To avoid the latency from RRC state transition procedure, legacy network maintains UE as RRC connected state for pre-defined time duration even without any traffic arrival. However, it consumes UE battery rapidly because UE should monitor PDCCH, and send CQI feedback periodically. In this paper, we design the RRC connection control to enhance energy efficiency with moderate control signaling overhead. In RRC INACTIVE state, the newly introduced in NR, both network and UE save UE context including bearer configuration and security even after UE released from network. Owing to the saved UE context, the RRC state transition from RRC INACTIVE to RRC CONNECTED requires fewer number of CN signalling. And thus network can release UE to RRC INACTIVE more aggressively with shorter timer. Furthermore, we propose the connectionless data transmission in RRC INACTIVE without RRC state transition to RRC CONNECTED. In our performance analysis, UE energy consumption can be improved by 50% for the modem only and 18% reduction for the total device including display. Furthermore, when the small data or background (keep-alive) traffic is transferred in RRC INACTIVE, the energy efficiency is increased up to double.

To enlarge the cell capacity of mobile network, LTE network offloads its traffic to WLAN network. However, User Equipment (UE) can run out of batteries rapidly due to the independent operation of multi-Radio Access Technologies (RAT), LTE and WLAN. In this paper, we propose new Multi-RAT power saving scheme, Single-RAT Monitoring which remove the duplication of periodic wake up operation. Because UE has both LTE and WLAN modem, LTE modem can monitor the traffic status for WLAN link and then activate or deactivate the WLAN modem. Our performance analysis shows that the WLAN modem applying proposed Single-RAT Monitoring consumes 11%∼99% less energy with various traffic pattern.

Achieving significant throughput enhancement is a fundamental technical challenge for next generation cellular communication systems. In order to meet the projected 1000 fold traffic increase by the year 2020, the cellular systems are intensely focusing on ultra-densification by deploying a large number of small cells in a geographical area as a practical means for aggressively increasing the areal capacity. However cell densification is inherently limited by the high inter-cell interference that it inadvertently generates which seriously jeopardizes its gain and renders it practically unsuitable to meet the projected huge traffic explosion. Furthermore, increased densification leads to increased cell edges which in turn lead to rugged user experience. In order to overcome these daunting limitations and make densification a practical reality, we propose novel mobility robustness solutions which significantly decrease the number of handover failures and minimize the interruption in the ongoing services thereby leading to an appreciably better quality of experience for the user. The performance of the proposed schemes are evaluated using computer simulations and also in the drive test with a commercial handset and network equipment in the Gang-nam station area where the network configuration is close to the dense small cell environment. The proposed schemes enhance mobility performance up to 77% and reduce the service outage by 38% in the simulation with typical handover configurations. The enhancement is also observed from the drive tests in the commercial LTE small cell network around Gang-nam station area.

To enhance the performance of a heterogeneous network, where many small cells are deployed in a macro cell, we consider two technical challenges. First, seamless handover between the same or different types of cells should be supported with a low hand over failure rate. Second, small cells should use their radio resources as much as possible to maximize a cell splitting gain. It was well known that there is a trade-off between the low handover failure rate and the high small cell utilization. To find a better operation considering such a tradeoff, we propose a state-dependent hand over decision algorithm. In the proposed scheme, a user sets a "state" for each small cell, and performs a "state transition" to distinguish between macro-to-small and small-to-macro handovers, and then uses suitable hand over parameters for each handover type. Through simulation, we observe that the proposed scheme can reduce the hand over failure rate significantly without degrading the small cell utilization and the throughput experienced by the user.

In this paper, we address the cross-tier interference problem in a two-tier network which consists of overlaid femtocells within a macrocell range. In the considered network, macro and femto base stations are assumed with the capabilities of power control and beamforming, differently from user terminals. Under such environments, we devise a decentralized solution where each femtocell determines whether to use shared spectrum or partitioned spectrum, considering users’ location information. Introducing two simple distance thresholds, we can optimally partition the spectrum and allocate them without detailed channel information. Simulation results demonstrate that our decentralized scheme achieves comparable performance to the centralized one with low overhead and feedback delay.

