In cellular systems, deploying a large number of antennas at the base station (BS), also called massive multiple-input multiple-output (MIMO), can offer a huge improvement in system throughput. This performance gain achieved through coherent beamforming highly depends on having accurate channel state information (CSI) at the BS. Initially, massive MIMO was considered promising only for time division duplexing (TDD) systems because the downlink training overhead in frequency division duplexing (FDD) systems could be large. To reduce the overhead in FDD systems, we propose efficient eigenspace training and precoding. In the downlink training, the prebeamforming matrix is designed based on the spatial fading correlation to probe the channel and minimize the mean squared error (MSE) in channel estimation. In data transmission, the precoding matrix is designed based on instantaneous CSI to manage the inter-user interference. Our algorithms achieve significant savings in the downlink training because the overhead is associated with the dimension of prebeamforming matrix and no longer limited by the number of BS antennas. Compared with joint spatial division and multiplexing, our algorithms offer much lower MSE and, under some conditions, higher sum rates.

Analytical expressions to approximate the expected per-user signal-to-interference-plus-noise-ratio (SINR) and ergodic sum-rate of a multiuser multiple-input-multiple-output system are presented. Our analysis assumes uncorrelated Ricean fading channels with regularized zero-forcing precoding on the downlink. The derived expressions are averaged with respect to the previously unknown arbitrary eigenvalue densities of the complex non-central Wishart distributed channel correlation matrix. To aid the derivation of the expected SINR, we derive analytical expressions for the joint density of two arbitrary eigenvalues of the complex non-central Wishart matrix. Unlike previous studies, our model caters to the presence of a unique Rice factor for each user terminal, making it suitable for analysis of modern systems, such as small cells and millimeter-wave. Our findings suggest that while the presence of strong line-of-sight has an adverse effect on the expected SINR and ergodic sum-rates, increasing the variability of Rice factors enhances the peak rate performance of the system. Our analysis can be applied to arbitrary system dimensions and is seen to remain tight across the signal-to-noise-ratio range considered.

This paper evaluates and compares the performance of single- user (SU) and multi-user (MU) transmission for downlink multiple-input multiple-output (MIMO) channels in terms of system energy efficiency (EE). We introduce power control algorithms to maximize EE. Specifically, to optimize the power allocation, we consider the problem of EE maximization with the satisfaction of the minimum spectral efficiency (SE) gain. Antenna selection is taken into account to further enhance the EE performance for both SU and MU systems. Our results reveal that we should turn off extra antennas at the transmitter, which are originally used for diversity gain but incur large circuit power consumption. Our comparisons between SU and MU differ from conventional comparisons, which focus on SE. Jointly considering EE and SE, we show that SU is more desirable when the transmit power is low, while MU is favored in the case of high transmit power.

We describe three in-field data collection efforts yielding a large database of RSSI values vs. time or distance from vehicles communicating with each other via DSRC. We show several data processing schemes we have devised to develop opportunistic Vehicle-to-Vehicle (V2V) propagation models from such data. The database is limited in several important ways, not least, the presence of a high noise floor that limits the distance over which good modeling is feasible. Another is the presence of interference from multiple active transmitters. Our methodology makes it possible to obtain, despite these limitations, accurate models of median path loss vs. distance, shadow fading, and fast fading caused by multipath. We aim not to develop a new V2V model, but to show the methods enabling such a model to be obtained from in-field RSSI data, without elaborate measurement design and the associated deployment cost. Finally, models based on field data allow for capturing the multiple effects of an increasing number of simultaneous V2V transceivers under typical extreme traffic scenarios.

Typically, a multipath channel response can be characterized as a sum of Rayleigh-fading ''rays'', each defined by a time delay and a mean-square amplitude. Therefore, the channel response can be largely described by a power delay profile (PDP), which is the set of mean-square ray amplitudes and relative delays. Here, we address the following question: Given an actual (or ''true'') PDP, PDP(τ), which may have many rays, is there a 3-ray (i.e. 3- tap) equivalent response, derivable from PDP(τ), that can be used to accurately estimate the average bit error rate,, vs. receiver input signal- to-noise ratio, SNR? The results reported here give an affirmative answer, e.g., forvalues down to = 10-4, the required SNR using a 3-tap equivalent channel response is less than 1.1 dB larger than that required for the ''true'' channel. This agreement can be improved upon, suggesting further work on deriving and evaluating equivalent 3-tap channels. We discuss the benefits of such simplifications for hardware emulators as well as for simulation and analysis.

Stochastic models for path loss are most often of the form PL = A + B log(d), where A and B are empirical constants derived from data via least-squares fitting, and d is the path distance. For in-building environments, however, many investigators have noted the added effects of transmission through interior walls and floors. Here, we represent this effect by a third term, which is exponentially related to log(d), and we model its impact on path loss. The database we use is obtained using a well-validated ray-tracing tool, which we apply to single floors of four very distinct office buildings. The context is an ad hoc wireless network, wherein both the transmit and receive locations can be anywhere on the floor. The resulting model consists of the median path loss, involving three fitting constants A, B and C; and a log-normal variation about the median, with its standard deviation being a function of distance. The structure and details of the model are shown to be remarkably similar across the four distinct buildings studied.

We describe a method for estimating achievable data rates in urban microcells using multiple-input/multiple-output (MIMO) techniques. Specifically, we use site maps and a versatile ray-tracing tool to compute MIMO gain matrices as a function of terminal location; and we use these matrices to determine achievable rates for various MIMO transmission modes (spatial multiplexing, beamforming, and diversity). Numerical results are generated for specific paths in Boston and Manhattan, though our results are shown to be fairly insensitive to neighborhood or city. We also show that, in urban microcells, data rate prediction using site-specific ray tracing is more informative than using stochastic models; and that adaptive switching among MIMO transmission modes as a terminal moves along its trajectory can help sustain high data rates. A new mode-switching algorithm is proposed that requires switching rates lower than those for the optimal scheme by a factor greater than 10, with little loss in average data rate. We also propose a novel software algorithm for optimally placing microcell bases. For a Manhattan neighborhood of area 0.5 km2, we find that full coverage can be obtained using only 5 bases, and that the highest total throughput is achieved using a frequency reuse factor of 1.

An orthogonal frequency division multiplexing system suffers performance degradation when the length of the cyclic prefix is less than the channel impulse response. The root cause of this degradation is the inter-carrier interference (ICI) and inter-symbol interference (ISI) introduced by the excessive multipath delay. Generally, multiple-input multiple-output (MIMO) beamforming is helpful in mitigating such interference because it can spatially suppress some of the multipath. However, the effectiveness of this suppression is very limited. In this paper, we propose an ICI/ISI-aware beamforming algorithm which explicitly takes into account the multipath characteristic of the channel. Optimal steering vectors are derived to maximize the signal-to-interference-plus-noise ratio. This technique not only achieves the beamforming benefit, but also significantly mitigates the ICI and ISI. We show, via simulations, that the proposed algorithm can dramatically reduce the block error rate, permitting good performance for channel delay profiles that would break conventional links. This is vitally important for the extension of indoor wireless LAN designs to outdoor uses.

Since the FCC's approval of unlicensed use of TV white spaces, the issue of how to use these white spaces has led to innovative technologies such as cognitive radios as well as a variety of spectrum policy proposals. There have been proposals to devise alternate rules for spectrum usage citing the overly conservative restrictions on secondary transmissions to protect incumbents. In this paper, instead, we propose to utilize white spaces for a backhaul network for internet traffic based on existing restrictions. Using the available white spaces and backhaul traffic demands in New Jersey as a case study, we evaluate the feasibility of such backhauling and present a methodology that can be used for other areas as well. Using a basic design involving fixed towers and directional antennas, our results show that the TV white spaces can be an effective medium for radio backhaul as an alternative to the costly laying of optical fiber. We also show that meeting FCC requirements on sensing and avoiding harm to wireless microphones will have only a minor impact on capacity.

We propose a PHY-authentication protocol to detect spoofing attacks in wireless networks, exploiting the rapid-decorrelation property of radio channels with distance. In this protocol, a PHY-authentication scheme that exploits channel estimations that already exist in most wireless systems, cooperates with any existing-either simple or advanced-higher-layer process, such as IEEE 802.11i. With little additional system overhead, our scheme reduces the workload of the higher-layer process, or provides some degree of spoofing protection for ``naked" wireless systems, such as some sensor networks. We describe the performance of our approach as a function of the spoofing pattern and the snapshot performance that can be easily measured through field tests. We discuss the implementation issues of the authentication protocol on 802.11 testbeds and verify its performance via field tests in a typical office building.

We present algorithms for adapting the MIMO transmission mode to varying channel conditions. Using real paths in Manhattan and Boston, we show that no single fixed transmission mode is superior to others over an entire path. Our two proposed switching algorithms, delayed feedback and probabilistic switching, achieve rates within 3% of the channel capacity. The delayed feedback algorithm requires less switching than the probabilistic switching algorithm. We compare the use of switching with the use of fixed mode transmissions over 50-m long paths. We show that 1) the use of switching rather than the best fixed mode gives at most a 9% gain in average data rate; and 2) the data rate gain relative to using the worst fixed mode can be as high as 90%. A very low rate switching (every 50 m) is thus attainable in theory but it is practical only if the best fixed mode can be accurately predicted.

In this study, we first quantify cell-wide mean throughputs in interference-limited single-input-single-output (SISO) and multi-element antenna (MEA) cellular systems along various system-level design dimensions, including: MEA array size; frequency-reuse factor; antenna pattern (omni-directional or sectorized); degree of coding; allowable constellation size; and transmitor receive-adaptation. Next, we examine the validity of a noise-model for co-channel interference present in MEA links. Using system simulations, we show that this model is valid under all conditions, with varying levels of accuracy.

Cell-wide mean throughputs on cellular system downlinks using multiple-input/multiple-output (MIMO) techniques is a topic of interest to researchers, cellular operators and equipment manufacturers. In previous work, the authors have tested the validity of the noise model (in the MIMO context) for the co-channel interference (CCI), wherein multipath-averaged CCI powers are added to the thermal noise power that results in an overall interference-plus-noise floor. This approximation simplifies throughput computation with only a minor loss of accuracy, while allowing the cell-wide mean throughput per user to be derived analytically. The key to the analysis is (i) the invocation of the noise model for CCI, and (ii) the demonstration that the overall CCI power has a distribution that is log-normal for any user distance from the base station (BS). This enables us to extend a previous analysis for single-cell (noise-only) MIMO systems to the interference-limited case.We present the analytical method and demonstrate its accuracy.

