We consider a wireless system with a multi-branch diversity receiver employing noncoherent reception. The receiver has information on the channel statistics instead of the instantaneous channel state information, and the average branch signal-to-noise ratios (SNRs) are all distinct. In such a scenario, a deterministically weighted square-law combining scheme can be used to improve the error performance over conventional square-law combining. We focus on such a scheme for a system with orthogonal signaling in uniformly correlated Rayleigh fading. We obtain suboptimal weights, by least squares fit of the matrix elements of the characteristic functions of the decision variables of the weighted square-law combining and the optimum quadratic combining receivers, in closed form that are computed easily using an iterative algorithm. For the case of a dual-diversity system, an implicit expression for the optimal weights, chosen to minimize the symbol error probability, is obtained; for binary signaling, we derive closed form expressions for the weights. It is found that weighted square-law combining offers significant performance improvement over conventional square-law combining at a cost of increased complexity, and the performance gap increases with increasing branch SNR imbalance and number of branches.

When a multi-branch diversity receiver has information on the channel statistics instead of the instantaneous channel state information and noncoherent reception is employed, a deterministically weighted square-law combining scheme can be used to improve the error performance over conventional square-law combining when the average branch signal-to-noise ratios are not equal. We consider such a scheme for a dual-diversity system with orthogonal signaling in correlated Rayleigh fading. The square-law output of each branch is multiplied by a deterministic weight and the weighted outputs are additively combined. The weights are chosen so as to minimize the symbol error probability (SEP). The SEP in terms of these weights is derived in closed form, and from it a method for computing the weights is obtained. The merit of this scheme when compared with conventional square-law combining is demonstrated through numerical examples.

We consider a picocell system that implements multiple input multiple output (MIMO) using cell bonding with one antenna per node. We show that this picocell system can initially have 1/2 or fewer access points (APs) with similar voice over IP coverage as a conventional system, yet can gracefully grow to the capacity of conventional systems with the same number of APs. Although the coverage is somewhat lower for MIMO (both 2X2 and 4X4), we show that through the use of dynamic antenna/node selection the picocell system with cell bonding can have similar coverage to conventional systems.

In this paper, linear combining receivers with deterministic weights are considered for a communication system employing receive diversity in flat Rician fading. We propose two receiver structures: (1) a modified maximum likelihood (ML) receiver in which detection is performed by maximizing the likelihood function of the combined received signal, (2) a deterministic maximal-ratio combining (MRC) receiver which uses the same structure as that of an MRC receiver but with deterministic weights. The deterministic weight vector is chosen such that it minimizes the union bound on the symbol error probability. Methods of computing this deterministic weight vector are presented. The error performance of the receivers is numerically compared with that of the square-law combining receiver and the equal-gain combining (EGC) receiver (which requires more complex phase estimation rather than using fixed, deterministic weights). Numerical results show that in the case of phase-shift keying, the EGC receiver performs better than the modified ML receiver (which has the same performance as that of the deterministic MRC receiver), but the performance gap decreases with increase of the Rician K-factor. It is also seen that in the case of orthogonal frequency-shift keying, the modified ML and deterministic MRC receivers outperform the square-law combining receiver.

For a communication system employing receive diversity in flat Rician fading, we consider linear combining receivers with deterministic weights. Two receiver structures are proposed: (1) a modified maximum likelihood receiver in which detection is performed by maximizing the likelihood function of the combined received signal, (2) a deterministic maximal-ratio combining (MRC) receiver which uses the same structure as that of an MRC receiver with deterministic weights. The deterministic weight vector is chosen such that it minimizes the union bound on the symbol error probability. The error performance of the receivers is compared with that of the square-law combining and the equal-gain combining receivers.

We consider a picocell system that implements multiple input multiple output (MIMO) using cell bonding with one antenna per node. We show that this picocell system can initially have 1/2 or fewer access points (APs) with similar voice over IP coverage as a conventional system, yet can gracefully grow to the capacity of conventional systems with the same number of APs. Although the coverage is somewhat lower for MIMO (both 2times2 and 4times4), we show that through the use of dynamic antenna/node selection the picocell system with cell bonding can have similar coverage to conventional systems.

A transmit-receive diversity system in correlated Rayleigh fading in which the receiver estimates the channel through pilot symbols, and feeds this information back to the transmitter through a feedback path, is considered. The imperfect channel state information (CSI) is used by the transmitter to obtain the transmit weight vector for data transmission. The optimum receiver in the maximum-likelihood (ML) sense obtained from the conditional distribution of the received signal vector, conditioned on the imperfect CSI and the transmit weight vector, is derived for the system. For the case ofM-ary phase-shift keying (MPSK), an analytical expression for the conditional symbol error probability (SEP), conditioned on the channel estimate and the transmit weight vector, is obtained, with the transmit weight vector chosen to minimize this conditional SEP. For the receive-only and transmit-only correlation scenarios with ill-conditioned eigenvalues of the receive and transmit covariance matrices (that is, some of the eigenvalues are very small), we derive expressions for the diversity gain. Numerical results are presented to compare the performance of our receiver with that of a conventional receiver in case of exponentially correlated fading. These results show that the optimum receiver typically has about a 0.5-dB gain over a conventional receiver when the correlation coefficient exceeds 0.5 and the number of receive antennas is much larger than the number of transmit antennas.

