We evaluate the performance of a low-power narrowband system utilising higher-order sectorisation (HOS) at the base stations (BS) and directional antennas (DA) at fixed user equipment (UE). We use a specific rotational frequency reuse scheme (RFR) for directional UEs and extend it for HOS (3, 6 and 12 sectors) by considering antenna orientations for interference avoidance. We propose two novel schemes for directional UEs with a cluster size less than one, named double frequency reuse (DFR) and triple frequency reuse (TFR). These schemes are tested and compared to conventional frequency reuse (CFR) through simulations in a realistic geographic terrain. The simulation results indicates that the proposed schemes are helpful to achieve higher capacity while using different BS and UE antenna beamwidths. It is shown that scheme RFR provides a performance enhancement compared to CFR only with wider UE antenna beamwidths. The implementation of DFR achieves reduced outage probability and offers a large increase in average spectral efficiency per cell (ASE) of up to 84.8-81.6% in 6 and 12 sectored system. The scheme TFR provides a maximum ASE of up to 149-153% in 6 and 12 sectored system. However, in a six-sectored system, TFR only meets the coverage criteria with a narrow beamwidth antenna of 20°.

We consider a coordinated multi-cell (CMC) system and the associated problem of independent carrier frequency offsets (CFOs) at the basestations (BSs). These BS CFOs cause accumulated phase errors that compromise downlink beamforming accuracy, and consequently degrade the spectral efficiency of the CMC system. Since the inherent structure of coordinated downlink beamforming techniques makes it impossible to correct for the BS CFOs at the mobile subscriber (MS), our topic is estimation and correction of the BS CFOs at the BSs. Our method begins with the formation of MS-side estimates of the BS CFOs, which are then fed back to the coordinated BSs. We then derive an optimum maximum likelihood (ML) estimator for the BS CFOs that uses independent MS-side CFO estimates and average channel signal-to-noise ratios. However, it is demonstrated that the CFOs of the MSs themselves introduce a bias to the optimal BS CFO estimator. To compensate for this bias, a joint BS and MS CFO estimator is derived, but shown both to require high computation and have rank deficiency. This motivates an improved technique that removes the rank problem and employs successive estimation to reduce computation. It is demonstrated to overcome the MS CFO bias and efficiently solve the joint BS and MS CFO problem in systems that have low to moderate shadowing. We term the full BS CFO estimation and correction procedure "BS CFO tightening".

In this paper, we investigate the performance of a single-relay assisted cooperative link, where the source, relay, and destination terminals are equipped with multiple transmit and receive antennas. We focus on the so-called transparent amplify-and-forward (T-AaF) for MIMO relay channels, in which the relay requires neither channel state information (CSI) nor synchronization of symbols or carrier. Specifically, we propose and analyze new selection schemes, where the relay, based on the received signal energies, selects the "best" received signal on the S → R hop and forwards it to the destination terminal with either repetitive or selective transmission. Through the derivation of pairwise error probability (PEP) expressions and asymptotic order of diversity (AOD) analysis, we demonstrate that the proposed selection schemes preserve full diversity order, while maintaining low complexity implementation, which makes them attractive from a practical point of view. Furthermore, we show that adopting selective transmission on the R → D hop provides better performance, better throughput, and significantly better energy efficiency than repetitive transmission. A Monte Carlo simulation study is also presented to corroborate the analytical results and to provide detailed performance comparisons between the two schemes under consideration.

In this paper, we consider a single-relay cooperative scenario, where the source, relay, and destination terminals are equipped with multiple transmit and receive antennas. We focus on the so-called transparent amplify-and-forward (T-AaF) for MIMO relay channels, in which the relay requires neither channel state information (CSI) nor synchronization of symbols or carrier. Specifically, we propose and analyze new selection schemes where the relay, based on the received signal energies, select the "best" received signals on the S rarr R hop and retransmits them to the destination terminal with either repetitive or selective transmission. Through the derivation of pairwise error probability (PEP) expressions and asymptotic order of diversity (AOD) analysis, we demonstrate that the proposed selection schemes maintain full diversity order. Furthermore, we show that selective transmission on R rarr D hop has better performance, better throughput, and significantly better energy efficiency than repetitive transmission.

