Janne Riihijärvi

Performance study of IEEE 802.15.4 using measurements and simulations

IEEE 802.15.4 was developed to meet the needs for simple, low-power and low-cost wireless communication. In the past couple of years it has become a popular technology for wireless sensor networks. It operates primarily in the 2.4 GHz ISM band, which makes the technology easily applicable and worldwide available. However, IEEE 802.15.4 is potentially vulnerable to interference by other wireless technologies working in this band such as IEEE 802.11 and Bluetooth. This paper gives a short overview of the IEEE 802.15.4 and carefully analyzes the properties and performance of IEEE 802.15.4 through measurement of the RSSI, PER and run lengths distribution using real off-the-shelf hardware. Furthermore we present simulation results from the evaluation of the IEEE 802.15.4 MAC protocol. Finally, we address the coexistence between IEEE 802.11 and IEEE 802.15.4 and measure the impact these two wireless technologies have on each other when operating concurrently and in range

Full length article: Empirical time and frequency domain models of spectrum use

Dynamic spectrum access (DSA) has been proposed as a solution to the spectrum scarcity problem. However, the models for spectrum use, that are commonly used in DSA research, are either limited in scope or have not been validated against real-life measurement data. In this paper we introduce a flexible spectrum use model based on extensive measurement results that can be configured to represent various wireless systems. We show that spectrum use is clustered in the frequency domain and should be modelled in the time domain using geometric or lognormal distributions. In the latter case the probability of missed detection is significantly higher due to the heavy-tailed behaviour of the lognormal distribution. The listed model parameters enable accurate modelling of primary user spectrum use in time and frequency domain for future DSA studies. Additionally, they also provide a more empirical basis to develop regulatory or business models.

TV White Space in Europe

In this paper, we study the availability of TV white spaces in Europe. Specifically, we focus on the 470-790 MHz UHF band, which will predominantly remain in use for TV broadcasting after the analog-to-digital switch-over and the assignment of the 800 MHz band to licensed services have been completed. The expected number of unused, available TV channels in any location of the 11 countries we studied is 56 percent when we adopt the statistical channel model of the ITU-R. Similarly, a person residing in these countries can expect to enjoy 49 percent unused TV channels. If, in addition, restrictions apply to the use of adjacent TV channels, these numbers reduce to 25 and 18 percent, respectively. These figures are significantly smaller than those recently reported for the United States. We also study how these results change when we use the Longley-Rice irregular terrain model instead. We show that while the overall expected availability of white spaces is essentially the same, the local variability of the available spectrum shows significant changes. This underlines the importance of using appropriate system models before making far-reaching conclusions.

Interference Measurements on Performance Degradation between Colocated IEEE 802.11g/n and IEEE 802.15.4 Networks

We have made detailed measurements, on the impact of modern Wireless LAN technologies on the IEEE 802.15.4 in the 2.4 GHz ISM band. We have specifically focused on IEEE 802.11g and pre-standard IEEE 802.11n products as potential interferers. Our measurements show that high levels of network traffic interference from either of these technologies has disastrous impact on the performance of IEEE 802.15.4. Our results also indicate that these interference effects are especially difficult to avoid in the (pre-standard) 802.11n case due to the significantly increased channel bandwidth compared to previous Wireless LAN technologies. Widespread adoption of IEEE 802.11n especially in applications involving high data rates (such as backbones for wireless mesh networks) could thus have serious impact on the usability of IEEE 802.15.4 as well as other low-power 2.4 GHz ISM band technologies. This indicates that low-power building automation, consumer electronics and sensor networks may be vulnerable to the interference from the future IEEE 802.11n high-data rate WLAN deployments.

Frequency allocation for WLANs using graph colouring techniques

At present, no standard frequency allocation mechanism exists for wireless LAN access points. In this article, we introduce a number of techniques based on graph colouring algorithms, and demonstrate their effectiveness using simulations. We also suggest a preliminary message format the access points could employ to exchange information regarding the wireless channel and elaborate on the possible protocol architectures that could be used in the actual channel allocation process.

