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Personal, Indoor and Mobile Radio Communications, 1995. PIMRC'95. Wireless: Merging onto the Information Superhighway., Sixth IEEE International Symposium on

Date 27-29 Sept. 1995

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  • Antenna architectures for CDMA integrated wireless access networks

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    The main resource in the design of a wireless multiple access bandwidth is the amount of radio spectrum allocated to the network. The requirement to provide mobile telephone services in a limited bandwidth has resulted in the development of the cellular network architecture. We consider different antenna sub-systems for integrated wireless access networks (IWAN). The various antenna sub-systems present alternatives to the standard architecture of a cellular system where the transmission/reception occurs via omnidirectional or directional antennas with each antenna located at a base station. The appropriate antenna sub-system is dependent on the modulation scheme used. We consider a CDMA scheme along with various antenna sub-systems such as a distributed antenna, an antenna sub-system which we refer to as a sectorized distributed antenna, and a distributed antenna utilizing sub-carrier multiplexing. These concepts lead to a network architecture consisting of network switches, radio controllers, base stations, and the various antenna subsystems. The mix of different approaches to antenna subsystem design leads to various system design alternatives with trade-offs in design issues such as capacity, system complexity, coverage, and robustness against fading View full abstract»

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  • Hopfield neurobeamformer for spread spectrum communications

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    In cellular or mobile systems, provisions must be made for a high-quality reception. Therefore, adaptive beamforming can be combined with spread spectrum modulation in order to provide interference protection for users within a cell or users in co-cells. Hopfield (1984) neural network is used to carry out adaptive beamforming with direct sequence spread spectrum signals. A reference signal generator is employed to derive the desired signal for the adaptive processor. A PN code is used to spread the data signal which is biphase modulated. At the receiver end, this code is used to extract the desired signal component from the array output and reject any narrowband interference signals. Simulations are carried out for 8-element linear and circular arrays. The performance of the neurobeamformer is evaluated in the presence of interference from directions very close to the desired signal. Simulations also include a scenario where a multipath signal impinges on the array View full abstract»

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  • A subspace method for signature waveform estimation in synchronous CDMA systems

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    Synchronous-CDMA techniques possess intrinsic protection against co-channel interference due to orthogonal codes employed and thus allow multiple users to operate within the same frequency band. In the presence of multipath, however, each signal is subject to frequency-selective fading and the code orthogonality condition does not hold leading to increased cross correlation. In these scenarios, multiuser detection techniques need to be employed to suppress interference and recover the message symbols. To implement these algorithms, explicit knowledge of the (nonorthogonal) signature waveforms of all users is required. We propose a blind estimation scheme that provides a closed-form solution to the estimation of signature waveforms by exploiting the structure information of the data output. In particular, we show that the subspace of the data matrix contains sufficient information of the signature waveforms. Based on this observation, a MUSIC-like algorithm which can accomplish blind estimation using a minimum number of data samples is derived View full abstract»

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  • Interference cancellation using antenna arrays

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    A limiting factor for the performance of the conventional CDMA receiver is the near-far effect. Practical CDMA systems use power control to overcome this problem, which requires additional circuitry at the mobile. On the other hand, optimal and suboptimal multiuser detectors have been proposed which are shown to be near-far resistant and thus increase the capacity of the system significantly. The major drawback of these detectors is their computational complexity which makes them less attractive for implementation. This paper describes a simple detector which is a spatio-temporal interference canceller. The receiver employs an array of M antennas which allows it to discriminate between the users based on their spatial diversity. The received signal goes through a bank of K beamformers each matched to one user, followed by a bank of matched filters. The K outputs are compared to rank the users in the order of their strength. Multiple access interference from each user is cancelled successively from the received signal based on this order. This detector is compared with a single antenna interference canceller and also the conventional detector and it is shown that using multiple antennas improves the performance of the interference canceller and compensates for nonzero crosscorrelations between the users' signature waveforms View full abstract»

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  • Adaptive multiuser receiver schemes for antenna arrays

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    This paper develops a new multiuser detector for antenna arrays. The receiver uses an adaptive multiuser receiver in a multiple sensor environment. Using this receiver, two schemes are proposed for (CDMA communications) use with antenna arrays. The first scheme is a simple extension of the multiuser detection scheme to a multisensor environment, and has a performance better than the single channel decorrelator detector. The second scheme adaptively implements an array-decorrelator detector, which exploits both spatial and code diversity. It is different from the first scheme in that it uses the array response vector estimates of each user to reduce the correlation between users's transmitted signal. It therefore, avoids the reduction in performance when signature correlations become significant View full abstract»

