The complete presentation was not made available for publication as part of the conference proceedings. Provides a brief professional biography of the presenter.

High speed communication systems play an important role in the development of advanced networks. The optimum design of these communication links under bandwidth and power constraints needs the use of efficient approaches. In this paper, a novel power efficient modulation scheme is introduced. For this purpose, the modulator at the transmitter uses simple and effective pulse shaping functions to change the amplitude and width of the transmitter pulse. Using this simple method, a high power efficient transceiver structure, with the same performance as typical NRZ signaling, can be designed. Our results show that not only we can reduce the power consumption by more than 10\% without loosing the performance but also the probability of error is improved in the presence of jitter and interference.

Due to their ease of deployment, 802.11 networks are widely used. To cope with increasing throughput requirements, and to take advantage of improvements in hardware performance, 802.11n has been introduced. One of the extra features is the 20/40 functionality, which allows 802.1 In devices to operate using one or two 802.11 channels. In this paper, we study the contention rules for such 20/40 operation on top of the 802.11 Distributed Coordination Function (DCF). We then study the performance of double-channel users in presence of single-channel 802.11 users. Similar to the 802.11 model introduced by Bianchi, we introduce a Markov model for both the legacy 802.11 and double-channel 802.11n users. To couple the Markov models of both types of users, we have to understand how to relate virtual time slots of both types. Next, we introduce collision rules that capture traditional in-channel 802.11 collisions as well as collisions between the different types of users. This allows us to complete the analytical throughput model for 802.11 and double-channel 802.11n users when they coexist. The conclusions are important for understanding 802.11 networks in particular, but also for contention of heterogeneous devices in general.

We present a new scheme for maximizing the signal-to-noise ratio (SNR) in wideband multi-antenna receivers that employ time-interleaved analog-to-digital converters (ADCs). Current wideband receivers interleave a fixed number of slower ADCs into one fast ADC and assign this latter to one antenna according to an antenna selection algorithm. However, this does not always guarantee an optimal trade-off between thermal noise and quantization noise. This results in an overall SNR that is lower than what could be obtained with the same number of ADCs, assigned in a more optimal way. Therefore, we propose to adjust the number of slower ADCs assigned to a certain fast, interleaved ADC dynamically, according to the SNR of every individual antenna. Our proposed algorithm can be implemented at the expense of a very limited hardware complexity increase. The SNR gain of our new scheme can exceed 7 dB, depending on channel conditions and ADC specifications.

This work comprehensively compares four possible multichannel medium access control (MAC) approaches for opportunistic spectrum access (OSA). One of important conclusions to be drawn from the analysis is that multichannel OSA MACs that spread both control and data between all available channels, e.g., hopping MACs, perform best among possible OSA MAC implementations when the PU traffic has long ON/OFF periods compared to the time-scale of the SU channel access.

In this paper, we present a novel algorithm for distributed transmit beamforming enabling multiple single-antenna nodes to simultaneously transmit a common message such that they constructively interfere at the receiver. In order to constructively interfere, the nodes iteratively estimate and adjust their carrier phases with the help of receiver feedback. The receiver's feedback is based on composite channel estimates calculated from two simultaneous transmissions initiated by the nodes at each iteration. We prove that starting from arbitrary carrier phases, the algorithm converges to the maximum gain almost surely. The algorithm is compared to the 1-Bit Feedback algorithm presented in [3] through simulation, and is shown to outperform it by margins of 4:1 and 3:1 on metrics reflecting power consumption and convergence rate, respectively. We simulate the impact of phase estimation errors on the algorithm's steady-state gain and show it to be robust to moderately large errors of plusmn 20deg. We analyze the causes of asynchrony in simultaneous message transmissions and simulate its contribution to phase estimation errors in the context of IEEE 802.15.4 packets. We show that the contribution falls within the acceptable plusmn 20deg range.

