Dithering is a technique commonly used to improve the perceptual quality of lossy data compression. In this work, motivated by low-complexity wearable applications, we analytically and experimentally justify the use of dithering for Automatic Speech Recognition (ASR) input compression, evaluated on OpenAI's Whisper Model. We hypothesize a relationship between the mean squared error, the autocorrelation vector of the quantization error evaluated at a specific lag$(\tau=5)$, and ASR performance under lossy compression. Using this hypothesis, we propose a parametric dithering technique for a low-complexity speech compression pipeline. Specifically, we define two dithers indexed by$m$as:$f_{V_{m, \alpha}}(v)=\alpha \Lambda_{2 \Delta((\alpha-1) m+2-\alpha)}(v)+(1-\alpha) \delta(v), \quad m \in \{1,2\}$where$\Lambda_{2 a}(v) \triangleq \frac{1}{a^{2}}(a-\vert v\vert)$for$\vert v\vert \leq a$, with$\Delta$representing the quantization interval size and$\alpha$as the control parameter.

This paper investigates covert multi-hop communication in wireless networks where an adversary employs a cyclostationary (cycle) detector to reveal hidden transmissions. The covert route employs direct sequence spread spectrum (DSSS) signaling to ensure either maximum end-to-end covertness maximization or minimum latency minimization-under quality-of-service (QoS) and link budget constraints. Optimal bandwidth, transmit power, and spreading gain for each hop jointly satisfy reliability and either rate or covertness requirements. We show the equivalence between the covertness and the detection SNR gain-based widest-path formulations, and, hence, enabling efficient route computation. Numerical simulations in a realistic 3D environment illustrate that (i) end-to-end latency increases exponentially with the covertness requirement, (ii) the end-to-end latency increase is super-linear with the packet size M, and (iii) cycle and energy detectors impose different latency behavior as a function of the message length and the covertness requirement. The proposed framework provides important insights into resource allocation and routing design for covert networks against advanced detection adversaries.

We consider a three party wireless network- Alice is a legitimate transmitter, Bob is a legitimate receiver, and Willie is a malicious adversary. Alice's goal is to communicate with Bob with a certain minimum Quality of Service (QoS) while evading detection from Willie. Alice transmits a Direct Sequence Spread Spectrum (DSSS) signal where the processing gain and transmit power are selected with the aim of evading detection by spreading the bit energy over a larger bandwidth while maintaining required QoS. Concurrently, Willie tries to detect the legitimate transmission by using either an energy or a cycle detector. We study this scenario within an adversarial optimization framework that aims at achieving a robust max-min covertness under link performance constraints. The framework is common to both detectors and hence allows for a comparative performance analysis that identifies common key performance parameters. The adversarial signal-to-noise ratio (SNR) detection gain enables direct trading of the DSSS processing gain for the (squared) channel quality ratio when aiming to improve link covertness, regardless of which detector Willie uses. The DSSS processing gain and SNR gain (and, hence, covertness) are limited by the bit rate and bit error rate link requirements, respectively. While both detectors' performance is limited by the SNR at Willie, the cycle detectors benefit significantly more from longer observation time of the legitimate transmission.

Dithering is a technique that can improve human perception of low-resolution data by reducing quantization artifacts. We hypothesize that the perceptual prominence of quantization artifacts is proportional to the magnitude of the quantization error autocorrelation vector. Under this hypothesis we derive two parametric dither distributions that trade-off between minimizing mean square error and minimizing an upper bound on the quantization error autocorrelation vector magnitude in the ℓ 1 sense $\left( {{f_{{V_{1,\alpha }}}}(v) = \alpha {\Pi _{\alpha \Delta }}(v) + (1 - \alpha )\frac{1}{2}\left[ {\delta \left( {v - \frac{{\alpha \Delta }}{2}} \right) + \delta \left( {v + \frac{{\alpha \Delta }}{2}} \right)} \right]} \right)$ or ℓ 2 sense $\left( {{f_{{V_{2,\alpha }}}}(v) = {\Pi _{\alpha \Delta }}(v)} \right)$ where ${\Pi _a}(v) \triangleq \frac{1}{a}, - \frac{a}{2} \leq v \leq \frac{a}{2}$ and ∆ is the width of the quantization region. The application of these distortion-controlling dithers to an example low-rate image recompression problem (using Lena) reveals optimal performance with partial dithering (0 < α ∝ λ < 1) as per Fig. 1 while our novel ℓ 1 -optimized dither produces a new Pareto front for the quality-entropy trade-off shown in Fig. 2 .

In this paper, we develop an approach to exploit multiple disparate wireless communication technologies simultaneously to enhance covertness of a communication link. Specifically, given two available communication modalities between a pair of friendly nodes (Alice and Bob), the goal is to evade detection by an adversary (Willie) who is equipped with a radiometer covering the frequency bands of both modalities. We propose a joint detection threshold optimization technique from Willie's point of view. We also develop a joint transmit power optimization strategy for Alice to maximize covertness while meeting the throughput requirement at Bob. Through numerical simulations we show that the proposed scheme matches the performance of exhaustive search method while reducing the computational time by 98% and also improves the covertness by 56% compared to a naïve benchmark scheme.

Low-resolution analog-to-digital converters (ADCs) are prevalent in both low-cost applications where memory is highly constrained and in wideband spectrum analyzers and receivers with highly constrained latency. In both applications it is desirable to implement a fast, memory-efficient frequency estimator that operates on the output digital samples, whether for post-correction indexing or estimation of the location of spectral power in the ADC bandwidth. However, such ADCs are susceptible to non-linearities due to manufacturing tolerances and high-speed operation. We present a method for fast frequency estimation via low-resolution quantization and table look-up with table entry estimates analytically optimized for ADC-specific nonlinearity patterns, as well as a zero crossing-based indexing method with superior memory efficiency for slightly higher latency and implementation complexity. The accuracy of these estimators is compared for various resolutions, window lengths, and evaluation SNRs, as well as in the presence of ADC nonlinearity. Our results indicate that the zero-crossing estimator far outperforms the conventional look-up table estimator in every SNR range for both memory and accuracy.

In this paper, we explore covert communication in a heterogeneous network where a transmitter sends a confidential message to a receiver by utilizing two different radio frequency (RF) bands, low-very high frequency (low-VHF) and microwave frequency modalities. Since two RF modalities exhibit different characteristics in terms of channels and available bandwidths, it is important to efficiently leverage the modalities based on the wireless environment. Therefore, we develop a new algorithm that optimizes the transmit powers and bandwidths for the modalities and selects one modality with the goal of maximizing the detection error probability at a warden while guaranteeing a quality-of-service requirement of the receiver. We first derive a closed-form joint optimal power and bandwidth solution for a given modality, and then provide a modality selection method. From numerical simulations, it is validated that the proposed scheme achieves the optimal performance and the covertness can be enhanced by judiciously choosing one of the two modalities based on the channel condition.

This paper presents the link-level performance of sparse code multiple access (SCMA) for PC5-based cellular-vehicle-to-everything (C-V2X with different channel estimation errors (CEEs) for every user, called a heterogeneous CEE, due to user mobility. Such a CEE may cause decoding error propagation for users experiencing no CEEs through the message passing algorithm used in SCMA due to the non-orthogonality. First, we analyze the error propagation in a basic heterogeneous CEE model, in which a specific user provides a Gaussian CEE. Our analysis shows that the error propagation depends on the edge connectivity with the specific user on the Factor graph; its directly connected users’ signals show worse decoding performance than its indirectly connected users’ signals. Next, we evaluate the link-level performance of each user in the heterogeneous CEE model to confirm the impacts of the error propagation. Our evaluation results demonstrated that the error propagation degraded 8.1 times worse bit error rate performance of the indirectly connected user at a CEE variance of 0.1 than no CEE for all users. Through our simulation results, this paper highlights that error propagation is a potential challenge for SCMA in PC5-based V2X.

