In this contribution, time varying threshold sequential detectors are employed for energy detection-based spectrum sensing in low- SNR regimes. Sequential detection is proven to be faster (on average) than any other multi-sample detector for a set of given probabilities of detection and false-alarm. In this report, exact performance of a sequential detector for spectrum sensing is analyzed using the direct method. The theoretical results presented herein are verified with Monte-Carlo simulations. It is shown that for a SNR of -10dB, among tests with Wald and triangular thresholds with similar probabilities of mis-detection and false- alarm, triangular performs 54% faster in terms of maximum detection time (90 percentile).

In this work, we present an accurate analysis of the probability of successful transmission in a slotted Aloha network with an arbitrary topology, provided that the channel can be accurately modeled as Rayleigh block fading channels. The obtained expression also takes into account the effect of different physical layer parameters such as modulation and coding methods. However, its computational complexity grows quickly as the network size increases. To address this, we also present an accurate approximation method in which the probability of success for a link is predicted by considering only a subset of the interfering nodes. A sufficient condition for the accuracy of this prediction is also presented. The validity of the proposed methods are verified by a series of simulations.

There are few analytical formulas that can be used for calculating the block error rate (BLER) in block fading channels. Thus, an estimate of the BLER is often obtained using numerical methods. One such method is the threshold method which assigns 0 or 1 to the instantaneous BLER given the signal to noise ratio (SNR) level. It has been shown that utilizing such a method results in an accurate approximation of the BLER in Nakagami-m block fading channels for a wide range of m. In this work, we consider a recently proposed simple method of obtaining the threshold and study the effect of adopting different physical layer and channel parameters on that threshold. We show that, while the value of this threshold depends on the modulation, coding, and block size, it is almost unaffected by the m parameter of Nakagami-m channels for a wide range of practical values. In addition, for a given modulation and coding method, the threshold is shown to be a simple function of block size. As a result, the computational complexity required to obtain the threshold can be significantly reduced.

In this paper we formally analyze the interleaver and code design for QAM-based BICM transmissions using the binary reflected Gray code. We develop analytical bounds on the bit error rate and we use them to predict the performance of BICM when unequal error protection (UEP) is introduced by the constellation labeling. Based on these bounds the optimum design of interleaver and code is found, and numerical results for representative configurations are presented. When the new design is used, the improvements may reach 2 dB, and they are obtained without any increase on the transceiver's complexity. We also introduce the concept of generalized optimum distance spectrum convolutional codes, which are the optimum codes for QAM-based BICM transmissions.

With few exceptions, an analytical closed-form expression for the block error rate (BLER) is lacking in block fading channels. Thus, the BLER is often obtained by numerical methods, such as Monte-Carlo simulations, resulting in additional computational complexity. In this paper, we propose a single-parameter analytical approximation for the BLER in flat, block-fading Nakagami-m channels, which significantly reduces the computational overhead. The low computational cost of the approximation makes it feasible to include the BLER in the objective function of larger optimization problems.

In this paper we present a general methodology for the interleaver and code design for QAM-based BICM transmissions. We develop analytical bounds on the bit error rate and we use them to predict the performance of BICM when unequal error protection (UEP) is introduced by the constellation labeling. Based on these bounds, the optimum design of interleaver and code is presented. The improvements obtained reached 2 dB for the analyzed cases, and are obtained without complexity increase. Although previous works noted the influence of the interleaver design and the UEP, to the best of our knowledge, this paper is the first to analyze formally this problem for BICM transmissions.

In this tutorial paper we analyze the capacity of bit-interleaved coded modulation (BICM) with quadrature amplitude modulation (QAM) constellations, and we pay special attention to different bit-to-symbol labeling strategies. The relation between the BICM capacity and the capacity of other CM schemes such as trellis coded modulation (TCM) and multilevel codes (MLC) is analyzed. Motivated by the fact that for BICM with some particular labelings, the same Eb/No maps to more than one BICM capacity value, we study the relation between the capacity and Eb/No. In particular, we present some analytical results on this relation, and we also give an intuitive explanation for the somehow contradictory behavior of these curves.

