Parameter identifiability improvement in multi-frequency array radar

We provide a compact characterization of the outage rates for wideband wireless communication, quantifying the effects of frequency and spatial diversity. Our information-theoretic computations yield the following findings. (a) Complex models typically employed in simulations can be replaced by simple, bandwidth-dependent, tap delay line models without loss of generality. (b) The spectral efficiency (i.e., the achievable rate, divided by the bandwidth) is well approximated as a Gaussian random variable, so that it is only necessary to specify its mean and variance to obtain outage rates. (c) The mean spectral efficiency depends only on the spatial diversity, while the variance depends on the spatial and frequency diversity via a product. Our definitions of frequency and spatial diversity have physically motivated interpretations, and are not based on high signal-to-noise ratio (SNR) asymptotics as in prior work.

International audience

Southern Railway have collected extensive data on microwave propagation over their 2000 KM net-work for over five years. The main problem of long path communication link is the occurrence of momentary deep fades of 10 to 40 dB for very short duration of 1 to 2 seconds. These dropouts. account for 80% of the total severe fading and about 60% of the total outage occur. in pre-summer months viz., February, March and April. The probability occurrence of deep fades increases very rapidly with distance and a computerised study indicate. that it is proportional to (Distance)10. Comparative study of Space Diversity and Frequency Diversity for long path indicates that Space Diversity reduces the outage time by 50% whereas Frequency Diversity wih even a small frequency separation of 1% reduces the outage by as much as 90%. Frequency Diversity, therefore, appears to be definitely more suited to tropical areas subjected to heavy momentary outages tban Space Diversity.

In this paper, we propose a space-time coding and carrier block spreading based multiple-input-multiple-output (MIMO) orthogonal-frequency-division-multiplexing (OFDM) system. The proposed system exploits the potential spatial and frequency diversity of MIMO systems in frequency-selective fading environments through a two-stage procedure. In the first stage, we use space-time coding to exploit the spatial diversity, and in the second stage, we adopt block spreading among blocks of OFDM carriers to obtain frequency diversity. Moreover, by using rotated Hadamard matrix or rotated discrete Fourier transform (DFT) matrix as the carrier spreading matrix, the system can also possess a certain degree of modulation diversity. Our proposed system enjoys the advantage of high flexibility and low complexity. It allows the use of any existing space-time coding scheme as its component space-time code, and the diversity gains are obtained with only moderate transmit and receive processing complexity. Simulation results show that our proposed system can achieve desired spatial and frequency diversity with low detection complexity. Comparing to some existing space-frequency coding schemes, our proposed system enjoys the merit of low complexity, although it could suffer from some performance degradation, which is caused by the separation of space-time coding and frequency-domain spreading process.

In this paper, an enhanced scheme of Index Modulation for Orthogonal Frequency Division Multiplexing (IM-OFDM), called Repeated IM-OFDM with Diversity Reception (ReIM-OFDM) is proposed. ReIM-OFDM achieves performance improvement over the convetional IM-OFDM at the same spectral efficiency by providing additional space and frequency diversity. Similar to IM-OFDM, the proposed ReIM-OFDM also activates K out of total N subcarriers to convey information bits using both the active sub-carriers and their indices. However, it differs from IM-OFDM in that all activated sub-carriers are modulated by the same M-ary data symbol. For signal combining, either Maximal Ratio Combining (MRC) or Selection Combining (SC) can be used in the spatial and sub-carriers domain to help ReIM-OFDM achieve both space and frequency diversity gain. In order to analyze performance of the system, the Moment Generating Function (MGF) is used to obtain the closed-form expressions for the pairwise index error probability (PEP) and the symbol error probability (SEP) in both cases: ReIM-OFDM-MRC and ReIM-OFDM-SC. Effects of various system parameters on the SEP are analyzed to select the best ReIM-OFDM configuration with minimum error probability. Our analysis proves the effectiveness of ReIM-OFDM over IM-OFDM at the same spectral efficiency.

