Weighted K nearest neighbor (WKNN) algorithm provides good result for indoor localization by searching for matching fingerprints in the dataset. However, due to the nature of this method, computation load increases as the size of dataset. In this paper, we propose a hierarchical localization method to reduce the computation load during the online phase. A two-level structure first localizes the user to a smaller subset and then position estimation is obtained by WKNN with feature extraction. On a public accessed WiFi fingerprinting dataset, the proposed method achieves a 98% reduction without sacrificing localization performance.

With the large deployment of WiFi networks, in-door localization using WiFi fingerprinting with received signal strength has been widely studied. One of the common localization methods is weighted K-nearest neighbor method (WKNN), which localizes the user to the weighted center of the K best matching points. The performance of this method is affected by the choice of parameter K. Once tuned, this parameter is usually applied to all test samples. In this paper, we study how far localization performance can be improved if this parameter is adapted for different test points. We show with two public access datasets that adapting parameter K for different test points can potentially improve localization performance by over 45% compared to the baseline of only choosing the closest neighbor. Additionally, we analyze the dataset to obtain some stochastic thresholds for dataset filtering and K selection.

Wifi fingerprinting using received signal strength has been widely studied for indoor localization. Classic similarity-based methods like weighted K-nearest neighbor (WKNN) localize targets by searching for the best matching fingerprint in the dataset. Performance of these methods suffers from RSS variance and they are slow under a large size of fingerprint dataset. In this paper, we propose a WKNN localization strategy using k-means clustering radio mapping that balances localization precision and computational complexity.

Time reversal (TR) is an effective solution in both single user and multiuser communications for moving complexity from the receiver to the transmitter, in comparison to traditional postfiltering based on Rake receivers. Imperfect channel estimation may, however, affect pre‐ versus postfiltering schemes in a different way; this paper analyzes the robustness of time reversal versus All‐Rake (AR) transceivers, in multiple access communications, with respect to channel estimation errors. Two performance indicators are adopted in the analysis: symbol error probability and spectral efficiency. Analytic expressions for both indicators are derived and used as the basis for simulation‐based performance evaluation. Results show that while TR leads to slight performance advantage over AR when channel estimation is accurate, its performance is severely degraded by large channel estimation errors, indicating a clear advantage for AR receivers in this case, in particular when extremely short impulsive waveforms are adopted. Results however also show a stronger non‐Gaussianity of interference in the TR case suggesting that the adoption of a receiver structure adapted to non‐Gaussian interference might tilt the balance towards TR.

Space-Time Block Codes (STBCs) suffer from a prohibitively high decoding complexity unless the low-complexity decodability property is taken into consideration in the STBC design. For this purpose, several families of STBCs that involve a reduced decoding complexity have been proposed, notably the multi-group decodable and the fast decodable (FD) codes. Recently, a new family of codes that combines both of these families namely the fast group decodable (FGD) codes was proposed. In this paper, we propose a new construction scheme for rate-1 FGD codes for 2atransmit antennas. The proposed scheme is then applied to the case of four transmit antennas and we show that the new rate-1 FGD code has the lowest worst-case decoding complexity among existing comparable STBCs. The coding gain of the new rate-1 code is optimized through constellation stretching and proved to be constant irrespective of the underlying QAM constellation prior to normalization. Next, we propose a new rate-2 FD STBC by multiplexing two of our rate-1 codes by the means of a unitary matrix. Also a compromise between rate and complexity is obtained through puncturing our rate-2 FD code giving rise to a new rate-3/2 FD code. The proposed codes are compared to existing codes in the literature and simulation results show that our rate-3/2 code has a lower average decoding complexity while our rate-2 code maintains its lower average decoding complexity in the low SNR region. If a it time-out sphere decoder is employed, our proposed codes outperform existing codes at high SNR region thanks to their lower worst-case decoding complexity.

In this paper we propose a new construction method for rate-1 Fast-Group-Decodable (FGD) Space-Time-Block Codes (STBC)s for 2atransmit antennas. We focus on the case of a = 2 and we show that the new FGD rate-1 code has the lowest worst-case decoding complexity among existing comparable STBCs. The coding gain of the new rate-1 code is then optimized through constellation stretching and proved to be constant irrespectively of the underlying QAM constellation prior to normalization. In a second step, we propose a new rate-2 STBC that multiplexes two of our rate-1 codes by the means of a unitary matrix. A compromise between rate and complexity is then obtained through puncturing our rate-2 code giving rise to a new rate-3/2 code. The proposed codes are compared to existing codes in the literature and simulation results show that our rate-3/2 code has a lower average decoding complexity while our rate-2 code maintains its lower average decoding complexity in the low SNR region at the expense of a small performance loss.