In this letter, we discuss the problem of positioning an unknown number of amplify-and-forward relays or point scatterers using a wireless network of wideband transceivers. We propose to model this positioning problem as an assignment problem. In doing so, we avoid the problems associated with an exhaustive gridsearch approach, which has recently been proposed to solve the problem. Numerical simulations show promising results in terms of performance and robustness.

Orthogonal Frequency Division Multiplexing has become a key element of todays wireless communication systems. However, its sensitivity to oscillator phase noise is responsible for common phase error (CPE) and inter-carrier interference (ICI) which greatly degrades the overall system performance. In this contribution, we address the problem of reducing the effects of phase noise in an OFDM system operating over a frequency selective fading channel. We propose a method for jointly estimating the channel and CPE in a first step and removing ICI in a second step, using a power series expansion of the phase noise process. The algorithm is simulated on both coded and uncoded systems with phase noise over a fading channel.

In this invited paper, we discuss positioning of road-users in a traffic environment. Once accurate and reliable position information can be made available, many interesting and innovative telematics applications may be realized. Some of the problems facing radio-frequency (RF) based positioning systems today include blocked RF channels and multipath propagation, complicating distance estimation between network members and between network members and the infrastructure. In this work, we propose a novel estimator of range operating on an ultra-wideband (UWB) physical layer. This distance estimator has been designed specifically with a novel type of positioning algorithm in mind, based on the method of projections onto convex sets. Also, we investigate the feasibility of positioning users that are not members of the telematics network based on arbitrary transmitted RF signals. Numerical simulations, using channel models recently adopted by the IEEE 802.15.4a working group, confirm the robustness and accuracy of the proposed approach in the presence of both non-line-of-sight (NLOS) channels, and severe multipath propagation

High data rates, high spectral efficiency, flexibility, and low delays over the air interface will be important features in next-generation wireless systems. The overall challenge will be packet scheduling and adaptive radio transmission for multiple users, via multiple antennas and over frequency-selective wideband channels. This problem needs to be structured to obtain feasible solutions. The basic simplifying assumptions used here are clustering of antennas into cells, orthogonal transmission by use of cyclic-prefix orthogonal frequency-division multiplexing (OFDM) and a time-scale separation view of the total link adaptation, scheduling and intercell coordination problem. Based on these assumptions, we survey techniques that adapt the transmission to the temporal, frequency, and spatial channel properties. We provide a systematic overview of the design problems, such as the dimensioning of the allocated time-frequency resources, the influence of duplexing schemes, adaptation control issues for downlinks and uplinks, timing issues, and their relation to the required performance of channel predictors. Specific design choices are illustrated by recent research within the Swedish Wireless IP program and the EU IST-WINNER project. The presented results indicate that high-performance adaptive OFDM transmission systems are indeed feasible, also for challenging scenarios that involve vehicular velocities, high carrier frequencies, and high bandwidths.

A major disadvantage with fixed modulation (nonadaptive) on channels with varying signal-to-noise ratio (SNR) is that the bit-error-rate (BER) probability performance is changing with the channel quality. Most applications require a certain maximum BER and there is normally no reason for providing a smaller BER than required. An adaptive modulation scheme, on the contrary, can be designed to have a BER which is constant for all channel SNRs. The spectral efficiency of the fixed modulation is constant, while it, in general, will increase with increasing channel SNRs for the adaptive scheme. This in effect means that the average spectral efficiency of the adaptive scheme is improved, while at the same time the BER is better suited to the requirement of the application. Thus, the adaptive link becomes much more efficient for data transmission. The major disadvantage is that the transmitter needs to know the channel SNR such that the best suitable modulation is chosen and the receiver must be informed on the used modulation in order to decode the information. This leads to an increased overhead in the system as compared with a fixed modulation system. In this paper, we introduce adaptive modulation systems by presenting some of the simpler adaptive quadrature amplitude modulation schemes and their performance for both perfectly known and predicted channels.