Dynamic spectrum access has been proposed as a means to share scarce radio resources, and requires devices to follow protocols that use resources in a proper, disciplined manner. For a cognitive radio network to achieve this goal, spectrum policies and the ability to enforce them are necessary. Detection of an unauthorized (anomalous) usage is one of the critical issues in spectrum etiquette enforcement. In this paper, we present a network structure for dynamic spectrum access and formulate the anomalous usage detection problem using statistical significance testing. The detection problem is classified into two subproblems. For the case where no authorized signal is present, we describe the existing cooperative sensing schemes and investigate the impact of signal path loss on their performance. For the case where an authorized signal is present, we propose three methods that detect anomalous transmissions by making use of the characteristics of radio propagation. Analytical models are formulated for two special cases and, due to the intractability of the general problem, we present an algorithm using machine learning techniques to solve the general case. Our simulation results show that our approaches can effectively detect unauthorized spectrum usage with high detection rate and low false positive rate.

Cognitive radios are expected to work in bands below about 3.5 GHz and may be used for a variety of applications, e.g., broadband fixed wireless access, mobile and nomadic access, etc. Cognitive radio system designers must have access to a wide range of channel models covering a wide span of operating frequencies, carrier bandwidths, deployment conditions, and environments. This paper provides a comprehensive overview of the propagation channel models that will be used for the design of cognitive radio systems. We start with classical models for signal loss versus distance and discuss their dependence on the physical properties of the environment and operating frequency. Here we also introduce the concept of log-normal shadowing resulting from signal blockage by man-made and natural features. Next, we discuss the time-varying nature of the wireless channel, introduced as a result of the motion of objects in the channel. This is followed by a discussion on the dispersion of the signal caused by various effects of propagation, especially in the time and frequency domains. Angular dispersion, which is discussed next, is important because cognitive radios may be based on modems that exploit the spatial domain. Lastly, we summarize channel models that have been standardized for fixed and mobile systems.

Due to the broadcast nature of the wireless medium, wireless networks are especially vulnerable to Sybil attacks, where a malicious node illegitimately claims a large number of identities and thus depletes system resources. We propose an enhanced physical-layer authentication scheme to detect Sybil attacks, exploiting the spatial variability of radio channels in environments with rich scattering, as is typical in indoor and urban environments. We build a hypothesis test to detect Sybil clients for both wideband and narrowband wireless systems, such as WiFi and WiMax systems. Based on the existing channel estimation mechanisms, our method can be easily implemented with low overhead, either independently or combined with other physical-layer security methods, e.g.,spoofingattack detection. The performance of our Sybil detector is verified, via both a propagation modeling software and field measurements using a vector network analyzer, for typical indoor environments. Our evaluation examines numerous combinations of system parameters, including bandwidth, signal power, number of channel estimates, number of total clients, number of Sybil clients, and number of access points. For instance, both the false alarm rate and the miss rate of Sybil attacks are usually below 0.01, with three tones, pilot power of 10 mW, and a system bandwidth of 20 MHz.

We investigate the reliability of radio channel simulators in predicting channel responses throughout a well-specified environment. Indoor environments for which the geometric layout and material properties of surfaces are known lend themselves to such site-specific simulation. We assess the performance of this approach by comparing its predictions with measurements in a specific static environment. The good agreement on path loss, Ricean K-factor and RMS delay spread, over the set of paths measured and simulated, suggests that a well-designed radio simulator can be used reliably to predict system behavior. Typically, wireless channel models obtained through this or similar techniques do not capture the temporal variability in the channel response due to people movement in the environment. We treat the time-varying part of the channel response using stochastic processes. Using channel sounding experiments for several typical office scenarios, we show that autoregressive processes can be used to model the time-varying tap gains for several different motion scenarios.

Energy harvesting from the working environment has received increasing attention in the research of wireless sensor networks. Recent developments in this area can be used to replenish the power supply of sensors. However, power management is still a crucial issue for such networks due to the uncertainty of stochastic replenishment. In this paper, we propose a generic mathematical framework to characterize the policy for single hop transmission over a replenishable sensor network. Firstly, we introduce a Markov chain model to describe different modes of energy renewal. Then, we derive the optimal transmission policy for sensors with different energy budgets. Depending on the energy status of a sensor and the reward for successfully transmitting a message, we prove the existence of optimal thresholds that maximize the average reward rate. Our results are quite general since the reward values can be made application-specific for different design objectives. Compared with the unconditional transmit-all policy, which transmits every message as long as the energy storage is positive, the proposed optimal transmission policy is shown to achieve significant gains in the average reward rate.

Network simulation packages such as NS-2 and OPNET have been shown to be a limited option for cross-layer experimentation in wireless networking because they cannot faithfully capture the propagation and interference characteristics of wireless channels [1]. Recent research on network cross-layer optimizations further raises this concern due to the close interaction between physical layer feedback and higher layer protocols. To overcome this shortcoming, wireless testbeds have been used wherein novel protocols and application concepts can be assessed in a realistic environment under controlled and repeatable conditions. Since average signal-to-noise-ratio (SNR) often determines the performance of a wireless link, our goal is to seek link SNR mapping methods that replicate real-world link SNRs onto an indoor testbed. Specifically, we devise and assess link SNR mapping methodologies for two different applications: hierarchical networks with a fixed access point (AP), and mesh networks. For the AP-based networks, we employ the minimum weight matching algorithm to minimize the root-mean-square (RMS) mapping error between the testbed and real-world SNRs. For the mesh networks, to avoid the technical difficulties inherent in “forward mapping”, we develop a “reverse mapping” method by which we turn a testbed configuration with specified link SNRs into a real-world configuration. By inducing the link gain difference between the testbed and the real-world distance-dependent path loss to have a log-normal distribution, a very close approximation to real-world shadow fading is achieved. We present results for a variety of indoor and outdoor real-world scenarios to demonstrate the generality of our method.

Fixed wireless multipoint communication systems are increasingly being deployed in suburban macrocell environments, either to provide voice and data services to residences (wideband systems) or to supply automated meter reading and related supervisory control and data acquisition services (narrowband systems). Here, we show that a particularly simple yet complete first-order statistical characterization of two-branch diversity reception over fixed wireless channels can be given in terms of just five channel parameters: the average path gains and Ricean K-factors for each branch and the complex envelope correlation coefficient between the time-varying parts of the two path gains. We further show that all five parameters can be estimated from amplitude-only measurements of path gain vs. time, subject only to the assumption that the time-varying component of the path gain is complex Gaussian. We present representative sets of model parameters based upon data collected in typical suburban macrocell environments over time, location, and/or frequency using polarization diversity antennas. Because each of the model parameters, or its logarithm, is close to Gaussian, the set may be cast as a five-element vector of jointly random Gaussian processes. Despite obvious physical differences between the environments, the results show a remarkable consistency.

We compute cell-wide mean throughputs for a wide range of multiple-input-multiple-output (MIMO) system design parameters, in both single- and multi-cell scenarios, using realistic models for channel dispersion, path correlations, and cochannel interference. We assume the use of single-carrier modulation, and quantify the degradation in downlink throughput as the channel dispersion increases. Our results highlight the potential harm even from mild dispersion in MIMO links. In studying path correlations, we assume linear arrays at each end of the link, and quantify the interplay among device size, number of antenna elements, path correlation, and carrier frequency. We show that, under practical size constraints, the use of additional antenna elements can be counterproductive.

The radio channel response decorrelates rapidly as the transmitter changes location in an environment with rich scatterers and reflectors. Based on this fact, a channel-based authentication scheme was previously proposed to discriminate between transmitters at different locations, and thus to detect spoofing attacks in wireless networks. In this paper, we study its application in frequency-selective Rayleigh channels, considering channel time variations due to environmental changes and terminal mobility, as well as the channel estimation errors due to the interference from other radios. We propose a generalized likelihood ratio test (GLRT) that is optimal but computationally cumbersome, and a simplified version that requires no a priori knowledge of channel parameters and is therefore more practical. We verify the efficacy of the channel-based spoofing detectors via numerical analysis, showing how performance is improved by using multiple antennas, higher transmit power, and wider system bandwidth. We show that, under a wide variety of practical conditions, spoofing can be detected with better than 90% probability while keeping the probability of falsely rejecting valid transmissions below 10%.

We investigate the reliability of radio channel simulators in capturing the important properties of radio channels throughout a well-specified environment. Indoor environments for which the geometric layout and material properties of surfaces are known lend themselves to such site-specific simulation. Our aim is to assess the performance of this approach by comparing its predictions with measurements in a specific environment. The measurements are made on 18 paths in the ORBIT Laboratory of Rutgers University's WINLAB; and the simulator we use is the WiSE ray-tracing tool developed by Bell Labs. The comparisons are made for three parameters that largely characterize a radio path's behavior: Path loss; Ricean K-factor; and RMS delay spread. The measurements are made over a 1-GHz bandwidth centered on 3.5 GHz. The comparisons show good agreement over the set of paths measured and simulated, establishing confidence that a well-designed radio simulator can be used reliably in system studies.

In the planning of large-scale distributed wireless networks, the knowledge about the link gain matrix is required to facilitate a better match between system design and channel characteristics. Nevertheless, stochastic path loss models fail to produce an accurate estimation of link gains since they cannot faithfully capture the propagation characteristics of a specific environment. Furthermore, exhaustive measurement of all links in a large-scale network is unrealistic due to the complexity involved. This motivates us to develop an accurate link gain estimation methodology based on a small fraction of measurements. We propose here to partition all transmit-receive links into mutually exclusive categories - with each category defined by the number of obstructions on the direct path - and then derive a separate link gain model (equivalently, pathloss model) for each category. Thus, for each category, one would measure only a subset of all transmit-receive paths (selected on the basis of a "maximum entropy" principle) and, for each such path, record the link gain and distance. A least-squares linear fit of dB gain vs. long-distance for the measured links produces the model for that category, which can then be used to predict the remaining "unmeasured" path gains. We use ray- tracing programs to evaluate this approach for different network geometries, and we discuss its superiority to spatial interpolation methods for determining the network's link gains.