In this paper, we introduce a novel power control strategy based on the total received power in the uplink of cooperative base stations (CBSs). Power control via channel inversion is common practice in the uplink of cellular systems as it maintains constant received signal power at the controlling base station (BS) regardless of the position of the mobile station (MS). Unlike the conventional cellular system, CBSs share information in detecting the MSs and they are shown to increase the system capacity. We propose a power control approach based on sustaining the total received power in CBSs. We investigate the performance of this method through simulations and show that it reduces the intercell interference in the CBS systems, saves power and improves the BER performance in both coded and uncoded scenarios.

In this paper we present an analytical solution to the problem of jointly optimizing transmit power and scheduling for a star network of wireless energy-limited sensors around the base station. We have included factors which are usually neglected in modeling sensors and their communication channel. These include: accounting for power utilized for activities other than transmission, i.e. processing power, and time variations of the channel gains throughout the lifetime of the sensor. With an information theoretic approach to this problem, we have avoided the details of modulation and pulse shaping, and accounted for the mutual interference in simultaneous sensor transmissions. We employ the volume of data received at the base station over the lifetime of all sensors as the system performance measure and prove that in the optimal scheduling scheme sensors should avoid simultaneous transmissions. In addition, we show the optimal transmit powers that maximize the data volume can be determined via water-filling over the sensor's lifetime once the processing power and the single sensor transmission schedule are accounted for. In this work we have assumed prior knowledge of the channel gains throughout the lifetime of all sensors. Nevertheless, the solution will provide an optimistic value for the network's maximum data volume achievable by a causal scheduler.

We introduce a novel framework for soft-input, soft-output (SISO) equalization in frequency selective multipleinput multiple-output (MIMO) channels based on the well-known belief propagation (BP) algorithm. As in the BP equalizer, we model the multipath channels using factor graphs (FGs) where the transmitted and received signals are represented by the function and variable nodes respectively. The edges connecting the function and variable nodes illustrate the dependencies of the multipath channel and soft decisions are developed by exchanging information on these edges iteratively. We incorporate powerful techniques such as groupwise iterative multiuser detection (IMUD), probabilistic data association (PDA) and sphere decoding (SD) in order to reduce the computational complexity of BP equalizer with relatively small degradation in performance. The computational complexity of this new reduced-complexity BP (RCBP) equalizer grows linearly with block size and memory length of the channel. The proposed framework has a flexible structure that allows for parallel as well as serial detection. We will illustrate through simulations that the RCBP equalizer can even handle overloaded scenarios where the channel matrix is rank deficient, and it can achieve excellent performance by applying iterative equalization using the low-density parity check codes (LDPC).

We present a minimal energy packet scheduling and rate control scheme with a strict maximum delay constraint. This problem occures natually in real-time wireless multimedia transmission where a hard delay limit is imposed on each packet. In general, the communication link is assumed to be a diversity channel with multiple parallel sub-channels(e.g. OFDM, or MIMO eigenchannels.) We present both the theoretical optimal solution, which assumes prescient traffic and channel knowlodge, and a causal scheduler where the future is only known statistically. We show that the causal scheduler performs well, to within 3 dB of the prescient optimal in a single channel, and is even better with diversity channels.

Mutual interference and battery are the main transmission issues in a wireless sensor network. Since all the data in the network flows towards the base station, the region around the base experiences the highest levels of mutual interference and data congestion. In this work we isolate and study the star network of sensors connected to the base. To facilitate our investigation we introduce a new performance measure, the data volume, which is in tune with the goal of the network. This criterion takes into account both the limited energy and the efficiency in which the energy is utilized. With the objective of maximizing the data volume received at the base station, cross layer optimization, i.e. joint scheduling and power control, is employed. We present an analytical solution to show that to maximize the data volume the best policy is for sensors to transmit sequentially. Once the optimum transmit policy for the star network is resolved, it sheds light on the optimization process for more complicated scenarios and system layouts.

