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Licensed assisted access (LAA) is a new feature for 3GPP LTE systems to operate in the unlicensed spectrum. Under LAA, licensed carriers will be aggregated with unlicensed carriers in order to opportunistically enhance user throughput, while still offering seamless mobility and outdoor coverage. In order to coexist with other technologies in the unlicensed bands, several new functionalities for LAA LTE have been introduced, including longterm channel selection, short-term channel sensing based on listen-before-talk (LBT), and discontinuous transmission on a carrier with limited maximum transmission duration. In this article, we present research findings behind the designs for LAA systems. We discuss the impact of several parameters of the LAA LBT framework on the channel access opportunities of LAA, and its coexistence performance toward co-channel networks based on extensive system-level simulation results. The investigation covers both single-channel as well as multi-channel operation and coexistence scenarios. In addition to the finalized designs for downlink LAA operations in Release 13, our findings in this article further shed light on the uplink LAA operations to be introduced in Release 14.

Today's 4G LTE systems bring unprecedented mobile broadband performance to over a billion of users across the globe. Recently, work on a 5G mobile communication system has begun, and next to a new 5G air interface, LTE will be an essential component. The evolution of LTE will therefore strive to meet 5G requirements and to address 5G use cases. In this article, we provide an overview of foreseen key technology areas and components for LTE Release 14, including latency reductions, enhancements for machine-type communication, operation in unlicensed spectrum, massive multi-antenna systems, broadcasting, positioning, and support for intelligent transportation systems.

LAA is a new operation mode of LTE in the unlicensed spectrum, which will be featured in LTE Release 13. Under LAA, licensed carriers will be aggregated with unlicensed carriers in order to opportunistically enhance downlink user throughput while still offering seamless mobility support. In order to coexist with WiFi, some of the new functionalities required of LAA LTE include a mechanism for channel sensing based on listen-before-talk, discontinuous transmission on a carrier with limited maximum transmission duration, and multicarrier transmission across multiple unlicensed channels. This article presents a detailed overview of the design agreements for LAA, the impact of unlicensed spectrum operation on the LTE physical layer architecture, and the scope of additional enhancements beyond LTE Release 13. A range of simulations for indoor and multicarrier scenarios show that fair coexistence between LAA and WiFi can be achieved, and that deployment of LAA can provide a boost in WiFi performance.

The immediate availability of Uplink (UL) time-frequency resources for transmission of new data is a key factor in reducing latency and improving UL throughput in cellular systems. This is particularly important for unlicensed spectrum operation where access to the channel is subject to Listen-Before-Talk (LBT), and scheduling UL transmissions with a preceding Downlink (DL) transmission as in Long-Term Evolution (LTE) can be inefficient. Therefore, this work proposes and studies autonomous UL transmissions (AUL) for LTE-based systems in unlicensed spectrum, where UL transmissions are allowed without requiring a prior scheduling request or an explicit scheduling grant from the Evolved Node B (eNB). The implications of AUL transmissions for the design of hybrid automatic repeat request (HARQ) and link adaptation mechanisms are investigated. Detailed performance evaluations in the 5 GHz unlicensed band show that dynamic switching between scheduled and autonomous UL transmissions offers the best throughput performance, while also coexisting fairly with co-channel Wi-Fi networks.

The uncertainty of channel availability for wireless systems in unlicensed bands necessitates new and robust designs for the feedback of uplink control information and channel state information. This work discusses the design of hybrid automatic repeat request (HARQ) feedback mechanisms and channels for unlicensed spectrum LTE. We describe in detail the motivation for new uplink physical channels that carry downlink HARQ feedback while adhering to unlicensed spectrum regulations. Finally, the performance of various control channel designs is evaluated by means of link-level simulations in order to provide insights on the trade-offs between required signal-to-noise ratio (SNR), HARQ-feedback payload size, transmission duration, and multi-user multiplexing capacity.

