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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.

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.

To evolve the LTE network to cope with ever and rapidly increasing demand for mobile broadband services and traffic volumes, interference avoidance designs have been identified as one major area to address in the next LTE release. In both traditional homogeneous macro network and heterogeneous network with mix of macro and small cells, intercell interference can be substantially reduced by temporarily halting unneeded downlink transmission. In this paper, we analyze several design aspects of such base station downlink discontinuous transmission (DTX) operations. We provide extensive performance evaluation results and analysis on (1) the time scale of such DTX operations; and (2) whether the DTX operations should be restricted to the small cells only. The results provide guideline for an ideal combination of these design dimensions.

Carrier aggregation has enabled LTE to efficiently utilize wide and fragmented spectrum to provide high data rate services to end users' benefits. We investigate the challenges of providing reliable multi-carrier hybrid automatic repeat request (HARQ) and timely periodic channel state information (CSI) feedbacks from terminals aggregating multiple component carriers in LTE networks. We consider unified and reliable uplink control channel designs to carry multi-carrier HARQ and periodic CSI feedback messages simultaneously. Care is taken in the designs to reduce the detection complexity on the network side, which also improves its detection reliability relative to existing LTE signalling schemes.

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.

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.

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.

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.