We consider the energy efficient scheduling of packets for a single user wireless link. We propose packet schedulers that meet individual per-packet maximum-delay constraints and present their performance. The main emphasis is on deriving an easy-to-implement scheduler with low average power consumption in a Rayleigh fading channel, and to identify areas of potential improvements. We firstly outline the structure of the optimal scheduler with prescient knowledge of the channel and the traffic pattern, and an efficient algorithm to derive this optimal schedule. This provides a baseline for comparison for all other schedulers. From the insight gained in the study of the prescient optimal scheduler, we remove algorithmic dependency on the future to derive a practical scheduler that has energy usage at most 6 dB away from the prescient optimal at 0.1% probability of bit outage under Rayleigh fading conditions.

The joint estimation of multiple carrier frequency offsets (CFOs) and channel state information (CSI) in a multibase MIMO downlink system is considered here for the case of a fading channel. The multiple CFOs result from independent local oscillators at each basestation (BS), and cause accumulation of phase errors that compromise beamforming accuracy. Using previously devised multiple CFO estimation techniques for simultaneously transmitted training sequences in a non-fading channel, we begin by examining the effects of fading on a linear channel state estimate. An example illustrates that a fade rate threshold can be defined after which linear channel estimation is infeasible. Because of the dependence of the CFO estimators on the CSI estimate, this threshold applies to the CFO estimates as well. As a way to combat the channel variation, the maximum likelihood metric is modified to accommodate a parameterized fading channel estimate.

The effect of carrier frequency offset (CFO) in a coordinated multibase MIMO downlink system is considered. We begin with evidence that uncompensated base station (BS) CFOs can significantly reduce the achievable capacity of such systems. Correction of the CFOs is desirable, and a critical step in doing so is estimation of their values. We present a method by which mobile subscribers (MSs) can estimate multiple CFOs from training sequences received from the BSs. Compared to existing estimators, the proposed estimator gives better accuracy with a lower transmit power.

System-wide multiuser detection (MUD), in which all base stations (BSs) of a cellular system cooperate to detect the data of all mobile stations (MSs), has much promise. However, little is known at present about practical techniques or their performance. An attractive method is belief propagation (BP), with message exchange between nearby cooperating BSs over a backbone landline network, but its performance is known only for a greatly simplified network model and its computation load grows exponentially with the number of interfering MS symbols at each BS. In this paper, we present a reduced complexity variation of BP (RCBP) and show that its performance is close to or identical to that of BP in the simplified network. We also observe excellent performance by iterative multiuser detection and decoding of low-density parity check codes (LDPC). Furthermore, we examine RCBP performance in a realistic wireless network model, with path loss, shadowing, fading and power control. These results, though poorer than those of the simplified network, show that system-wide MUD with cooperating BSs provides great improvement compared with conventional systems.

We consider downlink MIMO beamforming from several coordinated basestations (BSs), and the associated problem of in dependent carrier frequency offsets (CFOs) at the BSs which cause accumulated phase errors to compromise beamforming accuracy. Correction of the CFOs requires estimation of their values, so our topic is multiple CFO estimation, a little-explored area. We present a robust and easily generalized estimator that accounts for the training sequence (TS) correlations caused by the CFOs, and show that it meets the Cramer-Rao lower bound (CRLB) at moderate signal-to-noise ratios (SNRs). The performance of the estimator is contingent upon TSs short enough to ensure convexity of the log-likelihood over the allowable CFO ranges. For combinations of TS length and CFO range that violate this constraint, we present two suboptimal estimators based on segmentation of the TS, both of which also meet the CRLB at moderate to high SNRs.

We propose a reduced-complexity belief propagation (RCBP) receiver for joint detection in the uplink of cellular multi-access networks with base station cooperation. Using local message passing algorithms based on belief propagation (BP) has been shown to be very promising for distributed base station processing. Unfortunately, the computational complexity of the BP algorithm grows exponentially with the number of interfering users at each base station. Reducing the computational load of marginalizing the a posteriori probabilities (APP) in BP nodes is challenging because this problem is generally rank deficient, and most well-known sub-optimal detection methods are not efficient in such conditions. In this paper we use an iterative group-wise multiuser detection algorithm to efficiently reduce the complexity of BP receiver. We evaluate the performance and complexity of the proposed RCBP algorithm under decomposed and clustered base station scenarios via simulations.

