In this paper, a novel binary waveform with low peak-to-average power-ratio (PAPR) and low out-of-band (OOB) emissions is presented, targeting at extreme uplink coverage solutions towards the 6G era. It uses the discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-s-OFDM) signal model, as the baseline, togetherwith quadrature phase-shift keying (QPSK) modulation symbols, similar to the widely considered $\pi / 2$ binary phase-shift keying ($\pi / 2$-BPSK). The proposed approach further incorporates trellis-coding in a way which avoids sharp phase transitions in the generated signal, resulting in greatly reduced PAPR and OOB emissions. Additionally, while linear filtering based spectrum shaping has been widely considered in recent low-PAPR waveform studies, we propose to use phase interpolation to increase the sampling rate. This is nonlinear operation, but it helps to significantly reduce the PAPR compared to linear spectrum shaping. This can be achieved both with and without bandwidth extension. Further enhancement to the spectrum localization is achieved by improving the phase continuity of the generated signal, avoiding excessive phase transitions between consecutive transform-precoded OFDM symbols and between each main symbol and its cyclic prefix (CP).

Orthogonal frequency-division multiplexing (OFDM) has been selected as a baseline waveform for long-term evolution (LTE) and fifth-generation new radio (5G NR). Fast-convolution (FC)-based frequency-domain signal processing has been recently considered as an effective tool for transmitter and receiver side subband filtering of OFDM-based waveforms. However, for the original continuous FC-based model, the filtering can, in general, be configured in time-direction only with the granularity of half subframe, corresponding to 7, 14, or 28 symbols with 15kHz, 30kHz, or 60kHz subcarrier spacing, respectively. In this paper, we present a symbol-synchronous FC-processing scheme flexibly allowing filter re-configuration with the time resolution equal to one OFDM symbol while supporting tight carrier-wise filtering for 5G NR in mixed-numerology scenarios with adjustable subcarrier spacings, center frequencies, and subband bandwidths, as well as providing co-existence with LTE. Proposed approach segments each stream of time-domain OFDM symbols into overlapping processing blocks of fixed size. Symbol synchronous processing is achieved by dynamically adjusting the overlap between the processing blocks while aligning the payload part of the processing blocks with the boundaries of the OFDM symbols. The proposed scheme is demonstrated to support the envisioned use cases of 5G NR and provide a flexible starting point for sixth generation (6G) development.

In this paper, we investigate the effects of RF transceiver’s imperfections on the multi-antenna interference rejection combing (IRC) based black-space cognitive radio (BS-CR) operation. In particular, we explore the effects of power amplifier (PA) nonlinearities and carrier frequency offset (CFO) on the blind IRC technique. The BS-CR operation mode supports effective reuse of the primary user (PU) spectrum, especially for relatively short-distance CR communication. We assume that both the PU system and the BS-CR use orthogonal frequency division multiplexing (OFDM) waveforms with common numerology. In this case the PU interference on the BS-CR signal is strictly flat-fading at subcarrier level, and it can be suppressed using subcarrier-wise IRC processing. Spatial sample covariance matrix-based IRC adaptation is applied during silent gaps in CR operation. We propose an analytical framework for modeling CFO effects, together with experimental study of CFO and PA nonlinearity effects. The performance of the IRC scheme is tested considering terrestrial digital TV broadcasting (DVB-T) as the primary service. The validity of the offered expressions for CFO effects are justified through comparisons with respective results from computer simulations. The effect of CFO between the primary and secondary systems is found to be critical for BS-CR operation, while the effect of CR transmitter’s nonlinearity is no worse than in basic OFDM schemes, and the PU transmitter’s nonlinearity has minor effect on BS-CR operation.

Orthogonal frequency-division multiplexing (OFDM) has been selected as the basis for the fifth-generation new radio (5G NR) waveform developments. However, effective signal processing tools are needed for enhancing the OFDM spectrum in various advanced transmission scenarios. In earlier work, we have shown that fast-convolution (FC) processing is a very flexible and efficient tool for filtered-OFDM signal generation and receiver-side subband filtering, e.g., for the mixed-numerology scenarios of the 5G NR. FC filtering approximates linear convolution through effective fast Fourier transform (FFT)-based circular convolutions using partly overlapping processing blocks. However, with the continuous overlap-and-save and overlap-and-add processing models with fixed block-size and fixed overlap, the FC-processing blocks cannot be aligned with all OFDM symbols of a transmission frame. Furthermore, 5G NR numerology does not allow to use transform lengths shorter than 128 because this would lead to non-integer cyclic prefix (CP) lengths. In this article, we present new FC-processing schemes which solve or avoid the mentioned limitations. These schemes are based on dynamically adjusting the overlap periods and extrapolating the CP samples, which make it possible to align the FC blocks with each OFDM symbol, even in case of variable CP lengths. This reduces complexity and latency, e.g., in mini-slot transmissions and, as an example, allows to use 16-point transforms in case of a 12-subcarrier-wide subband allocation, greatly reducing the implementation complexity. On the receiver side, the proposed scheme makes it possible to effectively combine cascaded inverse and forward FFT units in FC-filtered OFDM processing. Transform decomposition is used to simplify these computations, leading to significantly reduced implementation complexity in various transmission scenarios. A very extensive set of numerical results is also provided, in terms of the radio-link performance and associated processing complexity.

This paper proposes a cyclic prefix (CP) free spectral efficiency frequency division multiplexing (SEFDM) wireless signal transmission and reception method based on alignment signal (AS). The method needs the cooperation of both the transmission and reception sides. At the transmitter, time-domain AS is designed to prevent inter-symbol interference (ISI) caused by multipath propagation in the received signal. At the receiver, the channel circularity convolution providing processing is used to enable high accurate frequency domain one-tap equaliser. The extensive computer simulation results under the 5G new radio (5G-NR) channel model, TDL-D, show that the AS aided SEFDM has similar bit error rate performance as CP-SEFDM without energy and latency cost on CP. The AS-SEFDM capability to mitigate the ISI and to enable the one-tap equaliser in SEFDM systems makes it a promising technique for future wireless communication systems.

Cyclic-prefix orthogonal frequency-division multiplexing (CP-OFDM) is a widely adopted modulation scheme for broadband wireless communication systems. The use of CP makes the scheme robust and facilitates simple and effective channel equalization since it makes the frequency-selective multipath fading to appear as flat-fading at subcarrier level. However, CP causes overhead in the spectrum efficiency and power efficiency. Various schemes have been proposed in the literature to avoid the use of CP or other forms of guard intervals (GIs) between OFDM symbols, but none of those has been fully satisfying. One recent proposal is based on adding a so-called alignment signal to GI-less OFDM signal in such a way that (i) intersymbol interference is avoided and (ii) the multipath channel effect appears as the cyclic convolution between the effective channel impulse response and each OFDM symbol. While assuming channel knowledge on the transmitter side, this approach allows basic OFDM signal processing to be used on the receiver side for channel estimation, equalization, and synchronization. In this work, we provide enhancements to the alignment signal structure and generation, which provide clear improvement in the link performance and reduce the complexity of transmitter processing. Also, new insights about the characteristics of alignment signal based CP-free OFDM waveforms are provided.

In cyclic-prefix orthogonal frequency-division multiplexing (CP-OFDM)-based radio access, the coexistence of different technologies without precise time-frequency synchronization is limited due to high out-of-band (OOB) emissions. Therefore, the spectrum enhancement techniques play a key role in relaxing the synchronization and power control requirements. This allows a higher degree of opportunistic spectrum use with minimized interference. In addition, all the transmitting devices have to fulfill specific transmitted signal quality requirements, including the maximum OOB radiated signal power. With the orthogonal frequency-division multiplexing (OFDM)-based radio access, some additional signal processing for improved spectrum containment is commonly needed to achieve these requirements. The filtering and time-domain windowing are two fundamentally different approaches for spectrum enhancement. The filtered OFDM (F-OFDM) provides better spectrum localization than the time-windowing schemes [such as windowed overlap-add (WOLA)], with the cost of higher complexity. This article introduces low-complexity solutions for spectrally enhanced narrowband OFDM transmitters based on the use of lookup tables (LUTs). The proposed LUT approach, requiring only memory units and a low number of additions, allows to avoid all computationally expensive operations in online transmitter processing, as it builds the transmitted signal by summing the stored partial waveforms optimized offline. In certain cases, completely multiplication- and summation-free designs are possible. The transmitters of narrowband Internet of Things (NB-IoT) devices are natural applications for the proposed LUT approach, as they require additional digital baseband signal processing to reach the emission requirements. It is shown that the proposed LUT schemes can provide significant savings in real-time computations of NB-IoT devices, while fulfilling the 3GPP requirements.

This paper investigates the 5G new radio (NR) uplink (UL) performance with CP-OFDM and DFT-s-OFDM based waveforms. The effects of highly non-linear PA behavior, inter-allocation interference, and UL multi-user MIMO on the relative performance of these waveforms are addressed. It is shown that with relaxed EVM and inband emission requirements for CP-OFDM, the coverage limited operation can be improved to achieve better link budget than DFT-s-OFDM based UL without performance penalty in the multi-user uplink. For high-throughput user equipment, the assumed highly non-linear PA behavior restricts the CP-OFDM based transmit power, which may limit the coverage compared to DFT-s-OFDM based waveform in UL without multi-user MIMO support. In UL multi-user MIMO scenario the CP-OFDM based waveform provides clearly better link performance and achieves better link budget than DFT-s-OFDM based waveform. Within the multi-user MIMO UL scenario, the requirement for substantially better PA linearity in high-throughput cases is also observed.

