We study how UE-to-UE interference impacts the performance, considering transmitter out-of-band (OOB) emissions and receiver blocking characteristics for homogenous 20 MHz LTE systems. For co-existing TDD systems on different carrier frequencies within the same band, UE-to-UE interference may occur in certain situations. The impact of such interference is evaluated for a hotspot scenario and compared to a uniform user distribution scenario, and is found to be significantly worse. When the number of scheduled users in the aggressor cell is high (e.g. >; 2), then receiver blocking impact, rather than the OOB emission, dominates the interference performance in hotspot scenario. Consequently, guard bands cannot be used to mitigate the receiver blocking impacts.

We investigate non-codebook-based multiple-input multiple-output (MIMO) precoding schemes for the time division duplex (TDD) mode of LTE where channel reciprocity could be exploited. Previously proposed non-codebook-based precoding schemes typically use UE specific reference signals for demodulation. Cell specific reference signals are however still transmitted for the transmission of common control signaling, mobility measurements and downlink channel quality measurements. In order to save the resources occupied by UE specific reference signals, and to simplify UE implementation, a non-codebook-based precoding scheme using cell-specific reference signals is considered. Link throughput simulations indicate that such scheme outperforms the scheme using UE specific reference signals in the scenario with high transmit antenna correlation and low UE velocity.

In LTE TDD there is typically no one-to-one association between UL and DL subframes, and for DL heavy asymmetries with more DL than UL subframes, ACK/NACK reports for multiple DL subframes need to be transmitted in an UL subframe. To improve uplink control channel performance, ACK/NACK bundling, where multiple ACK- NACKs are combined to a single ACK/NACK response for several DL subframes, is supported. In this paper, we study the impact on downlink performance of ACK/NACK bundling on link-level for LTE TDD systems both for SIMO and MIMO systems. It is found that the expected loss due to ACK/NACK bundling is rather small and hence bundling can be a good solution in many scenarios.

In long term evolution (LTE) specification, physical uplink control channel (PUCCH) consists of two basic transmission formats, i.e. formats 1a/1b and format 2, carrying some control signals fed back to eNodeB, e.g. ACK/NACK related to downlink data transmission blocks and downlink channel quality indicator (CQI). At eNodeB, conventional receiver detection algorithm is to do coherent data detection by utilizing reference signals (RS) based channel estimation, also called non-blind algorithm, whose detection performance is greatly limited by RS density and data coding rate. In this paper, a semi-blind algorithm is introduced to improve channel estimation accuracy by also using data energy besides RS energy at a cost of a higher computational complexity. The analyzes is done for both formats 1a/1b and format 2. Simulation results show that semi-blind algorithms efficiently improve the performance of format 2 because format 2 has lower RS density and higher coding rate than formats 1a/1b.

This paper outlines and evaluates a simplified interference model applicable in multi-cell multi-antenna radio network simulations. Based on the path-loss, the model classifies interferers as either strong or weak and the channels of strong interferers together with the channel of the desired signal are accurately modeled using a spatial channel model (SCM). The SCM assures that the spatial signature of the signals is accounted for in the evaluations. The channels of weak interferers are simply characterized by the path-loss and interference is modeled as additive white Gaussian noise (AWGN). The model is verified by means of simulations of a 57 sector OFDM/TDMA network and by comparing results achieved using the simplified model to results from simulations with full interference modeling, i.e., to the case when all interferers are accurately modeled. The verification results demonstrate that the simplified model with at least eight links accurately modeled provides a high modeling accuracy and results are comparable to results achieved with full interference modeling. Moreover, in the employed simulation tool this reduces the simulation time by up to a factor of four. The model may hence be used as a means to speed up simulations of multi-cell multi-antenna radio networks.

Advanced multiple antenna techniques are currently being considered in the evolution of today's wireless communication systems as well as in the development of future systems. The identification, assessment, comparison and selection of relevant techniques do in general require performance evaluations at adequate operating points in various deployments and this calls for system level evaluations. Modeling of individual links' performance is then needed and this is also of interest for the development of link adaptation strategies. The present paper considers link performance modeling of bit interleaved coded modulation (BICM) in combination with multiple transmit antennas and non-iterative receiver processing. More specifically, previous studies of the single antenna case have shown that a model based on mutual information for BICM is suitable, and the present work demonstrates that this is the case also for BICM with multiple transmit antennas. However, the calculation of the BICM mutual information is not trivial and this topic deserves further attention.