Easily and flexibly deployable ad-hoc communication networks emerging in tactical military or even civilian contexts, frequently suffer from poor synchronization due to lack of coordinating infrastructure. In addition to synchronization issues, and especially in military settings, security from the aspect of detectability is also of crucial importance. Imperfect synchronization can be dealt with by making use of Quasi-Synchronous Code Division Multiple Access (QS-CDMA), relying on Loosely Synchronous Codes to maintain orthogonality in the presence of limited time delays. Security, in terms of low probability of detection (LPD) from the standpoint of a malicious adversary, can be improved (reduced detection) by employing randomization techniques that disrupt the inherent structure of the transmitted QS-CDMA signals. This is based on the fact that QS-CDMA signals are Cyclostationary, having (almost) periodic Auto-Correlation functions (ACF) due to eminent signal periodicities (such as spreading code repetition). In this paper, we propose techniques to disturb the ACF and equivalently the Spectral Correlation function, and reduce the Degree of Cyclostationarity (DCS), our LPD measure. Specifically, we investigate randomization via 1) random per symbol time dithering and 2) random selection of spreading sequences, as well as a hybrid approach combining time dithering and code randomization. In all proposed techniques knowledge of the randomization pattern is not required at the legitimate receiver. We derive the Spectral Correlation function of the QS-CDMA signal under the proposed randomization schemes and compare it to simulations. We show through analysis and extensive numerical simulations that the proposed technique can reduce the DCS by almost two orders of magnitude. We also show that enhanced LPD can be achieved using the proposed techniques while sacrificing a part of the reduced time synchronization requirement. We further analyze the implications of the friendly receiver not knowing the randomization pattern and present results on the resulting communication performance.

Ad-hoc flexibly deployable networks destined for tactical military or civilian applications often suffer from poor synchronization which may lead to unreliable communication. Quasi-Synchronous (QS) CDMA employing Loosely Synchronous (LS) codes allows weak synchronization due to the Zero-Correlation Zone of the code's correlation functions. Apart from reliability, an essential characteristic of military networks is having low probability of detection (LPD) in the presence of adversaries employing sophisticated detection techniques. Such techniques are based on the fact that manmade signals are cyclostationary, meaning that they have some periodic structure (e.g. spreading sequence repetition) that can be exploited for improved detection. Disturbing those recurring patterns can reduce the probability of detection measured in terms of the Degree of Cyclostationarity (DCS). We aim to achieve that by employing some techniques that will perturb the signal structure by randomly selecting spreading sequences, random time dithering or a combination of the two. We study the trade-off between the communication performance and DCS reduction of such a system under these different techniques and compare the results.

Supporting networks with a large number of nodes in infrastructure-poor and complex propagation environments is an important challenge for military and civilian applications. A major problem when using classical approaches, such as code division multiple access (CDMA), is maintaining inter-link coordination while mitigating multi-user interference (MUI). By contrast, loosely synchronous (LS) codes have perfect code orthogonality within a window of inter-link delays at a cost of the number of available spreading codes. Since sporadic communications naturally involves a low probability of transmission, we investigate the potential for LS code reuse to effectively support more users. We study this problem by simulating inter-user channels using a high-fidelity physics-based model. We focus our study of the channel on the low-VHF band, which has improved penetration and channel coherence in complex environments. We perform initial characterization of different levels of code reuse, synchronization and coordination. Of particular interest is a purely random (uncoordinated) spreading code assignment. The results illustrate good performance of a scalable medium access scheme.

A Boolean generator for a large number of standard complementary QAM sequences of length 2Kis proposed. This Boolean generator is derived from the authors' earlier paraunitary generator, which is based on matrix multiplications. Both generators are based on unitary matrices. In contrast to previous Boolean QAM algorithms which represent complementary sequences as a weighted sum, our algorithm has a multiplicative form. Any element of a sequence can be generated efficiently by indexing the entries of unitary matrices with the binary representation of the discrete time index (which is easily implemented as a binary counter). Our 1Qum (based on one QAM unitary matrix) and 2Qum (based on two QAM unitary matrices) algorithms generate generalized Case I-III sequences and generalized Case IV and V sequences, respectively, as specified by Liu et al. in 2013, in addition to many new 2Qum sequences. The ratio of the numbers of sequences that are generated by our new construction and the previous construction increases with the constellation size. For example, for a 1024-QAM sequence of length 1024, this ratio is 4.4. However, if we compare only 2Qum sequences to Case IV and V sequences, this ratio is 267.

Exhaustive search beamforming (BF) protocol of IEEE 802.11ad and IEEE 802.15.3c millimeter wave (mm-wave) communication standards consume time and power. A time-efficient iteration-based Binary Search Beamforming (BSB) protocol was introduced as a replacement for the BF clauses in the standards. In this paper, we propose an improved method, Enhanced BSB (EBSB), to support NLOS environments in which BSB doesn't work efficiently. We also define a new "single-side approach" to remove the local maxima from which earlier solution methods suffer. Our proposed method aims to select the best beam pair in fewer steps using an iteration-based algorithm which reduces the BF setup time. We provide performance comparison of exhaustive search, BSB and EBSB in terms of step analysis, success probability, BF setup time and power loss performances. In NLOS case, for smaller than 100 sectors at both devices' antennas, EBSB reaches higher than 0.9 probability of successfully pairing the best beams. On the other hand, training time reduces to around %34 for 330 sectors at both antennas. For the cases when EBSB fails, power loss analysis shows that the average gain loss is less than 1 dB.

Symmetric block partitioned tensors (SBPT) are a useful structure in signal processing applications, often generated from computing higher-order statistics on observed data. Such tensors often follow the rank (Rm, Rm, 1) SBPT structure, but in some applications the partitioning of the factor matrices is not known a priori. We propose a both blind and non-blind column-wise SBPT decomposition algorithms that are better scalable to high-dimensional tensors because they avoid large matrix inversions. We apply the algorithms to simulated SBPTs and demonstrate that they estimate factor matrices having high congruence with the originals across a range of collinearity values for the columns of the original factor matrices.

If Alice must communicate with Bob over a channel shared with the adversarial Eve, then Bob must be able to validate the authenticity of the message. In particular we consider the model where Alice and Eve share a discrete memoryless multiple access channel with Bob, thus allowing simultaneous transmissions from Alice and Eve. By traditional random coding arguments, we demonstrate an inner bound on the rate at which Alice may transmit, while still granting Bob the ability to authenticate. Furthermore this is accomplished in spite of Alice and Bob lacking a pre-shared key, as well as allowing Eve prior knowledge of both the codebook Alice and Bob share and the messages Alice transmits.

We propose without loss of generality strategies to achieve a high-throughput FPGA-based architecture for a binary Quasi-Cyclic Low-Density Parity-Check (QC-LDPC) code based on a circulant-1 identity matrix construction. We present a novel representation of the parity-check matrix (PCM) providing a multi-fold throughput gain. Splitting of the node processing algorithm enables us to achieve pipelining of blocks and hence layers. By partitioning the PCM into not only layers but superlayers we derive an upper bound on the two-layer pipelining depth for the compact representation. To validate the architecture, a decoder for the IEEE 802.11n (2012) QC-LDPC is implemented on the Xilinx Kintex-7 FPGA with the help of the FPGA IP compiler available in the NI LabVIEW Communication System Design Suite (CSDS). It offers an automated and systematic compilation flow where an optimized hardware implementation from the LDPC algorithm was generated, achieving an overall throughput of 608Mb/s (at 260MHz). As per our knowledge this is the fastest implementation of the IEEE 802.11n QC-LDPC decoder using an algorithmic compiler.

