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In this paper, a novel low-complexity scheme is proposed for cyclic-prefix based OFDM systems to reduce inter-carrier interference (ICI) caused by time-varying channel. Under the assumption that channel delay spread is smaller than the guard interval (GI) of an OFDM symbol, the suitably selected inter-symbol interference (ISI) free received signals are exploited through compensating the corresponding linear phases and taking the average in the frequency domain. Therefore, some ICI terms can be automatically cancelled with less number of FFT operations. Moreover, the theoretical results of bit error rate performance are derived. The simulation results are shown that the ICI effect can be significantly removed by the proposed scheme for high-mobility OFDM systems when the ISI-free region q is adequate. Meanwhile, the proposed scheme has the advantages of increasing the mobile speed up to N/(N

In this paper, a novel low-complexity scheme is proposed for current OFDM systems to reduce inter-carrier interference (ICI) caused by time-varying channel. Under the assumption that channel delay spread is smaller than the guard interval period of an OFDM symbol, the suitably selected inter-symbol interference (ISI) free received signals are exploited through compensating the corresponding linear phases and taking the average in the frequency domain, and in consequence some ICI terms can be automatically cancelled. However, the performance of this self-ICI cancellation scheme is restricted by the inadequate ISI-free region in current OFDM systems. Therefore, a concatenated scheme, which consists of the self-ICI cancellation scheme and a residual ICI reconstruction scheme, is proposed to further improve the BER performance. The simulation results are provided to support that the ICI effect can be significantly removed by the proposed low-complexity concatenated scheme for current high-mobility OFDM systems.

Indoor localization is used in many applications like security, healthcare, location based services, and social networking. Fingerprinting-based methods are widely used for indoor localization. But received signal strength (RSS) variation due to device diversity and change of conditions in the localization environment (e.g., distribution of furniture, people presence and movement, and opening and closing of doors) induce a significant localization error. To overcome this, we propose a hybrid indoor localization system using radio frequency (RF) and pyroelectric infrared (PIR) sensors. Our localization system has two stages. In the first stage, the zone of the target person is identified by PIR sensors. In the second stage, we apply K-nearest neighbor ( K-NN) algorithm to the fingerprints within the zone identified and estimate position. Zone based processing of fingerprints will exclude deviated fingerprints because of RSS variation. We proposed two localization methods: Proposed_1 and Proposed_2 which use signal strength difference (SSD) and RSS, respectively. Simulation results show that the 0.8-meter accuracy of Proposed_1 achieves 84% and Proposed_2 achieves 65%, while traditional fingerprinting and SSD are 46% and 28%, respectively.

Handover is essential for seamless communication since subscribers move from cell to cell. This paper proposes an efficient handover decision based on user mobility to suppress the ping-pong effect. By using GPS positioning, the proposed algorithm adds a hysteresis parameter associated with distance between the mobile and surrounding base stations and a hysteresis parameter associated with the mobile heading direction in Event 1A and 1C. The performance was verified by applying the proposed method to a WCDMA system in urban Taipei city. The result shows that by using the proposed algorithm, handover number and mean active set number are decreased compared to the traditional algorithm

This paper proposes SADOA scheme to combine with TDOA method for mobile location estimation. Based on the ratio of distances between the mobile and base stations derived from differences of signal attenuations, each SADOA measurement yields a circle on which the mobile may lie. Meanwhile, each TDOA measurement defines a hyperbola on which the mobile may reside. The proposed hybrid SADOA/TDOA scheme uses Taylor-series expansion to linearize the circles and hyperbolas and iteratively computes the mobile position based on least-squares estimation. Without perfect path loss modeling and hardware modification, the proposed scheme reduces location errors compared with either technique separately. Simulations demonstrate encouraging performance with 50% improvement over the conventional TDOA method in shadowing and non-line-of-sight propagation environments.

Intersystem handover is essential for seamless communication since subscribers move between different systems. This paper proposes an efficient GSM/WCDMA intersystem handover decision based on user mobility to suppress the ping-pong effect. By using GPS positioning, the proposed algorithm adjusts the power threshold associated with distance between the mobile and surrounding base stations. The performance was verified by applying the proposed method to an existing network in urban Taipei city. The results show that the proposed algorithm outperforms traditional method by decreasing the number of intersystem handover and increasing the data throughput.