In this paper we deal with the problem of minimizing the complexity of an IR-UWB system by the introduction of Time-Reversal, under a power constraint and fixing a reachable performance in terms of BER. An approximate trade-off in the choice of the number of taps at the transmitter and the number of fingers at the receiver is proposed.

Complex Orthogonal Design (COD) codes are known to have the lowest detection complexity among Space-Time Block Codes (STBCs). However, the rate of square COD codes decreases exponentially with the number of transmit antennas. The Quasi-Orthogonal Design (QOD) codes emerged to provide a compromise between rate and complexity as they offer higher rates compared to COD codes at the expense of an increase of decoding complexity through partially relaxing the orthogonality conditions. The QOD codes were then generalized with the so called g-symbol and g-group decodable STBCs where the number of orthogonal groups of symbols is no longer restricted to two as in the QOD case. However, the adopted approach for the construction of such codes is based on sufficient but not necessary conditions which may limit the achievable rates for any number of orthogonal groups. In this paper, we limit ourselves to the case of Unitary Weight (UW)-g-group decodable STBCs for 2atransmit antennas where the weight matrices are required to be single thread matrices with non-zero entries ∈{± 1,± j} and address the problem of finding the highest achievable rate for any number of orthogonal groups. This special type of weight matrices guarantees full symbol-wise diversity and subsumes a wide range of existing codes in the literature. We show that in this case an exhaustive search can be applied to find the maximum achievable rates for UW-g-group decodable STBCs with g>;1. For this purpose, we extend our previously proposed approach for constructing UW-2-group decodable STBCs based on necessary and sufficient conditions to the case of UW-g-group decodable STBCs in a recursive manner.

The use of Space-Time Block Codes (STBCs) increases significantly the optimal detection complexity at the receiver unless the low-complexity decodability property is taken into consideration in the STBC design. In this paper we propose a new low- complexity decodable rate-5/4 full-diversity 4 × 4 STBC. We provide an analytical proof that the proposed code has the Non-Vanishing-Determinant (NVD) property, a property that can be exploited through the use of adaptive modulation which changes the transmission rate according to the wireless channel quality. We compare the proposed code to the best existing low-complexity decodable rate-5/4 full-diversity 4 × 4 STBCs in terms of performance over quasi-static Rayleigh fading channels, worst-case complexity, average complexity, and Peak-to-Average Power Ratio (PAPR). Our code is found to provide better performance, lower average decoding complexity, and lower PAPR at the expense of a slight increase in worst-case decoding complexity.

Space-time coding techniques have become common-place in wireless communication standards as they provide an effective way to mitigate the fading phenomena inherent in wireless channels. However, the use of Space-Time Block Codes (STBCs) increases significantly the optimal detection complexity at the receiver unless the low complexity decodability property is taken into consideration in the STBC design. In this letter we propose a new low-complexity decodable rate-1 full-diversity 4 × 4 STBC. We provide an analytical proof that the proposed code has the Non-Vanishing-Determinant (NVD) property, a property that can be exploited through the use of adaptive modulation which changes the transmission rate according to the wireless channel quality. We compare the proposed code to existing low-complexity decodable rate-1 full-diversity 4 × 4 STBCs in terms of performance over quasi-static Rayleigh fading channels, detection complexity and Peak-to-Average Power Ratio (PAPR). Our code is found to provide the best performance and the smallest PAPR which is that of the used QAM constellation at the expense of a slight increase in detection complexity w.r.t. certain previous codes but this will only penalize the proposed code for high-order QAM constellations.

Fast Maximum Likelihood Decodable Space-Time Block Codes (STBCs) have recently gained a lot of interest for use in wireless communication systems. Several strategies have been introduced in the literature to build such codes, among them we mention the conditional detection strategy, where the code matrix can be expressed as the sum of two Orthogonal STBCs by means of a unitary matrix. Thus, the decoder benefits from the orthogonality by estimating an orthogonal set of symbols assuming knowledge of the other set of symbols. Even if this strategy permits high rates, numerical optimization of the coding gain becomes unrealizable for high-order constellations. Another strategy has been proposed in the literature, the g-group decodable codes, where the minimization of the Maximum Likelihood (ML) metric can be made over several independent subsets of symbols. Moreover, the g-group decodability strategy enables us to optimize the coding gain for each subset of symbols separately which permits the optimization of the coding gain for high-order constellations. On the other hand, the rate of g-group decodable codes is limited. In this paper, we reformulate the problem of finding Unitary Weight (UW)-2-group decodable codes in a simpler way based on necessary and sufficient conditions. Then, a simple routine is used to search for the desired code. We determine the maximum achievable rate for a specified type of weight matrices.