As we are surrounded by an ever-larger variety of post-PC devices, the traditional methods for identifying and authenticating users have become cumbersome and time consuming. In this paper, we present a capacitive communication method through which a device can recognize who is interacting with it. This method exploits the capacitive touchscreens, which are now used in laptops, phones, and tablets, as a signal receiver. The signal that identifies the user can be generated by a small transmitter embedded into a ring, watch, or other artifact carried on the human body. We explore two example system designs with a low-power continuous transmitter that communicates through the skin and a signet ring that needs to be touched to the screen. Experiments with our prototype transmitter and tablet receiver show that capacitive communication through a touchscreen is possible, even without hardware or firmware modifications on a receiver. This latter approach imposes severe limits on the data rate, but the rate is sufficient for differentiating users in multiplayer tablet games or parental control applications. Controlled experiments with a signal generator also indicate that future designs may be able to achieve data rates that are useful for providing less obtrusive authentication with similar assurance as PIN codes or swipe patterns commonly used on smartphones today.

We propose an efficient application layer coding scheme suitable for time-limited wireless broadcast framework of the MBMS standards. The scheme, referred to as doped broadcast, is based on Fountain codes, and uses feedback to implement and control the tradeoff between the reconstruction delay, broadcast overhead, and decoding time/complexity. As the standardized schemes, our doped broadcast operates in two phases consisting of the limited time broadcast followed by an individualized repair phase in order to ensure (possibly prioritized) quality of service (QoS) to most users. The goal of the scheme is not to improve on any particular performance metric where highly optimized standard recommendations already perform exceptionally well, but rather to enable flexible and transparent mechanisms to implement and control tradeoff between different performance metrics. Toward this goal, we develop an analytically tractable model for doped broadcast with Ideal Soliton based codes leading to a repair strategy parameter estimation. The impact that inactivation and doping mechanisms employed by the decoder have on the complexity and overhead metrics is quantified and discussed for the proposed model. Hence, our approach guides a practical design tradeoff, which is important in today's highly heterogeneous environments requiring individualized QoS.

As we are surrounded by an ever-larger variety of post-PC devices, the traditional methods for identifying and authenticating users have become cumbersome and time-consuming. In this paper, we present a capacitive communication method through which a device can recognize who is interacting with it. This method exploits the capacitive touchscreens, which are now used in laptops, phones, and tablets, as a signal receiver. The signal that identifies the user can be generated by a small transmitter embedded into a ring, watch, or other artifact carried on the human body. We explore two example system designs with a low-power continuous transmitter that communicates through the skin and a signet ring that needs to be touched to the screen. Experiments with our prototype transmitter and tablet receiver show that capacitive communication through a touchscreen is possible, even without hardware or firmware modifications on a receiver. This latter approach imposes severe limits on the data rate, but the rate is sufficient for differentiating users in multiplayer tablet games or parental control applications. Controlled experiments with a signal generator also indicate that future designs may be able to achieve datarates that are useful for providing less obtrusive authentication with similar assurance as PIN codes or swipe patterns commonly used on smartphones today.

Today's identification and authentication mechanisms for touchscreen-enabled devices are cumbersome and do not support brief usage and device sharing. To address this challenge, this work explores a novel form of "wireless" communication that exploits the capacitive touchscreens which are now used in laptops, phones, and tablets, as a signal receiver. Using a custom built hardware token, in the form of a wearable ring, we show a proof-of-concept system that transmits a user identification code to the mobile device through the touchscreen. This mechanism works without any modification to the hardware or the firmware of the mobile device.

This paper studies source location anonymity in a large monitoring wireless sensor network with a single data collector, and under a global attack. The qualifier ”global” indicates the capability of the eavesdropper (attacker) to capture all network transmissions, and to discern their time and location. We propose a scheme for generating fake network traffic to disguise the real event notification. This scheme is particularly effective for the protection of the monitored asset in delay-intolerant applications monitoring rare and spatially sparse events. Unlike earlier work on this topic, we jointly consider the protection strength, events' dynamics, probability of the attacker's exposure during the attack, notification latency, network overhead, and scalability. The efficiency of the scheme that provides statistical source anonymity is achieved by partitioning network nodes randomly into several node groups. Members of the same group collectively emulate both temporal and spatial distribution of the event. Under such framework, we aim to better model the global eavesdropper, especially her way of using statistical tests to detect the real event. In addition, our approach aims to reduce the per-event work spent to generate the fake traffic while, most importantly, providing a guaranteed latency in reporting the event. The latency is controlled by decoupling the routing from the fake-traffic schedule. A good dummy-source group design also provides a robust protection of event bursts. This is achieved at the expense of the significant overhead as the number of dummy-source groups must be increased to the reciprocal value of the false alarm parameter used in the statistical test. Ultimately, our message is that designing a protection scheme to meet multiple requirements, imposed by realistic application scenarios, involves trade-offs among several performance measures, and calls for an evaluation framework that recognizes these challenges. We believe that the proposed source anonymity protection strategy, and the evaluation framework, are well justified by the abundance of the applications that monitor a rare event with uniform spatial distribution.

Vehicular networking has significant potential to enable diverse range of applications, including safety and convenience. As the number of vehicles and applications using wireless spectrum grow, one can expect to see a shortage of either spatially or temporally available spectrum. In this paper, we advocate that dynamic spectrum access for vehicles be the first step towards solving the spectrum shortage. For this, vehicles must be able to sense the availability of spectrum before attempting to transmit. The existence of other transmitters should be detected in order not to cause or experience interference. However, spectrum sensing in vehicular environments is a challenging task due to mobility, shadowing and other factors that govern vehicular environments. Therefore, spectrum sensing by a single vehicle may not be able to provide accurate information about the spectrum vacancies. Cooperative spectrum sensing, on the other hand, uses spatial diversity and can be employed to overcome the limitations associated with a single sensor/vehicle. In this paper, we investigate cooperative spectrum sensing performance in a vehicular environment for sensing signals transmitted from i) a roadside infrastructure and ii) radios located on other vehicles, by using energy-based detection of a transmitted pilot tone as an example. Our goal is to characterize the limits on detection speed and reliability of simple hard and soft cooperative energy-based schemes for this environment. We show how cooperation reduces sensing time by a factor of five in an AWGN channel. The cooperative sensing time reduction is far more significant in a vehicular environment with fading and shadowing. Finally, we illustrate how infrastructure-to-vehicle scenario favors soft equal gain combining while vehicle-to-vehicle scenario favors hard fusion OR rule.

This paper describes the signal-to-noise ratio (SNR) estimation based on the preamble in tri-sectored OFDM systems. Several SNR estimators from previous literature suffer degradation in several important fading channel models. This paper proposes an estimation method that takes advantage of the fact that the preamble sequence symbols are assigned to every third subcarrier for each BS sector. Typically the closer the MS is to the BS, the more corrupted the noise power estimate is due to the higher signal power term in the estimator. The proposed method lowers the noise estimate by exploiting the differences in the powers of the orthogonal preambles received from the three spatially separated BS sectors. The simulation results show that the performance of our estimator improves as the power received from the two non-serving sectors decreases relative to the received power of the serving sector.

CDMA (Code Division Multiple Access) allows distinct users to communicate simultaneously on the same frequency band using binary sequences uniquely allocated to each user. For a given sequence length, we are confronted with a tradeoff between large set size (maximizing user capacity) and low maximum absolute inner product (minimizing instantaneous interference). Significant effort has been devoted to constructing upper bounds on set size and lower bounds on maximum inner products or correlation. Here we examine a common lower bound on inner products [1], show its inadequacy in certain cases, and improve it using an upper bound on set size.