Propagation delay spread and inter-symbol interference impact the transmission performance of WLAN systems. This paper presents performance enhancement using beamforming techniques operated at the access point. The performance by using omni-directional, switched beam system and phase array system in an indoor office are analyzed and compared. Based on the IEEE 802.11a physical layer specification, numerical simulations indicate that adopting directional antennae at the access point can increase diversity gain in non-line-of-sight propagations channels, while the packet error rate remains below 10-2. Besides, this paper also presents a link adaptation algorithm, based on the Ricean k-factor and signal to noise ratio, for throughput enhancement in IEEE 802.11a systems. Simulation results show that the average throughput reaches 33.7 Mbps in the signal-strength-based conventional algorithm, but rises to 43.1 Mbps when adopting the proposed scheme

An analytic study applying an M-ary PSK adaptive modulation scheme in a Rayleigh-lognormal fading channel is proposed. The closed-form expressions of the average BER and the average data throughput (BPS) of the proposed adaptive modulation system with constant transmitted power over the fading channel are presented. The effect of shadowing on BER and throughput in the adaptive modulation system is also investigated. The results show analytical expressions for the average BER and data throughput derived for transmission over a Rayleigh-lognormal fading channel. Our analyzed results show good agreement with Monte Carlo simulation ones. The proposed method is useful in enhancing the data throughput of a communication system, such as a third generation wireless communication system, where a huge amount of bandwidth is needed for high-speed multimedia services.

This paper proposes a mobile location scheme using weighted centroid method based on the ratios of distances between mobile and base stations derived from the differences of signal attenuations. The proposed method does not require perfect path loss and shadowing models, and is capable to be applied in existing systems without hardware modifications. Simulations demonstrate encouraging performance with only two base stations being available in shadowing environments. Additionally, the results of driving measurement show that the proposed method outperforms the cell-ID method in an existing GSM system

An IEEE 802.11a WLAN system physical layer use coded orthogonal frequency division multiplexing (COFDM) to combat frequency selective fading channel. The WLAN system with omni antenna suffers excess delay spread and inter-symbol interference in indoor radio propagation channel. The impact of frequency selective fading channel on WLAN performance is evaluated. Wireless communication usually utilizes smart antenna to enhance performance. This paper presents Monte Carlo simulation based on the IEEE 802.11a physical layer specification and utilizes switch-beam smart antenna operation at the access point (AP) to enhance WLAN performance. The paper analyzes the performance of using omni and switch-beam antenna in a real office indoor environment. Compared with using omni antenna, the simulation results show that utilizing switch-beam antenna in AP the BER performance improve about 2 dB in light-of-sight (LOS) case, and 6 dB in non-light-of-sight (NLOS) case.

The variation of signal strength caused by shadowing is a random process, and handover decision mechanisms based on measurements of signal strength induce the "ping-pong effect". The paper proposes an improved handover algorithm, which identifies the correlation among shadowing components based on an estimation of mobile velocity, to suppress the ping-pong effect. The impact of velocity estimation errors on handover performance is investigated. Simulation results indicate that the proposed approach can reduce the number of unnecessary handovers by 9-17% compared to the conventional method, while the signal outage probability remains similar.

A handover decision mechanism considering only the signal strength induces the "ping-pong effect". This paper proposes an improved handover algorithm, based on the estimates of location and velocity of the mobile station, to suppress the ping-pong effect in cellular systems. A practical location technique was used to estimate the position and velocity of mobile station to identify the correlation among shadowing components of the received signals. The proposed handover algorithm was applied to a real GSM system in urban Taipei city. The results indicate that the number of unnecessary handovers can be reduced by 18/spl sim/26 percent by the proposed approach, compared to the conventional method, while the signal outage probability remains similar. Besides, the computational complexity of the proposed algorithm is low without using any database or lookup table.

This paper proposes a mobile location estimation and tracking technique for wireless communication systems. The location estimation is based on the differences of downlink signal attenuations, which are used to determine circles composed by possible mobile locations. Then the actual location is given by the intersection of the circles. The great advantages of this method are the non-necessity of a known and accurate path loss modelling and the reduction of shadowing effect. Furthermore, a mobile tracking technique via piecewise linear optimization using a simple genetic algorithm is applied to improve the locations estimation. As the results are shown, the estimation errors are much smaller than the errors from cell-ID method in a real GSM system.

