Estimation of downlink (DL) channel state information (CSI) is necessary in massive multiple-input multiple-output (MIMO) systems to enable precoding and achieve high spectral efficiency. However, CSI estimation (for both the uplink (UL) and DL) is challenging in an environment with highly-mobile users due to rapidly-varying fading. The estimation becomes even more challenging when UL CSI is incomplete due to system constraints. In this work, we combine two machine learning techniques to tackle the twofold problem of predicting upcoming DL CSI from earlier UL CSI estimates and estimating full UL CSI from its incomplete form. For the first sub-problem, we employ long short-term memory (LSTM) to capture the spatio-temporal correlation between CSI at different time instances and user positions. For the second sub-problem, we use a conditional generative adversarial network (CGAN) to estimate the full UL CSI from varying amounts of incomplete CSI. We examine the normalized mean square error performance of the proposed CGAN-LSTM method and compare the spectral efficiency of the system with what is maximally achievable with complete up-to-date CSI. Furthermore, we extend our machine learning methodology to directly estimate precoding matrices from partial CSI and similarly compare the performance with that achievable using complete up-to-date CSI.

In ambient backscatter communications, backscatter devices (BDs) utilize ambient “legacy” radio signals as both a harvested energy source and a carrier on which to modulate data. Symbiotic radio, a subtype in which the legacy system assists both its own users and underlaid BDs, has attracted much research interest recently. Furthermore, cell-free massive multiple-input multiple-output (CF mMIMO) systems are promising for beyond-5G networks from both spectral and energy efficiency perspectives. To reap the benefits of both, we consider a primary CF mMIMO system where the access points (APs) aid an underlaid BD layer to both harvest energy and reflect information toward the primary receivers (PRs) (which receive data from both layers). To acquire separate channel state information (CSI) of the direct and backscattered channels at the APs, a two-phase uplink pilot training method is proposed, with the effects of pilot contamination and spatial correlation between antennas accounted for. However, the receivers are assumed to only have partial CSI (statistical knowledge plus instantaneous phase information for partially coherent reception). Assuming uplink/downlink radio channel reciprocity, the CSI is used to design downlink precoding vectors for the APs such that channel hardening is enhanced and both PRs and BDs benefit. We derive expressions for the average signal-to-interference-plus-noise ratios of both primary and backscatter signals, accounting for the effects of imperfect CSI, spatial correlation, pilot contamination, and channel hardening. Furthermore, the average power harvested in the BDs is derived. Simulation results demonstrate that the performance of the proposed scheme is much more uniform across all devices with CF mMIMO than with conventional co-located mMIMO. The use of CF mMIMO also largely removes the need for special consideration of the BD layer in the symbiotic system, which is not the case with co-located mMIMO.

For implementation of massive multiple-input multiple-output (MIMO) cellular systems in frequency division duplex (FDD) mode, accurate estimation of downlink channel state information (CSI) is necessary, but full radio channel reciprocity between the uplink and downlink does not exist in that mode. Existing work on estimating downlink CSI in FDD massive MIMO systems has considered such approaches as angle-of-arrival reciprocity, compressive sensing, using second-order channel statistics (particularly the channel covariance matrix (CCM)), and machine learning using deep neural networks (DNNs). Typical DNN-based approaches are unsuitable for this problem because DNNs require large datasets, thousands of training epochs, and are susceptible to environmental variations. To overcome these shortcomings, we develop a conditional generative adversarial network (CGAN) approach to uplink-to-downlink mapping of both CCMs and CSI. To apply this method, we convert the uplink and downlink CCMs/CSI to images and employ CGAN techniques previously applied to image translation. The normalized mean square error performance of the proposed CGAN is evaluated for several array sizes for both CCM and CSI mapping. For uplink-to-downlink CSI mapping, we also examine the spectral efficiency performance of our CGAN-based method, as well as the impact of pilot reuse; both simulated and measured CSI data are considered. Our results demonstrate performance improvement over existing algorithms.

Conventional massive multiple-input multiple-output (MIMO) systems provide high spectral efficiency, throughput, and energy efficiency, but suffer from high inter-cell interference and poor cell-edge coverage. Cell-free massive MIMO addresses these shortcomings by geographically distributing access points (APs), each with one or more antennas, that form a virtual massive MIMO array instead of co-locating all antennas at a base station. This distributed AP placement significantly reduces the average distance between a user equipment (UE) and an AP. Existing work mainly considers the ideal canonical case where each UE is served by all APs, which is impractical because of limited fronthaul capacity and finite computational resources. Therefore, to make the network scalable to an arbitrary size, a user-centric approach should be adopted, where each UE is served by a personalized cluster of nearby APs. However, the clustering problem is combinatorially complex and may be too time consuming to solve optimally when the UEs are in motion. Therefore, in this work, we develop a multi-agent reinforcement learning (MARL) algorithm for AP selection and clustering, where each AP is an agent in the MARL algorithm and trained to near-optimally select for itself which UEs to serve. Simulation results demonstrate our MARL algorithm outperforms typical AP selection algorithms such as greedy selection as well as more sophisticated ones from the literature, and also provides performance comparable to the ideal canonical case.

Estimating or predicting the downlink channel state information (CSI) is extremely important for practical implementation of frequency division duplex (FDD) massive MIMO. Estimation of downlink CSI from uplink CSI using second order channel statistics, namely the channel covariance matrix (CCM), is a promising approach. However, published work so far has rarely applied machine learning techniques to solve this problem using CCMs, most probably due to the unavailability of a direct mapping function or parametric model for supervised learning to convert from uplink to downlink CCMs. In this paper, we develop a conditional generative adversarial network (CGAN) method for uplink-to-downlink CCM conversion. To apply the CGAN-based method, we convert the uplink and downlink CCMs to images and use image translation techniques for CGANs. The normalized mean square error performance of the proposed CGAN is evaluated for several antenna array sizes and with both perfect and imperfect knowledge of the CCMs. Our results demonstrate performance improvement over existing algorithms.

Most current stochastic geometric modeling of heterogeneous cellular networks (HetNets) assumes independent deployment of small-cell base stations (SBSs) with respect to macro base stations (MBSs), which leads to limited enhancement in network coverage and capacity. Therefore, in this paper we propose a new HetNet model where the locations of SBSs are correlated with those of the MBSs. We place the SBSs at the vertices of each macrocell, where the macrocells are modeled by a Poisson-Voronoi tesellation with the MBSs as seeds. Theoretical analysis of this deployment scheme is studied using the tools of stochastic geometry. A novel distribution is also derived for the distance between the typical user and its closest SBS. One significant advantage of this deployment scheme is that any given SBS can serve users from multiple macrocells, thus offloading the traffic from the MBSs and also improving network coverage. The performance of the proposed model is evaluated for several network parameters and our results concretely demonstrate the improvement in coverage probability compared to other schemes in the literature.

Millimeter wave (mmWave) communication systems are considered to be a promising technology enabling gigabit-per-second data rates, as the mmWave band offers large transmission bandwidth. However, precoding in mmWave systems cannot be performed in digital domain, due to the large number of radio frequency chains required. Deploying a hybrid precoding transceiver architecture, where mmWave precoding is divided among digital and analog processing domains, is an attractive and cost-efficient alternative. Yet, the design of near-optimal hybrid precoders is a non-trivial optimization problem. In this paper, we consider transmit precoding and receive combining in mmWave systems with large antenna arrays in a partially-connected structure, then we make use of projection algorithms to greatly simplify the design problem of digital baseband and analog radio frequency precoders into two sub-optimization problems whose optimal solutions can be found. We also develop a channel estimation algorithm to estimate mmWave channel parameters via a codebook of beamforming vectors obtained through the discrete Fourier transform design. The results illustrate that the system using the proposed projection hybrid precoding and channel estimation algorithms approaches the spectral efficiency achievable when perfect channel knowledge is available.

Massive multiple‐input multiple‐output (MIMO) transmission/reception is a very promising enablingtechnique for future cellular systems. The performance of massive MIMO systemsrelies on the availability of channel state information (CSI) at thetransmitter. However, due to estimation errors and delay this CSI is imperfect. Additionally, the use of many radio frequency (RF) chains to drive a largenumber of antennas at the transmitter quickly becomes impractical when thatnumber increases. Thus, reducing the number of RF chains in massive MIMO systemsis essential in order to reduce the system complexity and cost. Considering amassive MIMO system with a single‐RF‐chain transmitter, in this study, theauthors design a precoding technique that is robust to the channel uncertainty. To reflect realistic restrictions in the authors' design, they consider the peaktotal transmitted power rather than the average power constraint. Also, theyconsider imperfect CSI and model the uncertainty region as a bounded one, whichis a reasonable assumption. In this transmitter structure, there is only onepower amplifier and load modulation rather than voltage modulation is used togenerate the desired signals on the antenna elements. They demonstrate that whena very simple fixed equaliser is used at all user terminals, the problem ofminimising the mean‐square error of the received signals at user terminals underthe worst‐case channel uncertainty can be transformed into a convex optimisationproblem. They provide simulation results and demonstrate that the proposedrobust precoding technique outperforms non‐robust techniques in terms of powerefficiency and signal‐to‐interference‐plus‐noise ratios.

Power control for uplink transmission in a randomly laid-out cellular network operating over an environment with path loss and composite Rayleigh-lognormal shadowing is investigated. Each mobile station (MS) adjusts its transmit power to completely remove shadowing and to partially invert the effect of path loss. Using stochastic geometry tools, we derive an approximate coverage probability expression and validate it via simulations. With the power control scheme considered, shadowing with lower standard deviation improves the coverage probability. Also the severity of shadowing of local and surrounding environments has the same effect on the coverage probability. It was also observed that at low signal-to-interference-plus-noise ratio (SINR) thresholds, complete compensation of shadowing and partial compensation of path loss improves coverage, while at high SINR thresholds inverting only the effect of shadowing gives a better coverage probability.

