Troels Bundgaard Sørensen

Carrier load balancing and packet scheduling for multi-carrier systems

Abstract-In this paper we focus on resource allocation for next generation wireless communication systems with aggregation of multiple Component Carriers (CCs), i.e., how to assign the CCs to each user, and how to multiplex multiple users in each CC. We first investigate two carrier load balancing methods for allocating the CCs to the users- Round Robin (RR) and Mobile Hashing (MH) balancing by means of a simple theoretical formulation, as well as system level simulations. At Layer-2 we propose a simple cross-CC packet scheduling algorithm that improves the coverage performance and the resource allocation fairness among users, as compared to independent scheduling per CC. The Long Term Evolution (LTE)-Advanced is selected for the case study of a multi-carrier system. In such a system, RR provides better performance than MH balancing, and the proposed simple scheduling algorithm is shown to be effective in providing up to 90% coverage gain with no loss of the overall cell throughput, as compared to independent scheduling per CC.

Extension of the ITU channel models for wideband (OFDM) systems

This paper outlines a procedure to extend, or upsample, the ITU power delay profiles (PDP) to improve the frequency correlation properties while keeping the same mean delay and almost the same rms delay spread. Realistic frequency correlation properties are of particular importance for the evaluation of wideband system concepts with frequency dependent characteristics, e.g. frequency domain link adaptation and packet scheduling, both of which are likely to be part of future wideband systems such as based on OFDM. With the suggested procedure the frequency correlation can be kept approximately at or below 0.6 for frequency separations up to 25 MHz for the exemplified ITU Vehicular A and Pedestrian B channel profiles. The compatibility in terms of link level performance with WCDMA rake processing and the representation of the profiles on different sampling grids is briefly discussed in the paper. ITU Channel models; frequency correlation function; wideband systems; power delay profile; beyond 3G system concepts

Initial Performance Evaluation of DFT-Spread OFDM Based SC-FDMA for UTRA LTE Uplink

In this paper we present an initial performance evaluation of the 3GPP UTRA long term evolution (UTRA LTE) uplink with baseline settings. The performance results are obtained from a detailed UTRA LTE uplink link level simulator supporting OFDMA and SC-FDMA schemes. The basic transmission scheme for uplink direction is based on single-carrier transmission in the form of DFT spread OFDM with an MMSE receiver. Two antenna configurations, SISO and 1times2 SIMO are considered in the analysis of spectral efficiency in addition to adaptive modulation and coding (AMC) and L1-HARQ. For assessment purposes, the performance results of SC-FDMA are compared with OFDMA. It is shown that 1times2 SIMO greatly increases the spectral efficiency of SC-FDMA making it comparable to OFDMA, especially for high coding rate. Furthermore, SC-FDMA has a flexibility to increase BLER performance by exploiting frequency diversity.

Improving SC-FDMA Performance by Turbo Equalization in UTRA LTE Uplink

Turbo equalization is known as an advanced iterative equalization and decoding technique that allows to enhance the performance of the data transmission over a frequency selective fading channel. The turbo equalizer will result in extra receiver complexity, but isolated to the base-station, which does not have stringent power constraints. In this paper, a turbo equalization technique to improve Single Carrier Frequency Division Multiple Access (SC-FDMA) performance is proposed. A new adaptive coefficients solution for frequency domain equalization is considered. The work is in the context of UTRA Long Term Evolution (LTE) Uplink. The performance is evaluated for 1x2 Single Input Multiple Output (SIMO) antenna configuration in a 6 paths Typical Urban (TU-06) channel profile. For assessment purpose, the results are compared with SC-FDMA MMSE and OFDMA schemes. Simulation results show that the turbo equalizer can improve the BLER performance around 1 dB with only a few iterations, and improve the SC-FDMA performance over OFDM, especially at high coding rate.

Experimental investigation of multipath richness for multi-element transmit and receive antenna arrays

The multi-element antenna arrays concept with M elements at the mobile station (MS) in combination with N elements at the base station (BS) is experimentally investigated. Forschini (1996) has shown very promising results to improve the spectral efficiency in a rich scattering environment. The performance of the M/spl times/N concept is evaluated in terms of the number of independent parallel channels, diversity gain and total capacity in an outdoor to indoor microcellular environment. It is shown that the eigenanalysis provides a tool to describe the effective number of parallel channels in a multi-element array configuration. Practical results on spectral efficiency are presented for different antenna setups applied to different propagation scenarios. Also it is shown that polarization diversity is an attractive solution to achieve decorrelated antenna elements and subsequently provide a more robust system in terms of spectral efficiency within the microcell. Results show that a total capacity of 27.9 b/s/Hz can be achieved for an uncorrelated propagation environment and 17 b/s/Hz for a correlated one with a mean signal to noise ratio (SNR) of 30 dB in the case of a 4/spl times/4 antenna set-up.

