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In this paper, we present a frame structure that can be viewed as a preamble for the detection and synchronization process (leading to low cost receiver) and as an encoded codeword carrying the transmitted message (leading to reliable transmission). This duality facilitates an ALOHA protocol avoiding preamble overhead. The frame structure, named Quasi-Cyclic Short Packet (QCSP), is based on the association of a Cyclic Code Shift Keying (CCSK) modulation and a non-binary error control code. The detection/correction algorithm of the QCSP system is presented, its performance is theoretically derived and discussed for different parameters. A QCSP frame can be transmitted and received correctly with an error probability of $\mathbf {10^{-4}}$ , distanced by 1.2 dB from Polyanskiy’s bound (an estimated Shannon’s limit for small packet size) at −11 dB of SNR for a payload of 360 bits. Compared to a classical preamble-based frame using a modern binary error control code, the size of a QCSP frame is reduced by 23%. Moreover, detecting a QCSP frame requires a lower complexity than detecting a longer classical preamble. [ABSTRACT FROM AUTHOR]

This article proposes an opportunistic scheduling scheme based on adaptive modulation for users in a mixed noise environment, where some users are under additive white Gaussian noise (AWGN) and other users are exposed to impulsive noise. Unlike the scenario, where all the users are in an AWGN environment, the maximum signal-to-noise ratio (SNR) scheduler does not provide the maximum capacity if users are in a mixed noise environment. In the proposed scheduling scheme, called maximum rate scheduler, the user with the highest rate (or highest modulation order), instead of highest SNR, is selected. To evaluate the performance of the proposed scheduling scheme, the analyses of average spectral efficiency, outage probability and system capacity are provided along with a simple calculation of SNR thresholds for adaptive modulation in an impulsive noise environment satisfying target symbol error rate (SER). Simulation results illustrate that system capacity with the proposed scheduling given target SER = 10−2 and the average spectral efficiency are improved by 156% and 124%, respectively, at SNR = 5 dB when a strong impulsive noise is present. [ABSTRACT FROM AUTHOR]

Nonlinear channel codes are employed to protect two information fields that are related to the daylight saving time (DST) and leap second notifications within the enhanced WWVB broadcast format. The general codebook design criteria and the construction of these codes using integer linear program (ILP) is formulated. Specifically, two different variations are presented: i) minimizing the union bound of probability of word error (PWE), which is a tighter bound but has high computational complexity and ii) minimizing an analytical closed form approximation to the PWE, which has much lower complexity but is a looser bound on the exact PWE. By leveraging the highly non-uniform a-priori probabilities of the different messages being transmitted in these two fields, the proposed nonlinear codes provide coding gains of about 2–3 dB in the presence of additive white Gaussian noise (AWGN). When compared to checksum codes with the same rate, the nonlinear codes offer performance advantages of about 1–2 dB for the DST notification field. In the presence of impulsive noise, simulations and analysis show that the probability of word error when employing the proposed nonlinear code is lower by many orders of magnitude compared with that of the uncoded system. [ABSTRACT FROM AUTHOR]

Clarkepsilas model and Jakespsila spectrum have been traditionally accepted in wireless channel modeling. In comparison with measured spectra, Jakespsila spectrum has limitations - it is unbounded and does not incorporate the effect of temporal phase fluctuations. Previous work extended Clarkepsilas model to yield a theoretical power spectrum that is consistent with measured data. The modified spectrum, which includes the effects of phase fluctuations explicitly, is more appropriate as a theoretical basis for channel spectrum analysis and simulations. We develop here a state-space model that represents a wireless channel with these modified spectral characteristics. This is achieved by developing the relationship between a continuous-time state-space model and the theory of the rational transfer function. A novel method for the design of a rational transfer function of a linear system is proposed. The system input is a Gaussian white noise process, which generates a wireless channel with a desired arbitrary power spectrum. We represent the rational transfer function via the observable canonical form (OCF) to obtain the continuous-time state-space model. A discrete-time version of the state-space model is then provided to represent and simulate a discrete-time flat fading wireless channel. [ABSTRACT FROM PUBLISHER]

