Showing papers by "Preben Mogensen published in 2009"

Autonomous component carrier selection: interference management in local area environments for LTE-advanced

Abstract: Low-power base stations such as femtocells are one of the candidates for high-data-rate provisioning in local areas, such as residences, apartment complexes, business offices, and outdoor hotspot scenarios. Unfortunately, the benefits are not without new challenges in terms of interference management and efficient system operation. Due to the expected large number of user-deployed cells, centralized network planning becomes impractical, and new scalable alternatives must be sought. In this article we propose a fully distributed and scalable solution to the interference management problem in local areas, basing our study case on LTE-Advanced. We present extensive network simulation results to demonstrate that a simple and robust interference management scheme, called autonomous component carrier selection, allows each cell to select the most attractive frequency configuration; improving the experience of all users and not just the few best ones, while overall cell capacity is not compromised.

An overview of downlink radio resource management for UTRAN long-term evolution

Abstract: Radio resource management algorithms ranging from bearer admission control to semi-persistent and dynamic packet scheduling, fast link adaptation, and transmission control of multi-antenna transmission modes are addressed in this article for UTRAN long-term evolution. First, a high-level system overview of LTE is given, with special emphasis on the important components related to RRM. The quality of service parameter framework is outlined, as one of the main objectives for the families of RRM algorithms is to maximize system capacity while serving all users according to their minimum QoS constraints. It is demonstrated how the collocation of the RRM algorithms at the base station with easy access to air interface measurements offers opportunities for efficient cross-functional optimization between layers 1, 2, and 3. Examples of performance results for different traffic mixes and antenna transmission schemes are also presented, and the article is concluded with recommendations on how to operate the various RRM options under different load and traffic conditions.

LTE-Advanced: The path towards gigabit/s in wireless mobile communications

Abstract: This paper addresses the performance targets and the technology components being studied by 3GPP for LTE-Advanced. LTE-Advanced is the next major step in the evolution of UTRAN Long Term Evolution (LTE) release 8, currently being finalized by 3GPP. The high level targets of LTE-Advanced are to meet or exceed the IMT-Advanced requirements set by ITU-R and furthermore, meet any additional operator requirements. This for instance includes the target of supporting more than one gigabit/s data rates, higher cell throughput and lower cost per bit. The technology components being identified as part of the LTE-Advanced Study Item include component carrier aggregation to enable up to 100MHz bandwidth, advanced MIMO options up to 8×8 in DL and 4×4 in UL, coordinated multiple point transmission and reception (CoMP), relay nodes (RN) and autonomous component carrier selection (ACCS) for uncoordinated femto cell deployment.

Resource allocation considerations for multi-carrier LTE-Advanced systems operating in backward compatible mode

Abstract: In this paper we focus on LTE-Advanced with backward compatibility, which serves a mixture of LTE-Advanced and LTE-Rel'8 users. Aggregation of multiple component carriers (CCs) to form a wide spectrum is assumed in order to fulfill the bandwidth requirement for the next generation systems, thereby leading to a multi-carrier system. Although a LTE-Advanced user can simultaneously access all the CCs, a LTE-Rel'8 user is restricted to operate on a single CC at a time. Different methods for balancing the load across these CCs will affect the system performance. This is investigated through both analytical methods and system level simulations. Bearing in mind that the LTE-Advanced users are scheduled on more CCs than the LTE-Rel'8 users, we propose a simple cross CC packet scheduling algorithm that improves the resource allocation fairness among the two categories of users. This simple scheduling algorithm is shown to be effective in providing better coverage performance with no loss of the overall cell throughput, as compared to independent scheduling per CC.

Autonomous Component Carrier Selection for Local Area Uncoordinated Deployment of LTE-Advanced

Abstract: Uncoordinated deployment of eNBs in local area environments will benefit from having support for dynamic frequency re-use mechanisms also known as "autonomous com- ponent carrier selection", where each eNB dynamically selects to use only a subset of the available component carriers (i.e. using from one component carrier up to the maximum num- ber of available component carriers). Component carriers are selected autonomously by each eNB depending on the offered traffic, interference coupling with neighboring cells, etc. In this contribution we propose a simple and distributed scheme based on so-called background interference matrices (BIMs) for autonomous component carrier selection. Extensive numerical simulation campaigns indicate that the proposed scheme effec- tively allows each eNB to select the most attractive frequency configuration; improving average user capacity and boosting cell edge performance considerably. Index Terms—Spectrum-sharing, LTE-Advanced, Local Area, Self-organizing, Inter-cell Interference Coordination (ICIC).