LTE UE Power Consumption Model: For System Level Energy and Performance Optimization

In this work a novel LTE user equipment (UE) power consumption model is presented. It was developed for LTE system level optimization, because it is important to understand how network settings like scheduling of resources and transmit power control affect the UEs battery life. The proposed model is based on a review of the major power consuming parts in an LTE UE radio modem. The model includes functions of UL and DL power and data rate. Measurements on a commercial LTE USB dongle were used to assign realistic power consumption values to each model parameter. Verification measurements on the dongle show that the model results in an average error of 2.6 %. The measurements show that UL transmit power and DL data rate determines the overall power consumption, while UL data rate and DL receive power have smaller impact.

Zero-tail DFT-spread-OFDM signals

In the existing scheduled radio standards using Orthogonal Frequency Division Multiplexing (OFDM) or Discrete Fourier Transform-spread-OFDM (DFT-s-OFDM) modulation, the Cyclic Prefix (CP) duration is usually hard-coded and set as a compromise between the expected channel characteristics and the necessity of fitting a predefined frame duration. This may lead to system inefficiencies as well as bad coexistence with networks using different CP settings. In this paper, we propose the usage of zero-tail DFT-s-OFDM signals as a solution for decoupling the radio numerology from the expected channel characteristics. Zero-tail DFT-s-OFDM modulation allows to adapt the overhead to the estimated delay spread/propagation delay. Moreover, it enables networks operating over channels with different characteristics to adopt the same numerology, thus improving their coexistence. An analytical description of the zero-tail DFT-s-OFDM signals is provided, as well as a numerical performance evaluation with Monte Carlo simulations. Zero-tail DFT-s-OFDM signals are shown to have approximately the same Block Error Rate (BLER) performance of traditional OFDM, with the further benefit of lower out-of-band (OOB) emissions.

Utility Maximization in LTE-Advanced Systems with Carrier Aggregation

Long Term Evolution (LTE)-Advanced is expected to aggregate multiple Component Carrier (CC)s to fulfil the high data rate requirement. It may serve users with different capabilities in accessing these CCs, e.g., some can access all CCs, whereas some may operate on only one CC. This gives challenges to the packet scheduler to maximize the system performance over all CCs. In this paper we provide a mathematical model of the log-measure utility in an LTE-Advanced system, and give proof that our previously developed cross-CC Proportional Fair (PF) packet scheduler maximizes this utility. System level simulations are performed, which confirm that cross-CC PF scheduling offers much higher utility than independent PF and channel blind schedulers. This scheduler is then generalized to adjust the resource sharing among users. It can trade off between average cell throughput and cell edge user throughput. However, any adjustment in the resource sharing leads to a loss in utility.

Turbo Receivers for Single User MIMO LTE-A Uplink

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 receivers 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.

Performance of MIMO with Frequency Domain Packet Scheduling in UTRAN LTE Downlink

This paper addresses the performance of spatial division multiplexing (SDM) multiple-input multiple-output (MIMO) techniques together with frequency domain packet scheduling (FDPS) based on UTRAN long term evolution (LTE) downlink. Two SDM concepts currently considered in 3GPP are the single-user (SU-) and the multi-user (MU-) MIMO. For MU-MIMO, multiple users can be scheduled on different streams on the same time-frequency resource, while SU-MIMO restricts one time-frequency resource to a single user. Both concepts are analyzed against the 1times2 maximal ratio combining (MRC) case, using the proportional fair (PF) criterion extended to frequency and spatial domains. The realistic finite buffer best effort traffic model is used in the study. Results show that in the macrocell scenario, the combination of FDPS and SDM (SDM-FDPS) without preceding achieves only marginal gain in cell throughput, due to limited SINR dynamic range. The gain is increased to around 20% when precoding is applied. In the micro-cell scenario, the gain of SDM-FDPS over the reference scheme is increased up to 35% with precoding.