In wireless networks, an essential step for precise range-based localization is the high-resolution estimation of multipath channel delays. The resolution of traditional delay estimation algorithms is inversely proportional to the bandwidth of the training signals used for channel probing. Considering that typical training signals have limited bandwidth, delay estimation using these algorithms often leads to poor localization performance. To mitigate these constraints, we exploit the multiband and carrier frequency switching capabilities of wireless transceivers and propose to acquire channel state information (CSI) in multiple bands spread over a large frequency aperture. The data model of the acquired measurements has a multiple shift-invariance structure, and we use this property to develop a high-resolution delay estimation algorithm. We derive the Cramér-Rao Bound (CRB) for the data model and perform numerical simulations of the algorithm using system parameters of the emerging IEEE 802.11be standard. Simulations show that the algorithm is asymptotically efficient and converges to the CRB. To validate modeling assumptions, we test the algorithm using channel measurements acquired in real indoor scenarios. From these results, it is seen that delays (ranges) estimated from multiband CSI with a total bandwidth of 320 MHz show an average RMSE of less than 0.3 ns (10 cm) in 90% of the cases.

In this paper, we focus on the problem of blind joint calibration of multiband transceivers and time-delay (TD) estimation of multipath channels. We show that this problem can be formulated as a particular case of covariance matching. Although this problem is severely ill-posed, prior information about radio-frequency chain distortions and multipath channel sparsity is used for regularization. This approach leads to a biconvex optimization problem, which is formulated as a rank-constrained linear system and solved by a simple group Lasso algorithm. Numerical experiments show that the proposed algorithm provides better calibration and higher resolution for TD estimation than current state-of-the-art methods.

Achieving high resolution time-of-arrival (TOA) estimation in multipath propagation scenarios from bandlimited observations of communication signals is challenging because the multipath channel impulse response (CIR) is not bandlimited. Modeling the CIR as a sparse sequence of Diracs, TOA estimation becomes a problem of parametric spectral inference from observed bandlimited signals. To increase resolution without arriving at unrealistic sampling rates, we consider multiband sampling approach, and propose a practical multibranch receiver for the acquisition. The resulting data model exhibits multiple shift invariance structures, and we propose a corresponding multiresolution TOA estimation algorithm based on the ESPRIT algorithm. The performance of the algorithm is compared against the derived Cramér Rao Lower Bound, using simulations with standardized ultra-wideband (UWB) channel models. We show that the proposed approach provides high resolution estimates while reducing spectral occupancy and sampling costs compared to traditional UWB approaches.

A clock synchronization scheme is crucial for obtaining accuracy in time-based positioning systems. Existing clock synchronization schemes are mostly based on a simplified linear clock model, which unfortunately have a poor long-term synchronization accuracy. Assuming a two-way time transfer protocol, we propose a novel clock synchronization algorithm based on a precise physical clock model to realize joint clock synchronization and propagation delay estimation. The Cramer-Rao lower bound (CRLB) is derived for this clock model and shows that the estimation is asymptotically efficient and converges to the lower bound. For long time spans, this model performs better than a linear clock model.

This paper presents the channel measurements performed within a closed metal cabinet at 60 GHz covering the frequency range 57-62 GHz. Two different volumes of an empty metal cupboard are considered to emulate the environment of interest (an industrial machine). Furthermore, we have considered a number of scenarios such as line of sight, non line of sight, and placing absorbers. A statistical channel model is provided to aid short-range wireless link design within such a reflective and confined environment. Based on the measurements, the large- and small-scale parameters are extracted and fitted using the standard log-normal and Saleh-Valenzuela models, respectively. The obtained results are characterized by a very small path loss exponent, a single cluster phenomenon, and a significantly large root-mean-square (RMS) delay spread. The results show that covering a wall with absorber material dramatically reduces the RMS delay spread. Finally, the proposed channel model is validated by comparing the measured channel with a simulated channel, where the simulated channel is generated from the extracted parameters.

A theoretical model for non-coherent start-of-frame-delimiter (SFD) detection in IEEE 802.15.4a is presented and closed form solutions for SFD threshold and signal-to-noise ratio (SNR) estimation are derived. The derived results are implemented in an impulse radio ultra wide band (IR-UWB) demonstrator operating in the 6-10.6 GHz UWB band. The demonstrator is designed for low power operation (in the mW range for combined digital base band (DBB) & radio frequency (RF)). The transmitter (TX) RF is duty cycled at pulse level and the receiver (RX) RF is switched to pulse level duty cycling when possible. Moreover, digital TX and RX resources are only switched on when needed. Measurement results are in close agreement with the derived closed form solutions.