By the introduction of the fifth generation (5G) mobile communication networks and its physical layer entitled as new radio (NR), the question of link performance in coexistence scenario between the 5G NR and fourth generation (4G) mobile communication networks based on long term evolution (LTE) has been raised. In this paper, we evaluate the uplink (UL) performance of 5G NR and LTE links operating within a common channel. The need for subcarrier shift in 5G NR UL similar to LTE UL is addressed and analyzed, and the effect of guard band (GB) in frequency domain between 5G NR and LTE is studied. It is shown that with a single physical resource block GB no subcarrier shifting is required in 5G NR, as long as the power control accuracy is sufficient, in which case LTE performance is unaffected. From the 5G NR performance point of view no GB is required.

We study positioning of high-speed trains in 5G new radio (NR) networks by utilizing specific NR synchronization signals. The studies are based on simulations with 3GPP-specified radio channel models including path loss, shadowing and fast fading effects. The considered positioning approach exploits measurement of Time-Of-Arrival (TOA) and Angle-Of-Departure (AOD), which are estimated from beamformed NR synchronization signals. Based on the given measurements and the assumed train movement model, the train position is tracked by using an Extended Kalman Filter (EKF), which is able to handle the non-linear relationship between the TOA and AOD measurements, and the estimated train position parameters. It is shown that in the considered scenario the TOA measurements are able to achieve better accuracy compared to the AOD measurements. However, as shown by the results, the best tracking performance is achieved, when both of the measurements are considered. In this case, a very high, sub-meter, tracking accuracy can be achieved for most (>75%) of the tracking time, thus achieving the positioning accuracy requirements envisioned for the 5G NR. The pursued high-accuracy and high-availability positioning technology is considered to be in a key role in several envisioned HST use cases, such as mission-critical autonomous train systems.

Large databases with Received Signal Strength (RSS) measurements are essential for various use cases in mobile wireless communications and navigation, including radio resource management algorithms and network-based localization. Because of the constantly increasing number of radio transmitters with various wireless technologies and with the advent of 5G cloud computing and Internet of Things (IoT), the required size of the RSS databases are becoming unmanageably large. Thus, the requirements for the bandwidth and data rates for accessing the memory might become too costly. Therefore, in order to reduce the size of the RSS database, while maintaining the data quality, we have previously proposed the method of spectrally compressed RSS images, which are able to achieve considerable data compression of up to 70 percent. In this paper, we deeply analyze the process of spectral compression and introduce error sources, which affect the compression performance. Based on the analysis, we propose a novel theoretical framework and methods to optimize the performance of the spectral compression. In addition, we derive the Cramer-Rao Lower Bound (CRLB) for the RSS-based localization error and compare the CRLB between separate baseline localization approaches. The theoretical analysis is justified and compared with experimental RSS measurements taken from several multi-storey buildings.

Multirate fast convolution (FC) has recently been introduced as an effective tool for communication waveform processing, especially for advanced multicarrier systems targeting at well-contained spectrum. These include filter bank based multicarrier waveforms and filtered OFDM schemes which are receiving increasing attention in the 5G radio access development. Recalling that the key idea of FC is effective implementation of high-order linear filtering through frequency-domain processing, this paper investigates possibilities to reduce the complexity of FC based waveforms. Special focus is on scenarios where a relatively small part of the bandwidth is in active use, which could be the case, e.g., in low-rate machine-type communication devices. A new variant of fast-convolution filter bank (FC-FB) is developed which uses circular convolution decomposition. The narrowband variant of decomposed structure, called D-FC-FB, achieves significantly reduced complexity, which is proportional to the active bandwidth, while maintaining filtering performance equivalent to FC-FB. Therefore, this variant is considered as a low-complexity solution for low-rate devices. D-FC-FB can be used in any multicarrier scheme that utilizes filtering at subcarrier or resource block level. This paper develops closed-form complexity expressions for the case of filter bank multi-carrier with offset-QAM subcarrier modulation (FBMC/OQAM) demonstrating significant complexity reduction in a case study.

For the 5G new radio physical layer the CP-OFDM waveform has been chosen as the baseline for communications below 40 GHz. The requirement for multicarrier waveforms used for uplink is to achieve similar coverage as achieved by SC-FDMA in LTE uplink. In this paper, multiple candidate waveforms with enhanced CP-OFDM processing proposed for 5G incorporating realistic 3GPP compliant power amplifier model and peak clipping are evaluated in uplink transmission, and compared against SC-FDMA in terms of maximum average power amplifier output power and coded block error rate. It is shown that multicarrier waveforms have minor disadvantage in single-PRB transmission, but as the allocation size increases to encounter frequency selective fading the multicarrier waveforms provide similar or even improved link budget compared to SC-FDMA uplink. This implies that given the expected cell edge throughput requirements for 5G mobile broadband services and expected power amplifier development, enhanced CP-OFDM waveforms can achieve the uplink coverage requirement.

In this paper, subband filtered CP-OFDM and windowed CP-OFDM waveforms proposed for 5G new radio (NR) are compared to LTE-like downlink and uplink waveforms under different power amplifier (PA) models. The effects of subband filtering or windowing are evaluated in terms of out-of-band and inband emissions, average error vector magnitude (EVM) and maximum uplink PA output power when increasing the bandwidth efficiency from 90% to 97.2%. The evaluations include recently proposed PA models by 3GPP TSG-RAN WG1 for downlink and uplink. It is shown that the cost of increased bandwidth efficiency in fullband downlink transmission is mainly complexity increase required by the steeper channelization filter. On the other hand, the increased EVM caused by channel filter induced inter-symbol-interference may limit the usability of modulation 256-QAM and above. In uplink with fullband transmission using 64-QAM modulation, the spectral containment is not an issue because the required backoff with highly non-linear PA model limits the Tx power to a very low level independently of the maximum allocation size. In the 1 PRB uplink, increased bandwidth utilization decreases the maximum PA output power while increased backoff improves the inband ACLR.

Cognitive radio (CR) is considered an important solution to the current spectral scarcity, which is expected to be a significant issue in the next generation of wireless communication systems, namely, 5G. Wideband spectrum sharing and sensing constitute highly desirable features of CR systems as they aim to increase the probability of identifying available spectral bands, which ensures a more efficient resource utilization. This paper proposes an efficient frequency-domain cyclic prefix autocorrelation-based wideband spectrum sensing and sharing method that can provide accurate detection of orthogonal frequency-division multiplexing (OFDM)-based primaries in wideband CR systems. Novel analytic expressions are derived for the corresponding threshold, probability of false alarm, and probability of detection in the presence of noise uncertainty (NU) and frequency selectivity. The derived models are validated by extensive comparisons with respective results from computer simulations. It is demonstrated that the introduced autocorrelation-based sensing method is able to counteract NU and the frequency-selective multipath channel effects in realistic wideband communication scenarios. Furthermore, the method facilitates partial band sensing, allowing the sensing of weak OFDM-type primary user (PU) signals in channels, which are partly overlapped by other strong PU or CR transmissions. This is considered a crucial element in practical spectrum sharing scenarios. Since the proposed sensing method makes use of sparsity in the spectral domain, it can be technically considered as compressed sensing method. The flexibility of this approach supports robust wideband multi-mode, multi-channel sensing with low complexity. Finally, it is shown that the offered results are particularly useful in the context of spectrum sharing as their high performance and reduced complexity can enable the co-existence of non-exhaustive yet highly efficient algorithms.

This paper investigates the application of fast-convolution (FC) filtering schemes for flexible and effective waveform generation and processing in the fifth generation (5G) systems. FC-based filtering is presented as a generic multimode waveform processing engine while, following the progress of 5G new radio standardization in the Third-Generation Partnership Project, the main focus is on efficient generation and processing of subband-filtered cyclic prefix orthogonal frequency-division multiplexing (CP-OFDM) signals. First, a matrix model for analyzing FC filter processing responses is presented and used for designing optimized multiplexing of filtered groups of CP-OFDM physical resource blocks (PRBs) in a spectrally well-localized manner, i.e., with narrow guardbands. Subband filtering is able to suppress interference leakage between adjacent subbands, thus supporting independent waveform parametrization and different numerologies for different groups of PRBs, as well as asynchronous multiuser operation in uplink. These are central ingredients in the 5G waveform developments, particularly at sub-6-GHz bands. The FC filter optimization criterion is passband error vector magnitude minimization subject to a given subband band-limitation constraint. Optimized designs with different guardband widths, PRB group sizes, and essential design parameters are compared in terms of interference levels and implementation complexity. Finally, extensive coded 5G radio link simulation results are presented to compare the proposed approach with other subband-filtered CP-OFDM schemes and time-domain windowing methods, considering cases with different numerologies or asynchronous transmissions in adjacent subbands. Also the feasibility of using independent transmitter and receiver processing for CP-OFDM spectrum control is demonstrated

Cognitive radio (CR) has received increasing attention and is considered an important solution to the spectral crowding problem. The main idea behind CR technology is to utilize the unused spectral resources which are determined to be available for secondary user by effective spectrum sensing techniques. However, CR technology significantly depends on the spectrum sensing techniques which are applied to detect the presence of primary user (PU) signals. This paper focuses on detecting OFDM primaries using novel frequency-domain cyclic prefix (CP) autocorrelation based compressive spectrum sensing algorithms. To counteract the practical wireless channel effects, frequency domain approaches for PU signal detection are developed. The proposed spectrum sensing method eliminates the effects of both noise uncertainty and frequency selective channels. Using the frequency domain autocorrelation approach results in highly increased flexibility, facilitating robust wideband multi-mode, multi-channel sensing with low complexity. It also allows to sense weak PU signals which are partly overlapped by other strong PU or CR transmissions.