The number of wireless networks that coexist in space is increasing steeply. To allow coexistence while avoiding interference, it becomes important to properly characterize the propagation contour where the received power of wireless transmitters reaches a certain threshold. Detailed channel modeling taking into account the specificities of typical urban scenarios is however a very complex task. Thanks to the widespread use of wireless access technology, it becomes feasible to use network nodes to estimate and communicate these propagation contours. In this paper, we propose a lightweight practical scheme for local contour estimation of a given transmitter. The local estimate is efficiently propagated to all other secondary transmitters that can then meet interference constraints optimally, i.e., without having to consider large safety margins that limit spatial reuse gains. This optimality is obtained through iterative power control based on true propagation contour distances and local pathloss estimates. The overhead of the estimation and communication phases is simulated to be close to linear in the number of nodes, so that the solution scales well. The scheme can be used for optimal power control in practical wireless networks, or for the deployment of secondary networks in areas with primary transmitters that should be protected.

We present a GPS-enabled channel sounding platform for measuring both vehicle-to-vehicle and vehicle-to- roadside wireless channels. This platform was used to conduct an extensive field measurement campaign involving vehicular wireless channels across a wide variety of speeds and line-of-sight conditions. From the data, we present statistical characterizations of several classes of these channels at 5.9 GHz. This analysis suggests that while the proposed DSRC standard may account for Doppler and delay spreads in vehicular channels, large packets may face higher error rates due to time-varying channels.

Advances in low-power and low-cost sensor networks have led to solutions mature enough for use in a broad range of applications, requiring various degrees of reliability. To facilitate this, a broad range of options are possible to tune reliability, throughput or energy cost in the IEEE 802.15.4 standard defining the medium access control (MAC) and physical layer for sensor networks. Knowing how to tune those knobs however requires detailed models of the protocol behavior under different conditions. In our earlier work, we have proposed a very accurate model for the slotted Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) access scheme of the IEEE 802.15.4 standard for the unacknowledged transmission mode. Because of the design of the 802.15.4 carrier sensing mechanism, modeling the performance of the network in case of acknowledged transmissions is not a trivial extension. In this paper, we hence derive such model and illustrate through simulations that it is extremely accurate. Next, using the model, guidelines are derived to optimize the energy or throughput performance of sensor networks using the IEEE 802.15.4 standard.

Advances in low-power and low-cost sensor networks have led to solutions mature enough for use in a broad range of applications varying from health monitoring to building surveillance. The development of those applications has been stimulated by the finalization of the IEEE 802.15.4 standard, which defines the medium access control (MAC) and physical layer for sensor networks. One of the MAC schemes proposed is slotted carrier sense multiple access with collision avoidance (CSMA/CA), and this paper analyzes whether this scheme meets the design constraints of those low-power and low-cost sensor networks. The paper provides a detailed analytical evaluation of its performance in a star topology network, for uplink and acknowledged uplink traffic. Both saturated and unsaturated periodic traffic scenarios are considered. The form of the analysis is similar to that of Bianchi for IEEE 802.11 DCF only in the use of a per user Markov model to capture the state of each user at each moment in time. The key assumptions to enable this important simplification and the coupling of the per user Markov models are however different, as a result of the very different designs of the 802.15.4 and 802.11 carrier sensing mechanisms. The performance predicted by the analytical model is very close to that obtained by simulation. Throughput and energy consumption analysis is then performed by using the model for a range of scenarios. Some design guidelines are derived to set the 802.15.4 parameters as function of the network requirements.

An adaptive beamforming algorithm that enhances the quality of the signal recorded by a microphone array is presented. The system described here attenuates the interference from spatially scattered sources while preserving the desired signal quality in the speech frequency range. A novel method for defining additional constraints within the adaptation process and performing beamforming in parallel across several frequency bands achieves undistorted recording even with slight errors in localizing the speaker. Simulation results show nearly optimal improvement of speech quality in various noisy environments. The algorithm is modified to perform in a real environment by introducing two modes of adaptation that switch according to the speaker activity. The implementation of this modified system demonstrates SNR gain close to that of the original simulated system. It is robust to localization errors and non-ideal environmental effects.