A solution to the problem of finding a minimum energy packet scheduling and rate control scheme with per- packet maximum delay constraints is presented. This problem occurs naturally when QoS constraints are present in systems that support heterogeneous traffic types. The solution is shown to have the form of piecewise waterfilling between active arrival and delay constraints. A novel geometric interpretation is given, and shown to lead to an efficient iterative offline rate calculation algorithm that has a physical analogy of pulling a string through a set of pegs.

Many investigations of hybrid selection (HS) diversity assume maximal ratio combining (MRC) of the selected branches. However, a coherent detector needs to dwell on the received signal for some time before it can produce accurate channel estimates for fading compensation, a requirement that appears inconsistent with the branch switching that occurs in a selection diversity receiver. Motivated by this observation, we derive in this letter analytical results on the switching rate and average dwell time of a selection diversity receiver where M out of a total of N independent branches are selected for combining. We show that the switching rate can be many times the Doppler frequency, while the average dwell time can be a small fraction of the reciprocal Doppler frequency. The brevity of the dwell times suggests difficulty in obtaining channel state information, which in turn calls into question performance analyses of idealized HS/MRC structures. Our results also suggest that HS/MRC should be frame-based, rather than continuously acting in time

We consider a narrowband multiuser system in which several transmitting users are received through a synchronous, flat fading channel at an antenna array. The paper introduces a multiuser detection technique that combines groupwise detection with iteration. It is well suited to overloaded conditions, where there are more transmitter antennas than receive antennas. The soft decisions it uses internally also make it suitable as a detector in a concatenated structure. Its performance and computation can be traded off through selection of group size. A variant of the algorithm achieves further computation reduction by incorporating a soft sphere detection core.

This paper applies extrinsic information (EXIT) analysis to an internally iterative multiuser detector (IMUD) in serial concatenation with convolutional codes. The focus is overloaded conditions, in which the number of transmitters exceeds the number of receiver antennas. The study seeks to verify if EXIT charts can be used as a design tool under these circumstances. The use of non-Gaussian LLR distributions in performance estimation is discussed. The mutual information of the extrinsic information from IMUD is characterized under a variety of situations. It is shown that the EXIT charts do provide a means to determine both the minimum SNR and necessary number of iterations for convergence. However, estimating the bit error rate from the EXIT chart does not seem to be possible except in special circumstances.

Many investigations of hybrid selection (HS) diversity assume maximal ratio combining (MRC) of the selected branches. However, a coherent detector needs to dwell on the received signal for some time before it can produce accurate channel estimates for fading compensation, a requirement that appears inconsistent with the branch switching that occurs in a selection diversity receiver. Motivated by this observation, we derive in this letter analytical results on the switching rate and average dwell time of a hybrid selection diversity receiver where M out of a total of N branches are selected for combining. As one would expect, the switching rate is proportional to the Doppler frequency, while the average dwell time is proportional to the duration of a fade cycle. More importantly, the results in the paper suggest caution in interpreting the results of performance analysis of idealized HS/MRC structures. They also shed some light on the demodulator design in these systems

Since wideband systems can operate with single cell clusters they outperform existing narrowband systems in providing capacity. Current narrowband systems typically operate with cluster sizes of three or seven cells. In this work, we propose joint maximum likelihood (JML) multiuser detection as a means to reduce the cluster size and approach the capacity of wideband systems. To our knowledge, this is the first time a single cell cluster has been achieved in narrowband systems. Unlike previous work, in this research, ML detection is performed jointly on the desired user and a few of the other-cell users in the system which are received with the highest signal power at the desired user's base station. Due to the selection technique, the computation is kept manageable, while the performance of the single cell cluster is improved to match that of the conventional system

We introduce a symbol by symbol, soft-input soft-output (SISO) multiuser detector for frequency selective multiple-input multiple-output (MIMO) channels. The basic principle of this algorithm is to extract a posteriori probabilities (APP) of all interfering symbols at each symbol interval and then feed these updated APP as a priori probabilities (apP) for joint APP extraction in the next symbol interval. Unlike near-optimal block oriented sphere decoding (SD) and soft decision equalization (SDE), the computational complexity of this updating APP (UA) algorithm is linear in the number of symbols. However, the exponential computational load of optimal joint APP extraction makes the basic UA impractical. To decrease computations we replace the optimal joint APP extractor by a groupwise SISO multiuser detector with a soft sphere decoding core. The resulting reduced complexity updating APP (RCUA) equalizer is flexible in different situations and outperforms the traditional sub-optimal MMSE-DFE without increasing the computational costs substantially.

