Showing papers in "Wireless Personal Communications in 1998"

On Limits of Wireless Communications in a Fading Environment when UsingMultiple Antennas

Abstract: This paper is motivated by the need for fundamental understanding of ultimate limits of bandwidth efficient delivery of higher bit-rates in digital wireless communications and to also begin to look into how these limits might be approached. We examine exploitation of multi-element array (MEA) technology, that is processing the spatial dimension (not just the time dimension) to improve wireless capacities in certain applications. Specifically, we present some basic information theory results that promise great advantages of using MEAs in wireless LANs and building to building wireless communication links. We explore the important case when the channel characteristic is not available at the transmitter but the receiver knows (tracks) the characteristic which is subject to Rayleigh fading. Fixing the overall transmitted power, we express the capacity offered by MEA technology and we see how the capacity scales with increasing SNR for a large but practical number, n, of antenna elements at both transmitter and receiver. We investigate the case of independent Rayleigh faded paths between antenna elements and find that with high probability extraordinary capacity is available. Compared to the baseline n = 1 case, which by Shannon‘s classical formula scales as one more bit/cycle for every 3 dB of signal-to-noise ratio (SNR) increase, remarkably with MEAs, the scaling is almost like n more bits/cycle for each 3 dB increase in SNR. To illustrate how great this capacity is, even for small n, take the cases n = 2, 4 and 16 at an average received SNR of 21 dB. For over 99% of the channels the capacity is about 7, 19 and 88 bits/cycle respectively, while if n = 1 there is only about 1.2 bit/cycle at the 99% level. For say a symbol rate equal to the channel bandwith, since it is the bits/symbol/dimension that is relevant for signal constellations, these higher capacities are not unreasonable. The 19 bits/cycle for n = 4 amounts to 4.75 bits/symbol/dimension while 88 bits/cycle for n = 16 amounts to 5.5 bits/symbol/dimension. Standard approaches such as selection and optimum combining are seen to be deficient when compared to what will ultimately be possible. New codecs need to be invented to realize a hefty portion of the great capacity promised.

On Schemes for Multriate Support in DS-CDMA Systems

Abstract: Different modulation schemes supporting multiple data rates in a Direct Sequence Code Division Multiple Access (DS-CDMA) system are studied. Both AWGN and multipath Rayleigh fading channels are considered. It is shown that the multi processing-gain scheme and the multi-channel scheme have almost the same performance. However, the multi-channel scheme may have some advantages due to easier code design and multiuser receiver construction. The drawback though, is the need for linear amplifiers also in mobile terminals. A multi-modulation scheme is also possible, but the performance for the users with the high data rates are significantly worse than for the other schemes. Furthermore multi chip-rate, parallel combinatory spread spectrum (PC/SS), pulse position modulation (PPM) and variable duty cycle schemes are discussed briefly.

Distributed Discrete Power Control in Cellular PCS

Abstract: Transmitter power control has proven to be an efficient method to control cochannel interference in cellular PCS, and to increase bandwidth utilization. Power control can also improve channel quality, lower the power consumption, and facilitate network management functions such as mobile removals, hand-off and admission control. Most of the previous studies have assumed that the transmitter power level is controlled in a continuous domain, whereas in digitally power controlled systems, power levels are discrete. In this paper we study the transmitter power control problem using only a finite set of discrete power levels. The optimal discrete power vector is characterized, and a Distributed Discrete Power Control (DDPC) algorithm which converges to it, is presented. The impact of the power level grid on the outage probability is also investigated. A microcellular case study is used to evaluate the outage probabilities of the algorithms.

RLS Algorithm with Variable Fogetting Factor for DecisionFeedback Equalizer over Time-Variant Fading Channels

Abstract: In a high-rate indoor wireless personal communication system, the delay spread due to multipath propagation results in intersymbol interference (ISI) which can significantly increase the transmission bit error rate (BER). Decision feedback equalizer (DFE) is an efficient approach to combating the ISI. Recursive least squares (RLS) algorithm with a constant forgetting factor is often used to update the tap-coefficient vector of the DFE for ISI-free transmission. However, using a constant forgetting factor may not yield the optimal performance in a nonstationary environment. In this paper, an adaptive algorithm is developed to obtain a time-varying forgetting factor. The forgetting factor is used with the RLS algorithm in a DFE for calculating the tap-coefficient vector in order to minimize the squared equalization error due to input noise and due to channel dynamics. The algorithm is derived based on the argument that, for optimal filtering, the equalization errors should be uncorrelated. The adaptive forgetting factor can be obtained based on on-line equalization error measurements. Computer simulation results demonstrate that better transmission performance can be achieved by using the RLS algorithm with the adaptive forgetting factor than that with a constant forgetting factor previously proposed for optimal steady-state performance or a variable forgetting factor for a near deterministic system.