We investigate the measurement, interference and capacity issues of the overhead medium-voltage (MV) broadband over power line (BPL) systems operating between 1.7 and 80 MHz. The MV power lines are modeled as a 3-phase set of parallel wires with infinite length and above a lossy earth. Electric field strength is calculated. Two important parameter settings are addressed for the measurement of BPL emission field, namely, the measurement height and the extrapolation factor. The latter is defined as the falloff of field strength, in dB/decade, with lateral distance from the power line. BPL interference is evaluated in terms of the 3-dB critical distance, i.e., the lateral distance at which the total noise into a radio receiver is increased by 3 dB. Using this metric, we explore the relationship between capacity and BPL interference potential.

We analyze and quantify the radio interference to aeronautical receivers from a massively deployed broadband over power line (BPL) system. Based on our previous semi-finite multi-line model, a closed-form solution is derived for the far field using the saddle-point method. We investigate the aggregate radiation power from a circular area in which randomly oriented BPL spans are uniformly placed. A scaling rule for the radiation power is discovered, with respect to both the number of BPL spans and the altitude of the receivers. The level of interference is numerically evaluated for a baseline case. The result can be generalized using the scaling rule derived here.

We propose an enhanced physical-layer authentication scheme for multi-carrier wireless systems, where transmission bursts consist of multiple frames. More specifically, it is based on the spatial variability characteristic of wireless channels, and able to work with moderate terminal mobility. For the authentication of the first frame in each data burst, the legal transmitter uses the saved channel response from the previous burst as the key for authentication of the first frame in the next burst. The key is obtained either via feedback from the receiver, or using the symmetric channel property of a TDD system. Then the authentication of the following frames in the burst is performed either by a Neyman-Pearson hypothesis test, or a least-squares adaptive channel estimator. Simulations in a typical indoor building show that the scheme based on the Neyman-Pearson test is more robust against terminal mobility, and is able to detect spoofing attacks efficiently with small system overhead when the terminal moves with a typical pedestrian speed.

We investigate the use of distributed measurements for estimating and updating the performance of a cellular system. Specifically, we discuss the number and placement of sensors in a given cell for estimating its signal coverage. Here, an "outage" is said to occur at a location if a mobile receiver there has inadequate signal-to-noise ratio (SNR -based outage) or, using another criterion, inadequate signal-to-interference ratio (SIR- based outage); and the "outage probability" is the fraction of the cell area over which outage occurs. A design goal is to improve measurement efficiency (i.e., minimizing the required number of measurement sensors) while accurately estimating the outage probability and mapping the coverage holes. The investigation uses a generic path loss model incorporating distance effects and spatially correlated shadow fading. Our emphasis is on the performance prediction accuracy of the sensor network, rather than on cellular system analysis per se. Through analysis and simulation, we assess several approaches to estimating the outage probability. Applying the principle of importance sampling to the sensor placement, we show that a cell outage probability of Pocan be accurately estimated using ~ 10/Popower-measuring sensors distributed in a random uniform way over the area with base-sensor distances from 50% to 100% of the cell radius. This result applies to both SNR-based and SIR-based outage estimation for both indoor and outdoor environments.

The wireless medium contains domain-specific information that can be used to complement and enhance traditional security mechanisms. In this paper we propose ways to exploit the spatial variability of the radio channel response in a rich scattering environment, as is typical of indoor environments. Specifically, we describe a physical-layer authentication algorithm that utilizes channel probing and hypothesis testing to determine whether current and prior communication attempts are made by the same transmit terminal. In this way, legitimate users can be reliably authenticated and false users can be reliably detected. We analyze the ability of a receiver to discriminate between transmitters (users) according to their channel frequency responses. This work is based on a generalized channel response with both spatial and temporal variability, and considers correlations among the time, frequency and spatial domains. Simulation results, using the ray-tracing tool WiSE to generate the time-averaged response, verify the efficacy of the approach under realistic channel conditions, as well as its capability to work under unknown channel variations.

We analyze the radio interference from a Broadband Power Line (BPL) system operating between 2 MHz and several tens of MHz. The overhead medium-voltage (MV) power line is modeled as a 3-phase set of parallel wires above a lossy earth. A near-exact solution, based on previous approaches for infinitely long lines, is presented for the fields from arbitrarily long lines. Emissions are computed by considering a specified BPL model of semi-infinite length, and the maximum allowable excitation voltage vs. frequency is computed by assuming compliance with FCC field strength limits. These calibration results are used to study the interference to local terrestrial services and to quantify both the theoretical capacity and practical throughputs for BPL systems operation over [2, 30] MHz and [32, 60] MHz. We conclude that practical BPL throughputs are attainable while meeting current FCC requirements, but that these limits may not be stringent enough to avoid serious interference to neighboring radio services.

The wireless medium contains domain-specific information that can be used to complement and enhance traditional security mechanisms. In this paper we propose ways to exploit the fact that, in a typically rich scattering environment, the radio channel response decorrelates quite rapidly in space. Specifically, we describe a physical-layer algorithm that combines channel probing (M complex frequency response samples over a bandwidth W) with hypothesis testing to determine whether current and prior communication attempts are made by the same user (same channel response). In this way, legitimate users can be reliably authenticated and false users can be reliably detected. To evaluate the feasibility of our algorithm, we simulate spatially variable channel responses in real environments using the WiSE ray-tracing tool; and we analyze the ability of a receiver to discriminate between transmitters (users) based on their channel frequency responses in a given office environment. For several rooms in the extremities of the building we considered, we have confirmed the efficacy of our approach under static channel conditions. For example, measuring five frequency response samples over a bandwidth of 100 MHz and using a transmit power of 100 mW, valid users can be verified with 99% confidence while rejecting false users with greater than 95% confidence.

We perform a set of measurements of channel frequency responses at different points in the room that accommodates the ORBIT indoor testbed using a vector network analyzer (VNA). To validate the data collected with VNA we use spectrum analyzer (SA) measurements, as well. Four measurements are performed over non-overlapping 100 MHz bands in the industrial, scientific, and medical (ISM) band, and in the unlicensed national information infrastructure (UNII) band. The fifth measurement spans the coarsely sampled .4 to 6 GHz band. From the measured frequency responses we calculate path loss model parameters. The path loss exponent is between 1.1 and 2 and the dynamic range of the signal is around 25 dB across different bands. Based on the frequency responses measured over the .4 to 6 GHz band we determine multipath intensity profiles (MIP) with fine time granularity. The comparison of MIPs to the ray tracing simulations generated with WiSE software indicates that the surfaces perpendicular to the plane defined by the transmit and receive antennas represent a significant source of reflections. However, the reflections from the floor, dropped ceiling, and roof are suppressed by the antenna elevation patterns.

We compare three approaches for modeling the ultra-wideband (UWB) indoor channel delay profile, including the one adopted by the IEEE 802.15.3a Task Group. We do this using a large database we collected (and have reported on previously) spanning numerous indoor environments. Where appropriate, we recalculate model parameters so as to be compatible with the database. We test the models against the database and against each other by computing certain statistical `attributes' of the ensemble of channel delay profiles, e.g., the probability distribution, across the ensemble, of the root-mean-square (rms) delay spread. We show that each of the modeling approaches yields reasonable agreement with the database for most or all of the attributes tested, and we discuss the relative merits of the three approaches

We study power allocation for the decode-and-forward cooperative diversity protocol in a wireless network under the assumption that only mean channel gains are available at the transmitters. In a Rayleigh fading channel with uniformly distributed node locations, we aim to find the power allocation that minimizes the outage probability under a short-term power constraint, wherein the total power for all nodes is less than a prescribed value during each two-stage transmission. Due to the computational and implementation complexity of the optimal solution, we derived a simple near-optimal solution. In this near-optimal scheme, a fixed fraction of the total power is allocated to the source node in stage I. In stage II, the remaining power is split equally among a set of selected nodes if the selected set is not empty, and otherwise is allocated to the source node. A node is selected if it can decode the message from the source and its mean channel gain to the destination is above a threshold. In this scheme, each node only needs to know its own mean channel gain to the destination and the number of selected nodes. Simulation results show that the proposed scheme achieves an outage probability close to that for the optimal scheme obtained by numerical search, and achieves significant performance gain over other schemes in the literature

We investigate the use of an auxiliary network of sensors to locate mobiles in a cellular system, based on the received signal strength at the sensor receivers from a mobile's transmission. The investigation uses a generic path loss model incorporating distance effects and spatially correlated shadow fading. We describe four simple localization schemes and show that they all meet E-911 requirements in most environments. Performance can be further improved by implementing the MMSE algorithm, which ideally reaches the Cramer-Rao bound. We compare the MMSE algorithm and the four simple schemes when the model parameters are estimated via inter-sensor measurements.

We investigate the use of an auxiliary network of sensors to assist radio resource management in a cellular system. Specifically, we discuss the number and placement of sensors in a given cell for estimating its signal coverage. Here, an "outage" is said to occur at a location if the mobile receiver there has inadequate signal-to-noise ratio (SNR-based outage) or, using another criterion, inadequate signal-to-interference ratio (SIR-based outage); and the "outage probability" is the fraction of the cell area over which outage occurs. A design goal is to confine the number of sensors per cell to an acceptable level while accurately estimating the outage probability. The investigation uses a generic path loss model incorporating distance effects and spatially correlated shadow fading. Our emphasis is the performance prediction accuracy of the sensor network, rather than cellular system analysis per se. Through analysis and simulation, we assess several approaches to estimating the outage probability. Applying the principle of importance sampling to the sensor placement, we show that a cell outage probability of ~ Po can be accurately estimated using ~ 10/Po power-measuring sensors distributed in a random uniform way over base-mobile distances from 50% to 100% of the cell radius. This result applies to both SNR-based and SIR-based cases, in both indoor and outdoor environments.

