This paper investigates the impact of Access Point (AP) or user positioning imperfections and channel estimation errors on the performance of a smart antenna system serving multiple users through Space Division Multiple Access (SDMA) in an indoor Wireless Local Area Network (WLAN). A ray-tracing tool is used to predict the Power Delay Angle Profile (PDAP) of the users in an arbitrary indoor environment. A Sample Matrix Inversion (SMI) algorithm is used to calculate the optimal smart antenna weights. The impact of a small displacement of the AP or the user positions on the SINR value from their initial positions is first investigated and compared for the Line of Sight (LOS) and multipath cases. It is shown that, in the multipath case, small AP or user displacements (in the order of λ/4) will cause a variation of the smart antenna patterns and consequently the Signal to Interference plus Noise Ratio (SINR) due to the change of the phase and amplitude of the multipath components (rays) arriving at the AP. The impact of several degrees of channel estimation errors is then investigated. It is shown that small errors in the estimation of the channel transfer matrix coefficients could lead to a large degradation in the SINR value.

This paper presents a novel approach for the estimation of the local average signal level in an arbitrary indoor environment based on a wall imperfection model. A ray-tracing tool based on the method of images with angular information, is used to estimate the distribution of field strength (or coverage) in an arbitrary environment. The spatial sampling approach for signal level distribution prediction is studied. The wall imperfection model is then introduced to study the sensitivity of the received signal level at an arbitrary location to imperfect wall positioning and electromagnetic material properties. An alternative approach to estimate the local mean signal level at a particular point is proposed based on the introduced wall imperfection model to reduce the computation time compared to the spatial sampling approach.

This work presents a novel protocol for fair exchange of DSA-based signatures to support an e-commerce activity of digital contract signing. The protocol employs an off-line semitrusted third party (STTP), which participates in the protocol execution only in exceptional circumstances and which involvement in the protocol is transparent. The protocol can tolerate the STTP's own misbehaviour (i.e. if it does not collude with other participants of the protocol) and it prevents the STTP from gaining the exchanged signatures or the signed document. The protocol is based on the principle of verifiable and recoverable signature encryption, and in comparison with related protocols is more efficient.

This work presents a downlink pattern optimisation algorithm for a smart antenna optimum combiner operating in an infrastructure wireless local area network (WLAN) following the IEEE 802.11a or HIPERLAN/2 standards and serving the multiple users through space division multiple access (SDMA). An indoor infrastructure WLAN scenario with four users communicating with an access point is modeled using a deterministic ray-tracing tool. The performance of the smart antenna system is studied in both the uplink and downlink for the line of sight (LOS) and multipath cases to achieve optimum performance in terms of the maximisation of signal to interference plus noise ratio (SINR) for each user. It is shown that the proposed downlink optimisation algorithm achieves better SINR values for the worst case user compared to the case where uplink optimum weights are used for downlink and that the optimal downlink patterns are not necessary the same as the optimal uplink patterns.

This paper investigates the effects of directed retry (DR) in cellular networks characterized by a nonuniform environment, that is, with unequal distribution of users and/or radio resources. We propose an analytical model able to take both propagation and network aspects into account. The nonuniform environment is modeled assuming the superposition on a uniform scenario of a circular area characterized by different offered load or radio capacity. The case of a hot spot of users is included. Path loss and lognormal shadowing have been considered. This is done through a statistical approach which is very general, and provides an expression for the probability of receiving a suitable power level from a given number of base stations within a fixed distance from the mobile. A comparison to simulation results is performed to validate the model. We show that the impact of DR on the blocking probability depends largely on the propagation parameters considered.

This paper considers points which can communicate with one another and which are uniformly and randomly distributed on the plane, assuming an inverse power law for attenuation and log-normal shadowing. First, a probability distribution is obtained for the distance between a pair of such points. Communication is assumed to be possible if the attenuation of a signal transmitted from one point, say a mobile phone, does not exceed some specified value by the time it reaches the other, say a base station. The probability distribution of the number of base stations which can hear such a signal from a given mobile is found. An example shows how the probability that a mobile is in a handover region between two or more base stations can be evaluated. The analysis is extended to the case where the power law changes at a specified distance from the mobile, and also to consider the effect of an area within the plane containing a greater density of mobiles and/or base stations (a hot spot). A final result gives the conditional distribution for the number of base stations within some specified area of the plane, given the number within some larger area containing that specified area. The results also apply to the number of mobiles within range of a base station and, in an ad hoc network, to the number of nodes within range of a given node.