This work presents a cell planning scheme using the genetic algorithm with the help of the propagation model and digitized building information to achieve the acceptable solutions based on impact of background noise for the WCDMA systems. Among these solutions, the best one is selected by a performance simulation platform with a RAKE receiver, which evaluates the BER performance with delay profiles obtained from ray-tracing simulator. The required coverage and maximum throughput can be achieved with the optimum solution for base station number, locations, antennas heights, and transmitting power. Thus, an easy and efficient cell planning for WCDMA systems in the initial stage of system development could be delivered.

In this paper, the fading properties of satellite-to-earth propagation channels are predicted using different Markov models. The proposed methods are validated and evaluated using mobile satellite link measurements data at typical mobile user environments. Compared with multi-states Markov models and traditional hidden Markov model, a better match between simulated and measured data shows that the non-stationary hidden Markov model predicts most satisfactorily the first and second order statistics of propagation properties. Besides, the proposed Markov models can be applied to evaluate the performance of error probability using coherent QPSK modulation schemes. This simple and efficient method extends the applicability of the Markov model approach in propagation channel modeling for satellite mobile communication services.

In this paper, the error probability performance of an OFDM system with multiple receiving antennas in a frequency selective fading channel is presented. In the receiving system, we use the adaptive direction of arrival (DOA) algorithm, MUSIC, to track each user's location. According to the DOA information, the antenna array creates a directive beam pattern to each user. Then, the received user's signal can get an additional antenna processing gain to beat frequency selective fading impairment and other direction interferences. By utilizing multiple antennas in the OFDM system, as shown in the simulation results, the error probability performance could be sufficient to improve even a DOA estimation with several degrees of error.

Radio location by time advance for GSM systems had been published. However, the resolution of time advance in GSM systems is too coarse to locate a mobile's position. The paper proposes a mobile location estimation based on the differences of downlink signal attenuations. This provides a possible mobile location method if the relationship between distance and signal attenuation is determined. Then, the mobile location can be estimated from those possible locations. The error of the proposed method is much smaller than the error of the cell-ID method in the practical microcell system. The greatest advantage of this method is the non-necessity of a known and accurate path loss model and the reduction of the shadowing effect.

The hidden Markov models are widely used for modeling random processes. Drawbacks in a standard hidden Markov model for modeling state occupancy have often been pointed out. In this paper, we proposed a propagation channel model for predicting the fading properties of satellite-to-Earth propagation channels using non-stationary hidden Markov model approach. An accurate prediction model was developed to provide better performance in modeling channel properties than a standard hidden Markov model and validated by several measured fading signal series. A close match between simulated and measured data shows that the proposed model predicts satisfactorily the first and second order statistics of propagation properties in several user environments. Considering the state stay duration issues, our proposed method can produce more accurate simulation results and extend efficiently the applicability of the HMM approach in modeling propagation channel properties for land mobile satellite communication services.

Microcell planning is quite different from the cell planning used for macrocells. For achieving efficient cell planning, it is necessary to understand the propagation characteristics of microcell systems. In this paper, we present the propagation measurement results at the campus area located at urban Taipei city. We verify that the Walfisch-Ikegami model is an accurate model for predicting path loss in microcell systems. Based on the Walfisch-Ikegami model and digitized building information, cell planning using a simple genetic algorithm is presented. The optimum solution for base station numbers, locations, antenna heights, and transmitting power can be achieved. On other hand, due to capacity demand and coverage requirement, the existing system must be enhanced. We also present the simulation results for system enhancement using the simple genetic algorithm with the help of digitized building information and the Walfisch-Ikegami model.

In previous work, environmental images were processed to derive Markov matrices and state transition matrices, and the conventional three-state propagation channel simulation algorithm was used to obtain a statistical representation of the communication channel. However, images take times and memory spaces for processing. For more accuracy and easier prediction, increasing the number of states and adopting a less complicated propagation channel model are proposed. We give a description of our multi-state Markov propagation channel model, and we compare the simulation results with measured data in several typical mobile user environment.