We consider intelligent transport systems which aim at providing more safety to vehicles by forming a wireless ad-hoc network among the vehicles. To provide safety, each vehicle informs its neighbors about its status by periodically broadcasting data packets. For such a network to provide safety, having an efficient broadcast MAC layer that can provide a low end-to-end delay, regular packet receptions, and fast adaptation to topology changes is essential. We consider CSMA-based broadcast MAC protocols and argue that schemes based on RTS-CTS handshake are not suitable for this type of networks as they suffer from high overhead, long end-to-end delay, and poor adaptation to topology changes. The IEEE 802.11 approach of ignoring RTS-CTS handshake for broadcast packets solves the overhead, delay, and topology adaptation problems. However, it suffers from the hidden terminal problem resulting in low delivery ratio. To address this problem, we propose the long range busy tone (ERBT) protocol. In this protocol, similar to IEEE 802.11, we ignore the RTS-CTS handshake completely. To provide protection against the hidden terminals, the transmitter sends a packet along with a longer range busy tone to block the hidden terminals. The simulation results confirm that this method offers higher delivery ratio and less inter-packet delay compared to the simple broadcast scheme

In this paper, we address the issue of imperfect channel estimation in coded systems on fading channels. Since performance of channel codes is influenced in different ways by different components of channel-estimation errors, we develop a simplified model which separates the estimation errors of a Wiener-filtered received signal into the amplitude error and the phase error. Based on the model, we derive tight bounds on component error variances. Moreover, we prove that the classical Wiener filter results in a biased estimate of the channel amplitude. We also show that the probability of having a phase-estimation error large enough to cause decision errors in the receiver is significant. Using our model, we derive an approximate upper limit on the optimum pilot-symbol spacing and approximate lower limit on bit-error rate performance of coded systems with a given pilot-symbol separation. The proposed model and derivations are confirmed by extensive simulations.

We address the subject of reducing the impact of phase noise on a QAM-OFDM system transmitting over an AWGN channel. Phase noise is known to have two effects on OFDM systems, rotating each symbol by a different common phase rotation (CPR) and producing an intercarrier interference term (ICI) that adds to the channel noise. We present two novel algorithms that remove CPR and reduce the amount of ICI using different approaches to the ICI problem. Their performance in terms of bit error rate (BER) are evaluated by simulation using symbol by symbol or maximum likelihood sequence detection (MLSD). The results show that these algorithms can significantly reduce the BER floor of the system while still maintaining an acceptable throughput.

The main focus of this paper is to investigate how the co-operative nature of an ad-hoc sensor network can be exploited in order to reduce the complexity of accurate node locationing algorithms in sensor networks. We propose a novel approach to target tracking, called node-aided tracking. The node-aided tracking algorithm is assisted in the tracking process by the tracked nodes in the network, allowing for a reduction in filter complexity. This approach is shown to be sensitive to network delays. The effects of such delays are investigated, and simple countermeasures are presented that stabilize the algorithm. Further, we present a novel way of evaluating the performance of tracking filters used in automotive safety applications. This new measure, called the time margin difference, takes not only the mean-squared-error into account but also latency in providing location estimates and other important filter characteristics for a fair comparison between different tracking algorithms designed for automotive safety applications. At the end of the paper we present a brief comparison, based on simulation, between the node-aided tracking filter and a traditional high-order Kalman filter with respect to this new performance measure.

When adaptive modulation is used to counter short-term fading in mobile radio channels, signaling delays create problems with outdated channel state information. The use of channel power prediction will improve the performance of the link adaptation. It is then of interest to take the quality of these predictions into account explicitly when designing an adaptive modulation scheme. We study the optimum design of an adaptive modulation scheme based on uncoded M-quadrature amplitude modulation, assisted by channel prediction for the flat Rayleigh fading channel. The data rate, and in some variants the transmit power, are adapted to maximize the spectral efficiency, subject to average power and bit-error rate constraints. The key issues studied here are how a known prediction error variance will affect the optimized transmission properties, such as the signal-to-noise ratio (SNR) boundaries that determine when to apply different modulation rates, and to what extent it affects the spectral efficiency. This investigation is performed by analytical optimization of the link adaptation, using the statistical properties of a particular, but efficient, channel power predictor. Optimum solutions for the rate and transmit power are derived, based on the predicted SNR and the prediction error variance.