We consider a transmit-receive diversity system in correlated Rayleigh fading in which the receiver estimates the channel through pilot symbols, and feeds this information back to the transmitter through a feedback path. The imperfect channel state information is used by the transmitter to obtain the transmit weight vector for data transmission. The optimum receiver in the maximum likelihood sense obtained from the conditional distribution of the received signal vector, conditioned on the imperfect channel estimate and the transmit weight vector, is derived for the system. For the case of M-ary phase-shift keying, an analytical expression for the conditional symbol error probability (SEP), conditioned on the channel estimate and the transmit weight vector, is obtained. The transmit weight vector is chosen to minimize this conditional SEP. Numerical results are presented to compare the performance of our receiver with that of a conventional receiver.

We derive a simple closed-form upper bound on the symbol error probability for coherent detection of M-ary phase-shift keying using antenna arrays with optimum combining, in the presence of multiple uncorrelated equal-power cochannel interferers and thermal noise in a Rayleigh fading environment. The new bound, based on Laguerre polynomials, is valid for an arbitrary number of antenna elements as well as arbitrary number of interferers, and it is proven to be asymptotically tight. Comparisons with Monte Carlo simulation are also provided, showing that our bound is useful in many cases of interest.

We derive an upper bound and investigate some approximations on the symbol error probability (SEP) for coherent detection of M-ary phase-shift keying, using an array of antennas with optimum combining in wireless systems in the presence of multiple uncorrelated equal-power cochannel interferers and thermal noise in a Rayleigh fading environment. Our results are general and valid for an arbitrary number of antenna elements as well as an arbitrary number of interferers. In particular, the exact SEP is derived for an arbitrary number of antennas and interferers; the computational complexity of the exact solution depends on the minimum number of antennas and interferers. Moreover, closed-form approximations are provided for the cases of dual optimum combining with an arbitrary number of interferers, and of two interferers with an arbitrary number of antenna elements. We show that our bounds and approximations are close to Monte Carlo simulation results for all cases considered in this paper.

Closed-form expressions that lower and upper bound the penalty of hybrid selection/maximal ratio combining relative to maximal ratio combining (MRC) for M-ary phase-shift keying (MPSK) modulation are proved. The bounds offer simple-to-evaluate explicit expressions, and are typically within 0.6 dB for hybrid systems with diversity order up to eight that use at least two branches, yet are independent of signal-to-noise ratio (SNR). Contrary to conclusions conjectured in a previously published paper, it is proved that the SNR penalty is not a constant, independent of SNR. It is also shown that previous estimates of the performance losses of selection diversity relative to MRC underestimate or lower bound the losses for MPSK modulation systems, and that the true loss can be significantly larger than previously believed. An upper bound to this loss is also obtained.

In this paper, we derive an exact expression for the symbol error probability (SEP) for coherent detection of M-ary PSK signals using array of antennas with optimum combining in a Rayleigh fading environment. The proposed analytical framework is based on the theory of orthogonal polynomials and we give an effective technique to derive the SEP involving only one integral with finite integration limits. The result is general and valid for an arbitrary number of receiving antennas or co-channel interferers.

Wireless communication systems with transmit and receive antenna arrays are studied when antenna selection is used. A case with very limited feedback of information from the receiver to the transmitter is considered, where the only information fed back is the selected subset of transmit antennas to be employed. It is shown that the optimum signaling, for the largest ergodic capacity of a single isolated link with given interference and antenna selection, is generally different from that which is optimum without antenna selection for some range of signal-to-noise ratios (SNRs). In cases with interference, the improvement obtained from using the true optimum signaling approach tends to increase for this range of SNRs. Further, the optimum approach for cases without antenna selection tends to be optimum in fewer cases as interference power is increased.

We derive a simple closed-form upper bound on the symbol error probability for coherent detection of M-ary PSK using an array of antennas with optimum combining. We assume multiple equal-power cochannel interferers and thermal noise in a Rayleigh fading environment. The new bound applies for an arbitrary number of antenna elements as well as arbitrary number of interferers, and it is proved to be asymptotically tight. Based on the simplicity of the bound, the signal-to-noise ratio penalty due to cochannel interference is evaluated. Comparisons with simulation is also provided, showing that our bound is useful in a large number of practical interesting cases.