The analysis of the received data of GPS to study the path diversity effects of NGO satellite systems is presented. Measurements are conducted in the downtown Taipei vicinity. Three cases of different user mobility (fixed-point, pedestrian, and vehicular) are examined, and the switching and maximum ratio combining method are compared for path diversity gain. The results imply that significant fade reduction and great diversity gain can be expected in NGO satellite PCS. By combining the received signals of 2 satellites, significant diversity gain can be obtained at any given user scenario.

We present a system that uses a Markov chain approach to model the communication channels using videos of the environments in land mobile satellite (LMS) personal communication systems (PCS). Our method is based on capturing fish-eye lens images of the environments and, using very effective image and signal processing algorithms, modeling the communication channel characteristics using the theory of Markov stochastic processes. The resulting Markov chain models provide a full characterization of the narrowband LMS PCS propagation channel statistics in the selected environments. We present our imaging system and propagation channel modeling procedure in detail and discuss the results.

We present a system that substitutes video image processing for propagation measurements and permits flexible fade simulation of land mobile satellite communication (LMSC) systems. Our method is based on deriving environmental characteristics from fisheye lens videos, using image processing algorithms. We employ the processed images to track the propagation state (clear, shadowed by vegetation or blocked) along the satellite-Earth communication path. Analyzing sequences of images results in a propagation state time series which is input to a channel simulator. In this paper, we present the optical measurement system, the image processing algorithms and channel simulation procedures. Finally, we compare simulation results with actual satellite fade measurements.

The photogrammetric satellite service prediction method is an efficient way to predict the signal fading for land mobile satellite service (LMSS) and low-Earth-orbit (LEO) satellite personal communications system (S-PCS) in typical mobile user environments. Most of the work so far has been applied to urban areas, where satellite-Earth paths are either clear or blocked by building, mountains or other solid objects. To validate the feasibility of using photogrammetric satellite service prediction method in predominantly rural environments, where paths are either clear or shadowed by trees, a series of environmental pictures has been taken near Ottawa, Canada in 1995 and used to generate simulated fading time series for that environment. The photogrammetrically derived propagation statistics are compared with measurements taken along the same roads in 1982 and 1983. Our results show that the satellite service prediction method gives a good prediction of fading in the shadowed environment.

Antenna arrays can be employed in mobile communications to increase channel capacity as well as communication quality via spatially selective reception/transmission at base stations. In most wireless communications systems, directions of arrival of multipath signals need to be found for spatial selective transmission. Unfortunately, due to the coherent nature of multipath signals, it is quite difficult to find their directions of arrival. In this paper, we will present a subspace smoothing algorithm for finding the directions of arrival of multipath signals based on the mobile terminal signals received at different time instances. More importantly, we will present our experimental results to demonstrate that the spatial diversity is present for slight movements of a mobile terminal and that the subspace smoothing approach is effective in real wireless scenarios. All of the experiments were performed using the smart antenna testbed at the University of Texas at Austin.

This paper presents some preliminary results of experimental studies of space-division-multiple-access (SDMA) systems for wireless communications to expand capacity, increase coverage, and improve quality. Although SDMA schemes have been studied by a number of researchers, most of these studies are based on theoretical analyses and computer simulations. Very few real RF or microwave experiments have been conducted to validate the feasibility of various signal processing algorithms, such as direction finding and signal copy techniques. Also, no extensive experiments have been conducted to study the channel propagation associated with multiple antennas. The purpose of this paper is to present our preliminary experimental results using our recently developed antenna array testbed. We will also discuss the implications of these results on various array signal processing algorithms.

Propagation channels can be considered as operating in one of a finite number of possible states. The propagation fading is treated as fast fading and slow fading. Each of these two levels can be separated into several propagation states. Two Markov transition matrices are introduced for simulating the variation of the received signal strength due to multipath fading. Our method provides more states while limiting the transition matrix size to save processing time and memory requirements. We obtain accurate predictions of fading properties and error probability for typical mobile user environments. Comparing with the method of using one large Markov transition matrix, this simple and efficient method not only provides higher accuracy and saves memory requirements, but also extends the applicability of the Markov process approach in propagation channel modeling for land mobile satellite services.