Data archiving networks and electronic media libraries for video and audio broadcasting have gained a large interest recently. This paper proposes the use of smart (adaptive) antennas for HIPERLAN/2 (HIgh PERformance Radio Local Area Network/2) in order to increase the data rate and user capacity targeting wired LAN replacement in modem TV broadcasting studio environments. Spatial channel models are necessary for the proper design, analysis and implementation of smart antenna systems. A simple 2D ray tracing indoor channel model for predicting propagation characteristics inside a room of arbitrary dimensions and wall properties is presented. The simulation model gives fast estimates of the power-delay profile (PDP) and direction of arrival (DOA) information based on the geometrical optics (GO) theory taking into account double reflections. The effects of the relative positions of the transmitter and the receiver, room dimensions as well as the number of transmitters are investigated.

This paper considers two sets of points which can communicate with one another and which are uniformly and randomly distributed, one set within a confined area, a hot spot, on the plane, and the other with a different density over the rest of the plane: an inverse power law with lognormal shadowing is assumed for attenuation. Communication is assumed to be possible if the attenuation of a signal transmitted from one point, say a mobile telephone, does not exceed some specified value by the time it reaches the other, say a base station. The probability distribution of the number of base stations which can hear such a signal from a given mobile is found, taking account of the effect of the hot spot. The conditional distribution of the number of such base stations within the hot spot, given the total number over the whole plane, is deduced, and this result is applied in an example to finding the blocking probability for a directed retry algorithm. The results also apply to the number of mobiles within range of a base station and, in an ad hoc network, to the number of nodes within range of a given node.

In this paper we present an analytical model for the estimation of the blocking probability as a function of the offered traffic per user in a cellular environment, where capacity is determined by hard blocking and the average number of users per cell is small. By the term "hard" capacity we mean the maximum number of users that can be served by a base station. Using a statistical model, the paper shows that the number of mobiles audible to a base that have that base as the strongest one has a Poisson distribution whose mean is given in terms of the mean densities of mobiles and base stations and the parameters of the attenuation law. The model proposed here does not represent a tool for detailed network planning, but it is a helpful (and from the computational viewpoint simple) means for network dimensioning.

This article presents a near-far resistant detection scheme for asynchronous code-division multiple access with frequency offset. Based on a one-shot technique and Taylor expansion, a zeroth-order and a first-order one-shot linear decorrelating detector (LDD) are proposed. The zeroth-order LDD has simple architecture but suffers performance degradation for large frequency offset. The first-order one-shot LDD, with increased complexity, has very good near-far resistant property even for large frequency offset. Two versions of the first-order one-shot LDD are investigated according to different Taylor expansion approaches. The feasibility of the proposed detectors is demonstrated by computer simulations.

We derive the exact closed form expression for symbol error probabilities in slow frequency hopping multiple-access spread spectrum communications employing MFSK modulation schemes. The new expression is easy to compute and permits signal levels to be unequal or randomly distributed. However, it is exact only in the case of synchronous hopping and the Rayleigh fading channel.

This paper considers the effects of base station diversity on the capacity of a cellular CDMA system. Closed loop power control is assumed to maintain the received power constant at the base station receiving the second or third strongest signal for dual or triple diversity, as opposed to the strongest for conventional CDMA. The probability distributions of the power ratios between signals received at two such base stations (ranked in descending order of received signal strength) are obtained, and these lead to a measure of the reduction in the overall capacity of a system brought about by high power interferers affecting one base station but controlled by a neighbouring one.

The well-known linear decorrelating detector (LDD) for direct-sequence code-division multiple-access (CDMA) systems provides near-far resistant performance when the timing of each user is accurately known. Traditional CDMA acquisition techniques suffer from high differences in power levels. The estimation accuracy for a user overwhelmed by stronger ones is likely to be unsatisfactory; at the same time, the interference from a user undergoing acquisition or tracking is not removed by the standard LDD. In this paper, a fully near-far resistant technique for acquisition and tracking for asynchronous CDMA systems applying the LDD is proposed, considering realistic band-limited signals. This technique is based on the adoption of a pair of special sequences equivalent to a dedicated access channel and is shown to provide a relatively fast and robust means to perform channel estimation both in case of single- and multipath channels.