Device-to-Device communication (D2D) has been a hot research topic in recent years, especially concerning the integration of D2D and conventional cellular networks. In general, the integration improves the efficiency and flexibility of the overall system. Furthermore, D2D can provide partial solutions for some inherent problems of cellular systems, such as cell edge related problems. Currently, most of the D2D integration studies assume that D2D UEs and cellular UEs use identical multiple access (MA) technology for the convenience of integration and backward compatibility. In this paper, besides the conventional OFDMA based D2D, we investigate the use of multicarrier code division multiple access (MC-CDMA) as the MA scheme for D2D UEs. The idea is to use processing gains obtained by spreading against critical interference, especially from cellular UEs. The study is emphasized on interference related issues, including interference analysis of the integrated system, and overall performance enhancement, mainly for D2D UEs, by utilizing frequency domain spreading. Network simulations show the gain against critical interference from cellular UEs. Meanwhile, it is shown that intra-cell cross mode interference control/elimination is the key scheme to guarantee performance of the integrated system. Furthermore, we discuss the potential benefits and procedures of MC-CDMA D2D based cluster integration.

Forthcoming fifth generation (5G) mobile networks need to support a wide variety of services, from mobile broadband (MBB) communications supporting up to 100 Gbps throughput and millisecond class latency to ultra-low power machine-to-machine (M2M) communications supporting very low bit rates and tens of years operation with single battery or operation together with new energy harvesting solutions. In this paper, we concentrate on the MBB aspect and consider two new OQAM-OFDM based waveform candidates proposed for 5G communications while noting the strict time latency requirements. Also, we compare different preamble designs used for channel estimation and show that some of the pseudo-pilot power maximizing interference approximation method based preamble designs are not suitable for communications with any nonlinear power amplifier (PA) model and that an ad-hoc random OQAM based preamble design performs well compared to specific optimized preamble designs.

The present work is devoted to the comprehensive analysis of the detrimental effects of noise uncertainty in wide-band multimode subband based spectrum sensing over frequency selective channels. Unlike existing analyses that are mostly limited to single-band detection, an analytical model is firstly formulated for the case of wide-band multimode subband based spectrum sensing, considering both noise uncertainty and frequency selectivity. This model is also extended for the case of detecting a reappearing primary user (PU), as well as for the sensing scenarios where the frequency range of a PU is unknown. Novel closed-form expressions are derived for the corresponding probabilities of false alarm and probabilities of detection. The derived expressions are subsequently employed in quantifying the effects of noise uncertainty and frequency selectivity. A subband based scheme for the sensing of multi-channel primary systems is presented and analyzed. The proposed method is found to exhibit significantly reduced sensing time and greatly improved robustness against noise uncertainty compared to the basic wideband energy detection method.

Spectrum scarcity has become a critical concern in wireless communication systems due to the limited availability of frequency spectrum. Hence, cognitive radio (CR) has been introduced as a solution for more effective use of the spectrum resources. Spectrum sensing (SS) is one of the key elements in the implementation of effective and reliable CR systems. Energy detection (ED) based SS is the most common sensing algorithm due to its low complexity. The main drawback of ED based SS is that it is highly dependent on the precise knowledge of the receiver noise variance. Hence, the performance of the ED algorithm is degraded significantly, when there is noise uncertainty in the estimation of the noise variance. In this study, we apply a recently proposed enhanced ED based algorithm to the sensing of Wireless Microphone (WM) signals, demonstrating robustness to noise uncertainty in real-time testing with actual WM signals. This so-called Max-Min ED algorithm is based on subband division of a wideband signal using an analysis filter bank (AFB) and utilizing the difference of maximum and minimum subband energies as the test statistic. Following the introduction of analytical models and scenarios of ED based SS algorithms, the sensing algorithms are implemented and tested using National Instruments (NI) Universal Software Radio Peripheral (USRP) and the NI-LabVIEW software platform, together with the necessary toolboxes.

Exponentially-modulated filter bank (EMFB) is an extension of odd-stacked cosine-modulated filter bank (CMFB) for subband processing of complex-valued signals. Paraunitary EMFB can be implemented using complex extended lapped transform (CELT) which is structurally critically sampled perfect reconstruction system. In this paper we introduce the necessary modifications to make the CELT algorithm suitable for invertible complex-valued integer-to-integer transform. The effect of alternative scaling approaches are compared in terms of subsignal integer range growth and maintaining pulse waveform in signal synthesis.

This paper investigates the use of effective and flexible fast-convolution (FC) implementation for two main types of filter bank multicarrier (FBMC) waveforms, FBMC/OQAM and filtered multitone (FMT). The generated waveforms are spectrally well-contained, with very small power leakage to adjacent frequencies, and are thus good candidates for opportunistic and heterogeneous spectrum use scenarios. On the receiver side, the FC filter bank (FC-FB) approach can be used for simultaneously processing multiple channels with individually tunable bandwidths, center frequencies, and timing offsets, supporting asynchronous multi-user operation. FC-FB implementations are compared with traditional polyphase designs in terms of spectral characteristics and computational complexity.

Device-to-Device (D2D) communication has been a hot research topic in the recent years. Among different D2D use cases, the integration of D2D and conventional cellular network is one of the most attractive schemes. This approach has been investigated in recent academic and standardization studies, since such a combination improves the efficiency of the overall system in both resource and power domains. It also improves the coverage of the cellular network and improves the cell edge performance. In this paper, such a system combining D2D and OFDMA/SC-FDMA based cellular network is investigated, targeting at simple and effective resource management algorithms. The network level performance of a system using proposed algorithms is evaluated by simulations. We propose the use of separate resource pools for cellular and D2D UEs, with in-cell resource reuse utilizing the D2D resource pool. This is shown to provide significant gains in terms of spectrum efficiency and network level performance, especially at the cell edge.

In this article, we focus on the RF and digital front-end design and implementation challenges associated with future 5G radio access systems with special emphasis on spectrally contained waveforms and small-cell system scenarios. In general, filter bank based multicarrier (FBMC) type of techniques have various potential benefits due to their excellent spectral containment compared to classical OFDM(A). However, these spectrally contained waveforms lose their intriguing spectral properties when power amplifier (PA) nonlinearities are considered. Flexible and efficient digital predistortion (DPD) algorithms are thus considered an interesting solution in order to restore the spectral containment of such advanced 5G waveforms. Noncontiguous spectrally-agile transmission is another key feature of future 5G systems for increasing data rates and spectral allocation flexibility. However, the PA nonlinearities impose even more severe challenges in such noncontiguous transmission scenarios due to the resulting spurious intermodulation emissions that can easily violate the emission limits or even desensitize the own receiver in frequency division duplexing based systems. Furthermore, at the network deployment level, different small-cell concepts are expected to play a major role in future 5G networks. Unlike the ordinary macro base-stations, the digital computing capabilities in small-cell base-stations are much more limited. Furthermore, they should also adopt lower-cost and small-size analog RF components, while still maintaining high energy-efficiency. The afore-mentioned constraints, along with advanced 5G waveforms, call for flexible and low-complexity DPD solutions, a challenge addressed in this article. We report novel DPD methods with built-in capability to direct the linearization performance to pre-defined frequencies or subbands in a flexible manner, and demonstrate their good performance and complexity benefits in the context of non-contiguous FBMC transmission.

Filter bank multicarrier (FBMC) is considered as one of the strong 5G waveform candidates due to better spectral containment compared to orthogonal frequency division multiplexing (OFDM). However, when considering the spectral leakage effects appearing in practical devices due to transmitter non-idealities, such as power amplifier (PA) nonlinearity and inphase-quadrature (I/Q) imbalance, the nice spectral properties of FBMC may be severely degraded, thus questioning the additional complexity needed to generate such waveforms in baseband. In this paper, digital pre-distortion (DPD) is investigated as a practical solution for both of these problems. We extend the study also to potential applications of spectrum sensing based cognitive radio in future wireless communications. We utilize the benefits of the analysis filter bank of FBMC receiver in wideband multichannel spectrum sensing. In this context, the power leakage due to transmitter non-idealities also makes it difficult to identify spectral gaps amongst relatively strong primary transmissions. It is demonstrated that effective DPD greatly facilitates both spectrum sensing and spectrum utilization functions and thus enhances the overall spectral efficiency in opportunistic dynamic spectrum use scenarios.

Cognitive radio is an emerging wireless technology that is capable of efficiently coordinating the use of the currently scarce spectrum resources, and spectrum sensing constitutes its most crucial operation. This paper proposes wideband multichannel spectrum sensing methods utilizing fast Fourier transform or filter-bank-based methods for spectrum analysis. Fine-grained spectrum analysis facilitates optimal energy detection in practical scenarios where the transmitted signal, channel frequency response, and/or receiver frequency response do not follow the commonly assumed boxcar model, which typically assumes, among other things, narrow-band communications with flat spectral characteristics. Such sensing schemes can be tuned to the spectral characteristics of the target primary user signals, allowing simultaneous sensing of multiple target primary signals with low additional complexity. This model is also extended to accounting for the specific scenario of detecting a reappearing primary user during secondary transmission, as well as in spectrum sensing scenarios where the frequency range of a primary user is unknown. Novel analytic expressions are derived for the corresponding probability of false alarm and probability of detection in each case, while the useful concept of the area under the receiver operating characteristics curve is additionally introduced as a single scalar metric for evaluating the overall performance of the proposed spectrum sensing algorithms and scenarios. The derived expressions have a rather simple algebraic representation, which renders them convenient to handle both analytically and numerically. The offered results are also extensively validated through comparisons with respective results from computer simulations and are subsequently employed in evaluating each technique analytically, which provides meaningful insights that are anticipated to be useful in future deployments of cognitive radio systems.