Gaussian mixture (GM) receivers have been recently introduced in the context of ultra wide band (UWB) impulse radio (IR) as a robust alternative to commonly employed techniques. By statistically modeling interference as a GM, as opposed to the ordinary AWGN assumption, such receivers are able to mitigate the effect of multiple user interference (MAI). However, this is achieved at the cost of an increased complexity, commonly thought as an high complexity. In this paper we prove that a suitable reformulation of the GM approach leads to a receiver which is low complexity, while providing the same performance of standard GM. Hence, the GM paradigm is a usable, and welcome, technique for IR applications. [ABSTRACT FROM PUBLISHER]

We derive tight closed-form approximations of symbol and bit error probability on maximum-likelihood sequence detection (MLSD) of Nyquist-rate and oversampled OFDM systems over the AWGN, flat and frequency-selective quasi-static Rayleigh fading channels. Contrary to common knowledge, we show that on a frequency-selective channel, clipping at the transmitter leads to achievable frequency-diversity and performance gain over unclipped systems with MLSD. We derive closed-form expressions of the diversity and the performance gain over the no-clipping case, as well as the optimal clipping ratio for a given system. We show that such diversity and performance gain are a result of both clipping and channel frequency-selectivity, and they do not occur on the AWGN and flat fading channels.We also present a new near-optimum detection algorithm that achieves the MLSD performance bound with a few iterations for the clipped OFDM systems with both high and low clipping levels on the AWGN and flat Rayleigh fading channels. For the frequency selective channel, we present a receiver design that utilizes the advantage of clipping to some extent. We consider rectangular QAM signaling in this paper. [ABSTRACT FROM PUBLISHER]

In this letter, we propose a generalized version of the window-based pseudonoise (PN) acquisition scheme verifying one or more best hypotheses rather than the best one per search window. The false alarm probability, detection probability, and mean acquisition time (MAT) of the proposed scheme are derived by using the well-known flow graph approach in additive white Gaussian noise (AWGN) and fading channels. Numerical results demonstrate that the acquisition performances are improved through the proposed generalization, particularly when the pilot energy to total received energy ratio (pilot Ec/Io) is low. [ABSTRACT FROM PUBLISHER]

This paper studies the mutual information transfer characteristics of a novel iterative soft interference cancellation (SIC) aided beamforming receiver communicating over both additive white Gaussian noise (AWGN) and multipath slow fading channels. Based on the extrinsic information transfer (EXIT) chart technique, we investigate the convergence behavior of an iterative minimum bit error rate (MBER) multiuser detection (MUD) scheme as a function of both the system parameters and channel conditions in comparison to the SIC aided minimum mean square error (SIC-MMSE) MUD. Our simulation results show that the EXIT chart analysis is sufficiently accurate for the MBER MUD. Quantitatively, a two-antenna system was capable of supporting up to K=6 users at Eb/Na=3dB, even when their angular separation was relatively low, potentially below 20deg. [ABSTRACT FROM PUBLISHER]

Location awareness is one of the essential characteristics of cognitive radios. In this paper, an innovative cognitive positioning system (CPS) that achieves accuracy adaptation in both indoor and outdoor environments is proposed as a step towards realization of location awareness in cognitive radios. The proposed CPS is composed of two modes; bandwidth determination and enhanced dynamic spectrum management (EDSM). Bandwidth determination equations are derived through Cramer-Rao Lower Bound (CRLB) for both additive white Gaussian noise (AWGN) and multipath channels. An EDSM system providing the optimum available bandwidth to the CPS is proposed. Overlay spectrum access based EDSM (O-EDSM) and hybrid overlay and underlay spectrum access based EDSM (H-EDSM) are two schemes that are introduced. A switching mechanism that manages the transition between underlay and overlay spectrum usage modes for the H-EDSM algorithm is presented. The theoretical analysis of the mechanism is carried out using Two-slope model. Simulation results, challenges and complexity options for the implementation of the CPS are outlined. Our study reveals the existence of a trade-off between the accuracy and complexity in the cognitive positioning systems. [ABSTRACT FROM PUBLISHER]

This letter provides closed-form expressions for the outage probability, the average level crossing rate (LCR) and the average fade duration (AFD) of a dual diversity selection combining (SC) system exposed to the combined influence of the cochannel interference (CCI) and the thermal noise (AWGN) in Nakagami fading channel. The branch selection is based on the desired signal power SC algorithm with all input signals assumed to be independent, while the powers of the desired signals in all diversity branches are mutually equal but distinct from the power of the interference signals. The analytical results reduce to known solutions in the cases of an interference-limited system in Rayleigh fading and an AWGN-limited system in Nakagami fading. The average LCR is determined by an original approach that does not require explicit knowledge of the joint PDF of the envelope and its time derivative, which also paves the way for similar analysis of other diversity systems. [ABSTRACT FROM PUBLISHER]