In this paper, we analyze the error probability of equal gain combining with quantized channel phase compensation for binary phase shift keying signalling over Rayleigh fading channels. The probability density and characteristic functions of the combined signal amplitude are derived and used to compute the analytic expressions for the bit error probability in dependance of the number of quantization levels L, the number of diversity branches NRand the average received signal-to-noise ratio. The analysis is utilized to outline the trade-off between NRand L and to compare the performance with non-coherent binary frequency shift keying and differential binary phase shift keying schemes under diversity reception.

In this paper a novel signaling protocol for co-existence and spectrum sharing among cognitive radio nodes is proposed. This protocol allows the radios to rendezvous with each other in a statically allocated spectrum band through on-off keying signaling and reliable spectrum sensing. It enables the radios to have non-cooperative communication in the sense that they do not need to exchange their modulation schemes and frequency channel. The proposed protocol requires no synchronization and is augmented with power control mechanisms. Analysis of the proposed protocol in terms of timing, probability of a successful connection and energy efficiency is also given.

In this paper, an Ultra-Wideband (UWB) receiver scheme with very low sampling rate is proposed for ranging in indoor multipath environments. The idea underlying the proposed scheme is that, instead of processing the whole UWB band, only partial sub-bands are considered at the receiver. The missing sub-bands are estimated by exploiting an estimate of the frequency correlation properties of the channel to reconstruct the signal over the whole UWB band. For the simulations, the IEEE UWB channel model is used and the range error using the proposed receiver scheme is obtained and compared to that of conventional UWB receivers which require very high sampling rates. Furthermore, a large number of UWB channel measurements in an office environment has been used to validate the accuracy of the obtained results. These results show that the range accuracy strongly depends on the number of received sub-bands and the spacing between them. For instance, using 1% of the total UWB bandwidth, an average range error of less than 10 cm is obtained. Furthermore, unlike conventional UWB receivers, the proposed technique performs better (in terms of range error) for low signal-to-noise ratios.

In this paper, a new method is proposed for low complexity localization based on measured/estimated ranges. First, it is proved that the method provides a better estimator than the well known non-iterative direct methods documented in literature, i.e. the Spherical Interpolation and the Linear Least Squares method. It does so by exploiting two similar full constrained models. The proposed estimator is better, in the sense that it corresponds to an equal or smaller value of the original least-squares objective function. Second, the method goes without iterations, and therefore requires considerably less computation power than the iterative techniques such as the Gauss-Newton method. Validation is performed based on actual Ultra-WideBand (UWB) radio measurements conducted in typical office environments, with signal bandwidths varying from 1 to 7.5 GHz. Results show that the position estimates obtained with the proposed non-iterative method have lower root mean squared errors than any other tested well-known direct methods.

Wireless personal area networks applications may benefit from the use of ultra-wideband (UWB) technology. In these applications transmit and receive antennas are very close to each other and the far-field condition assumed in most of the link budget models may not be satisfied. Under near- field conditions, variations in the link budget and pulse shape compared to the far-field can be observed. In this work, a new UWB link budget model for very short distances is proposed and validated with measurements using different types of antennas. The measurements have been performed in the time domain by exciting the antenna with very short pulse of 35 ps width covering a large frequency band. The proposed model, which includes frequency, antenna size and orientation as parameters, shows a good agreement with the simulations and the measurements.

In this paper, the possibility of using UWB channel impulse response statistics, other than the TOA, for ranging applications, is investigated. Models for the total received signal power and for the first path power are proposed, based on an extensive UWB measurement campaign conducted in typical office environments. The use of the models is twofold: to obtain a best linear unbiased estimation of the distance and to identify the environment in which the transmitter and receiver are operating. Finally, the bandwidth dependency of the proposed approach is discussed and compared with that of the signal strength method, and a possible low complexity hardware implementation is suggested. The described strategy does not need synchronization between the system nodes, and only modest sampling rates are required. At the same time, the achieved standard deviation of the range error is 0.40 m, about 2.5 times less than obtained with the classical signal strength estimation.

Noisy radio frequency (RF) circuits tend to degrade the system performance, especially in high frequency communication systems. The performance analysis of such systems is normally based on Monte Carlo simulations which are rather time consuming and provide little insight for system performance bottle necks. In this paper, an exact closed form expression for the bit error rate (BER) of M-ary pulse amplitude modulated (MPAM) single carrier (SC) schemes as a function of analog to digital converter (ADC) word length is presented. This expression is then used for the performance evaluation of MPAM schemes perturbed by Gaussian and ADC quantization noise (QN) in various fading scenarios. The expressions presented can also be used to determine the system performance in the presence of other RF imperfections such as phase noise in addition to ADC noise and thus provide the system designer with a handy tool for RF system design and optimization.

