This paper introduces a novel method to minimize the peak-to-average power ratio (PAPR) and at the same time enhance the data rates of orthogonal frequency-division multiplexing (OFDM) systems by combining tone reservation (TR) and index modulation (IM). Unlike conventional TR methods, in which a number of tones (or subcarriers) with fixed positions are reserved for canceling the peaks in OFDM signals, the TR-IM method treats the positions of the reserved tones (TR tones) as random and embeds extra information in their positions using IM. In the proposed system, the amplitudes of the TR tones are quantized with a novel quantization method, which not only helps the receiver distinguish between data tones and TR tones, but also enables the TR tones to carry data on their amplitudes. Based on that, we propose a novel forward error correction (FEC) structure to increase the reliability of detection without requiring extra overhead. The proposed FEC design encodes the IM activation pattern rather than the index data bits, and carries the resulting parity bits with a novel mechanism that exploits the extra bits carried on the amplitudes of the TR tones. Simulation results show that, not only does our proposed system have significantly higher data rates, but it can also achieve remarkable PAPR reduction performance as well as a lower bit error rate than the original OFDM systems under the influence of the non-linear distortion caused by power amplifiers.

This article investigates the effect of uncertainty in the positions of a set of spatially separated receivers (gateways) that are used to localize a device (target) that transmits a burst signal intermittently. Such systems have applications in radar, sonar, and wireless sensor networks, and have been extensively studied, at least in the context of point estimation. This article extends the limited work on region estimation to include the effect of uncertainty in the locations of the gateways on the probability that the target is in a specified region. Specifically, this article extends previous work that found the a posteriori probability density function (pdf) of the target’s coordinates from estimates of the times the transmitted message arrives at the receivers [Time of Arrivals (ToA)] under the assumption the positions of the gateways were known exactly. The expression for the a posteriori pdf is redeveloped to include the uncertainty in the measurement of the receivers’ positions. Three practical scenarios are considered: 1) the target transmits a single message and the gateways measure the ToAs as well as their own positions; 2) the target transmits multiple messages and the gateways measure the ToAs as well as their own positions upon each reception; and 3) the target transmits multiple messages and the gateways measure the ToAs upon each reception, but the positions of the gateways are measured once, at the time of installation. Various numerical examples are given to corroborate, provide insights, and illustrate the utility of the obtained results.

This article was motivated by the need to localize a sensor (signal source) in an Internet-of-Things (IoT) network to an area with a predetermined probability (credibility). The source is assumed to transmit a short time duration (burst) signal in a homogeneous line-of-sight environment. The source's signal is known to an array of spatially separated receivers, which can measure the times of arrival of the source's signal subject to Gaussian measurement errors. The time that the signal leaves the source (i.e., the transmit time) is, however, unknown. The authors develop a method for computing the exact a posteriori probability density functions (pdfs) of the coordinates of the source's in both 2-D and 3-D spaces. The obtained a posteriori pdfs incorporate arbitrary a priori densities, which makes them very useful in many practical scenarios. Unlike existing point-estimate methods, the probabilistic method does not simultaneously solve a set of equations so there is neither a lower nor upper limit on the number of receivers. Various examples are provided to demonstrate the superiority and usefulness of the proposed method. In particular, it is shown that the joint a posteriori pdf of the target's location is not always approximately jointly Gaussian, especially when the target is in close proximity to one of the gateways, or when the gateways are located in close proximity to each other.

This paper presents an OFDMA channel estimation technique that jointly considers the effects of coarse timing error and multipath propagation. Many conventional approaches only consider an optimistic scenario where timing synchronization is perfect and each of the channel delays is an integer number of system samples. In realistic scenarios, timing offsets and echo delays are not integer multiples of the system's sampling period. This threatens sub-carrier orthogonality and causes leakage in the discrete Fourier transform (DFT)-based channel estimation method. Such leakage leads to poor estimation and consequently reduces the system's overall performance. Proposed in this paper is a novel iterative channel estimation technique that considers the practical scenario of fractional timing error and non sample-spaced echo delays. The proposed method does not require channel state information (e.g. second-order statistics of the channel impulse responses or the noise power). Simulation shows that, when comparing OFDMA channel estimation techniques under DOCSIS 3.1 realistic channel conditions, the proposed algorithm significantly outperforms all conventional methods known to the authors.

In this paper, the asymptotic performance of a precoded bit-interleaved coded modulation with multiple-input multiple-output (BICM-MIMO) system in a block-fading environment is first evaluated. Specifically, the asymptotic pairwise error probability is derived for imperfect channel state information (CSI) and of the minimum mean squared-error (MMSE) detector. The analysis is then used to evaluate the impact of two different training schemes. It is shown that inserting training symbols before precoder is a better option than multiplexing training symbols with data symbols after the precoder. The theoretical result is verified by simulation.