In this paper, a rotating clustering scheme to increase the average achievable rates for the downlink of a coordinated multicell multiple-input multiple-output (MIMO) system is proposed. The users' data signals are transmitted cooperatively via the base stations of a cluster within the sectorized multicell heterogeneous cellular network. The performance of the system with the proposed clustering methods using a simulated annealing algorithm as the user scheduler is investigated. The simulations demonstrate the effectiveness of the proposed methods in conjunction with the throughput maximization and proportionally fair scheduling metrics.

We propose a novel base stations (BSs) - mobile stations (MSs) association policy for cellular networks. In this policy, the BS which provides the highest signal-to-interference ratio (SIR) among those located within a predetermined maximum association distance of the MS is selected as the serving BS. This policy encompasses the conventional highest-SIR association as a special case. Application of the new policy in 2- and 3- dimensional single-tier (homogeneous) and 2-dimensional two-tier (heterogeneous) networks is discussed. Coverage probability expressions are derived assuming BSs in each tier are distributed according to an independent homogeneous Poisson point process (PPP). Rayleigh fading and exponential path-loss radio channels are assumed. Analysis is validated by Monte-Carlo simulations. For single-tier networks, two methods are proposed for the selection of the maximum association distance. With such selection, the proposed association policy performs similarly to the highest-SIR association. It is shown that this policy can also be used to manage user offloading to small cells in two-tier heterogeneous networks.

In this paper we study a cellular network, in which multiple-antenna users communicate with multiple-antenna base stations (BSs) through fixed infrastructure-based multiple antenna relay stations (RSs). With cooperation among the BSs and linear processing at the RSs, we aim to find the optimal precoding matrices at the users and the beamforming matrices at the RSs that jointly maximize the system sum rate. Unlike for the conventional uplink (without relays), the sum-rate optimization is non-convex. More so, there is cross-coupling of the RSs' channels due to the forwarded interferences by the RSs. Firstly, we incorporate interference pre-cancelation into the RSs' beamforming designs. Secondly, we match each RS's beamforming matrix to the corresponding backward and forward channels such that the end-to-end channel is diagonalizable. Furthermore, we propose an iterative alternating minimization based algorithm to maximize the system sum rate. Finally, we consider two user-RS scheduling/mapping schemes namely the “random” and “channel-aware” schemes. Simulation results show that the channel-aware scheme outperform the random scheme with the performance gap unchanged with increasing number of antennas at the nodes.

Recently, studies on sub-optimal precoding techniques for multiple-input multiple-output broadcast channels (MIMO-BC), which achieve performance near to that of the dirty paper coding (DPC), have drawn attention to vector perturbation (VP). In practice, each antenna or more generally each antenna group has its own limit on the transmitted power, which makes per-antenna-group (per-AG) power constraints more meaningful than the sum power constraint. In this paper, we assume the per-antenna-group constraints and first investigate antenna-group power-constrained VP inspired by the p-sphere encoder for the case when channel inversion is used by the front-end linear precoder. Furthermore, we consider joint optimization of the front-end precoder and the perturbing vector subject to per-AG power constraints and propose precoding based on a minimum mean square error (MMSE) criterion. The results show that the proposed algorithm outperforms conventional VP and the p-sphere encoding in the case of per-AG power constraints.

In this work, we propose progressive user scheduling (PUS) and distributed successive zero-forcing (SZF) precoding for clustered coordinated multi-point transmission/reception (CoMP) systems. With joint transmission (JT) across a cluster of cells, centralized processing of precoding and user scheduling requires significant bandwidth resources for information exchange among coordinated base stations (BTSs) and the central controller, and channel state information (CSI) feedback from all users in the cluster to the central controller. In this work, we propose a greedy progressive user scheduling (PUS) and distributed SZF precoding to reduce CSI feedback and information exchange. With PUS, each BTS schedules and precodes its users in the coverage area of its cell separately from other BTSs in a cluster, but takes limited amount of feedback information available from other BTSs into account for user scheduling. Once the users are scheduled in that cell, BTS passes precoder information of the scheduled users to the next BTS. The second BTS schedules users in its cell taking the information from the first BTS into account. This process continues until the last BTS in the cluster schedules its users. With PUS, users in a cell do not completely estimate CSI from other cells in the cluster, which may significantly reduce the CSI feedback. Also, central controller is not necessarily required, but it may be utilized for limited amount of control signalling, e.g. an order in which the BTSs should schedule their users. Due to its distributed implementation, the proposed technique enables the use of various existing single-cell multiuser MIMO techniques that have been already standardized or proposed in the literature. In this work, we investigate the performance of greedy PUS and compare it with the greedy user selection for JT. It is shown that PUS can achieve sum rate close to the JT technique.

Vector perturbation is a sum-capacity approaching non-linear precoding technique. Vector perturbation uses sphere encoding to perturb data such that the unscaled sum power is minimized. Unfortunately, this minimization does not guarantee that the peak to average power ratio (PAPR) remains in the acceptable range for the transmitter. A p-sphere encoder was proposed in literature to reduce the PAPR by using p-norm minimization instead of the 2-norm one used typically in vector perturbation and its performance was analyzed in terms of bit error rate (BER). In this paper we focus on the spectral efficiency of the p-sphere encoding and investigate its sum rate for multiple-input multiple-output broadcast channel (MIMO-BC) with multiple-antenna users and uniformly distributed input. The results show that for p=5, the PAPR is reduced by 25%, while the sum rate is just slightly less than that of vector perturbation employing 2-norm sphere encoding (only 1% reduced sum rate).

We address joint design of robust precoder and equalizer in a MIMO communication system using the minimization of weighted sum of mean square errors. In addition to imperfect knowledge of channel state information, we also account for inaccurate awareness of interference plus noise covariance matrix and power shaping matrix. We follow the worst-case model for imperfect knowledge of these matrices. First, we derive the worst-case values of these matrices. Then, we transform the joint precoder and equalizer optimization problem into a convex scalar optimization problem. Further, the solution to this problem will be simplified to a depressed quartic equation, the closed-form expressions for roots of which are known. Finally, we propose an iterative algorithm to obtain the worst-case robust transceivers.

A cellular multiple-input multiple-output (MIMO) downlink system is studied in which each base station (BS) transmits to some of the users, so that each user receives its intended signal from a subset of the BSs. This scenario is referred to as network MIMO with partial cooperation, since only a subset of the BSs are able to coordinate their transmission towards any user. The focus of this paper is on the optimization of linear beamforming strategies at the BSs and at the users for network MIMO with partial cooperation. Individual power constraints at the BSs are enforced. It is first shown that the system is equivalent to a MIMO interference channel with generalized linear constraints (MIMO-IFC-GC). The problem of minimizing the weighted sum mean square error (WSMSE) of the data estimates is non-convex, and suboptimal solutions with reasonable complexity need to be devised. Novel designs that aim at minimizing the WSMSE are then proposed. Extensive numerical simulations are provided to compare the performance of the considered schemes for realistic cellular systems.

In this paper, we apply vector perturbation (VP) precoding to a network multiple-input multiple-output (MIMO) scheme, in which downlink transmissions from base stations are coordinated. We propose a multi-cell VP by introducing a common power scaling factor for all base stations in order to satisfy per base station power constraint. In our scenario, we consider multiple-antenna users with heterogeneous signal-to-noise ratios (SNRs). Our work is an extension of earlier work on VP to a multi-cell network with multiple-antenna users. The sum rate for the multi-cell VP in the case of uniformly distributed input is obtained and an asymptotic upper bound for it is proposed. The results show that the multi-cell VP is superior to the multi-cell block diagonalization (BD). By using the upper bound on the sum rate, we propose a user scheduling algorithm, which provides better performance and is less complex than semi-orthogonal user selection in the case of multiple-antenna users (SUS-MA).

We consider a coordinated multicell multiple-antenna downlink transmission in a cellular network also known as \textit{network MIMO}, which is expected to play a key role in future broadband cellular systems. While clustered network MIMO is a practical scheme to benefit from multicell cooperation, it still suffers from inter-cluster (uncoordinated) interference especially for the cell-edge users. In this work, a multicell network scheduling scheme is proposed to minimize the uncoordinated interference, which introduces overlapping clusters. It also offers a novel model to study network MIMO systems. The block diagonalization (BD) scheme is revised so that it can be employed as a coordinated transmission method. Further, a subgradient iterative algorithm is derived to determine the precoder matrices for BD scheme. The proposed multicell network scheduling is shown to outperform that in the conventional clustered network MIMO under the per-cell power constraints.

In this paper, a new reference symbol mapping scheme is proposed to improve downlink channel estimation performance over the standard reference symbol mapping scheme defined in the 3GPP LTE standard. The proposed scheme is based on the concept of guarding reference symbols (i.e., pilot symbols) with null sub-carriers. This helps mitigate the effect of inter-carrier interference (ICI) at pilot sub-carriers. In addition, the proposed scheme allows the ICI gain parameters to be estimated via a simple estimator. To assess the relative merits of the proposed reference symbol mapping scheme over the standard scheme, we compare normalized mean square error and uncoded bit error rate performances of the two schemes under different mobile user speeds. These comparisons show excellent performance improvements achieved by the proposed scheme over the standard one at high mobile user speeds.

In this paper we consider simplified greedy user scheduling algorithms for clustered network multiuser multiple-input multiple-output (MIMO) systems with successive zero-forcing (SZF) precoding. The optimal user scheduling involves an exhaustive search, which is very complex. Among various suboptimal but lower complexity algorithms, greedy algorithms with heuristic scheduling metrics have been shown to achieve performance close to the exhaustive search. In this paper, we propose two simplified algorithms: interference-aware greedy user scheduling (IA-GUS) and interference-whitening greedy user scheduling (IW-GUS). IA-GUS schedules the users based on the information on interference power of the selected users in surrounding cooperating clusters, and IW-GUS schedules the users based on the information on whitened channels of all users requesting the service in the cluster. IA-GUS has lower complexity and performs better than IW- GUS. Simplified metrics for proportionally fair (PF) scheduling are also proposed. Performance of the proposed algorithms is analyzed and compared with dirty paper coding (DPC), block diagonalization (BD) and systems without network coordination. Simulation results demonstrate that the proposed algorithms for SZF achieve much higher outage capacity compared to BD with previously proposed algorithms.

