FCC "final" rules. Cognitive radio or just unlicensed access?

In the previous post I presented the situation in both the US and Europe regarding the use of unlicensed white space devices in the television band. As it was expected, the FCC released the "final" rules for the cognitive use of TV white spaces in the US (press release, report FCC-10-174).

This solution is based on a database architecture only. That is, the white space devices must have geolocation capabilities and download from a database the occupancy tables while they are not required to perform additional sensing before transmitting. Moreover, as opposed to 2008 rules, the low power devices which could relay only on sensing are banned:

"Eliminating the requirement that TV bands devices that incorporate geo-location and database access must also listen (sense) to detect the signals of TV stations and low power auxiliary service stations (wireless microphones). As part of that change we are also revising and amending the rules in several aspects to reflect use of that method as the only means for determining channel availability. While we are eliminating the sensing requirement for TVBDs, we are encouraging continued development of this capability because we believe it holds promise to further improvements in spectrum efficiency in the TV spectrum in the future and will be a vital tool for providing opportunistic access to other spectrum bands."

Note that the final rules encourage further research in cognitive radio sensing techniques, since this may be useful for other spectrum bands. However the final rules discard the idea of cognitive radios: is it cognitive to download from a database a list of free channels?

On the other hand, wireless microphones, low-power dumb devices that cannot be guaranteed to be registered in the database, will have two channels for exclusive use. Extra channels can be temporarily reserved in the database during special events for the use of these devices.

To finish with, note that the central part of the architecture is still missing. While the FCC already received different proposals for the database architecture, interface and specifications of the database need to be defined yet.

All these issues motivate the use of quotation marks when I write FCC "final" rules.

Final FCC rules for the unlicensed use of the TV white spaces today?

The FCC is expected to release today the final rules for the use of TV white space spectrum in the US. The previous version of these rules, published almost two years ago, generated several protests by (both aerial and cable TV) broadcasters and raised additional technical issues seeking reconsideration of the 2008 rules. However during these two years the FCC authorized several tests and trials in order to evaluate the benefits and possible problems the unlicensed access to the TV band. The FCC also received database proposals from different companies to enable the use of geolocation based devices, and allowed the time for the digital transition process to complete.

While this happens in the US, Europe is still a step behind. Recently the CEPT SE43 workgroup, which is developing recommendations for the unlicensed use of 470-790 MHz in Europe, participated in the 57th WG SE meeting. A final version of the Draft rules for European White Space Devices (draft ECC Report 159) has been approved for public consultation (see pages 17-18 of this document). After a two weeks pre-consultation period within administration, the public consultation will start on the 30th of September. However, from the documents currently available, additional studies will be probably required before the definitive report is published.

In the (hopefully) near future I will go over the FCC decision and the ECC Report.

Edit: Here my comments on the FCC decision.

Spectral Sensing at CIP 2010

The International Workshop in Cognitive Information Processing (CIP) is a small conference focused on Cognitive Radio. This year's conference was on Elba Island, Italy. While it took place a while ago (in June) I did not have the opportunity to write over it yet.

Surveys and comparisons of spectrum sensing strategies

The works included in this review will present a slightly different classification to the previous ones (ICASSP, ICC). I will start discussing four works that either survey or compare different spectrum sensing techniques:

In "Overview of Spectrum Sensing for Cognitive Radio", Erik Axell, Geert Leus and Erik G. Larsson present a survey of state-of-the-art algorithms for spectrum sensing. The algorithms discussed range from energy detection to feature detectors exploiting some known structure of the transmitted signal (cyclostationarity properties, known eigenvalue structure of the signal's covariance...), including cooperative detection schemes.

A more involved review on cooperative detection schemes is presented by Luca Bixio, Marina Ottonello, Mirco Raffetto, Carlo S Regazzoni and Claudio Armani in the paper "A Comparison among Cooperative Spectrum Sensing Approaches for Cognitive Radios". This paper compares the three main fusion rules in cooperative spectrum sensing, i.e. OR, AND and optimal, in terms of required processing capabilities at the fusion center and at the secondary terminals, and required control channel capacity.

In "Multiantenna spectrum sensing for Cognitive Radio: overcoming noise uncertainty" Roberto López Valcarce, Gonzalo Vazquez-Vilar and Josep Sala propose a novel multiantenna spectrum sensing paradigm for detection of rank-1 primary signals with uncalibrated multiantenna detectors. However this paper also includes in the discussion most of the previously proposed multiantenna detectors derived under several assumptions for both calibrated and uncalibrated receivers.