Characterization and modelling of spectrum for dynamic spectrum access with spatial statistics and random fields

There is need to develop better models and characterization methods for spectrum usage and radio environments of cognitive radios. Currently different theoretical and simulation based approaches towards enabling dynamic spectrum access would greatly benefit from the possibility to generate synthetic data for testing purposes. Such Radio Environment Maps must statistically exhibit the characteristics of realistic environments. Previous and on-going spectrum measurement campaigns are generating a vast amount of such data. In this paper we provide a partial answer to the spectrum modelling problem by showing how one can characterize and model spectrum maps with spatial statistics and random fields. We present the basic mathematical premises for building models and also through examples outline how one can generate useful statistics from real measurement data.

Performance evaluation of IEEE 1609 WAVE and IEEE 802.11p for vehicular communications

In this paper we study the performance of the IEEE 1609 WAVE and IEEE 802.11p trial standards for vehicular communications. We have implemented key components of these standards in a simulation environment also supporting realistic vehicular mobility simulation. We study both the overall capacity of vehicular networks utilizing the said standards, as well as delay performance, which is an extremely important performance metric especially for safety applications. Our results show that the traffic prioritization schemes selected for the standards work well, and even in the presence of multi-channel operation implemented by the IEEE 1609.4 the delay of control messages of highest priority remains on the order of tens of milliseconds. Thus even with high densities of vehicles these standards should yield a stable platform a variety of vehicular applications can be built on.

UHF white space in Europe — A quantitative study into the potential of the 470–790 MHz band

In this paper we study the availability of TV white spaces in Europe. We focus specifically on the 470–790 MHz UHF band that is still predominantly used for TV broadcasting also after the digital dividend (frequency reallocation as part of transition to digital TV), which has taken place or is ongoing in several European countries. We find that in the countries used in our studies, approximately 56% of the TV channels are unused, when averaged over the whole geographic area. Considering the average over population instead of geographic area reduced the mean available white space to 49% of the channels. Our results confirm quantitatively the often stated expectation that there are indeed fewer white spaces available in Europe compared to, for example, the United States. We also study the influence of different modeling assumptions on our results, especially focusing on the impact of the choice of propagation model. Our results show that changing from statistical ITU-R model to the Longley-Rice irregular terrain model does not cause a major change in the overall estimated availability of white spaces. However, changing the propagation model can significantly affect especially the local variability of the estimated availability of spectrum. This underlines the importance of using right system models in various studies before making too far-reaching conclusions.

Automatic channel allocation for small wireless local area networks using graph colouring algorithm approach

Classical graph colouring and its generalisations have been used to model various frequency and channel allocation processes in different radio and wireless network contexts for some time now. However, most of this work has targeted cellular networks and graphs with a relatively large number of nodes. In this article we demonstrate how graph colouring can be used as a theoretical basis for a protocol to effectively assign channels to WLAN access points. We also give the outline of the protocol operation, and show its effectiveness with real-life wireless networks.

Spatial statistics and models of spectrum use

In order to opportunistically exploit unused radio spectrum nodes of dynamic spectrum access (DSA) networks monitor the spectrum around them. Such cognitive radios can greatly benefit from a spatial characterization of spectrum use. However, there is need to find an efficient way to describe spatial use, something which has not been studied in details so far. In this paper, we introduce spatial statistics techniques as promising methods to describe spectrum use and enable optimization of DSA networks. We discuss two approaches to spatial modelling of spectrum, namely a deterministic approach based on a system model of the complete radio environment and an empirical approach that exploits passive measurements of the spectrum use. We elaborate on the impact of different network properties on the models and provide realistic parameter sets for generation of simulation scenarios. Additionally, we investigate cooperative sensing as a use case for spatial statistics based runtime optimization of the network configuration.