Carrier load balancing methods with bursty traffic for LTE-Advanced systems

Abstract: In this paper we focus on LTE-Advanced performance under bursty traffic conditions, and devote our effort to the different methods for balancing the load across multiple carriers. These carriers are the component carriers (CCs) that belong to the same carrier frequency. They are bonded together in order to fulfill the requirement of wide spectrum in LTE-Advanced. We first derive the analytical model for a bursty birth-death traffic model with fixed payload size for OFDMA with different frequency domain packet schedulers. Applying this model for a multi-carrier system, we compute the performance for different system setups. The obtained analytical results are verified using extensive system level simulations. Based on the analytical and simulation results, it is suggested to assign the users on all CCs if a cell is not heavily loaded. Otherwise, assign each user with only one CC using the load balancing method of Round Robin is preferable, in the sense that it maintains good performance with low uplink overhead.

Turbo Receivers for Single User MIMO LTE-A Uplink

Abstract: The paper deals with turbo detection techniques for Single User Multiple-Input-Multiple-Output (SU MIMO) antenna schemes. The context is on the uplink of the upcoming Long Term Evolution - Advanced (LTE-A) systems. Iterative approaches based on Parallel Interference Cancellation (PIC) and Successive Interference Cancellation (SIC) are investigated, and a low-complexity solution allowing to combine interstream interference cancellation and noise enhancement reduction is proposed. Performance is evaluated for Orthogonal Frequency Division Multiplexing (OFDM) and Single Carrier Frequency Division Multiplexing (SC-FDM) as candidate uplink modulation schemes for LTE-A. Simulation results show that, in a 2times2 antenna configuration, the turbo processing allows a consistent improvement of the link performance, being SC-FDM the one having higher relative gain with respect to linear detection. The turbo receiver's impact is however much reduced for both modulation schemes in a 2times4 configuration, due to the higher diversity gain provided by the additional receive antennas.

Fixed Frequency Reuse for LTE-Advanced Systems in Local Area Scenarios

Abstract: LTE-Advanced systems, which aim to provide high data rate wireless services, have received world-wide researching interests nowadays. In this paper, the performance of fixed frequency reuse with different reuse factors is studied in LTE-Advanced systems. Performance is measured in terms of both average cell throughput and cell edge user throughput. It is found that a properly chosen reuse factor with respect to cell size (which leads to different level of inter-cell interference), can offer up to 30% gain in average cell throughput and much higher gain for cell-edge user throughput in Local Area (LA). This high gain from frequency reuse makes it attractive for future LTE-Advanced systems.

UTRAN LTE Downlink System Performance under Realistic Control Channel Constraints

Abstract: In the UTRAN Long Term Evolution (LTE), the downlink data channel is shared among the active users through the Orthogonal Frequency Division Multiple Access (OFDMA) technique, and high gains can be achieved by using dynamic packet scheduling. Control channels introduce an additional overhead, but also play a key role in exploiting such gains. This paper focuses on the Physical Downlink Control Channel (PDCCH) and introduces the main PDCCH related features which need to be analyzed in order to minimize its impact on the system performance.

Performance of proportional fair frequency and time domain scheduling in LTE uplink

Abstract: Channel-aware scheduling is one of the advanced features introduced in the uplink of Long Term Evolution (LTE) in order to improve the system spectral efficiency. In the uplink of LTE channel-aware scheduling is possible thanks to the transmission of sounding reference signals. In this paper the performance of frequency and time domain scheduling in LTE uplink is investigated. In particular, we focus on the performance impact of various scheduling metrics which try to achieve proportional fairness in throughput or resource allocation. Our analysis shows that a combination of a throughput-based metric in the time domain and an SINR-based metric in the frequency domain is able to provide the highest performance. For example, in a noise-limited propagation environment with high inter-site distance the sector throughput gain compared to performing throughput-based proportional fair scheduling in both time and frequency domain is above 20%.