In "Performance Comparison for Low Complexity Blind Sensing Techniques in Cognitive Radio Systems" Bassem Zayen, Wael Guibène and Aawatif Hayar classify and compare different low complexity sensing strategies. Specifically, two blind sensing algorithms: the distribution analysis detector and the algebraic detector, which are compared with the energy detector as reference algorithm.

Mixing topics

Estimating the rank of the covariance matrix of a given random process is a recurrent topic in the literature. However it received limited attention in the context of Cognitive Radio systems. In "Estimating the Number of Signals Observed by Multiple Sensors" Marco Chiani and Moe Win show how the exact (non asymptotic) distribution of the eigenvalues of the empirical covariance matrix can be used to find the ML estimate of the actual eigenvalues. However, since this procedure presents a high complexity, they finally propose a rank estimator based on the usual eigenvalue estimate.

Erik Axell and Erik G. Larsson (authors of the overview previously commented) also present "Optimal and Near-Optimal Spectrum Sensing of OFDM Signals in AWGN Channels". In this work they derive the OFDM signal GLR detector for unknown noise and signal powers, which exploits the non-stationary correlation structure of the OFDM signal. Additionally they discuss the optimality of Energy Detection when the noise level is known.

In every conference focused on Cognitive Radio there are some papers on Compressed sensing. In "Compressive sampling based MVDR spectrum sensing" Ying Wang, Ashish Pandharipande and Geert Leus use the minimum variance distortionless response (MVDR) estimator to perform detection from a set of compressed measurements. As opposed to other works they derive the probability distribution of the CS MVDR spectrum estimate, which can be used to determine the detection thresholds.

Finally, Miguel López-Benítez and Ferran Casadevall explore in "On the Spectrum Occupancy Perception of Cognitive Radio Terminals in Realistic Scenarios" the perceived spectrum occupancy in different practical scenarios via empirical measurements. Nice to see that things work in practice.

Cooperative Spectrum Sensing

To finish with, just a list of the works presented in the session on cooperative spectrum sensing for cognitive radio networks:

• Sensor Fusion by Two-Layer Conflict Solving. Volker Lohweg, Uwe Mönks. Approach to data fusion which provides a stable conflict scenario handling, extendable to fuzzy classification.
• On Multi-Step Sensor Scheduling via Convex Optimization. Marco Huber. Two efficient multi-step sensor scheduling approaches are proposed in this paper for optimization over long time horizons.
• Bayesian Joint Recovery of Correlated Signals in Distributed Compressed Sensing. Pablo Viñuelas-Peris and Antonio Artés-Rodríguez. Distributed Compressed Sensing (DCS) of sparsely correlated signals.
• A Robust Approach for Optimization of The Measurement Matrix in Compressed Sensing Vahid Abolghasemi, Delaram Jarchi and Saeid Sanei. Optimized matrices can improve the quality of reconstruction and satisfy the conditions for efficient sampling.
• A novel adaptive algorithm for diffusion networks using projections onto hyperslabs. Symeon Chouvardas, Konstantinos Slavakis and Sergios Theodoridis. A new diffusion based algorithm to implement cooperation among neighboring nodes and the corresponding analysis.
• Node Localization and Tracking Using Distance and Acceleration Measurements. Benjamin R. Hamilton, Xiaoli Ma, Robert John Baxley and Brett Walkenhorst. Algorithm to combine acceleration measurements with RSS readings to achieve accurate localization of a distributed sensor network.

While the CIP is just a small workshop compared to the main conferences on signal processing, we have seen that the CIP 2010 proceedings include very trendy papers.

IEEE 802.16h - the cognitive Wimax

The final 802.16h IEEE Standard was recently published [802.16h]. This extension aims for license-exempt operation of 802.16 networks, that is, [802.16h] defines a set of cognitive radio capabilities for Wimax networks.

The proposed global framework is divided in two separated profiles. The first one provides uncoordinated coexistence mechanisms (WirelessMAN-UCP), i.e., without requiring much interaction among the different systems and hence adequate for heterogeneous systems, while the second provides coordinated coexistence mechanisms (WirelessMAN-CX), which addresses the required coordination of neighboring systems in order to reduce the interference generated to each other.