Recent papers have shown that space-time (ST) coding using continuous phase modulation has a number of advantages relative to linear modulation, among which is its constant envelope, which allows transmission with inexpensive and power-efficient Class C amplifiers. The present paper presents a particularly simple variant, ST coding based on minimum shift keying (MSK), an offset format. ST codes with offset modulation give much better performance than quadrature phase shift-keying (QPSK), for the same number of states, and the MSK ST decoding computational load is smaller than that of QPSK by a ratio that improves exponentially with the number of transmitter antennas. The relationship between offset and nonoffset modulation formats is explored and the effect of pulse shape is shown explicitly.

We consider a multiuser system in which the interfering users have the same pulse shape and are received in a synchronous, flat fading channel at an antenna array. This paper introduces a new iterative group-wise multiuser detection technique (IMUD) that gives a tradeoff of complexity and performance between joint maximum likelihood (JML) detection and zero forcing (ZF) MUD. It is shown that in the overloaded case the new IMUD algorithm significantly outperforms MMSE V-BLAST. Even with an equal number of users and receive antennas, IMUD gives better performance within two iterations. An additional feature of IMUD is that it inherently produces soft-output decisions.

In a general wireless system, cells are loosely defined and user signals appear at multiple antennas with various powers and delays. Despite the enormous performance benefits of system-wide maximum-likelihood multiuser detection (ML MUD), its application to such systems is hampered by the lack of a regular structure. Prior work usually dismisses the possibility on computational grounds as exponential in the total number of users, at least. This paper is the first to address efficient computation of system-wide ML MUD. We present a computational organization that achieves dramatic reduction in complexity through exploitation of the partial overlap of user sets at different antennas. This algorithm, which applies to code-division multiple access or narrowband systems, can be viewed as a spatio-temporal extension of the well-known Viterbi algorithm (VA), and, like the VA, it is derived from dynamic programming principles.

Multiuser detection (MUD) of signals occupying the same time/frequency slot has been shown to provide significant increases in the capacity of a single cell or coverage area. In a cellular system, however, increasing the number of users per cell also produces increased interference from other cells. Compensation by increasing the co-channel cell spacing partially offsets the capacity gain due to MUD, and the net effect on system capacity then rests on the detection method, such as maximum likelihood (ML) or minimum mean-squared error (MMSE). This paper is the first to analyze the net capacity gains due to ML-MUD in time division multiple access (TDMA), and the first to compare ML-MUD with MMSE-MUD on this basis. We demonstrate that ML-MUD with M cochannel users per cell and four base station antennas gives a net system capacity that is M times greater than that of single-user TDMA system with two antennas. In contrast, we show that MMSE-MUD gives only a modest net capacity increase, and that the system capacity may actually decrease with more than one or two users per cell, making it poorer than the original single-user system.

The authors present a performance analysis for the reverse link of a wireless DS-code division multiple access (CDMA) system that exploits macrodiversity reception while adopting the maximum likelihood (ML) multiuser detector for the basic detection. This appears to be the first analytical treatment of the ML multiuser macrodiversity detector in asynchronous DS-CDMA systems. They also include the effects of frequency selective fading, imperfect synchronization, and imperfect channel state information. Power control and connection control companion algorithms are presented to assess the value of this detector in a system context. The authors have shown that the ML multiuser-macrodiversity detector is capable of reducing the bit-error rate for many users by several orders of magnitude compared with multiuser detectors that operate on each antenna separately.