We consider a mixed-signal ultra wideband (UWB) direct-sequence spread-spectrum (DS-SS) receiver. The receiver expands the signal over a basis set, and then operates on the basis coefficients. An analog computation of the basis coefficients efficiently parallelizes the signal for digital processing, relaxing the sampling requirements and enabling parallel digital processing at a much lower rate. Bit error rate (BER) performance is evaluated, as well as distortion due to basis truncation. The parallel processing also facilitates efficient interference mitigation. Various models for interference including adjacent multicarrier interference are developed and analyzed. Several simulation results confirm the analysis and provide insight into quantization error, basis truncation distortion, and the impact of the UWB channel. We show that in the AWGN uncoded case for SNR < 10 dB, the receiver can be operated at almost half the Nyquist rate with only 1 dB of performance loss. Moreover, when a common convolutional code is employed, the aggregate sampling rate can be 37.5% lower than Nyquist rate with little loss in receiver performance [ABSTRACT FROM PUBLISHER]

We study pulse-based ultra wideband (UWB) communications over multipath channels using asynchronous spread spectrum (SS) multiple access (MA) based on time-hopping (TH) and pulse position modulated (PPM) signals. More specifically, we analyze the signal-to-interference (SIR) degradation in the presence of additive white Gaussian noise (AWGN), multi-user interference (MUI), and dense multipath effects (DME) with line-of-sight (LOS) and non-line-of-sight (NLOS). In particular, we define a degradation margin factor for the combined MUI and multipath effects and also find an expression for the maximum number of simultaneous radio links Nu in terms of the operating SIR, the SS processing gain, and the bit transmission rate Rb. We consider both cases with perfect and imperfect power control. [ABSTRACT FROM PUBLISHER]

This paper considers signaling schemes for heterogeneous ultrawideband communications networks that contain both coherent (rake) and transmitted-reference (TR) receivers. While coherent receivers are capable of receiving TR signals, they do so with a 3 dB penalty, because they cannot make use of the energy invested into the reference pulse. We propose a new signaling scheme that avoids this drawback, by encoding redundant information on the reference pulse. The resulting scheme does not affect the operation of a TR receiver, while recovering the 3 dB penalty and furthermore providing an additional 1.7 dB coding gain to a coherent uncoded binary scheme. This can be explained by interpreting the scheme as a trellis-coded modulation. We also provide an alternative implementation that can be viewed as a recursive systematic convolutional encoder. Combining this version further with a simple forward error correction encoder results in a concatenated code that can be decoded iteratively, providing a bit-error rate of 10-3 at 2.8 dB signal-to-noise ratio in additive white Gaussian noise. The convergence behavior of this iterative code is analyzed by using extrinsic information transfer charts. [ABSTRACT FROM PUBLISHER]

In this paper, a multiple-rate, multicarrier direct- sequence code-division multiple-access (MC/DS-CDMA) system with the use of two-dimensional orthogonal variable spreading factor (2-D OVSF) codes is studied. They are the first 2-D codes that can be generated by the tree structure commonly used by one-dimensional OVSF codes in wideband-CDMA. The 2-D OVSF codes also preserve orthogonality among code matrices with different spreading factors in the code tree. The performances of the proposed system with RAKE receivers employing equal-gain and maximal-ratio combining methods are analyzed and compared. Our results show that our multiple-rate MC/DS-CDMA system with the 2-D OVSF codes is more suitable for a non-fading additive white Gaussian noise channel or a Rician weak-fading channel. However, in a Rayleigh fading channel, the system performance gets worse because the orthogonality of the 2-D OVSF codes is destroyed by the strong fading effect. [ABSTRACT FROM PUBLISHER]

In this paper, we propose a new pilot tone placement scheme and a novel pilot sequence design to estimate the carrier frequency offset (CFO) in OFDM systems. Unlike the conventional approach in which isolated pilot tones are used, we cluster two pilot tones together as a group and these tone groups are equally spaced along the subcarrier axis. The pilot sequence of each group is carefully designed, in which the pilot symbol on the left side in each cluster is made antipodal to the one on the right. The performance in terms of the signal-to-interference ratio (SIR) can be significantly improved by the proposed scheme. Theoretical analysis and simulation results show that the clustered pilot tones can give a substantially lower variance of CFO estimate than that of the isolated pilot tones. For a given CFO, it demonstrates that the variance of the estimate can be reduced by 3 - 14 dB. The bit error rate (BER) is reduced by about 1 dB both in AWGN and multipath fading channels in the simulated cases [ABSTRACT FROM PUBLISHER]