Multirate filter banks can be implemented very efficiently using fast-convolution (FC) processing. The main advantage of the FC filter banks compared with the conventional polyphase implementations is their increased flexibility, that is, the number of channels, their bandwidths, and the center frequencies can be independently selected. In this paper, an approach for optimizing the FC filter banks is proposed. First, a subband representation of the adjustable FC filter banks is derived. Then, the optimization problems are formulated with the aid of the subband model. Finally, these problems are conveniently solved with the aid of a nonlinear optimization algorithm. Two examples are included to illustrate the performance of the proposed overall scheme.

Direct-conversion radio receivers can offer highly integrated low-cost hardware solutions for cognitive radio (CR) devices. Such receivers are, however, also very sensitive to various radio frequency (RF) impairments such as IQ imbalance, which can considerably limit the spectrum sensing capabilities. Most of the existing spectrum sensing studies in literature assume an ideal RF receiver and hence neglect the impacts of such practical RF hardware limitations. In this article, we study energy detection (ED) based spectrum sensing in both single-channel and multi-channel direct-conversion receiver scenarios impaired by IQ imbalance. With complex Gaussian primary user (PU) signal models, we first derive the detection and false alarm probabilities in closed-form for both receiver scenarios. The analytical results, confirmed through extensive simulations, show that while the single-channel receiver scenario is fairly robust to IQ imbalance, the wideband multi-channel sensing receiver is very sensitive to the image channel crosstalk induced by IQ imbalance. More specifically, it is shown that the false alarm probability of multi-channel energy detection increases significantly, compared to ideal RF receiver case, and the exact performance depends on the image channel power level and IQ imbalance values. In order to prevent such degradation in the ability to identify free spectrum, a waveform level interference cancellation method is then proposed to mitigate the image channel crosstalk. The optimum cancellation coefficient yielding interference-free signal is first derived, being also complemented with a practical coefficient sample estimator. An explicit condition is also derived for which the proposed cancellation scheme deploying the practical coefficient sample estimator provides performance gain in the sensing decisions compared to uncompensated energy detection. Extensive computer simulations with various signal and imbalance conditions are provided which demonstrate that the proposed enhanced energy detection can suppress the image channel crosstalk efficiently, yielding detection and false alarm probabilities essentially identical to those of an ideal RF receiver.

In cognitive radio networks, the unoccupied frequency bands licensed to primary users can be opportunistically accessed by secondary (low-power or hidden) cognitive users. Secondary users can be authorized low-power users, or unauthorized (hidden) users that occupy the band illegally transmitting under the noise level. More in details, the devised method is able to detect unknown low-power constant-modulus signals in noise of unknown variance, exploiting higher order moments of the received signal. The decision variable used in the detection stage is represented (only) by the estimated power of the hidden signal. Performance analysis is carried out in comparison with conventional energy detection, in the presence of noise uncertainty. In particular, the detection probabilities of the proposed and conventional methods as well as the mean detection time are evaluated and compared. The numerical results, obtained from wide simulation trials, have evidenced the efficiency of our method for detecting the hidden user's low-power signal in cognitive communications.

Direct-conversion radio receivers can provide highly-integrated and low-cost hardware solutions for cognitive radio (CR) devices. However, such receiver structures are also known to suffer from various RF circuit imperfections, especially IQ imbalance. When studying the performance of various spectrum sensing methods, most existing work in literature neglect the effects of such RF imperfections. In this paper, we analyze the performance of a well-known eigenvalue based sensing method, namely maximum-to-minimum eigenvalue test, in the challenging multichannel spectrum sensing context under sensing receiver RF IQ imbalance. It is analytically shown that under RF IQ imbalance, the false alarm probability of the spectrum sensor is always higher compared to ideal RF front-end case. Then, in order to compensate for such degradation in the ability to identify free spectrum, a waveform level interference cancellation solution is deployed to enhance the performance, and optimum cancellation coefficient together with a practical sample estimator are formulated. Extensive computer simulations are provided to illustrate the degradation of false alarm probability in the presence of practical IQ imbalance values. Furthermore, the simulations also show that the proposed interference cancellation method can efficiently suppress the IQ imbalance effects, achieving identical false alarm probability with the ideal RF front-end case.

OFDM as multicarrier scheme provides adaptive functionality that is required to implement waveform processing for cognitive radio, dynamic spectrum use and fragmented coexistence scenarios. However, OFDM faces some difficulties due to the relatively strong sidelobes around the active subcarriers, which introduces interference effects between adjacent frequency slots intended to be used independently in non-synchronized way. In this paper we consider edge windowing and cancellation carrier approaches for controlling the spectrum leakage in a practicadl 5 MHz 3GPP LTE scenario. The focus is on minimizing the OFDM signal leakage to narrow unused gaps in the used frequency band, using low-complexity algorithms and minimized overheads in terms of power and spectral efficiency. We develop a simplified version of the cancellation carrier scheme which is suitable for non-contiguous subcarrier allocation in OFDM. It is shown that with relatively narrow unused gaps in the spectrum, it is possible to reach good level of sidelobe suppression with significantly reduced computational complexity.

Our main emphasis is on the use of enhanced OFDM and filter bank based multicarrier (FB-MC) waveforms for utilizing effectively the available fragmented spectrum in heterogeneous radio environments. Special attention is on the broadband-narrowband coexistence scenario of the Professional Mobile Radio (PMR) evolution. The target here is to provide broadband data services in coexistence with narrowband legacy services of the TETRA family. The core idea is a multi-mode radio platform, based on variable filter bank processing, which is able to perform modulation/detection functions simultaneously for different signal formats with adjustable center frequencies, bandwidths and subchannel spacings.

A Bayesian method for dynamical off-line estimation of the position and path loss model parameters of a WLAN access point is presented. Two versions of three different on-line positioning methods are tested using real data. The tests show that the methods that use the estimated path loss parameter distributions with finite precisions outperform the methods that only use point estimates for the path loss parameters. They also outperform the coverage area based positioning method and are comparable in accuracy with the fingerprinting method. Taking the uncertainties into account is computationally demanding, but the Gauss-Newton optimization method is shown to provide a good approximation with computational load that is reasonable for many real-time solutions.

Wireless Local Area Networks (WLAN) and Wireless Personal Area Networks (WPAN) such as the Bluetooth are designed to operate in 2.4 GHz ISM band. Since both IEEE 802.15 based Bluetooth and IEEE 802.11 WLAN devices, among various others, use the same frequency band, interference may lead to significant performance degradation. Hence, Cognitive Radio (CR) technology has been considered for coordinating better the spectrum use in this band. Spectrum sensing is one of the most important functions in a CR. In this study, energy detector based spectrum sensing techniques are optimized for detecting Bluetooth signals, considering both the effect of non-flat power spectrum and frequency hopping characteristics. To reduce complexity and required number of samples for effective spectrum sensing, optimum weighting process is proposed for subband based spectrum sensing. Bluetooth sensing is analyzed also in the presence of WLANs at nearby frequencies.

The everlasting public interest on location and positioning services has originated a demand for a high performance Global Navigation Satellite System (GNSS), such as the Global Positioning System or the future European satellite navigation system, Galileo. The performance of GNSS is subject to several errors, such as ionosphere delay, troposphere delay, receiver noise and multipath. Among all these errors, multipath is the main limiting factor in precision-oriented GNSS applications. In order to mitigate the multipath influence on navigation receivers, the multipath problem has been approached from several directions, including the development of novel signal processing techniques. Many of these techniques rely on modifying the tracking loop discriminator in order to make it resistant to multipath. These techniques have proved very efficient against multipath having a medium or large delay with respect to the Line-Of-Sight (LOS) signal. In general, the multipath errors are largely reduced for multipath delays greater than around 0.1 chips (which is about 29.3 meters for Galileo E1 Open Service (OS) signal). Theoretically, this constitutes a remarkable improvement as compared to simpler techniques such as narrow Early-Minus-Late (nEML) tracking loop. However, in practice, most of the multipath signals enter the receiver with short-delay with respect to LOS signal, making most of these mitigation techniques partially ineffective. In this paper, we propose a new multipath estimation technique, namely the Slope-Based Multipath Estimation (SBME), which is capable of mitigating the short-delay multipath (i.e., multipath delays less than 0.35 chips) quite well compared with other state-of-the-art mitigation techniques, such as the nEML and the High Resolution Correlators (HRC). The proposed SBME first derives a multipath estimation equation by utilizing the correlation shape of the ideal normalized correlation function of a Binary Phase Shift Keying (BPSK)- or Multiplexed Binary Offset Carrier (MBOC)-modulated signal, which is then used to compensate for the multipath bias of a nEML tracking loop. It is worth to mention here that the SBME requires an additional correlator at the late side of the correlation function, and it is used in-conjunction with a nEML tracking loop. The multipath performance of the above-mentioned mitigation techniques is presented for Galileo E1 OS and GPS L1 C/A signals from theoretical as well as simulation perspective.