The authors investigate hybrid automatic repeat request (H-ARQ) schemes for spatially multiplexed multiple-input multiple-output (MIMO) systems with channel state information available only at the receiver. In particular, the authors compare the multiple H-ARQ scheme and the single H-ARQ scheme with repetition. The authors first propose a system model for symbol detection for the multiple H-ARQ processes, and then discuss joint and separate detection algorithms for both multiple H-ARQ and single H-ARQ. Simulation results show that with linear detection the single H-ARQ outperforms multiple H-ARQ in the high signal-to-noise ratio region. With the vertical Bell Labs space-time (V-BLAST) architecture, multiple H-ARQ always outperforms single H-ARQ. Additionally, joint detection always outperforms separate detection.

In this paper we investigate optimised power allocation over two-hop multiple-input multiple-output (MIMO) fixed multiple relays for a given power budget. Optimum beamforming weights under the total sum power constraint for all relays, as well as maximum per-relay power constraint, are found to maximise the received SNR at destination. Results show that optimising the allocation of power improves system performance, especially foe highly unbalanced links. The system with optimised power allocation can outperform a two-hop multiple relay system using uniform power allocation and distributed beamforming at the expense of increased computational complexity. We also study the threshold decode-and-forward fixed relay network with beamforming, which is more reliable than conventional decode-and-forward relaying. The impact of multiple antennas on the outage probability of cooperating fixed relays is considered. It is determined that increasing the number of relays and antennas at each relay increases capacity. The outage probability of threshold maximal-ratio combining and threshold selection combining for multiple-antenna multiple fixed relays is also derived. It is observed that the performance of the relay network with selection combining is close to that of the network with maximal-ratio combining, but the former is less complex and less expensive to implement.

In this paper we propose a simplified user scheduling algorithm for the downlink of multiuser multiple antenna systems with limited feedback channel state information available at the transmitter (CSIT) and block diagonalization (BD) precoding. In similar work, the perfect CSIT for all users is generally assumed, but it is normally not available. Optimal user scheduling involves exhaustive search, which becomes very complex for realistic numbers of users and transmit antennas. We employ existing vector quantization algorithms to obtain quantized feedback of CSIT. A simplified heuristic user scheduling metric is proposed, which is shown to achieve performance close to that of the exhaustive search method. Further simplification of the greedy scheduling algorithm is obtained with an intermediate user grouping technique. A user-side single antenna/mode selection technique in conjunction with the proposed user scheduling algorithm is also proposed. The proposed algorithm is of low complexity, but provides performance close to a highly complex exhaustive search.

Multi-hop relaying will play a central role in next generation wireless systems. In this paper a novel relaying strategy that uses multiple-input multiple-output (MIMO) relays in a two-hop wireless network supporting multiuser transmission is proposed. The fixed relays linearly process the received signal, decode and forward it to multiple users. This relaying strategy employs MIMO spatial multiplexing to achieve high spectral efficiency and improve link capacity in cellular networks. In this paper the case when source (base station) and all the relays employ multiple antennas and each user has only one antenna is studied. New lower and upper bounds on the achievable sum rate for this architecture are derived. Zero-forcing dirty paper coding (ZF-DPC) at the base station is assumed and the direct link between the base station and users is neglected. We propose to jointly design precoding at the base station and linear processing at the relays to improve throughput subject to power constraints at the source and relay transmitters. The impact of multiple relaying on the achievable sum rate lower bound is investigated. The proposed lower bound improves on earlier sum rate lower bounds that were derived for simpler cases of relaying.

Relaying has become a very active area of wireless research due to its ability to overcome wireless channel impairments. Wireless relaying, adopted in emerging standards such as 802.16j, can increase the data throughput, improve system coverage, and lower outage probability in wireless systems by reducing path loss, mitigating shadowing, and enabling spatial reuse. In cooperative relaying, numerous stations (either user stations or dedicated relay stations) create diverse paths in order to mitigate the effects of fading during the transmission of data from the source to destination. On the other hand, multihop relaying (non-cooperative) employs intermediate stations in order to relay around obstacles, mitigating the effects of shadowing and fading due to multipath scattering. In this paper, we look at the relative performance of these techniques in a Ricean channel, in which the path loss and Rice factor are modeled as functions of distance. In order to obtain results that are relevant to potential actual deployments, we use realistic system parameters and a system model appropriate on dual Rayleigh/Ricean (NLOS/LOS) channels.

In this paper, we consider a multiuser Gaussian broadcast channel where the base station and the users both are equipped with multiple antennas. The problem of optimal zero-forcing (ZF) precoding design subject to sum power constraint is discussed. Block diagonalization (BD) is applied as the ZF precoding method. We consider two common optimization criteria: maximum throughput and maximal fairness, where by maximizing fairness we mean maximizing the minimum rate among all users. Under these criteria, the structure of the optimal BD precoders is presented and analyzed. It is shown that the conventional BD precoding design with sum power constraint is indeed optimal under the throughput and fairness criteria.

In this paper, we discuss channel adaptation techniques in linear multi-user multiple-input multiple-output (MUMIMO) systems with imperfect Channel State Information (CSI) caused by limited feedback bandwidth and feedback channel delay. We assume that such systems will operate in environments, where channel characteristics constantly change due to the movement of mobile terminals. We consider an approach where the impact of delayed CSI is partially mitigated by channel prediction performed at the base station side. In this paper, we use a dictionary based predictor that operates on phrases consisting of indices of quantized channel state information. To address variations in channel characteristics, we propose an algorithm that adapts the prediction dictionary to the current channel state. We demonstrate the performance of our algorithms in broadcast MU-MIMO systems with and without CSI prediction operating on a channel with changing Doppler rates. The simulation results show that significant sum-rate gains are achievable for a practical range of feedback delays. The additional advantage of our proposed adaptive prediction algorithms is that they can be used with any type of CSI quantization implementations and contribute very little extra complexity to the base station.

In this paper, we consider efficient and low complexity scheduling algorithms for multiuser multiple-input multiple-output (MIMO) systems. Due to the dimensionality constraint imposed by linear precoding techniques like block diagonalization (BD), user scheduling is required. Optimal user scheduling involves an exhaustive search, which becomes very complex for realistic numbers of users and transmit antennas. Hence, various suboptimal but low complexity algorithms have been considered in the literature. Among them, greedy algorithms with heuristic scheduling metrics have been shown to achieve performance close to an exhaustive search. Meanwhile, genetic algorithms (GAs) are a rapid, though suboptimal, option of performing a utility (i.e. scheduling) metric optimization. In this paper, we propose and analyze the performance and complexity of greedy and genetic scheduling algorithms for multiuser MIMO systems with BD precoding. We demonstrate that except at low SNR with a smaller number of transmit antennas, the genetic algorithm outperforms the greedy algorithm. A detailed complexity analysis shows that the order of complexity of the genetic algorithm is higher than that of the greedy algorithm by a factor equal to Ko, where Ko denotes the maximum number of simultaneously supported multiple-antenna users.

In this paper we consider user scheduling problem for linearly preceded multiuser multiple-input multiple-output (MIMO) downlink, where base station as well as the mobile receivers are equipped with multiple antennas. Optimal precoding involves dirty paper coding (DPC) technique, and it is highly nonlinear and complex. On the other hand, complete inter-user interference cancellation using linear zero-forcing or block diagonalization precoding are suboptimal. Hence, we consider successive zero-forcing precoding, which achieves improved system throughput compared to block diagonalization by allowing users to work under limited interference. Due to the dimensionality constraint of linear precoding techniques user scheduling is required. The optimal user scheduling involves exhaustive search, which becomes very complex for realistic numbers of users and transmit antennas. In addition, for successive zero-forcing precoding the order in which users are precoded successively is important for sum rate maximization, which further increases the complexity of exhaustive search. In this paper we develop a low complexity greedy user scheduling algorithm for successive zero-forcing precoding, which incorporates various user ordering techniques. Simplified heuristic scheduling metrics are proposed, which are shown to perform close to the exhaustive search method. A suboptimal user ordering technique that is similar to the order, in which the proposed greedy user selection selects users, is proposed. Further simplification of regular greedy scheduling algorithm is obtained with the proposed intermediate user grouping technique. The proposed algorithm is of low complexity, but provides performance close to the highly complex exhaustive search algorithm.

Spatially multiplexed multicarrier code-division multiplexing (SM-MC-CDM) is a multiple-input multiple-output, orthogonal frequency division multiplexing (MIMO-OFDM) communication technique with multiple antennas used for spatial multiplexing and with frequency domain spreading on each antenna. Unified successive interference canceller (U-SIC) is an efficient detector recently introduced for SM-MC-CDM. This paper presents an analytical approach to the performance of zero-forcing (ZF) U-SIC for SM-MC-CDM communications. For a system with an equal number of transmit and receive antennas, an approximation for the probability density function of post-detection signal-to-noise ratio (SNR) is used to derive a closed-form analytical upper bound and approximations for the probability of error and ergodic capacity. It is shown that SM-MC-CDM with ZF U-SIC is able to achieve higher diversity order than that achieved by ZF and minimum mean squared error (MMSE) V-BLAST detectors used on each subcarrier of a MIMO-OFDM system with the same number of subcarriers. The diversity order obtained increases with the number of subcarriers. It is also shown that the ergodic capacity of the system decreases with increasing number of subcarriers.