With the increasing pervasiveness of the geo-positioning technologies, there is an enormous amount of spatio-textual objects available in many applications such as location based services and social networks. Consequently, various types of spatial keyword searches which explore both locations and textual descriptions of the objects have been intensively studied by the research communities and commercial organizations. In many important applications (e.g., location based services), the closeness of two spatial objects is measured by the road network distance. Moreover, the result diversification is becoming a common practice to enhance the quality of the search results. Motived by the above facts, in this paper we study the problem of diversified spatial keyword search on road networks which considers both the relevance and the spatial diversity of the results. An efficient signature-based inverted indexing technique is proposed to facilitate the spatial keyword query processing on road networks. Then we develop an efficient diversified spatial keyword search algorithm by taking advantage of spatial keyword pruning and diversity pruning techniques. Comprehensive experiments on real and synthetic data clearly demonstrate the efficiency of our methods.

Literature indicates that frequency diversity can be utilized to compensate channel uncertainties such as multipath fading. Therefore, in this paper it is exploited for improving accuracy in locating stationary and mobile objects in the indoor environment. First, frequency diversity technique is introduced for small scale and temporal variation compensation of received signals and demonstrated analytically that it in fact enhances location accuracy. A novel metric is introduced in selection combining in order to achieve location accuracy through the addition of frequency diversity upon two of the available location determination schemes. The results are evaluated experimentally against the case where there is no frequency diversity for reception by using low cost wireless RF devices such as motes. An asset location tracking system is then devised to both improve accuracy and predict asset movement. Frequency diversity in terms of channel spacing of 55 MHz is evaluated and shown to provide a reduction in location determination error between 18 and 23% when compared to a system without frequency diversity. Finally, results from frequency diversity are compared against the spatial diversity techniques in terms of improvement in location accuracy, power consumption of the transmitter, and hardware and software costs.

The global navigation satellite systems (GNSS) receiver is likely to experience performance degradation in the presence of multipath interferences in indoor and urban canyon environments. There will be multiple reflected signals arriving at the receiver antenna from different paths and directions regarding the different locations of the nearby constructions, causing significant fluctuations and distortions on the amplitude and phase of the direct signals. This is the so-called multipath fading effects and usually exhibit with the random features of temporal, spatial, and frequency diversity across the signal bands. To have a better understanding of the multipath fading effects, a live test was conducted near Lujiazui CBD area in Shanghai. The collected data were processed to characterize the signal attenuations and phase fluctuations on multi-frequency GNSS signals via the detrend code delay, carrier phase, and navigation bit errors. The resulting scintillation index on amplitude, code delay, and carrier phase, e.g., S4, $${\sigma }_{\varphi }$$ and $${\sigma }_{\tau }$$ can be calculated. Based on these multipath features, the inter-frequency aiding strategy will be utilized for the multipath mitigations in the multi-frequency tracking loop design. The results demonstrate the effectiveness of the multi-frequency tracking algorithm in the urban environment.

We propose a space-frequency (SF) block coding scheme for a multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) system using antennas with reconfigurable radiation patterns. In this system, each element of the antenna array at the transmitter side is assumed to be reconfigurable so that it can independently change the physical characteristics of its radiation pattern. The proposed block code is full rate and benefits from spatial, frequency, and reconfigurable radiation pattern state diversity over frequency-selective fading channels. We provide simulation results to demonstrate the performance of the proposed block coding technique and make comparisons with that of the previous SF coding schemes in MIMO-OFDM systems. The results indicate that the proposed code achieves higher diversity and coding gain compared to other available SF codes.

In this paper, a planar multiple input multiple output MIMO UWB array is proposed for 3D microwave imaging applications. According to the virtual array concept, the advantage of the MIMO array lies on improving the angular resolution with a reduced number of antennas, and achieve the same resolution as a fully populated conventional arrays. In addition, the UWB systems provide a better range resolution thanks to their large bandwidth. The frequency diversity and its influence on the system's resolution is also studied. For the MIMO array, an optimal UWB antenna element is designed with more than 128% of fractional bandwidth with no considerable distortion of the transmitted impulse. For the imaging algorithm, the Back-projection algorithm is implemented and used for the reconstruction of the images. In order to study the performance of the designed system, the full-wave electromagnetic simulator CST MWS is used which allow the modelling in three-dimensions (3D). The resulting images of a different target materials and dielectric properties confirm the reliability of the proposed system and the impact of the spatial and frequency diversity in the resolution improvement while considerably reducing the cost of the MIMO UWB array.