We explore the feasibility of designing an outdoor cellular network based on the IEEE 802.11 specification. Since the standard is intended for wireless local-area networks (WLAN), there are many technical challenges when applying the air interface to the outdoor environment. We study how the 802.11 medium access control (MAC) protocol can be applied and how it performs in the outdoor network. By exploiting the fact that timeout intervals are not explicitly specified, without modifying the standard, we propose a new timing structure for the distribution coordination function (DCF) and the handshake of request-to-send (RTS) and clear-to-send (CTS) to handle increased signal propagation delay in the outdoor network. We find that the DCF and RTS/CTS protocols as specified in the standard continue to work properly for a link distance up to 6 km. Our analysis reveals that the DCF performance degrades slightly in the 802.11 network with a cell size of 6 km when compared with the 600 m WLAN. Thus, as far as the MAC protocol is concerned, the 802.11 outdoor, cellular network with 6 km cell size is feasible.

Multiple transmit and receive antennas can be used to form multiple-input multiple-output (MIMO) channels to increase the capacity by a factor of the minimum number of transmit and receive antennas. In this paper, orthogonal frequency division multiplexing (OFDM) for MIMO channels (MIMO-OFDM) is considered for wideband transmission to mitigate intersymbol interference and enhance system capacity. The MIMO-OFDM system uses two independent space-time codes for two sets of two transmit antennas. At the receiver, the independent space-time codes are decoded using prewhitening, followed by minimum-Euclidean-distance decoding based on successive interference cancellation. Computer simulation shows that for four-input and four-output systems transmitting data at 4 Mb/s over a 1.25 MHz channel, the required signal-to-noise ratios (SNRs) for 10% and 1% word error rates (WER) are 10.5 dB and 13.8 dB, respectively, when each codeword contains 500 information bits and the channel's Doppler frequency is 40 Hz (corresponding normalized frequency: 0.9%). Increasing the number of the receive antennas improves the system performance. When the number or receive antennas is increased from four to eight, the required SNRs for 10% and 1% WER are reduced to 4 dB and 6 dB, respectively. Therefore, MIMO-OFDM is a promising technique for highly spectrally efficient wideband transmission.

The bit error probability (BEP) for coherent detection of binary signals with dual-diversity predetection equal gain combining is derived using the Beaulieu (1991) series. In particular, we consider a correlated Rayleigh fading channel with unequal branch signal-to-noise ratios. The BEP expression is in terms of the power correlation coefficient of the branches, is easy to compute, and depicts clearly the effect of correlated fading on the error performance.

We derive the exact symbol error probability for coherent detection of MPSK signals with optimum combining in the presence of multiple uncorrelated equal power co-channel interferers and thermal noise in a Rayleigh fading environment. The expression is general and valid for arbitrary numbers of receiving antennas or co-channel interferers. The complexity of the analytical model depends on the smaller of the number of antennas and the number of interferers.

In this paper, orthogonal frequency division multiplexing (OFDM) for MIMO channels (MIMO-OFDM) is considered for wideband transmission to mitigate intersymbol interference and enhance system capacity. The MIMO-OFDM system uses two independent space-time codes for two sets of two transmit antennas. At the receiver, the independent space-time codes are decoded using pre-whitening followed by maximum-likelihood (ML) decoding or ML decoding based on successive interference cancellation. Computer simulation shows that for 4-input and 4-output systems transmitting data at 4 Mbits/sec over a 1.25 MHz channel, the required SNRs for a 10% and 1% WER are 10.5 dB and 13.8 dB, respectively, when each codeword contains 500 information bits and the channel's Doppler frequency is 40 Hz.

We consider the capacity of multiple-input-multiple-output (MIMO) systems with reduced complexity. One link end uses all available antennas, while the other chooses the "best" L out of N antennas. As "best", we use those antennas that maximize capacity. We derive an upper bound on the capacity that can be expressed as the sum of the logarithms of ordered chi-squared variables. This bound is then evaluated analytically, and compared to results from Monte Carlo simulations. As long as L is at least as large as the number of antennas at the other link end, the achieved capacity is close to the capacity of a full-complexity system. We demonstrate, for example, that for L=3, N=8 at the receiver, and 3 antennas at the transmitter, the capacity of the reduced-complexity scheme is 20 bits/s/Hz compared to 23 bits/s/Hz of a full-complexity scheme.

It is shown that the capacity of a single, isolated, multiple transmit and receive antenna array link, with given interference, is maximized by transmitting an independent data stream from each antenna for a quasistatic and flat Rayleigh fading channel with independent fading coefficients for each path. However, if such links mutually interfere, in some cases the overall system capacity can be increased by transmitting fewer streams.