To reduce computational complexity and delay in randomized network coded content distribution, and for some other practical reasons, coding is not performed simultaneously over all content blocks, but over much smaller, possibly overlapping subsets of these blocks, known as generations. A penalty of this strategy is throughput reduction. To analyze the throughput loss, we model coding over generations with random generation scheduling as a coupon collector's brotherhood problem. This model enables us to derive the expected number of coded packets needed for successful decoding of the entire content as well as the probability of decoding failure (the latter only when generations do not overlap) and further, to quantify the tradeoff between computational complexity and throughput. Interestingly, with a moderate increase in the generation size, throughput quickly approaches link capacity. Overlaps between generations can further improve throughput substantially for relatively small generation sizes.

Wireless communication is susceptible to eavesdropping attacks because of its broadcast nature. This paper illustrates how interference can be used to counter eavesdropping and assist secrecy. In particular, a wiretap channel with a helping interferer (WT-HI) is considered. Here, a transmitter sends a confidential message to its intended receiver in the presence of a passive eavesdropper and with the help of an independent interferer. The interferer, which does not know the confidential message, helps in ensuring the secrecy of the message by sending an independent signal. An achievable secrecy rate and several computable outer bounds on the secrecy capacity of the WT-HI are given for both discrete memoryless and Gaussian channels.

To reduce computational complexity and delay in randomized network coded content distribution (and for some other practical reasons), coding is not performed simultaneously over all content blocks but over much smaller subsets known as generations. A penalty is throughput reduction. We model coding over generations as the coupon collector's brotherhood problem. This model enables us to theoretically compute the expected number of coded packets needed for successful decoding of the entire content, as well as a bound on the probability of decoding failure, and further, to quantify the tradeoff between computational complexity and throughput. Interestingly, with a moderate increase in the generation size, throughput quickly approaches link capacity. As an additional contribution, we derive new results for the generalized collector's brotherhood problem which can also be used for further study of many other aspects of coding over generations.

This paper describes experiments which study the effects of both a stationary and a moving vehicle on the 22 MHz 802.11b and the 20 MHz 802.11a channel transfer functions. Our setup enables concurrent measurements of: (1) the complex channel transfer function with a Vector Network Analyzer (VNA), and (2) the Received Signal Strength Index (RSSI) obtained using off-the-shelf 802.11 cards. Our results show that, for the case of the stationary channel, the average RSSI is a several dBs away from the channel loss measured by the VNA, even with meticulous calibration of the 802.11 cards. The VNA measurements show that a vehicle moving in an environment resembling a multi-lane highway can cause a change in the magnitude of the transfer function on the order of tens of dB. The results of the RSSI measurements show that changes in RSSI caused by the passing vehicle are in the order of magnitude of the ones measured with the VNA.

Generally, the capacity-achieving signaling design for a specific channel should consider the joint optimization of channel coding and modulation. Nevertheless, coding theorists and practitioners have recognized that a well-designed LDPC code can achieve capacity-approaching performance universally across a range of data transmission and storage channels. A pronounced example is the forward error correction scheme adopted recently by the second generation standards for digital video broadcasting (DVB), wherein the same LDPC codes are expected to be reused over satellite, terrestrial and cable channels. However, to accommodate the spectral efficiency of different channel type, the coded bits should be mapped to the modulator judiciously. The well-known BICM strategy employs a large random bit interleaver, which typically yields good performance for an arbitrary choice of constellation mapper. However, it is problematic for high-speed coding and modulation due to the large amount of memory and circuits routing required. This motivates us to impose structural simplicity on the bit interleaver configuration so that the coded bits are de-multiplexed systematically into parallel groups to feed the constellation mapper. In this paper, we focus on the design of bit demultiplexers for multi-level modulations by applying the framework of multi-edge type (MET) LDPC. Since the channel-dependence of a given code ensemble is dominated by the mutual information between the input and output of the effective channel, we propose to simplify the analysis of the decoding behavior by using a set of surrogate binary erasure channels (BEC). Simulation results indicate that the proposed bit demultiplexer surpasses the performance of the heuristic interleaving strategy specified in second generation DVB standard for terrestrial channels (DVB-T2).

A secret-key sharing strategy based on layered broadcast coding is introduced for slow fading channels. In the model considered, Alice wants to share a key with Bob while keeping the key secret from a passive eavesdropper, Eve. Both Alice-Bob and Alice-Eve channels are assumed to undergo slow fading, and perfect channel state information (CSI) is assumed to be known only at the receivers during the transmission. Layered coding facilitates adapting the reliably decoded rate at Bob to the actual channel state without CSI available at Alice. The index of a reliably decoded layer is sent back to Alice via a public and error-free channel, which is exploited by Alice and Bob to generate the secret key. In this paper, the secrecy key rate is derived. In addition, the optimal power distribution over coded layers is characterized. It is shown that layered coding can increase the secrecy key rate significantly compared with single-level coding.

This paper studies decentralized, Fountain and network-coding based strategies for facilitating data collection in circular wireless sensor networks, which rely on the stochastic diversity of data storage. The goal is to allow for a reduced delay collection by a data collector who accesses the network at a random position and random time. Data dissemination is performed by a set of relays which form a circular route to exchange source packets. The storage nodes within the transmission range of the route's relays linearly combine and store overheard relay transmissions using random decentralized strategies. An intelligent data collector first collects a minimum set of coded packets from a subset of storage nodes in its proximity, which might be sufficient for recovering the original packets and, by using a message-passing decoder, attempts recovering all original source packets from this set. Whenever the decoder stalls, the source packet which restarts decoding is polled/doped from its original source node. The random-walk-based analysis of the decoding/doping process furnishes the collection delay analysis with a prediction on the number of required doped packets. The number of doped packets can be surprisingly small when employed with an Ideal Soliton code degree distribution and, hence, the doping strategy may have the least collection delay when the density of source nodes is sufficiently large. Furthermore, we demonstrate that network coding makes dissemination more efficient at the expense of a larger collection delay. Not surprisingly, a circular network allows for a significantly more (analytically and otherwise) tractable strategies relative to a network whose model is a random geometric graph.

For wireless sensor networks with many locationunaware nodes, which can be modeled as a planar Poisson point process, we investigate a protocol, dubbed BeSpoken, which steers data transmissions along a straight path called a spoke. BeSpoken implements a simple, spatially recursive process, where a basic set of control packets and a data packet are exchanged repeatedly among daisy-chained relays that constitute the spoke. Hence, a data packet originated by the first relay makes a forward progress in the direction of the spoke. Despite the simplicity of the protocol engine, modeling the spoke process is a significant challenge. Bespoken directs data transmissions by randomly selecting relays to retransmit data packets from crescent-shaped areas along the spoke axis. The resulting random walk of the spoke hop sequence may be modeled as a two dimensional Markov process. Based on this model, we propose design rules for protocol parameters that minimize energy consumption while ensuring that spokes propagate far enough and have a limited wobble with respect to the spoke axis. The energy efficiency is demonstrated through simulations of the BeSpoken-based data search, and a comparison with the energy consumption of a search based on directed diffusion.

The focus of this paper is an information-theoretic study of retransmission protocols for reliable packet communication under a secrecy constraint. Thehybridautomaticretransmission request (HARQ) protocol is revisited for a block-fading wiretap channel. Here, two legitimate users communicate over a block-fading channel in the presence of a passive eavesdropper who intercepts the transmissions through an independent block-fading channel. In this model, the transmitter obtains a 1-bit ACK/NACK feedback from the legitimate receiver via an error-freepublicchannel. Both reliability and confidentiality of secure HARQ protocols are studied through the joint consideration of channel coding, secrecy coding, and retransmission protocols. In particular, the error and secrecy performance ofrepetition time diversity(RTD) andincremental redundancy(INR) protocols are investigated based on Wyner code sequences. These protocols ensure that the confidential message is decoded successfully by the legitimate receiver and is kept completely secret from the eavesdropper for a set of channel realizations. This paper illustrates that there exists a rate-compatible Wyner code family which ensures a secure INR protocol. Further, it defines theconnection outageandsecrecy outageprobabilities to characterize the tradeoff between the reliability of the legitimate communication link and the confidentiality with respect to the eavesdropper's link. For a given connection/secrecy outage probability pair, an achievable throughput of secure HARQ protocols is derived for block-fading channels. Finally, both asymptotic analysis and numerical calculations demonstrate the benefits of HARQ protocols to throughput and secrecy.