Parallel Interference Cancellation in Multiuser CDMA ChannelEstimation

Abstract: Parallel interference cancellation (PIC) based channel parameter estimators for frequency selective fading channels are proposed for the uplink in code-division multiple-access (CDMA) mobile communication systems. The performance of PIC based algorithms depends heavily on the quality of the multiple-access interference estimates, which can be improved by using adaptive channel estimation filters. The performance of two adaptive complex channel coefficient estimation filters has been verified in a fading channel by computer simulations. According to the results, the PIC based adaptive channel estimators outperform clearly conventional, successive interference cancellation, and decorrelation based adaptive channel estimators. The PIC method is also used in delay tracking. By using the principles of sample-correlate-choose-largest (SCCL) delay trackers, a robust algorithm for multiuser delay tracking in fading channels is obtained.

Analysis of an Approximate Decorrelating Detector

Abstract: In this paper an approximate decorrelating detector is analyzed on the basis of a first order approximation to the inverse crosscorrelation matrix of signature waveforms. The approximation is fairly accurate for systems with low crosscorrelations and is exact in the two-user synchronous case. We present an exact as well as approximate analysis of the bit-error-rate performance of this detector on a channel that is subject to flat fading, and also specifically for the case of random signature waveforms. The detector outperforms the conventional matched filter receiver in terms of BER. The approximate decorrelator (while not being near-far resistant like the decorrelating detector) is fairly robust to imperfections in power control. Power trade-off regions are identified which characterize the significant advantage that the approximate decorrelator provides over the matched filter receiver. The reduced complexity of the approximate decorrelator and performance gains over the conventional matched filter makes it a viable alternative for implementation in practical CDMA systems, in particular in those where the signature waveforms span many symbol intervals.

Linear Differentially Coherent Multiuser Detection for Multipath Channels

Abstract: By combining multipath processing, differential signal detection, and multiuser detection techniques, we develop a class of near-far resistant linear detectors for differentially coherent multipath signals. We derive and establish performance relationships among the following detectors: an optimally near-far resistant detector, a suboptimum detector which does not require knowledge of the signal coordinates, and a minimum mean square error (MMSE) detector which achieves near-optimum asymptotic efficiency. We present an adaptive multiuser detector which converges to the MMSE detector without training sequences and which requires less information than the conventional single user rake receiver.

Performance Comparison of Multiuser Detectors with Channel Estimation for Flat Rayleigh Fading CDMA Channels

Abstract: Since the conventional detector performs poorly in bandwidth-efficient Code Division Multiple Access (CDMA) systems, several multiuser detectors were proposed recently. In this paper, we compare performance of the decorrelator, the two-stage detector (2S), and the decision-feedback detector (DF) for the Rayleigh flat fading synchronous CDMA channel. First, assuming perfect channel estimation, we show that the 2S and the DF have much lower bit error rate (BER) than the decorrelator, and prove that the ideal DF has unity Asymptotic Multiuser Efficiency (AME). Furthermore, we evaluate the performance of these detectors in the presence of channel mismatch. We model the Rayleigh flat fading channel as the second order Auto Regressive (AR) process, and use the Kalman filter as the channel estimator. The lower bounds on the BER of the 2S and the DF are derived. The analytical results and the simulations show that the estimation error accumulated due to cancellation of other users limits the performance of the 2S and the DF. Therefore, in the presence of channel mismatch, the decorrelator offers comparable or even better performance than more complex decision-feedback and two-stage detectors.

Blind Adaptive Suppression of Narrowband Digital Interferers from Spread Spectrum Signals

Abstract: The application of the minimum-mean-square-error (MMSE) multiuser detection technique to the problem of suppressing the digital narrowband interference (NBI) from spread-spectrum signals is considered. The MMSE multiuser detector can be implemented using a blind adaptive method, which is ideally suited for use in the NBI suppression framework. The optimal linear filter for the recovery of the spread-spectrum signal is derived, and its performance is analyzed in terms of probability of error and signal-to-interference ratio (SIR). It is shown that the performance of this optimal filter is very close to the situation when there is no narrowband interference present, even at the presense of very strong interference. This application requires the treatment of a single narrowband digital signal as a group of related, virtual spread-spectrum signals with very simple spreading codes. This model gives a special structure to the matrices appearing in the optimization problem implied by the MMSE criterion, and this structure is exploited herein to develop and analyze a practical adaptive algorithm. The major contribution of this paper beyond the previous work in the field of NBI suppression is the development of this adaptive algorithm that can exploit the advantages of multiuser detection in suppressing narrowband digital interference from spread-spectrum networks.