The performance of bit-interleaved coded modulation in multiple-input multiple-output (BICM-MIMO) systems using an iterative channel estimator is analysed. In a conventional iterative channel estimator, after initialisation with the training phase, the channel estimator switches to the data phase. However, such a conventional iterative channel estimator does not always improve the performance of the receiver. In order to guarantee the performance improvement, a condition on when the output of the decoder should be used by the estimator is determined. Such a condition is related to the reliability of the soft information utilised by the channel estimator. The key in establishing this relationship is to use the mutual information (MI) that the observation vector has about the channel gains given the output of the decoder at each iteration. In this switch-augmented conventional iterative channel estimator, referred to as SAICE, the condition is theoretically found and indicates the needed reliability of the soft information for the channel estimator at the switching time. The switch-augmented scheme guarantees performance improvement of the iterative receiver with each iteration, however, it might need many iterations to converge for moderate to low signal-to-noise ratios (SNRs). A less computationally intensive approach is to use both the training and data segments of the observation. This approach produces a combined iterative channel estimator (CICE) for BICM-MIMO systems. The performance behaviour of the BICM-MIMO system is illustrated through the extrinsic information transfer (EXIT) chart with imperfect channel state information (CSI). Analytical results are verified with computer simulations.

In order to improve bandwidth efficiency and bit error rate (BER) performance, a new training scheme is proposed for bit-interleaved coded modulation in multiple-input multiple-output (BICM-MIMO) systems. Typically, in a block fading channel, the training overhead used for obtaining channel knowledge is proportional to a power of two of the number of transmit antennas. However, this overhead can be reduced by embedding pilot symbols within data symbols before precoding. The values and positions of pilot symbols are theoretically found by minimizing the Cramer-Rao bound (CRB) on channel estimation error. Computer simulations are presented to demonstrate the advantage of the proposed scheme over other training methods, in terms of both the mean square error (MSE) of the channel estimation and the system's BER.

There are applications in spread spectrum channels where spatial diversity is required to mitigate fading. The spreading gain in such channels is often limited to reduce the chances of a nearby interferer being in-band. With modest spreading gains the signal bandwidth may be less than the coherence bandwidth. In such cases receiver-based multi-path diversity is not feasible, and spatial diversity is used to combat fading. This study improves and extends the work presented in US patent #6389085 B1, which exploits the structure of spread spectrum signals to implement Nth order spatial diversity with simple incoherent radio frequency combining. An improved combining scheme is proposed here together with a special pseudo-random noise (PN) sequence to reduce intra symbol interference. A unique coherent all-digital demodulator is also provided. The signal-to-noise ratio (SNR) of the new system is about 3.5 dB higher than that of the original system described in the patent. The new receiver is comparable in cost to selection diversity, but does not switch antennas and therefore, does not suffer the momentary signal drop-outs. However, the penalty for eliminating the drop outs is a 1.5 dB lower output SNR.

This study improves the fading channel estimation in bit-interleaved coded modulation systems with iterative decoding (BICM-ID) and signal space diversity (SSD) by embedding a training sequence. Existing training schemes work well at high signal-to-noise ratio (SNR) or slowly time-varying channels whereas the applications of BICM-ID are beneficial at low SNR and fast time-varying fading channels. Motivated by the power/bandwidth efficiency of the SSD technique and the fact that superimposed training outperforms pilot symbol-assisted modulation (PSAM) training over relatively fast time-varying channels, a new superimposed training sequence is explored. The proposed training sequence inserts pilot bits into the coded bits prior constellation mapping and signal rotation. This becomes a superimposed training sequence in the rotated symbols and helps the estimator to track fast variation of the channel gains. A soft iterative channel estimator is developed to work with the superimposed training sequence. The performance of the proposed scheme, namely SSD-pilot, is shown to be superior to PSAM scheme. To gauge the performance improvement achieved with the proposed channel estimation, an analytical bound on the asymptotic bit error probability for BICM-ID using SSD over correlated fading channels is provided. The Cramer–Rao bound on the mean-square error of the channel estimator is also derived to evaluate the performance of the iterative channel estimator.

Recently, non-data aided synchronizers for QAM with a feedforward structure have been significantly improved by incorporating the prefllter originally proposed by Franks to mitigate the self-noise in analog clock regenerators. Prior to this, D'Andrea proposed a synchronizer with a feedback structure that used Franks' prefllter. Although not reported in the literature, the earlier D'Andrea's synchronizer outperforms the more recent feedforward synchronizers, and consequently is believed by the authors to be the optimum non-data aided synchronizer for QAM. An all digital version of D'Andrea's synchronizer is presented in this paper. The steady state timing jitter is thoroughly analyzed by deriving a closed form expression for the noise power spectrum, as well as a simple expression to estimate the variance of the jitter. Performance comparisons are made.

Signal space diversity (SSD) has been recently applied to improve the error performance of bit-interleaved coded modulation with iterative decoding (BICM-ID) systems over wireless fading channels. Previous works, however, only consider perfect channel estimation at the iterative receiver of the system. Performing channel estimation is an important task in a practical BICM-ID system and is the focus of this paper. To exploit the technique of SSD for improving channel estimation, a new method of using training sequences is presented. The proposed method inserts pilot bits between the coded bits, i.e., before constellation mapping and signal rotation. It is therefore can be considered as superimposed training. Developed is a soft iterative channel estimator to work with the new arrangement of the training signal. The performance of the system with the proposed training and channel estimation is shown to be superior that with pilot symbol assisted modulation (PSAM). To benchmark the achievable error performance of the system with channel estimation, an analytical bound of the asymptotic bit error probability for BICM-ID using SSD over correlated fading channels and perfect channel state information (CSI) is also derived.