It has been shown that link adaptation and power control can improve the performance of our future wireless packet networks. Realizing the expected performance gain of these techniques requires accurate prediction of future interference power. We propose a new method based on Kalman filtering for interference estimation. The new method is devised by observing: (a) it is possible to identify fairly accurately the number of active co-channel interferers in the cellular networks and (b) interference power is positively correlated with the number of active interferers. The new technique uses a two-dimensional Kalman filter to exploit that correlation to enhance the prediction accuracy. Using a cellular network with 1/3 frequency reuse and partial traffic loading, the performance of the new method is compared with a simplified method using a one-dimensional Kalman filter where the number of active interferers is not considered. Further, the new method is compared with the traditional exponential filtering. Since the proposed method and the simplified method track interference and measurement errors separately, their predictions represent closely the best estimation by exponential filtering with the optimal parameter. In addition, for a typical network environment, the two-dimensional method yields the lowest prediction errors for a wide range of parameters, and provides a 0.5 dB improvement for the 90th percentile estimation error over the simplified method due to exploitation of the positive correlation between interference and the number of active interferers.

In this paper we present results from the first field test to characterize the mobile multiple-input multiple-output (MIMO) radio channel. We measured the capacity, normalized to a single antenna system for different antenna configurations using 4 transmit and 4 receive antennas at both the base station and terminal in a mobile environment. We compare results for a base station rooftop antenna array consisting of dual-polarized spatially separated antennas, a vertically-polarized multibeam antenna array, and a dual-polarized multibeam antenna array. The field test results show that close to the theoretical 4 times the capacity of a single antenna system can be supported in a 30 kHz channel with dual-polarized, spatially-separated base station and terminal antennas.

We compare measured fading data with the popular Rayleigh model. Our results show that this model does not agree with the experimental data. As an alternative, we approximate the experimental data using autoregressive moving average (ARMA) models. To validate the models, we compare several characteristics of fading obtained analytically, by simulation, and from measured data.

Improved space-time coding for multiple-input multiple-output orthogonal frequency division multiplexing is studied for wireless systems using QPSK modulation for four transmit and four receive antennas. A 256-state code is shown to perform within 3 dB of outage capacity (and within 2 dB with perfect channel estimation), which is better than any other published result without using iterative decoding.

We derive analytical expressions for the symbol error probability (SEP) for a hybrid selection/maximal-ratio combining (H-S/MRC) diversity system in multipath-fading wireless environments. With H-S/MRC, L out of N diversity branches are selected and combined using maximal-ratio combining (MRC). We consider coherent detection of M-ary phase-shift keying (MPSK) and quadrature amplitude modulation (MQAM) using H-S/MRC for the case of independent Rayleigh fading with equal signal-to-noise ratio averaged over the fading. The proposed problem is made analytically tractable by transforming the ordered physical diversity branches, which are correlated, into independent and identically distributed (i.i.d.) "virtual branches," which results in a simple derivation of the SEP for arbitrary L and N. We further obtain a canonical structure for the SEP of H-S/MRC as a weighted sum of the elementary SEPs, which are the SEPs using MRC with i.i.d. diversity branches in Rayleigh fading, or equivalently the SEPs of the nondiversity (single-branch) system in Nakagami fading, whose closed-form expressions are well-known. We present numerical examples illustrating that H-S/MRC, even with L/spl Lt/N, can achieve a performance close to that of N-branch MRC.

The radio interface EDGE (Enhanced Data rates for Global Evolution) is currently being standardized as an evolutionary path from GSM and TDMA-IS136 for third-generation high-speed data wireless systems. For the EDGE system with multiple antennas, spatial-temporal equalization (STE) can reduce intersymbol interference and co-channel interference, thereby increasing the capacity and range. In this paper, we propose two new techniques to improve the performance of a previously proposed STE: a fast timing recovery algorithm for a selective time-reversal equalizer and a two-stage soft-output equalizer. The new timing recovery algorithm determines the estimated burst timing and processing direction by computing the minimum mean-square error (MMSE) for decision feedback equalizers in both the forward and reverse time directions. The two-stage soft-output equalizer is the cascade of a delayed decision-feedback sequence estimator (DDFSE) and maximum a posteriori probability (MAP) estimator. The DDFSE provides better noise variance estimation and channel truncation for the following MAP. The performance of the new STE is evaluated for the EDGE. At 10% block error rate, the two-branch receiver requires a 3-7-dB lower signal-to-interference ratio (SIR) than the previous approach. Compared with the one-branch receiver, the two-branch receiver requires a 4-dB lower SNR with noise only, and a 10-27-dB lower SIR with a single interferer.