This paper discusses the requirements on future radio access and, based on the requirements, proposes a framework for such a system. The proposed system based on orthogonal frequency-division multiplexing supports very low latencies and data rates up to 100 Mb/s with wide area coverage and 1 Gb/s with local area coverage. Spectrum flexibility is identified as one main requirement for future systems, and the proposed framework can be deployed in a wide range of spectrum allocations with bandwidths ranging from 2.5 to 100 MHz. Multihop relaying, useful to extend the range for the high data rates, and multiple-antenna configurations are integral parts of the framework. A packet-centric approach is taken for the dataflow processing, implying that the scheduling mechanism and the retransmission protocol operate on complete packets rather than segments thereof, thus allowing for cross-layer optimization. Finally, numerical evaluations are provided, illustrating the feasibility of future very wideband radio access.

This paper gives an overview of some so-called link performance models used in system level simulations to determine the link packet error rate (PER) at reduced complexity. A subset of link performance models is evaluated in terms of PER prediction accuracy focusing on a single receive and transmit antenna OFDM link with different coding options and channel characteristics. The results demonstrate that a mutual-information based metric which accounts for the modulation alphabet is preferable in the considered cases and, furthermore, applicable to the large class of MIMO-OFDM transmission techniques with linear pre- and post-processing.

This paper summarizes current work within the IST-Project WINNER (Wireless World Initiative New Radio) towards a spatial processing solution as an integral part of the overall system concept. Major requirements for the WINNER multi-antenna concept include high performance, robustness, and adaptation to a wide range of scenarios and terminal classes, as well as efficient interworking with the basic transmission and network concept. Based on these guidelines, a first proposal for the WINNER multi-antenna concept is elaborated upon and presented in the form of a generic downlink transmitter that will serve as baseline for further investigation and system integration

This paper considers the problem of estimating the time delays of multiple replicas of a known signal received by an array of antennas. Under the assumptions that the noise and co-channel interference (CCI) are spatially colored Gaussian processes and that the spatial signatures are arbitrary, the maximum likelihood (ML) solution to the general time delay estimation problem is derived. The resulting criterion for the delays yields consistent and asymptotically efficient estimates. However, the criterion is highly non-linear, and not conducive to simple minimization procedures. We propose a new cost function that is shown to provide asymptotically efficient delay estimates. We also outline a heuristic way of deriving this cost function. The form of this new estimator lends itself to minimization by the computationally attractive iterative quadratic maximum likelihood (IQML) algorithm. The existence of simple yet accurate initialization schemes based on ESPRIT and identity weightings makes the approach viable for practical implementation.

In wireless communication scenarios, multipath propagation from local scatterers in the vicinity of mobile sources may cause angular spreading as seen from a base station antenna array. This paper studies the effects of such local scattering on direction of arrival (DOA) estimation with the MUSIC and ESPRIT algorithms. Previous work has considered rapidly time-varying scenarios, and concluded that local scattering has a minor effect on DOA estimation in such scenarios. This work considers the case in which the channel is time-invariant during the observation period. The distribution of the DOA estimates is derived, and the results show that local scattering has a significant impact on DOA estimation in the time-invariant case. In addition, numerical examples are included to illustrate the analysis, and to demonstrate that the results may be used to formulate simple estimators of angular spread.

In wireless communication scenarios, local scatterers in the vicinity of the mobile sources cause angular spreading. As a result, the spatial signatures will not belong to the conventional array manifold parameterized by direction of arrival (DOA) alone. A parameterized model for spatial signatures applicable in scenarios with local scattering is presented. Several algorithms that exploit this model are proposed, and the performance of signal waveform estimators using the model is investigated via simulations. It is demonstrated that considerable gain may result as compared with using the conventional plane wave model.