In this paper an iterative, interference cancelling receive structure for single carrier transmission using superimposed training is studied. We derive the analytical MSE limits for LMMSE channel estimator using perfect channel knowledge and show that the simulated values follow well the analytical ones. Then, we utilise these results to analyse the MSE performance of a combined ML-LMMSE chanel estimator structure, where the initial channel estimates are obtained from the ML channel estimator

This paper studies the impact of one important radio transceiver impairment, namely the I/Q imbalance, on the link performance of spatial multiplexing MIMO-OFDM systems. A closed-form solution for the effective signal-to-interference-plus-noise-ratio (SINR) at the input of the receiver detection stage due to frequency-selective transmitter and receiver I/Q imbalances is derived, taking also the effects of fading multipath radio channel and additive noise into account. As will be shown, the derived SINR can be directly mapped to the achievable detection error rate at high SNR regime, yielding a valuable analytical tool for radio transceiver designers to analyze the imbalance effects at link-level without any actual data simulations. The analytical outcomes are also verified using extensive computer simulations backing up the theoretical studies. In general, the obtained results indicate that I/Q imbalance can easily become a limiting factor to the achievable link performance in future spatial multiplexing MIMO-OFDM systems, and thus should be carefully mitigated using proper digital and/or analog signal processing.

This paper presents comparatory studies of two different peak-to-average power-ratio (PAPR) reduction methods when applied to orthogonal frequency division multiplexing (OFDM) and filter bank based multicarrier (FBMC) communication signals. The studied methods are based on clipping with windowing, and they were originally devised for OFDM transmitters. The performance comparisons are done by evaluating bit error rate and spectral regrowth behavior. The study also investigates the near-far effect for adjacent signals considering both the synchronized and unsynchronized cases. The results show that the methods designed for OFDM can be fully applied to FBMC with similar performance in the synchronized case. Further, the well-known advantage of high spectral selectivity of FBMC in unsynchronized scenarios is also clearly visible when applying the studied techniques.

In earlier studies, we have considered filter bank based frequency domain processing for channel equalization both in multicarrier and single-carrier transmission. With mildly frequency selective subband processing and modest number of subbands, these approaches are able to provide better performance than the conventional FFT-based multicarrier technique OFDM, on one hand, and FFT-based frequency-domain equalizer in single-carrier transmission, on the other. The benefits in transmission link efficiency are due to the lack of guard-interval overhead and, in the multicarrier case, also due to reduced guard- band. Another significant advantage of this approach is that the same filter bank can be utilized to implement significant part of the receiver selectivity, in a very flexible way. In this paper, we extend the filter bank approach to cover also multi- antenna reception schemes. We show that the subband equalizer structure of the basic single-input scheme can be used for the different antennas of a multi-antenna receiver while introducing frequency domain combining of the diversity branches. We also show how to extend the frequency-sampling based techniques for channel equalizer design to the multi-antenna case. Performance evaluation of a simple receiver antenna diversity scheme in single-carrier transmission with maximum ratio combining is included to verify the ideas.

This work presents a new technique for blind and joint estimation of symbol timing and carrier frequency offset in wireless orthogonal frequency division multiplexing (OFDM) systems. The joint estimation is achieved by sensing the interference introduced at the fast Fourier transform (FFT) output when a carrier frequency or timing offsets exist. The synchronization parameters are selected such that the interference is minimized. The proposed joint estimator is highly efficient because it does not require any overhead or channel state information. Simulation results show that the system is effective and robust even at low signal-to-noise ratios.

I/Q imbalance is one of the main practical obstacles in the implementation of direct-conversion receivers. This paper presents novel DSP-based techniques for the estimation and compensation of frequency-selective receiver I/Q imbalances in OFDM systems. The estimation is based on a special pilot or preamble structure, and frequency-domain smoothing is utilized to effectively reduce the effect of noise. Reliable estimation is attained with a minimum of two OFDM symbols. Further, it is shown that estimation of frequency-selective I/Q imbalance and the frequency-selective radio channel can be decoupled, which is beneficial from the computational complexity point of view. Simulation analysis shows impressive performance with fast convergence.

In this paper, we study the performance of our single carrier receiver model using turbo coding and regular low density parity check (LDPC) coding. The single carrier receiver utilizes filter bank based frequency domain equalizer, which is then followed by a low complexity, iterative noise predictive decision feedback equalizer (NP-DFE). Furthermore, the average computation rate as a function of Eb/N0ratio is observed for different coding methods, thus characterizing the performance vs. complexity trade-offs in the considered receiver structure in terms of the channel coding. Clear performance benefits from the NP-DFE loop can be achieved and the averaged complexity increase in our system is tolerable in the Eb/N0range where frame error rates below 10-1are achieved. In addition, by fixing the maximum decoding complexity to be the same between turbo and LDPC codes, the performance of these codes is similar but the averaged complexity of the LDPC code saturates to a clearly lower level.

This paper presents a pilot-based compensation algorithm for mitigation of frequency-selective I/Q imbalances in direct-conversion OFDM transmitters. By deploying a feedback loop from RF to baseband, together with a properly-designed pilot signal structure, the I/Q imbalance properties of the transmitter are efficiently estimated in a subcarrier-wise manner. Based on the obtained I/Q imbalance knowledge, the imbalance effects on the actual transmit waveform are then mitigated by baseband pre-distortion acting on the mirror-subcarrier signals. The compensation performance of the proposed structure is analyzed using extensive computer simulations, indicating that very high image rejection ratios can be achieved in practical system set-ups with reasonable pilot signal lengths.

Gain and phase differences between the analog in-phase (I) and quadrature (Q) branches of a quadrature receiver are unavoidable, and seriously degrade its image rejection capabilities. Furthermore, this so-called I/Q imbalance problem is in general a frequency-dependent phenomenon, which is often ignored in many otherwise excellent work. In this paper, we take this frequency-dependency into account and study a class of I/Q imbalance compensators based on widely linear (WL) processing of the received mismatched signal, under the assumption that the ideal baseband signal is proper (or circular). In other words, the complementary autocorrelation function of the ideal baseband signal is assumed to vanish, which is a valid assumption for most practical communications signals. Under I/Q imbalance the observed baseband equivalent signal becomes improper, and I/Q imbalance compensation can be performed by making the observation proper again. We propose a simple blind (non-data aided) WL compensator structure for suppressing the mirror-frequency interference. It shows impressive performance and has many additional desirable features, such as immunity to channel noise and the fading channel.

The combination of orthogonal frequency division multiplexing (OFDM) and multiple-input multiple-output (MIMO) techniques has been widely considered as the most promising approach for building future wireless transmission systems. In such systems, the limited implementation resources for the radio parts cause big restrictions on the size, cost, and quality of the individual radio transceivers. This implies that there are several imperfections at the analog radio front-end stages, which in turn can severely limit the overall system performance. One good example is the so called I/Q imbalance problem related to the amplitude and phase matching of the transceivers I and Q chains. This paper studies the performance of space-time coded (STC)-OFDM systems under I/Q imbalance. As a practical example, a 2 × 1 STC-OFDM system is examined in detail and a closed-form solution for the resulting signal-to-interference ratio (SIR) due to I/Q imbalance at the output of the receiver combining stage is derived. The analytical outcomes are verified using extensive computer simulations, and can easily be extended to multi-antenna receiver cases as well. In general, the obtained results indicate that I/Q imbalance can easily become a limiting factor in practical STC-OFDM systems and should be carefully mitigated using proper digital and/or analog signal processing.

Line-of-sight (LOS) delay estimation with high accuracy is a pre-requisite for reliable location via satellite systems. The future European satellite positioning system, Galileo, uses spread-spectrum signals modulated via binary-offset-carrier (BOC) modulation. The receiver for a BOC-modulated spread spectrum signal has to cope not only with multipath effects, but also with possible lost of lock due to additional peaks in the envelope of the correlation function. Traditionally, code tracking is implemented at the receiver side via feedback delay locked loops. Feedforward methods have also been presented as alternatives for increased delay estimation accuracy, especially for short-delay multipaths. The increase in the delay estimation accuracy is typically counter-balanced by a faster mean time to lose lock (MTLL). In this paper we introduce a new algorithm, namely the peak tracking (PT) algorithm, which combines the feedback technique with the feedforward technique, in such a way that it increases the delay estimation accuracy while preserving a good MTLL.

The so-called single-carrier FDMA (SC-FDMA) waveform class, also known as DFT-spread OFDM, is a special form of multicarrier modulation, and has received a lot of interest in 3.9G system context recently. This paper addresses the radio implementation and RF impairment issues related to the reception of SC-FDMA waveforms. The main emphasis is on the so-called I/Q imbalance problem which in general results in imperfect attenuation of the mirror-frequencies. Here, the role of mirror-frequencies and mirror-frequency interference is addressed in detail from the SC-FDMA signals point of view, covering both the localized and distributed transmission modes. Furthermore, under perfect I/Q balance, it is shown that SC-FDMA signals satisfy certain circularity conditions related to the second-order statistics of complex random signals. This circularity, on the other hand, is lost due to I/Q imbalance which can then be used as a basis for developing efficient digital signal processing (DSP)-based algorithms to compensate for the I/Q imbalance effects. Comprehensive system simulations are carried out in both the localized and distributed spectral deployment cases, with and without digital compensation, to illustrate the relative effects of I/Q imbalances and the compensation stage on the overall system performance.