Using a series approach, expressions for the average symbol error probability (SEP) of coherent binary signals over a correlated Rayleigh fading channel with dual predetection equal gain combining (EGC) and selection diversity (SD) are derived. For both EGC and SD cases, the SEP is in terms of the correlation coefficient of the branch amplitudes, which is easy to compute and depicts clearly the effect of correlated fading on the error performance.

We use a virtual branch technique to derive higher order statistics of the output signal-to-noise ratio (SNR) of hybrid selection/maximal-ratio combining (H-S/MRC) in a multipath fading environment. In particular, the cumulants, central moments, skewness and kurtosis are derived. We consider the case of independent Rayleigh fading with equal receive SNR averaged over the fading on each diversity branch.

We propose a new 2-stage soft-output equalizer (TSSOE) structure for spatial-temporal processing in EDGE. The TSSOE is the cascade of a delayed decision-feedback sequence estimator (DDFSE) and maximum a posteriori probability (MAP) estimator. The TSSOE uses the final-decision symbols from the DDFSE to estimate the noise variance and truncates the channel memory for the following MAP estimator. Compared with the soft-output DDFSE, the TSSOE reduces both the feedback symbol errors and the noise variance estimation error. At a 10% block error rate and 20 dB signal-to-noise ratio, the TSSOE requires about 10 dB lower signal-to-interference ratio than the soft-output DDFSE for the GSM typical urban channel profile.

Enhanced Data Rates for Global Evolution (EDGE) is currently being standardized as an evolution of GSM in Europe and of IS-136 in the United States as an air interface for high speed data services for third generation mobile systems. In this paper, we study space-time processing for EDGE to provide interference suppression. We consider the use of two receive antennas and propose a joint equalization and diversity receiver. This receiver uses feedforward filters on each diversity branch to perform minimum mean-square error cochannel interference suppression, while leaving the intersymbol interference to be mitigated by the subsequent equalizer. The equalizer is a delayed decision feedback sequence estimator, consisting of a reduced-state Viterbi processor and a feedback filter. The equalizer provides soft output to the channel decoder after deinterleaving. We describe a novel weight generation algorithm and present simulation results on the link performance of EDGE with interference suppression. These results show a significant improvement in the signal-to-interference ratio (SIR) performance due to both diversity (against fading) and interference suppression. At a 10% block error rate, the proposed receiver provides a 20 dB improvement in SIR for both the typical urban and hilly terrain profiles.

We consider optimum space-time equalizers with unknown dispersive interference, consisting of a linear equalizer that both spatially and temporally whitens the interference and noise, followed by a decision-feedback equalizer (DFE) or maximum-likelihood sequence estimator (MLSE). We first present a unified analysis of the optimum space-time equalizer, and then show that, for typical fading channels with a given signal-to-noise ratio (SNR), near-optimum performance can be achieved with a finite-length equalizer. Expressions are given for the required filter span as a function of the dispersion length, number of cochannel interferers, number of antennas, and SNR, which are useful in the design of practical, near-optimum space-time equalizers.

The performance of a hybrid selection/maximal ratio combining (H-S/MRC) diversity system in a multipath-fading environment is analyzed. With H-S/MRC, L out of N diversity branches are selected and combined using maximal-ratio combining (MRC). This technique provides improved performance over L branch MRC when additional diversity is available, without requiring additional electronics and/or power. In particular, we consider independent Rayleigh fading on each diversity branch with equal signal-to-noise ratio averaged over the fading. We analyze this system using a "virtual branch" technique which results in a simple derivation and formula for the mean and the variance of the combiner output SNR for any L and N.

Hybrid selection/maximal-ratio combining (H-S/MRC) is a reduced-complexity diversity combining scheme, where L out of N diversity branches (with the largest signal-to-noise ratio (SNR) at each instant) are selected and combined using maximal-ratio combining (MRC). We derive closed-form expressions for the symbol error probability (SEP) of a H-S/MRC diversity system with arbitrary L and N. We consider coherent detection of M-ary phase-shift keying (MPSK) for the case of independent Rayleigh fading with equal SNR averaged over the fading.

We develop an analytical framework to study the performance of wireless systems using maximal ratio combining (MRC) with an arbitrary number of diversity branches in correlated multipath fading. We consider the coherent detection of digital signals received over correlated Nakagami (1960) fading channels, where the instantaneous signal-to-noise ratios (SNRs) of the diversity branches are not necessarily independent or identically distributed. Specifically: (1) these SNRs can be arbitrarily correlated; (2) the SNR distributions can be from different Nakagami families, i.e., fading parameters (ms) are not necessarily equal; and (3) the average SNRs (averaged over the fading) of the branches are not necessarily equal. We derive closed-form expressions for three performance measures of a MRC diversity system: (1) probability density function (p.d.f.) of the combiner output SNR; (2) symbol error probability (SEP) for coherent detection; and (3) outage probability. We obtain a canonical structure for these performance measures as a weighted sum of the corresponding expressions for a non-diversity (single-branch) system with appropriately-defined parameters. This result is fundamental: the canonical structure depends only on the properties of the channel and diversity combiner, and not on the specific modulation technique. Calculations of the SEP for specific modulation techniques are illustrated through examples.