This paper examines the uplink user capacity in a two-tier code division multiple access (CDMA) system with hotspot microcells when user terminal power is limited and the wireless channel is finitely-dispersive. A finitely-dispersive channel causes variable fading of the signal power at the output of the RAKE receiver. First, a two-cell system composed. of one macrocell and one embedded microcell is studied and analytical methods are developed to estimate the user capacity as a function of a dimensionless parameter that depends on the transmit power constraint and cell radius. Next, novel analytical methods are developed to study the effect of variable fading, both with and without transmit power constraints. Finally, the analytical methods are extended to estimate uplink user capacity for multicell CDMA systems, composed of multiple macrocells and multiple embedded microcells. In all cases, the analysis-based estimates are compared with and confirmed by simulation results.

The number of simultaneous users (or user capacity) supportable on the uplink of a multiple-macrocell code division multiple-access (CDMA) system with multiple "hotspot" microcells embedded within is studied. These microcells operate on the same frequency as the macrocells and are installed in regions of high user demand. It is shown that the user capacity depends on how the users are distributed among cells, and that the maximum (called the attainable capacity) occurs when all cells serve roughly the same number of users. The approach builds on a two-cell analysis published previously, for a single microcell embedded in a single macrocell. First, this analysis is expanded upon to estimate the attainable capacity for M macrocells, where the center one contains L microcells. Then the case in which L microcells are distributed randomly among the M macrocells is analyzed. In each case, the formula for attainable capacity is very simple and highly accurate (as demonstrated via simulations) up to reasonably high values of L. For example, with L microcells distributed among M macrocells, the analysis is accurate at least up to eight microcells per macrocell. The analysis and results are general with respect to cell geometries, propagation parameters, and other variables of the two-tier CDMA system

The orthogonality factor (OF) quantifies the loss of orthogonality between the signals transmitted simultaneously on a wideband code division multiple access (WCDMA) downlink due to multipath dispersion. It is one of the fundamental parameters that determines the signal-to-interference-plus-noise ratio at the output of a Rake receiver and, consequently, it has a significant impact on downlink system capacity. The OF depends greatly on the delay profile of the multipath channel between the mobile and its serving base station; this profile varies considerably from one location in the cell to another. In this paper, we study how the statistical properties of the small-scale-fading-averaged OF vary over the entire macrocellular area. We use an ensemble of channel profiles at different locations in a cell generated from an implementation of the comprehensive COST259 channel model, which incorporates results from several experimental investigations. We show that the small-scale-fading-averaged OF is itself a random variable whose statistics depend on the mobile's distance from its serving base station. The large observed variance of the OF indicates that using a single value for all users in downlink capacity analyses and simulations, as has been the practice, may lead to erroneous conclusions. Finally, we propose a simple model that closely matches the statistics of the OF as a function of mobile-to-base distance, thus obviating the need to set up the complicated channel model every time OF values are to be generated

Packet detection is the first task that a receiver has to perform in a random access communication scheme. The evaluation of different packet detection methods depends on the probability density functions of the decision variables and their construction process. This paper provides a new probability density function (PDF) that enhances evaluation and implementation of one method, due to Schimidl and Cox (SC method), in the low signal-to-noise ratio (SNR) region as required for ultra-wideband (UWB) systems. The new PDF is accurate from high SNR, where it is well-matched by a Gaussian approximation, to SNR = 0, where the Gaussian approximation breaks down. It is thus useful for analyzing packet detection, while the Gaussian approximation is not. We use the new PDF for this purpose, and we compare the packet detection performance of the SC method with that of other candidate methods. The SC method is shown to provide a good tradeoff between performance (including robustness to multipath) and complexity.

We study power allocation for the space-time-coded decode-and-forward cooperative diversity protocol in a wireless network under the assumption that only mean channel gains are available at the transmitters. In a Rayleigh fading channel with uniformly random node locations, a near-optimal power allocation that minimizes the outage probability is derived under a short-term power constraint, wherein the total power is fixed for each two-stage transmission. This near-optimal scheme allocates one half of the total power to the source node and splits the remaining half equally among selected relay nodes; a node is selected for relay if it is able to decode the signal from the source and its mean channel gain to the destination is above a threshold. Numerical results show that this scheme significantly outperforms the constant-power scheme, wherein all nodes use the same power at all times, and the best-select scheme, which employs one relay node with the largest mean relay-destination gain

Multipath dispersion leads to the loss of orthogonality between signals transmitted simultaneously on a wide-band code division multiple access (WCDMA) downlink. The orthogonality factor (OF), which models its impact in the link signal-to-interference-plus-noise ratio (SINR) equation, depends - to a large extent - on the power delay profile of the multipath channel between the mobile and its serving base station. We use the comprehensive and general COST259 channel model for urban cellular environments to evaluate the impact on the OF of multipath clusters and distance-dependence of the multipath delay decay time constant, both of which have been observed in several channel measurements. The observed large standard deviation of the OF indicates that using a single value for all users in downlink capacity analyses and simulations, as has been the practice, may lead to erroneous conclusions. We also propose an empirical model to analytically characterize the observed statistics of the OF

We consider wireless data services characterized by short distances, no shadowing, low power, and low antenna heights, deployed in places where a high frequency of potential users is expected, e.g., toll booths, parking lots, intersections, etc. Within such a system, we expect to see a well-defined geometry of base-to-user radio paths, as well as a predictable user trajectory, neither of which can be assumed for the wide-area cellular case. This offers the promise of a strong deterministic component of the channel response, in addition to a weaker stochastic component. Here, we combine analysis of the former with measurements and modeling of the latter for three typical outdoor scenarios. Comparisons between predicted and measured behavior show excellent agreement.

A signal transmitted over a multipath channel experiences fading, the variability of which is a function of: 1) signal bandwidth; and 2) power delay profiles of the channel's specular and diffuse components. We analyze this general relationship and, for several important classes of multipath channel, we derive simple, closed-form approximations for what we call the stability bandwidth, W/sub 0/. For signal bandwidths greater than W/sub 0/, the local area variation in the received signal power is acceptably small, e.g., less than 1 dB standard deviation of decibel power. We demonstrate the accuracy of our W/sub 0/ approximations for some representative cases. Moreover, we show that the root mean square delay spread-a statistic commonly used to characterize multipath channels-has limited utility in estimating W/sub 0/.

This paper examines the effect of soft handoff on the uplink user capacity of a code division multiple access system consisting of a single macrocell in which a single hotspot microcell is embedded. The users of these two base stations operate over the same frequency band. In the soft-handoff scenario studied here, both macrocell and microcell base stations serve each system user, and the two received copies of a desired user's signal are summed using maximal ratio combining. Exact and approximate analytical methods are developed to compute uplink user capacity. Simulation results demonstrate a 20% increase in user capacity compared to hard handoff. In addition, simple approximate methods are presented for estimating soft-handoff capacity and are shown to be quite accurate.

This paper studies a two-tier code-division multiple-access (CDMA) system in which the microcell base is converted into a data access point (DAP), i.e., is a limited-range base station that provides high-speed access to one user at a time. The microcell (or DAP) user operates on the same frequency as the macrocell users and has the same chip rate. However, it adapts its spreading factor, and thus its data rate, in accordance with interference conditions. By contrast, the macrocell serves multiple simultaneous data users, each with the same fixed rate. The achievable throughput for individual microcell users is examined and a simple accurate approximation for its probability distribution is presented. Computations for average throughputs, both per-user and total, are also presented. The numerical results highlight the impact of a desensitivity parameter used in the base-selection process.

This paper examines the effect of soft handoff on the uplink user capacity of a CDMA system consisting of a single macrocell in which a single hotspot microcell is embedded. The users of these two base stations operate over the same frequency band. In the soft handoff scenario studied here, both macrocell and microcell base stations serve each system user and the two received copies of a desired user's signal are summed using maximal ratio combining. Exact and approximate analytical methods are developed to compute uplink user capacity. Simulation results demonstrate a 20% increase in user capacity compared to hard handoff. In addition, simple, approximate methods are presented for estimating soft handoff capacity and are shown to be quite accurate.

In this study we provide a physical layer based analysis of the coexistence issues of ultra-wideband (UWB) with other devices in the same spectrum. Specifically, we have focused on the UWB interference to and from devices using the wireless local area network (WLAN) standard IEEE 802.11a. Our results indicate that a UWB interferer operating at the peak allowable power density induces minimal interference into such WLAN devices in line-of sight (LOS) scenarios, even at close range. However, in the non-LOS (NLOS) case, a UWB interferer can severely affect the data-rate sustainable by 802.11a systems. Moreover, 802.11a interference into UWB systems is shown to reduce the signal-to-interference ratio (SIR) by as much as 36 dB when the interferer is within LOS of the UWB receiver.

We present a statistical model for the multipath intensity profile (MIP) that can be used to construct an impulse response of a residential wireless channel. The model incorporates relevant parameters such as distance, global rms delay spread and the correlation between received multipaths. It describes the statistics of the MIP in detail. This model, together with a suitable path loss model, can be used to simulate residential wireless channels for performance evaluation of various communication systems. The MIP model is based on over 300,000 indoor frequency responses measured in a 1.25GHz bandwidth centered on 5-GHz, and simulation shows that it captures very well the main statistical properties of the measured data.

This paper examines the downlink user capacity in a CDMA system composed of a macrocell and a microcell, where both cells use the same frequency channel. The effect of channel dispersion on the number of simultaneous users supported is studied for three different downlink power control methods. Channel dispersion produces in-cell interference and variable fading of the received signal power. Novel analytical methods are developed to account for both effects; these techniques derive user capacity for any delay profile using the results for the uniform delay profile. The downlink user capacities for the three power control methods are compared to the uplink user capacity under the same channel conditions. The results point to the need for fast power control to obtain uplink-limited performance.

This paper examines the uplink user capacity in a CDMA system composed of one macrocell and one microcell when user terminal power is limited and the wireless channel is finitely-dispersive. A finitely-dispersive channel causes variable fading of the signal power at the output of the RAKE receiver. Analytical methods are developed to compute the user capacity as a function of a dimensionless parameter that depends on the transmit power constraint and cell radius. Additionally, a novel analytical method is developed to study the effect of variable fading due to a finite number of multipaths. This technique derives the user capacity for any delay profile using the results for the uniform multipath channel. Simulation results confirm the accuracy of these simple analytical solutions.