The widespread deployment of WiFi infrastructure has facilitated the development of Time-of-Flight (ToF) based sensing applications. ToF estimation, however, is a challenging task due to the complexity of multipath effect. In this paper, we propose a phase difference based method for ToF estimation and uncover the potential of signal cancellation to mitigate the impact of multipath and noise on phase differences among subcarriers. To separate the moving target path from the complex multipath for ToF estimation, we suggest employing specific elimination methods tailored to the characteristics of different signal components. For dynamic multipath, we observe that when a given subcarrier propagates along two paths to the receiver, with path lengths differing by half a wavelength, the phase difference introduced by these two paths cancels each other out. Therefore, we propose two metrics to identify signals that satisfy this condition, utilizing both frequency diversity and spatial diversity. Additionally, we propose leveraging time diversity to eliminate the static multipath component and reduce the impact of noise. We implemented the methods with off-the-shelf WiFi devices and achieved mean errors of 15.36 cm and 21.05 cm for distance estimation in outdoor and indoor scenarios, outperforming state-of-the-art ToF estimation method by 50% error reduction.

This paper deals with the study of received signal strength (RSS) as an input reference used in Bluetooth Low Energy (BLE) Indoor Positioning System (IPS) for location estimation. In the IPS where there are fixed scanners and portable advertisers which are sending no additional data, the RSS is the only reference for location process. Therefore, it is important to suppress RSS fluctuation caused by fading. The proposed method uses spatial diversity (SIMO approach) in combination with frequency diversity. Two different distances between four antennas were tested and for a quarter of the wavelength additional arrangement using back-side reflector was taken.

Orthogonal frequency and code division multiplexing (OFCDM) has been spotlighted as a next generation communication system. Cooperative OFCDM relaying system has advantages that provide the high reliability while maintaining high data rate because of getting the frequency diversity and the spatial diversity. However, this system decreases a throughput performance when it employs many relays. Therefore, in this letter, we present that the proposed system combines CDD due to complement throughput performance problem. In addition to that, the proposed system based on orthogonal frequency and code division multiple access (OFCDMA) which efficiently communicates each other through assigning the subcarrier and the channel code according to channel state or other environments.

Discrete Hartley transform (DHT) based orthogonal frequency division multiplexing (OFDM) outperforms the OFDM based on discrete Fourier transform (DFT) for its intrinsic frequency diversity. However, the space-frequency block coding (SFBC) cannot be applied since the symbols in Hartley domain are coupled. In this paper, by using the properties of DHT and DFT, we propose a DHT based SFBC-OFDM system design with better performance than and similar complexity to the DFT counterpart. Moreover, a low complexity algorithm is derived based on the properties of DHT to get the time domain signals of the DHT based SFBC code words with about halved complexity. Finally, analysis and simulations are provided to validate its feasibility and ability to exploit both the frequency and spatial diversity, as a promising alternative to the DFT counterpart.

Air-to-Ground (A2G) wireless communications using Unmanned Aerial Vehicles (UAVs) as Low Altitude Platforms (LAPs) have become a hot topic in recent decades. Among all the A2G channel models that can be used for A2G channel characterization, the two-ray ground reflection path loss model can be used for environments with fewer scatterers such as over water, suburban and mountainous. In such environments, signal down-fades appear due to the destructive combination of the direct component and the ground reflected component. This paper presents a frequency diversity approach to improve the coverage in such an environment. Our results show that frequency diversity can improve coverage at least by 17% when two frequencies are used assuming the threshold to be −80 dBm. Moreover, this work analyses the coverage improvement that can be obtained through polarization diversity. Our results show that having both horizontal and vertical polarizations can improve the coverage by 10%95%, assuming the transmission frequency to be 700MHz and UAV altitude is between 100 m300 m.