Performance Analysis of Simple Channel Feedback Schemes for a Practical OFDMA System

Abstract: In this paper, we evaluate the tradeoff between the amount of uplink channel feedback information and the orthogonal frequency-division multiple access (OFDMA) downlink performance with opportunistic frequency-domain packet scheduling. Three candidate channel feedback schemes are investigated, including practical aspects, such as the effects of terminal measurement errors, bandwidth measurement granularity, quantization, and uplink signaling delays. The performance is evaluated by means of system-level simulations with detailed modeling of various radio resource-management algorithms, etc. Our results show that the optimal tradeoff between the channel feedback and the downlink OFDMA system performance depends on the radio channel frequency coherence bandwidth. We conclude that the so-called average best-M scheme is the most attractive channel feedback solution, where only the average channel quality for the best M subbands is reported, together with a bit mask identifying those subbands. The latter scheme only has a channel feedback word size in the range of 10-20 bits for the considered OFDMA system with 10-MHz bandwidth.

Autonomous Inter Cell Interference Avoidance under Fractional Load for Downlink Long Term Evolution

Abstract: The main source of interference in OFDMA system in downlink is inter-cell interference, which can severely limit the throughput of users near the cell edge. The inter-cell interference coordination (ICIC) is one method to improve the performance. In this paper autonomous inter-cell interference avoidance schemes under Fractional Load (FL) conditions in the downlink for 3 rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) are proposed. The proposed schemes do not require any inter-cell signaling for ICIC; rather the decision about the allocation of the spectrum in order to avoid the inter- cell interference is taken based on the information available within the cell itself. We show that the schemes for spectral resource selection is important for FL scenario to avoid high BLER. The proposed schemes further improve the SINR condition therefore higher cell throughput and coverage are realized.

LTE Part I: Core network

Abstract: Part I of this Feature Topic covers several key aspects of the evolved packet system (EPS) architecture.

Uplink overhead analysis and outage protection for multi-carrier LTE-Advanced systems

Abstract: In a multi-component carrier LTE-Advanced system, a user can simultaneously be scheduled on all component carriers (CCs). For the base station to perform radio channel aware packet scheduling, each user ideally has to feedback information corresponding to all CCs. This results in a potentially high amount of uplink overhead. Furthermore, a user in a power limited situation may experience outage because of insufficient power for the required feedback. In this paper we first propose several techniques at different layers for the overhead reduction, as well as protecting the users from being in the outage situation. Afterwards, we show how these techniques can be integrated in a LTE-Advanced system. A weighted proportional fair scheduler is also proposed to maintain good performance with a reduced amount of overhead. Extensive simulation results are presented in the end for analyzing the proper trade-off between performance and overhead reduction and outage protection.

Single-user MIMO for LTE-A Uplink: Performance evaluation of OFDMA vs. SC-FDMA

Abstract: The 3rd Generation Partnership Project (3GPP) Long Term Evolution-Advanced (LTE-A) aims at very high peak data rates such as 1 Gbps in local areas and 100 Mbps in wide areas. To meet these targets, evolved multi-antenna techniques and large spectrum allocations are required. In this paper, we carry out a performance evaluation of uplink single-user multiple-input-multiple-output (SU-MIMO), both for orthogonal frequency division multiple access (OFDMA) and single-carrier frequency division multiple access (SC-FDMA) under LTE parameters. Different MIMO schemes such as single-input-multiple-output (SIMO), spatial multiplexing (SM) and transmit diversity with space-frequency coding (SFC) are investigated. Results show that OFDMA tends to outperform SC-FDMA for 2 receive antennas. However, this gap is reduced when 4 antennas are considered at the receiver. Furthermore, it is shown that the peak-to-average power ratio (PAPR) of the SC-FDMA signal is increased about 0.6 dB, when SFC is employed; nevertheless, a gain of 2 dB over OFDMA is still preserved.

LTE part II: Radio access [Guest Editorial]

Abstract: Part II of this Feature Topic provides insight into the inner workings of the long term evolution (LTE) air interface.

