Marko Höyhtyä

Performance improvement with predictive channel selection for cognitive radios

Prediction of future availability times of different channels based on history information helps a cognitive radio (CR) to select the best channels for control and data transmission. Different prediction rules apply to periodic and stochastic ON-OFF patterns. A CR can learn the patterns in different channels over time. We propose a simple classification and learning method to detect the pattern type and to gather the needed information for intelligent channel selection. Matlab simulations show that the proposed method outperforms opportunistic random channel selection both with stochastic and periodic channel patterns. The amount of channel switches needed over time reduces up to 55%, which reduces also the delay and increases the throughput.

Classification-Based Predictive Channel Selection for Cognitive Radios

The proposed method classifies traffic patterns of primary channels in cognitive radio systems and applies different prediction rules to different types of traffic. This allows a more accurate prediction of the idle times of primary channels. An intelligent channel selection scheme then uses the prediction results to find the channels with the longest idle times for secondary use. We tested the method with Pareto and exponentially distributed stochastic traffic and with deterministic traffic. The predictive method using past information improves the throughput of the system compared to a system based on instantaneous idle time information. The classification-based predictive method improves the performance compared to pure prediction when the channels of interest include both stochastic and deterministic traffic. The amount of collisions with a primary user can drop 60 % within a given interval compared to a predictive system operating without classification.

Application of cognitive radio techniques to satellite communication

Applicability of cognitive radio (CR) techniques to satellite communications is investigated and classification for the possible applications is foreseen in this paper. The proposed application scenarios include a) secondary use of satellite spectrum by a terrestrial system, b) secondary use of terrestrial spectrum by a satellite system, and c) a hybrid scenario where terrestrial network coverage is expanded by the use of satellite spots. Key challenges and enabling technologies for each scenario are discussed. Link budget analyses and system simulations are used to define operational limits for interference management in the mentioned scenarios regarding spectrum sensing, transmission power control, and directional antennas. The results show that CR techniques should be applied with caution in satellite bands. However, obvious potential to improve the spectrum efficiency can be seen in each scenario.

Spectrum Occupancy Measurements: A Survey and Use of Interference Maps

In order to provide meaningful data about spectrum use, occupancy measurements describing the utilization rate of a specific frequency band should be conducted over a specific area instead of a single location. This paper presents a comprehensive methodology for the measurement and analysis of spectrum occupancy. This paper surveys spectrum measurement campaigns and associated interference maps, introducing the latter as a tool for spectrum analysis and management based on measurement data. An interference map characterizes the spectrum use by defining the level of interference over an area of interest in a certain frequency band. Building on findings from practical measurement studies, guidelines for spectrum occupancy measurements are given. While many scientific spectrum occupancy measurement papers tend to be too optimistic about the significance and generality of the results, we propose a cautionary perspective on drawing strong conclusions based on the often limited amount of data gathered. The different phases of the spectrum occupancy measurement and analysis process are described and a thorough discussion of interpolation methods is provided. Means to improve the measurement accuracy are discussed, especially regarding spatial domain considerations and the impact of the sampling interval on the results. A practical example of an improved measurement system design covering all the phases of the measurement process and used at the Turku, Finland; Blacksburg, VA, USA; and Chicago, IL, USA, spectrum observatories is given. Using the improved design, more realistic spectrum occupancy data can be obtained to lay the foundation for spectrum management decisions.

Distributed and directional spectrum occupancy measurements in the 2.4 GHz ISM band

This paper presents distributed and directional spectrum occupancy measurements in the 2.4 GHz ISM band. Spectrum occupancy measurements can be used to assess how efficiently the spectrum bands are used today. Future cognitive radio systems can improve the spectrum occupancy by filling the gaps in the prevailing spectrum by opportunistically using unoccupied channels. Most of the spectrum occupancy measurements in the literature have been conducted by using a single measurement device with an omnidirectional antenna. The resulting spectrum occupancy values have presented an average of the overall situation. To characterize the influence of the spatial dimension on the spectrum occupancy in a given area, we introduce the directional spectrum occupancy metric. Directional spectrum occupancy is defined as the fraction of time that the received power in a channel exceeds a threshold in a given measurement direction. We have used two separately located measurement devices with directional antennas to measure the directional spectrum occupancy in an office area with heavy traffic load. The results indicate that the spectrum occupancy is heavily dependent on the measurement location and direction. The influence of the spatial dimension is therefore very crucial in the development of future cognitive radio systems.