Sharing of the frequency spectrum between licensed primary users and unlicensed secondary users (SUs) requires reliable detection of spectrum occupancy by the SUs. Due to fading, single terminal detection is unreliable and results in a high probability of missed detection. This problem is solved by applying cooperative detection. In this paper two novel energy-based cooperative detection methods using weighted combining for Dynamic Spectrum Access are presented and analyzed. Weighting is based on the local mean SNR and the optimum log-likelihood ratio. Simulation results show a substantial improvement for the proposed weighting methods compared to equal gain combining and hard decision combining.

UWB technology can reach centimetre level ranging and positioning accuracy in LOS propagation when time of arrival techniques are used. However, in a real positioning system, this accuracy poses high demands on hardware. In this paper, we investigate the possibility of using UWB channel impulse response statistics, other than the TOA, for ranging applications, thus avoiding the costly synchronization between the system nodes. First, models for the total received signal power, for the first path power, and for the mean excess delay of the power delay profile are proposed, based on an extensive UWB measurement campaign conducted in typical office environments. The distance dependency of these statistics is analyzed. These models are subsequently used to obtain a best linear unbiased estimation of the distance. Finally, it is shown how the knowledge of the model of the described statistics, can be used to identify the environment in which the transmitter and receiver are operating. The achieved standard deviation of the range error is 0.35 m, about three times less than obtained with the classical signal strength estimation.

For multi-level modulation methods, rotation of the signal constellation together with in-phase and quadrature phase channel interleaving (signal space diversity) are known to provide good performance gains over fading channels. This paper studies the extension of such schemes with a low density parity check (LDPC) code. It is shown that for both coded and uncoded Gray-mapped MPSK modulation formats with signal space diversity on a Rayleigh fading channel, a well-considered choice of the rotation angle may lead to a significant gain over the conventional unrotated constellation. However, the optimum rotation angle for the coded scheme may be different from the corresponding optimization angle of the uncoded scheme.

In this paper we give insight into the performance of Dynamic Spectrum Access Networks (DSAN), analyzing Quality of Detection of Primary User (PU) and DSAN blocking probability, when the channel is under the effect of log-normal shadowing. Specifically we propose a distributed power conserving PU detection architecture and investigate the impact of PU detection accuracy on DSAN performance. We measure DSAN blocking probability as a function of the number of PU channels and their utilization. Finally we propose two efficient DSAN channel access schemes called Least-used and Least-used with Channel Hopping which aim at minimizing packet dropping due to the arrival of the PU.

The autocorrelation receiver (AcR) is a suboptimum, low-complexity receiver architecture particularly suited to ultra-wideband (UWB) communication systems. As the bit rate increases, interference among pulses due to multipath propagation causes serious impairments of the AcR's performance. To mitigate this effect, we propose an appropriate design of the chip code and of the delay hopping (DH) code. We provide conditions to be satisfied by the DH code in order to reduce the nonlinear intersymbol interference (ISI) and the bias term, which are peculiar nuisance parameters of autocorrelation receivers. By extending the length of the chip code, we show that N/sub p/ transmitted pulses per symbol can be employed to suppress the average linear ISI of N/sub p/-1 previous symbols. Simulated results confirm the performance improvement in terms of bit-error rate. However, in previous work it has been shown that the noise power linearly increases with N/sub p/. Although a large number of pulses per symbol is favorable for ISI mitigation, we show that the transmission of a single pulse minimizes the probability of error, for bit rates lower than an upper bound depending on the channel root mean-square delay spread and on the noise power.

The performance of a conventional threshold detector for a frame differential autocorrelation receiver in IR UWB systems in a dense multipath channel are improved by means of appropriate chip code and delay hopping code design. We point out which conditions must be satisfied by the delay hopping code in order to reduce the nonlinear ISI and the bias term, which are peculiar nuisance parameters of autocorrelation receivers. Moreover, by extending the periodicity of the chip code, we show how to suppress linear ISI. Simulated results confirm the performance improvement in terms of BER. An analytical expression relating the noise power to the system parameters is also presented. These results can be extended to the well known transmitted reference systems.