Frequency-offset correction is considered for a HIPERLAN (HIgh-PErformance Radio LAN) system over the indoor radio channel. Since the multipath channel response is not known a priori, a viable frequency-offset estimator should not depend on such knowledge. Such an estimator, using a single sample per symbol, is derived for HIPERLAN. The estimator is shown to approach the Cramer-Rao bound for frequency-offset estimation over a multipath channel. A HIPERLAN system simulation example shows that the performance with an offset of 150 kHz is within 0.5 dB of that of a system with zero frequency offset.

This paper addresses the problem of selecting the optimum training sequence for channel estimation in communication systems over time-dispersive channels. By processing in the frequency domain, a new explicit form of search criterion is found, the gain loss factor (GLF), which minimizes the variance of the estimation error and is easy to compute. Theoretical upper and lower bounds on the GLF are derived. An efficient directed search strategy and optimal sequences up to length 42 are given. These sequences are optimal only for frequency domain estimation, not for time domain estimation.

This paper uses some computational results to show that the peak envelope power (PEP) of orthogonal frequency-division multiplex (OFDM) waveforms can be reduced very substantially by coding to select only those messages with low peak factor as valid codewords in the scheme. It is shown that the asymptotic limit of the PEP is between two and three, and that only four bits of redundancy are necessary to limit the PEP to within 10% of this optimum value as the number of carriers becomes large.

This paper presents a new approach to reduced complexity detection of partial response continuous phase modulated signals over channels with severe intersymbol interference (ISI). The detector combines the overall channel response for the CPM modulator with that of a multipath channel to calculate the branch metrics of a breadth-first sequential detection algorithm known as the T-algorithm. To our knowledge the reported work is the first attempt to make use of such a technique for equalisation of quaternary partial response CPM signals transmitted over multipath channels. Simulation of the performance of the receiver in multipath channels is reported.

A mobile radio demonstration system using spread spectrum and CDMA has been created by a joint industry/university project in the UK. This paper presents results obtained during testing of the reverse link (the link from mobile station to base station). These results illustrate the BER performance and the operation of the power control mechanism over a multipath channel simulator.

This paper presents an efficient key exchange protocol for cryptographically secure CDMA systems which uses no more than the single REQUEST-RESPONSE protocol of conventional systems. The protocol, based on the Diffie-Hellman scheme, makes synchronisation of the cryptographic spreading sequence easy and uses the existing cryptographic sequence generation hardware for the key protocol computations.

A fast computation method for the tap weights of a linear equaliser is developed. By approximating the channel autocorrelation matrix, which has a Toeplitz structure, by a circulant matrix, the tap weights can be solved via discrete fourier transform (DFT) techniques. For an equaliser with N taps, the approximate tap weights approach the exact tap weights as N/spl rarr//spl infin/. Excess mean-square error and bit error rate (BER) degradation caused by the approximate tap weights are computed for a representative channel.

To reduce the effects of intersymbol interference resulting from the dispersive nature of the indoor radio channel most HIPERLAN receivers will incorporate an adaptive equaliser. The computational complexity of several equaliser algorithms is estimated. The effect of frequency offset between the transmitter and receiver on the performance of such an equaliser is investigated. By employing a decision feedback equaliser incorporating a second order phase locked loop, the effect of both the intersymbol interference and frequency offset can be significantly reduced. Using such a technique, the packet error ratio (PER) of a HIPERLAN radio link in a multipath channel is found by simulation.

Little has so far been reported on the performance of the near-far resistant CDMA detectors in the presence of the synchronization errors. Starting with the general mathematical model of matched filters, this paper examines the effects of three classes of synchronization errors (i.e. time-delay errors, carrier phase errors, and carrier frequency errors) on the performance (bit error rate and near-far resistance) of an emerging type of near-far resistant coherent DS/SSMA detectors, i.e. the linear decorrelating detector (LDD). For comparison, the corresponding results for the conventional detector are also presented. It is shown that the LDD can still maintain a considerable performance advantage over the conventional detector even when some synchronization errors exist. Finally, several computer simulations are carried out to verify the theoretical conclusions.