We consider optimum space-time equalizers with unknown dispersive interference, consisting of a linear equalizer that both spatially and temporally whitens the interference and noise, followed by a decision-feedback equalizer or maximum-likelihood sequence estimator. We first present a unified analysis of the optimum space-time equalizer, and then show that, for typical fading channels with a given signal-to-noise ratio (SNR), near-optimum performance can be achieved with a finite-length equalizer. Expressions are given for the required filter span as a function of the dispersion length, number of cochannel interferers, number of antennas, and SNR, which are useful in the design of practical near-optimum space-time equalizers.

We use a novel virtual branch technique to succinctly derive the mean and variance of the combiner output signal-to-noise ratio for hybrid selection/maximal-ratio combining in a multipath-fading environment.

We investigate optimum spatial-temporal equalization for diversity receiving systems with co-channel interference. We first present the structure of the optimum spatial-temporal decision-feedback equalizer and linear equalizer, and derive closed-form expressions for the equalizer parameters and mean-square-error (MSE) for one-antenna systems. Then, we generalize the results to multiple-antenna systems through a single channel equivalent model. Finally, we obtain the general configuration of the minimum MSE spatial temporal equalizer for bandlimited systems, and show its application by a simulation example.

This paper presents upper bounds on the bit-error rate (BER) of optimum combining in wireless systems with multiple cochannel interferers in a Rayleigh fading environment. We present closed-form expressions for the upper bound on the bit-error rate with optimum combining, for any number of antennas and interferers, with coherent detection of BPSK and QAM signals, and differential detection of DPSK. We also present bounds on the performance gain of optimum combining over maximal ratio combining. These bounds are asymptotically tight with decreasing BER, and results show that the asymptotic gain is within 2 dB of the gain as determined by computer simulation for a variety of cases at a 10/sup -3/ BER. The closed-form expressions for the bound permit rapid calculation of the improvement with optimum combining for any number of interferers and antennas, as compared with the CPU hours previously required by Monte Carlo simulation. Thus these bounds allow calculation of the performance of optimum combining under a variety of conditions where it was not possible previously, including analysis of the outage probability with shadow fading and the combined effect of adaptive arrays and dynamic channel assignment in mobile radio systems.

We investigate parameter tracking of spatial-temporal equalization for IS-136 TDMA cellular/PCS systems. We present three parameter tracking algorithms: the diagonal loading minimum mean-square-error algorithm, which uses the diagonal loading to improve tracking ability, the 2-stage tracking algorithm, which uses diagonal loading in combination with a reduced complexity architecture, and the simplified 2-stage tracking algorithm, which further reduces the complexity to one M/spl times/M and one 3/spl times/3 matrix inversion for weight calculation with M antennas.

We investigate the capacity of cellular systems. In particular, we study how the reuse factor can be improved given the knowledge of the mobiles' locations; i.e., we evaluate the minimum number of channels required to support a cellular infrastructure with a given number of mobiles in each cell. We assume that the mobiles' locations are sampled from the uniform random distribution or are fixed on a uniform grid. Moreover, we show the effect of a number of parameters, such as the number of mobiles per cell, the minimum allowable signal-to-interference ratio, and limited knowledge of mobile location. The assumption of a single interferer, used in our study, is also justified.

The paper presents upper bounds on the bit error rate (BER) of optimum combining in wireless systems with multiple cochannel interferers in a Rayleigh fading environment. The authors present closed-form expressions for the upper bound on the bit error rate with optimum combining, for any number of antennas and interferers, with coherent detection of BPSK and QAM signals, and differential detection of DPSK. They also present bounds on the performance gain of optimum combining over maximal ratio combining. These bounds are asymptotically tight with decreasing BER, and results show that the asymptotic gain is within 2 dB of the gain as determined by computer simulation for a variety of cases at a 10/sup -3/ BER. The closed-form expressions for the bound permit rapid calculation of the improvement with optimum combining for any number of interferers and antennas, as compared with the cpu hours previously required by Monte Carlo simulation. Thus, these bounds allow calculation of the performance of optimum combining under a variety of conditions where it was not possible previously, including analysis of the outage probability with shadow fading and the combined effect of adaptive arrays and dynamic channel assignment in mobile radio systems.>