This paper extends the work on density evolution for binary low-density parity-check (LDPC) codes with Gaussian approximation to LDPC codes over GF(q) . We first generalize the definition of channel symmetry for nonbinary inputs to include q-ary phase-shift keying (PSK) modulated channels for prime q and binary-modulated channels for q that is a power of 2. For the well-defined q-ary-input symmetric-output channel, we prove that under the Gaussian assumption, the density distribution for messages undergoing decoding is fully characterized by (q-1) quantities. Assuming uniform edge weights, we further show that the density of messages computed by the check node decoder (CND) is fully defined by a single number. We then present the approximate density evolution for regular and irregular LDPC codes, and show that the (q-1) -dimensional integration involved can be simplified using a dimensionality reduction algorithm for the important case of q=2p. Through application of approximate density evolution and linear programming, we optimize the degree distribution of LDPC codes over GF(3) and GF(4). The optimized irregular LDPC codes demonstrate performance close to the Shannon capacity for long codewords. We also design GF(q) codes for high-order modulation by using the idea of a channel adapter. We find that codes designed in this fashion outperform those optimized specifically for the binary additive white Gaussian noise (AWGN) channel for a short codewords and a spectral efficiency of 2 bits per channel use (b/cu).

Two very important and active areas of wireless research are multi-hop relaying and multiple antenna techniques. Wireless multi-hop relaying, adopted in emerging standards such as 802.16j, can increase the data throughput and improve coverage of cellular systems by reducing path loss, mitigating shadowing, and enabling spatial reuse. On the other hand, multiple antenna techniques can take advantage of scattering in the wireless channel to achieve higher capacity. In this paper, we look at the combination of these two techniques in a cellular system, and show that they exhibit a fundamental tradeoff. In order to obtain results that are relevant to potential actual deployments, we use realistic system parameters and a system model appropriate for MIMO operating on dual Rayleigh/Ricean channels.

This paper investigates the zero-forcing (ZF) beamforming transmit strategy in the multiple-antenna multiuser downlink systems. We consider the case of mobile users equipped with multiple antennas. Although the capacity of such systems can be achieved by dirty paper coding (DPC), DPC is extremely difficult and challenging to implement. Thus, simple but suboptimal linear beamforming techniques like ZF beamforming can be deployed. However, the number of users that can be served using this strategy is limited by the number of transmit antennas at the base station. The solution for this limitation is user selection (scheduling), which also exploits multiuser diversity. Therefore, user selection can be used to enhance the throughput of the system. Recently, it has been shown that ZF beamforming strategy with user selection is asymptotically optimal for a large number of users. In this paper, a semi-orthogonal user selection (SUS) algorithm is extended to the system with multiple-antenna mobile users. This algorithm aims to select users, which are semi-orthogonal. The optimal ZF beamforming matrices are obtained and it is shown that the optimal ZF beamforming throughput is related to the eigenvalues of the user channels. Therefore, SUS is aimed to select the proper group of users based on maximizing the channel eigenvalues and therefore improving the optimal ZF beamforming throughput of the system.

In multiple-input multiple-output (MIMO) broadcast channels, multiuser diversity is exploited by scheduling data transmission to users with best channel conditions. To find the best set of users, the base station requires knowledge of user channels, which for the case of non-reciprocal uplink and downlink channels may lead to a very heavy feedback overhead. In particular, well known transmission schemes for asymmetric MIMO downlink channels with more transmit than per user receive antennas, such as beamforming, require availability of full channel state information (CSI) at the transmitter. In this paper, two multiuser MIMO downlink techniques are presented, which only require partial CSI at the base station. The proposed schemes represent a combination of zero-forcing (ZF) receiver processing with MIMO point-to-point eigenmode transmission and transmit antenna selection. The results show that the scheme which is able to suppress inter-stream interference while maintaining maximum spatial multiplexing and high multiuser diversity gain, achieves higher system throughput.

In multiple-input multiple-output (MIMO) broadcast channels, multiuser diversity is exploited by scheduling data transmission to users with best channel conditions. To find the best set of users, the base station requires knowledge of user channels, which for the case of non-reciprocal uplink and downlink channels will lead to a great increase in feedback overhead. On the other hand, scheduling based on partial channel state information (CSI) often results in a great loss in system throughput. In this paper, a multiuser MIMO technique is presented for MIMO broadcast channels which only requires partial CSI at the base station and achieves a relatively high system throughput. The proposed scheme is a combination of MIMO point-to-point eigenmode transmission with zero-forcing (ZF) zero-forcingat the receivers. Spatial multiplexing is considered and the optimum number of data streams assigned to each of these two schemes in order to maximize the sum-rate is derived. The results show that with a negligible increase in feedback overhead compared to the case where only ZF linear receiver processing is adopted, the proposed scheme leads to a significant increase in the sum-rate.

A major hindrance to the adoption of orthogonalized space-division multiplexing (SDM) via block diagonalization (BD) in multi-user MIMO downlinks is the need for channel state information (CSI) feedback from user terminals. Another drawback is lower than optimal sum rates, theoretically achievable with dirty paper coding (DPC). While multi-user diversity could be leveraged via user selection to narrow the sum- rate performance gap, it requires the presence of very large user pools. To help raise the practical feasibility of BD-SDM, we propose a scheme that jointly reduces CSI feedback while approaching optimal DPC sum rates with smaller user-pool sizes. Additionally, BD-SDM offers the flexibility for spatial mode allocation to cater for individual transmission rate requirements. This presents a challenging resource allocation problem because mode selection at one terminal affects the rates achieved at all other terminals and in turn, the overall sum rate. The proposed scheme offers a systematic means for resource allocation, while minimizing rate loss at the overall- and individual levels. It represents a streamlined process that simultaneously reduces CSI feedback while achieving sum rate maximization, user selection and systematic rate-loss minimizing resource allocation.

In this paper we study the threshold decode and forward (T-DF) fixed relays network with beamforming, which is more reliable than conventional decode and forward (DF) relaying. Deployment of a small number of antennas on fixed relays is easier than on mobile terminals, therefore, the impact of multiple antennas on the outage probability of cooperating fixed relays is considered. Under the total sum power constraint (TSPC) of all relays and also maximum per-relay power constraint (PRPC), the optimum beamforming weights have been found to maximize the received SNR at the destination. It is determined that increasing the number of relays and antennas at each relay increases capacity. The performance of threshold-maximal ratio combining (T-MRC) and threshold-selection combining (T-SC) of the multiple-antenna multiple fixed relays with beamforming is derived. It is observed that the performance of the network with selection combining (SC) configuration is close to the network in which maximal ratio combining (MRC) is used, in addition that it is less complex and less expensive to implement. The outage probability in Rayleigh fading channels is also analyzed.

We consider cooperative downlink transmission in multiuser, multi-cell and multiple-antenna cellular networks. It has been shown that multi-base coordinated transmission has significant spectral efficiency gains over that without coordination. The capacity limits can be achieved using a non-linear preceding technique known as dirty paper coding, which is still infeasible to implement in practice. This motivates investigation of a simpler linear preceding technique based on generalized zero-forcing known as block diagonalization (BD). In this paper, an enhanced form of BD is proposed for multiple-input multiple-output (MIMO) multi-base coordinated network. It involves optimizing the precoding over the entire null space of other users' transmissions. The performance limits of the multiple-antenna downlink with multi-base coordination are studied using duality of MIMO broadcast channels (BC) and MIMO multiple-access channels (MAC) under per-antenna power constraint, which has been established recently.

Frequency offsets negatively impact the performance of closed-loop multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems. Particularly, when multiple users are active, the impact can be high. Linear precoding and non-linear Tomlinson-Harashima precoding (THP) are thus developed for spatially-multiplexed multiuser OFDM and orthogonal space-time block-coded (OSTBC) OFDM. The proposed precoders employ a limited feedback structure, which is implemented with a shared codebook of precoding matrices, and only the index of the selected optimal matrix is fed back to the transmitter. The conventional limited feedback design criterion for flat-fading MIMO channels is only applicable to single-user OFDM without frequency offsets. We show that the ICI matrix due to frequency offset does not impact users precoding individually, and precoding on a per-subcarrier basis is possible. Exploiting this property, the conventional design is generalized to multiuser OFDM with frequency offsets. Nonlinear precoding uses a modulo arithmetic precoding matrix (which reduces the power efficiency loss inherent in linear precoding and leads to a lower error rate) and outperforms linear precoding. Our precoders not only offer significant bit error rate (BER) improvement for spatially-multiplexed multiuser MIMO OFDM with frequency offsets, but are equally effective for both OSTBC MIMO OFDM and spatially correlated channels.

This work presents precoding design for error-rate improvement in closed-loop multiuser orthogonal space-time block-coded (OSTBC) multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) downlink, where both mean feedback and covariance feedback are available. We derive adaptive linear precoding and nonlinear Tomlinson-Harashima precoding (THP) over a transmit-antenna-correlated, frequency-selective fading MIMO channel with estimation errors and feedback delay. In our precoder, mean-feedback precoding or covariance-feedback precoding, is adaptively chosen at the user terminal. The maximum achievable signal-to-noise power ratio (SNR) is used as the precoding-mode selection criterion. Each user calculates the selection metric and decides whether mean feedback is necessary. We confirm the intuition that mean-feedback precoding offers BER gains over covariance-feedback precoding when mean feedback becomes sufficiently accurate. Our adaptive precoding outperforms either mean-feedback precoding or covariance-feedback precoding in multiuser OSTBC OFDM, and considerably reduces the bit error rate (BER). Non-linear adaptive precoding is shown to outperform linear adaptive precoding.

MIMO systems are of interest to meet the expected demands for higher data rates and lower delays in future wireless systems. The introduction of multiple transmit antennas adds additional complexity to any scheduling algorithm for the multi-user system. It is optimal to transmit to multiple users simultaneously in contrast to a single user in a single-input single-output (SISO) system, resulting in a combinatorial optimization problem. In this paper, we analyze the performance of scheduling through utility functions implemented via a genetic algorithm. Namely, we investigate the maximum throughput and the proportionally fair utility functions. The analysis is in the context of a MIMO broadcast channel using dirty paper coding (DPC). This paper builds upon earlier work using zero-forcing beamforming instead of DPC. Under DPC, the order of user encoding affects the user data rates and hence the performance of the scheduling algorithm. We demonstrate that the genetic algorithm is able to achieve a near-optimal performance relative to an exhaustive search at a significant reduction in computational complexity.