When adaptive modulation is used to counter short-term fading in mobile radio channels, signaling delays create problems with outdated channel state information. The use of channel power prediction improves the performance of the link adaptation. It is then of interest to take the quality of these predictions into account explicitly when designing an adaptive modulation scheme. We study the optimum design of an adaptive modulation scheme based on uncoded M-QAM modulation assisted by channel prediction for the flat Rayleigh fading channel. The data rate, and in some variants the transmit power, are adapted to maximize the spectral efficiency subject to average power and bit error rate constraints. The key issues studied here are how a known prediction error variance affects the optimized transmission properties such as the SNR boundaries that determine when to apply different modulation rates, and to what extent it affects the spectral efficiency. This investigation is performed by analytical optimization of the link adaptation, using the statistical properties of a particular but efficient channel power predictor. Optimum solutions for the rate and transmit power are derived based on the predicted SNR and the prediction error variance.

This paper gives a broad description of a broadband communication system using the 60 GHz frequency band. We describe the channel and its inherent problems such as interference situations and body shadowing. We introduce the use of virtual cellular networks (VNC) and multiple receiving antennas (MRA) in order to solve these problems resulting in an overlapping cellular architecture. We explain how the diversity of the channel can be used to increase reliability and performance. Finally, we simulate the ideal link performance of a VCN using two receiving antennas for a particular set of OFDM parameters.

This paper discusses a realistic turbo coding system with the signal phase which has not been perfectly estimated. We propose improved decoding algorithms for the situations when the residual phase error can be modelled by the Gaussian probability distribution and a Markov chain, a model which can be used in many actual phase estimators. It is shown that increasing the state space of the decoders can decrease the bit error probability.

The classical method for decoding turbo codes is the Bahl-Cocke-Jelinek-Raviv (1974) algorithm which, in the majority of existing papers, is analyzed under the assumption of perfect channel knowledge and synchronization. In reality, however, its properties change significantly when synchronization is not ideal. We show that the influence of timing synchronization offset can be modeled as a decreased effective signal-to-noise ratio, which provides results consistent with simulations. Furthermore, we present a general model for the soft bit output of the BCJR algorithm and apply it to the situation when the received signal is sampled with a timing offset. Finally, we use the derived properties of the algorithm in a simple timing synchronizer which is shown to outperform the classical maximum-likelihood non-data-aided symbol synchronizer without a need of using additional pilot symbols.

The optimum rate-compatible repetition convolutional (RCRC) codes with different parent encoder rates and different constraint lengths are presented. They are constructed according to the optimum distance spectrum (ODS) criterion. The obtained codes provide, e.g., significant throughput gains compared to simple repetition codes, when applied in hybrid type II automatic repeat request (ARQ) schemes.

New rate-compatible convolutional (RCC) codes with high constraint lengths and a wide range of code rates are presented. These new codes originate from rate 1/4 optimum distance spectrum (ODS) convolutional parent encoders with constraint lengths 7-10. Low rate encoders (rates 115 down to 1/10) are found by a nested search, and high rate encoders (rates above 1/4) are found by rate-compatible puncturing. The new codes form rate-compatible code families more powerful and flexible than those previously presented. It is shown that these codes are almost as good as the existing optimum convolutional codes of the same fates. The effects of varying the design parameters of the rate-compatible punctured convolutional (RCPC) codes, i.e., the parent encoder rate, the puncturing period, and the constraint length, are also examined. The new codes are then applied to a multicode direct-sequence code-division multiple-access (DS-CDMA) system and are shown to provide good performance and rate-matching capabilities. The results, which are evaluated in terms of the efficiency for Gaussian and Rayleigh fading channels, show that the system efficiency increases with decreasing code rate.

A parallel combinatory-orthogonal frequency division multiplexing (PC-OFDM) system is proposed and analyzed. The proposed system selects at each symbol interval a subset of the available subcarriers, and the selected subcarriers are modulated by points from an M-PSK signal constellation, PC-OFDM systems can be designed to have lower peak-to-average power ratio (PAPR), higher bandwidth efficiency, and lower bit error probability on Gaussian channels compared to ordinary OFDM systems. A bit mapping procedure using the Johnson (1993) association, together with a position algorithm for the PSK symbols, is proposed. Good analytical approximations of the BER for PC-OFDM systems are derived for AWGN and Ricean fading channels, and extensive simulation results are presented.