Overview and comparison of recent spectrum sharing approaches in regulation and research: From opportunistic unlicensed access towards licensed shared access

This paper reviews recent spectrum sharing models under study in the spectrum regulation and wireless communications research domains. An overview of different spectrum regulator forums and directions is presented and their activities related to the development of spectrum sharing models are reviewed. Special emphasis is put on the recent European and US regulatory approaches for spectrum sharing. In particular, Licensed Shared Access (LSA) and Collective Use of Spectrum (CUS) models from Europe and Three-Tier Hierarchy Model from the US are analyzed in detail. A comparison is made between these approaches to identify similarities and differences in the regulatory developments. Factors for developing a successful sharing model are also discussed. For a dynamic spectrum sharing model to be adopted, it must protect the rights of entrant users without impact to the legacy systems. It must also create a reasonable straightforward opportunity for an entity that wishes to access a shared spectrum to do so in a manner that is neither overly complex nor costly to implement. The practical implementation of dynamic spectrum sharing models is likely to require different national implementations because the regulatory approaches and the incumbent spectrum uses are different in various countries.

Relationship of Average Transmitted and Received Energies in Adaptive Transmission

This paper studies the analytical relationship between the average transmitted and received energies under several adaptive transmitter power control methods, including water filling, truncated power inversion, and downlink beamforming. The study is applicable to many fading channel scenarios, including frequency-nonselective, frequency-selective, and multiple-input-multiple-output (MIMO) channels. Both the average transmitted and received energies are commonly used in performance comparisons, and the selection depends on what one wants to investigate. The transmitted energy is known to be the basic system resource. In the case of adaptive transmission, the average transmitted energy should, in general, be used instead of the average received energy. The use of transmitted energy leads to the normalization problem of the channel. The ratio of received energy to transmitted energy is the energy gain of the channel. All physical systems follow an energy-conservation law, which implies that the energy gain of the channel is less than or equal to 1. The major approaches for normalization include the setting of either the average energy gain or the peak energy gain to unity. In the normalization, the average energy gain is defined for a signal whose energy is uniformly distributed across the frequency and spatial dimensions. The peak energy gain of many mathematical fading models is not bounded, and those models cannot be normalized by the peak energy gain. We show that the proper normalization of the mathematical model and the selection of the correct performance measure are of critical importance in comparative performance analysis of adaptive transmission systems.

Adaptive Inverse Power Control Using an FxLMS Algorithm

In this paper, a novel adaptive inverse power control method that is based on the filtered-x LMS (FxLMS) algorithm is introduced. Inverse power control minimizes the interference that a cognitive radio creates to licensed users and allows more users to share the spectrum. It is also needed in multi-access systems like code division multiple access (CDMA) to alleviate the near-far problem. The proposed variable step algorithm adjusts the step size in a nearly optimal way. Based on numerical analysis this new method clearly improves system performance compared to the algorithm where the well-known fixed or variable step adjustment power control is used. A normalized version of the FxLMS algorithm is needed in a fading channel. In a slowly fading channel (e.g., normalized Doppler frequency of 0.001) the FxLMS power control can keep the received signal-to-noise ratio (SNR) in the desired level with a good accuracy most of the time. The standard deviation of the received SNR is 1.92 dB when the received SNR is kept at 10 dB. The results with almost all different SNR values are better than the other methods can achieve.

Combination of short term and long term database for cognitive radio resource management

We propose a method that uses long term information on the use of primary channels to select the most auspicious ones to be sensed and exploited by cognitive radios at the requesting time. These channels are investigated in more detail over the short term. Sensing results are stored in the short term database and used to predict which channels are best for data transmission. The method makes the operation of cognitive radios more reliable and efficient in terms of delay and throughput, and decreases collisions with primary users.

A motivating overview of cognitive radio: Foundations, regulatory issues and key concepts

The radio frequency spectrum is a scarce natural resource and its efficient use is of the utmost importance. A fundamental problem facing the future wireless systems is where to find suitable spectrum bands to meet the demand of future services. A promising technique to improve the spectrum utilization under a lot of investigation is the cognitive radio (CR) concept. This paper provides an overview of a CR study with discussion on enabling techniques for CR, measurements on current spectrum use, forthcoming standardization activities and some findings from our recent research studies on spectrum sensing, frequency management, and power control. The measurements on current spectrum use have identified that in general the overall spectrum occupancy is low. However, the overall spectrum occupancy does not characterize the actual situation adequately and better measures are needed. International Telecommunication Union Radiocommunication sector (ITU-R) has started standards activities on CRs, in particular on the measurement of spectrum occupancy and CR systems in the mobile service, indicating that the inclusion of the CR concept into the future regulatory framework is topical. Reliable spectrum sensing is a crucial task of the CR and proper performance assessment is important. If the future availability times of spectrum holes can be predicted, intelligent channel selection with the predicted channel information can be useful. A power control candidate for the CR system is the truncated inverse power control.