Licensed-Assisted Access (LAA) is a new feature for 3rd Generation Partnership Project (3GPP) Long-Term Evolution (LTE) system to operate in the unlicensed spectrum. LAA utilizes the carrier aggregation technology to aggregate both licensed and unlicensed carriers in order to opportunistically boost user throughput, while maintaining seamless mobility, quality of service and good indoor and outdoor coverage. In order to coexist with other technologies in the unlicensed bands, several new coexistence functionalities for LAA LTE have been introduced, including long term channel selection, short-term channel sensing based on listen-before-talk (LBT), and discontinuous transmission on a carrier with limited maximum transmission duration. LAA will be supported for both Downlink (DL) and Uplink (UL) transmissions. This paper presents a detailed overview of the design challenges for UL LAA access on unlicensed bands due to the scheduling aspects of LTE. We present solutions to address grant and scheduling request (SR) overhead and delays. We also discuss possible UL LBT options to increase the performance and the flexibility of scheduled LTE when operating on unlicensed bands. A range of simulations show fair coexistence between LAA and Wi-Fi. Furthermore, this paper sheds lights on possible future enhancement for UL LAA using unscheduled UL transmission.

The next major step in the evolution of LTE targets the rapidly increasing demand for mobile broadband services and traffic volumes. One of the key technologies is a new carrier type, referred to in this article as a Lean Carrier, an LTE carrier with minimized control channel overhead and cell-specific reference signals. The Lean Carrier can enhance spectral efficiency, increase spectrum flexibility, and reduce energy consumption. This article provides an overview of the motivations and main use cases of the Lean Carrier. Technical challenges are highlighted, and design options are discussed; finally, a performance evaluation quantifies the benefits of the Lean Carrier.

Licensed-assisted access (LAA) is a new operation mode of Long-Term Evolution (LTE) in the unlicensed spectrum currently under study in the 3GPP standardization forum. In order to coexist with Wi-Fi, some of the new functionalities required of LAA LTE include a mechanism for clear channel assessment based on listen-before-talk (LBT), discontinuous transmission on a carrier with limited maximum transmission duration, and dynamic frequency selection (DFS) for radar avoidance in certain frequency bands. This paper presents a detailed overview of the impact of unlicensed spectrum operation on the LTE physical layer architecture, such as downlink physical channel design, scheduling, and radio resource management. System-level simulation results are then presented for indoor and outdoor scenarios, and show that fair coexistence between LAA and Wi-Fi can be achieved and that deployment of LAA can provide a boost in Wi-Fi performance.

We develop a unified performance metric and detailed analysis for hybrid automatic repeat request (HARQ) schemes based on incremental redundancy (IR) and Chase combining (CC). The general result is applicable to both symbol-based and bit-interleaved coded modulations, to HARQ processes based on rateless and fixed-rate mother codes, and IR schemes with and without a self-decodability restriction. The analysis shows that IR over CC coding gains tend to increase with the initial coding rate, but decrease with the signal-to-noise (SNR) variation between retransmissions. The gains can also be diminished when a fixed-rate mother code is used or when the self-decodability criterion is imposed. The theoretical prediction is compared with observed gains at 10% codeword error rates based on turbo-coding simulations. For scenarios with moderately varying SNR between retransmissions, the analytical model tracks actual simulation results very well. However, when the SNR varies widely and the systematic part of the turbo codeword is effectively erased, the CC scheme could, in fact, outperform some IR schemes. For these scenarios, which can be induced by fast time-varying fading or long retransmission delays, the self-decodable IR and the CC schemes prove to be more robust without much performance comprise. Finally, we discuss adaptive improvement to the conventional IR schemes based on the analytical result