In this paper, we discuss the influence of feedback channel errors on throughput of linear multi-user multiple-input multiple-output (MIMO) systems employing vector quantization (VQ) algorithms for encoding channel state information (CSI). We consider an approach where the receiver estimates the downlink channel and chooses one of the predefined channel characterization codewords whose indices are transmitted back to the transmitter. In practice, the feedback channel will introduce errors in transmission of the indices and we evaluate the resulting channel capacity loss for different system setups. We demonstrate the performance of two types of systems, one using time division multiplexing of individual users and another transmitting to multiple users. We evaluate two basic error mitigation approaches: the first one with user expurgation using error-detection codes and the second one, based on specially designed VQ indexing. We show that the proposed schemes are very robust to even relatively high bit error rates in the feedback link and that proper VQ indexing may suffice for actual system design, even if the feedback channel is not very reliable.

In this paper the authors investigate the hybrid automatic repeat request (H-ARQ) schemes for spatial multiplexing (SM) multiple-input multiple-output (MIMO) systems with channel state information available only at the receiver (CSIR). In particular, the authors compare the multiple H-ARQ scheme and the single H-ARQ scheme with repetition. The authors first propose a system model for symbol detection for the multiple H-ARQ processes, and then discuss joint detection algorithms and separate detection algorithms for both multiple H-ARQ and single H-ARQ. Simulation results show that with linear detection the single H-ARQ outperforms multiple H-ARQ in the high signal to noise ratio (SNR) region. With the vertical Bell Labs space time (V-BLAST) architecture, multiple H-ARQ always outperforms single H-ARQ and joint detection always outperforms separate detection.

This paper considers precoding for closed-loop multiuser orthogonal space-time block-coded (OSTBC) orthogonal frequency-division multiplexing (OFDM) downlink over multiple-input multiple-output (MIMO) channels. The authors present a general transmit-antenna-correlated, frequency-selective fading MIMO channel model with imperfect channel estimates and feedback delay, and derive the conditional means of the channel response. Exploiting the channel statistics, the authors develop new linear precoding and non-linear Tomlinson-Harashima precoding (THP) to maximize the signal-to-noise power ratio (SNR). By considering the conditional mean as the equivalent channel matrix, our precoding takes into account the estimation errors and channel time variations over feedback delay. The authors confirm the intuition that when channel state information at the transmitter (CSIT) becomes accurate, the full-CSIT precoder outperforms the covariance-feedback precoder. The proposed precoder has a bit error rate (BER) gain over unprecoded systems. As well, non-linear THP is shown to outperform linear precoding.

In this paper, we discuss the influence of feedback channel delay on throughput of linear multi-user multiple-input multiple-output (MIMO) systems employing vector quantization (VQ) algorithms for encoding channel state information (CSI). We consider an approach where the mobile receiver estimates the downlink channel and chooses one of the predefined channel characterization codewords whose indices are transmitted back to the base station transmitter. In practice, the feedback channel introduces delay in transmission of the indices and we evaluate the resulting channel throughput loss for different system setups. Moreover, we define two new parameters of VQ MIMO systems: the channel eigenmode coherence time and singular value coherence time, and discuss their influence on system design. We demonstrate the performance of two types of systems, one using time division multiplexing of individual mobile users and another transmitting to multiple users at the same time. The simulation results show that feedback delay sensitivity of the multiple-user system throughput is much higher than in the TDM approach. Based on those results we propose a simple method allowing a selection of the VQ resolution for the given channel characteristics and system setup.

In this paper, we study simplified algorithms for user scheduling in conjunction with receive antenna selection (RAS) for two downlink multiuser orthogonal space-division multiplexing techniques: block diagonalization (BD) and successive optimization (SO). Proposed algorithms select user and antenna subsets that maximize total power gain of the equivalent channel through mutual null-space projections of the active user channels. With this approach, algorithms add the best user at a time from the set of users not selected yet to the set of selected users until the desired number of users have been selected. To reduce the complexity further, users whose channel correlation to the signal subspace of the previously selected users is under some specified threshold are grouped at each step of the algorithm such that next user is selected from that group. This avoids the search through all K - i remaining users at the ith step of the algorithm. Two RAS algorithms are proposed, which further enhance the power gain of the equivalent channel by selecting a subset that contributes most towards the sum rate. For SO, user selection accounts for the interference of the previously selected users and maximizes the ratio of the total power of the equivalent channel to the approximate interference power on the desired signal subspace. Proposed algorithms perform close to the previously proposed algorithms with much less complexity.

A list decoder generates a list of more than one codeword candidates, and decoding is erroneous if the transmitted codeword is not included in the list. This decoding strategy can be implemented in a system that employs an inner error correcting code and an outer error detecting code that is used to choose the correct codeword from the list. Probability of codeword error analysis for a linear block code with list decoding is typically based on the “worst case” lower bound on the effective weights of codewords for list decoding evaluated from the weight enumerating function of the code.

We propose a two-stage precoder/equalizer to suppress intercarrier interference (ICI) and multiuser interference (MUI) in downlink multiuser OFDM with multiple transmit antennas. The first stage, non-linear Tomlinson-Harashima preceding (THP) at the base station (BS) transmitter, mitigates the effect of the spatial inter-stream interference caused by transmission from multiple transmit antennas to decentralized users. In the second stage, each user's receiver employs low- complexity iterative linear minimum mean-square error (MMSE) equalization to suppress the ICI due to frequency offset. Our proposed technique virtually eliminates the bit error rate (BER) degradation due to normalized frequency offsets as high as 10%.

In this paper, a type-II H-ARQ scheme with incremental redundancy is proposed for a turbo coded system that uses the sub-block structure. Sub-block recovery is applied to the decoder and a method to dynamically select the bits for retransmission according to knowledge of the decoding status is proposed. The new scheme benefits from both the frequent termination technique and the built-in CRC of the constituent recursive systematic convolutional codes of the turbo encoder. Simulations show that for a turbo coded system that employs the sub-block structure, the new type-II H-ARQ scheme outperforms both type-I H-ARQ with the sub-block recovery and the traditional type-II H-ARQ scheme without sub-block recovery on both the AWGN channel and the flat Rayleigh fading channel.

Spatially multiplexed multi-carrier code division multiplexing (SM-MC-CDM) communications with turbo coding is considered over correlated and uncorrelated multiple input multiple output (MIMO) fading channels. Two link transmission strategies for SM-MC-CDM are proposed and their bit error rate performances are compared. The proposed system models combine successive interference cancellation and turbo decoding at the receiver end to mitigate the effect of interference and noise. By comparing the performance of the proposed systems with that of turbo coded MIMO-OFDM with VBLAST detector, we evaluate the effect of frequency domain spreading on the system performance.

This paper presents non-linear precoding design in closed-loop multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) over spatially- correlated, frequency-selective fading channels. Our analysis takes into consideration receiver channel mismatch due to imperfect channel estimates, and transmitter channel mismatch due to estimation errors, channel variations over feedback delay and feedback noise. We present a general spatially-correlated, frequency-selective fading MIMO channel model and derive the conditional means of the channel response. Exploiting the channel statistics, which are only available at the receiver, we design new non-linear zero-forcing (ZF) Tomlinson-Harashima precoding (THP) for uncoded MIMO OFDM. The channel statistics do not need to be sent back to the transmitter, which avoids the possible maximum-Doppler-shift transmitter mismatch. Our proposed precoders are robust against time variations, channel estimation errors and antenna correlations, and offer a significant system performance gain over conventional THP.

The path and antenna correlations significantly degrade the system capacity in spatially-multiplexed multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems and increase bit error rate (BER) in orthogonal space-time block coded (OSTBC) OFDM. We develop adaptive dual-mode precoding to improve system performance with transmit antenna and path correlations. Both linear and non-linear Tomlinson-Harashima (TH) precoders are considered. In our proposed approach, precoding for maximizing capacity in spatially-multiplexed OFDM or precoding for minimizing error rate in OSTBC OFDM, is adaptively chosen at the receiver, and only one-bit decision information per subcarrier needs to be sent back to the transmitter. The precoding mode selection depends on the channel conditions. To determine the precoding matrix only the statistical knowledge of the channel is needed at the transmitter, which significantly reduces the feedback requirements. In spatially correlated channels, proposed adaptive dual-mode precoding individually outperforms either capacity- maximization precoding in spatially-multiplexed OFDM or error-rate-minimization precoding in OSTBC OFDM.

We demonstrate that receive antenna selection (RAS) provides significant increase in the achievable sum rates for multi-user MIMO wireless downlinks that employ block diagonalization (BD) to achieve orthogonal space division multiplexing (OSDM), where each user terminal has one or more antennas. Although dropping one or more receive antennas at a user terminal reduces its capacity and correspondingly, the system sum capacity, judicious RAS improves the projected channel spatial mode gains and provides additional degrees of freedom to all other terminals within the BD-OSDM context. In this way there is mutual benefit to be shared among users when RAS is applied to all users and numerical results show significant sum rate gains despite sum capacity loss due to RAS. In many cases, users with reduced array sizes also enjoy increased channel rates. When projected virtual channels are used as a means of spatial mode allocation, this RAS concept is also beneficial and may be referred to as spatial mode selection (SMS). RAS/SMS is therefore a necessary first step in any resource allocation and power control exercise for BD-OSDM. Further, the same RAS/SMS algorithms for sum rate maximization also provide a systematic means of resource allocation and power control. To avoid exhaustive RAS search, which has exponential complexity, efficient RAS algorithms with linear complexity and near optimal performance are proposed.