Spread Spectrum PN Code Acquisition Using Hybrid Correlator Architectures

Abstract: A hybrid correlator architecture is described which combines the serial structure of an active correlator with the parallel structure of a matched filter correlator. The mean PN code acquisition time performance of this hybrid serial-parallel correlator structure is analysed. Results are shown which compare the acquisition performance of the serial, parallel, and serial-parallel structures. The results are for a PN code length of 64 code chips and assumes a Gaussian channel with the receiver detection threshold set to obtain a constant false alarm rate. An enhancement to the serial-parallel acquisition algorithm is also described which can increase the acquisition time performance by about 15% for typical operating conditions. Overall the results demonstrate that the hybrid correlator can provide rapid code acquisition with a limited receiver complexity.

A Sequential Algorithm for Joint Parameter Estimation and Multiuser Detection in DS/CDMA Systems with Multipath Propagation

Abstract: A direct sequence (DS) spread spectrum code division multiple access (CDMA) communication system is considered where several users transmit data symbols over different multipath channels. The main objective of this work is the proposal of a sequential algorithm for joint parameter estimation and multiuser data detection. The computationally prohibitive maximization of the log-likelihood function is replaced by a sequential scheme using the multistage detector for recovering the data symbols and the expectation-maximization algorithm for estimating the channel parameters. The performance of the resulting multiuser receiver is evaluated via Monte Carlo simulations using both synthetic as well as measured channel impulse responses. A comparison of the performance with analytical expressions for the single-user single-path case shows that the proposed system is capable of eliminating the near-far effect existing in conventional DS/CDMA systems.

Multichannel Algorithms for Simultaneous Equalization andInterference Suppression

Abstract: This paper proposes techniques for simultaneous cancellation of intersymbol and interchannel or multi-access interference (ISI and ICI) that shows up in several multi-input, multi-output (MIMO) communication channels. Correlation and kurtosis based optimization criteria are derived for multi-channel decision feedback equalizers (MC-DFE) and compared with the popular Godard algorithm (CMA) and the minimum mean-square error in a decision directed mode (MMSE-DD). The proposed adaptive algorithms are easily extended to a scenario with more than two users with the computational complexity increasing linearly with the number of inputs. Simulation results show that the algorithms converge to the global minimum in a blind environment with channels that introduce moderate distortion.

Link and System Level Performance of Multiuser Detection CDMA Uplink

Abstract: The link level performance and the cellular system capacity in the uplink direction of a CDMA cellular system utilising multiuser detection base station receivers is analysed by simulation. In the receiver, parallel multistage multiuser detection is employed together with two-antenna diversity reception and fast closed-loop power control. A system level simulator is built to utilise the link level simulation results and to show the increase in cellular capacity obtained by using multiuser detection. The capacity is studied in urban micro and macro-cell environments utilising the channel models developed in the European CODIT project. The modelling of the environment specific and CDMA specific features is considered in the system simulator. The system level simulator is calibrated with analytical capacity calculations.

Measurements and Statistical Modeling of 900 MHz Radio Propagation Channels for Microcellular and Personal Communications in Tunnels

Abstract: This paper reports the results of measurements of 900 MHz narrow-band radio wave propagation in four tunnels with pedestrians and vehicles. Analysis of the measured data files shows that the power distance factor ranges from 1.87 to 4.44. The power attenation rate with distance is insensitive to the location of the transmit antenna in the cross section of a tunnel, but the transmit antenna insertion loss is. The log-normal distribution fits slow variation of the received signals. The Racian distribution closely describes fast fading in the straight sections of tunnels. However, the Rayleigh distribution does not fit the fast fading in the curved sections of tunnels, as expected.