An unequal importance communication approach, for cheap, reliable and real-time image communication over heterogeneous networks and its applications in mobile multimedia communication systems is presented. Here, an image is partitioned into different layers of importance: base layer and enhancement layers. The low-rate base layer description, which can be strongly protected, is transmitted through a low-bandwidth, but robust and more expensive channel, supporting delay-sensitive applications (e.g. GSM network), while the high-rate enhancement layer description(s) are transmitted through a high-bandwidth, but high/variable-delay and cheap channel which does not support delay-sensitive applications (e.g. WLAN). In the receiver, when the base layer description is quickly received, an acceptable quality image is reconstructed, and when the enhancement layer description(s) are received a high quality image can be reconstructed. Our practical results indicated that this effective, flexible, and highly compatible method not only facilitates high quality image distribution among heterogeneous networks and increases the coverage, quality of service, and bandwidth and power efficiency but also decreases the delay and cost of image distribution in mobile communication systems.

Almost a decade long research on the performance of TCP in wireless networks has resulted in many proposals and solutions to the problem of TCP throughput degradation. Several of these measures, however, have their share of drawbacks. With the continuing emergence of wireless technologies ever since the work on TCP performance over wireless began, smart link-layer mechanisms like adaptive modulation and coding, power control, and incremental redundancy have been designed and deployed. In this work, we outline a cross-layer optimization framework based on the congestion control dynamics of a bulk-transfer TCP flow and demonstrate its application to networks which offer link-layer adaptive measures. We begin by observing that the TCP's congestion window dynamics are comprised of certain recurring patterns which we term as cycles. We then overlay a TCP throughput optimization methodology that selects link-layer transmission modes (e.g. modulation scheme, coding rate, transmission power, or a combination thereof) in accordance with TCP dynamics and wireless channel conditions. We provide insights into the working of the optimization procedure which protects TCP segments against losses on the wireless channel when the TCP congestion window size (in bytes) is below the bandwidth-delay product of the network. The protection against wireless channel losses is rendered by the link-layer by employing robust modulation and coding schemes, high transmission power, etc. We show that TCP dynamics aware link adaptation measures lead to substantial enhancement of TCP throughput in EGPRS and IEEE 802.11a networks

The IEEE 802.15.4 standard defines the medium access control (MAC) and physical layer for sensor networks. One of the MAC schemes proposed is slotted carrier sense multiple access with collision avoidance (CSMA/CA), and this paper analyzes whether this scheme meets the design constraints of low-power and low-cost sensor networks. The paper provides a detailed analytical evaluation of its performance in a star topology network for both saturated and unsaturated periodic traffic.The form of the analysis is similar to that of Bianchi for IEEE 802.11 DCF only in the use of a per user Markov model to capture the state of each user at each moment in time. The key assumptions to enable this important simplification and the coupling of the per user Markov models are however different, as a result of the very different designs of the 802.15.4 and 802.11 carrier sensing mechanisms. The performance predicted by the analytical model is very close to that obtained by simulation. Throughput and energy consumption analysis is then performed and design guidelines are derived.

Transmit maximum-ratio combining (transmit MRC) is a popular antenna diversity technique that provides both spatial diversity and array gain in downlink multiple-input single-output (MISO) links. These gains, however, critically depend on the availability of the downlink channel state information (CSI). In time-division duplexing systems, channel reciprocity has been commonly put forth to justify the convenient use of the CSI already acquired from the uplink, in the calculation of the transmit-MRC weights. Recent work has questioned this practice, based on the non-reciprocity of multi-antenna RF transceivers, due to significant amplitude and phase mismatches across the antennas. Furthermore, expensive digital calibration solutions have been proposed to enforce the reciprocity of the multi-antenna RF transceivers. Both the impact of multi-antenna amplitude and phase mismatches and the performance of the proposed calibration approaches have only been assessed via simulations. In this contribution, we propose an alternative statistical analysis of the impact of these mismatches on transmit MRC. This analysis allows a faster and more reliable characterization as well as provides insight into the relative importance of these mismatches. Consequently, sufficient matching requirements can be extracted for the multi-antenna RF transceivers, for which simpler and cheaper calibration solutions can be devised.