In this paper, a simple structure is proposed for accommodating reuse-within-cell in the downlink of a time-division multiple-access system using a transmit antenna array at the base station and joint detection of the cochannel signals at the single-antenna mobile receivers. The transmit array is used solely for system capacity increase; diversity is obtained in the traditional way through standard coding and interleaving. Due to the spatial formatting of the coded symbols prior to transmission, a single multiantenna channel usage spans several trellis transitions - an unconventional situation which is handled through a "merged trellis". Using the merged trellis, a combined joint detector/soft-decision decoder is constructed, and an analytical expression is derived for the coded bit-error rate based on the exact computation of pairwise error probabilities. It is shown that while the system suffers a reduction in diversity order as the number of intracell cochannel users increases, the reduction may be countered by increasing the constraint length of the code. Although this increases complexity, it is shown that multiple users may be supported, all enjoying good performance at low signal-to-noise ratios, while keeping the complexity in check.

This article presents a new computational tool for use in general fading channel analysis when the detection scheme can be expressed as a quadratic form in zero-mean complex Gaussian random variables. We develop a simple numerical algorithm which is capable of inverting a characteristic function consisting of both simple and multiple poles. The approach benefits from the inherent symmetry in the residue calculations and uses the well-known Vandermonde matrix in order to take advantage of this symmetry. It is numerically stable, eliminates singularities, and circumvents the need for differentiation.

In this paper, a simple structure is proposed for supporting multiple intracell users in the same time/frequency slot in the downlink using a transmit antenna array at the base station and joint detection at the single-antenna mobile receivers. To support multiple users, the system requires the simultaneous transmission of several successive code symbols from different diversity antennas. As a consequence, a single multi-antenna channel usage spans several trellis transitions - an unconventional situation which is handled with a novel merged trellis. Using the merged trellis, the optimal decoder is identified, and an analytical expression for the average bit-error rate is developed, based on soft-decision joint decoding at the mobiles. Unusual behaviour is demonstrated in terms of diversity order: as the number of transmit antennas increases due to an increasing number of users, the diversity order actually decreases due to the simultaneous transmission of successive code symbols. Even with the loss in diversity order, the method provides a major increase in downlink capacity while maintaining good performance for all users at low signal-to-noise ratios with moderate computational load.

The combination of macrodiversity reception with maximum likelihood (ML) multiuser detection has the capability to reduce the bit error rate (BER) for many users by several orders of magnitude compared with multiuser detectors that operate on each antenna separately. In this paper, we present the conditional metric merge (CMM) algorithm which reduces the computational complexity of the ML multiuser-macrodiversity detector by an enormous factor. The CMM algorithm can be viewed as a spatial variant of the Viterbi algorithm. It is a new algorithm and is the first of its kind as ML multiuser-macrodiversity detection (MUMD) is a relatively new area of research.

Estimation of the channel impulse responses of multiple cochannel users is a key requirement of all multiuser detection and interference cancellation techniques, though little attention has been paid to the subject in the context of time-division multiple-access (TDMA) systems. This paper addresses a pilot-based MMSE technique for multiuser channel estimation in a TDMA system, and makes two key contributions. Firstly, it allows for time variation of the channels within and between training sequences, an essential feature in a multiuser environment even at moderate fading rates. Secondly, it addresses the design of training sequences for the multiple cochannel users.

We derive a fully analytical expression for the union bound on symbol-error rate for the joint detection of several cochannel fading signals using a diversity antenna array, putting numerical results previously published on a firm analytical base. We prove that with pilot-based multiuser channel estimation, any number of users can enjoy diversity order equal to the number of antennas, and we quantify the performance penalty relative to single-user binary phase-shift keying as a function of number of users, constellation density, and number of antennas. We also demonstrate the interdependence of all users participating in the detection process.

Estimation of the channel impulse responses of multiple cochannel users is a key requirement of all multiuser detection and interference cancellation techniques, though little attention has been paid to subject in the context of TDMA systems. This paper addresses a pilot-based technique for multiuser channel estimation in a TDMA system, and makes several new contributions: it allows for time variation of the channels within and between training sequences, it accounts for colouration of the sampled noise sequence as well as correlation between the channel taps, and it considers users to be asynchronous resulting in a technique whereby explicit timing recovery is unnecessary. In addition, it addresses selection of appropriate training sequences.