For a broad class of interference-dominated wireless systems including mobile, personal communications, and wireless PBX/LAN networks, the authors show that a significant increase in system capacity can be achieved by the use of spatial diversity (multiple antennas), and optimum combining. This is explained by the following observation: for independent flat-Rayleigh fading wireless systems with N mutually interfering users, they demonstrate that with K+N antennas, N-1 interferers can be nulled out and K+1 path diversity improvement can be achieved by each of the N users. Monte Carlo evaluations show that these results also hold with frequency-selective fading when optimum equalization is used at the receiver. Thus an N-fold increase in user capacity can be achieved, allowing for modular growth and improved performance by increasing the number of antennas. The interferers can also be users in other cells, users in other radio systems, or even other types of radiating devices, and thus interference cancellation also allows radio systems to operate in high interference environments. As an example of the potential system gain, the authors show that with 2 or 3 antennas the capacity of the mobile radio system IS-54 can be doubled, and with 5 antennas a 7-fold capacity increase (frequency reuse in every cell) can be achieved.>

Techniques are presented that can cancel ghosts in received analog TV (for IDTV, EDTV, and HDTV) signals. The authors utilize the fact that there are short periods of time without the analog signal (the horizontal flyback interval between the lines) to periodically cleanse a finite impulse response (FIR) or an infinite impulse response (IIR) equalizer. This line-by-line processing (cleansing) overcomes the limitations of standard equalizers to allow adequate suppression of ghosts, even with nulls in the spectrum.>

Issues in congestion control are discussed, and a novel congestion control scheme for high-speed networks is described. The scheme is based on periodic transmission of sample time-stamped packets through the network. Upon reception, the packet delays are calculated, averaged, and used to determine the state of the network. The information on the state of the network is then used to drive the network admission control. The major advantage of the proposed scheme over conventional congestion control techniques is that it copes with traffic surges that are shorter than the network round-trip delay. This is achieved by controlling traffic admission with a continuous estimate of the network state. The scheme is targeted towards networks that carry aggregated traffic, and can be applied to ATM-based networks.>

The authors present techniques for breaking the bottleneck caused by delays in the feedback loop, which can severely limit the maximum data rate of a detector, operating with nonlinear cancellation (NLC). The authors first show how to simplify the loop to avoid high-speed switching of analog signals by using multiple decision elements, each with a different threshold level. The authors then show how to use lookahead computation to increase the delay permissible in the feedback loop. The authors describe nonlinear cancellation, and outline some of the problems of implementing NLC at high data rates. Techniques for overcoming these problems are developed and are illustrated with the example of a simple one-tap NLC. The authors describe how to generalize the techniques to more complex NLCs and present some more examples.>

Techniques that can cancel ghosts in received analog TV (for improved-definition TV, extended-definition TV, and high-definition TV) signals are presented. The fact that there are short periods of time without the analog signal (the horizontal flyback interval between the lines) is utilized to periodically cleanse a finite impulse response (FIR) or an infinite impulse response (IIR) equalizer. This line-by-line processing (cleansing) overcomes the limitation of standard equalizers to allow for 40-50 dB of suppression of ghosts, even with nulls in the spectrum, as long as the ghost delay is less than the period of time without the analog signal. Furthermore, by using time inversion in combination with line-by-line processing, the stability problem of the conventional IIR equalizer can be eliminated. It is shown that it may be possible to implement this IIR equalizer on a single digital integrated circuit. Alternatively, an FIR equalizer can be used; although it requires multiple chips (i.e. more taps), it can acquire and adapt to the ghosted channel more rapidly than an IIR equalizer.>

The potential for electrical signal processing to mitigate the effect of intersymbol interference in long-haul fiber-optic systems is discussed. Intersymbol interference can severely degrade performance and consequently limit both the maximum distance and data rate of the system. Several techniques for reducing intersymbol interference in single-mode fiber systems with single-frequency lasers are presented, and those techniques which are appropriate at high data rates in direct coherent detection systems are identified. The performances of linear equalization (tapped delay lines), nonlinear cancellation (variable threshold detection), maximum-likelihood detection, coding, and multilevel signaling are analyzed. The results for a simulated binary 8-Gb/s system show that simple techniques can be used to reduce intersymbol interference substantially, thereby increasing the system margin by several decibels. A six-tap linear equalizer increases the dispersion-limited distance (due to chromatic or polarization dispersion) by 20% (or reduces the optical power penalty by as much as a factor of two) in direct detection systems, even when the distortion is nonlinear. A nonlinear cancellation technique (adjusting the decision threshold in the detector based on previously detected bits) can more than double the dispersion-limited distance and/or data rate.>