An exact expression for the bit error rate of a synchronous maximum-likelihood receiver used to detect a bandlimited binary phase-shift keying (BPSK) signal in the presence of a similar BPSK signal, additive white Gaussian noise and imperfect carrier phase recovery is derived. The users are assumed to employ pulse-shaping filters whose impulse responses satisfy Nyquist's first criterion. The receiver's error probability is compared with that of the perfect phase recovery case to show its sensitivity to carrier phase errors. It is shown that the receiver is more robust to carrier phase mismatch at high signal-to-interference ratios (SIR's) than low SIR's [ABSTRACT FROM PUBLISHER]

To mitigate intercarrier interference (ICI), a two-path algorithm is developed for multicarrier communication systems, including orthogonal frequency division multiplexing (OFDM) systems. The first path employs the regular OFDM algorithm. The second path uses the conjugate transmission of the first path. The combination of both paths forms a conjugate ICI cancellation scheme at the receiver. This conjugate cancellation (CC) scheme provides (1) a high signal to interference power ratio (SIR) in the presence of small frequency offsets (50 dB and 33 dB higher than that of the regular OFDM and linear self-cancellation algorithms (Y. Zhao and S.-G. Haggman, 2001), (J. Armstrong, 1999), respectively, at DeltafT = 0.1% of subcarrier frequency spacing); (2) better bit error rate (BER) performance in both additive white Gaussian noise (AWGN) and fading channels; (3) backward compatibility with the existing OFDM system; (4) no channel equalization is needed for reducing ICI, a simple low cost receiver without increasing system complexity. Although the two-path transmission reduces bandwidth efficiency, the disadvantage can be balanced by increasing signal alphabet sizes [ABSTRACT FROM PUBLISHER]

Maximum likelihood detector algorithms are developed for the matrix of transmitted symbols in a multiuser system in which the received signal is the sum of K cochannel continuous phase modulated (CPM) signals and additive white Gaussian noise. We illustrate that the maximum likelihood matrix detector, which provides optimum detector performance, consists of K sets of front-end matched filters followed by a Viterbi algorithm. We also derive two reduced complexity receivers, demonstrating through simulation that they perform within 1-2 dB of the optimal while substantially reducing complexity. The paper demonstrates how performance can be traded off against complexity, giving particular attention to cochannel Gaussian minimum shift keyed (GMSK) signals [ABSTRACT FROM PUBLISHER]

We present a simple design technique for extended irregular repeat-accumulate (eIRA) codes for flat Rayleigh fading channels, using simple channels as surrogates in the design. We show that eIRA codes designed for the burst-erasure channel (BuEC) or the burst-erasure channel with AWGN (BuEC-G) achieve essentially the same performance over Rayleigh fading channels as codes designed for the fading channel. Thus, to design good codes for Rayleigh fading channels, instead of implementing the complex design procedures targeted, specifically for this channel, we propose the simple approach of designing codes over surrogate channels, the BuEC or the BuEC-G. We also show that eIRA codes designed for the BuEC enjoy the advantage of efficient encodability and a lower error-rate floor. Finally, we demonstrate that it is the distribution of the number of faded bits per codeword which determines the difference between correlated and uncorrelated fading channel performance. Perfect channel state information is assumed in this paper. [ABSTRACT FROM PUBLISHER]

In this paper, we evaluate and enhance the performance of a Forward Error Correction (FEC) scheme for IEEE 802.11 Medium Access Control (MAC). A novel retransmission combining technique is proposed to enhance the performance of the MAC-level FEC scheme.,We also identify the problem with the IEEE 802.11a physical (PHY) layer when it is used with the MAC-level FEC. A new PHY frame format, backward compatible with the original format, is proposed to resolve the problem. Finally, we analytically evaluate the error performance of the MAC-level FEC, and its enhanced performance via retransmission combining and new 802.11a PHY frame format in AWGN environment. Additionally, we present and discuss the results from simulations using TCP/UDP traffic in more realistic channel environments. [ABSTRACT FROM PUBLISHER]