In this paper, we show that for STC (Space-Time Coded) Hybrid-ARQ (Automatic Repeat reQuest) schemes with (re)transmission power control over independent Rayleigh block fading channels, the problem of optimizing energy efficiency with a PER (Packet Error Rate) constraint can be solved as a geometric programming problem. The optimum transmit power increases super-linearly with each requested retransmission and the fraction of the average power optimally allocated to each ARQ round only depends on the incremental diversity gain. The energy savings increases with a decrease in the PER targets and decreases with an increase in the diversity gain.

Due to the broadcast nature of the wireless medium, wireless communication is susceptible to adversarial eavesdropping. This paper describes how eavesdropping can potentially be defeated by exploiting the superposition nature of the wireless medium. A Gaussian wire-tap channel with a helping interferer (WTC-HI) is considered in which a transmitter sends confidential messages to its intended receiver in the presence of a passive eavesdropper and with the help of an interferer. The interferer, which does not know the confidential message assists the confidential message transmission by sending a signal that is independent of the transmitted message. An achievable secrecy rate and a Sato-type upper bound on the secrecy capacity are given for the Gaussian WTC-HI. Through numerical analysis, it is found that the upper bound is close to the achievable secrecy rate when the interference is weak for symmetric interference channels, and under more general conditions for asymmetric Gaussian interference channels.

Wireless communication is susceptible to adversarial eavesdropping due to the broadcast nature of the wireless medium. In this paper it is shown how eavesdropping can be alleviated by exploiting the superposition property of the wireless medium. A wiretap channel with a helping interferer (WT-HI), in which a transmitter sends a confidential message to its intended receiver in the presence of a passive eavesdropper, and with the help of an independent interferer, is considered. The interferer, which does not know the confidential message, helps in ensuring the secrecy of the message by sending independent signals. An achievable secrecy rate for the WT-HI is given. The results show that interference can be exploited to assist secrecy in wireless communications. An important example of the Gaussian case, in which the interferer has a better channel to the intended receiver than to the eavesdropper, is considered. In this situation, the interferer can send a (random) codeword at a rate that ensures that it can be decoded and subtracted from the received signal by the intended receiver but cannot be decoded by the eavesdropper. Hence, only the eavesdropper is interfered with and the secrecy level of the confidential message is increased.

This paper investigates the problem of ensuring secure communication through error-correcting coding methods. A practical structured secure coding design is considered for a general wiretap channel, in which the main channel and the eavesdropper channel are binary-input symmetric-output memoryless (BISOM) channels. The proposed secure error-correcting code has a nested code structure. The nesting is based on cosets of a capacity-achieving sequence for binary erasure channels (BECs). The corresponding achievable secrecy rate is derived based on an erasure decomposition for the eavesdropper channel and an Bhattacharyya-equivalent channel construction for the main channel. Those two techniques allow a “degraded” erasure wiretap channel to be built and, hence, significantly simplify the practical coding design for secure transmission.

We studyinformation-theoretic securityfor discrete memorylessinterferenceandbroadcastchannels with independent confidential messages sent to two receivers. Confidential messages are transmitted to their respective receivers while ensuring mutual information-theoretic secrecy. That is, each receiver is kept in total ignorance with respect to the message intended for the other receiver. The secrecy level is measured by the equivocation rate at the eavesdropping receiver. In this paper, we present inner and outer bounds on secrecy capacity regions for these two communication systems. The derived outer bounds have an identical mutual information expression that applies to both channel models. The difference is in the input distributions over which the expression is optimized. The inner bound rate regions are achieved byrandom binningtechniques. For the broadcast channel, adouble-binningcoding scheme allows for both joint encoding and preserving of confidentiality. Furthermore, we show that, for a special case of the interference channel, referred to as theswitchchannel, derived bounds meet. Finally, we describe several transmission schemes for Gaussian interference channels and derive their achievable rate regions while ensuring mutual information-theoretic secrecy. An encoding scheme in which transmitters dedicate some of their power to createartificial noiseis proposed and shown to outperform both time-sharing and simple multiplexed transmission of the confidential messages.

We study anincremental redundancy(IR) cooperative coding scheme for wireless networks. To exploit the distributed spatial diversity we propose a cluster-based collaborating strategy for a quasi-static Rayleigh-fading channel model. Our scheme allows for enhancing the reliability performance of a direct communication over a single hop. The collaborative cluster consists of$M-1$nodes between the sender and the destination. The transmitted message is encoded using a mother code which is partitioned into$M$blocks each assigned to one of$M$transmission slots. In the first slot, the sender broadcasts its information by transmitting the first block, and its helpers attempt to decode this message. In the remaining slots, each of the next$M-1$blocks is sent either through a helper which has successfully decoded the message or directly by the sender where a dynamic schedule is based on the ACK-based feedback from the cluster. By employing powerfulgood codesincluding turbo, low-density parity-check (LDPC), and repeat–accumulate (RA) codes, our approach illustrates the benefit of collaboration through not only a cooperation diversity gain but also a coding advantage. The basis of our error rate performance analysis is based on a derived code threshold for the Bhattacharyya distance which describes the behavior of good codes. The new simple code threshold is based on the modified Shulman–Feder bound and the relationship between the Bhattacharyya parameter and the channel capacity for an arbitrary binary-input symmetric-output memoryless channel. An average frame-error rate (FER) upper bound and its asymptotic (in signal-to-noise ratio (SNR)) version are derived as a function of the average fading channel SNRs and the code threshold. Based on the asymptotic bound, we investigate both the diversity, the coding, and the transmission energy gain in the high and moderate SNR regimes for three different scenarios: transmitter clustering, receiver clustering, and cluster hopping. We observe that the energy saving of the IR cooperative coding scheme isuniversalfor all good code families in the sense that the gain does not depend on the sender-to-destination distance and the code threshold.

Motivated by the problem of reducing the peak-to-average power ratio (PAPR) of transmitted signals, we consider a design of complementary set matrices whose column sequences satisfy a correlation constraint. The design algorithm recursively builds a collection of$2^{t+1}$mutually orthogonal (MO) complementary set matrices starting from a companion pair of sequences. We relate correlation properties of column sequences to that of the companion pair and illustrate how to select an appropriate companion pair to ensure that a given column correlation constraint is satisfied. For$t=0$, companion pair properties directly determine matrix column correlation properties. The proposed companion pair-based design can construct binary complementary sets with either a minimum out-of-phase autocorrelation magnitude or a minimum sum-of-out-of-phase autocorrelation magnitude for column sequences of length at least up to$28$. For$t \geq 1$, reducing correlation merits of the companion pair may lead to improved column correlation properties. Exhaustive search for companion pairs satisfying a column correlation constraint is infeasible for medium length and long sequences. We instead search for two shorter length sequences by minimizing a cost function in terms of their autocorrelation and cross-correlation merits. In addition, by exploiting the well-known Welch bound, sufficient conditions for the existence of companion pairs which satisfy a set of column correlation constraints are given.