We describe the close connection between the now celebrated iterative turbo decoding algorithm of Berrou et al. (1993) and an algorithm that has been well known in the artificial intelligence community for a decade, but which is relatively unknown to information theorists: Pearl's (1982) belief propagation algorithm. We see that if Pearl's algorithm is applied to the “belief network” of a parallel concatenation of two or more codes, the turbo decoding algorithm immediately results. Unfortunately, however, this belief diagram has loops, and Pearl only proved that his algorithm works when there are no loops, so an explanation of the experimental performance of turbo decoding is still lacking. However, we also show that Pearl's algorithm can be used to routinely derive previously known iterative, but suboptimal, decoding algorithms for a number of other error-control systems, including Gallager's (1962) low-density parity-check codes, serially concatenated codes, and product codes. Thus, belief propagation provides a very attractive general methodology for devising low-complexity iterative decoding algorithms for hybrid coded systems.

This paper discusses the design, performance optimization and potential extensions of the LTE HARQ operations. The HARQ retransmission behavior of all 764 possible LTE code blocks is classified into 12 types based on two easily and already computed quantities in the standard. Using with the unified information-theoretical quantification of various HARQ gains discussed in the companion paper, the optimal retransmission settings for the LTE system are designed. The analytical framework is further applied to optimize potential modifications to the LTE standard for enhanced retransmission performance. Numerical results are provided to verify these optimized settings.

We develop a general model based on random coding bounds to explain and predict the coding performance of various type-I and type-II hybrid automatic repeat request (HARQ) schemes based on the bit-interleaved coded modulation (BICM) in 3GPP HSPA. We explore how the intrinsic properties of BICM interact with various transmitter-side (e.g., whether to use the same bit mapping for retransmission) and receiver-side HARQ options (e.g., bit vs. symbol level processing and Log-MAP vs. Log-Max demodulation). The accuracy of the theoretical model is verified with extensive link simulations. The performance model can be incorporated into radio resource management algorithms for efficient wireless network operations. In a companion paper, the unified analytical model is further applied to investigate performance of LTE HARQ operations, which consist of varying mixture of type-I/II and bit remapping HARQ features.

This paper discusses circular buffer rate matching (CBRM) algorithms for the turbo code in the long term evolution (LTE) of the WCDMA-based air interface. To enhance performance at high code rates, systematic bit puncturing is incorporated in conjunction with the CBRM. The RM algorithm is further optimized based on the algebraic properties of the QPP interleavers and the 8-state recursive systematic convolutional code of the LTE turbo code.

The design of LTE turbo coding chain suitable for flexible parallel and pipelined hardware implementations is presented. The hierarchical data structure further offers an opportunity to design an efficient two-level early stopping algorithm. An analytical model is developed to compute the distribution of actual decoding iterations performed by the proposed algorithm. It is then shown the scheme can reduce both the total hardware requirement at the design phase as well as the average power consumption at run-time of turbo decoders.

The error detection performance of CRC coding in LTE with general two-level early stopping algorithms for turbo decoding is investigated. Analytical models for the probability of block error and undetected block error at the code block and transport block levels were developed. Simulations were used to verify the model for shorter CRC lengths and block sizes. The analytical models show the setting of 24-bit CRCs in LTE allows low-complexity, robust and reliable early stopping algorithms to reduce turbo decoding complexity.

Work is ongoing in 3GPP to significantly extend the performance of UMTS in the work item long term evolution (LTE). LTE contains a new system architecture with fewer infrastructure nodes and it has been decided to terminate the ARQ functionality in the evolved node B. This paper describes the requirements that exists for an LTE ARQ concept and outlines a solution that fulfills these requirements in the new LTE architecture. The solution builds on two layered ARQ feedback mechanisms that provide sufficient reliability with a low resource cost for the feedback The paper contains thorough stepwise argumentation how we arrived at the proposed ARQ concept.

Performance of conventional decoders with inaccurate channel estimation input is analyzed to construct an accurate and computationally efficient model for dynamic system simulations. The model abstracts general channel conditions experienced by a codeword into a collection of repetitive states, caused by type-I HARQ or diversity receive antennas, and non-repetitive states, caused by fast fading, type-II HARQ or OFDM transmissions. An effective SNR accounting for losses due to channel estimation errors is derived for the repetitive states. The contribution of all non-repetitive states is summarized in the normalized conditional information rate, which can then used to read off the correct codeword error rate from a look-up table. The accuracy of the proposed model is verified with extensive simulation results.