Delay optimization is an important issue for ad hoc wireless networks supporting multimedia applications. Current medium access schemes do not take the application packet size into account for their operation. Long packets win the channel contention equally likely as the smaller packets; however, they occupy the channel for a longer time. This unfairness leads to increased delay and jitter for smaller packets. This paper attempts to resolve the above issue by proposing a fully distributed algorithm that adapts the contention process to the packet size as well. More specifically, we emulate the shortest job first scheduling policy by proposing a new resetting backoff regime. The proposed scheme is then compared to the distributed coordination function of the IEEE 802.11 standard. Analytical and simulation results show that the delay of the small packets is significantly improved.

Wireless systems where the nodes operate on batteries so that energy consumption must be minimized while satisfying given throughput and delay requirements are considered. In this context, the best modulation strategy to minimize the total energy consumption required to send a given number of bits is analyzed. The total energy consumption includes both the transmission energy and the circuit energy consumption. For uncoded systems, by optimizing the transmission time and the modulation parameters, it is shown that up to 80% energy savings is achievable over nonoptimized systems. For coded systems, it is shown that the benefit of coding varies with the transmission distance and the underlying modulation schemes.

We consider radio applications in sensor networks where energy is a limited resource so that energy consumption must be minimized while satisfying given delay and throughput requirements. In this context, we analyze energy-efficient data collection strategies where we minimize the total energy consumption necessary for collecting a certain amount of data from multiple sensors. The total energy consumption includes both the transmission energy and the circuit energy consumption. We propose a variable-length TDMA scheme where the slot length is adaptively assigned according to the number of bits in the transmitting queues and the distance between the transmitting nodes and the collecting node. The underlying goal is to finish the collection of information bits from the multiple sensor nodes before a deadline T with minimum energy cost. We show that the problem can be efficiently solved by convex relaxation methods, and in some special cases simple analytical solutions can be derived.

We consider radio applications in sensor networks, where the nodes operate on batteries so that energy consumption must be minimized, while satisfying given throughput and delay requirements. In this context, we analyze the best modulation and transmission strategy to minimize the total energy consumption required to send a given number of bits. The total energy consumption includes both the transmission energy and the circuit energy consumption. We first consider multi-input-multi-output (MIMO) systems based on Alamouti diversity schemes, which have good spectral efficiency but also more circuitry that consumes energy. We then extend our energy-efficiency analysis of MIMO systems to individual single-antenna nodes that cooperate to form multiple-antenna transmitters or receivers. By transmitting and/or receiving information jointly, we show that tremendous energy saving is possible for transmission distances larger than a given threshold, even when we take into account the local energy cost necessary for joint information transmission and reception. We also show that over some distance ranges, cooperative MIMO transmission and reception can simultaneously achieve both energy savings and delay reduction.

We consider radio applications where the nodes operate on batteries so that energy consumption must be minimized while satisfying given throughput and delay requirements. In this context, we analyze the best modulation strategy to minimize the total energy consumption required to send a given number of bits when error-control codes are used. The total energy consumption includes both the transmission energy and the circuit energy consumption. We show that for both MQAM and MFSK the total energy consumption may be reduced significantly if the transmission time T/sub on/ is optimized to reduce the sum of transmission energy and circuit energy consumption. Our optimization considers both delay and peak-power constraints. Numerical examples are given, where we exhibit up to 90% energy savings over modulation strategies that minimize the transmission energy alone. We also show that the benefit of coding varies with the transmission distance and the underlying modulation schemes.