A novel nonlinear joint transmitter-receiver processing algorithm is proposed for the downlink of multi-user MIMO systems with multiple-antenna mobiles. In this algorithm, linear receiver processing is applied at each mobile, while nonlinear pre-processing is applied at the base station. Using the zero-forcing (ZF) criterion, the transmitter and receiver processing matrices are designed jointly to optimize the performance of each mobile. When used on the downlink of multi-user MIMO systems with multiple-antenna mobiles, this algorithm achieves significantly better performance than the ZF criterion based pre-processing and joint transmitter-receiver processing algorithms known in the literature, because it successfully constrains the transmitted power increase and effectively utilizes the processing capabilities of the mobiles. For the proposed algorithm, it is found that better system performance can be achieved by suitably ordering the channel matrices of different mobiles, and a combined optimal diversity "best-first" (CODBF) ordering method is proposed to perform the ordering.

In this paper, we compare two selective-repeat automatic-repeat-request (SR-ARQ) protocols for spatial multiplexintiplexingg multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexed (OFDM) systems assuming perfect channel state information (CSI) only at the receiver. These two SR-ARQ protocols differ in the number of packets transmitted simultaneously. In one protocol, a single packet is transmitted on all subcarriers from all antennae at the same time. In the other protocol, multiple packets are simultaneously transmitted. For the latter protocol, we also consider schemes that differ in the way that the subcarriers transmitted from different antennae are grouped to support the transmission of multiple packets in parallel. We compare the throughput and the resequencing delay of these SR-ARQ protocols in a frequency selective fading channel. Simulation results suggest that in such a system, the single ARQ protocol is the best choice. If we have to transmit multiple packets in parallel, each packet should be transmitted over a subband of adjacent subcarriers emitted from all transmit antennae.

We propose a sub-optimum detection method for spatially multiplexed multicarrier code division multiplexing (SM-MC-CDM) communications. Spreading the data in frequency domain results in additional diversity gain while using multiple antennas at the transmitter and the receiver may result in capacity gain due to spatial multiplexing and/or further diversity gain. To take advantage of the diversity and the multiplexing offered by SM-MC-CDM, low complexity and efficient detectors have to be designed. In this paper, the performance of our proposed detector is compared with a number of sub-optimum detectors using simulations over uncorrelated and correlated frequency selective MIMO channels. The results demonstrate that the new algorithm outperforms existing methods of comparable complexity.

Orthogonal space-time block-coded (OSTBC) orthogonal frequency-division multiplexing (OFDM) links for frequency-selective multiple-input multiple-output (MIMO) channels with correlated paths and transmit antennas are considered. In such systems, optimal precoding with only covariance feedback is derived using the minimum pair-wise error probability (PEP) criterion; linear and non-linear precoders are designed. The proposed precoding only needs the statistical knowledge of the channel at the transmitter, which significantly reduces the feedback requirements. Both linear and non-linear precoders substantially improve the system bit error rate (BER) for OSTBC OFDM in transmit-antenna and path-correlated channels. The proposed non-linear precoder outperforms the linear precoder.

In this paper we look at the network throughput and efficiency of a cellular system employing multi-hop relaying. It is shown that multi-hop relaying can provide great improvement in the area averaged spectral efficiency (measured in bits/second/Hz per unit area of coverage) of a system. However, the number of relays introduced and the dimensions of coverage areas of relays must be carefully chosen for the specific propagation environment and system parameters, otherwise poorer efficiency may result. Some example designs for the 5 GHz unlicensed band are given to illustrate some design considerations. Realistic simulated throughput, considering physical and medium access control layer overhead, of a HiperMAN or 802.16 system together with noise and interference calculations are used to determine potential real-world performance of such systems. Calculations for one to four relay hops are presented and designs' area averaged spectral efficiencies are compared.

MIMO wireless downlinks using transmit zero-forcing beamforming (TZFBF) with MTtransmit antennas can serve up to K = MTreceivers, each equipped with one antenna. To maximize the sum rate, waterfilling can be used. It is shown that unlike classical waterfilling, TZFBF waterfilling cannot simply drop the poorer spatial modes during optimization. Instead, receive antenna selection (RAS) must be incorporated and achieving the maximum sum rate requires an exhaustive search over ∑|s|(MrC|s|) = 2MT−1 iterations to find the optimal subset S of active receivers where |s| = 1,..., MT. In principle, a separate RAS algorithm can be used in conjunction with waterfilling to reduce the exponential complexity O(2MT) of the exhaustive search to linear complexity O(MT). We develop optimization algorithms that emulate classical waterfilling by progressively reducing the effects of poor spatial modes in MTiterations. They do so by performing RAS jointly during the waterfilling process at little additional complexity. By avoiding a separate RAS process, complexity is thus further reduced. For the typical case where K>MT, we propose a 2-phase framework that helps reduce the overall complexity while meeting the TZFBF dimensional constraints. Numerical results over different channel conditions are given.

This paper focuses on signal processing algorithms for the downlink of multi-user multiple-input multiple-output (MIMO) systems with multiple antenna mobiles. A novel nonlinear joint transmitter-receiver processing algorithm is proposed. In this algorithm, linear receiver processing is applied at each mobile, while nonlinear Tomlinson-Harashima precoding (THP) is applied at the base station. Closed-form solution of the transmitter and receiver processing matrices is found based on the minimum mean-square error (MMSE) criterion. For this algorithm, it is found that better performance can be achieved by suitably ordering the channel matrices of different mobiles, and the "best-first" ordering method is proposed to perform the ordering. Simulation is used to show the advantages of the proposed algorithm and the "best-first" ordering method

We consider spatially multiplexed MIMO transmission in the presence of co-channel interference with subset transmit antenna selection. Several algorithms for antenna selection have been proposed in the literature for point-to- point MIMO systems. However, performance of such antenna selection techniques in interference-limited environments is less well understood. In this paper, we propose to use transmission techniques with antenna selection to minimize the effect of co-channel interference. We propose several simplified transmit antenna selection algorithms. V-BLAST detection is used and channel state information (CSI) is assumed to be known only at the receiver. A simple algorithm to adaptively obtain information about the number of transmitted streams and the best antenna subset that minimizes the symbol error rate is also proposed. Several examples to demonstrate the effectiveness of proposed selection algorithms in interference limited environments are presented. Simulation results show that for low to moderate interference power, significant improvement in the system performance is achievable with the use of transmit antenna selection algorithms. It is found that employing transmit antenna selection algorithms, and adaptation of the number of transmitted streams and the signal constellation sizes can significantly enhance the performance of MIMO systems with co-channel interference. The performance improvement is more significant in spatially correlated fading channels.

Irregular low-density parity-check (LDPC) codes have shown exceptionally good performance for single antenna systems over a wide class of channels. In this paper, we investigate their application to multiple antenna systems in flat Rayleigh fading channels. For small transmit arrays, we focus mainly on space-time coding with 2/sup p/-ary LDPC codes, where p equals the number of encoded bits transmitted by the transmit antenna array during each signaling interval. For large transmit arrays, we study a layered space-time architecture using binary LDPC codes as component codes of each layer: We show through simulation that, when applied to multiple antenna systems with high diversity order, LDPC codes of quasi-regular construction are able to achieve higher coding gain and/or diversity gain than previously proposed space-time trellis codes, space-time turbo codes, and convolutional codes in a number of fading conditions. Extending the work of density evolution with Gaussian approximation, we study 2/sup p/-ary LDPC codes on multiple antenna fading channels, and search for the optimum 2/sup p/-ary quasi-regular codes in quasi-static fading. We also show that on fast fading channels, 2/sup p/-ary irregular LDPC codes, though designed for static channels, have superior performance to nonbinary quasiregular codes and binary irregular codes specifically designed for fast fading channels.

In this paper, the concepts of effective weight enumerating function and generalized pairwise error event are introduced to predict the performance of linear block codes with list decoding. For the first time, a method to evaluate the actual effective codeword weights for a given code, rather than a lower bound on effective codeword weights, is proposed. Based on these actual effective codeword weights, the performance of a given code with list decoding can be predicted more accurately. We propose an analytical method to evaluate the performance of a given code with list decoding and consider terminated convolutional codes to show the validity of the analysis.

We consider preceding for intercarrier interference (ICI) reduction due to frequency offsets in closed-loop orthogonal frequency-division multiplexing (OFDM) systems. Since the complete channel state information (CSI) is difficult to obtain at the transmitter, we develop a novel non-linear limited-feedback Tomlinson-Harashima (LFB-TH) precoder to suppress ICI in closed-loop multiple-antenna OFDM. Our precoder applies a pre-designed codebook of matrices, which is available at both the transmitter and the receiver. The receiver selects the feedback matrix from the codebook according to a specific selection criterion and transfers the index of the chosen matrix to the transmitter. Hence the CSI is not required at the transmitter in the proposed precoder. The codebook design criterion and feedback matrix selection criteria are analyzed. Our LFB-TH precoder significantly reduces the bit error rate (BER) degradation due to frequency offset; the degradation is virtually eliminated for normalized frequency offsets as high as 10%.

In this paper, we consider a soft parallel decoding approach using probabilistic data association (PDA) algorithm for per-antenna-coded (PAC) transmission on multiple-input multiple-output (MIMO) channels. The proposed algorithm allows for decoding of layered signals in a variety of contexts: downlink and uplink in multi-user systems, single user transmission for both real-time and data traffic, systems with fewer receive than transmit antennas and with or without channel state information at the transmitter. We investigate performance of the proposed algorithm in turbo-coded MIMO systems using 16, 64, 256-QAM and spectral efficiencies ranging from 2 to 6 bpcu/layer. We present frame error rates for selected scenarios with the corresponding lower bounds on channel outage probabilities. We also briefly discuss the convergence properties of the PDA algorithm with turbo encoded signals.

