Daniel Catrein

Power control, capacity, and duality of uplink and downlink in cellular CDMA systems

Accurate power control is an essential requirement in the design of cellular code-division multiple-access (CDMA) systems. In this paper, we contribute three main themes to the power control problem. First, we derive an efficient algorithm for computing minimal power levels for large-scale networks within seconds. Nice and intuitive conditions for the existence of feasible power solutions follow from this approach. Second, we define the capacity region of a network by the set of effective spreading gains, or data rates, respectively, which can be supplied by the network. This is achieved by bounding the spectral radius of a certain matrix containing system parameters and mutual transmission gain information. It is shown that the capacity region is a convex set. Finally, we reveal an interesting duality between the uplink and downlink capacity region. In a clear-cut analytical way, it substantiates the fact that the uplink is the more restricting factor in cellular radio networks. The same methods carry over to certain models of soft handover. In the case that the channel gains are subject to log-normal shadowing, we introduce the concept of level-/spl alpha/ capacity regions. Despite the complicated structure, it can still be shown that this set is sandwiched by two convex sets coming arbitrarily close as variance decreases.

Accelerating Radio Wave Propagation Predictions by Implementation on Graphics Hardware

Fast radio wave propagation predictions are of tremendous interest, e.g., for planning and optimization of cellular radio networks. We propose the use of ordinary graphics cards and specialized algorithms to achieve extremely fast predictions. Our implementation of the empirical COST-Walfisch-Ikegami model allows the computation of several hundred predictions in one second in a 7 km urban area. Further, we present a ray-optical approach exploiting the programming model of graphics cards. This algorithm combines fast computation times with high accuracy.

Location tracking of mobiles: a smart filtering method and its use in practice

Accurate location tracking of mobile stations in cellular radio networks is of tremendous interest for many applications. In this work, we analyze the performance of a speed and location tracking algorithm using data from a field trial. The algorithm fits received signal strengths of surrounding base stations to corresponding predictions. These raw location estimates, in GSM available each 0.48 s, are subsequently smoothed by a model-based Kalman filter. An essential ingredient of our method is to find suitable initial parameters. The method is tested with measurement data from a field trial by Siemens. Although the field strength prediction method is rather simple, the location algorithm itself yields promising results. Typical average deviations from the true positions were 173.5 m for indoor, 117.7 m for walking, and 104.9 m for driving scenarios. This shows that the method is robust against moderate errors in the prediction model and leads to good results in a real GSM network.

DRM protected dynamic adaptive HTTP streaming

Dynamic adaptive HTTP streaming (DASH) is a new concept for video streaming using consecutive downloads of short video segments. 3GPP has developed the basic DASH standard which is further extended by the Open IPTV Forum (OIPF) and MPEG. In all versions available to date, only very simple content protection use cases are enabled. Extensions are needed to enable important advanced use cases like pay-per-view and license change in an ongoing video channel. In this publication, we analyze what is missing in the current DASH standards with regards to content protection, and propose changes and extensions to DASH in order to enable the application of DRM. This includes changes to the Media Presentation Description (MPD), and the file format. With a suitable key and license structure used together with DASH, even complex use cases like pay-per-maximum-quality are possible. Besides the analysis of required changes to DASH for content protection, and the description of suitable key and license structures applied to DASH, we also present a proof-of-concept implementation of the proposed concepts.

Analytic Evaluation of a Hybrid Broadcast-Unicast TV Offering

Today mobile TV services in cellular spectrum are delivered over unicast radio bearers as offered by 3G technologies like WCDMA and HSDPA. Since unicast transmission does not scale very well if mobile TV becomes a true mass service, 3GPP has defined a broadcast extension for UMTS, called multimedia broadcast multicast service (MBMS). MBMS introduces radio broadcast bearers serving all users in common, thus providing true broadcast capabilities. A not so well known feature of MBMS is its support for hybrid broadcast-unicast transmission. In this mode the more popular channels are broadcasted over point- to-multipoint radio bearers while the less popular channels are delivered over point-to-point bearers only on request. In this paper we will present a theoretical framework which allows an analytic evaluation of the capacity limits when delivering mobile TV services over hybrid broadcast-unicast transmission schemes provided by MBMS.