This paper deals with timing jitter reduction in the timing recovery loop of a digital QAM receiver. The main contribution, which is derived analytically, is an economical prefilter to reduce the timing jitter in timing recovery loops containing either the early–late or the Gardner timing-error detector (TED). The proposed prefilter has the advantage of being an infinite impulse response filter that is placed inside the TED and runs at the symbol rate. For small roll off factors, it is shown with a computer simulation that a single-pole filter placed inside either the early–late or the Gardner detector is quite effective in reducing the timing jitter.

In burst mode communication, the beginning of a burst is often estimated by comparing the power in a sliding window to a threshold. The simplicity of the algorithm makes it very attractive. The probability density function of the detection variable for this simple algorithm is the subject of (H. H. Nguyen et al., April 2003) where it is shown to be approximately Gamma distributed. This paper provides further analysis of this low complexity edge/burst detection DSP algorithm. The analysis yields two equations that relate "the probability of false detection" and "the probability of missing a detection" to window size and the decision threshold.

This paper proposes a two-phase hybrid algorithm for optimal placement of radioports. The placement strategy minimizes the dynamic range of the channel, and thus reduces the peak transmit power needed by a wireless handset. The performance of the proposed algorithm is compared with that of an exhaustive search, as well as with that of a simplex algorithm. Simulation results show that the two-phase hybrid algorithm, although suboptimal, outperforms the simplex algorithm with only a modest increase in computation time. It has the added advantage of being quite insensitive to the seed solution.

Code-division multiple-access (CDMA) implemented with direct-sequence spread spectrum (DS/SS) signaling is a promising multiplexing technique for cellular telecommunications services. The efficiency of a direct-sequence spread-spectrum code-division multiple-access (DS-CDMA) system depends heavily on the shape of the spectrum of the spread signal. Maximum efficiency is obtained with an ideal brick-wall bandpass spectrum. There are two approaches toward achieving such a spectrum. One is to use a simple spreader that produces a broad spectrum and then follow it with a precise, high order filter to band limit the spectrum. A second approach, which is the approach taken in this paper, is to use a spreader that produces a spectrum close to the ideal spectrum and then employ a simple filter to control the out-of-band power. The proposed spreader/despreader is based on a simple hybrid function and can be easily implemented. An analysis provides a compact expression for the signal-to-noise ratio (SNR) of a RAKE receiver. The expression includes the effects of baseband, intermediate frequency (IF) and RF filtering as well as the effects of the spectral densities of the spreading/despreading functions. The analysis shows that the proposed spreader/despreader yields superior performance over a conventional pseudo noise (PN) spreading/despreading mechanism.

A generalized covariance expression for time difference of arrival (TDOA) estimators is derived for an M-sensor arbitrary array in a two-path underwater environment. The resulting expression can be reduced to the variance and covariance expressions developed previously. The correlation among the time difference of arrival estimators and its effect on the localization error are investigated for a two-sensor vertical array. The results show that the correlation among the multipath TDOA estimators is significant and the degree of correlation depends on the power spectral density of the source signal and that of the noise, and only on two multipath TDOAs. Because of this correlation, every TDOA estimate contributes information to the localization and therefore a whole set of TDOA estimators should be used.

Direct-sequence spread spectrum systems utilizing RAKE receivers provide inherent diversity for in-building mobile CDMA radio. A new compact expression for the SNR of the decision variable of a RAKE receiver is derived for generalized offset quadrature carrier modulation in this multipath environment. This expression also applies to BPSK, QPSK, and MSK modulation. The novelty of the derived expression is that it includes the effects of filtering. Filtering must be used to eliminate out-of band radiation and can also be used to shape the in-band spectrum, It is shown that bandlimiting filters have a significant effect on the SNR. A comparison of bandlimited O-QPSK and MSK reveals that, under the same co-user load, MSK has a lower SNR-0.917 times that of O-QPSK. However, MSK has an offsetting advantage of being a constant envelope-type modulation that can be very efficiently generated using direct modulation of a power RF oscillator. It is further shown that the O-QPSK SNR can be increased by a factor of 1.63 by whitening the in-band spectrum.>

Battery power efficiency of the portable unit is a major issue in the design of indoor radio telecommunication systems. The efficiency of the portables depends on the modulation method employed. Digital modulation methods with constant envelope signals are more battery efficient than those with modulated envelope signals since the latter require inefficient linear radio frequency amplifiers. The improvement in efficiency is very significant when the constant envelope modulation method is minimum shift keying and a directly modulated power radio frequency oscillator is used to generate the modulated signal. This paper compares the performance of binary phase shift keying (BPSK) and minimum frequency shift keying (MSK) in a spread spectrum code division multiple access setting. The BPSK system can support about 8 percent plus 0.5 more simultaneous users for a given carrier to interference ratio but the MSK system has the advantage of being more power efficient.>