This paper describes different techniques for mitigation of high power and long duration interference in terrestrial digital video broadcasting (DVB-T/H) systems. The interference is assumed to distort the DVB-T/H signal severely so that on average one fourth of the DVB-T/H symbol is lost. We investigate the use of long enough time interleaver in combination with simple soft bits scaling. Additional mitigation of the interference effects is also achieved using the pilots carried by the DVB-T/H signal. The mitigation schemes are evaluated in the case where the interference occurs in additive white Gaussian noise (AWGN) and static frequency selective channels.

The OFDMA technology has been proposed as one of the main downlink schemes for the B3G systems. In this paper, the characteristics of a Wiener filter based channel estimation scheme have been investigated for the downlink OFDMA system. The basic pilot patterns have also been studied for the proposed channel estimator. In general, it has been shown that with proper pilot density, a 2D Wiener filter can provide good performance in different channel conditions. So such a channel estimator can be proposed as one of the suitable channel estimation schemes for the downlink OFDMA system.

For coded transmission over band-limited channels with inter-symbol interference, Douillard proposed the turbo equalization approach which has been studied widely. It is an iterative equalization/decoding algorithm and results in tremendous performance improvement. This paper studies a turbo equalization approach, which includes our recently proposed filter bank based frequency-domain equalizer, and a noise predictor inside of a decision-feedback loop, in coded single-carrier transmission. Its performance is simulated in a frequency-selective channel for QPSK and 16-QAM modulations. Different number of iterations and noise prediction orders are examined. The simulation shows that one iteration with 5-taps noise prediction filter is sufficient to obtain significant gain over linear equalizer.

Abstract An important aspect in designing the modulation scheme for various satellite systems, such as the modernized GPS and Galileo, is to obtain good spectral properties and suitable spectral shaping. For example, in the future satellite navigation systems, some of the main goals are: low interference with the existing GPS signals, good root‐mean‐square (RMS) bandwidth, good time resolution (in order to allow the separation between channel paths and to decrease the synchronization errors) etc. Starting from the recently proposed cosine‐ and sine‐BOC modulation families for GPS and Galileo systems, we introduce a new, generalized family, denoted here by double‐BOC (DBOC) modulation. We derive and analyze the properties of the power spectral densities (PSD) and autocorrelation functions (ACF) of the DBOC modulation with various orders, we show its relationship with BPSK, sine‐ and cosine‐BOC modulations, and we illustrate via several examples how to choose optimally the parameters of this new modulation family, according to different optimization criteria. The examples are targeting at applications such as the design of suitable modulations for Galileo open service (OS) and public regulated service (PRS) signals, but the authors believe that the DBOC concept might be useful to other satellite‐based applications, when the available bandwidth is large enough. Copyright © 2006 John Wiley & Sons, Ltd.

The Binary Offset Carrier (BOC) modulation which has been proposed for future Galileo and GPS M-code signals, provides a higher spectral separation from BPSK-modulated signals, such as GPS C/A code. The absolute value of the auto-correlation function of a BOC signal has a narrower main lobe, which may increase the resolution of delay estimates, but also presents deep fades, which may lead to a higher number of timing hypotheses to acquire the signal. In order to get rid of these ambiguities, several approaches have been proposed in literature, which provide an unambiguous BPSK-like shape of correlation function. In this paper we analyze, compare and develop further two BPSK-like methods which allow to acquire a BOC-signal unambiguously. The focus is on hybrid search, where several time-frequency bins are searched in parallel. We introduce here a modified version of a BPSK-like method which decreases the receiver complexity and is valid for both odd and even BOC orders. We analyze both single-side band (SSB) processing (i.e., only one band is used) and dual-side band (DSB) processing (i.e., upper and lower bands are combined non-coherently). While eliminating the ambiguities in auto-correlation function, both SSB and DSB processing present some performance degradation, induced by the band selection and non-coherent processing. The analysis is done in the presence of multipath fading channels. As a benchmark, we keep also the ambiguous BOC processing. We consider parameters specified in the proposals for Galileo system Open Service (OS), respectively Publicly Regulated Service (PRS).

In this paper we present a method to mitigate the effects of a narrowband interference present in an adjacent subchannel of a filter bank based multicarrier (FBMC) system. For this purpose, we use the spectral shaping capabilities of a low-complexity subchannel equalizer for filter banks. With this method, over 20 dB stronger narrowband interference (NBI) power can be tolerated as compared to a system without interference mitigation. These findings are relevant when improving the throughput / spectral efficiency of an FBMC system in environments with narrowband interference.

In this paper, the performance of an exponentially modulated filter bank based multicarrier (FBMC) modulation is compared with a cyclic prefix based OFDM (CP-OFDM). Both frame and bit error rates (FER/BER) are evaluated using the ITU-R vehicular-A channel model and LDPC forward error correction coding. For FBMC, a low complexity per-subcarrier equalizer called AP-ASCET is utilized to perform channel equalization. Simulation results indicate that, compared to FBMC, CP-OFDM requires about 1.5 dB higher source Eb/Noto operate on same information rate at frame error rate of 10-1and bit error rate of 10-3, respectively

This paper studies filter bank based multicarrier data transmission over wireless mobile channel. A complex modulated filter bank, based on cosine and sine modulated filter banks, is used as a transmultiplexer and a low complexity equalizer called AP-ASCET is used to compensate the channel distortion. At first, various filter bank design criteria are compared and analyzed based on their time-frequency localization (TFL) properties. Then, the system performance is simulated using the ITU-R vehicular-A channel model. TFL measures and uncoded bit error rate (BER) results indicate that even short subchannel filters can provide sufficient performance in narrowband interference-free channel conditions. Furthermore, the filter bank design criterion has only a small effect on the system performance if AP-ASCET approach is used

When targeting receiver integrability and flexibility, the choice of the front-end architecture is in key position. Instead of the traditional superheterodyne architecture, more recent I/Q or quadrature receiver front-ends are receiving increasingly more interest. One crucial aspect in quadrature receiver front-ends is the amplitude and phase matching of the analog I and Q signal branches. I/Q mismatches result in incomplete image signal or mirror frequency attenuation which must be enhanced using additional analog or digital signal processing. This paper presents and analyzes a novel DSP-based blind (non-data-aided) technique for I/Q imbalance compensation, utilizing a property of the ideal baseband equivalent called circularity. The weights of the compensator are computed directly from the estimates of the second-order moments of the mismatched baseband equivalent received signal. This algorithm is further simplified, with minimal loss in performance, yielding an extraordinarily simple yet effective compensation technique which ideally triples the dB value of the analog front-end image rejection ratio. The algorithms are applicable to any I/Q receiver structure, whether single-channel or multi-channel, and are in fact totally independent of any specific structure or characteristic (other than circularity) of the ideal baseband equivalent signal

The direct-conversion or homodyne receiver principle has received lots of interest in recent years for building flexible and compact radios. The homodyne architecture is, however, found to be rather sensitive to certain non-idealities of the analog front-end components. One big concern here is the amplitude and phase mismatches of the I and Q signal branches, resulting in insufficient rejection of the mirror frequency band. This paper presents an efficient low-order statistics based method (stat) for digital I/Q mismatch compensation, and compares the proposed technique with the blind signal separation (BSS) and hard decision (HD) based compensators proposed earlier by the authors. In addition to algorithm developments, big emphasis in the paper is on performance measurements and verifications carried out using measured true-world receiver front-end signals obtained from a laboratory homodyne measurement system. In general, the obtained results indicate that I/Q imbalance can be efficiently compensated using the proposed ideas. Even with really high-order modulation techniques, such as 64-QAM, close-to additive noise bound compensator performance is demonstrated using the measured signals

Complex modulated filter bank (FB) based frequency-domain equalization (FDE) provides an attractive single-carrier scheme for broadband wireless communications. With mildly frequency selective subband processing and modest number of subbands, it is able to provide better performance than the conventional FDE with FFT due to the lack of guard-interval overhead. Another significant benefit of this approach is that the same filter bank can be utilized to implement significant part of the receiver channel selectivity in a flexible way. This paper studies a hybrid frequency-time domain equalizer, which utilizes FB based equalizer for the feedforward part and a noise predictor (NP) for the feedback. The motivation is that the feedforward and feedback filter can be designed separately, in contrast to a conventional decision-feedback equalizer (DFE). The performance can be adjusted by the order of the NP. A significant advantage of NP based equalizer, over the traditional DFE, is that it is easier to include error control decoding in the equalizer feedback loop. This is able to provide significant performance enhancement over the DFE structure where the decoding is after the feedback loop

In this paper we study the effect of the transition band in designing both FIR- and IIR-types of digital filters, as possible bandwidth-limiting receiver filters, during the CDMA code acquisition of a BOC-modulated and over-sampled Galileo signal. It is shown that using an asymmetric transition band (i.e., the band between the passband and stopband frequencies) with respect to one fourth of the sampling rate, the performance, in terms of root mean square errors (RMSE), can be improved for both FIR and IIR filters compared to the situation with symmetric transition bands. As an optimum bound we have employed the ideal rectangular pulse (i.e. no bandwidth limitation). All investigated filtering methods still suffer from some performance degradation, compared to rectangular shaping. The analysis is done here for both static and fading multipath channels

This paper investigates the performance of LDPC coded MC-CDMA systems. Turbo multiuser detection is exploited by combining per-user MMSE-PIC detection and LDPC decoder together. In this way, error propagation can generally be averted for lower number of subcarriers. Simulation results show that the proposed scheme achieves about 4 dB coding gain for 256 subcarriers at BER= 10-4. The performance degrades only slightly with smaller spreading factors per MC-CDMA multiplex. Thus, from practical point of view, smaller spreading factors can be utilized to reduce the complexity of the detector as a whole.