Interference Cancellation Based on Divergence Minimization for MIMO-OFDM Receivers

Abstract: In this paper, we present a novel iterative receiver for MIMO-OFDM systems with synchronous interferers. The receiver is derived based on the Kullback-Leibler divergence minimization framework, and combines channel estimation, interference cancellation and residual noise estimation in an iterative manner. By using both the pilot and data symbols, the channel estimator improves the accuracy of the estimates in each iteration, which leads to a more effective interference cancellation and data detection process. A performance evaluation based on Monte-Carlo simulations shows that the proposed scheme can effectively mitigate the effect of interferers, and operates very close to the single-user performance even in severe interference scenarios.

Network Time-Synchronization in TDD Based LTE-Advanced Systems

Abstract: In Frequency Division Duplex (FDD) systems or Time Division Duplex (TDD) systems with perfect Uplink (UL)/Downlink (DL) alignment, all UL transmissions receive interference only from other UL links and vice versa. However, take into consideration one of the most important benefits that TDD offers, i.e., dynamic UL/DL switching points, the UL and DL transmission can be misaligned. This means UL might be corrupted by DL, and vice versa. The loss of network time-synchronization among different cells makes things even more complicated for TDD systems. Many techniques are developed to achieve network synchronization, e.g., GPS based or IEEE 1588 based techniques. In this paper, the effect that different synchronization and UL/DL switching points introduce to the system performance is studied. The purpose is to find out the benefit that synchronization can offer. Based on the observations, it is suggested to achieve at least loose network synchronization.

On the Feasibility of Precoded Single User MIMO for LTE-A Uplink

Abstract: The 3rd Generation Partnership Project (3GPP) is currently specifying the system requirements for Long Term Evolution - Advanced (LTE-A), having as a target peak data rates of 1 Gbit/s in local areas and 100 Mbit/s in wide areas. To meet these ambitious requirements for the uplink, multiple-input-multiple-output (MIMO) antenna techniques are expected to be deployed. In this paper, several channel-aware MIMO precoding techniques are presented. Specifically, precoded single user spatial multiplexing for both Orthogonal Frequency Division Multiplexing (OFDM) and Single Carrier Frequency Division Multiplexing (SCFDM) is studied, and its feasibility in a LTE-A uplink system is discussed. Particular emphasis is given to the limited feedback precoding, where a codebook generation method based on the Lloyd algorithm is proposed. Results show that, when full channel knowledge is available at both the transmitter and the receiver, precoding leads to a spectral efficiency gain up to 4 dB over open loop transmission; furthermore, OFDM slightly outperforms SC-FDM because of its higher robustness to the noise. Limited feedback precoding has been shown to be effective and consistently robust to the subcarrier grouping in a urban micro scenario. However, the performance is severely degraded in the blind precoding case, where transmitter and receiver compute the precoding matrix independently, due to the high sensitivity to the delay. Finally, the precoding operation over the SCFDM signal is shown to increase its Peak to Average Power Ratio (PAPR), thus reducing its advantage with respect to OFDM.

Channel Estimation Based on Divergence Minimization for OFDM Systems with Co-Channel Interference

Abstract: In this paper, we present a novel approach for pilot-aided channel estimation in OFDM systems with synchronous co-channel interference. The estimator is derived based on the Kullback-Leibler divergence minimization framework. The obtained solution iteratively updates both the desired user's and the interferer's channels, using a combination of linear minimum mean squared-error (LMMSE) filtering and interference cancellation, avoiding the complex matrix inversions involved in the full LMMSE channel estimation approach. Estimation of the noise variance is also included in the iterative algorithm, accounting for Gaussian noise and residual interference after each iteration. The estimates of both channels are used at the equalizer to reject the interfering signal, thus mitigating the degradation due to co-channel interference. Simulation results show that the receiver using the proposed estimator performs as good as the one employing the full LMMSE estimator and very closely to a receiver having perfect knowledge of the channel coefficients.