In this paper, the Tomlinson-Harashima precoding (THP) algorithm is studied for use on the downlink of multiple antenna multi-user systems. Based on the minimum mean-square error (MMSE) criterion, a new THP solution is derived, which provides significant performance improvement as compared to the zero-forcing (ZF) THP discussed in the literature. Ordering of the rows of the channel matrix is found to be important to the THP system performance. We reveal how this ordering reflects on the changing of system structure and prove the "best-first" ordering method is optimal for the ZF THP (or MMSE THP) in the minimax noise (or error) variance sense. Simulation results are used to show the performance advantage attained by the MMSE THP and the optimal ordering.

This paper compares diversity combining schemes for the downlink of spread spectrum orthogonal frequency division multiplexing (SS-OFDM) systems in frequency selective fading channels. In particular, symbol-level combining after despreading is compared to chip-level combining under maximal ratio combining (MRC) of signals from different diversity branches and minimum mean-square error (MMSE) equalization of spreading sequences. Symbol-level combining takes place after the operations of MMSE equalization and despreading, whereas the operations of equalization and despreading occur after MRC if chip-level combining is used. Chip-level combining combines diversity samples in an efficient manner while reducing inter-code interference (self-interference) that results from the loss of orthogonality of spreading sequences due to a frequency selective channel. This method is shown to be superior to symbol-level combining when the diversity branches are uncorrelated, and when the branches differ only due to subcarrier interleaving. Novel expressions for the bit error rate (BER) of the two methods, as well as the extension of the analysis to partially loaded systems are presented. The results are relevant to antenna diversity as well as temporal diversity achieved though re-transmission within an ARQ scheme.

This work improves channel decoder performance on the code-division multiple-access (CDMA) forward link through more accurate calculation of the channel code trellis path metrics used to decode convolutional and turbo codes. This is achieved by performing the metric calculations with better channel state estimates. Channel state estimation involves determining desired signal amplitude and interference plus noise variance at the channel decoder input. Two options are presented for performing this estimation. Standard estimation is a simple scheme that results from assuming all CDMA forward link interference and noise can be lumped into a single additive white Gaussian noise process. Improved estimation is a new and more accurate technique that separately accounts for the intracell interference and intercell interference plus thermal noise processes. Simulations show that the decoding of convolutional and turbo codes is significantly improved by the more accurate channel state information provided by improved estimation.

A new trellis path metric is derived for the CDMA mobile MAP and logMAP turbo decoders. This metric accounts for the non-Gaussian received symbol probability distribution seen at the output of a mobile RAKE receiver when the RAKE combining weights are subject to channel estimation error. Since an exact analytical expression for this distribution is difficult to find, the metric is instead derived by finding the characteristic function of an approximation to the distribution. Simulations are then used to compare how the CDMA forward link performs when the mobile uses either the conventional logMAP trellis path metric or the new metric. The results indicate that while the conventional metric is still the best choice when channel estimation error is small, the new metric provides a significant performance improvement when channel estimation error is large.

We propose and evaluate a novel method of constructing hybrid ARQ systems with turbo codes that addresses the convergence problems of the turbo decoders. The puncturing pattern is chosen in such a way that the parts of the trellis that present difficulties to the decoder are given priority in retransmission. This is different from the traditional construction of rate compatible punctured turbo codes (RCPT) which attempts to obtain optimal weight distance properties of the codes without taking the convergence properties of the turbo decoder into account. We show that in the 'turbo cliff' region our system outperforms the best known hybrid ARQ scheme in terms of the throughput, the number of retransmissions and the total number of iterations required to decode a frame at the receiver. Moreover, the design of the puncturing pattern is greatly simplified and requires no extensive computer simulations.

The focus of this research work is the sub-optimal list decoding algorithms for parallel concatenated convolutional codes (PCCCs) which improve the frame error rate (FER) performance. Error events and weight spectra for convolutional codes and PCCCs are analyzed with emphasis on their effects on list decoding. We explain the inefficiencies of list decoding algorithms for PCCCs that use a list generated from the component codes, and introduce a new algorithm based on the sub-block structure that generates a list directly for the PCCC. The additional complexity of the new algorithm is low and does not depend on the complexity of the component code. Simulations on the additive white Gaussian noise (AWGN) channel show that the new algorithm can lower the frame error floor by more than one order of magnitude.

Irregular low-density parity-check (LDPC) codes have shown exceptionally good performance for single antenna systems over a wide class of channels. In this paper, we investigate their application to multiple antenna systems in flat Rayleigh fading channels. For small transmit arrays, we focus mainly on space-time coding with 2/sup p/-ary LDPC codes, where p equals the number of encoded bits transmitted by the transmit antenna array during each signaling interval. For large transmit arrays, we study a layered space-time architecture using binary LDPC codes as component codes of each layer. We show through simulation that, when applied to multiple antenna systems with high diversity order, LDPC codes of quasiregular construction are able to achieve higher coding gain over quasistatic fading channels than previously proposed space-time trellis and convolutional codes.

This work uses the second order statistics of the signal received by a CDMA mobile to determine what proportion of that signal consist of in-cell interference and what proportion is out-of-cell interference plus thermal noise. A least-squares estimator is derived and compared to the Creamer-Rao lower bound. The interference estimated are incorporated into the CDMA mobile channel decoder in order to improve the performance of convolutional and turbo codes on the CDMA forward link.

This paper introduces and analyses a novel medium access control protocol for reverse links in wideband direct-sequence code-division multiple-access (DS-CDMA) mobile systems. The proposed protocol is particularly efficient when data consists of short packets. The new MAC protocol uses the time taken by the receiver to process the packets (the processing delay) to admit a new packet. When packet arrival is assumed to be Poisson, analysis and simulations show consistently that, for short packets, the new MAC scheme can handle a higher arrival rate than the conventional one, in which the processing delay is not used. The new MAC scheme shows only slight throughput degradation, a high utilization of the packet-switched data channel, and achieves a fair treatment of mobile stations.

The channel code decoder in a CDMA mobile performs channel state estimation to determine the reliability of each received encoded symbol. This reliability information is typically an estimate of received signal envelope and interference plus noise variance. Most papers that investigate CDMA forward link channel code performance assume that all interference and noise can be modeled as a single, stationary Gaussian process. This assumption leads to inaccurate channel state estimates since in-cell interference is non-stationary on the CDMA forward link. This paper presents a channel state estimation procedure for single antenna and Space Time Transmit Diversity (STTD) transmission that accounts for non-stationary in-cell interference. Simulations are used to illustrate how this scheme improves convolutional and turbo code performance.

This paper presents a comparison of the forward link performance of the HDR and 1XTREME systems. Both systems have been proposed as possible evolutionary paths for the 1.25 MHz cdma2000 system, and have been designed to enable high data rate packet transmission. Link-level performance is evaluated by chip-level time-domain simulation of the forward data traffic channel in the presence of additive white Gaussian noise, and is then used in a system-level simulator modeling a network of 19 three-sector cells. The system-level simulator accounts for slow and fast fading and two types of multipath channel profiles, as described by the pedestrian A and vehicular B ITU-R channel models. The packet scheduling is performed using a proportionally fair (PF) scheduler. The systems use bit rate adaptation and hybrid ARQ to achieve 1% packet error rate (PER). All users in the system are assumed to always have data packets available for transmission. System performance is evaluated for an embedded sector in terms of average sector throughput as a function of the number of users, distributions of data rates, and individual throughputs over a set of users. Results show the benefits of multiuser diversity exploited by the PF scheduler, and present throughput and fairness comparisons for the two systems.

Many IS-95 CDMA networks currently deployed in the field are forward link capacity limited. The introduction of IS-2000 is intended to improve forward link performance and increase system capacity. However, the performance improvement provided by IS-2000 will vary with propagation environment. A technique for collecting forward channel impulse response measurements using IS-95 pilot signals is presented. This technique is used to characterize the forward channel in residential and urban regions in Calgary, Canada. The propagation data is then processed in a simulation in order to generate a comparison of IS-95 and IS-2000 performance that is specific to these environments. The results illustrate how the propagation environment can affect the performance improvement provided by IS-2000.

One of the main improvements that the 1xEV-DO (IS-856) standard adds to the original HDR proposal is the hybrid automatic repeat request (H-ARQ) mechanism. The H-ARQ scheme combines error correction, error detection, soft packet combining and incremental redundancy. H-ARQ reduces the required forward link SINR and thereby improves overall system capacity, particularly in mobile environments. We present simulation results comparing the aggregate throughput of CDMA HDR systems with and without H-ARQ. We find that H-ARQ gives greatest gain at moderate to high mobile speeds and also at cell-edge locations, where the mean received Ec/Io is small. The addition of H-ARQ more than doubles the aggregate throughput for mobile speeds above 21 km/h.

Multicarrier systems are gaining much attention for their possible application to the design of wideband wireless networks for both indoor and outdoor environments. We present a method for the evaluation of the bit error rate (BER) degradation in orthogonal frequency division multiplexing (OFDM) systems when the sampling instant is affected by timing jitter. We model the timing jitter by a stationary random process with a known statistic and we describe the error caused by it as additive noise. This decomposition is useful because it allows for the analytical determination of the BER. The method may be applied for both uncoded and coded input data sequences.

The paper considers optimal forward link power allocation for data transmission in DS-CDMA systems. Acceptable quality voice communications in DS-CDMA can be achieved through the application of the forward error correction (FEC). Due to stringent requirements on signal delay, no retransmission is allowed in voice only systems. Highly reliable data transmission over wireless channels, however, can be achieved with hybrid automatic-repeat-request (ARQ) schemes. The average power allocated to data users then needs to be calculated as the actual link transmitted power times the average number of times that a data packet must be transmitted before it is accepted. Since a base station's total power available to mobiles is limited, there is an optimum power allocated to every data user in the cell, which maximizes the system capacity. The optimal power allocation can be described as the stochastic nonlinear programming optimization problem. Numerical results are presented to illustrate the problem. The powers allocated to data users are location dependent, but the numerical results demonstrate that the mean signal-to-interference ratio (SIR) is location independent when the optimal power allocation is achieved.