We describe a statistical model for the multipath intensity profile (MIP) in residential environments. The work is based on over 300,000 frequency response measurements at 712 locations in 23 homes. We show that, for non-line-of-sight (NLS) path, the MIP is well-modeled as a decaying exponential vs. delay, multiplied by a lognormal process over delay; and that a similar model applies to line-of-sight (LOS) paths. Moreover, we present simple statistical descriptions for the parameters of these functions.

Multiple-input-multiple-output (MIMO) systems generally fall into two categories, depending on the kind of gain they provide: spatial multiplexing (SM) methods yield capacity gain and diversity methods yield link quality gain [measured here by the post-processing signal-to-noise ratio (SNR)]. We consider a set of systems from each category, quantify their gains analytically or via simulations, and show how these gains vary with the receiver input SNR and the numbers of antennas. The contribution of this work resides in both the closed-form analytical results and the numerical comparisons. We both highlight the benefits of using additional transmit antennas and provide comparisons among diversity-based and SM-based MIMO schemes. The analytical results are for a diversity-based scheme that combines selection diversity at the transmit side with maximum ratio combining (MRC) at the receive side, which we show to upper bound the SNR performance of other diversity-based schemes. We find that, for practical system parameters, the relevant SNR metric is 6-12 dB higher for the diversity-based schemes than for SM-based schemes. At the same time, SM-based schemes yield capacity metrics which range from 30% higher to double that of diversity-based schemes.

The loss of orthogonality between the spreading codes in a wideband code-division multiple-access downlink is modeled in analysis and simulations by means of the orthogonality factor (OF). The OF, which is an important parameter limiting downlink capacity, increases with the multipath dispersion in the channel. We utilize this relationship between the OF and the channel dispersion to show a novel, simple, and accurate way to determine the time-averaged OF directly from the channel delay profile. To this end, we define the diversity factor (DF) of a channel profile, derive the expression for it, and show that the DF and the OF are related by a simple mathematical relationship. We then verify our approach over an entire ensemble of channel profiles generated by the general COST259 macrocellular channel model.

This paper studies the number of voice users (user capacity) supported on the uplink of a single-macrocell/single-microcell code-division multiple-access system. A "hotspot" microcell is embedded within a larger macrocell and operates over the same bandwidth as the larger cell. Analytic methods are presented for computing user capacity which account for propagation loss, multiple-access interference, power control, and random locations of user terminals, as well as two distinct methods by which users select base stations (tiers). Along with the exact user capacity, a technique for making accurate approximations is also presented. Simulation results verify both the exact and approximate analytical methods. This simulation is also employed to study the capacity gains of a third, more optimal, tier-selection scheme. These results point to differences in capacity performance based on the tier-selection method, as well as on the traffic density within the hotspot region.

The presence of multipaths leads to a loss of orthogonality between signals transmitted simultaneously on a wideband code-division multiple access (WCDMA) downlink. We derive general analytical expressions for the orthogonality factor (OF), which quantifies this loss of orthogonality, as a function of the instantaneous multipath fade realization of the channel. We show that the OF exhibits a significant temporal variation for the three channel profiles suggested in the WCDMA standard, namely, typical urban and rural areas and hilly terrain, which span a wide range of realistic cases. Moreover, its temporal variations and statistics vary significantly from one channel profile to another. Also, while the pulse shape and the number of RAKE fingers has only a marginal impact on the OF's statistics, the granularity in setting the finger positions has a considerable impact. The results of the work can be directly used for evaluation of the system performance of WCDMA cellular systems.

We study the data performance of WCDMA systems using the downlink shared channel (DSCH) by investigating the impact of loading, rate adaptation and power control for the typical urban (TU) channel. We conclude that, using DSCH, high speed data transmissions can be achieved if the offered traffic load is well controlled. We discuss a heuristic rate adaptation algorithm that adapts the transmission rate based upon the perceived user performance in previous frames. We show that the performance of the rate adaptation is significantly better than that for the fixed-rate allocation policy.

This paper studies the number of simultaneous voice users (or user capacity) supportable on the uplink of a multiple macrocell CDMA system with multiple "hotspot" microcells embedded within. These microcells operate over the same bandwidth as the macrocells and are assumed to be installed in regions of high user demand. First, conditions for optimal performance of such a system are established. This analysis shows that the total user capacity is maximum when there are equal numbers of active users in each cell. Along with these conditions, analytical approximations are presented for computing maximum user capacity as a function of the number of microcell bases in the system. In particular, the approximations show the total user capacity as growing linearly with the number of hotspot microcells. Simulation results verify this trend when the number of microcells in the system is not large. The numerical constants for this linear trend are related to basic system and propagation parameters. When the number of microcells gets large, it is shown that the same linear growth in capacity can be sustained by desensitizing the macrocells, i.e., raising the received power requirements at the macrocell bases.

The presence of multipath leads to a loss of orthogonality between the signals transmitted simultaneously on a WCDMA downlink. We derive general analytical expressions for the orthogonality factor (OF), which quantifies this loss of orthogonality. We show that the orthogonality factor exhibits a significant temporal variation for the three channel profiles suggested in the WCDMA standard, namely, typical urban, rural area, and hilly terrain, which span a wide range of realistic cases. Moreover, its temporal variations and statistics vary significantly from one channel profile to another. Also, while the pulse shape and the number of RAKE fingers has only a marginal impact on the OF's statistics, the granularity in setting the finger positions has a considerable impact. The results of the work can be directly used for evaluation of the system performance of WCDMA cellular systems.

We present a general framework to quantify the data throughput capabilities of a wireless communication system when it combines: (1) multiple transmit signals; (2) adaptive modulation for each signal; and (3) adaptive array processing at the receiver. We assume a noise-limited environment, corresponding to either an isolated cell or a multicell system whose out-of-cell interference is small compared with the thermal noise. We focus on the user data throughput, in bits per second/Hertz (bps/Hz), and its average over multipath fading, which we call the user spectral efficiency. First, an analysis method is developed to find the probability distribution and mean value of the spectral efficiency over the user positions and shadow fadings, both as a function of user distance from its serving base station and averaged over the cell coverage area. We assume fading conditions and receiver processing that lend themselves to closed-form analysis. The resulting formulas are simple and straightforward to compute, and they provide a number of valuable insights. Next, we run Monte Carlo simulations, both to confirm the analysis and to treat cases less amenable to simple analysis. A key contribution of this paper is a simple formula for the mean spectral efficiency in terms of the propagation exponent, mean signal-to-noise ratio at the cell boundary, number of antennas, and type of coding. Under typical propagation conditions, the mean spectral efficiency using three transmit and three receive antennas ranges from 19.2 bps/Hz (uncoded) to 26.8 bps/Hz (ideally coded), highlighting the potential benefits of multiple transmissions combined with adaptive techniques. This is much higher than the spectral efficiencies for a link using a single transmitter and a threefold receive diversity under the same conditions, where the range is from 8.77 bps/Hz to 11.4 bps/Hz. Moreover, the latter results are not nearly as practical to achieve, as they can for large signal constellations that would be highly vulnerable to impairments.

This paper studies the number of voice users supported on the uplink (user capacity) of a single-macrocell/single-microcell CDMA system. A "hotspot" microcell is embedded within a larger macrocell and operates over the same bandwidth as the larger cell. Analytic methods are presented for computing user capacity which account for propagation loss, multiple-access interference, power-control, and random locations of user terminals, as well as two distinct methods by which users select base stations (tiers). Along with the exact user capacity, a technique for approximations is also presented. Simulation results verify both the exact and approximate analytical methods. This simulation is also employed to study capacity gains of a third, more optimal, tier-selection scheme. The results point to differences in capacity performance based on the tier-selection method as well as on the traffic density within the hotspot region.

We investigate the high spectral efficiency capabilities of a cellular data system that combines the following: 1) multiple transmit signals, each using a separately adaptive modulation; 2) adaptive array processing at the receiver; and 3) aggressive frequency reuse (reuse in every cell). We focus on the link capacity between one user and its serving base station, for both uncoded and ideally coded transmissions. System performance is measured in terms of average data throughput, where the average is over user location, shadow fading, and fast fading. We normalize this average by the total bandwidth, call it the mean spectral efficiency, and show why this metric is a useful representation of system capability. We then quantify it, using simulations, to characterize multiple-input multiple-output systems performance for a wide variety of channel conditions and system design options.

We describe a simulation study of a cellular system using multiple-input multiple-output (MIMO) antenna techniques along with adaptive modulation and aggressive frequency reuse. We show, for the case of 3 transmit and 3 receive antennas, how much MIMO systems outperform systems with receive-diversity-only when noise dominates. When co-channel interference from surrounding cells dominates, the differences shrink, as do the absolute numbers. We quantify these reductions for the specific cases studied, and discuss further areas of research.

We present a statistical path loss model derived from 1.9 GHz experimental data collected across the United States in 95 existing macrocells. The model is for suburban areas, and it distinguishes between different terrain categories. Moreover, it applies to distances and base antenna heights not well-covered by existing models. The characterization used is a linear curve fitting the decibel path loss to the decibel-distance, with a Gaussian random variation about that curve due to shadow fading. The slope of the linear curve (corresponding to the path loss exponent, /spl gamma/) is shown to be a random variate from one macrocell to another, as is the standard deviation /spl sigma/ of the shadow fading. These two parameters are statistically modeled, with the dependencies on base antenna height and terrain category made explicit. The resulting path loss model applies to base antenna heights from 10 to 80 m, base-to-terminal distances from 0.1 to 8 km, and three distinct terrain categories.