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  • Intelligent antennas for DS-CDMA systems

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    This paper considers the performance of a DS-CDMA system which employs adaptive antenna technology at the basestation site. By utilising the capability of ray-tracing to provide the complex channel impulse response, a new ray-based simulation methodology for an adaptive antenna in a DS-CDMA system is presented. Results for a typical microcellular environment highlight the behaviour of the adaptive antenna. Finally, with the help of a DS-CDMA capacity analysis, the potential spectrum efficiency enhancement is evaluated and the improvement of some of the statistics of the system is discussed View full abstract»

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  • A spatially and temporally optimal multi-user receiver using an array antenna for DS/CDMA

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    The tandem structure of a matched filter (MF) and a maximum likelihood sequence estimator (MLSE) using the Viterbi algorithm (VA) has been considered to be an optimal receiver for digital pulse-amplitude sequences in the presence of intersymbol interference (ISI) and additive white Gaussian noise (AWGN). An adaptive array antenna has the capability of filtering received signals in the spatial domain as well as in the temporal one. In this paper, we propose a receiver structure using an adaptive array antenna, a digital filter and the VA that is spatially and temporally optimal for multi-user detection in a DS/CDMA environment. This receiver uses a tapped delay line (TDL) array antenna and the VA, which provides a maximum likelihood sequence estimate from the spatially and temporally whitened matched filter (ST-WMF) output. Performance of the proposed receiver is evaluated by theoretical analysis and computer simulations View full abstract»

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  • Power control for a spread spectrum system with multi-user receivers

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    The power control problem has received a lot of attention in CDMA systems employing single-user receivers because of the `near-far' effect under certain conditions. Multi-user receivers suffer less from this problem, but adaptive power control is still necessary for the following reasons: (i) to compensate for the motion of the mobile, (ii) the actual amount of power needed to achieve the required performance is also a function of the code correlations, (iii) it is necessary to conserve the battery power and (iv) if we assume that multi-user detection is employed only for users with known codes with the other interfering users being treated as noise, the amount of interference caused by these users is in terms of their powers, just as with single-user receivers. This case could occur, for example, if multi-user detection were used in the uplink for the users in the given cell with the interference from adjacent cells treated as stationary noise. In this paper, we consider multi-user receivers based on the minimum mean squared error (MMSE) approach for a synchronous CDMA system where external cell interference is treated as stationary noise. We consider the convergence of the algorithm and show sufficient conditions for convergence to a unique optimal power vector that minimizes the total transmitted power View full abstract»

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  • Performance of multistage multiuser detection in a fading multipath channel

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    In this paper we study the performance of multiuser detection in cellular uplink reception. The main motivation in this paper is not to study the performance in a near-far scenario, but to demonstrate the achievable capacity gain. To this end we apply the joint estimation and multistage detection algorithm developed previously (Fawer and Aazhang, 1995) in a multipath fading environment. We show that multiuser detection can be used effectively to increase the system capacity and range. The assessment is carried out by extensive simulations with up to 60 users using length 63 Gold codes in a two path Rayleigh channel, both with and without 1/2-rate coding View full abstract»

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  • Multi-stage interference cancellation in multi-rate DS/CDMA systems

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    We show that the one-stage IC scheme for M-level rectangular QAM can easily be extended to a multi-stage IC scheme. The performance of different single modulation systems, with equal throughput, is compared with systems employing conventional detectors and the single user bounds. In the analysis we consider two methods for handling multiple data rates, mixed modulation and parallel channels. These methods are analysed under the assumption of Rayleigh fading. The results show a considerable improvement in both performance, close to the single user bound, and capacity compared with systems employing a conventional detector View full abstract»

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  • A unified approach to multiuser detectors for CDMA and their geometrical interpretations

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    In this paper we align a class of known multiuser detectors for code division multiple access (CDMA) along with some new multiuser detectors in a unified group. The unified group structure is a binary tree obtained by splitting sequence and single symbol detection, maximum a-posteriori (MAP) and maximum likelihood (ML) detection, and unconstrained as well as constrained detection. Finally, this group structure is supported by a geometrical technique illustrating the detection process View full abstract»