In applications of wireless packet-oriented data networks, a special coding scheme, the hybrid automatic retransmission request (HARQ) exhibits high throughput efficiency by adapting its error correcting code redundancy to channel conditions. Motivated by the increasing importance of secure communication over wireless networks, we investigate secure packet communication based on HARQ over block-fading (BF) channels. More specifically, we consider two legitimate users communicating over a BF channel in the presence of a passive eavesdropper who intercepts the transmission through another independent BF channel. We assume that the transmitter can obtain a 1-bit ACK/NACK feedback from the receiver via a reliable public channel. Under this setting, we consider incremental redundancy (IR) and repetition time diversity (RTD) HARQ schemes based on rate-compatible Wyner secrecy codes from an information theoretic point of view. We study a good Wyner code sequence, with which the legitimate receiver can decode the message and the eavesdropper can be perfectly confused. For a given pair of reliability/secrecy outage probabilities, we derive an achievable secrecy throughput of HARQ protocols for block-fading channels. Finally, we illustrate numerically that HARQ can benefit both throughput and secrecy.

We perform a set of measurements of channel frequency responses at different points in the room that accommodates the ORBIT indoor testbed using a vector network analyzer (VNA). To validate the data collected with VNA we use spectrum analyzer (SA) measurements, as well. Four measurements are performed over non-overlapping 100 MHz bands in the industrial, scientific, and medical (ISM) band, and in the unlicensed national information infrastructure (UNII) band. The fifth measurement spans the coarsely sampled .4 to 6 GHz band. From the measured frequency responses we calculate path loss model parameters. The path loss exponent is between 1.1 and 2 and the dynamic range of the signal is around 25 dB across different bands. Based on the frequency responses measured over the .4 to 6 GHz band we determine multipath intensity profiles (MIP) with fine time granularity. The comparison of MIPs to the ray tracing simulations generated with WiSE software indicates that the surfaces perpendicular to the plane defined by the transmit and receive antennas represent a significant source of reflections. However, the reflections from the floor, dropped ceiling, and roof are suppressed by the antenna elevation patterns.

Recent real-world measurements in dense congested radio environments have pointed out the inefficiency of frame error-based bit-rate adaptation mechanisms, which significantly reduce network capacity by misinterpreting frame errors due to collisions. These effects are likely to be amplified with the heavy use of media applications. Fortunately, traditional SNR-based rate adaptation, and the more recently proposed throughput-based, and collision-aware rate adaptation algorithms are expected to provide more robust performance in these scenarios. To our knowledge, however, their performance has never been experimentally validated in a congested environment. In this paper, we report our implementation experiences with rate adaptation in a dense, congested IEEE 802.11 network. We find that throughput-based adaptation, contrary to expectations, also suffers from poor bitrate selection. Due to an increase in physical layer capture, while using lower bitrates, nodes can increase their individual throughput at the expense of cumulative network throughput. SNR-based rate adaptation performs well in static environments but the lack of sufficient precision in RSSI measurements makes accurate rate selection in dynamic radio environments dfficult. The use of RTS/CTS, in the spirit of collision-aware rate adaptation, shows throughput improvements for frame error-based algorithms and, additionally, for throughput-based algorithms as well. However, results are below expectations likely due to RTS/CTS implementation issues on the Atheros 5212 platform.

We study an adaptive transmission scheme based on variable-rate turbo bit-interleaved coded modulation (VR- Turbo-BICM). The proposed coding scheme employs punctured turbo codes. A continuously varying transmission rate can be obtained by changing the code rate through both puncturing of the coded bits and adapting of the modulation constellation size. The main results are elaborated in two parts. First, we derive a closed-form expression for a set of achievable rate bounds (called rate thresholds) for VR-Turbo-BICM by employing recent results on the parallel channel performance of turbo code ensembles and the BICM parallel channel analysis model. The derived rate threshold is expressed as a fraction of the capacity of BICM with Gray mapping, where this fraction is a turbo code weight spectrum parameter. Simulation results illustrate that introduced rate thresholds predict well the rate versus SNR performance of VR-Turbo-BICM for a wide range of codeword error probabilities and codeword lengths. Next, based on a simplified rate threshold, we derive a power, puncturing rate, and modulation constellation size assignment policy for a slow fading channel.

A multiple-access channel is considered in which messages from one encoder are confidential. Confidential messages are to be transmitted with perfect secrecy, as measured by equivocation at the other encoder. The upper bounds and the achievable rates for this communication situation are determined.

For a quasi-synchronous code-division multiple access (QS-CDMA) system, the relative time delays between the signals of different users can be maintained with a given time certainty. In this paper, the time delay is modelled as a uniformly distributed random variable on (-tau, tau). It is known that the multiple access interference (MAI) of the QS-CDMA system can be significantly reduced by employing spreading sequences with a zero-correlation zone (ZCZ). Hence, we propose a construction method for the design of ternary orthogonal variable-spreading-factor (OVSF) codes with a ZCZ of arbitrary lengths. Based on the knowledge of tau, we demonstrate how to select the OVSF code tree with different ZCZ to increase the aggregate throughput of DS-UWB systems with a loose synchronization. Once the OVSF code tree is selected, an adaptive date rate transmission scheme is proposed which can further increase the aggregate throughput by allocating OVSF codes to the active users

This work examines a transmission system which adapts a finite set of code rates and a continuously varying transmit power. We propose a technique for finding the average reliable throughput (ART)-maximizing policy satisfying an average power constraint for a slow fading additive white Gaussian noise (AWGN) channel. ART is a measure motivated by the information outage and can, for example, be argued to characterize the long-term average throughput of a data packet transmission system with a transmit queue and a feedback protocol which requests retransmission of erroneously received packets. Given the size of the code rate set L, the ART-maximizing policy has the following properties. 1. For a given set of code rates, the optimum allocation policy suggests quantizing the fading state space into a set of L+1 corresponding intervals. For each quantization interval the optimal policy specifies a minimum transmitted power assignment which guarantees zero information outage. The optimum average power assignments across quantization intervals have a waterfilling relationship with respect to the interval channel quality measure. 2. The joint optimization of quantization intervals and the corresponding rate assignments are shown to have multiple local maxima. Nevertheless, this optimization problem can be reduced to a simple one-dimensional search over a parameter which determines the outage interval. Numerical results show that, in a Rayleigh-fading channel, there is only a 1-dB gap between the ergodic capacity and the throughput of a two-rate adaptive transmission system when the throughput is less than 6 bits/s/Hz. A special case of our optimal policy assignment is the optimal power and rate policy for an adaptive M-QAM system.

We study the average error probability performance of binary linear code ensembles when each codeword is divided into J subcodewords with each being transmitted over one of J parallel channels. This model is widely accepted for a number of important practical channels and signaling schemes including block-fading channels, incremental redundancy retransmission schemes, and multicarrier communication techniques for frequency-selective channels. Our focus is on ensembles of good codes whose performance in a single channel model is characterized by a threshold behavior, e.g., turbo and low-density parity-check (LDPC) codes. For a given good code ensemble, we investigate reliable channel regions which ensure reliable communications over parallel channels under maximum-likelihood (ML) decoding. To construct reliable regions, we study a modifed 1961 Gallager bound for parallel channels. By allowing codeword bits to be randomly assigned to each component channel, the average parallel-channel Gallager bound is simplified to be a function of code weight enumerators and channel assignment rates. Special cases of this bound, average union-Bhattacharyya (UB), Shulman-Feder (SF), simplified-sphere (SS), and modified Shulman-Feder (MSF) parallel-channel bounds, allow for describing reliable channel regions using simple functions of channel and code spectrum parameters. Parameters describing the channel are the average parallel-channel Bhattacharyya noise parameter, the average channel mutual information, and parallel Gaussian channel signal-to-noise ratios (SNRs). Code parameters include the union-Bhattacharyya noise threshold and the weight spectrum distance to the random binary code ensemble. Reliable channel regions of repeat-accumulate (RA) codes for parallel binary erasure channels (BECs) and of turbo codes for parallel additive white Gaussian noise (AWGN) channels are numerically computed and compared with simulation results based on iterative decoding. In addition, an examp

Packet detection is the first task that a receiver has to perform in a random access communication scheme. The evaluation of different packet detection methods depends on the probability density functions of the decision variables and their construction process. This paper provides a new probability density function (PDF) that enhances evaluation and implementation of one method, due to Schimidl and Cox (SC method), in the low signal-to-noise ratio (SNR) region as required for ultra-wideband (UWB) systems. The new PDF is accurate from high SNR, where it is well-matched by a Gaussian approximation, to SNR = 0, where the Gaussian approximation breaks down. It is thus useful for analyzing packet detection, while the Gaussian approximation is not. We use the new PDF for this purpose, and we compare the packet detection performance of the SC method with that of other candidate methods. The SC method is shown to provide a good tradeoff between performance (including robustness to multipath) and complexity.