Spatial diversity techniques are now used in most wireless communications systems to improve robustness to signal fading. On the downlink of cellular systems, low-cost requirements of the user terminal tend to favor transmit diversity over receive diversity, and Alamouti's transmit diversity technique appears today in most wireless communications systems standards. But in terms of the total transmit power, Alamouti's technique loses 3 dB with respect to optimum transmit diversity which requires perfect channel state information at the transmitter. In this paper, we introduce a simple transmit diversity technique which performs significantly better than both Alamouti's transmit diversity and switched diversity schemes and comes very close to optimum transmit diversity performance. In this technique, the same power is transmitted from the two antennas and the phase difference of the signals received from the two channels is constrained to be smaller than π/4. Its performance is studied in both narrowband fading channels and in OFDM systems operating on frequency- selective channels.

We are motivated to exploit the broadcast nature of a radio medium in combating its unreliable and time-varying characteristics. This leads to cooperative transmission where a neighbor node keeps a copy of an overheard packet and helps in retransmitting the overheard packet to its intended destination when the original transmission fails. Cooperative transmission improve wireless network performance through diversity gain, and can be performed in the forms of amplify-and-forward. Existing amplify-and-forward schemes either: (a) use multiple relays but in a non-incremental way, or (b) use only a single relay in an incremental way. Non-incremental (simple) schemes do not provide a good spectral efficiency. Also, using multiple relays can further improve diversity. This paper aims at proposing an incremental amplify-and-forward scheme that uses n relays, where n >; 1. In the proposed scheme, a relay will retransmit an overheard packet in a time slot, only if all the previous transmissions are not successful. The transmission cycle of a packet ends as soon as a packet has been received successfully by the intended destination or all the n relays has retransmitted their overheard packets. Analytical and numerical results show that the proposed scheme outperforms both the simple amplify-and-forward scheme and the single-relay incremental scheme in terms of a lower outage probability for a given spectral efficiency.

From a practical point of view, fast Maximum Likelihood Decoding (MLD) and low Peak-to-Average-Power- Ratio (PAPR) are two important design criteria for Space-Time Block Codes (STBCs). The use of the principle of conditional detection allows for having high-rate STBCs while keeping the complexity of detection at a reasonable level. Recently, we proposed a structure for a STBC with fast MLD having a rate of 3/2 complex symbols per channel use. In this paper, the PAPR criterion is included in the code design, and a modified version is proposed which has a lower PAPR.

This paper describes the design of several simple IR-UWB receivers that could be used for sensor networks. The research is looking for a simple design with low complexity and good performances. To achieve this, a priori information is needed: the statistical knowledge of the Multi User Interference (MUI). In many systems the Gaussian distribution is a common presumption but due to the impulsive nature of the interference, in the Impulse Radio Ultra Wide Band (IR-UWB) systems this presumption is no longer possible to characterize MUI. We investigate here the use of mixtures of Gaussian, Laplacian, and generalized Gaussian random variables to model the MUI.

This paper considers a wireless sensor network with Time-Hopping-Impulse-Radio Ultra Wide Band as the physical layer. The network is structured as a group of interconnected cells that are not synchronized with each other. In such an unsynchronized environment, inter-cell interference is not Gaussian, but generalized Gaussian. For such a non-Gaussian random interference, its impulsiveness is determined by the Kurtosis of the distribution. This paper shows that the Kurtosis can be controlled by adjusting some practical parameters, such as pulse shape, pulse length, slot duration, number of slots per frame, number of users, etc. Further, this paper shows that network throughput can be maximized by controlling the impulsiveness. The finding has been validated through simulations.

Recently, Space-Time Block Codes (STBCs) with fast Maximum-Likelihood Decoding (MLD) have gained a lot of interest from a practical perspective. B. Rajan et.al proposed a systematic approach to build such codes from the representations of the real Clifford algebras. In the Sari-Sezginer code, another approach was proposed to build fast MLD STBC's. Indeed, the authors used the conditional detection principle and numerical optimization in order to have a large coding gain (thus full diversity) while maintaining a reduced complexity level at the receiver. In this paper we are interested in extending the principle of conditional detection to the case of four transmit antennas. We propose here a full-diversity 4×4 STBC achieving a rate of 3/2 complex symbols per channel use. To the best knowledge of the authors, the highest rate for full transmit diversity 4×4 codes with the same complexity level was reported to be 5/4 complex symbols per channel use (such codes were found independently).

This paper presents the impact of antenna design over the capacity of a IR-UWB system in a multi-user interference environment. A new antenna design is proposed and it is showed using a new performance criterion that for this design, the communication capacity given by the Sholtz's pulse might be outperformed by using some other types of pulses. More than that, the proposed criterion might be used in IR-UWB not only to compare the performance of different antennas but also to compare the performance of different pulse shapes from the communication capacity point of view.