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  • An improved multi-user CDMA decorrelating detector

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    Decorrelating detectors were proposed as tools to combat the near-far problem in multi-user CDMA communication systems. They were also used as the first stage of many two-stage, multi-user signal separators to facilitate tentative decisions for the canceller stage. These decorrelators result in a complete separation of signals, but also in relatively high additive noise. An improved multi-user detector is proposed. It may not totally reject other signals, but it does result in a better error probability, particularly in regions where interference energies are low. The proposed improved decorrelators can be implemented by an adaptive algorithm and hence are better suited for a changing environment View full abstract»

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  • Linear multiuser detector update in synchronous dynamic CDMA systems

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    For synchronous CDMA systems the decorrelating multiuser detector is the inverse of the correlation matrix of the signature waveforms. The noise-whitening multiuser detector is the inverse of the Cholesky factor of the correlation matrix. If the number of users or the signature signals changes, the detectors must be recomputed. The paper introduces order-recursive algorithms for detector update. The algorithms compute either the inverse or the Cholesky factorization of the correlation matrix. The numerical accuracy of the algorithms is found to be superior even with random spreading sequences View full abstract»

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  • A Bayesian receiver for asynchronous code division multiple access communications

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    An adaptive Bayesian receiver structure for use in asynchronous multi-user code division multiple access (CDMA) systems is presented. The structure consists of a bank of eigenfilters and a radial basis function (RBF) network. This receiver can be trained to form the Bayesian non-linear decision boundary. Simulations demonstrate that it outperforms conventional matched filter detectors. Bit-error-rate curves are presented which show that it is robust, near-far resistant and able to cope with multipath. Its performance is similar to that of optimal maximum likelihood detectors with the advantage that it is less complex and inherently adaptive View full abstract»

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  • Frequency acquisition in a synchronous CDMA cordless phone using fast Walsh transform

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    We have designed a CDMA cordless phone for fast acquisition and simple tracking. The standard acquisition proceeds through searching for the peak of correlation to get the delay offset, and correcting the frequency offset. We use the fast Walsh transform to estimate the frequency offset. The final correlations are tested within bounds so that the probability of false alarm is very small. The methods and bounds are derived from the formulas of the Walsh transform of sine functions. The method of frequency estimation is demonstrated by simulation its application in a spread spectrum receiver View full abstract»

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  • An improved hybrid PN code acquisition for CDMA personal wireless communications

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    This paper proposes an improved decision strategy for noncoherent hybrid parallel pseudonoise (PN) code acquisition and analyzes the acquisition performance in the presence of multiple access interference (MAI) over Rayleigh and Rician fading channels for code-division multiple access (CDMA) personal wireless communications. The hybrid acquisition scheme combines parallel search with serial search to cover the whole uncertainty region of the input code phase. The closed-form expressions of the detection and false-alarm probabilities and mean acquisition time are derived. It is shown that, (i) the scheme achieves both better acquisition performance and simpler acquisition hardware structure than total parallel acquisition in the case of severe MAI, and (ii) the increase of the mean acquisition time (due to MAI) depends on the number of users in the CDMA system, PN code search strategy, system design parameters, and channel fading statistics View full abstract»

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  • Mean time to lose lock for a second-order extended tracking range delay-locked loop

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    The delay-locked loop (DLL) is an appropriate device to guarantee fine synchronization for direct-sequence spread spectrum systems. For the performance analysis at low signal to noise ratios the mean time to lose lock (MTLL) represents the most important criterion. The MTLL of the second-order extended tracking range DLL is investigated. A coherent loop with a passive loop filter is considered. The MTLL is approximated by an explicit expression for the leading order term based on results by Kramers (1940). The investigations show significant differences for the MTLL of different loop structures. Furthermore, design considerations and optimal loop parameters are given View full abstract»

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  • Approximated probability density functions for the output signal of coherent matched filter and median filter code acquisition structure in an AWGN channel

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    Acquisition of code synchronization is the most difficult part in a spread spectrum receiver. This paper describes approximations for the probability density functions for the output signal of the coherent matched filter and median filter code acquisition structure in the AWGN channel. The analysis is based on Gaussian density functions. The important parameters for code acquisition, the probabilities of detection and false alarm, are used as performance measures. The results obtained by the analysis are compared with those achieved simulations. Comparisons are made using both fixed thresholds and fixed false alarm probabilities. The conclusion is that the probabilities can be calculated quite accurately using these approximations. The analysis very easily gives an insight of the density functions View full abstract»