Wireless positioning has received increased attention during the past few years, where several wireless applications have been envisaged and promoted such as the E-911/E-112 regulations. The positioning needs to be carried out in all the environments covered by the wireless communication services, including the most constraining areas such as dense urban areas and obstructed indoor environments. The most known positioning system is the global navigation satellite system (GNSS), which demonstrated quite reliable positioning capabilities when the receiver is in direct view with the sky. However, in indoor environments the signal characteristics are not well understood yet and positioning capabilities are quite poor. Therefore, understanding the multipath propagation indoors and fading characteristics are quite important to make GNSS works indoors. In this paper, we describe measurement-based modeling results of the level-crossing rate (LCR) and average duration of fades (ADF) with the purpose of giving further insight on the real satellite-to-indoor channel characteristics.

This work presents a novel efficient method for conversion between arbitrary sampling rates. The method allows arbitrary number of zeros at multiples of both input and output sampling rates, thus it has both good anti-imaging and good anti-aliasing properties. Further, two equivalent efficient implementation structures are presented. The implementation is done either by using the modified Farrow or transposed modified Farrow structure. By choosing one of these two alternatives appropriately, it is possible to shift most of the operations to lower sampling rate.

This letter addresses the blind signal estimation problem in the so-called conjugate signal model, where the observed signal is a linear combination of the desired signal and its complex conjugate. It will be shown that blind signal recovery in this kind of signal model is feasible using only the second-order statistics of the observed signal, under the assumption of circular or proper complex signals. Furthermore, one practical example application in the field of communications receiver signal processing will be given, where the image signal interference caused by amplitude and phase mismatches of the receiver analog branches is digitally attenuated. In addition to the analytical results and practical implementation algorithms, the efficiency of the proposed estimation concepts in the digital image rejection application is evaluated using computer simulations, showing impressive performance results.

Delay estimation in direct-sequence code-division multiple-access (DS-CDMA) systems is necessary for accurate code synchronization and for applications such as mobile phone positioning. Multipath propagation is among the main sources of error in the DS-CDMA delay estimation process, together with multiple access interference and non-line-of-sight (NLOS) propagation. This paper provides a review of main delay estimation techniques, existing in the literature so far, which are able to cope with multipath propagation, together with our novel delay estimation techniques proposed in the context of DS-CDMA systems. The performance of all these techniques is compared through analysis and simulations, considering also their relative computational complexity and required prior information. Starting from the traditional delay locked loops (DLL) and their improved variants, we discuss several recently introduced delay estimation techniques able to cope with multipath propagation. The characterization of these methods is given in a unified framework, suited for both rectangular and root raised cosine pulse shapes. The main focus in the performance comparison of the algorithms is on the closely-spaced multipath scenario, since this situation is the most challenging for achieving diversity gain with low delay spreads and for estimating LOS component with high accuracy in positioning applications.

In most positioning techniques, two or more non-serving base stations (BS) are involved in the location procedure. The position is usually derived from the estimates of the time of arrival of the line-of-sight (LOS) components from these different BSs. In the downlink transmission, the received signal strength from a remote BS is quite low, especially when the mobile is close to the serving BS, and also it is composed of a sum of multiple propagation paths, which may arrive at sub-chip delay intervals, generating closely spaced multipaths. For accurate location estimates, some measures should be taken to solve the closely spaced paths and diminish the intercell interference. In this paper, we introduce a constrained deconvolution approach aided with an intercell interference estimation scheme to solve the closely spaced paths and to enhance the estimation of the delay of the first arriving path from the desired BS.

The acquisition of CDMA signals in adverse mobile communication channels has been studied for more than five decades. However, the introduction of new standard proposals for the future European satellite system (i.e., Galileo) and for the modernized GPS has triggered new interest in fast and reliable acquisition strategies for CDMA systems with very high spreading factors (e.g., length of 10230 chips or higher). Typically, the double-dwell serial search strategies have been preferred for CDMA signal acquisition. Few papers have addressed also the problem of hybrid and parallel search strategies, but typically, the choice of the best number of dwells has not been discussed. The goal of this paper is to introduce a generic method for the computation of the mean acquisition time (MAT) for multiple-dwell hybrid-search acquisition blocks and to compare the performance of several multiple-dwell structures for CDMA systems with high spreading factors. It will be shown here that increasing the number of dwells does not always increase the performance from the point of view of the MAT. We also discuss the influence of various parameters on the selection of the multiple-dwell strategy and we present simulation results for a realistic Galileo signal.

In many signal processing applications, it is beneficial to use polynomial-based interpolation filters. Actual implementations of these filters can be performed effectively by using the Farrow structure or its modifications. In the literature only polynomial-based filters having even number of polynomial pieces N have been considered. This letter presents a modification of the implementation form which allows realizing polynomial-based filters having an odd number of polynomial pieces. The condition that N must be even integer is not required any more. Further, a frequency-domain filter optimization method is developed, based on the methods for even-length filters. We illustrate through examples the performance of the odd-length polynomial-based filters.

The problem of closely-spaced paths in DS-CDMA systems is a challenging task at the baseband receiver for applications such as mobile location and RAKE receivers. Previously, we introduced a method based on Teager-Kaiser (1990) operator for resolving paths spaced at less than one chip distance. We compare the performance of TK operator with that of the subspace based MUSIC (multiple signal classification) algorithm. In order to make a fair comparison, an extension of MUSIC algorithm to the systems with long codes is derived and the performance of both algorithms is assessed via simulations.

In this paper, we introduce an efficient and simple technique for estimating closely-spaced multipath delays in an asynchronous multiuser CDMA systems. The subchip resolution is achieved via a nonlinear quadratic operator called Teager-Kaiser operator, which exploits the structure of the cross-correlation function between the received signal and the reference code. Simulation results in the presence of multiple interfering users and Rayleigh fading multipath channels are presented. It is shown that the proposed technique is near-far resistant, and its performance in the presence of closely spaced multipaths is much better compared to the peak tracking with subtraction method. Moreover, it has the advantage of a very simple implementation, compared to other maximum likelihood approaches.

We propose an alternative technique to the implementation of discrete wavelet multitone systems based on fractional-delay filter banks. The technique yields orthogonal subband signals and allows independent equalization for each subband at the receiver end. When compared to systems employing cosine-modulated filter banks, our technique shows better performance in terms of bit-error rate, particularly for low signal-to-noise ratios and highly selective channels.

This paper presents an efficient way to implement flexible multirate signal processing systems with high oversampling ratio and adjustable fractional or irrational sampling rate conversion ratio. One application area is a multi-standard communication receiver which should be adjustable for different symbol rates utilized in different systems. The proposed decimation filter consists of parallel CIC (cascaded integrator-comb) filters followed by a linear interpolation filter. The idea is to use two parallel CIC filters to calculate the two needed sample values for linear interpolation. These samples occur just before and after the final output sample. This corresponds to a system where the linear interpolation is done at the higher input sampling rate.

The discrete multi-tone (DMT) modulation is considered a viable technique for high-speed digital transmission over a subscriber loop. It was selected as the standard for ADSL modems and it is also a strong candidate in the VDSL area. This paper investigates a programmable solution for DMT-based VDSL modems using the Texas Instruments' processor family TMS320C62xx. A structure for both transmitter and receiver sides is presented. Synchronization and equalization issues are also discussed.

Filter bank-based transmultiplexer systems have certain advantages compared with existing DFT-based multicarrier systems and they are promising candidates for data transmission in frequency-selective channels. We have recently proposed a novel and efficient channel equalization idea to be used with critically decimated perfect reconstruction cosine modulated transmultiplexer systems. The equalizer utilizes parallel cosine and sine modulated filter banks in the receiver end. This paper explores efficient realization structures for the needed parallel filter bank system, which finds applications also in other areas.

In this paper, we study the quality of the terrestrial digital video broadcasting (DVB-T) transmission over the cable TV network. Usually, when terrestrial digital TV signals are distributed in the cable TV network a conversion is needed in the head-end, from DVB-T (OFDM) to DVB-C (single carrier). In this work we study the possibility of using the DVB-T signal without any conversion. At first, a simple model for the channel is designed taking into account the main impairments. Secondly, the performance of the DVB-T signal is simulated and compared to that of digital cable TV standard DVB-C that is based on single carrier technique. The comparison is based on the bit error ratio that each system can achieve, in the same channel environment. Channel interference problems are also simulated.

Wideband CDMA systems employ pilot symbols for coherent detection. When the pilot symbols are time-multiplexed with data symbols, their number is usually limited, due to overhead considerations and rate matching requirements. Therefore, channel estimation based only on the pilot symbols is usually not accurate enough. It was shown previously that, in the absence of code synchronization errors, a pilot-aided decision-directed (PADD) approach exhibits better BER performance than pilot-aided estimation over fading channels, because it uses both pilot and data symbols for channel estimation. This paper shows the impact of the delay estimation errors on the PADD algorithm. The multipath delays are estimated using pilot-aided (PA), non-data aided (NDA), or decision-directed (DD) algorithms, and the comparison with ideal channel estimation case is done via simulation. The PADD algorithm in the presence of multipath delay errors is also compared with the pilot-aided plus linear interpolation channel estimation. The simulation results show that the PADD algorithm is not very sensitive to the delay estimation errors when the delays are estimated via an NDA or DD approach, and it exhibits better performance than the traditional pilot-aided methods also when the delay estimation errors are present.