Worldwide interoperability for microwave access (WiMAX) suffers from inter-carrier interference (ICI) since it adopts orthogonal frequency division multiplexing in physical layer. This study applies a repetition coding scheme, which encodes the same data on a subcarrier pair, to the hierarchical modulation to suppress the ICI and thus the inter-layer interference. Consequently, the proposed method mitigates the effects of fading and interference and provides a finer granularity of modulation order for link adaptation. Besides, the repetition coding scheme is also applied to a hybrid automatic repeat request (HARQ) protocol to recover error packets because of poor channel conditions. The proposed method incorporates the ICI cancellation into the conventional incremental redundancy HARQ and automatically activates the cancellation of ICI without Doppler shift/mobility estimation. Simulations show that the proposed algorithms outperform the conventional schemes in time-varying Rayleigh fading channels, especially in high-mobility environments.

Based on XOR coding, proposed is a proactive broadcasting for wireless broadcast systems where multiple faulty packets can be rectified at the cost of transmitting an additional coded packet. Since it greatly outstrips the conventional one in terms of BER performance, the proposed scheme can aggressively allocate the transmission rate to compensate for its throughput penalty. Numerical simulations show that the proposed scheme outperforms its conventional counterpart for about 5 dB SNR at a target BER of 10 -3 for an OFDM system.

Hidden Markov models (HMMs) of propagation fading channels are widely used. However, the standard HMM cannot effectively predict the state duration of a propagation channel. This investigation study develops a propagation channel model using a non-stationary hidden Markov approach to accurately predict the characteristics of the satellite-to-earth propagation channel. The proposed model can not only describe the propagation state characteristics but also capture the relationship between the times associated with the states. Typical measured data are used to derive optimal model parameters and generate a fading signal simulator. A close match between the simulated and measured data shows that the proposed model satisfactorily predicts the first- and second-order statistics of the propagation characteristics in several user environments. This new method produces more accurate simulation results than the standard HMM, in predicting the fading properties of the state duration statistics.

A signal attenuation difference of arrival (SADOA) scheme is proposed to combine with the time difference of arrival (TDOA) method for mobile location estimation. On the basis of ratio of distances between the mobile and base stations derived from differences of signal attenuations, each SADOA measurement yields a circle on which the mobile may lie. Meanwhile, each TDOA measurement defines a hyperbola on which the mobile may reside. The proposed hybrid SADOA/TDOA scheme uses Taylor-series expansion to linearise the circles and hyperbolas and iteratively computes the mobile position based on least-squares estimation. Without perfect path loss modelling and hardware modification, the proposed scheme reduces location errors compared with either technique separately. Simulations demonstrate encouraging performance with 50% improvement over the conventional TDOA method in shadowing and non-line-of-sight propagation environments.

WCDMA is an interference-limited system and its coverage and data throughput are sensitive to background noise. The results of background noise measurements in urban Taipei city for the licence bands of 3G systems issued in Taiwan are presented. The measurements involve FDD mode uplink and downlink frequency bands measured on building tops and at street level, respectively. The severeness of spectrum pollution of these bands is evaluated by extracting three statistical parameters from the measurement data, and the impact of the background noise on coverage and throughput is analysed for WCDMA systems. Also, based on measurement results, a better solution using a genetic algorithm with the help of a propagation model and digitised building information is proposed for the deployment of the base stations of WCDMA systems, by which the required coverage can be met with a suitable number of base stations, locations, antenna heights and transmitting power. A system is obtained that suffers less impact from background noise and achieves a higher data throughput with minimum cost.

Space-division-multiple-access is one of the solutions for increasing the system capacity as well as improving the system performance in wireless communications. Experimental studies have been reported; using a smart antenna system to exploit spatial diversity in low-Earth-orbit (LEO) satellite personal communication services (S-PCS). The paper presents direction-of-arrival (DOA) estimation of satellite signals, variation of spatial signature and DOA with frequencies, elevation angle and motion, and diversity gain achieved by using an antenna array in line-of-sight (LOS) and shadowing scenarios. For typical environments, such as LOS and light or heavy shadowing cases, the experimental results reveal rich spatial diversity and high diversity gain when using a smart antenna system in LEO S-PCS.

A series of hemispherical pictures was taken near Ottawa, Canada and used to generate simulated fading time series for that environment. Photogrammetric satellite service prediction (PSSP) derived propagation statistics are compared with radio frequency measurements taken along the same roads. The results show that PSSP provides a convincing way to predict the elevation angle dependence of fading properties in the shadowed environment.