We present a new methodology for constructing accurate physical-layer (radio link) performance models for dynamic wireless network system simulations. Our approach emphasizes the consistent treatment of the residual fading effects caused by inaccurate channel estimation across slot, transmission time interval, and hybrid automatic repetition request levels. The proposed performance models can he implemented using simple manipulations of certain pre-calculated values. Numerical results are provided to illustrate the accuracy for individual models as well as for multiple models combined together.

We develop an information-theoretic model to explain and predict the coding gains of the incremental redundancy (IR) over the chase combining (CC) scheme using random coding bounding techniques. From extensive simulations, the model is found to provide accurate coding gain predictions in conjunction with either optimal or practical repeated low-rate codes. Contrary to a widely held belief that IR is superior to CC for all circumstances, we demonstrate that this is not always the case when parallel concatenated turbo codes with severe puncturing are used in IR on fading channels. Based on the theoretical model, we further propose an adaptive algorithm to remedy this anomaly.

We consider the maximum-likelihood (ML) decoding performance of concatenated space-time (ST) random coded communication utilizing multiple input and multiple output (MIMO) antennas on quasi-static fading channels. The ensemble-averaged codeword error rates of such systems can be upper bounded by the outage probabilities of the corresponding conditional cutoff rates. We refer to these as the information cutoff probabilities and evaluate them numerically for a wide range of ST coding schemes. The slope behaviors of these cutoff probabilities are found to be consistent with the maximum achievable diversity orders given by a new unified bound. With simulations of a class of ST coding schemes based on bit-interleaved coded modulations and iterative spatial/temporal decoders, we further show these cutoff probabilities can provide accurate predictions for total performance gains.

For pt. see ibid., p.1177-81(2002).We continue the consideration of the maximum-likelihood decoding performance of MIMO space-time random coded communication on quasi-static fading channels. First, we generalize the formulation to frequency-selective fading channels. By the equivalence of multipath fading and delay diversity coding, we show the system can be analyzed with an equivalent flat fading representation. We then present numerical results of the p% cutoff capacities, which are defined as the highest supportable rates for target cutoff probabilities at p%. Our discussion show how this new notion can provide a unified view of three main theoretic results of MIMO space-time coding: rate enhancements, achievable diversity gains, and performance of specific codes. As a result, this analysis of cutoff capacities reveals the relative strengths and weaknesses of several space-time coding schemes in interesting and surprising ways.

We develop an information-theoretic model to explain and predict the coding gains of the incremental redundancy over the Chase combining scheme. From this analysis, we identify a new approach to improve the spectral efficiency of the conventional incremental redundancy scheme, which uses the same modulation and coding formats and, most notably, the same amounts of channel resources for retransmissions as those for the initial transmissions. Depending on the relative channel conditions between the transmissions, fewer incremental redundant bits than those employed in the first transmission might be enough to guarantee high probability of successful decoding. Hence, we propose a new scheme that adapts the modulation and coding formats according to simple feedback information to minimize unnecessary channel usage. Our simulation results demonstrate system throughput gains up to 25% in certain conditions can be achieved by the proposed scheme without modifying the existing WCDMA specifications.

A new coding design with large coding gains and complexity-reduced decoders is proposed. The coding strategy is based on a multilevel coding construction with rate-compatible punctured convolutional codes as building blocks. A new sub-optimal decoding is proposed where only one decoder using Viterbi algorithm is drafted. Typical example codes for quadrature amplitude shift keying (QASK) constellations an constructed and shown to outperform Ungerboeck's codes, but with higher decoding delay. Computer simulation is also performed to verify the results. >

Several parallel concatenated coding schemes (turbo codes) based on multimemory (MM) convolutional codes (more specifically, a (2,1,4,7) code) were proposed to achieve near Shannon-limit error correction performance with reasonable decoding complexity. On the other hand, in many cases of interest, unit-memory (UM) codes have been demonstrated to have larger free distances than the MM codes with the same rate and the same number of memory elements. New turbo codes based on the (8, 4, 3, 8) UM Hamming code are developed and shown to possess better performance potential in some senses. The standard turbo decoding algorithms, however, do not appear to achieve this potential.