This paper deals with the measurement and modeling of multipath delay on fixed wireless paths at 1.9 GHz in suburban environments. The primary focus is on the delay profile, which is the normalized plot of received power versus delay in response to an RT "impulse." We describe measurement campaigns in the western suburbs of Chicago, IL, and in suburban north-central New Jersey. Our analysis of the data suggests to us that, for directive terminal antennas, the delay profile can be modeled as having a "spike-plus-exponential" shape, i.e., a strong return ("spike") at the lowest delay, plus a set of returns whose mean powers decay exponentially with delay. This delay profile can be characterized by just two parameters (both variable over the terrain), namely, the ratio (K/sub 0/) of the average powers in the "spike" and "exponential" components and the decay time constant (/spl tau//sub 0/) of the "exponential" component. No such simple structure appears to apply for delay profiles using omnidirectional antennas. For a directive antenna with a 32/spl deg/ beamwidth, we find that: (1) the statistical correlation between the profile parameters K/sub 0/ and /spl tau//sub 0/ is negligible; (2) these parameters are relatively insensitive to antenna height and path length; and (3) over each measured region (Illinois and New Jersey), K/sub 0/ and /spl tau//sub 0/ have median values close to 8 dB and just below 0.2 /spl mu/s, respectively. Moreover, we have found simple probability distributions that accurately portray the variability of K/sub 0/ and /spl tau//sub 0/ over the terrain.

This paper presents an adaptive tap assignment technique for improving the performance and robustness of a reduced-complexity decision-feedback equalizer (DFE) for broadband wireless mobile communications. The spacing of individual feedforward taps of the DFE is made variable such that, when the channel consists of sparsely-distributed multipath with a large delay spread (e.g., "hilly terrain" delay profiles), the feedforward taps can be sparsely assigned to increase the equalizer span without increasing the total number of the feedforward taps. We provide an analysis of the optimum tap assignment problem and propose a practical channel-estimation-based tap assignment algorithm which can be executed prior to equalizer training. Simulation results are obtained for various outdoor delay profiles with dispersions ranging up to several tens of symbol periods and while using only a small number of feedforward taps (e.g., 5 taps), compared to a conventional contiguous-tap DFE, the proposed tap-selectable DFE is found to give relatively stable performance (measured in terms of required SNR) over all profiles, and also to provide improved robustness to fast fading.

We consider the effects of imperfect average power measurements on adaptive MQAM modulation, where the transmit power and data rate are varied relative to the received signal power. The channel varies with both fast Rayleigh fading and slow log-normal shadowing. We assume that the fast fading is estimated perfectly, and that the estimation error of the shadowing is log-normally distributed. This estimation error leads to a change in the average transmit power and rate of the adaptive modulation. We characterize these changes for two adaptive modulation schemes: variable-rate variable-power MQAM, where the average data rate is maximized, and fixed-rate MQAM, where the transmit power is adapted to invert the signal fading. The shadowing estimation error affects these two modulation techniques in opposite ways: the data rate and dB power changes resulting from the error are positive for variable-rate variable-power MQAM, and negative for fixed-rate MQAM. In both cases, however, the changes are very small.

This paper presents reduced-complexity equalization techniques for broadband wireless communications, both outdoors (fixed or mobile wireless asynchronous transfer mode (ATM) networks) and indoors [high-speed local-area networks (LANs)]. The two basic equalization techniques investigated are decision-feedback equalization (FE) and delayed decision-feedback sequence estimation (DDFSE). We consider the use of these techniques in highly dispersive channels, where the impulse response can last up to 100 symbol periods. The challenge is in minimizing the complexity as well as providing fast equalizer start-up for transmissions of short packets. We propose two techniques which, taken together, provide an answer to this challenge. One is an open-loop timing recovery approach (for both DFE and DDFSE) which can be executed prior to equalization; the other is a modified DFE structure for precanceling postcursors without requiring training of the feedback filter. Simulation results are presented to demonstrate the feasibility of the proposed techniques for both indoor and outdoor multipath channel models. The proposed open-loop timing recovery technique plays a crucial role in maximizing the performance of DFE and DDFSE with short feedforward spans (the feedforward section of DDFSE is a Viterbi sequence estimator). A feedforward span of only five is quite sufficient for channels with symbol rate-delay spread products approaching 100. The modified DFE structure speeds up the training process for these channels by 10-20 times, compared to the conventional structure without postcursor precancellation. The proposed techniques offer the possibility of practical equalization for broadband wireless systems.

This paper presents an adaptive tap assignment technique for improving the performance of a previously reported reduced-complexity decision-feedback equalizer (DFE) for broadband band wireless systems. The spacings of individual feedforward taps of the DFE are made selectable so that, when the channel consists of a sparsely distributed multipath with a large delay spread (e.g. "hilly terrain" (HT) delay profiles), the equalizer span can be increased without increasing the total number of taps. We propose simple tap selection algorithms and show that they provide: (1) performance gains over a contiguous-tap approach in various outdoor delay profiles and (2) improved robustness against fast fading.

Predictions of uplink space-diversity gain in the cellular and personal communications systems (PCS) bands (near 850 MHz and 1.9 GHz, respectively), suffer from incomplete modeling of multipath angular spread /spl sigma/. Using previously published measurements to reduce the gap, we show that /spl sigma//spl sim/[distance]/sup -1/2/ and that diversity gains are about 2 dB higher for PCS than for cellular.

Broadband analog transport facilities using fiber or fiber/coax cable can play a significant role in the evolution of the network infrastructure for personal communications services (PCSs). Low-power PCS systems require a dense grid of radio ports to provide connectivity to the telephone network. Analog transport has a number of important advantages over digital transmission facilities, including the flexibility to support a variety of air interface formats, shared infrastructure cost with other services such as video distribution, and centralized call processing allowing the use of low cost and simple radio ports. A simulcast technique can be used in such systems to permit low rates of handoff (no handoff within each simulcast area) and sharing of hardware resources among multiple radio ports. This paper provides a detailed model and a simulation analysis of the cochannel interference and noise performance as well as the resource sharing benefit of a simulcast PCS system. Several potential PCS air interfaces are considered, including time division multiple access (TDMA) and code division multiple access (CDMA) techniques. Our investigation shows that the impact of multiple antenna noise in a simulcast system is offset by the improved signal-to-interference (SIR) ratio brought about by distributed antennas. Even with distributed antennas, multiple antenna noise places a limit on the maximum number of radio ports that can be assigned to each simulcast group. This limit, however, is shown to have little impact on the achievable resource sharing benefit of simulcasting (i.e., grouping beyond this limit has diminishing returns). A saving of 40% to 60%, in terms of the required central hardware resources, is typical for both TDMA and CDMA systems in suburban environments.

We consider high-speed wireless indoor links in which narrowbeam antennas are used to limit multipath delay spread. For this kind of link, the performances of simple 2- and 4-level modulation schemes are analyzed. The amplitudes and phases of all multipath rays are assumed to be static (no time variation), as expected for fixed links in an indoor environment. We consider coherent PSK with data-directed and pilot tone carrier recovery, differential PSK, and coherent and noncoherent forms of 2-level ASK. Our objective is to find techniques for which factors other than multipath limit the attainable bitrate. Thus for each scheme, analysis is used to identify regions of the multipath parameter set over which data eye closure occurs. Results are given for the 2- and 3-ray multipath channels which can arise in an indoor environment with narrowbeam antennas. Specifically, we assume two parallel walls approximately perpendicular to the direct ray and a narrowbeam antenna at one or both ends of the link. The 2-level schemes are found to be quite robust with respect to 2-ray multipath, and can mostly tolerate a second ray amplitude equal to that of the direct ray. The 4-level schemes are sensitive to 2-ray multipath, showing eye closure at many phases with a second ray power about 7.5 dB below the first (direct) ray. A similar sensitivity is found for 2-level schemes with 3-ray multipath. Finally, 4-level schemes with 3-ray multipath are significantly more sensitive, showing eye closure with a second ray power about 11 db below the direct ray. Thus, considerations of simplicity and robustness favor 2-level PSK schemes (coherent with pilot tone, or differential) for narrowbeam high speed transmission indoors.>

Resource auction multiple access (RAMA) is a technique for rapidly assigning communication resources (e.g., frequency carriers and/or TDMA time slots) in a wireless environment. This scheme is deterministic in that, barring errors caused by fading and noise, a resource assignment is made in each and every assignment cycle. Previous papers reported on the ultimate call assignment capacity of this technique, limited only by propagation delays, and also demonstrated its capabilities for statistical speech multiplexing in a PCS environment. We extend the previous work in two directions: (1) we examine the effects of channel fading and noise; and (2) we consider both GSM and IS-54 cellular environments. Propagation is modeled, for purposes of this study, as flat Rayleigh fading, with mean propagation loss as a parameter. We focus on the subscriber-to-base link and consider several forms of diversity at the base. We also consider two RAMA modes (call assignment without and with statistical speech multiplexing). We show that a simple form of two-branch diversity can be very effective. In both GSM and IS-54, for example, using simple diversity and an average transmit power of 100 mw yields assignment error probabilities of only 1% for losses as high as 151 dB.>

The authors have performed data reductions on 900 MHz signal attenuations measured on numerous streets in Manhattan. The database consists of both local spatial averages of signal attenuation and the short-term fluctuations about this average. The former, which is termed the local mean attenuation (LMA), is the primary focus. The database is used to obtain contours of constant LMA for two neighborhoods. It is shown that the contours have the shapes of convex diamonds. The authors propose that squares inscribed within these contours be used as the building blocks of microcellular environments. A theory is developed that explains the contours and predicts, with reasonable accuracy, the sizes of the inscribed squares. It is also shown that the prediction method can be applied without the need for measured data. The short-term fluctuation statistics of the signal attenuation are examined. They are shown to be Rayleigh-like in the non-line-of-sight regions of a microcell and Rice-like in the line-of-sight region. Possible extensions to other frequency bands and other urban environments are discussed.>

Th authors explore the use of a wireless network having a two-tier architecture to serve both conventional mobile subscribers and quasi-stationary (e.g., PCN (personal communications network)) subscribers. The latter are served by microcells which are embedded within macrocells that serve the mobile users. This provides a balance between maximizing the number of users per unit area (which favors small cells) and minimizing the network control associated with handoff (which favors large cells). Four approaches to sharing the spectrum between the two tiers, using per-cell capacity as the measure, are evaluated. The first two feature spread-spectrum sharing, i.e., they use TDMA (time-division multiple access) among microcell users and CDMA (code-division multiple access) among macrocell users (System I), or vice versa (System II). The other two approaches feature orthogonal sharing, i.e., they use TDMA in both tiers, with time slots (System III) or frequency channels (System IV) partitioned so there is no overlap between tiers. Analysis shows that the capacity tradeoffs are poor for Systems I and II because of the large amounts of cross-tier interference: and that System IV gives the best capacity tradeoffs.>