Comparison of Spectrum Sharing Techniques for IMT-A Systems in Local Area Networks

Abstract: The explosive growth of mobile communications on one hand and the overly crowded and expensive spectrum on the other hand have fueled hot debates on spectrum sharing techniques, anticipating fundamental changes in spectrum regu- lation. A key point of the discussions has been the need for empirical tests and validation of even the simplest spectrum sharing proposals already available. In this paper, inspired by IEEE 802.11 WLANs, the authors employ carrier sense multiple access with collision avoidance (CSMA/CA) in an LTE-Advanced framework and compare the achieved performance with that of an ordinary LTE-Advanced system employing static frequent reuse schemes. The analysis is performed by means of system level simulations in Local Area networks where the e-NodeBs (eNB) may be randomly placed without any prior considerations to minimize inter-cell interference. Average and cell-edge user throughput results show that the contention based approach, despite its beautiful simplicity, wastes resources due to random back-off timers and hidden nodes. In nearly all cases, well known frequency reuse alternatives are more interesting options. Index Terms—Spectrum-sharing, LTE-Advanced, Local Area, Contention, Frequency reuse, WLAN.

An Interference Aware Dynamic Spectrum Sharing Algorithm for Local Area LTE-Advanced Networks

Abstract: This paper presents a dynamic spectrum sharing algorithm to minimize the inter-cell interference, so as to achieve a high system performance. This algorithm operates in a self-organized manner without the need of any centralized control, thereby is especially useful in Local Area (LA) where the enhanced-NodeBs (eNB) are close and coordination among them is hard or expensive to achieve. Results show that with very limited signaling between the eNBs, the proposed algorithm can effectively improve the system performance, and it can even

Precoded Multirank Transmission with Linear Receivers for LTE-A Uplink

Abstract: The 3rd Generation Partnership Project (3GPP) defined the requirements for the upcoming Long Term Evolution Advanced (LTE-A) systems, aiming at peak spectral efficiencies of 30 bit/s/Hz in the downlink and 15 bits/s/Hz in the uplink. These ambitious targets can be accomplished only by using advanced Multiple Input Multiple Output (MIMO) antenna techniques. In this paper, we evaluate closed loop precoded multirank transmission for Single Carrier Frequency Division Multiplexing (SC-FDM) as selected uplink modulation scheme by using the precoding codebook defined for LTE downlink. Its performance is compared with Orthogonal Frequency Division Multiplexing (OFDM). Results show that OFDM outperforms SC-FDM for medium-high ranks, but the gap can be reduced by adding further diversity branches. While narrowband precoding shows considerable gain over open loop transmission for all ranks, wideband precoding works with a very small feedback overhead but results to be a valid option mainly for lowrank transmission, where the low Peak-to-Average Power Ratio (PAPR) of SC-FDM signals is also preserved.

Light Cognitive Radio Enabled Flexible Spectrum Usage in Local Area Deployment

Abstract: In this paper we propose a mechanism to enable flexible spectrum usage (FSU) in local area indoor deployment scenario with several operators in the given geographical area. The proposed scheme is referred to as policy assisted light cognitive radio (CR) enabled FSU, because it follows the CR cycle and considers policy as an important element to assist FSU. It facilitates allocation of spectral resources from a common pool in flexible manner and ensures coexistence of several operators in the given geographical area on the shared spectrum. It also provides an autonomous, self adjustable and scalable solution for the emerging large scale Local Area (LA) indoor deployment of Home eNode-Bs (HeNBs).

Integrated Fractional Load and Packet Scheduling for OFDMA Systems

Abstract: This paper study the performance of an orthogonal frequency division multiple access (OFDMA) system under fractional load based in long term evolution (LTE) downlink. Fractional load is defined by a situation where only a fraction of the bandwidth is used for transmission due to lack of traffic. This type of situation should result in a global increase of signal to interference and noise ratio (SINR) conditions in the network. We propose different methods integrating the transmission pattern selection to the packet scheduling functionality of the enode-B depending only on the channel quality indicator (CQI) reports from the user equipments. Fractional load handling is operated without inter eNode-B coordination. We conclude that the system can operate under fractional load conditions if the CQI is calculated with wideband interference. Further gain can be obtained if the CQI is calculated with local interference, however, in that case, it is necessary to inject a certain time correlation in the transmission patterns due to reporting delay.