Optimality Properties, Distributed Strategies, and Measurement-Based Evaluation of Coordinated Multicell OFDMA Transmission

The throughput of multicell systems is inherently limited by interference and the available communication resources. Coordinated resource allocation is the key to efficient performance, but the demand on backhaul signaling and computational resources grows rapidly with number of cells, terminals, and subcarriers. To handle this, we propose a novel multicell framework with dynamic cooperation clusters where each terminal is jointly served by a small set of base stations. Each base station coordinates interference to neighboring terminals only, thus limiting backhaul signalling and making the framework scalable. This framework can describe anything from interference channels to ideal joint multicell transmission. The resource allocation (i.e., precoding and scheduling) is formulated as an optimization problem (P1) with performance described by arbitrary monotonic functions of the signal-to-interference-and-noise ratios (SINRs) and arbitrary linear power constraints. Although (P1) is non-convex and difficult to solve optimally, we are able to prove: 1) Optimality of single-stream beamforming; 2) Conditions for full power usage; and 3) A precoding parametrization based on a few parameters between zero and one. These optimality properties are used to propose low-complexity strategies: both a centralized scheme and a distributed version that only requires local channel knowledge and processing. We evaluate the performance on measured multicell channels and observe that the proposed strategies achieve close-to-optimal performance among centralized and distributed solutions, respectively. In addition, we show that multicell interference coordination can give substantial improvements in sum performance, but that joint transmission is very sensitive to synchronization errors and that some terminals can experience performance degradations.Comment: Published in IEEE Transactions on Signal Processing, 15 pages, 7 figures. This version corrects typos related to Eq. (4) and Eq. (28

Analysis of Radio Channel Measurements Using Multiple Base Stations

Future wireless communication systems will utilize the spatial properties of the wireless channel to improve the spectral efficiency and thus increase capacity. This is realized by deploying multiple antennas at both the transmitter and receiver. Utilizing the spatial properties of the channel demands channel models that properly reflect these characteristics. Due to the unpredictable nature of the wireless channel, a common approach is to model its effects in a statistical fashion. A few large world wide cooperations, like the 3GPP or WINNER projects, have developed channel models intended for reference and standardization use. These models are partly based on bulk parameters which describe the characteristics of the channel over larger areas of several wavelengths. These parameters include shadow fading, angle spread, and delay spread among others, and are within the WINNER project called large scale (LS) parameters. Considering the variations of these parameters, it is likely that they are correlated between closely located areas. Therefore it is of key interest to model their correlation properties instead of randomizing them independently for each new simulation. In multi-cell scenarios, when considering macro diversity or evaluation other properties such as interference and/or handover schemes, the intra site correlation of the shadow fading parameters should be taken into account for proper simulations. Neglecting this correlation the interference could be over/under estimated, hence the system capacity can be over/under estimated. Further, reliable estimations of the interference levels will increase the frequency reuse, and may even permit reuse within the same cell, on spatially separated links. The thesis focuses on modelling the statistics and variations of these parameters to increase accuracy of channel models. Previous studies have looked at the autocorrelation properties of the shadow fading in outdoor environments and found it to follow an exponential decay. This means that an first order autoregressive function would be sufficient to model this effect. Research of the angular spread parameter is more scarce, and therefor this issue is studied herein. In addition to the autocorrelation properties the intra-site correlation between the different large scale parameters is of interest. It is herein found that the shadow fading is negatively correlated with angle spreads, while the angle spread at the base station and the angle spread at the mobile show positive correlation. Finally, the inter site correlation between one mobile and multiple base stations is studied as a function of the angular separation between the links. For closely located base station, ie with small angular separation as seen from the mobile, substantial inter-site correlation is found. It is further seen that the correlation of the shadow fading is more prominent than the correlation of angle spread.QC 2010110

Investigation of Simple Algorithms for Estimation of Delay-Spread and Angle-Spread

In this paper we describe two simple methods for estimation of delay-spread and angle-spread, respectively. The algorithms are simple in the sense that the transmitted signal may consist of only three superimposed CW tones -and the receiver need only two antennas. The algorithms are also simple in the sense that the computational cost is very low. We verify the algorithms by applying them to wideband and multi-antenna measurement data, respectively

Analysis of multi-cell MIMO measurements in an urban macro cell environment,” URSI GA

In this paper we analyze the properties of Multiple Input Multiple Output (MIMO) channels in urban macro-cell environments based on a narrow-band measurement campaign with simultaneous measurements from two sites in the GSM-1800 band. We focus here on bulk parameters such as shadow-fading, angle-spread, and their variation in space and cross-correlation among different sectors. We also look on the dependence between Direction of Arrival and Direction of Departure, (DoA/DoD). Our analysis result shows virtually no cross-site correlation of the studied parameters, while cross-sector correlation is substantial. I

Correlation properties of large scale fading based on Indoor measurements

Statistical channel models are attractive for their simplicity but sometimes lack in precision. In to improve modelling accuracy, statistical model parameters, which are dependent on the environment, are extended to include spatial and temporal correlation. In outdoor scenarios these parameters are assumed constant over large areas of several wavelengths hence the name large scale parameters. This paper studies the large scale fading (LSF) and the applicability of bringing this previously used outdoor variable to the indoor case. The impacts of the model of the LSF on the outdoor system-performance have been studied and several relevant models have been proposed for outdoor cases. We present the intra-site autocorrelation as well as the inter-site cross correlation of the LSF for an indoor channel. The results are based on two separate measurement campaigns conducted at KTH, Stockholm and TUI, Ilmenau, using a single mobile station (MS) and multiple base stations (BSs) to investigate such models. We observe that the areas under which the LSF could be assumed constant are, in indoor scenarios, so small that it can be assumed independent from one local area to another. Furthermore, we find results that point towards the existence of inter-site correlation in some specific scenarios

Measurements of MIMO Indoor Channels at 1800 MHz with Multiple Indoor and Outdoor Base Stations

Abstract—This paper proposes several configurations for multiple base stations in indoor MIMO systems and compares their performance. The results are based on channel measurements realized with a MIMO testbed. The receiver was moved along several routes and floors on an office building. Both outdoor and indoor locations are considered for the transmitters or base stations, which allows the analysis of not only indoor but also outdoor to indoor environment. The use of 2 base stations with different system level combinations of the two is analysed. We show that the 2×4 configuration with base station selection provides almost as good performance as a 4 × 4 full water-filling scheme when the 2 base stations are placed at different locations. Also the spatial correlation properties for the different configurations are analysed and the importance of considering path loss when evaluating capacity is highlighted. Index Terms—Multiple-input multiple-output (MIMO), radio channel measurement, spatial diversity, base station. I