The Performance of Space-Time Processing for Suppressing NarrowbandInterference in CDMA Communications

Abstract: The requirement to suppress narrowband interferences in CDMA communications stems from the overlay concept, ie, coexistence of different types of signals in the same frequency band The conventional approach to rejecting the narrowband interferences has been to whiten the received signal containing the interference, prior to spread spectrum demodulation In this paper, it is proposed to achieve the interference rejection through spatial processing The main benefit of this approach is its robustness with respect to the interference bandwidth Stepping up from single domain spatial processing to space-time processing provides degrees of freedom for both overlay interference cancellation and diversity combining Two space-time architectures, cascade and joint-domain, are studied and compared to a Rake receiver preceded by a whitening filter Main contributions of the paper are the development of analytical expressions of (1) the efficiency of each method, (2) the pdf‘s of the output SNR in a Rayleigh fading environment, and (3) the error probability associated with each method The analysis therein demonstrates that the joint-domain architecture outperforms the cascade configuration, which in turn is superior to the whitening filter-Rake combination

Adaptive Antennas for Microcellular and Mixed Cell Environments withDS-CDMA

Abstract: Adaptive antenna technology is now regarded as one of the key system components in future generation cellular networks. This paper considers the performance of a DS-CDMA system which employs adaptive antennas at the base station sites of both microcell and mixed micro/umbrella cell operational environments. A microcellular ray tracing propagation tool has been employed in order to ascertain site specific propagation data. Results for a typical small cell environment highlight both the propagation characteristics and the behaviour of the adaptive antenna, as well as the sensitivity of the system upon parameters such as the mutual coupling effect. This work does not assume idealised propagation conditions or antenna radiation patterns for the small cell analysis. In summary, the results indicate that microcellular base stations employing adaptive antenna signal processing can offer a significant capacity enhancement and improved channel characteristics when compared with their omnidirectional counterparts. In addition, it is shown that umbrella cells overlaying microcells within the same RF bandwidth allocation, do not impact upon system capacity providing that adaptive antenna technology is employed at the umbrella cell site.

Switched Dual Diversity Reception of M-ary DPSK Signals on Nakagami Fading Channels

Abstract: The paper deals with dual diversity reception of M-ary differential phase-shift keying modulated signals in the presence of additive white Gaussian noise and Nakagami-distributed slow and nonselective fading. The performance of a switched diversity system is analysed and compared to that of the predetection selection diversity combining scheme. The general case of correlated diversity branches is considered, without restrictions on the fading severity parameter. Average symbol error rate formulas are analytically derived in terms of integral expressions that can be easily computed via numerical integration routines. Moreover, the numerical evaluation of the optimum switching threshold is carried out and the influence of the fading severity parameter, the branch correlation, and the cardinality of the symbol alphabet is analysed. Finally, three fixed switching threshold strategies that allow to obtain a satisfactory diversity gain are considered.

The Generalized Delay-Locked Loop

Abstract: The delay-locked loop (DLL) is a synchronization device that is widely used for PN-code tracking in spread spectrum systems. The error detector characteristic (S-curve) of the DLL has a major impact on the performance. Using more than two correlators will extend the tracking range of the S-curve. The Generalized DLL (GenDLL) theory provides a concept to analyze a large class of DLL configurations including the classical DLL. The focus of the performance criteria is on tracking jitter and the mean time to lose lock (MTLL). It is shown that the MTLL can be considerably improved by using extended S-curves. However, the tracking jitter is increased by additional correlators. The tradeoff between the two criteria is explained. With the GenDLL theory loop configurations can be designed having both low tracking jitter and high loop robustness against loss of lock.

On the Capacity Enhancement of a Cellular CDMA Channel with Asymmetrical Bandwidth Allocation

Abstract: Capacity enhancement of cellular CDMA is analyzed using an asymmetrical-bandwidth-allocation approach. Cellular CDMA systems with and without successive interference cancellation are considered. The main source of interference is interuser interference and, in particular, a 9-cell configuration is employed to account for interference from surrounding cells. By transferring more bandwidth or, equivalently, processing gain from the forward link to the reverse link, we have effectively balanced their performance and raised the overall capacity of the cellular system. The optimum bandwidth allocation is easily obtained from the performance curves of both links. For a typical cellular CDMA with a bit error rate of 10^-3, the capacity gain of this approach is about 40%. If successive interference cancellation is employed and both links have the same quality, then both links should have equal bandwidth. However, in a situation where imperfect power control occurs, our studies indicate that using interference cancellation alone achieves a 25% increase in capacity, while enhancement with an asymmetrical bandwidth allocation overlay raises this gain to 68%.