We propose an algorithm for the construction of ternary orthogonal variable-spreading-factor (OVSF) codes with zero-correlation zone (ZCZ); this is of particular interest for ternary sequence based UWB (TS-UWB) systems. The conventional approach is to design OVSF codes based on Walsh codes and, in this case, the orthogonality of the corresponding OVSF codes is easily lost when synchronism is lacking or in a multipath scenario. 2D OVSF codes possess ideal correlation properties which significantly improve the interference-rejection capability of multicarrier DS-CDMA systems. However, 2D OVSF code schemes, applied over multiple orthogonal channels, suffer from the disadvantages of a high peak-to-average power ratio (PAPR) and high complexities on transceiver design. The proposed OVSF codes, derived from two-dimensional (2D) OVSF codes, present a ZCZ which allows for significant suppression of multipath and multiuser interference and keeps the transceiver of an impulse based UWB system simple.

The weight spectrum of sequences of binary linear codes that achieve arbitrarily small word error probability on a class of noisy channels at a nonzero rate is studied. We refer to such sequences as good codes. The class of good codes includes turbo, low-density parity-check, and repeat-accumulate codes. We show that a sequence of codes is good when transmitted over a memoryless binary-symmetric channel (BSC) or an additive white Gaussian noise (AWGN) channel if and only if the slope of its spectrum is finite everywhere and its minimum Hamming distance goes to infinity with no requirement on its rate growth. The extension of these results to code ensembles in probabilistic terms follows in a direct manner. We also show that the sufficient condition holds for any binary-input memoryless channel.

The service outage based allocation problem explores variable-rate transmission schemes and combines the concepts of ergodic capacity and outage capacity for fading channels. A service outage occurs when the transmission rate is below a given basic rate r/sub o/. The allocation problem is to maximize the expected rate subject to the average power constraint and the constraint that the outage probability is less than /spl epsi/. A general class of probabilistic power allocation schemes is considered for an M-parallel fading channel model. The optimum power allocation scheme is derived and shown to be deterministic except at channel states of a boundary set. The resulting service outage achievable rate ranges from 1-/spl epsi/ of the outage capacity up to the ergodic capacity with increasing average power. Two near-optimum schemes are also derived by exploiting the fact that the outage probability is usually small. The second near-optimum scheme significantly reduces the computational complexity of the optimum solution; moreover, it has a simple structure for the implementation of transmission of mixed real-time and non-real-time services.

We study a multiple channel direct sequence ultrawideband (DS-UWB) system which transmits the same information bit over a set of parallel channels corresponding to a set of orthogonal pulses. It is known that by employing a mutually orthogonal (MO) ternary complementary set of spreading sequences, one can efficiently suppress both multipath and multiuser interference. Similar to a multicarrier DS-CDMA system, the multichannel DS-UWB system can have a high peak to average power ratio (PAPR) which may limit its application. In this paper, we analyze the PAPR of the multichannel UWB system by studying its upper bound. The bound illustrates how column sequences of MO complementary set matrices with small out-off-phase aperiodic autocorrelation functions (ACF) allow for lowering the PAPR. Hence, we develop an algorithm to construct the spreading sequence sets which may result in data sequences with reduced PAPR, while at the same time, preserving the complementarity and orthogonality which alleviate the multipath and multiuser interference.

This paper describes a given ensemble of good binary codes and a codeword-symbol with channel assignment rule and error probability performance. The reliable channel regions based on the parallel-channel Gallager bound achieves all functions of the code weight enumerators, parallel-channel transition probabilities, and the channel assignment rates. The channel model consists of parallel binary-input symmetric-output (BISO) discrete memoryless channels. The uniform codeword partition and decoded iteratively of a reliable channel regions for good codes transmitted over parallel channels is studied.

This paper studies the performance of variable-rate turbo bit-interleaved coded modulation (Turbo-BICM) with random puncturing. A union-Bhattacharyya rate threshold for the variable-rate Turbo-BICM is derived. A closed form approximation of this rate threshold is determined for an AWGN channel and shown to predict the Turbo-BICM iterative decoding performance very well.

In multi-hop networks, messages are traditionally relayed over a set of sequential point-to-point communication links. An overheard message is typically discarded since the noisy packet is below the detection threshold. However, an overheard packet still contains useful information about the original message, and its consideration can improve the energy efficiency of a transmission scheme. Hence, we study an incremental redundancy multi-hop transmission scheme which enhances the overheard information hop-by-hop. The j-th sequential node combines the previously (over)heard hop transmissions which together form a codeword of a "good" code of rate sufficient for reliable decoding. The analysis of punctured codes whose symbols are distributed over a number of hops is based on a random hop assignment technique. This technique allows for a performance threshold behavior description as a function of the hop rates and a derivation of the asymptotic (as the number of relays goes to infinity) reliable hop rate threshold as a function of channel and mother code parameters. The significant energy savings of the cooperative transmission relative to schemes that discard overheard packets are a function of only the channel parameters.

Without perfect channel state information at the transmitter, it is possible for adaptive transmission systems to experience information outage. In this paper, we formulate the throughput maximization with both an average power constraint and an information outage constraint. It is verified that, for the optimal transmission policy, the transmission only needs to adapt to a sufficient statistic for the channel state. For a Rayleigh fading channel with a simple training scheme, numerical results show that, with a reasonable amount of training and a small set of code rates, the adaptive transmission can achieve a performance very close to the ergodic capacity.

The service outage based allocation problem explores variable rate transmission schemes and combines the concepts of ergodic capacity and capacity versus outage for fading channels. A service outage occurs when the transmission rate is below a given basic rate r/sub o/. The allocation problem is to maximize the expected rate subject to the average power constraint and the constraint that the outage probability is less than /spl epsiv/. A general class of probabilistic power allocation schemes is considered in this problem. In an M-parallel fading channel model, the optimum power allocation scheme is derived and is shown to be deterministic except at channel states of a boundary set. The resulting service outage average rate is between the outage capacity times 1-/spl epsiv/ and the ergodic capacity.

We analyze the asymptotic performance of punctured turbo codes. The analysis is based on the union bound on the word error probability of maximum likelihood decoding for punctured turbo code ensembles averaged over all possible puncturing patterns and interleavers. By using special probabilistic puncturing, we prove that, for a given mother turbo code ensemble, [C], with a finite noise threshold, c/sub 0//sup [C]/, if the asymptotic puncturing turing rate, /spl lambda/, satisfies log /spl lambda/ < -c/sub 0//sup [C]/, there exists a finite noise threshold, c/sub 0//sup [Cp]/, for the punctured turbo code ensemble which is bounded by a function of c/sub 0//sup [C]/ and /spl lambda/. Based on this result, we prove that, on any binary-input memoryless channel whose Bhattacharyya noise distance is greater than c/sub 0//sup [Cp]/, the average ML decoding word error probability of the punctured turbo code ensemble approaches zero at least as fast as n/sup -/spl beta//, where /spl beta/ is the well known "interleaver gain" exponent. This enables us to answer an important question in the practice of HARQ (hybrid ARQ) schemes, namely, up to which puncturing rate "good" turbo codes give rise to "good" punctured codes.