This paper presents a low-complexity iterative receiver for coded OFDM systems. We present an EM-based iterative algorithm for combined channel estimation and decoding that makes collective use of the available data and system constraints. Minimum numbers of pilots are sent only in the first symbol of the packet to acquire the channel; then the iterative algorithm is used to track the channel time variation, which is assumed to follow a state-space model, using an EM-based Kalman filter. Data recovery can be achieved within a single OFEM symbol. We also propose the use of an optional outer LDPC code in serial concatenation to offer a trade-off between latency and performance, especially for multi-amplitude modulations, without affecting the complexity of the core iterative algorithm.

We consider radio applications where the nodes operate on batteries so that energy consumption must be minimized while satisfying given throughput and delay requirements. In this context, we analyze the best modulation strategy to minimize the total energy consumption required to send a given number of bits. The total energy consumption includes both the transmission energy and the circuit energy consumption. We show that for both MQAM and MFSK the transmission energy decreases with the BT/sub on/ product while the circuit energy consumption increases with T/sub on/, where B is the modulation bandwidth and T/sub on/ is the transmission time. Thus, in short-range applications where the circuit energy consumption is nonnegligible compared with the transmission energy, the total energy consumption is minimized by using the maximum system bandwidth along with an optimized transmission time T/sub on/. We derive this optimal T/sub on/ for MQAM and MFSK modulation in both AWGN channels and Rayleigh fading channels. Our optimization considers both delay and peak-power constraints. Numerical examples are given, where we exhibit up to 68% energy savings over modulation strategies that minimize the transmission energy alone.

Orthogonal frequency division multiplexing (OFDM) is robust against inter-symbol interference (ISI) due to the increase of the symbol duration. However, for mobile applications, channel variations during one OFDM symbol introduce inter-carrier-interference (ICI), which degrades the performance. This gets more severe as mobile speed, carrier frequency or OFDM symbol duration increases. We introduce two new methods to mitigate ICI in an OFDM system with coherent channel estimation. Both methods use a piece-wise linear model to approximate channel variations. The first method extracts channel variations information from the cyclic prefix. The second method estimates these variations utilizing the next symbol. Since mathematical analysis of the whole system becomes intractable in a long delay spread environment, we provide a mathematical analysis that investigates the effect of linearization for the case of a time-variant one path channel. Then our simulation results show how these methods would improve the performance in a highly time-variant environment with a long delay spread.

Uplink and downlink load equations for a snapshot of users in a UMTS network are presented in unified and general matrix form. Capacity and coverage constraints of the system are presented in the form of inequalities to he satisfied by users and network loadings. It is shown that the static simulation of the network is the joint solution of the load equations and capacity and coverage constraints carried out with an iterative method.

OFDM modulation combines the advantages of high achievable rates and relatively easy implementation. However, for proper recovery of the input, the receiver needs accurate channel information. In this paper, we propose an expectation-maximization (EM) algorithm for joint channel and data recovery. The algorithm makes use of the rich structure of the underlying communication problem - a structure induced by the data and channel constraints. These constraints include pilots, the cyclic prefix (CP), and the finite alphabet constraints on the data, and sparsity, finite delay spread, and the statistical properties of the channel (time and frequency correlation). Channel identification and equalization is performed optimally and recovery is achieved within the same OFDM symbol using an EM based Kalman filter.

OFDM systems typically use coding and interleaving across subchannels to exploit frequency diversity on frequency-selective channels. This paper presents a low-complexity iterative algorithm for combined blind and semi-blind channel estimation and soft decoding in coded OFDM systems. Channel estimation is performed in the time domain using the expectation maximization (EM) algorithm to take advantage of the channel-length constraint and the extra observation offered by the cyclic-prefix. The proposed technique converges within a single OFDM symbol and, therefore, has a minimum latency and is suitable for fast time-varying channels.

This work proposes an iterative least-/ mean-squares approach to channel identification and equalization in OFDM. This is achieved by exploiting the natural constraints imposed by the channel (sparsity and maximum delay spread) and those imposed by the transmitter (pilots, cyclic prefix, and the finite alphabet constraint). These constraints are used to reduce the number of pilots needed for channel and data recovery and also to perform this task within one packet. The diagonal nature of the OFDM channel makes it possible to perform optimal (nonlinear) mean-square detection of the data.