In adaptive transmitter diversity, the complex weights applied to the downlink antennas are obtained from the instantaneous complex gains in the uplink. This paper provides the first analysis of the effects of decorrelation of gains between the uplink and downlink and of number of antennas, and the first assessment of the effect of fading when the method supports multiple users.

Errors and erasures decoding has the capability to improve the performance of a Reed-Solomon decoder if each error location is correctly identified, or "flagged", and is considered as an erasure. However, the erasure flagging must be extremely reliable. In this paper, we present a new way to flag erasures, based on the side information provided by pilot signals. We derive a Bayesian method for optimum character erasure flagging, using both the received data signals and the channel estimates provided by pilot signals, and we present a simpler method which flags erasures on a modulation symbol basis. The simulation results demonstrate that the method results in significant transmit power savings compared to errors-only decoding.

This paper presents a method for the exact calculation of the union bound on the error performance of trellis-coded modulation (TCM) in fading channels. Previous analyses have approximated the error expression to allow the use of the transfer function, or restricted their attention to the most common error events, or both. In contrast, we combine the transfer function, which counts all error events, with exact calculation of event probabilities, and produce an exact expression for the union bound. Although computationally intensive, the technique produces the tightest bound yet published. In addition, it is very general and is applicable to coherent, differential, and pilot-based detection, and to phase-shift keying (PSK)- and quadrature amplitude modulation (QAM)-types of constellations.

Joint detection based on exploiting differences among the channels employed by several users allows a receiver to distinguish cochannel signals without reliance on spectrum spreading. This paper makes a number of new contributions to the topic; it provides an analytical expression for the union bound on the average symbol-error rate for an arbitrary number of users and diversity antennas in a fading environment, for both perfect and imperfect channel state information (CSI), and it compares the performance of joint detection with diversity antennas against classical minimum-mean-square-error (MMSE) combining. The performance is remarkable. With accurate CSI, several users can experience good performance with only a single antenna; moreover, for perfect CSI, only a 2-dB penalty is incurred for each additional user. With several antennas, many more users than the number of antennas may be supported with a slow degradation in performance for each additional user. Furthermore, high accuracy is not required from the channel estimation process. In all cases, the performance of joint detection exceeds that of MMSE combining by orders of magnitude.

Pilot symbol assisted modulation (PSAM) has previously been shown to give good performance in flat fading, noise and cochannel interference. The present paper analyzes its performance in ISI due to frequency selective fading, and provides a similar analysis of differential detection for comparison. The paper also introduces a method for performing the formidable average over transmitted data patterns simply, and with an analytical result, PSAM is shown to be sensitive to RMS delay spread, though it always gives better performance than differential detection.>

The paper presents a quick and accurate method for calculating the total intermodulation (IM) power at an amplifier output as a function of the modulation format and the amplifier nonlinearity. Although it does not give the spectral distribution, it does not require lengthy simulations. First, each modulation is characterized by the probability density function (pdf) of its instantaneous power. Next the candidate amplifiers are represented by their measured AM/AM and AM/PM characteristics (low degree polynomials are not required). In an application of linear estimation principles, a simple numerical integration gives the IM power resulting from any combination of modulation and amplifier.>

Feedforward linearization has advantages in bandwidth and generality over other linearization methods. However, it is based on the subtraction of nearly equal quantities, so its major parameters must adapt to changes in environmental or operating conditions. The paper is the first published analysis of adaptation behaviour in a linearized feedforward amplifier. It presents an analytical framework and several new results, including convergence time and coefficient jitter, a bias effect that leads to extreme accuracy requirements in one coefficient, and the effect of delay mismatch.>

This work presents an exact expression for the pairwise error event probability of trellis-coded modulation (TCM) transmitted over Rayleigh-fading channels. It includes phase shift keying (PSK) and multilevel quadrature amplitude modulation (QAM) codes, as well as coherent and partially coherent (e.g. differential, pilot tone, etc.) detection. Due to the form of the exact pairwise error event probabilities, this calculation technique cannot be used with the transfer function technique to obtain an upper (union) bound on the overall bit error probability. For this reason, the authors estimate the bit error probability by considering only a small number of short error events. Through simulations, they found that the estimation is usually very accurate at high signal-to-noise ratios but not as accurate at lower signal-to-noise ratios. They study several coded modulation schemes this way. Among the results are the fact that TCM provides significant improvement in the error floor when detected differentially, and an asymmetry in the pairwise error event probability for 16 QAM.>