The required frequency spacings between channels in an optical frequency division multiplexing (FDM) network are considered. The minimum permissible spacings consistent with meeting bit error rate (BER) objectives are derived. The assumed transmission uses on-off keying (OOK), at a data rate 1/T (in bits per second), via external modulation of a laser source having linewidth beta (in hertz). The assumed receiver consists of an optical channel selection filter followed by a p-i-n photodiode and a postdetection integrate-and-dump circuit. The analysis estimates the adjacent channel interference (ACI)-induced floor on BER for the middle of three FDM channels, as a function of frequency spacing and linewidth-to-bit rate ratio ( beta T). For BER=10/sup -9/ and beta T ranging from 0.32 to 5.12, the required channel spacing ranges from 5.2 to 27.5 bit rates. The multiplying factors associated with using (wide-deviation) frequency shift keying (FSK), coherent (heterodyne) detection, and infinitely many FDM channels, respectively, are estimated to be 2.0, at most 3.0, and at most 1.37.>

The paper analyzes and quantifies the performance of a space diversity combining receiver operating in a digital cellular radio environment with quaternary phase shift keying (QPSK), frequency-selective Rayleigh multipath fading, and co-channel interference (CCI). The receiver has an infinite length filter in each branch of the diversity combiner, and the filters are jointly optimized according to the minimum mean-square error (MMSE) criterion. The link bit-error-rate (BER) is accurately estimated using Metzger's algorithm (Metzger, 1987) which approximates the probability density function of the combined intersymbol interference (ISI) and CCI. The authors present numerical performance results showing the influence of the diversity order, the number of dominant co-channel interferers, the multipath channel's delay spectrum, and the root-mean square delay spread. The results show that, under certain conditions, the optimum linear receiver can almost completely eliminate all ISI and dominant CCI while providing near-optimum noise filtering.>

A study of minimum mean-square error (MMSE) diversity combining for wide-band digital cellular radio, designed to combat intersymbol interference (ISI) caused by frequency selective fading is outlined. The systems analyzed use binary phase-shift keying (BPSK), quarternary phase-shift keying (QPSK) or 16-level quadrature amplitude modulation (16-QAM) with cosine rolloff spectral shaping, and space diversity with selection, maximal ratio or MMSE combining. A set of performance curves is presented for selected combining schemes showing the influence of the following system parameters: the diversity order (1 to 4); the cosine rolloff factor; the power delay spectrum (with its associated delay spread); the signal-to-interference ratio; and the number of modulation levels (2, 4 and 16).>

Consideration is given to the case of an optical pulse containing phase noise which is passed through either an optical filter or (following heterodyne lightwave detection) an electrical filter. Because of the phase noise, the envelope of the filter output at any instant is a random variable. An analytical method is developed for estimating the probability density function (pdf) of this envelope for different kinds of filter responses and for realistic combinations of phase noise severity and filter bandwidth. Obvious applications are to detection analyses of coherent lightwave systems, wherein finite laser linewidths constitute an important source of impairment. For each of the several types of filters considered, the envelope PDF can be accurately fitted by an exponential function approximation, where the decay constant is related in a simple way to known system parameters.>

The decision of on-off keying (OOK) and frequency-shift keying (FSK) is considered for coherent lightwave communications in which the optical pulses are corrupted by the phase noise associated with nonzero laser linewidths. Heterodyne lightwave detection is assumed, followed by intermediate frequency filtering, envelope detection, and postdetection low-pass filtering. Using analytical results published previously, theoretical minimum values of the required average power are obtained in photons/bit, for achieving a bit error rate of 10/sup -9/. Numerical results are given as functions of laser linewidth-to-bit-rate ratio for OOK, binary FSK (or 2-FSK), and 8-FSK. It is shown that heterodyne detection for lightwave signals of this kind can be made highly robust to phase noise, in contrast to the case of binary phase-shift keying.>

Waves are treated that modulate by either on-off keying (OOK) or binary frequency-shift keying (FSK) and are further impaired by additive Gaussian noise. Heterodyne detection of such a waveform produces an electronic bandpass signal, which, to ease demodulation in the presence of phase noise, is noncoherently demodulated to extract the baseband pulse stream. The treatment goes beyond previous bit error rate (BER) analyses of optical heterodyne receivers for OOK and FSK. First, there is full adherence to the standard (Brownian motion) model of phase noise. Also, the receiver structure is formulated in such a way that the probability density function of the receiver output samples can be accurately determined. This permits calculations of the additive noise and phase noise tolerable when achieving bit error rates as small as 10/sup -9/. Finally, the study is comprehensive regarding the range of parameters explored. Filtering at an intermediate frequency (IF) alone, as well as IF filtering plus postdetection low-pass filtering, is considered. When the receiver parameters decision threshold (for OOK) and IF filter bandwidth are optimized, large amounts of phase noise can be accommodated with only minor increases in required signal-to-noise ratio. This is especially important when the bit rate is moderate compared to the laser linewidth.>

This paper evaluates the effects of transmit amplifier nonlinearities on digital radio link performance. Special emphasis is given to links carrying 256-level modulations, for which the impact of even mild nonlinearities can be severe. Performance is measured in terms of the flat fade margin (F) normalized by its theoretical maximum (Fmax). Curves ofF / F_{max}versus amplifier input drive level are used to evaluate and compare numerous design strategies. A major contribution of this work is the development of a fast, accurate method of computer analysis, in contrast to the more cumbersome Monte Carlo simulations generally used for these assessments. Moreover, the new method is used to study a variety of techniques devised to combat the nonlinearity, particularly signal predistortion. It is also used to investigate various approaches to pulse shaping and filtering, to quantify the degradation in performance with increasing numbers of modulation levels, and to examine the possible benefits of linear equalization.

We have investigated the performance of the times-four method of carrier recovery in a digital radio link transmittingM-level quadrature amplitude modulation (M-QAM), whereMcan be 16, 64, 256, etc.. With the help of a recently built multipath fade simulator, we measured rms phase jitter for various combinations of fade notch frequency and fade depth. This paper reports on the experiment and the associated analysis. The times-four method has the virtues that it does not depend upon accurate data decisions, and it does not interact with the receiver's adaptive equalizer. Our results show that the rms phase jitter using this method, for all but the most severe and improbable fades, could be held to a few degrees. Although the measurements were done only for 16 QAM, the analysis predicts only modest increases for higher numbers of levels. The analysis also predicts, and the measurements verify, that the recovered carrier amplitude shrinks to zero for four distinct combinations of fade notch frequency and fade depth. To our knowledge, this phenomenon has not been reported in the literature heretofore. We show that, from a statistical viewpoint, the damage associated with it (measured in terms of outage time) should be quite small.

We analyze a digital radio link in which rectangular RF data pulses drive the high-power amplifier (HPA) and spectral shaping of the transmission is done after the HPA. The purpose of the analysis is to quantify detection performance for a variety of design possibilities. These include two approaches to predistortion in the transmitter and two ways of setting decision boundaries in the receiver. Also considered are different signal constellations and numbers of levels(M). We show that performance differences among practical system designs increase significantly withM. For example, the spread of minimum distance values among the various designs studied here grows from 4.4 dB forM = 16to 14.4 dB forM = 256.

Recent analysis/simulation studies have quantified the multipath outage statistics of digital radio systems using ideal adaptive equalization. In this paper, we consider the use of finite-tap delay line equalizers, with the aim of determining how many taps are needed to approximate ideal performance. To this end, we assume anM-level QAM system using cosine rolloff spectral shaping and an adaptive equalizer with either fractionally spaced or synchronously spaced taps. We invoke a widely used statistical model for the fading channel and computer-simulate thousands of responses from its ensemble. For each trial, we compute a detection signal-to-distortion measure, suitably maximized with respect to the tap gains. We can thereby obtain probability distributions of this measure for specified combinations of system parameters. These distributions, in turn, can be interpreted as outage probabilities (or outage seconds) versus the number of modulation levels. A major finding of this study is that, for the assumed multipath fading model, very few taps (the order of five) are needed to approximate the performance of an ideal infinite-tap equalizer. We also find that a simple, suboptimal form of timing recovery is generally quite adequate, and that fractionally spaced equalizers are more advantageous than synchronously spaced equalizers with the same number of taps. This advantage is minor for rolloff factors of 0.5 and larger but increases dramatically as the rolloff factor approaches zero.

In the past few years, a great deal has been learned about the modeling and equalization of multipath fading in microwave digital radio systems. This paper adds a number of new results to this body of knowledge. We begin with a description of two published multipath fading models that were derived from the same collection of data, i.e., the threepath and polynomial models. We then derive outage probability characteristics for a digital radio link usingM-level quadrature amplitude modulation, cosine rolloff spectral shaping, and some form of equalization. We obtain results for both models, several kinds of equalization, and various combinations of system parameters. We find that the theoretically optimal equalizer, which is nonlinear, is not measurably better in performance than the best of the linear equalizers, which is simpler. Also, the performance of a properly equalized receiver can be very accurately estimated by knowing just the probability distribution, over many fades, of the gain amplitude at a single frequency. Finally, we show the influences on performance of such parameters as cosine rolloff factor, symbol rate, and unfaded carrier-to-noise ratio.

we analyze the effects of frequency-selective fading in a cellular mobile radio system that uses 1) phase-shift keying (PSK) with cosine rolloff pulses, and 2) space diversity with maximal-radio combining. The distorting phenomena with which we deal are multipath fading (which produces the frequency selectivity), shadow fading, and cochannel interference. The relevant quality measure is defined to be the bit error rate averaged over the multipath fading, denoted by (BER). The relevant system performance characteristic is defined to be the probability distribution for (BER), taken over the ensemble of shadow fadings and locations of the desired and interfering mobiles. To obtain numerical results, we use a combination of analysis and Monte Carlo simulation, invoke widely accepted models for the multipath and shadow fadings, and assume a cellular system with seven channel sets and centrally located base stations. The outcome is a set of performance curves that reveal the influences of various system and channel parameters. These include: the number of modulation levels (two or four), the diversity order, the shape of the multipath delay spectrum, and the standard deviation (or delay spread, τ0) of the multipath delay spectrum. Practical factors accounted for in these assessments include fading- and interference-related timing recovery errors and combiner imperfections. Our results highlight the importance of the ratio\tau_{0}/T, whereTis the digital symbol period. They show that the delay spectrum shape is of no importance for\tau_{0}/T \leq 0.2, but can have a profound influence for\tau_{0}/T \geq 0.3. We also find that using 4-PSK leads to better detection performance, in certain cases, than using 2-PSK.