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  • A frequency and timing period acquisition technique for OFDM systems

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    Orthogonal frequency division multiplexing (OFDM) has been receiving a lot of attention in the field of broadcasting because of its robustness against the effects of multipath propagation which are main obstructions for mobile reception and realization of the single frequency network. This paper introduces a frequency and timing period acquisition technique for OFDM systems. The proposed technique estimates both the frequency and timing period offsets at the time by using only one pilot symbol with its suitable frequency assignment. Pseudo noise (PN) sequences are introduced to assign these frequencies of the pilot symbol so that the acquisition range can be widened. Numerical examples are given to show the estimate variances for both additive white Gaussian noise (AWGN) and multipath fading channels View full abstract»

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  • Synchronization for an indoor wireless communication system utilizing TDMA and commutation signaling

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    A new synchronization scheme for a high speed indoor data link is proposed. The frame and bit timings as well as the channel delay spread are estimated in the presence of multipath induced intersymbol interference. A mixed narrow-band and spread spectrum synchronization scheme is found efficient for low processing gain spread spectrum signaling in higher data rates. The synchronization scheme is described in detail and the simulation results are given View full abstract»

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  • Indoor propagation measurements at 94 GHz

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    Experiments of 94 GHz millimeter wave propagation were conducted in indoor environments where the propagation path loss and log-normal fading are investigated in a typical lecture room and corridor. The results show that in a LOS channel the path loss exponents for a room and corridor are found to be 1.8~2.5 and 1.7~1.8, respectively. The artificial reflection by a mirror reflector is also suggested under non-LOS environments in order to avoid significant channel degradation, thereby the power level is found to be improved by 15~20 dB View full abstract»

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  • Transmission and isolation of signals in buildings at 60 GHz

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    This article presents a model, based on reflection and transmission of plane waves in a dielectric slab, for the estimation of transmission and isolation through building walls and floors at 60 GHz. The internal reflections is negligible for the usual thickness of building materials, with the exception of glass, acrylic glass and plasterboard. In the same conditions, the estimation of the isolation can be done by using the product of the attenuation coefficient and the wall thickness, and it is seen that at least a value of 20 dB is obtained. As a general conclusion, only glass and plasterboard are not effective in isolating cells with the same frequency group View full abstract»

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  • Effects of antenna directivity on indoor multipath propagation characteristics at 60 GHz

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    In millimeter-wave indoor communications systems, the radiation patterns of the antennas at base stations and remote terminals have a significant influence on the channel characteristics. The work reported in this paper investigated the effects of the radiation patterns of the antennas at remote terminals on multipath propagation characteristics. These effects were investigated by indoor propagation measurements at 60 GHz conducted in a modern office room and by ray-tracing simulations based on geometrical optics. Multipath channel characteristics are compared in terms of impulse responses and their RMS delay spreads for an omnidirectional antenna and for three directive antennas with different beam widths. From the results of the measurements and ray-tracing simulations, the use of a directive antenna at the remote terminal is demonstrated to be an effective method of reducing the effects of multipath propagation View full abstract»

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  • High data rate indoor wireless communications using antenna arrays

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    In this paper, we consider the feasibility of indoor wireless communications at very high data rates (up to multi-Gbps). In particular we wish to use one base station to cover the entire floor of an office building, which may have in excess of 60 dB propagation loss relative to 1 meter. This feasibility depends on two factors: received signal power margin and delay spread. Based on results using the propagation-prediction techniques of Valenzuela (1993), supported by experimental results up to 622 Mbps, we conclude that neither multicarriers, equalization, nor antenna arrays with less than 1600 elements at one end of a communication link are economical methods for increasing the data rate substantially above 10 to 20 Mbps for multiple room indoor wireless coverage. However, based on the propagation-prediction techniques of Valenzuela and verified by our experimental measurements using directive antennas (15° beamwidth) at both ends of a link between the center of the Crawford Hill building to an end laboratory, we have shown that high-speed ubiquitous communication is feasible. Using antenna arrays with 50 to 200 elements at both the transmitter acid receiver, we expect to obtain entire floor coverage at data rates in excess of 1 Gbps View full abstract»

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