In mobile communications, there exists increasing demand to develop small sized mobile terminals with the ability to operate more flexibly in different systems. In this paper, we examine the requirements for wideband bandpass sampling, which is seen as an important tool together with multirate signal processing, in implementing more flexible devices. We consider the case, where sampling is done straight away at a high IF frequency. It is shown how the requirements for quantization and jitter noise depend in the GSM case on the bandwidth and filter type of the analog front-end, as well as on the sampling rate.

The DSP based solutions have become the main concern of new modems design. At present, most of the voice band modems use a programmable solution and most of the telecommunication systems tend to do the same. For the xDSL modems, the same trend is expected and the DSP based implementation becomes the challenge for new designs. We investigate a programmable solution for the DMT (discrete multitone) based VDSL modems using Texas Instrument processor family TMS320C64x. We present structures for both the transmitter and receiver sides. We also evaluate the complexity and the hardware needed for both parts.

The RAKE receiver is the most common receiver employed in wideband CDMA (W-CDMA) systems. Traditionally, multiple access interference (MAI) in spread spectrum systems is modelled as white Gaussian noise. This presumption is valid when the number of users is large and interference signals have large spreading factors. But the W-CDMA air interface for 3rd generation mobile communication systems supports low spreading factors, high and variable data rates, which make MAI non-Gaussian. We analyze the bit error rate (BER) performance of a downlink receiver in the presence of an interfering user when the Gaussian assumption does not hold any more. The receiver is a maximum ratio combining RAKE receiver. First we study the performance of the ideal RAKE receiver, which has exact estimates of delays, amplitudes and phases of the multipath components. Then we consider the effect of MAI when non-data aided (NDA) and pilot aided decision directed (PADD) algorithms are used for delay and channel coefficients estimation, respectively. It is shown that imperfect power control can significantly affect the BER performance, while practical single-user channel estimation algorithms, such as the NDA-PADD algorithm, behave well also in the presence of non-Gaussian MAI.

In quadrature receivers, unavoidable imbalances in the analog front-end between the I- and Q-branches result in finite and usually insufficient rejection of the image frequency band. This causes the image signal to appear as interference on top of the desired signal. Both analog and digital techniques to compensate the effects of I/Q imbalance have been presented in the literature. In this paper, we carry out a detailed performance analysis of the interference cancellation based imbalance compensation structure utilizing baseband digital signal processing. Also simulation results are provided for comparison. The results indicate that the interference canceller based solution can offer adequate performance for most communication applications.

We consider filter bank based multicarrier systems which have been considered for fast digital subscriber line applications and possibly could be interesting also for broadband wireless LAN applications. In addition to being resistant to narrowband interference, these systems provide better spectral shaping than DFT-based OFDM and DMT systems for the subchannels, as well as for the overall signal. Furthermore, they provide better efficiency because the time-domain guard interval is not needed and the guard-band in frequency domain can be reduced. The channel equalization for such systems has been an open problem, but the paper demonstrates that it can be solved, at least in principle, even though effective adaptive equalization algorithms are still under development. Another open question is the performance when nonlinearities are included in the chain, especially in the transmitter power amplifier. In the paper, it is demonstrated that the performance of filter bank based systems with nonlinear power amplifiers is only slightly worse than in the corresponding OFDM/DMT systems. When the number of channels is the same, no more than 1 dB higher input backoff is required.

Quadrature downconversion to a very low-IF frequency is a promising idea but it has one serious problem. Unavoidable imbalances in the analog front-end between the I- and Q-branches result in finite and usually insufficient rejection of the image frequency band which causes interference. We carry out a signal analysis of an imbalanced low-IF receiver and propose a novel baseband correction structure utilizing an adaptive interference canceller. Theoretical and simulation results are provided in order to evaluate the performance of the proposed idea. The results indicate that the I/Q imbalance can be effectively compensated during the normal operation of the receiver, even in the rapidly changing case, as long as a linear system model for the imbalance is valid.

We investigate the performance of two delay estimation algorithms, non-data aided (NDA) and decision-directed (DD), in the presence of multipath fading and additive Gaussian noise. The comparison is made assuming perfect knowledge about the amplitudes and phases of channel multipaths, in order to see the maximum achievable performance with these two algorithms. The results indicate that NDA delay estimation offers but little degradation of performance compared to DD algorithm and has the advantage of a reduced complexity and higher spectral efficiency. We also study the impact of data errors on DD delay estimation. The algorithms are applied for a downlink wideband CDMA (WCDMA) system, using Walsh spreading codes and long Gold scrambling codes.

We evaluate three decision-directed algorithms for complex multipath channel tap estimation in the downlink wideband CDMA (WCDMA) system. These algorithms are tested with indoor and vehicular channel models at different mobile velocities and their performance is compared with ideal channel estimation, as well as with estimation based on linear interpolation.

Recently, a new all-digital technique for symbol timing and carrier phase synchronization of digital receivers was introduced by the authors. The's technique is based on a polynomial approximation of the likelihood function by using the Farrow structure. In this contribution, we propose a new Tretter-based frequency estimator that is directly derived from the carrier phase estimate. The proposed estimator is simple to implement and is particularly suited for digital receiver architectures. This new Tretter-based estimator performs fairly well when compared with the Fitz and the Luise-Reggiannini frequency estimators, in the presence of substantial carrier frequency offset.

Maximum likelihood (ML) estimation theory provides a general framework for developing optimum timing recovery schemes for digital communication systems. Feedforward schemes based on ML-theory have been shown to have rapid acquisition characteristics. Another important trend is to use digital interpolation for symbol timing adjustment. The so-called Farrow (1988) structure has become popular in this context because it offers an efficient and flexible realization structure for such an interpolation task. In a previous paper Vesma and Tuukkanen (see Proc. IEEE NORSIG 96, Espoo, Finland, p.183-6, 1997) proposed a new feedforward timing estimation scheme which utilizes the Farrow structure efficiently. The scheme is based on a low-order polynomial approximation for the log-likelihood function and also the derivative of this function is available with a small additional computational cost. The performance of the new timing recovery technique was found to be close to ideal even with short symbol block-lengths which is very important in mobile systems. In this paper, we show that this new scheme gives much better results when optimized training sequences are used. The scheme works also with severe multipath distortion channel.

Efficient IIR filter structures based on allpass subfilters have been presented earlier for Nyquist pulse shaping applications. In a previous paper, we presented a receiver front-end architecture which efficiently combines the half-Nyquist filtering and special prefiltering required to achieve jitter-free frequency-error detection. In this paper we utilize the same kind of prefiltering to develop an all-digital jitter-free symbol timing error tracker suitable for multiamplitude signals. The proposed structure is an approximation to a timing error tracker based on the maximum likelihood principle. It is also shown that fast convergence of the timing recovery loop can be achieved due to the short filter delay in the feedback loop.

Quincunx sub-sampling and interpolation techniques are widely used in many image and video signal bandwidth compression techniques, like HDTV transmission systems. In this paper we consider the basic quincunx interpolation problem: interpolation of the missing samples of an orthogonally sampled image after quincunx sub-sampling with a factor of two. New median-based nonlinear interpolation techniques are developed and compared with traditional linear designs. It is shown that nonlinear interpolators result in considerably lower hardware complexity than good quality linear designs and, in many respects, even better image quality can be achieved.

This paper introduces a new class of uniform digital filter banks based on DFT and recursive Nth-band filters. We consider both even and odd types of channel stacking arrangements. Due to the use of the DFT, the basic filter bank is complex. Real filter banks are obtained by adding the output signals from two symmetrically located complex channels. The filter banks are designed to satisfy the power complementarity and allpass complementarity constraints with high accuracy. Furthermore, nearly alias-free analyzer-synthesizer system can be obtained in the critically sampled case.

This paper extends the recently introduced class of multistage FIR Nyquist filters to cover also the multi-stage IIR and hybrid IIR/FIR Nyquist filters. The filter stages, as well as the overall system, have zero intersymbol interference. The filter stages are simultaneuosly designed to provide a Chebyshev stopband behavior for the overall filter, thereby band-limiting the pulses optimally. Computationally efficient IIR filter stages are obtained by using the polyphase structure with allpass subfilters. Selecting one of the subfilters to be a pure delay results in zero intersymbol interference. Efficient procedures are presented for simultaneously optimizing the filter stages for both IIR and hybrid IIR/FIR cases. Examples demonstrate how the proposed multistage IIR filters provide significant advantages over other existing IIR designs and a slight reduction in the multiplication rate compared to multistage FIR designs.

Error-feedback is an effective technique for reducing multiplier wordlengths in fixed-point recursive digital filter implementations. In this paper, a new roundoff noise formula for error-feedback state-space filters is derived and used in optimizations.

This paper presents a method for finding efficient multiprocessor realizations of digital filters. It provides a framework for studying the tradeoffs between the sampling rate and the number of arithmetic units or processors. A simple method for determining the minimal sampling period as a function of addition and multiplication times is introduced. Also the effect of delays in data transfers on the sampling period can be analyzed for different hard-ware configurations. Some examples are presented to show the use of the method in obtaining efficient multiprocessor realizations.