A major source of impairment in microwave digital radio is multipath fading, a phenomenon sufficiently prevalent on many paths to frustrate, the attainment of reliability objectives. Performance during multipath fading depends on such system parameters as channel bandwidth and type of modulation, and on details of the receiver processing (e.g., adaptive schemes, synchronization schemes). This paper compares the performance of different digital radio techniques in the presence of multipath fading. The measure used is the expected fraction of time, conditioned on the occurrence of a fading event, that the eye pattern at the receiver detector is closed. This measure is plotted against spectral efficiency (bit rate/channel bandwidth) for various types of modulation (QPSK, offset QPSK, 8-PSK, 16-PSK, 16-QAM, and offset 16-QAM) and for various forms of carrier recovery, timing recovery, transmit/receive filtering, and adaptive equalization. The computations assume a multipath fading frequency response described by a first-order complex polynomial, where the coefficients obey an empirically derived probability law reported previously. The comparisons show clearly the superiority of high-level modulation and the inferiority of offset modulation. The comparisons also demonstrate significant improvements for each of two adaptive equalizers postulated here.

To maximize the average power of a multitone bandpass signal under a peak power constraint, the tones must be phased so as to minimize the RF peak factor. Results are given here forM= 3, 5, 7, and 9 where theMtones are of equal power and uniformly spaced in frequency. The two methods of phasing studied lead to similar minimized peak factors of roughly 6 dB. Optimum parameter values, improvements over worst case peak factors, and sensitivities to phasing errors are given, and extensions to other values ofMare discussed.

Widely used digital radio modulations such as quaternary phase shift keying (QPSK), minimum shift keying (MSK), and sinusoidal frequency shift keying (SFSK) usually require some form of transmit filtering to ensure compliance with spectrum emission rules. Typically, radio transmit filters are of a specified type, such as fourthorder Butterworth, where the bandwidth parameter(\beta)is a design variable. We specify β to be optimal for a particular filter when it has the largest value for which the transmitted signal satisfies the emission rules, and we present numerical solutions for β over a wide range of conditions. These data can be used in design studies that compare contending modulation/transmit filter combinations. The solutions given are for systems subject to the spectral limits of FCC Docket 19311. The types of modulation considered are generalizations of those cited above; the types of transmit filters considered are Butterworth and Chebyshev filters of various orders; and β is tabulated as a function of channel width-to-symbol rate ratio for numerous modulation/filter combinations. While computed specifically, for the 6 GHz common carrier band, the results are also valid, to within a few percent, for the 4 and 11 GHz bands.

Radio signals derived using constant-envelope digital modulations (e.g., PSK, FSK and variations thereon) are impervious to the nonlinear characteristics of high-power RF amplifiers such as TWT's. However, constant-envelope modulations that must be heavily filtered to meet spectrum emission requirements, or intrinsically spectrumefficient modulations such as Quadrature Partial Response Signaling (QPRS), exhibit envelope fluctuations that render them vulnerable to amplifier nonlinearities. This paper describes a study, based on the use of Monte Carlo computer simulations, of the RF envelope variations associated with different digital modulations. Simulations and statistical analyses of these variations have been conducted for both filtered PSK and QPRS. The PSK category includes such special cases as Staggered Quaternary PSK (SQPSK); Minimum Shift Keying (MSK);L-ary PSK with cosine rolloff shaping (divided equally between transmitter and receiver); and linear phase modulation using shaped (nonrectangular) baseband pulses. The primary fluctuation measure used in the study is the dynamic range, in decibels, between the 10th and 90th percentiles of the RF envelope. This quantity,D, is presented for the various modulations considered and for various types of transmit filtering, with filter bandwidth-to-symbol rate ratio treated as a variable. The numerical results show the tradeoff between envelope constancy and spectrum efficiency among the most widely used digital radio approaches. For QPRS modulations,Dis typically in the range 11-16 dB; for heavily filtered PSK modulations, the range ofDlies roughly 6 dB lower, but other sources of distortion are more prominent.

An analytical study to predict multipath fading outages in terrestrial digital radio systems is described. The method, which is quite general, makes specific use of previously reported statistics on multipath fading. It is applied, in this study, to 6- and 11-GHz systems carrying 90 Mbits/s per RF channel. For purposes of comparing alternate approaches, three different modulations are considered, namely, (i) conventional QPSK at 45 Mbits/s, using two polarizations per channel; (ii) Offset-QPSK at 45 Mbits/s using two polarizations per channel; and (iii) 8-level PSK at 90 Mbits/s, using one polarization per channel. In all cases, a fourthorder Butterworth transmit filter is assumed which enforces FCC spectral emission requirements. The channel is treated as a two-path propagation medium, the receiver is assumed to be coherent, and various practical assumptions are invoked regarding the recovered carrier phase, timing phase, receive filter response, bit error rate objectives and fade margin. Results are given in terms of expected yearly fraction of multipath outage per repeater hop vs. hop length. We find that the choice of operating band has only a minor effect on the results; that OffsetQPSK systems suffer somewhat more multipath outage than conventional QPSK systems; that 8-level systems suffer more multipath outage than 4-level systems (although this difference may be offset by cross-polarization effects in the latter); and that all the approaches considered require repeater spacings of 15 mi or less in order to meet short-haul outage objectives over average terrain.

Frequency-selective fading in a radio channel, caused by multipath propagation or any other phenomenon, can seriously degrade the effectiveness of digital transmission. Receiver processing that adapts in an appropriate way to the prevailing channel response can strongly reduce the harmful effects of such fading. We examine here the theoretical possibilities of adaptive processing by deriving and analyzing receiver responses for three different criteria. The criteria used permit an analytical approach that is simple and exact. The performance characteristics derived for the three receivers bracket the "best" performance that is possible in practice and reveal possible tradeoffs between performance and practicality. The analysis applies generally to the broad class of quadrature amplitude modulation (QAM) signals, which are assumed to be filtered in the transmitter to enforce spectral emission requirements. For each receiver response we derive a formula for the "power penalty", defined as the increase in transmitter power (over some theoretical minimum) needed to compensate for transmit filtering and channel fading. The power penalty formulas are evaluated for a two-ray multipath fading channel and for each of two common forms of modulation/transmit filtering. Graphical results are given for numerous combinations of the multipath parameters and the transmitter bandwidth-to-symbol rate ratio. The results of this study can be used to predict the probability (time fraction) of multipath outage, provided that the statistics of the channel parameters are known. Some cursory comparisons are made between the new results and those of a previous analysis for fixed receivers. The outage reductions made possible by using appropriate receiver responses are found to be quite large, possibly two or more orders of magnitude.

The frequency transfer function of a multipath fading channel is examined in terms of its effects on digital radio signals. The transfer function is expanded into a power series about the channel center frequency and the coefficients are related to the multipath structure. It is then shown that, in the channelized common carrier bands below 15 GHz, the first two (complex) terms of the power series are usually sufficient for characterizing multipath effects. This demonstration is based on a mean-square error-of-fit measure which is applied to the multipath fading response and evaluated under some worst-case assumptions.

This paper derives a new power spectrum formula for constant-envelope PSK signals. The only signal constraints are that the baseband pulses be of finite duration and the pulse amplitudes be mutually independent. The computer program used to implement the formula is made highly efficient by the use of fast Fourier transform (FFT) algorithms. The key contribution of the new result is the high degree of baseband pulse overlap permitted before the computation cost becomes excessive. In some computation approaches, this cost grows as LN, whereLis the number of modulation levels andNis the number of data periods spanned by the pulse. In the approach developed here, the cost is independent ofLand grows roughly linearly withN. The cost improvement, which is substantial for largeLandN, is exploited here by using the new approach for a wide variety of baseband pulses. Results are given which demonstrate the potential value, in terms of narrower spectra and lower sidelobes, of using overlapping pulses.

A computerized procedure for obtaining the pull-in frequency of a phase-locked loop is described. The procedure, which consists of solving for the limit cycle of the out-of-lock loop and finding the frequency offset below which no solution exists, is quite general with respect to phase detector function and loop filter. It is applied in this study to the case of a second-order loop with a sinusoidal phase detector. The results of the study give normalized pull-in frequencyXas a function of the damping factor\zetaand normalized natural frequency β of the linearized closed loop. A review of several previously published investigations of this case is given, and the limitations identified point the way to the more exact solution described here. In addition, an experimental study is described which produces excellent agreement with the new results, and a curve-fitting procedure is outlined which leads to a highly accurate functional description ofXin terms of β and\zeta.

Methods that increase the speed of digital detection comparators can also produce hysteresis, wherein the effective threshold in a given signaling interval is influenced by the previous output. If the comparator is fed by a peak-limited IF detector output followed by a detection filter with unity dc gain, the hysteresis can be a significant fraction of the peak comparator input. This concise paper derives the detection filter impulse response for which the resulting eye-opening-to-rms-noise ratio is maximized. The impulse response is taken to be either unconstrained in time or confined to a signaling interval (as in filter-and-dump detection). The results for each case are applied to 2- and 4-level coherent phase shift keying (CPSK). Expressions are obtained for the maximized eye-opening-to-rms-noise ratio as a function of comparator hysteresis and phase modulation pulse shape. These expressions are normalized to represent the excess in required power above the theoretical minimum, and graphical results are given. They show the suboptimality of filter-and-dump detection when hysteresis is high, the insensitivity of performance to hysteresis when unconstrained optimal filters are used, and the relative effects of phase modulation pulse shape on required power.

The averaging of target-detection probability over random target strength is more difficult when multiple-trial detection logic is used. This correspondence examines an approximation method sometimes used, and determines the theoretical bounds on the true average in terms of the estimated average. These are derived for a two-trial detection scheme and then generalized toNtrials. An improved method of approximation is suggested and a specific example is given for a Swerling Type-I target.