A novel algorithm called velocity estimation using the power spectral density (VEPSD), which uses the Doppler spread in the received signal envelope to estimate the velocity of a mobile user (MU), is introduced in this paper. The Doppler spread is estimated using the slope of the power spectral density (PSD) of the received signal envelope. The performance of the VEPSD algorithm is evaluated in both Rayleigh and Rician fading environments. The sensitivity of the estimation error to additive white Gaussian noise (AWGN), Rice factor (K), and the angle of arrival of the line-of-sight (LOS) component is analyzed and compared with the level crossing rate (LCR) and covariance-based velocity estimators. Simulation results show that VEPSD estimates the velocity of MUs accurately. It is also shown that VEPSD can be used for velocity estimation under nonisotropic scattering and is well suited for next-generation wireless systems (NGWSs). [ABSTRACT FROM PUBLISHER]

A new signal-space partitioning method is proposed for the calculation of transition probabilities of arbitrary two-dimensional (2-D) signaling with polygonal decision regions. This enables straightforward formulation of exact bit error probability, symbol error probability, mutual information (MI), and error probabilities of individual bits in the constellation. Previously published symbol error rate (SER) results for additive white Gaussian noise (AWGN) and various fading channels, diversity reception, and imperfect channel estimation (ICE) can all be used in conjunction with the new signal-space partitioning method to obtain the desired performance measures. Numerical results of the bit error rates (BERs) of several 8-ary and 16-ary constellations in AWGN and in Rayleigh, Ricean, and Nakagami fading channels with diversity reception are presented. Pilot-symbol-assisted 2-D signaling is also studied. The proposed new approach is both exact and numerically efficient. It provides new insights into the effect of bit mapping, constellation parameter optimizations, unequal error protection (UEP) at different bit positions, and performance comparisons of different constellations. [ABSTRACT FROM PUBLISHER]

Interference from coexisting narrowband services is a critical factor affecting performance of ultra-wideband (UWB) radio communications. There is clearly a need to quantify interference and compare UWB systems on that basis. In this paper, we develop a general Rake reception model along with a unifying transmission framework for low duty-cycle UWB multiple access encompassing existing and novel spreading codes, including direct sequence, digital singleand multi-carrier, time-hopping, and combinations of them. Our unifying framework relies on a digital model, which leads to closed-form performance analysis expressions. Different from existing alternatives that require oversampling, our general model is developed directly from the samples of the Rake receiver output and allows for various Rake finger delay selections. Signal-to-interference-plus-noise ratio analysis and simulations are carried out to assess the relative merits of several UWB systems in the presence of narrowband interference, multipath and additive white Gaussian noise, for both matched-filter and minimum mean square error Rake receivers. [ABSTRACT FROM PUBLISHER]

Nearly all multirate detection schemes proposed for IS-95 and CDMA2000 rely heavily on the assumption of an additive white Gaussian noise (AWGN) channel model. The idea is to exploit the property of symbol repetition and to test, in one way or another, the equality of the mean value of the received samples within an appropriate data block. In a fast fading environment as encountered in cellular mobile systems, however, the time-varying channel gain destroys the block-wise constant-mean structure in the received process lending the use of an AWGN channel model no longer justified. It is the correlation structure, rather than the mean value, of the received process that carries the information for multirate detection. To fully reflect this nature, we formulate the problem of multirate detection in a completely new framework ending up with a simple maximum-likelihood-based algorithm to examine the canonical correlation of the received sequence. The new test possesses the property of constant false-alarm rate thereby allowing for thresholds to be analytically determined. Computer results are also presented to demonstrate the superior performance of the new algorithm in Rayleigh fading. [ABSTRACT FROM PUBLISHER]

We derive an exact bit-error probability (BEP) expression for coherent detection of binary signals with optimum combining in wireless systems in the presence of multiple cochannel interferers and thermal noise. A flat Rayleigh fading environment with space diversity, uncorrelated equal-power interferers, and additive white Gaussian noise is considered. The approach is to use the chain rule of conditional expectation together with the joint probability density function (pdf) of the eigenvalues of the interference correlation matrix. This joint pdf is related to the Vandermonde determinant. Let NA denote the number of antennas and NI the number of interferers. We consider both the cases of an overloaded system, in which NI≥NA, and an underloaded system, in which NI