Considers a vision system that uses the echo of a transmitted ultrasonic pulse as the basis for identifying objects. The return of a single pulse from an object generates a three-dimensional acoustical characteristic signature across the aperture of a receiving antenna which can be directly used for object identification. Thus, there is no need to reproduce an accurate visual representation to recognize which of a class of objects is present. Since the signal bandwidth is narrow relative to that of an optical imaging system, faster execution should be possible. The need to remove clutter which can arise in a conventional imaging system, caused by the compression of three dimensions into two, is totally avoided. The authors calculate a fundamental limit (in the Shannon sense) on the number of objects that can be distinguished by an ultrasonic system and show that, for modest signal-to-noise ratios and an object space comparable to the beamwidth, an enormous number of different objects can still, in principle, be resolved.>

We propose and analyze a wide-band indoor communication system that uses radio as the transmission medium either on a stand-alone basis or to supplement a hard-wired network for those situations where complete portability is desired. One principal impairment to such a system is intersymbol interference caused by frequencyselective fading. A novel media-access scheme is proposed which permits the use of resource sharing, wherein a small pool of time slots is effectively shared among all users to provide added protection against channel impairments on an as-needed basis. Our results show that the use of resource sharing and diversity provide excellent protection against intersymbol interference caused by frequency-selective fading with negligible impact on throughput. Furthermore, resource sharing plus diversity can permit significantly higher data rates without large queueing delays. For example, a wireless network with a 10 Mbit/s data rate in a 10 MHz bandwidth using four antennas at the base station has a less than 10-4outage probability at a 10-4BER in buildings with less than 58 ns rms delay spread. A loading of 75 percent is permitted for a queueing delay of less than 20 packet transmission times all but 0.01 percent of the time.

In this paper, we study the fundamental limits on the data rate of multiple antenna systems in a Rayleigh fading environment. WithMtransmit andMreceive antennas, up toMindependent channels can be established in the same bandwidth. We study the distribution of the maximum data rate at a given error rate in the channels between up toMtransmit antennas andMreceive antennas and determine the outage probability for systems that use various signal processing techniques. We analyze the performance of the optimum linear and nonlinear receiver processor and the optimum linear transmitter/receiver processor pair, and the capacity of these channels. Results show that with optimum linear processing at the receiver, up toM/2channels can be established with approximately the same maximum data rate as a single channel. With either nonlinear processing at the receiver or optimum linear transmitter/receiver processing, up toMchannels can be established with approximately the same maximum data rate as a single channel. Results show the potential for large capacity in systems with limited bandwidth.

In this paper we study differential detection ofM-ary differential phase-shift-keyed (DPSK) signals with intersymbol interference and noise correlation. With these impairments there can be a wide variation in the error rate of the individual symbols, with this variation increasing withM. However, by adding a phase shift offset and adjusting the decision boundaries in the detector, this variation can be reduced and the average symbol error rate can be decreased. For example, for quaternary DPSK with a signal to adjacent (from the previous and next symbols) intersymbol interference power ratio of 17 dB, these methods can reduce the variation in symbol error rates by more than two orders of magnitude and decrease the required signal-to-thermal noise ratio for a 10-6symbol error rate by more than 2 dB.

This paper studies the use of optimum combining to increase the capacity of narrow-band in-building radio communication systems with multiple users. We consider systems consisting of a base Station with numerous remotes in a Rayleigh fading environment and study the problem of more users requiring channels than the number of channels available. A system is described that, with multiple antennas at the base station but only one antenna at each remote, uses optimum combining to suppress interfering signals. We show that this system, withMantennas at the base station, can achieve anM-fold increase in the number of users or tolerateM - 1interferers from other systems. Thus, with optimum combining, radio communications can be used in high-density, multiple-user environments, such as within buildings, even when only limited bandwidth is available.

In this paper we study differential detection of binary differential phase shift keyed (DPSK) signals with several impairments typical in satellite communication systems. In particular, the bit error rate is determined for bit timing error and frequency offset error at the receiver. The effect of a hard-limiting, band-limited repeater is also considered. The differential detector performance is determined for both conventional DPSK signals (with 0 and π phase shifts) and symmetrical DPSK signals (with- \pi/2and+ \pi/2phase shifts). With symmetrical DPSK, adjacent bit signals are orthogonal rather than either in or out of phase. Therefore, with intersymbol interference, the error rates for all bits are equal and the average bit error rate is less with symmetrical DPSK, as shown in this paper.

This paper discusses the use of spread spectrum in a four-phase communicaton system employing adaptive antennas. A system is described that provides protection against both conventional (i.e., noise and CW) and smart (in particular, repeat) jamming with rapid acquisition of the signal at the receiver. A method is shown for generating reference signals required by the adaptive array through the use of spread spectrum. With these reference signals, the received antenna pattern can be adapted to maximize desired signal to interference and noise power ratio at the receiver. The signal acquisition technique is also described and analyzed. Analytical and experimental results demonstrate both the rapid acquisition and protection against jamming with this system.