This paper presents a quick and accurate performance evaluation technique for direct-sequence spread-spectrum multiple-access (DS-SSMA) systems with deterministic signature sequences over Rician flat-fading and frequency-selective fading channels (with RAKE receivers), without resorting to any assumptions with respect to the distribution of the multiple-access interference (MAI). The method is based on a reduced-complexity formulation of the characteristic function method and its computational complexity is simply proportional to the spreading gain thus rendering it attractive for the study of future wideband CDMA systems. The paper also presents novel results with respect to the accuracy of a Gaussian approximation of MAI as a function of various channel and system parameters. It is notably observed that the type of signature sequence has a strong influence on the Gaussian approximation's accuracy which can degrade severely under a variety of conditions (e.g. with Walsh sequences over frequency-selective fading channels) even for a moderate to large number of users.

Two rapid synchronization acquisition algorithms applicable to spread spectrum links of code division multiple access (CDMA) personal communication systems are proposed and evaluated. The algorithms operate within a self-referencing matched filter synchronizer structure, and are particularly useful in reducing synchronization overhead on links designed to carry packet-type services. The main distinguishing characteristic between the two schemes is that one uses hard-decision while the other uses soft-decision detection. The proposed schemes are especially applicable to reverse link transmissions in quasisynchronous CDMA systems in which timing at portable terminals is established via pilot and synchronization signals received on respective code-division channels from the home base. If discontinuous (bursty) transmission is used on reverse links, the acquisition process is required for each transmission burst because of the propagation time uncertainty. Analysis of the algorithms on additive white Gaussian noise (AWGN) and Rayleigh fading channels reveals that their performance depends significantly on the choice of synchronizer parameters and the average despread signal-to-interference ratio (SIR). When this choice is proper, acquisition over a single preamble of relatively short duration can be achieved with high probability. The soft-decision scheme introduces a performance advantage of between 4-9 dB depending on the length of the synchronizing preamble.

A novel multistage successive interference cancellation scheme is proposed which operates on the reverse code-division multiple-access (CDMA) link using binary phase shift keying (BPSK) modulation, coherent detection and reference symbols to obtain channel estimates. The results of a single cell multiuser investigation demonstrate increased traffic capacity. However, this increase is initially very moderate due to corruption of the channel estimates by interference from symbols not yet demodulated and cancelled. A modification in the transmitted signal structure addressing this problem is proposed. The results of a single cell analysis of the modified cancellation scheme demonstrate that the system's traffic capacity reaches approximately 80% of that of a multistage successive interference cancelling receiver operating with the perfect knowledge of channel parameters. A subsequent multicell investigation shows that for a hexagonal cell geometry with a path loss exponent of four and without any forward error correction (FEC) coding or cell sectorization, the system capacity compares very favorably with that of the IS-95 system, which employs powerful error control coding. Capacities with other path loss exponents and cell geometries are also investigated. The results show a substantial traffic capacity increase over that of a comparable receiver without interference cancellation.

A simple negative impedance amplifier has been developed that can bidirectionally amplify a wideband signal on a telephone line. The amplifier uses a multibranch RC network to accurately match the negative impedance to the characteristic impedance of the line over a wide range of frequencies. An operating range extension of 1 km has been achieved which gives an increase of 40% in area coverage. Advantages of this amplifier are low complexity and good tolerance to component variation. A 2 wire/4 wire amplifier has also been developed. It has achieved a range increase of 1.5 km which is 27% more range or 61% more area coverage. The amplifier can also operate closer to either end of the line than a negative impedance amplifier of the same gain. A disadvantage of the 2w/4w amplifier is the increase in circuit complexity by approximately a factor of four. These techniques are not dependent on the modulation scheme and thus a single unit could be used in ADSL, HDSL, or BR-ISDN loops.

The paper proposes and analyzes a combined framing and error control scheme for the DS3 format in which the primary framing bits are replaced with parity check bits of a rate 0.9912 extended and shortened Hamming code. A fast syndrome computation scheme is developed for the code to facilitate faster reframe times. The proposed scheme has better overall reframing performance than conventional framing, and reduces the output BER to approximately 1360P/sub e//sup 2/. This is effective against error-rate floors or "dribble error" phenomena in fiber optical transmission. Primary applications are in advanced in-service performance monitoring and for throughput enhancement in ATM networking applications using leased DS3 carrier signals.>

An improved synchronization acquisition algorithm applicable to spread-spectrum links of code division multiple access (CDMA) personal communication systems is proposed. The algorithm operates within a self-referencing matched filter synchronizer structure, and is particularly useful to reduce synchronization overhead on links designed to carry packet-type services. The proposed scheme involves a soft-decision detection algorithm which improves the acquisition performance by between 4 and 9 dB (depending on the choice of system parameters) in comparison to the one described previously by the authors (1994).

The bit error rate (BER) performance of a mobile or personal radio digital communications system at a high bit rate can be increased significantly by intersymbol interference (ISI) due to multipath propagation. In this paper, a novel adaptive channel precoder is proposed to improve the system performance with-out increasing the complexity of the portable unit. In the forward link, both the amplitude and phase of the signal carrier are predistorted at the transmitter of the base station, so that the signal received at the portable unit is ISI free (in the ideal case). The precoding is based on channel information estimated (from the reverse link) at the receiver of the base station. Computer simulation results demonstrate that, over a two-path Rayleigh fading channel, differential QPSK with the precoder has significantly lower BER values than those with a conventional decision feedback equalizer (DFE), because the precoder is not subject to error propagation.

In this paper, adaptive channel equalization is investigated in order to reduce the bit error rate of trellis-coded continuous-phase frequency shift keying (CPFSK) in an indoor radio environment. A new equalizer structure is proposed, which makes use of fractionally spaced soft-decision signal samples instead of symbol-spaced hard decisions on transmitted symbols in the decision feedback loop, in order to remove the intersymbol interference (ISI) resulting from the frequency-selective fading channel.>

Fixed rate, variable rate, and feed forward pointer spreading as well as stuff threshold modulation (STM) are analyzed as methods for reducing jitter caused by SONET pointer adjustments. Each method of jitter reduction is described, theoretical analysis is developed to predict RMS jitter performance, and simulation results are presented to illustrate RMS and peak-to-peak jitter performance. A new method of implementing feed forward pointer spreading is described in which STM is used to transmit the synchronizer elastic store phase information.>

The authors report simulation results and prototype control protocols for a digital subscriber loop transmission system, which will find and operate at the highest common bidirectional transmission rate that is viable on a given loop while preserving a specified noise margin. A prototype implements the rate-adaptation and transmission protocols over a captive multigauge cable facility. Simulation results predict the accompanying symbol rate versus reach characteristics for a transceiver with the same structure as basic rate access (BRA) systems under RA-DSL control. Results show that 1.5 Mb/s could be delivered to loops of up to 2 km while subscribers out to 10 km could receive 80 kb/s. For a number of specialized or short-notice situations, RA-DSL could be the basis for a fast and flexible response vehicle. If RA-DSL cannot serve an individual combination of distance and capacity, existing methods remain. However, a statistical assessment with metro population data shows the potential for RA-DSL to satisfy 4-6 times more n*DS-0 access requests than BRA on existing copper, without carrier serving area constraints.>

The technique introduced has relatively simple encoding and decoding procedures which can be implemented at the high bit rates used in optical fiber communication systems. Because it is similar to the established technique of self-synchronizing scrambling but is also capable of guiding the scrambling process to produce a balanced encoded bit stream, the technique is called guided scrambling, (GS). The concept of GS coding is explained, and design parameters which ensure good line code characteristics are discussed. The performance of a number of guided scrambling configurations is reported in terms of maximum consecutive like-encoded bits, encoded stream disparity, decoder error extension, and power spectral density of the encoded signal. Comparison of guided scrambling with conventional line code techniques indicates a performance which approaches that of alphabetic lookup table codes with an implementation complexity similar to that of current nonalphabetic coding techniques.>

The derivation of a signaling channel using the redundant justification bits in the DS-3 signal is proposed to remedy the DS-3 networks lack of overhead for support of intelligent network functions. In the DS-3 network each frame of 680 time slots contains one time slot (a stuff slot) that can be used to either transmit information from an appropriate tributary or to fill out the format. In the latter case, the redundant time slot can be used to carry auxiliary signaling information. It is proposed to use the random sequence of all these redundant time slots as the basis for the new signaling channel. It is shown that the information capacity of the resulting channel is 23 kb/s. This capacity is more than sufficient to implement the network management techniques in DS-3 networks and has the advantage of complete transparency to the existing payloads and to existing or new equipment not intended to process such signaling channels.>

A family of mB(m+1)B binary, nonalphabetic, balanced line codes is presented that is suitable for high bit rate (>or=135 Mb/s) optical fiber transmission due to its relatively simple encoding and decoding rules. Here, B represents a block of m bits, where m is an odd number. The coding, decoding, and bit error rate (BER) performance of the codes are discussed. Statistical and spectral analysis for the specific case in which the number of bits, m, equals seven, is presented. This makes possible a detailed comparison of the proposed code with conventional 7B8B codes.>

A discussion is presented of the selection of a subset of the ISDN (integrated-services digital network) signalling system's network-layer protocol for point-to-point links suitable for processing in VLSI. A symmetric subset of the CCITT Q.931 recommendation as it stood in August 1987 is proposed. The need for bearer-channel negotiations is eliminated by using a 'two-way independent channel selection' technique in which each end of the link is master in the transmit direction and slave in the receive direction. A logical model for the signalling system's network layer processor is developed to aid in understanding the Q.931 protocol. The model also serves as the initial functional partitioning for the VLSI implementation. The key element of the model is the separation of the network-layer processor into a protocol-specific module (PSM) and an application-specific module (ASM).>