channel access procedure in the form of virtual carrier sensing and binary exponential backoff. This thesis proposes centralized and distributed methods to estimate the active time of links. The centralized approaches have the knowledge of the entire network topology and use graph theoretic approaches to derive individual link active times. The distributed approaches on the other hand only have the partial knowledge of network topology to derive the link active times. This thesis proposes another approach to the link capacity problem using the analytical modeling of multi-hop network for string and grid topologies. The analytical modeling uses Markov models to derive node throughputs and successful transmission probabilities of individual nodes in multi-hop ad hoc networks. The advantages of the estimation methods are topology independent nature of the solution, a wide spectrum of applications using the estimates and real-time applications using the distributed approach. It also provides insight into the performance evaluation of multi-hop networks. The accuracy of the estimations is validated through simulations and it is observed that there is a difference between the estimated and simulated values only of the order of 10e-02. The values obtained through analytical model too matches the values obtained in simulations to a good degree.

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  3. CHANNEL ESTIMATION AND LOW COMPLEXITY DETECTION FOR HIGH SPEED WLANs - MUTHURAJA N

    The IEEE 802.11 based wireless local area networks (WLANs) have enjoyed popularity in many segments. Enhancements to the standards have been ongoing resulting in various flavors of the standard. Recent activities have centered around IEEE 802.11n which is concerned with speed and throughput upgrades to the standards. This thesis discusses and proposes new techniques for channel estimation (CE) in multiple input multiple output – orthogonal frequency division multiplexing (MIMO-OFDM) systems with the orientation towards IEEE 802.11n, which is the standard for high speed WLANs. This thesis provides an exhaustive study on different CE schemes for MIMO-OFDM systems with different preamble structures. New CE schemes are proposed by modifying the existing CE schemes to obtain better performance with less computational complexity.
    The detection of data in MIMO-OFDM systems is also another key challenge as we need parallel MIMO detectors on the subcarriers. This results in a system with a large computational complexity. We propose low complexity MIMO-OFDM detectors whose idea is to reduce the number of MIMO detectors applied for detecting the data in all the subcarriers by considering the IEEE 802.11n channel conditions. The proposed detector offers a complexity reduction of about 50%. The BER and PER performance show that the low complexity (LC) MIMO-OFDM detectors are suitable for the IEEE 802.11n based WLANs in practical channel conditions.

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  1. TIME SYNCHRONIZATION AND LOW COMPLEXITY DETECTION FOR HIGH SPEED WIRELESS LOCAL AREA NETWORK - V.SATHISH

    In this thesis, we propose a low complexity time synchronization algorithm and a low complexity spatial detection technique for high-speed WLANs based on the 802.11n standard. The two major goals of the 802.11n standard are achieving a higher data rate and providing backward compatibility with the existing legacy 802.11a/g systems. To achieve the first goal, the proposals for the 802.11n standard use multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) technology. This necessitates the use of spatial detection techniques at the receiver. The conventional spatial detection techniques exhibit a tradeoff between their complexity and their performance. Since the 802.11n system uses forward error correcting (FEC) codes at the transmitter and the viterbi decoder is typically used at the receiver which further increases the complexity of the system.

    To achieve the backward compatibility goal, we have to address two issues. They are the design of a new preamble and the use of protection mechanism to avoid interference from the legacy systems. The new preamble should be understandable by the legacy stations and it should work well for the MIMO-OFDM systems. The protection mechanism can be provided either in the physical (PHY) layer level or in the medium access control (MAC) layer level. At the PHY layer, the header which carries the length and rate field is decoded by the non- transmitting stations and they defer the channel access for that duration. To decode the header information of the new preamble successfully, the legacy system should be able to use its existing initial receiver algorithms. In addition, the new receiver algorithms have to be proposed for MIMO-OFDM systems which use this new preamble.

    In the first part of the thesis, we propose a low complexity time synchronization algorithm for the legacy stations and for the MIMO-OFDM stations in a typical 802.11n network. We first study the different ways of extending the legacy 802.11a preamble to the MIMO-OFDM systems because using the legacy preamble in some form for the MIMO systems can help in achieving the backward compatibility. We study the performance of automatic gain control (AGC) algorithm using these preambles in the MIMO stations. We show that sending the cyclically shifted versions of the legacy preamble from the 802.11n transmitter provides better power measurements at the receiver compared to simply repeating the legacy preamble at the transmitters. As a first receiver task, we review the method of simply extending the single-input single-output OFDM (SISO-OFDM) start of packet detection (SOP) algorithm to MIMO-OFDM systems. We study the performance of MIMO-OFDM SOP detection algorithm under the spatially correlated and uncorrelated channels. We propose a new coarse timing estimation algorithm that can be used in legacy systems and in MIMO-OFDM systems. We study the performance of the different preambles in the legacy systems and in the MIMO-OFDM systems. We show that the proposed coarse timing estimation algorithm in SISO systems performs well for legacy preamble and for the new preambles as compared to the threshold based algorithm. Since the proposed coarse timing estimation algorithm works well even in lower SNRs, we propose a low complexity fine timing estimation algorithm for the SISO-OFDM and MIMO-OFDM systems. We compare the performance of this algorithm with the popular correlation based techniques. We evaluate the performance of the proposed fine timing estimation algorithm for all the preamble types under all the channel models. From the simulation results, we have shown that the cyclically shifted preamble with cyclic shift of 8 samples for a 2x2 system seems to be good choice for mixed mode and green field operations.

    In the second part of the thesis, we propose a low complexity spatial detection technique for bit interleaved coded modulated (BICM) MIMO-OFDM system. We first review the different spatial detection techniques, their performance, and their complexity requirements. Based on the moderately complex and moderately performing ordered successive interference cancellation (OSIC) technique, we propose a new grouping and detection technique. The grouping can be done in a fixed manner or in an adaptive manner. We study the performance of the proposed techniques in the uncoded and coded systems. We show that the performance in the coded system matches with the performance of the OSIC technique and has lesser complexity compared with the OSIC technique. We also study the performance of the proposed system under various channel models. For the simulation, we have considered the BICM MIMO-OFDM system proposed in draft 802.11n standard. The results show a 40% reduction in complexity can be achieved by using the proposed technique as compared to complexity requirement of OSIC technique and achieves similar performance.

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    2.