Direct-sequence code-division multiple access (DS-CDMA) is a popular wireless technology. In DS-CDMA communications, all of the users' signals overlap in time and frequency and cause mutual interference. In this paper a novel multi-user decoding scheme for coded synchronous DS-CDMA is proposed. The decoder performs joint successive interference cancellation (SIC) and Viterbi decoding (VD) in each step of the interference cancellation. In a conventional SIC scheme, SIC is performed first, followed by VD that uses the outputs of the last stage of the SIC scheme for metric calculation. Simulated bit error probability for the proposed scheme is presented and compared with bit error probability of a conventional scheme. A capacity increase of at least 4 compared to SIC followed by VD is reported in one example.

The performance for sequential decoding on Rayleigh fading channels has been obtained theoretically and by simulations. The results show that sequential decoding of long constraint length convolutional codes is a good candidate coding scheme for data oriented services in future mobile communication systems. The applicability of such a coding and decoding scheme for future mobile systems is discussed.

A parallel combinatory OFDM system (PC-OFDM) is proposed. The proposed system is able to obtain a higher bandwidth efficiency than an ordinary OFDM system. The bit error rate performance is analyzed and simulated. On a Gaussian channel and on known time invariant channels the PC-OFDM system can obtain lower bit error rates than ordinary OFDM. In some cases we can at the same time increase the bandwidth efficiency compared to an ordinary OFDM system. Further, the peak to mean power ratio of the PC-OFDM system is lower than for OFDM, which reduces the problem of designing linear power amplifiers.

Abstract: This paper presents multistage interference cancellation (IC) schemes for the uplink of multirate directsequence code division multiple access (DS/CDMA) systems.The performance is evaluated both analytically and via computer simulations for two multiple data rate schemes, mixed modulation and parallel channels, assuming flat Rayleigh fading and multipath environments.

Star 16QAM is a modulation method that transmits 4 bits per symbol and has the advantage that it may be differentially encoded and detected. It is very robust to fast multiplicative Rayleigh fading and is suitable for mobile telephone systems and personal communication networks. In this paper we study maximum likelihood differential detection for bit decisions using L diversity branches as well as much simpler post detection combining techniques. The bit error probability is simulated for both diversity detectors on a multiplicative Rayleigh fading channel with additive white Gaussian noise. Both detectors are found to perform almost identically. Large performance gains are obtained with diversity. 1. Introduction Future personal communication systems require bandwidth efficient digital modulation schemes due to the limited radio spectrum available. At the same time the bit error probability of the modulation method must be small also in fading. Differential encoding and detection i...

Presents some improvements to practical realizations of detectors for the Japanese mobile telephony system. A differential detector is improved by using maximal ratio combining, taking into account issues of limited dynamic range in soft metric calculations. Feed forward and feedback coherent detection are discussed and compared for fading channels. Finally two different frame and symbol synchronisation methods are presented and compared.>

The authors consider the error probability at large signal-to-noise ratios of rate-1/2 convolutionally encoded CPFSK (continuous-phase frequency-shift keying) with an optimum noncoherent detector on an additive white Gaussian noise channel. The performance is given in terms of a parameter called the minimum squared normalized equivalent Euclidean distance which plays the same role mathematically as the minimum squared normalized Euclidean distance used for coherent detectors. It is shown that by introducing convolutional coding the error performance is significantly improved. The authors propose a decoding algorithm for these convolutionally encoded CPM schemes which is based on a limited tree search algorithm and uses the maximum-likelihood decision rule for noncoherent detection. Computer simulations show that the degradation in error performance compared to the performance of the optimum coherent Viterbi detector is less than 1 dB with a relatively simple noncoherent detector on an additive white Gaussian noise channel for most of the schemes considered.>

The effects are studied on the symbol error probability of the Viterbi detector for continuous phase modulations with constant amplitude, when the desired signal is received in multiple interfering signals and white Gaussian noise. The detection is assumed to be coherent and, furthermore, perfect timing in the detector is also assumed. An upper bound on the symbol error probability is derived for the Viterbi detector. The bound is based on the union bound technique, and it is shown to be tight for small error probability levels. It is found that the smoothed binary and all quaternary modulations sustain much larger interference power in adjacent channel interference than minimum-shift keying (MSK) does and, thus, the adjacent channels can be spaced closer in frequency for these modulations, In particular, the smoothed quaternary modulations seem to be significantly more efficient than MSK. In cochannel interference the difference in error performance between the schemes is relatively small.>