The huge potential of ultra wide band impulse radio (UWB IR) with multiple access was initially evaluated using the standard Gaussian approximation which assumed that multi user interference could be approximated as classical Gaussian noise. Later the standard Gaussian approximation has been proved to be invalid in several cases. Thus the performance of the classical UWB IR correlation receiver dramatically reduces. We show in this paper how the non Gaussianity of the multi user interference can be exploited, turning the non Gaussianity into an advantage, increasing the performance with respect to the Gaussian interference case which was supposed to be too optimistic until now.

The generalized Gaussian probability density function is shown to better approximate the probability density function of the multiple access interference in ultra-wide bandwidth systems than the Gaussian approximation and the Laplacian density approximation. Two ultra-wide bandwidth receiver structures based on this new approximation using a p-order metric receiver decision statistic are investigated for the detection of time-hopping ultra-wide bandwidth wireless signals in multiple access interference channels. The first receiver outperforms both the conventional matched filter ultra-wide bandwidth receiver and the soft-limiting ultra-wide bandwidth receiver when only multiple access interference is present in UWB channels. The second new receiver with adaptive limiting threshold outperforms the conventional matched filter ultra-wide bandwidth receiver, the soft-limiting ultra-wide bandwidth receiver, and the adaptive threshold soft limiting ultra-wide bandwidth receiver in all multiple access interference-plus-noise environments. In multipath channels, a new Rake receiver based on the porder metric receiver is proposed for signal detection. Mathematical analysis and numerical results show that this new Rake receiver can achieve larger signal-to-interference-plus-noise ratio than the standard matched filter Rake receiver when multipath components are resolvable in UWB channels.

We propose in this paper a simple receiver adapted to multi user interferences (MUI) in impulse radio ultra wide band (IR-UWB). It is known that MUI distribution is not Gaussian in several cases. The classical correlation receiver which is adapted to Gaussian noise, is not adapted to the particular form of UWB MUI and is often far from the performance computed through the standard Gaussian approximation. We show that modeling MUI by a generalized Gaussian distribution helps in deriving a simple receiver which increases the performances and may also outperform the standard Gaussian approximation which was considered formerly to be too optimistic.

We propose in this paper to show how a one-bit sampling receiver can be more efficient in some cases than the classical correlation receiver for impulse radio ultra wide band (IR-UWB) communications with high multi user interferences (MUI). This one-bit sampling receiver can also be seen as a hard decoding process of the classical repetition code used in UWB IR. Thus hard decoding can be more efficient than soft decoding in some cases. Such a result is based on the special distribution of UWB MUI. It is known that MUI distribution is not Gaussian in several cases. The classical correlation receiver which is adapted to Gaussian noise, is not adapted to the particular form of UWB MUI. We show that modeling MUI by a generalized Gaussian distribution helps in deriving a receiver which in some cases is nearly equivalent to the one-bit sampling receiver

Future ultra-wideband systems will be applied for Internet access, multimedia services, wireless interfaces and location-aware services. This paper deals with the impulse radio (IR) UWB systems with high spectral efficiency. Though the IR systems are based on the use of baseband signals, the introduction of a proper orthogonal basis permits to derive higher order constellations as a linear combination of the elements of the basis with no additional bandwidth and a higher system throughput

UWB (ultra wide band) is an emerging technology for short-range wireless communications, such as WPAN (wireless personal area networks). In the context of UWB impulse radio, one of the crucial challenges remains the acquisition of synchronization in terms of time, precision and number of operations. In this paper, we present a study on the acquisition time for hybrid strategies; the analysis relies on common assumptions on the fading statistics and the multipath intensity profile, while the simulations consider the complete channel models adopted by IEEE 802.15.4a. The analysis and simulations highlight the advantages and drawbacks of this type of search strategies

This paper investigates the signaling scheme of time-hopping impulse radio ultra wide band (TH-IR-UWB). With a simple repetition code and without scrambling code, at a given spreading factor and a given system load, a trade-off has to be done in the choice of the frame length and the repetition factor. If the repetition factor on frame length ratio is too small, multiuser interferences (MUI) are not Gaussian, if it is too high the MUI becomes Gaussian but its variance becomes too high also. We show how the introduction of a scrambling code breaks this trade-off, we can increase the repetition factor on frame length ratio without increasing the MUI variance. Thus we will see that such a scrambling code should be applied to UWB networks with high density of users and low data rates like ad-hoc sensor networks, as well as networks with higher data rate and small number of users.

This paper investigates the parameters of time-hopping impulse radio ultra wide band (TH-IR-UWB). At a given spreading factor and a given system load, a trade-off has to be done in the choice of the frame length and the repetition factor. This trade-off lays on the condition of validity of the standard Gaussian approximation for multi user interferences (MUI). This study concerns both Pulse Amplitude Modulation and Pulse Position Modulation and may be applied to UWB networks with high density of users and low data rates like adhoc sensor networks, as well as networks with higher data rate and small number of users.