Kalle Ruttik

Detection of Unknown Signals in a Fading Environment

In this letter we consider the effect of combined slow and fast fading on the energy detection. We model the received signal power distribution and propose a simplified approximation to this distribution. The approximation allows us to derive the distribution of the decision variable at the detectors output in closed-form. By using the suggested distribution, we find that in the block fading channel the impact of slow fading is not significant. In the case of multiple independent fast fading realizations with commo.n slow fading value we can show how the slow fading starts to dominate the detection performance.

Model for computing aggregate interference from secondary cellular network in presence of correlated shadow fading

The estimation of the total interference generated from a secondary system is essential for protecting the primary receivers. However, interference level estimation from a large number of secondary transmitters is a challenging problem. In this paper, we estimate the aggregate interference as an integration over the power spatial density in the secondary systems deployment area. We modify such integration-based model to contain the correlation in shadow fading. We apply the model on a cellular system downlink and study how well the proposed analytical model describes the interference level in correlated and non-correlated slow fading environment. The analysis indicates that for cell size less than five km the integration based model describes interference relatively well. For larger cell sizes, the interference is dominated by a few strong sources and has to be computed by summing over the power of all individual transmitters. It is also illustrated that the full correlated and independent slow fading provide two extremes of the amount of generated interference. The complex estimation of the impact of the fading cross correlation can be avoided by using those two extreme models as bounds to the interference level.

Controlling the interference from multiple secondary systems at the TV cell border

The main requirement for secondary operation over the TV white spaces is interference control. In this paper, we propose an interference control scheme that allows multiple secondary systems to maintain their aggregate interference at the TV receivers under specific protection limits. The main characteristic of the proposed scheme is the small signaling overhead required for the cooperation between the secondary systems. The secondary systems need only to communicate their locations to a central network controller. The controller can then allocate the transmission power levels to the different systems such that the TV receivers are sufficiently protected. At the same time, the sum of the transmission power levels allocated to the secondary systems is maximized.

Computation of Aggregate Interference from Multiple Secondary Transmitters

We describe how to compute the secondary systems generated aggregate interference in a shadow fading environment. We express the interference as an integral over secondary systems service area. For numerical computation of this integral we quantize its polar coordinate representation. We outline how the proposed numerical method can be used to compute the interference generated from a cellular network. For making this connection we parameterized the model by the cell size and the frequency reuse distance of the cellular secondary system.

Implementing TD-LTE as software defined radio in general purpose processor

Cloud radio access networks use servers that are connected to Remote Radio Heads (RRH). Base station (BS) implementation with this concept is challenging. The strict real-time nature of baseband (BB) processing seems to rule out usage of General Purpose Processors (GPP) with non-real time Operating Systems (OS). In this paper, we propose a BS architecture where most of the real-time processing is confined into a Virtual Hardware Enhancement Layer (VHEL). VHEL hides the hardware non-idealities from the software and vice versa. Possible errors due to the non-real-time OS and RRH appear as channel errors, which makes software development easier. We demonstrate the benefits of our architecture by implementing a Time-Division LTE system (TD-LTE) in C++ and running it as a user process in an Intel i7 class PC. Over-the-air transmissions are realized using USRPs. We report the performance of the implemented platform. We observe that with the given VHEL the transmitter and receiver never lose synchronization. Also the PC tends to be quick enough to feed the data; and the loss rate of subframes due to the non-real-time nature of the platform is relatively low. The proposed platform provides the possibility to implement TD-LTE on GPPs and virtual machines.

Distributed Sensing in Multiband Cognitive Networks

We consider a short range cognitive network searching for spectrum holes from very wide bandwidth. In practice, one cognitive user can sense only a small portion of spectrum. Unfortunately, in fading environment a reliable detection scheme requires measurements collected by multiple users. Because of that, it is unreasonable to expect a small-sized network to sense the complete candidate bandwidth. In this paper we propose an algorithm for optimal sensing of multiple spectrum bands by multiple cognitive users. The user allocation is optimized so that the expected opportunistic throughput is maximized and the total power spent for spectrum measurements is controlled. As a constraint we use the detection performance requirements imposed by the primary systems. For a small number of spectrum bands the optimal solution can be found by exhaustive search. For a large number of spectrum bands we view the spectrum sensing as a multiple choice knapsack problem. By using algorithms for this class of problems we propose two heuristics that are suitable for optimizing spectrum sensing in multiband cognitive networks. These algorithms provide quick, near optimal solutions and are therefore suitable for practical spectrum sensing systems.

Distributed Power Detection in Shadowing Environment and with Communication Constraint

In this paper we analyze the performance of the distributed power detection algorithm operating in a shadowing environment and with communication constraints. A well performing detection algorithm is critical for detecting a primary users signal for dynamic spectrum allocation purposes. Dynamic spectrum allocation allows a secondary user to use a spectrum chunk provided that the primary user is not disturbed. In a shadowing environment one secondary user could not guarantee a reliable detection of primary users signal with acceptable false detection probability. The detection performance can be greatly enhanced by combining measurements from secondary users at a fusion centre. Such combining requires communication bandwidth. We provide equations and numerically compute the bounds for two extremes of communication requirements. In the first scheme the secondary users send a full loglikelihood ratio, which in principle would require infinite amount of communication capacity. In the second scheme the secondary users send to the fusion centre only one bit, describing the hard decision made based on the loglikelihood ratio. These two cases provide performance bounds for possible practical implementations.

Spectrum sensing with multiple antennas

The primary signal detection is an essential operation for the secondary spectrum usage. In this paper we extend the covariance based detection for multiple-antenna receiver. The proposed method uses the noise power estimation and does not suffer from the noise level uncertainty. We analyze the detection algorithm and compute the distribution of the decision variable not only for the pure noise case but also for the jointly presence of primary signal and noise. By deriving the distribution of the primary signal in noise we are able to use the detection probability as the detector design constraint. Our analysis takes advantage of the high amount of samples available at the detector. The proposed method sets the foundation for the analysis of any detector that involves cross correlation, summing and squaring operations of samples.

Design of adaptive modulation and coding system using ideal performance codes

A major issue in feedback systems is to use the feedback bits most efficiently. In this paper, we use information theory to get an upper limit on the value of feedback bits in an optimum AMC system. The AMC optimization problem is simplified by using ideal error performance (i. e. a step function like performance curve) characteristic of capacity-reaching codes. Given a strong enough code such as turbo code similar step function based approximation can be used. The results show nice match between the solution sets from the ideal error performance approximation and the real error performance.

Interference Canceling Power Optimization for Device to Device Communication

We consider D2D communication underlying cellular uplink communication when single stage interference cancellation receivers are available to improve local service. The interference cancelation configurations and transmission powers are jointly optimized in the network to maximize a network utility. Each receiver may or may not cancel the signal from the dominant interferer. With N-1 D2D pairs and one cellular transmitter, there are 2N possible IC state combinations. A scheduling problem is formulated to allocate resources to these combinations, and network utility is maximized by iterating between scheduling weight and transmit power. In a simulation where sum rate or proportionally fair network utility is maximized, we observe significant gains in the spectral efficiency enjoyed by the users.