Spectrum shaping codes introduce redundancy and correlation into a transmitted data stream in order to alter the transmitted spectrum without affecting the pulse shape or peak-to-average power ratio. Most are based on algebraic properties, or on a running digital sum. A new and very general approach to spectral shaping, in which the Viterbi algorithm is used at the transmitter to minimize the transmitted power in specified bands, is introduced. The principal advantage is greatly increased flexibility of width, depth, and placement of spectral notches. The computational load at the transmitter is greater than that of earlier algorithms, although the receiver is unaffected.>

Transparent tone-in-band (TTIB) creates a spectral gap by splitting a signal into frequency bands, which are then separated by modulation. Phase locked transparent-tone-in-band (PLTTIB), in its original form, contained an unacceptably high level of self-noise in the coherent band recombination process. A performance analysis of PLTTIB with symmetric phase detection is given. Several new results are highlighted including the effect of recombination phase error on bit-error rate, the mean-square phase error, the probability density function of phase error, and the pull-in time of the phase tracker. Although the detailed results are obtained only for static channels, a qualitative assessment of the effect of fading is given.>

Since fading channels are characterized by frequent signal dropouts which are long compared to packet duration, substantial buffer space must be provided at both ends of the link. Systems which rely on selective, on/off transmission to achieve low bit error rates are seriously affected by the special conditions which must be imposed on them to prevent buffer overflow or underflow. This effect can be reduced by a method of buffer control developed and analyzed in this paper, in which the average transmission rate is varied as a function of queue length in an attempt to keep the queue away from the ends of the buffer. It is shown that buffer control provides the same improvement in average error probability as does doubling the buffer size, but without the associated doubled storage cost and doubled delay. In order to simplify the analysis and keep the discussion relevant, the system is oriented to transmission of fixed length blocks or packets.

Transmission of data with receiver-controlled bit rate is an effective way to reduce the effect of signal-to-noise ratio (SNR) fluctuations, as in a fading channel. All previous publications on this subject ignore the critical topic of buffer size. This concise paper summarizes the results of the first study of the effect of finite buffers on a variable-rate system for fading channels.

The problem of calculating the reliability of a communication network of given topology is discussed. The definition of failure is generalized from simple disconnection to the event that terminal capacity falls below some threshold, and simple methods are developed for bounding the probability of such failures and for approximating the mean time between failures (MTBF). It is shown that the cumulative distribution function of terminal capacity is readily calculated, and that the dependence of network reliability on individual link reliability is substantially different when the generalized (and more realistic) definition of failure is used.

A feedback scheme for regulating error probability on slowly fading communication channels is described and analyzed. In this selective diversity system, only the bestRofLavailable diversity subchannels are used, the selection being revised from moment to moment. Comparison with conventional diversity transmission on an 8-channel system shows a performance improvement ranging from 20.8 to 9.5 dB, where the low figure corresponds to constant power operation of the transmitter and a typical value of round-trip delay. A novel feature of selective diversity is that, for moderate SNR, the error probability decreases to zero as a strong inverse power ofL, the order of diversity, which contrasts with behavior of conventional systems in which increasingLcauses the error probability to decrease to a nonzero lower bound. The implication is that the figures quoted above can be improved indefinitely by increasing the order of diversity.

A modulation system is proposed that continuously adjusts its data rate in response to signal strength variations in a fading channel. The optimum variation of data rate with channel conditions is determined, and includes the effects of feedback channel time delay, the interval between rate changes, and restriction of the number of allowable rates. Application of these results to a full duplex situation allows determination of the optimum fraction of the data stream to be devoted to service information. Comparison of this scheme with diversity transmission on the basis of error probability and bandwidth utilization reveals a reduction on the order of 14 dB in transmitter power for a typical duplex link.