In this paper we address the problem of radio resource allocation for QoS support in the downlink of a cellular OFDMA system. The major impairments considered are cochannel interference (CCI) and frequency selective fading. The allocation problem involves assignment of base stations and subcarriers, bit loading, and power control, for multiple users. We propose a three-stage, low-complexity, heuristic algorithm to distribute radio resources among multiple users according to their individual QoS requirements, while at the same time maintaining the QoS of already established links in all the cochannel cells. The allocation objective is to minimize the total transmit power, which adds to reducing CCI. Simulation results show a superior performance of the proposed method when compared to classical radio resource management techniques. Our scheme allows us to achieve almost 6 times higher capacity (sum data rate) than the method based on FDMA with power control, at a blocking probability of 0.02.

In this paper we propose two innovative detection schemes for IR-UWB communication systems based on the autocorrelation receiver and differential detection. The proposed schemes avoid the need for a complex analog multiplier by employing: (i) a limiter in the reference branch only; or (ii) employing also a limiter in the signal branch. Analytical models for the statistics of the decision variables are presented for the receivers, and the analytical and simulated BER exhibit an excellent agreement. We point out the dependence of the performance on some important system parameters, that shows the system design trade-off between receiver complexity and performance.

A low-complexity, suboptimal algorithm to distribute subcarriers and transmit power among multiple users according to their individual QoS requirements in an OFDMA system employing embedded modulation is proposed. Application of embedded modulation allows to increase the system capacity in the downlink by stacking K independent, QAM modulated signals on each of the N available OFDM subcarriers. In this way, the maximum number of independent links is N/spl middot/K compared to N when no subcarrier sharing is allowed. The larger number of links can be used for serving more users or increasing the users' data rates. Simulation results show that signal stacking is more power efficient than addressing users with a higher order modulation and applying time sharing (TDMA). The performance of the proposed algorithm expressed in total transmit power is very close to the optimal solution found by exhaustive search.

With a narrowband multiuser detector (MUD), multiple users can simultaneously share the same channel without using a bandwidth-expanding signature code when sufficient power differences at the receiver are maintained. In this paper, we assess the capacity gain that can be achieved in a hypothetical cellular system with centralized power control, using a narrowband MUD based on successive cancellation. The power-control algorithm is adapted to multiple users per channel in a cell. Simulation results are presented for two heuristic carrier-to-interference ratio-based channel-assignment schemes. The MUD allows for a substantial capacity gain due to a tighter packing of users in a channel. The observed gain compared with a conventional single user-per-channel system, ranges from 1.4 to 5 for cluster sizes C = 1 and 7, respectively.

With a narrowband multiuser detector (MUD), multiple users can simultaneously share the same channel without using a bandwidth expanding signature code when sufficient power differences at the receiver are maintained. In this paper, we estimate the capacity that can be achieved in a hypothetical cellular system with centralized power control, using a narrowband MUD based on successive cancellation. The power control algorithm is adapted to multiple users per channel in a cell. Capacity results from simulations based on user removal are presented for three removal algorithms. It is shown that the narrowband MUD has a significant potential for capacity gain due to a tighter packing of users in a channel.

This paper proposes a soft-iterative multiuser receiver of low complexity, based on the principle of turbo decoding. A serially concatenated system is used, where every user employs a convolutional code, interleaver, space-time block code (STBC) and the spreading operation. The proposed receiver uses a soft-input soft-output (SISO) multiuser detector-decoder, which is basically a soft interference canceller for the space-time coded wideband code division multiple access (WCDMA) signal. In each iteration, extrinsic information is interchanged between the different SISO blocks giving asymptotically unbiased estimates. The introduction of STBC adds decorrelation between the different user signals, relaxing the interleaver design and less iterations are needed for the system to converge. The receiver allows the use of a less complex soft matched filter instead of a minimum mean square error (MMSE) filter. It achieves nearly single-user performance, and gives an excellent trade-off between implementation complexity and performance for advanced WCDMA multiuser receivers.

A narrow-band multiuser receiver based on successive signal detection and subtraction, is considered. The symbol error probability (SEP) for M-PSK modulated signals is evaluated and analytical approximations for the SEP of the individual signals are presented and compared with results obtained from simulations. For geometrically related signal amplitudes, a constant minimum distance can be guaranteed independent of the number of signals. The required amplitude ratio is shown to be related to M and the number of co-channel signals. Optimizing the transmitted power for the different signals while ensuring the same SEP is then addressed and closed-form expressions of the signal amplitude ratios are derived. The effect of inaccurately estimated signal parameters due to noise is also analyzed. SEP results are presented for synchronous signals in an additive white Gaussian noise environment.

