Jorma Lilleberg

Spectrum Sharing Scheme Between Cellular Users and Ad-hoc Device-to-Device Users

In an attempt to utilize spectrum resources more efficiently, protocols sharing licensed spectrum with unlicensed users are receiving increased attention. From the perspective of cellular networks, spectrum underutilization makes spatial reuse a feasible complement to existing standards. Interference management is a major component in designing these schemes as it is critical that licensed users maintain their expected quality of service. We develop a distributed dynamic spectrum protocol in which ad-hoc device-to-device users opportunistically access the spectrum actively in use by cellular users. First, channel gain estimates are used to set feasible transmit powers for device-to-device users that keeps the interference they cause within the allowed interference temperature. Then network information is distributed by route discovery packets in a random access manner to help establish either a single-hop or multi-hop route between two device-to-device users. We show that network information in the discovery packet can decrease the failure rate of the route discovery and reduce the number of necessary transmissions to find a route. Using the found route, we show that two device-to-device users can communicate with a low probability of outage while only minimally affecting the cellular network, and can achieve significant power savings when communicating directly with each other instead of utilizing the cellular base station.

Chip-level channel equalization in WCDMA downlink

The most important third generation (3G) cellular communications standard is based on wideband CDMA (WCDMA). Receivers based on TDMA style channel equalization at the chip level have been proposed for a WCDMA downlink employing long spreading sequences to ensure adequate performance even with a high number of active users. These receivers equalize the channel prior to despreading, thus restoring the orthogonality of users and resulting in multiple-access interference (MAI) suppression. In this paper, an overview of chip-level channel equalizers is delivered with special attention to adaptation methods suitable for the WCDMA downlink. Numerical examples on the equalizers′ performance are given in Rayleigh fading frequency-selective channels.

Genetic algorithms for multiuser detection in synchronous CDMA

The optimal multiuser detector for communication systems can be characterized as an NP-hard optimization problem. The application of genetic algorithms to suboptimal multiuser detection is studied. The performance of the genetic algorithm multiuser detection is studied by computer simulations. They demonstrate that a good initial guess vector is needed to obtain a superior performance. If the decorrelating detector output is included in the set of initial guess vectors, the performance of the genetic algorithm detection is close to that of the single user system.

Iterative implementation of linear multiuser detection for dynamic asynchronous CDMA systems

Several linear multiuser detectors for code-division multiple access (CDMA) systems can be characterized as an inverse of some form of correlation matrices. If the correlations change, the detectors must be redesigned. An ideal computation of the decorrelating or the linear minimum mean-squared-error (LMMSE) detector requires order K/sup 3/ flops, where K is the number of users. To alleviate the computational complexity, iterative decorrelating and LMMSE detectors are proposed. The iterative detectors use steepest descent (SD), conjugate gradient (CG), and preconditioned conjugate gradient (PCG) algorithms, and require order K/sup 2/ flops per iteration. Their main advantages are the reduced number of flops and their suitability to highly parallel implementations. The correlation coefficient computation can also be embedded into the CG algorithm, which is an advantage with time-varying signature waveforms. The performance of the iterative algorithms is studied via computer simulations.

Downlink channel decorrelation in CDMA systems with long codes

We develop linear detectors suitable for a code division multiple access (CDMA) mobile receiver using long codes. The special signal structure in the downlink transmission is exploited to obtain a simple detection rule. A least-squares (LS) detector, a best linear unbiased estimator (BLUE) detector, and a linear minimum mean-square error (LMMSE) detector are derived. For the LMMSE detector we consider an adaptive implementation. The results show that improvement can be achieved using the proposed detectors compared with that of the conventional RAKE receiver.

Inter-Operator Spectrum Sharing in a Broadband Cellular Network

Inter-operator spectrum sharing in a wide-area broadband network is considered in this paper. A packet-based cellular model is developed, emphasizing the shift in the telecommunications industry towards IP-based services. In the framework of this architecture, we show the ideal maximum load that the network can handle without excessive delays, and present our main result: that even with sharing, this operating point is rarely achieved. This is the consequence of geographical and physical layer constraints, which limit the gains achievable by spectrum sharing. Using a physical layer cellular model with idealistic resource management, we quantify the achievable sharing gains. We further analyze these gains in the context of variable data rates, quality of service guarantees, and number of operators

Spectrum sharing in a cellular system

The cost of long term leasing of spectrum has proven to be a major road block in broad deployment of 3rd generation cellular radio systems. Similar economical road blocks can be predicted for systems beyond 3G. In order to alleviate the burden of spectrum cost incurred by service providers, there must be a paradigm shift from fixed allocations to a more flexible spectrum management. Barring practical implementations, this is indeed a feasible economical solution since the spectrum as a commodity does not depreciate with use. The search for novel spectrum management techniques has been ongoing for a decade or so and has taken many different forms from dynamic channel assignment to dynamic spectrum allocation, software defined radio, and cognitive radio. These different methodologies attack a common problem, efficient spectrum management, with various tools and complexities and have had varying degrees of success. This paper introduces a simple concept of spectrum sharing in a cellular radio system. We propose two protocols to manage spectrum sharing by redistributing excess users to spectrum bands with excess user capacity. We analyze the performance of ideal sharing by evaluating the probability of call block. We also simulate the performance of the two protocols in a realistic cellular environment. Both analyses show great gains in performance when the base-stations decide to share the spectrum as opposed to only assign users to their licensed band.

Adaptive channel equalization and interference suppression for CDMA downlink

In the synchronous CDMA downlink using orthogonal codes, the intracell multiple access interference (MAI) is essentially caused by the frequency selective multipath channel. Therefore, intracell MAI may be suppressed by simple linear channel equalization. Similarly, intercell interference can be suppressed by means of linear space-time filtering. We compare two different adaptive receiver algorithms in the context of one or several receive antenna elements. The first one is based on adaptive LMMSE chip estimation, whereas the other one performs simple interchip interference cancellation by adaptive chip separation. The presented receiver structures are suitable for systems with long code scrambling, such as 3rd generation wideband CDMA standard. The results show considerable performance gains when compared to the conventional RAKE receiver.

Parallel Interference Cancellation in Multiuser CDMA ChannelEstimation

Parallel interference cancellation (PIC) based channel parameter estimators for frequency selective fading channels are proposed for the uplink in code-division multiple-access (CDMA) mobile communication systems. The performance of PIC based algorithms depends heavily on the quality of the multiple-access interference estimates, which can be improved by using adaptive channel estimation filters. The performance of two adaptive complex channel coefficient estimation filters has been verified in a fading channel by computer simulations. According to the results, the PIC based adaptive channel estimators outperform clearly conventional, successive interference cancellation, and decorrelation based adaptive channel estimators. The PIC method is also used in delay tracking. By using the principles of sample-correlate-choose-largest (SCCL) delay trackers, a robust algorithm for multiuser delay tracking in fading channels is obtained.

CDMA downlink code acquisition performance in frequency-selective fading channels

In this paper, the performance of some code acquisition schemes are studied in the downlink of DS-CDMA systems. The conventional matched filter, the minimum variance method, the MUSIC algorithms and the eigenvector method based delay acquisition schemes are studied in a frequency-selective fading channel. The three last algorithms are based on different estimation schemes for the inverse of the sample-covariance matrix. Based on the results, the minimum variance based method can be used to improve the acquisition performance in comparison to the conventional non-coherent matched filter method. The MUSIC algorithm becomes useless in highly loaded systems. The best performance, however, was obtained by using a longer despreading interval in the conventional delay estimator. Unfortunately, it would require either unmodulated pilot channel or a control channel with lower data rate, which may not be available in all applications.