This paper presents a novel scheme for near-far resistant CDMA detection: isolation bit insertion (IBI). At the transmitter, isolation bits are inserted into the information bit sequence before modulation, and a practical linear decorrelating detector (LDD) is obtained at the receiver. All the advantages that an LDD theoretically offers are retained and realised in practice. >

The performance of a conceptual digital cellular radio system which employs frequency hopping CDMA has been analysed. The bit error probability and the system outage are used as performance indicators, while the system load in terms of users per cell per MHz is varied as a parameter. The results suggest that the capacity of a basic FH-CDMA system with full frequency reuse is poor, but that there is potential for dramatic improvement through the use of antenna diversity. Without diversity a capacity of between 5 and 10 users/cell/MHz is predicted. With a basic model of dual antenna diversity the capacity improved to more than 100 users/cell/MHz.

In this paper we present the system design for a BPSK demodulator suitable for earth station implementation for satellite communications. The primary design goal has been to obtain a demodulator that has fast acquisition in a low signal-to-noise ratio environment. To this end the implementation uses structures and algorithms that are entirely feedforward. The system design has been analysed and verified by the use of software simulation and an implementation suitable for Inmarsat-C applications bas been developed using a Motorola DSP56001 target system. The key requirement was for a demodulator implementation that would operate both in a burst-mode and a continuous mode according to the signal types and specifications applicable for Inmarsat-C 'coast earth stations' and 'network control stations'. The design has resulted in a flexible implementation that could be easily adapted for many applications.

The European Telecommunications Standards Institute (ETSI) is currently defining a European standard for high performance radio LANs (HIPERLAN). To operate as wired LAN replacements, these systems will need to offer data rates as high as 20 Mb/s. To counteract the time dispersive nature of the indoor radio channel, adaptive equalisation is proposed at the receiver. In this paper a number of possible modulation and equalisation techniques are presented and, in particular, the BER performance of quasi-coherent GMSK detection is investigated. Important issues such as frame and symbol synchronisation, frequency offset correction and equaliser implementation are also addressed from a practical standpoint.

This paper proposes a new signaling scheme: orthogonal on-off BPSK (O/sup 3/BPSK), for near-far resistant detection in the asynchronous DS/CDMA systems (up-link). The temporally adjacent bits from different users in the received signals are decoupled by using the on-off signaling, and the original data rate is maintained with no increase in transmission rate by adopting an orthogonal structure. The detector at the receiver is a one-shot linear decorrelating detector, which depends upon neither hard-decision nor specific channel coding. Some computer simulations are shown to confirm the theoretical analysis.

It has been suggested that the transmit powers required for a mobile unit in a cellular radio system are lower for DS-CDMA based systems than for systems using narrowband modulation. This paper investigates whether this is an intrinsic property of DS-CDMA. Ideal systems, all with the same thermal noise spectral density are considered. Coiling and interleaving are included. The results suggest that when systems are operating near capacity, DS-CDMA mobiles require similar transmit powers to mobiles in other systems. For system loads less than capacity, however, the power requirement of DS-CDMA mobiles is smaller than that of mobiles in other systems.>

Hussain and Barton (see Seventh IEE European Conference on Mobile Personal Communications, p.95-98, 1993) have shown that noncoherent binary frequency shift keying (BFSK) is subject to an irreducible error rate in the presence of phase noise. This paper extends the analysis of the performance degradation caused by oscillator phase noise to non-coherent orthogonal M-ary frequency shift keying (MFSK). It is shown that for a given bit rate, a particular level of MFSK may prove more robust than others before being subject to an irreducible enter rate. Results are presented in the form of bit error rate (BER) versus Eb/No plots for M=2, 4, 8 and 16. The performance degradation for higher M can also be determined by this analysis.>

Standardisation bodies in Europe and North America are currently developing standards for high data rate packet radio LANs. Radio LANs will have two main applications: wired LAN replacement and ad-hoc networking. In contrast to wired LAN replacement, ad-hoc networks will operate without control from a central node. As a consequence, no central synchronisation is provided and nodes must have the capability of acquiring all synchronisation from a received packet. This synchronisation can be achieved by matched filtering synchronisation codes inserted into each data packet. This paper discusses these synchronisation requirements and outlines synchronisation sequence selection criteria.>