Ilkka Harjula

Analysis of and compensation for non-ideal RoF links in DAS [Coordinated and Distributed MIMO]

Distributed antenna systems have been found to be an elegant solution for the problems arising in high-data-rate wireless communication, particularly in large service areas. This article considers radio over fiber links as an essential part of the DAS, connecting the central unit with the remote antenna units. In particular, we analyze and discuss delays and nonlinearities stemming from the RoF links. In addition, we study the compensation for these impairments. Our studies indicate that the RoF links are a viable and cost-effective solution for implementing the DAS, although some of the RoF link non-idealities require compensation.

Performance of IEEE 802.11 based WLAN devices under various jamming signals

IEEE 802.11 based WLAN systems have gained interest to be used in the military and public authority environments, where the radio conditions can be harsh due to intentional jamming. The radio environment can be difficult also in commercial and civilian deployments since the unlicensed frequency bands are crowded. To study these problems, we built a test bed with a controlled signal path to measure the effects of different interfering signals to WLAN communications. We use continuous wideband noise jamming as the point of comparison, and focus on studying the effect of pulsed jamming and frequency sweep jamming. In addition, we consider also medium access control (MAC) interference. Based on the results, WLAN systems do not seem to be sensitive to the tested short noise jamming pulses. Under longer pulses, the effects are seen, and long data frames are more vulnerable to jamming than short ones. In fact, even a small amount of long frames in a data stream can ruin the performance of the whole link. Under frequency sweep jamming, slow sweeps with narrowband jamming signals can be quite harmful to WLAN communications. The results of MAC jamming show significant variation in performance between the different devices: The clear channel assessment (CCA) mechanism of some devices can be jammed very easily by using WLAN-like jamming signals. As a side product, the study also revealed some countermeasures against jamming.

Channel estimation algorithm for space-time block coded OFDM systems

Accurate channel estimates are needed in space-time coded OFDM systems for decoding purposes. In this paper, a novel channel tracking algorithm based on space-time coded training and information symbols is presented. The algorithm takes advantage of the orthogonality properties of the space-time block codes and it is therefore capable of providing channel estimates without performance degradation due to estimation errors from adjacent channels. The algorithm is tested via Monte Carlo computer simulations using both slowly Rayleigh fading channels and time-invariant frequency selective channels and Alamouti space-time block coding. Computer simulations also show that this simple algorithm outperforms LMS-based approach without additional complexity.

A vendor-independent resource control framework for WiMAX

In this paper, a novel solution to dynamically control the resources of a WiMAX system is proposed. The presented solution is aligned with the NGN trends, as well as with the WiMAX Forum guidelines and architecture. The interaction with the WiMAX systems is performed through SNMP, supporting the standardized IEEE 802.16 f MIB. Furthermore, the paper defines an horizontal abstraction layer, hiding the WiMAX equipments specific functionalities from the network control plane, thus providing robustness and vendor independency. In order to verify the performance of the proposed framework, a WiMAX testbed has been installed, and a set of measurements have been made, showing that the proposed system ensures a quick resource allocation, even for a large number of simultaneous resource reservation requests.

Performance Evaluation of Spectrum Sensing Using Welch Periodogram for OFDM Signals

Reliable spectrum sensing is one of the key technologies for enabling spectrum coexistence. In this paper, co-operative spectrum sensing based on the Welch periodogram is studied in frequency selective fading environment for quadrature phase shift keying (QPSK) and orthogonal frequency division multiplexing (OFDM) signals. Parameter selection for sensing OFDM signal is discussed, and near-optimal values for frequency averaging for QPSK and OFDM signals are derived. The results, given as receiver operating characteristic (ROC) curves, indicate that the frequency-selective fading degrades the performance of the sensing. This could be compensated by co-operation of several sensing nodes. Both analytical and simulation results indicate that best performance of spectrum sensing is achieved with different combining rules in the case of signal passing through additive white Gaussian noise (AWGN) or fading channels.