Effect of Time Diversity on the Forward Link of the DS-CDMA CellularSystem

Abstract: A direct-sequences code division multiple access system has been accepted as a digital cellular standard (IS–95) in North America [1]. This digital cellular standard employs a powerful rate 1/2, constraint length 9, convolutional code in its forward link. It is well-known that in a Rayleigh fading channel the performance of a channel code depends very heavily on the interleaving depth and the relative variations of the channel characteristics. In slow fading channels, since the input symbols to the channel decoder are highly correlated, the bit-error-rate at the output of the channel decoder may be unacceptably high. Interleavers of large dimensions can reduce the correlation of the input signal to the channel decoder at the expense of an intolerable delay. In this paper we examine the performance of the IS–95 system, at the mobile receiver, for different channel fade rates. Also, we present a simple time diversity technique which employs multiple receive antennas. The multiple receive antennas in this case generate a fast fading effect and thus improves the performance of the channel decoder significantly.

Reference Symbol Assisted Multi-Stage Successive Interference Cancellation for CDMA Wireless Communication: Robustness toParameter Imperfectionswith Biphase and Quadriphase spreading

Abstract: This paper investigates the sensitivity to system imperfections of a reference symbol assisted multi-stage successive interference cancelling (RAMSIC) receiver. Reverse link of a CDMA system with binary antipodal modulation and coherent detection is considered. Performance of systems using either biphase and quadriphase spreading is compared under different operating conditions. Analysis of a conventional matched filter receiver operating on an AWGN channel reveals that when the number of users is small (such that the multiple access interference cannot be accurately modelled as Gaussian), quadriphase spreading has a significant advantage over biphase spreading. This advantage, however, disappears when the number of users per sector is large (of the order necessary for the multiple access interference to be considered Gaussian). Results for the RAMSIC receiver with quadriphase spreading, on the other hand, show that for hexagonal cell geometry with path loss exponent of 4 and without any forward error correction coding, the traffic capacity is between 1.17 and 1.67 times that of the IS-95. These numbers represent a significant increase over those obtained with biphase spreading. Further investigation with nonidealized cell geometries and other path loss exponents also shows substantial capacity improvement over that of conventional correlator receivers. Performance losses due to nonideal transmitter power amplifier gating, imperfect power control and synchronization errors in the RAKE receiver are also determined. The results for biphase spreading show that for path loss exponent of 4, imperfect amplifier gating causes relatively minor decrease in the traffic capacity, while no such effect is observed for path loss exponents of 2 and 3. As expected, relaxing of power control for both biphase and quadriphase spreading has a similar capacity reducing effect. In spite of these two effects the resultant capacity is still significantly higher than that with the conventional matched filter receiver. Capacity increase with quadriphase over biphase spreading is between 1.4 and 2.0 times. Chip synchronization errors of the order to be expected in a properly designed conventional CDMA system have only minimal effect on performance. Therefore, we conclude that conventional synchronization algorithms should perform adequately with successive interference cancelling receivers considered in the paper.

The Linear Prediction Method of Fading Channel Estimation for the RakeReceiver with Impulsive Interference

Abstract: Parameter estimation of the multipath and fading channel is investigated for the Rake combiner used in a CDMA system with coherent demodulation. A linear predictor is proposed to obtain a better estimation. Since the industrial interference environment is generally impulsive, a parallel LMS adaptive algorithm is proposed to reduce the impact of impulsive noise on predictor weights adjustment. Performance analysis and computer simulations are presented.

Spreading Codes Generator for Wireless CDMA Networks

Abstract: The prime characteristic of spread spectrum modulated signals is that their bandwidth is greater than the information rate. In this way a redundancy is introduced that allows the severe levels of inteference inherent in the transmission of digital information over radio and satellite links to be overcome. Current spread spectrum applications are primarily in military communications; nevertheless, there is growing interest in this technique for third generation mobile radio networks (UMTS, FPLTS, etc.) with open discussion regarding the practicality of using a multiple access system based on spread spectrum techniques (CDMA). However, in order to support as many users in the same bandwidth as other multiple access techniques such as TDMA or FDMA, it is important how to generate large families of sequences that present low cross-correlation. The aim of this paper is to describe a spreading codes generator that can produce a large number of PN sequences with good properties of auto- and cross-correlation. Moreover, the codes generated shows high unpredictability and good statistical behaviour. This also allows the implementation of some features that are common on military networks such as message privacy (increasingly important in commercial networks) without additional cost. The structure presented shows itself to be advantageous for high speed generation of codes at a low cost, low power consumption (allowing longer life for batteries), small size and simplicity of implementation, essential ingredients for commercial equipment. Another attractive feature is its structural parallelism, useful in VLSI implementations. All of these features render it potentially suitable for the implementation of channel bandwidth sharing systems in future wireless personal communications networks.