In modern digital communication systems, quality of service is specified in terms of average frame error rates (FER). Accurate and fast estimation of the FER is thus a crucial element of these power control or adaptive systems. This paper proposes an accurate and fast FER estimation method for any receivers equipped with iterative processors and the ability to stop the iterative processing early. Based on the observation that the average number of processing iterations is strongly correlated to the operating FER, the proposed method estimates the FER through the average number of actual processing iterations. As confirmed by extensive simulation results, the proposed estimation algorithm is much more accurate and responsive to changing operating conditions than traditional methods.

The GSM/EDGE Radio Access Network (GERAN) is presently being evolved as a full complement to the 3G UMTS network. One part of this evolution is the introduction of efficient support of real-time packet data and alignment to the UNITS Core Network. Another part will introduce performance-enhancing features aimed at improving spectral efficiency, peak and average user-throughput and capacity. One such enhancement being considered is the use of Higher Level Modulations to enhance the user peak rate in the GERAN, in much the same way as the data rate for GPRS was enhanced with the 8PSK modulation in EDGE. By introducing new coding schemes based on QAM the maximum bit rate per time slot could be increased up to 88.8 kbit/s. This paper explains the GERAN concept and investigates possible system level gains, from both spectrum efficiency and perceived user quality perspective, with Higher Level Modulation in GERAN.

EDGE has enabled GSM networks to offer third generation services as part of the UMTS core network. This has been possible through two steps: the introduction of 8PSK modulation and the specification of interfaces from the GERAN (GSM/EDGE radio access network) to the UMTS core network. This paper studies further improvements to spectral efficiency and capacity for EDGE through the use of more sophisticated adaptive modulation schemes, and the use of newer coding schemes (see Nanda, S. et al., IEEE Commun. Mag., p.54-64, 2000; Ikeda, T. et al., IEEE Trans. Vehic. Tech., vol.49, p.404-12, 2000). It is possible to use theoretical predictions of outage analysis to predict the gain in performance due to the use of higher order modulations.

The following topics were dealt with: Shannon theory; universal lossless source coding; CDMA; turbo codes; broadband networks and protocols; signal processing and coding; coded modulation; information theory and applications; universal lossy source coding; algebraic geometry codes; modelling analysis and stability in networks; trellis structures and trellis decoding; channel capacity; recording channels; fading channels; convolutional codes; neural networks and learning; estimation; Gaussian channels; rate distortion theory; constrained channels; 2D channel coding; nonparametric estimation and classification; data compression; synchronisation and interference in communication systems; cyclic codes; signal detection; group codes; multiuser systems; entropy and noiseless source coding; dispersive channels and equalisation; block codes; cryptography; image processing; quantisation; random processes; wavelets; sequences for synchronisation; iterative decoding; optical communications.

EDGE has enabled the GSM network to offer third generation services as part of the UMTS core network. This is due to the introduction of 8PSK modulation and the recent specification of interfaces from the GERAN to the UMTS core network. This paper studies further improvements to the spectral efficiency and capacity of EDGE through the use of more sophisticated adaptive modulation schemes based on QAM, and the use of novel turbo coding schemes. Furthermore, this work considers a novel transmit diversity scheme adapted for use in dispersive channels. We provide performance results for the enhanced system showing tremendous gains. We also use theoretical predictions of outage analysis to predict the gain in performance obtained by the use of higher order modulations and transmit diversity. The gains from theoretical predictions match our simulations almost exactly.