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 discuss the methods of improving sum-rates 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 CSI codewords whose indices are transmitted back to the base station transmitter. The algorithms described here are aimed at combating bit errors, erasures and delays occurring in CSI feedback channels. The proposed solution is to design transmitter-side CSI index prediction algorithms that allow the system to alleviate feedback channel imperfections. Moreover, the proposed techniques can be used to lower the required feedback rate, while keeping the system sum-rate at the required level. We demonstrate the performance of our algorithms in broadcast MIMO systems with and without CSI prediction. The simulation results show that significant sum-rate gains are achievable for a large range of channel Doppler rates and feedback delays.

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, 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.

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.

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.

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.

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, we address the issue of imperfect channel estimation in coded systems on fading channels. Since performance of channel codes is influenced in different ways by different components of channel-estimation errors, we develop a simplified model which separates the estimation errors of a Wiener-filtered received signal into the amplitude error and the phase error. Based on the model, we derive tight bounds on component error variances. Moreover, we prove that the classical Wiener filter results in a biased estimate of the channel amplitude. We also show that the probability of having a phase-estimation error large enough to cause decision errors in the receiver is significant. Using our model, we derive an approximate upper limit on the optimum pilot-symbol spacing and approximate lower limit on bit-error rate performance of coded systems with a given pilot-symbol separation. The proposed model and derivations are confirmed by extensive simulations.

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.

This paper discusses a realistic turbo coding system with the signal phase which has not been perfectly estimated. We propose improved decoding algorithms for the situations when the residual phase error can be modelled by the Gaussian probability distribution and a Markov chain, a model which can be used in many actual phase estimators. It is shown that increasing the state space of the decoders can decrease the bit error probability.

The classical method for decoding turbo codes is the Bahl-Cocke-Jelinek-Raviv (1974) algorithm which, in the majority of existing papers, is analyzed under the assumption of perfect channel knowledge and synchronization. In reality, however, its properties change significantly when synchronization is not ideal. We show that the influence of timing synchronization offset can be modeled as a decreased effective signal-to-noise ratio, which provides results consistent with simulations. Furthermore, we present a general model for the soft bit output of the BCJR algorithm and apply it to the situation when the received signal is sampled with a timing offset. Finally, we use the derived properties of the algorithm in a simple timing synchronizer which is shown to outperform the classical maximum-likelihood non-data-aided symbol synchronizer without a need of using additional pilot symbols.

This study is a comparison of three routing protocols proposed for wireless mobile ad-hoc networks.The protocols are: Destination Sequenced Distance Vector (DSDV), Ad-hoc On demand Distance Vector (AODV) and Dynamic Source Routing (DSR).Extensive simulations are made on a scenario where nodes moves randomly.Results are presented as a function of a novel mobility metric designed to reflect the relative speeds of the nodes in a scenario.Furthermore, three realistic scenarios are introduced to test the protocols in more specialized contexts.In most simulations the reactive protocols (AODV and DSR) performed significantly better than DSDV.At moderate traffic load DSR performed better than AODV for all tested mobility values, while AODV performed better than DSR at higher traffic loads.The latter is caused by the source routes in DSR data packets, which increase the load on the network.routers and hosts, thus a node may forward packets between other nodes as well as run user applications.Mobile ad-hoc networks have been the focus of many recent research and development efforts.Ad-hoc packet radio networks have so far mainly concerned military applications, where a decentralized network configuration is an operative advantage or even a necessity.Networks using ad-hoc configuration concepts can be used in many military applications, ranging from interconnected wireless access points to networks of wireless devices carried by individuals, e.g., digital maps, sensors attached to the body, voice communication, etc. Combinations of wide range and short range ad-hoc networks seek to provide robust, global coverage, even during adverse operating conditions.Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the tirst page.To copy otherwise, to republish, to post on servers or to redistrihute to lists.requires prior specific permission and/or a fee.