Modeling and Simulation of Mixed Queueing and Loss Systems

Abstract: In this paper, we investigate a multi-rate network in which wide-band calls are allowed to wait if insufficient resources are available at the time of the call arrival. On the link level, an analytical model is presented and simulations have been carried out on the network level. The results indicate that allowing a few wide-band calls to queue can give a significant improvement in performance in terms of network revenue , as well as a means to level out the blocking probabilities of the different traffic classes. This improvement becomes significant when the service discipline of the waiting calls (of different bandwidth requirements) is adaptive in the sense that longer queues get served first. This observation motivates the investigation of the impact of various buffer space assignment and queueing disciplines on network revenue and call blocking probabilities. The study of such mixed delay and queueing networks is motivated by its possible applications to traffic problems in future Broadband Integrated Services Digital Networks as well as in multi-rate cellular radio networks.

Code Combining of Reed–Muller Codes in an Indoor Wireless Environment

Abstract: Radio communications are often desirable for data and voice communications in an indoor environment. Wireless technology offers an improved mobility since the users are not confined to any particular location by wires. This paper investigates the use of Reed–Muller codes in an indoor wireless environment. Code combining is used to exploit the multiple signal copies resulting from the inherent frequency diversity of spread spectrum. The performance of Reed–Muller codes is compared with that of convolutional codes of similar rates and comparable decoding complexity. The complete weight distribution of various Reed–Muller codes is presented.

A Novel Blind Channel Identification and Equalisation AlgorithmBased on Maximum Likelihood

Abstract: A novel blind non-decision directed maximum likelihood algorithm for fractionally-spaced nonminimum phase FIR channel identification and equalisation is presented. The algorithm results from using the low signal to noise approximation to the average of the likelihood function with respect to the transmitted data sequence. The channel estimation equation is derived in a closed form. The resulting algorithm has two distinct advantages. The first is that the channel estimates are asymptotically consistent, and the second is that the algorithm is computationally efficient since it only requires the calculation of one eignevector. Simulation results are presented to show the performance of the proposed algorithm.

Application of Adaptive Filtering to Direct-Sequence Spread-Spectrum Code Acquisition

Abstract: A novel hybrid scheme utilizing an adaptive FIR filter is proposed for acquisition of DS-SS signals. Timing information on the delay offset between the incoming DS-SS signal and the locally generated replica of the spreading code is extracted from the tap-weight vector of the acquisition adaptive filter. Expressions for the mean acquisition time, detection, and false alarm probabilities for a coherent, chip synchronous DS-SS system in AWGN are derived. The improvement in acquisition performance over serial search techniques is twice the length of the adaptive filter. This is similar to that gained by other hybrid schemes that search the same number of cells at a time. However, a significant reduction in hardware complexity is obtained. The proposed system is also compared to a system utilizing a partial matched filter structure. Moreover, the same hardware could be used for code tracking and, hence, eliminating the need for a separate tracking loop.

Wide-Band UHF Radio Propagation Characteristics in a Tunnel Environment

Abstract: This paper presents the measured wide-band characteristics of 900 and 1800 MHz band radio propagation channels in a tunnel environment for microcellular and personal communications. The measurements were made using the swept frequency technique. The analysis of the average measured power delay profiles reveals that the rms time delay spread is less than 25 ns when the tunnel is empty and can be as large as 103 ns when the tunnel is blocked by vehicles.

Blind Multiuser Detection with Array Observations

Abstract: Cochannel interference is usually a major limitation to the performance of mobile wireless systems. Examples of different forms of cochannel interference include multi-access interference in CDMA systems and cochannel interference resulting from frequency reuse in TDMA systems. In order to mitigate the interference from other users we present a blind multiuser receiver which utilizes array observations and performs both spatial and temporal processing of the received signal. The presented technique is completely blind in the sense that no signature sequences, channel state or spatial location needs to be known a priori, nor use of a training sequences, channel state or spatial location needs to be known a priori, nor use of a training sequence is required for the adaptation. The diversity introduced by the array observations can be efficiently combined with the use of CDMA signature sequences. After initial convergence, a reliable estimate of the combined temporal and spatial signature for each user is provided that can be employed by a multiuser receiver of lower complexity.

An Efficient Implementation OF Bandlimited Dithering

Abstract: A simple implementation of bandlimited dithering using halfband interpolation filters is described. Measurements show that the performance of a 9 bit DAC is improved by 4 dB at the expense of a small increase, 3.4, in total die area of a halfband interpolator ASIC.