A general joint source-channel rate allocation scheme for a dependent video coding environment is presented. It is assumed that the source coders rely on motion-compensated prediction, and thus error propagation will contribute significantly to the degradation of the reconstructed video. The rate allocation methodology is based upon generation of operational distortion-rate characteristics. In order to reduce computational complexity, these surfaces and the channel code performance are modeled. An analytic method for computing optimal rate allocation across frames in a video sequence is then introduced. Comparisons are made between equal error protection and unequal error protection over video frames. Results are shown for H.263 compressed video used in conjunction with rate-compatible punctured convolutional codes. [ABSTRACT FROM PUBLISHER]

This paper deals with a modified version of a blind adaptive multiuser detector for direct sequence code-division multiple-access (DS-CDMA) wireless communication systems which is named precombining blind adaptive multiuser detector (PBA-MUD). The main properties of this receiver are low complexity, multiple-access interference mitigation and remarkable near-far resistance in time-varying multipath fading scenarios. In particular, the PBA-MUD is based on a blind adaptive algorithm that allows the detector to avoid deep fading impairments and the cancellation of the desired signal due to signature sequence mismatch. Nevertheless, such a receiver experiences a performance degradation in fast fading channels. In order to overcome this problem, a window reprocessing technique (Del Re et al., 2001) was introduced which yielded a new blind detection algorithm. It will be shown that this receiver, together with this new algorithm [precombining reprocessing window blind adaptive multiuser detector (PWBA-MUD)] allows a high performance without requiring training sequences or the knowledge of the interfering signature waveforms in multipath fast fading channels. [ABSTRACT FROM PUBLISHER]

This paper presents a space-time turbo (iterative) equalization method for trellis-coded modulation (TCM) signals over broadband wireless channels. For fixed wireless systems operating at high data rates, the multipath delay spread becomes large, making it impossible to apply trellis-based equalization methods. The equalizer proposed here consists of a broadband beamformer which processes antenna array measurements to shorten the observed channel impulse response, followed by a conventional scalar turbo equalizer. Since the applicability of trellis-based equalizers is limited to additive white noise channels, the beamformer is required to preserve the whiteness of the noise at its output. This constraint is equivalent to requiring that the finite-impulse response (FIR) beamforming filters must have a power complementarity property. The power complementarity property imposes nonnegative definite quadratic constraints on the beamforming filters, so the beamformer design is expressed as a constrained quadratic optimization problem. The composite channel impulse response at the beamformer output is shortened significantly, making it possible to use a turbo equalizer for the joint equalization and decoding of trellis modulated signals. The proposed receiver structure is simulated for two-dimensional TCM signals such as 8-PSK and 16-QAM and the results indicate that the use of antenna arrays with only two or three elements allows a large decrease in the channel signal-to-noise ratio needed to achieve a 10-4 bit-error rate. [ABSTRACT FROM PUBLISHER]

The performance of digital cellular radio systems employing maximal ratio combining diversity is analyzed in a flat-fading channel with cochannel interference and additive white Gaussian noise. It is assumed that the desired signal may experience Rice fading (due to the presence of a line-of-sight component), while the interferers are Rayleigh-faded and may have similar or dissimilar average powers. Exact expressions are derived for the average symbol-error probability of M-ary phase-shift keying modulation in the presence of multiple independent Rayleigh-faded interferers. [ABSTRACT FROM PUBLISHER]

The convergence behavior of linear parallel interference cancellation is investigated. Especially the so-called ping-pong effect, where the bit-error rate performance is found to oscillate between two different convergence patterns is studied in detail. This effect is shown to be a direct consequence of the extreme eigenvalues of the correlation matrix, allowing for an analytical approach. Intervals for the dominating eigenvalues within which ping-pong effects can occur are specified and illustrated by examples. It is shown that the decision statistic for traditional parallel cancellation will always exhibit oscillating behavior with either short or long codes. Relaxation factors, leading to weighted cancellation, are shown to be effective for alleviating oscillations and ping-pong effects at the expense of convergence rate. Asymptotic analysis for large systems is applied to uncover the convergence behavior for long code systems. [ABSTRACT FROM PUBLISHER]

A novel pseudo noise code acquisition combined with the newly proposed adaptive sampling rate and threshold control (ASTC) algorithm is derived for low-power spread-spectrum systems with complementary metal-oxide-semiconductor implementations. Low-power performance can be achieved by reducing the sampling rate of the proposed system while maintaining the system performance. The sampling rate is dynamically updated due to the change of the channel noise level. Under the assumptions that the channel is additive white Gaussian noise slow-fading channel, up to 74.3% reduction in power consumption compared to the conventional fixed-sampling rate and fixed-threshold architecture is demonstrated with insignificant increase of system complexity. The proposed architecture can be applied to the design of low-power and controllable-performance spread-spectrum communication systems. [ABSTRACT FROM PUBLISHER]