This article examines spectrum allocation and partitioning schemes to mitigate cross-tier interference under downlink beamforming environments. The enhanced SIR owing to beamforming allows more femtocells to share their spectrum with the macrocell and accordingly improves overall spectrum efficiency. We first design a simplified centralized scheme as the optimum and then propose a practical decentralized algorithm that determines which femtocells to use the full or partitioned spectrum with acceptable control overhead. To exploit limited information of the received signal strength efficiently, we consider two types of probabilistic femtocell base station (HeNB) selection policies. They are equal selection and interference weighted selection policies, and we drive their outage probabilities for a macrocell user. Through performance evaluation, we demonstrate that the outage probability and the cell capacity in our decentralized scheme are significantly better than those in a conventional cochannel deployment scheme. Furthermore, we show that the cell utility in our proposed scheme is close to that in the centralized scheme and better than that in the spectrum partitioning scheme with a fixed ratio.

Two-tier overlay networks that consist of a conventional macrocell network and femtocell hotspots offer an economical solution for high capacity and extended coverage. However, wireless interference across tiers significantly degrades network performance by restricting spectrum reuse. In this paper, we explore schemes to mitigate cross-tier interference using beamforming in overlay networks. In our model, femtocells can operate with frequency spectrum that is either shared with or separated from the macrocell. The enhanced signal to interference ratio (SIR) from beamforming can contribute to the shared-spectrum population of femtocells and thus improve the spectrum efficiency. We develop a decentralized scheme that solves the spectrum sharing problem to maximize total cell capacity.Through performance evaluation, we show that our proposed scheme significantly improve cell capacity in two-tier overlay networks.

Two-tier overlay networks that consist of a conventional macrocell network and femtocell hotspots offer an economical solution for high user capacity and extended coverage. However, wireless interference across tiers causes significant performance degradation and restricts spectrum reuse. In this paper, we explore schemes to mitigate cross-tier interference with beamforming antennas for overlay networks. In our model, femtocells can operate with frequency spectrum that is either shared with or separated from the macrocell. The enhanced SIR from beamforming contributes to the population of femtocells with the shared spectrum, and thus improve the spectrum efficiency. Given a required SIR level, we show that which femtocells can use the shared spectrum and how much spectrum can be shared to maximize total utility. We show through a numerical performance evaluation that proposed schemes improve spectrum utilization for two-tier overlay networks.

As wireless networks evolve, the spatial reuse becomes important because it heavily affects overall network throughput. Especially in a dense network, the selection of transmission nodes highly influences the number of simultaneous transmissions. In this paper, we propose a spatial reuse enhanced medium access control (SRE MAC) that gives transmission priority to the nodes that have a small number of interferers. Both centralized and distributed approaches are considered with a hierarchical network model. The performance comparison shows that, with a perfect MAC control, the proposed SRE MAC enhances the network-wide throughput through allowing more number of transmissions in parallel.

Over the last decade multiple-input and multiple-output (MIMO) systems have been actively researched and started to be deployed in wireless communications owing to the significant increase in channel capacity. In this paper, we propose an Energy Saving MAC protocol in MIMO (ESMACM) systems by focusing on energy efficiency instead of capacity maximization. ESMACM considers the energy consumption together with the tradeoff between reliability (i.e., diversity) and throughput (i.e., multiplexing gain), and dynamically chooses an appropriate number of antennas for transmission. In computing the total energy consumption, ESMACM counts circuit energy as well as transmission energy. Naturally the circuit energy consumption is directly proportional to the number of active antennas. Through numerical analysis, we confirm that our ESMACM considerably saves energy consumption compared to conventional capacity maximization schemes that use a fixed number of MIMO channels, for a given outage constraint. Our finding is that the capacity maximizing communication which possibly can be regarded best in terms of energy efficiency gives a different solution from the energy minimizing communication.

As wireless networks evolve, the spatial reuse is becoming an important issue because it heavily affects overall system throughput. Concerned with the IEEE 802.11 standard, media access control (MAC) schemes have been researched actively for obtaining high throughput by increasing spatial reuse. In this paper, we propose a power controlled clear-to-send (CTS) scheme that aims at increasing spatial reuse while using the fixed power for data transmission. Other competitive schemes mainly control the data transmission power, but our scheme controls the transmission power for CTS message which level is determined to cover the receiver's interference range. The spatial reuse incense in proposed scheme which reduces the occupied area of each communication pair, thereby improving overall throughput performance. We perform the comparison work through mathematical analysis. The results show that our scheme improves throughput performance.

This paper presents an efficient layered coding scheme using block turbo codes for multimedia broadcasting and multicasting services (MBMS). Because of unidirectional nature of MBMS, we cannot use adaptive coding and modulation schemes using channel quality information from a return link. The layered coding scheme consists of more than two codes that are concatenated and can be separated, and thus the receiver selects a suitable demodulation and decoding scheme for the channel condition. In this paper we demonstrate an example of layered coding scheme using 2D/3D block turbo codes combined with M-ary modulation.