Throughput maximization of an adaptive transmission system with a finite number of transmission power levels and code rates for communication over slow fading channels is analyzed, based on the concept of information outage. Properties of throughput maximizing policies lead to an iterative algorithm that yields good system designs. Numerical results show that carefully designed discrete adaptive transmission systems with a small number of power levels and code rates can achieve throughput values close to ergodic capacity.

This article combines the concepts of ergodic capacity and capacity versus outage for fading channels, and explores variable-rate transmissions under a service outage constraint in a block flat-fading channel model. A service outage occurs when the transmission rate is below a given basic rate. We solve the problem of maximizing the expected rate subject to the average power constraint and the service outage probability constraint. When the problem is feasible, the optimum power policy is shown to be a combination of water filling and channel inversion allocation, where the outage occurs at a set of channel states below a certain threshold. The service outage approach resolves the conflicting objectives of high average rate and low outage probability.

This work examines an adaptive transmission system that supports a discrete set of code rates and continuously variable transmit power. Based on the concept of information outage, we maximize the system throughput over a slow fading channel. Properties of the throughput maximizing policies result in an iterative algorithm that yields good system designs. Numerical results show that in a Rayleigh fading channel, there is only a gap of 1 dB between the ergodic capacity and the throughput of a 2-rate adaptive transmission system when the throughput is less than 4 bits/sec/Hz.

In this work, we address optimal adaptive transmission policies in slow varying wireless environments. Continuous rate and power assignments that achieve the ergodic capacity for these channels have been derived previously. Nevertheless, from a practical point of view, use of a finite number of power and rate levels is imperative. Here, we address the mapping from channel states of an arbitrary distribution to a discrete set of power level and code rate pairs. Unlike earlier work, our design does not require that the transmitter knows the exact value of the current channel state. We show that our design yields results close to the well-known water-filling result.

The optimum receiver to detect the bits of multiple CDMA users has exponential complexity in the number of active users in the system. Previous work showed that the successive and parallel soft interference cancellers correspond to nonlinear programming relaxations of the optimum multiuser detection problem. We use this approximation method combined with the slowest descent approach to improve the performance of soft interference cancellers. The aim is to achieve a performance closer to the performance of the optimum receiver without significantly compromising the low complexity of the resulting receiver. We derive the resulting detectors and evaluate their performance. Results show that they can achieve near-optimum performance and outperform several previously proposed multiuser detectors.

A new technique is proposed for robust multiuser detection in the presence of non-Gaussian ambient noise. This method is based on minimizing a certain cost function (e.g., the Huber penalty function) over a discrete set of candidate user bit vectors. The set of candidate points are chosen based on the so-called "slowest-descent search." starting from the estimate closest to the unconstrained minimizer of the cost function and along mutually orthogonal directions where this cost function grows the slowest. Simulation results show that this new technique offers substantial performance improvement over the previously proposed robust multiuser detectors with little attendant increase in computational complexity.

A new approach to sequence estimation is proposed and its performance is analyzed for a number of channels of practical interest. The proposed approach, termed the slowest descent method, comprises as a special case the zero-forcing equalizer for intersymbol interference channels and the decorrelator for the multiuser detection problem. The latter two methods quantize the unconstrained sequence that maximizes the likelihood function. The proposed method can be viewed as a generalization of these two methods in two ways. First, the unconstrained maximization is extended to nonquadratic log-likelihood functions; second, the decorrelator estimate can be "refined" by comparing its likelihood to a set of discrete-valued sequences along mutually orthogonal lines of the least decrease in the likelihood function. The gradient descent method for iterative computation of the line of least likelihood decrease (i.e., slowest likelihood descent) and its relationship to the expectation-maximization (EM) algorithm for unconstrained likelihood maximization is discussed. The slowest descent method is shown to provide a performance comparable to maximum-likelihood for a number of channels. These problems can be described by either quadratic or nonquadratic log-likelihood functions.

A nonlinear group-blind technique is developed for joint detection of some given users' data in a CDMA uplink environment with the presence of unknown interference. This method performs the so-called "slowest-descent search" over a likelihood function of the desired users, starting from the estimate closest to the unconstrained maximizer of the likelihood function, and along mutually orthogonal directions where this likelihood function drops to the slowest. Simulation results show that this new nonlinear technique offers substantial performance improvement over the previously proposed linear group-blind multiuser detectors with little attendant increase in computational complexity. The problem of group-blind multiuser detection in the presence of both unknown interference and impulsive ambient noise is also treated under the framework of slowest-descent search, with the aid of a novel subspace-based robust interference cancellation scheme. It is seen that this robust group-blind method significantly outperforms the robust blind multiuser detection scheme proposed previously.

A merit factor based on the sequence autocorrelation function, whose minimization leads to the reduction in the Cramer-Rao lower bound (CRLB) for the variance of "two-sided" intersymbol interference (ISI) channel estimation is introduced. Pairs of binary pilot symbol sequences (a preamble and a postamble) for channel estimation are jointly designed to minimize this merit factor. Given that the number of channel taps is L and the length of a pilot symbol sequence is (N+L-1), where N/spl ges/L, we distinguish between the case when N is even and the case when it is odd. For even N, we show that complementary sequences not only minimize the merit factor, but also the CRLB. For a subset of odd N we construct almost-complementary periodic sequence pairs that minimize the merit factor. The optimal pilot symbol block signaling requires alternating between two (in most cases) different binary sequences that form the merit-minimizing pair.

This paper considers the problem of multiuser detection for CDMA systems where the codes of a group of users are known while others are unknown, called group-blind multiuser detectors. An example is the uplink in cellular communication system with interference from both in-cell users, with known codes, and out-of-cell users, with unknown codes. In previous papers linear detectors for this situation were developed. In the current paper, we combine these methods with low-complexity non-linear post processing, using the method of slowest descent, a general method for non-linear optimization. The performance of this new group-blind detector is compared to that of the non-blind MMSE detector as well as the linear group blind detector, and the non-linear detector is seen to have a considerably better performance, in particular in heavily loaded systems.

Frequency offset due to Doppler shift and/or oscillator instabilities degrade the receiver performance. A family of frequency detectors for frequency offset estimation and compensation in digital receivers is introduced. The proposed detectors are best suited for frequency offset compensation of a subclass of binary continuous phase modulation with h=1/2 that includes modulation schemes with nonnegative frequency pulses. For the considered modulation schemes, the modulation-induced self-noise term is absent from the variance of the frequency estimate. The estimator is nondata- and nontiming-aided and its estimation range is either half or a quarter of the bit rate (R). With larger frequency offsets, the estimators that have a /spl plusmn/R/2 estimation range introduce a frequency ambiguity of R that is of no relevance to the performance of a differential detection based receiver.

An iterative pilot symbol aided procedure for detection in multipath fading channels that does not neglect intersymbol interference (ISI) and achieves diversity combining with unresolved signal path components is introduced. The algorithm resolves the received signal into components corresponding to each path. The estimated components are combined for obtaining the next sequence estimate, and, thus, no signal energy is lost. The proposed approach attempts iterative unconstrained maximization of the likelihood function using the expectation-maximization (EM) algorithm. The EM solution is based on the approach to parameter estimation for superimposed signals introduced by Feder and Weinstein (1988). It allows for symbol-by-symbol maximization in the M-step for orthogonal signaling pulses.