Spectrum sensing in cognitive femto base stations using welch periodogram

Reliable spectrum sensing is one of the key technologies for enabling cognitive communications. In this paper, spectrum sensing based on Welch periodogram is applied to a femtocell scenario where cognitive femto base station (FBS) is located in the same frequency band as the surrounding macrocell and neighboring femtocells. Parameters for the femtocell simulations are selected based on 3G Partnership Project (3GPP) contributions to tie this work to on-going femtocell standardization work. Different interference scenarios are also discussed. Spectrum sensing based on Welch periodogram is presented, and two scenarios including a user equipment (UE) located indoor and outdoor are selected for further study. The results, given as receiver operation characteristic (ROC) curves, clarify the practical regions inside of which spectrum sensing is capable of providing reliable results for sensing the presence of the MUE.

Empirical Evaluation of Mobile WiMAX with MIMO

Mobile WiMAX is claimed to have the greatest potential of all wireless access technologies already in the markets with respect to network capacity, range, mobility, and quality of service support. Mobile WiMAX has also adopted MIMO as an essential element for increasing capacity and data propagation reliability. However, there are few publicly available studies demonstrating what is possible with Mobile WiMAX in practice and none that empirically evaluates the performance gains attained with MIMO. We fill this gap by comparing spacetime coding based 2x2 MIMO with single antenna mode in our Mobile WiMAX testbed. We conduct thorough measurements that involve vehicular mobility with different types of traffic. We find that MIMO significantly increases the reliability of data propagation over the wireless link, which is particularly evident when we consider TCP traffic. Since 3GPP LTE, the main rival to Mobile WiMAX, equipment and publicly deployed networks are yet to become available, HSPA and the soon-tobe-deployed HSPA+ amendments to UMTS are the only wireless technologies challenging Mobile WiMAX. Thus, we also compare Mobile WiMAX with HSPA, providing the first publicly available side-by-side measurement study.

Multihop Relaying for Local Area Access

We consider multi-hop forwarding in a Beyond 4G local area network, where in addition to nodes with wired backhaul, there is a high density of self-backhauling relay nodes acting simultaneously as access points towards users. The nodes apply Time Division Duplexing, access is framed and synchronized along a multi-hop flow, and there is a corresponding reuse factor for active hops along a multihop route. Interference cancelation, as well as power and resource optimization, is performed within along a route. Simulations are performed in a local area network consisting of multiple multi-floor buildings. Multi-hop self-backhauling is found to significantly improve the coverage of high data rates in the system.

Full duplex relaying for local area

We consider full-duplex multi-hop forwarding in a Beyond 4G local area network. In the network, there is a high density of self-backhauling relay nodes that simultaneously act as access points towards the users, in addition to few nodes with wired backhaul. The access is framed and synchronized along the multi-hop flow, and the nodes apply time division duplexing. Interference cancelation as well as power optimization is performed within the multihop route. Simulations are carried out in a local area network consisting of multiple multi-floor buildings. The propagation channel is modelled using static pathloss, log-normal distributed random variable, or static pathloss with Rayleigh fading. The simulation results indicate that full-duplex relaying improves the network performance over half-duplex relaying, if self-interference channel attenuation is kept over 80 dB. The means of achieving tolerable self-interference levels in full-duplexing relays via physical design of the relay, and analog and digital interference cancellation are discussed.

WiMAX backhaul for environmental monitoring

In environmental monitoring, wirelessly connected sensors form local networks and collect data, which are sent to a central repository for further analysis and permanent storage. In this paper we empirically evaluate the use of fixed WiMAX to backhaul data from an environmental monitoring application. We find that aggregating sensor readings is crucial for taking full advantage of the WiMAX backhaul. We consider application-layer aggregation for scenarios where sensor communication is programmable or at least configurable. For scenarios where sensors cannot be modified, we consider network-layer aggregation using a performance enhancing proxy. Finally, we consider the case where application- and network-layer aggregation are combined. Our results show that without aggregation, goodput is only a small fraction of the maximum achieved in our testbed. Furthermore, we examine the effect of different modulation schemes on performance and quantify in practice the number of wireless sensor nodes that could be served with different modulation and data aggregation combinations.