This paper analyzes the outage probability of multi-branch dual-hop decode-and-forward (DF) cooperative relaying systems over non-identical Nakagami/Nakagami fading channels. The paper derives an exact closed-form expression for the outage probability with maximum ratio combining considering both the effect of co-channel interference (CCI) and addictive white Gaussian noise. The results of this paper show that a DF cooperative relaying system is more vulnerable to noise than interference for low and moderate SINR, whereas it is more susceptible to interference than noise for high SINR and high diversity order. Moreover, this paper shows that the robustness of the destination against CCI is more important than that of the relay. The results of this paper are verified by Monte Carlo simulations. [ABSTRACT FROM PUBLISHER]

Frequency offset estimation for time-hopping (TH) ultra-wide-band (UWB) is addressed in the literature by relying on an AWGN assumption and by exploiting a periodic preamble appended to each packet. In this paper we generalize these techniques with two aims. First, we identify a solution which does not rely on any periodic structure, but can be implemented with a generic TH format. Second, we identify a solution which is robust to multiple access interference (MAI) by assuming a Gaussian mixture (GM) model for MAI. In fact, GMs have recently been identified as good descriptors of UWB interference, and they provide closed form and limited complexity results. With these ideas in mind, we build a data aided maximum likelihood (ML) estimator. The proposed ML solution shows quasi optimum performance in the Cramer-Rao bound sense, and proves to be robust in meaningful multiple user scenarios. [ABSTRACT FROM PUBLISHER]

This paper is concerned with classification of quadrature amplitude modulation (QAM) signals by using noncoherent maximum likelihood (ML) schemes, which are robust to frequency mismatch and phase shift. To reduce the high complexity involved in optimal noncoherent ML classification, we propose two suboptimal alternatives that perform well under various conditions. In particular, one suboptimal scheme, derived from high signal-to-noise ratio (SNR) approximation, is shown to provide close-to-optimal performance. We also present a general performance analysis for noncoherent ML classification of K types of QAM constellations, ending up with simplified integration for K <= 5 and a mixed formula for K > 5. In addition, we investigate the asymptotic behavior of noncoherent ML classification assuming the number of samples used increases to infinity. The asymptotic performance of optimal as well as suboptimal noncoherent ML classification is established. [ABSTRACT FROM PUBLISHER]

In cognitive radio networks, each cognitive transmission process typically requires two phases: the spectrum sensing phase and data transmission phase. In this paper, we investigate cognitive transmissions with multiple relays by jointly considering the two phases over Rayleigh fading channels. We study a selective fusion spectrum sensing and best relay data transmission (SFSS-BRDT) scheme in multiple-relay cognitive radio networks. Specifically, in the spectrum sensing phase, only the initial spectrum sensing results, which are received from the cognitive relays and decoded correctly at a cognitive source, are selected and used for fusion. In the data transmission phase, only the best relay is utilized to assist the cognitive source for data transmissions. Under the constraint of satisfying a required probability of false alarm of spectrum holes (for the protection of the primary user), we derive an exact closed-form expression of the spectrum hole utilization efficiency for the SFSS-BRDT scheme, which is used as a measure to quantify the percentage of spectrum holes utilized by the cognitive source for its successful data transmissions. For the comparison purpose, we also examine the spectrum hole utilization efficiency for a fixed fusion spectrum sensing and best relay data transmission (FFSS-BRDT) scheme, where all the initial spectrum sensing results are used for fusion without any refined selection. Numerical results show that, under a target probability of false alarm of spectrum holes, the SFSS-BRDT scheme outperforms the FFSS-BRDT scheme in terms of the spectrum hole utilization efficiency. Moreover, the spectrum hole utilization efficiency of the SFSS-BRDT scheme always improves as the number of cognitive relays increases, whereas the FFSS-BRDT scheme's performance improves initially and degrades eventually after a critical number of cognitive relays. It is also shown that a maximum spectrum hole utilization efficiency can be achieved through an optimal allocation of the time durations between the spectrum sensing and data transmission phases for both the FFSS-BRDT and SFSS-BRDT schemes. [ABSTRACT FROM PUBLISHER]