The paper compares performance of finite impulse response (FIR) adaptive linear equalizers based on the recursive least-squares (RLS) and least mean square (LMS) algorithms in nonstationary uncorrelated scattering wireless channels. Simulation results, in terms of steady-state mean-square estimation error (MSE) and average bit-error rate (BER) metrics, are found for the frequency-selective Rayleigh fading wireless channel experienced in a mobile ad hoc network where nodes are lognormally shadowed from each other. For the nonstationary channel models considered, RLS is always found to outperform LMS.

We motivate the need for a complexity based design for performing joint iterative equalization and decoding. This joint iterative process, which requires the exchange of soft information, incurs a huge complexity increase over hard-decision based algorithms. We introduce complexity as a design parameter and provide two different methodologies. The first approach is a combination of SOVA (soft output Viterbi algorithm) and DFSE (decision feedback sequence estimation), and is called soft-output DFSE (SO-DFSE). The second approach, called soft-decision DFSE (SD-DFSE) generalizes the notion of reliability to soft-decisions through the use of appropriately chosen functions. By varying the design parameters in both approaches, the module can range from being as simple as a soft output DFE to being as complex as a SOVA or APP (a posteriori probability). We conclude by presenting performance curves of iterative algorithms that utilize these modules.

Multicarrier signals are known to suffer from a high peak-to-average power ratio, caused by the addition of a large number of independently modulated subcarriers in parallel at the transmitter. When subjected to a peak-limiting channel, such as a nonlinear power amplifier, these signals may undergo significant spectral distortion, leading to both in-band and out-of-band interference, and an associated degradation in system performance. This paper characterizes the distortion caused by the clipping of multicarrier signals in a peak-limiting (nonlinear) channel. Rather than modeling the effects of distortion as additive noise, as is widespread in the literature, we identify clipping as a rare event and focus on evaluating system performance based on the conditional probability of bit error given the occurrence of such an event. Our analysis is based on the asymptotic properties of the large excursions of a stationary Gaussian process, and offers important insights into both the true nature of clipping distortion, as well as the consequent design of schemes to alleviate this problem.

Previous proposals for next generation wireless communication technology have prompted numerous research efforts comparing different technologies for next generation systems. Variations of wide-band CDMA and TD-CDMA have been the strongest candidates for the physical layer. Third generation technology promises data rates of up to 2 Mbps in small coverage areas for low mobility users, whereas, for some applications higher data rate in larger area is very desirable. That brings about another challenge as to flexibility of third generation proposals to accommodate new services such as higher speed packet data, particularly in downlink, for wireless Internet access. We discuss technical challenges and requirements of these proposals and briefly study complexity of network and terminals for multi-mode, multi-band systems. Moreover, we propose the concept of integrating asymmetric high-speed wireless packet services with current proposed systems for third generation wireless services.

Optimum detection in randomly time-varying channels requires an efficient adaptive receiver structure. The complexity of signal processing in the receiver is limited by the amount of processing power and power consumption of the receiver. Therefore, efficiency and convergence of adaptive matched filtering and equalization techniques are very important. We extend common results in tracking theory for system identification to the equalization case for systems with short impulse response. Unlike in system identification where a steady-state error energy is minimized, the optimization criterion here is the minimum of the BER. However, they are related by a monotone function and therefore minimizing the BER is equivalent to minimizing the steady-state-error energy. Optimum parameters for LMS as well as RLS algorithms are derived and simulation results indicate that under the conditions defined in the TDMA standards and small delay spread, the performance of the two methods is comparable.

In this paper a new parameter estimation technique based on channel impulse response estimation is introduced which is desirable in randomly time-varying systems such as burst transmission mobile systems since it is robust in environments with considerable delay spread, thus ideal for wireless channels, and has a fast convergence, thus desirable in burst transmission systems. The shortcomings of some of the existing algorithms are also discussed here.