A narrowband multiuser receiver based on successive signal detection and subtraction is proposed. The symbol error probability (SEP) for M-PSK modulated signals is evaluated and analytical approximations for the SEP of the individual signals are presented and compared to results obtained from simulations. The effect of inaccurately estimated signal parameters due to noise is analyzed. The SEP results are presented for synchronous signals for the additive white Gaussian noise (AWGN) channel. For geometrically related amplitudes the minimum required amplitude ratio is shown to be dependent on the modulation level M and the number of cochannel signals.

The dual-signal receiver (DSR) for simultaneous reception of two co-channel signals, shows large performance degradation for SIR/spl cong/0 dB. In this paper, antenna diversity techniques are investigated to overcome this problem. Computational results for BPSK modulated signals in narrowband and wideband fading multipath channels, show that antenna combining results in a significant performance enhancement for the DSR; the largest improvement is achieved by minimum mean square error combining (MMSEC). For the narrowband Rayleigh fading channel and a four element array, MMSEC performance with two signals is nearly equal to the one signal case for SIR=0 dB. In the wideband channel, the SIR required to achieve good performance, increases with increasing symbol rate due to intersymbol-interference.

The dual-signal receiver (DSR), proposed for simultaneous reception of two narrowband modulated co-channel signals, shows large performance degradation when both signals have about equal strength. The performance enhancement of the DSR using binary phase shift keying (BPSK) modulation is obtained with forward error correction (FEC) coding. Rather short linear block codes were evaluated using hard and soft decision decoding techniques. With soft decision decoding, large coding gains are obtained in the channel with co-channel interference and additive white Gaussian noise (AWGN). The gain for the channel with interference is larger than for the AWGN-channel, but the absolute performance is worse. Hard decision decoding gives no significant enhancement. The coding gain for the small signal is about the same as for the large signal due to the occurrence of correlated errors.

Results from propagation measurements, conducted in an indoor office environment at 2.4, 4.75, and 11.5 GHz, are presented. The data were obtained in small clusters of six measurements, using a coherent wideband measurement system. The channel characteristics for the three frequencies are compared by evaluating path loss, rms delay spread, and coherence bandwidth. An analytical model for evaluation of the bit-error rate (BER) of the stationary frequency selective indoor channel is developed for a coherent binary phase shift keying (BPSK) receiver, based on the complex impulse response of the channel. Computational BER results are obtained for data rates up to 50 Mb/s, using the measured multipath channel impulse responses. The BER results for a number of clusters are presented and compared for the maximum reliable data rate as inferred by the measured rms delay spread of the channel.

Currently a lot of research is done on a future Universal Mobile Communication System (UMTS), which is a third generation system. An important research topic is to determine a suitable and flexible multiple access technique. The performance of a multi-user receiver in combination with a groupwise successive interference cancellation (CSIC) scheme for a DS/CDMA system is investigated. An analysis of the GSIC scheme is presented for BPSK modulation under Rayleigh fading in an asynchronous channel. The GSIC scheme results in a considerable reduction of the hardware complexity. A performance comparison with a SIC scheme without grouping is done. Numerical results show that the system performance of the GSIC scheme approaches the performance of the SIC scheme, when the group size is not too large.

The paper analyses the performance of a dual-signal receiver in a Rician fading channel. The receiver is designed for simultaneous reception of two narrowband binary phase shift keying (BPSK) modulated co-channel signals, e.g. two desired signals, of different strength. The dual signal receiver consists of two demodulators. The principle of operation is based on separation of both signals using the data signal of the large signal which is estimated by the first demodulator. After cancellation of the large signal, the small signal is demodulated. The bit error rate (BER) and receiver outage performance in the Rician fading channel with additive white gaussian noise are derived for both signals. Numerical results are presented for two receiver structures with different second demodulator: (i) the coherent BPSK demodulator; (ii) the differentially coherent (DPSK) demodulator. It is shown that the dual-signal receiver outperforms the conventional receiver significantly, especially in the interference limited channel with high signal-to-noise ratio.

The paper analyses the performance of quasi-coherent combining of signals from a small circular antenna array in a Rician fading indoor multipath channel at microwave or millimetre wave frequencies. Instead of maximum ratio combining, where the complex weighting of signals results in an optimum for a single frequency, the proposed concept is based on quasi-coherent combining of the dominant component of the multipath signal of each antenna. The resulting channel impulse response, shows less delay spread and a higher signal to noise ratio. A scheme for this combining technique is given for a coherent binary phase shift keying receiver and evaluated by simulation. Cumulative BER results for the Rician channel and normalised bit rates are presented for circular arrays of 1, 2 and 4 antennae with a diameter of one wavelength. It is shown that this combining technique results in a significant improvement of the BER or application of higher bit rates,.