Gaussian Random Fields as a Model for Spatially Correlated Log-Normal Fading

Slow fading or shadowing on a wireless channel is commonly modeled by stochastically independent, log-normally distributed random variables. However, as slow fading is caused by buildings and large size obstacles, spatial correlations occur. In this paper, Gaussian random fields are used as a model for correlated slow fading in urban environments. An exponential correlation function is employed. The according parameters are estimated from path gain predictions by an accurate ray-optical propagation algorithm, named CORLA. Furthermore, a multidimensional model is suggested to describe correlated shadowing of the path gains from different base stations to a single receiver.

Private Domains in Networks of Information

The networking research community has recently started looking into so-called information-centric networks, which regard information objects as the primary elements in the network. Users store and retrieve the information objects, denoted by object identifi- ers, from the network without knowing or bothering which host serves the actual request, comparable to earlier initiatives in networking that separate node identifiers from network locations. The capability to address content directly takes many optimizations, such as content- optimized routing or transparent caching, to the next level, while at the same time enabling novel services. In order to prevent unautho- rized access to the information objects that roam around freely in the global network, objects shall be encrypted. We argue that for some use cases, e.g. when dealing with sensitive company-internal information, a stricter partitioning of the network is required. We propose and discuss two solutions to this problem, one focusing on securing the name resolution procedure, the other concentrating on the actual re- trieval. Both mechanisms complement existing proposals by separating information objects in the private network domain while maintaining global reachability.

Feasibility and Power Control for Linear Multiuser Receivers in CDMA Networks

This paper is concerned with linear multiuser receivers for code division multiple access (CDMA). Mainly three are of practical relevance: the matched filter (MF), the successive interference cancellation (SIC) and the minimum mean squared error (MMSE) receiver. For the first time, an explicit representation of the signal-to-interference-plus-noise (SINR) ratio for the linear SIC receiver under general random spreading is given. For the MMSE receiver the SINR is obtained by using the asymptotic large system regime as an accurate approximation. Once the SINR for each receiver type is known, the question arises what quality-of-service (QoS) requirements can be supported by an adequate power assignment. A vector of QoS requirements (gammai) is called feasible, if there exists some power assignment such that the SINR of each user i does not fall short of threshold gammai. It is shown that for each receiver type there exists a componentwise minimal power assignment such that the SINR of each user equals this threshold. This minimal vector may be determined for the MF and SIC receiver by solving a system of linear equations with coefficients explicitly known. For the MMSE receiver in the large system regime an iterative algorithm is derived, which converges to the optimum power allocation.

An Analysis of Web Caching in Current Mobile Broadband Scenarios

The web caching business has significantly changed over the recent years. The percentage of content that is suitable for caching has constantly decreased, e.g. due to unfavorable content formats like Flash or varying URLs due to usage of CDNs. While this of course has a negative impact on the effectiveness of caching, also other parts of the equation have changed: Transmission cost has dropped significantly, and so has the cost to run a cache, especially in times where Cloud Computing approaches commoditize such operations. At the same time, the topic of caching as such remains highly relevant, as the data volumes in the networks continue to grow exponentially and net-work operators are seeking ways to cope with this demand in an efficient manner. We have therefore analyzed a realistic mobile network topol-ogy consisting of primary and secondary telecommunication sites that can be found in many deployments, and have determined the efficiency of web caching, taking into account the latest changes in the ecosystem. An abstract telecommunication site model has been developed and parameterized with a realistic traffic and cost structure for a Western European operator. We find that, contrary to other research that suggests the days of caching are over, there is a clear business case for introducing web caches in primary sites. With current hosting costs and economies of scale, a caching system can already be efficient if only 5.1% of the traf-fic is suitable for caching. For the secondary sites the same effect can be observed and, although the effect is not that distinct, also here web caching can bring clear benefits in certain scenarios.

Segregating in- and other-cell interference with applications to decentralized admission control

This paper deals with the intertwined problems of feasibility, power allocation and call admission control on the basis of the signal-to-interference-ratio in multi-cell CDMA networks. Positively homogeneous in-cell interference functions are introduced. Under this general concept optimum power allocations are determined by first solving in-cell interference problems with fixed background noise, and subsequently a small system of linear coupling equations. This approach simultaneously clarifies the existence of some power allocation such that minimum SIR requirements are fulfilled. The decomposition is also utilized for designing a decentralized admission control algorithm. Finally, guidelines are given on how required parameters may be estimated in existing network structures.