A matched-filter differential detector for continuous phase modulation is proposed and simulated error probability for this detector is presented. The detector consists of a conventional differential detector operating with a delay, a bank of matched filters, and a Viterbi processor to perform sequence decision. The author shows that quaternary schemes outperform binary schemes, when the schemes are compared in a power-bandwidth tradeoff. Schemes that lose about 3.4 dB compared to coherently detected minimum shift keying (MSK) at 10/sup -3/ on a Gaussian channel, but with a bandwidth of about 0.6 times the MSK bandwidth, are presented.>

For binary continuous-phase modulation with modulation indexh = 1/2(MSK being a rather familiar example), quadrature matched-filter reception is possible with only minor degradation relative to the maximum likelihood (trellis-based) receiver. Thus this class of modulations provides good spectral attributes together with reception techniques capable of being implemented simply at high speeds. As usual, the constant-envelope property is also present. We study here the effects of interfering signals with the same modulation format, located at the same frequency (cochannel interference), or at a nearby frequency (adjacent channel interference). Coherent reception is assumed, and we analyze two channel situations: both signals nonfading with additive Gaussian noise; and both signals independently Rayleigh fading, with additive Gaussian noise. As one would expect, the smoothed phase modulation schemes have larger tolerable adjacent channel levels, while there is little differentiation in the cochannel case. We also find that MSK filters are usable for other formats with negligible loss in performance relative to the optimal linear filters, provided the filter parameters are rescaled appropriately.

In this paper, the performance of continuous phase modulation (CPM) transmitted on a two-ray fading channel and received in white Gaussian noise is studied. The optimum coherent maximum likelihood (ML) detector and approximations thereof and their performance are studied by means of minimum Euclidean distance and simulated symbol error probability. A linear detector optimum at large signal-to-noise ratios is also studied and the performance is given by means of error probability. It is assumed that measurements on the channel provide information about the channel parameters. It is found that the loss in signal power due to the channel is small when an ML detector or an approximation thereof is used for binary schemes with modulation indexh =1/2. The loss for these schemes with a linear detector becomes significantly larger, especially when MSK is transmitted. The performance for this linear detector can, however, be improved significantly by using decision feedback, but still, the performance of the ML detector is superior.

Partial response continuous phase modulation (CPM) schemes have a high spectrum utilization and also a high immunity to nonlinear distortion, since they have a constant envelope. Schemes with high power efficiency can also be designed when they are coherently detected with a Viterbi detector. These schemes are sometimes complex, however. In this paper two minimum-shift-keyed (MSK)type receivers are studied. The MSK-type receiver can be implemented as a serial receiver and as a parallel receiver. Both receivers are useful for binary modulation schemes with modulation indexh = 1/2. Only coherent detection of signals transmitted over an additive white Gaussian noise channel is considered. The serial receiver can be implemented with only two filters and simple decision logic. The decisions are made serially in one decision eye pattern. Two types of receiver filters are considered. Error probability results are presented for the receiver, both with and without phase and timing errors present in the receiver. It is shown that, assuming perfect phase and time synchronization, the serial and parallel receivers have equal performances. The advantages of the serial receiver over the parallel receiver are the same for partial response continuous phase modulation as for classical MSK; i.e., both in implementation aspects and in the reduced sensitivity to phase errors.

Partial response continuous phase modulation (CPM) gives constant envelope digital modulation schemes with excellent power spectra. Both narrow main lobe and low spectral tails can be achieved. When these signals are detected in an optimum coherent maximum likelihood sequence detector (Viterbi detector), power efficient schemes can also be designed, sometimes at the expense of receiver complexity. This paper describes a general class of simple Viterbi detectors with reduced complexity compared to the optimum case. The key idea is that the approximate receiver is based on a less complex CPM scheme than the transmitted scheme. The asymptotically optimum reduced-complexity receiver is found for a variety of transmitted schemes and various complexity reduction factors, for a specific class of receivers and modulation indexes. A new distance measure is introduced for the performance analysis. Smooth schemes based on raised cosine pulses are analyzed and simulated for the case of simplified reception. A graceful performance degradation occurs with the reduction of complexity.