In this paper, we propose a distributed turbo-like coding scheme for a multi-source relay scenario where multiple sources communicate with a destination with the help of a common relay, which uses the decode-and-forward strategy and operates in half-duplex mode. The proposed distributed code can be viewed as a serially concatenated code (SCC). Thus, at the destination decoding is performed in an iterative fashion which resembles the decoding of a classic SCC. We consider the two scenarios where the sources transmit over orthogonal channels and where they do not. For the latter, interleave-division multiple-access is used for multiuser detection. For both scenarios we optimize the transmission time allocated to the sources and to the relay and compute the achievable rates. The proposed scheme achieves very low error rates and offers significant performance gains with respect to non-cooperation, even for a very large number of sources. Furthermore, it provides a high flexibility in terms of code rate, number of sources, overall system rate and error protection. [ABSTRACT FROM PUBLISHER]

This paper proposes a new concept of multiple-input multiple-output (MIMO) detection, aiming at minimizing the average computational cost. The detection methods are adapted according to the estimated channel state information to deliver just-acceptable error rate (JAER) performance. Error rate models for two popular MIMO detection algorithms are derived given the channel matrix. From these models, a channel-adaptive-MIMO (CA-MIMO) receiver with detector-switching strategies is proposed. Simulation results demonstrate that the proposed CA-MIMO detector meets the JAER criterion efficiently. Compared with the sophisticated Sphere Search (SS) MIMO detector, the average saving at moderate signal-to-noise ratio (SNR) is around 58% to 72%, depending on different modulation alphabets. At high SNR, the CA-MIMO detector almost always switch to Zero-Forcing (ZF) detection, where the complexity is several orders lower than the SS detector. [ABSTRACT FROM PUBLISHER]

In this paper, we combine opportunistic transmission and source/destination cooperation in a decode-and-forward (DF)-based two-hop cooperative diversity network consisting of multiple source-destination pairs and a single relay. Firstly, we consider a special scenario with two source-destination pairs. For this network, there are two possible strategies in each hop: no-cooperation or cooperation. Considering the combination of the two strategies in two cascaded hops, we investigate four different end-to-end transmission strategies, and we find that the four strategies complement one another depending on channel conditions in terms of outage performance. To maximize the mutual information, we propose an optimum joint selection of source-destination pair and end-to-end transmission strategy. Then we show that the optimum joint selection scheme can be simplified without loss of outage performance, and derive its exact outage probability in closed-form. Secondly, we generalize the proposed transmission strategy into a scenario with multiple source-destination pairs. For this network, we first propose an optimum end-to-end transmission strategy to maximize the mutual information, then a suboptimum end-to-end transmission strategy to reduce the signaling overhead and computational complexity. For the suboptimum strategy, we derive the outage probability and diversity order. [ABSTRACT FROM PUBLISHER]

In this letter, a design metric of the interleaver is derived from the union bound of the bit error rate (BER) of linearly precoded orthogonal frequency division multiplexing (OFDM) systems, assuming that the power-delay profile of the channel is known at the transmitter. A new cyclic shift interleaving scheme is proposed to improve the BER performance of OFDM systems. The proposed interleaving scheme is parameterized, and it incorporates existing interleaving schemes. The simulation results indicate that the proposed interleavers outperform the existing interleavers by up to 1.5 dB in a practical scenario. [ABSTRACT FROM PUBLISHER]

The number of coefficients for estimating time-varying channel is generally much larger than the maximum number of observable data. Linearly time-varying channel (LTV) models have been widely used as a way of reducing the number of these channel parameters. This approach allows estimation using the channel slope and channel frequency response between symbols. A fatal problem with this approach, however, is that before it is even initiated, the estimated channel in the frequency domain has already been degraded by inter-carrier interference (ICI). We demonstrate this by analyzing the channel estimation mean square error (MSE) and signal to interference and noise ratio (SINR) after ICI cancellation with respect to the LTV model. We then propose a new channel estimation technique using dual-ICI cancellation which resolves this problem by pre-canceling ICI for the channel estimation using the channel of the previous symbol and post-canceling ICI for symbol detection with the more accurate channel information. The process is initialized using the first symbol as a preamble to estimate the channel in the time domain. Performance comparisons of MSE and SINR show that the proposed method is better suited to fast-fading channels than the conventional method, where ICI dominates the interference and noise power. [ABSTRACT FROM PUBLISHER]