Uncoordinated coexistence mechanism

In an real scenario a Wimax network may coexist with licensed users (denoted in the amendment as specific spectrum users) and other unlicensed users (denoted as non specific spectrum users) sharing the same frequency band. In such case the interference that the Wimax system may cause to each of the users can be different. [802.16h] defines three possible levels of interference:

• Acceptable interference: This level of interference does not cause degradation in the receiver performance for a given choice of modulation and/or coding. This interference is admissible for both licensed and unlicensed users.
• Harmful interference: Strong interference that decreases the link performance in terms of modulation/coding. While this interference must be avoided in licensed links, some amount of communication is still possible and thus could be acceptable for unlicensed users.
• Destructive interference: The receiver is not capable of decoding the received signal for any available modulation at the transmitter. It must be avoided.

In order to achieve these acceptable interference levels, both to licensed users and unlicensed users, the standard provides a set of mechanisms. These include:

• Testing channels for other users.
• Discontinuing operations after detecting channel activity.
• Detecting other users.
• Scheduling for channel testing.
• Requesting and reporting measurements by different nodes.
• Selecting and advertising a new channel.

While all these procedures are described in detail in the standard, the timing and threshold parameters used are left open and must be specified by each regulatory administration. In the case of unlicensed users the proposed procedures are still valid with minor variations. For example, instead of performing a search for channels free of licensed users, the cognitive network looks for the best set of channels for operation either when certain unlicensed users are present.

The standard [802.16h], in the uncoordinated profile, permits distributed architectures for the radio resource management within the network formed by one 802.16 base station (BS) and its associated subordinated nodes. Each BS has a Distributed Radio Resource Management entity to execute the spectrum sharing policies of 802.16h and to build up a database for sharing information related to actual and intended future usage of radio spectrum. This database can be recovered from a master entity with the required information or from different devices (e.g. using the GPS, IP address, operator information, radio signature scheduling info...).

In order to avoid the regulation infringement the different interferers must be identified by their radio signature, which can be a short preamble, peak power, relative spectral density... Every transmitter will send the radio signature
during an interference-free slot. The time position of this slot (frame_number , sub-frame, time-shift) will be used for identification.

Once the environmental data is available the Radio Resource Management entity performs a real-time, adaptive scheduling, which can be done in terms of channel or even interference free regions within a MAC frame.

Coordinated coexistence mechanism

When multiple secondary networks coexist in the same region, they can collaborate in order to coordinate their transmissions and build a neighbor relationship. In the standard [802.16h] the following three basic mechanisms for achieving coexistence are provided:

• MAC Frame Synchronization, including Tx and Rx intervals, for separating transmissions and enabling operation in synchronized zones.
• Dynamic Channel Selection (DCS) and Adaptive Channel Selection (ACS) for finding a less interfered or less used frequency (similar to the uncoordinated case).
• Separation of the remaining interference in the time domain, by using a Coexistence Frame, coordinated scheduling, and a fairness approach, thus allowing the usage of a frequency channel by more than one system.

Note the high degree of awareness and cooperation required for implementing these mechanisms. To this end the standard defines a Coexistence Control Channel based on a series of globally synchronized time-slots and used for inter-network coordination.

In this post I tried to summarize the main capabilities supported by the IEEE 802-16h amendment. In my opinion this standard on the one hand address the FCC rules for unlicensed access to unused television spectrum and on the other hand is flexible enough to allow inter system collaboration and future regulations, hence the title of the post: cognitive Wimax.

[802.16h]

IEEE Standard for Local and metropolitan area networks Part 16. Air Interface for Broadband Wireless Access Systems Amendment 2: Improved Coexistence Mechanisms for License-Exempt Operation. IEEE Std 802.16h-2010 (Amendment to IEEE Std 802.16-2009). July 30 2010. doi: 10.1109/IEEESTD.2010.5538195

How does a Cognitive Radio job look like?

As you have noticed I took a one month vacation of "blog-posting". Now I'm back and I'll try to keep posting regularly again. These days I was curious about the kind of jobs companies are offering related to cognitive radio. While there exist several research positions for PhD students and postdocs, there are not so many open positions from private companies.

Academy is looking for a profile oriented to pure research: knowledge in stochastics, random graph theory, advanced signal processing, optimization... together with an important mathematical background and programming skills. Some of the topics the candidates will work on include:

• Information networks.
• Cross-layer optimization.
• Dynamic Spectrum Allocation Algorithms.
• Throughput analysis for complex wireless networks.
• Reconfigurable MAC-layer.
• Application of cognitive radio techniques to embedded networks.
• Development of Cognitive Engines.
• Network security.
• ...

On the other hand, companies like Qualcomm, Motorola, Mitre Corporation, General Electric or Sabre Systems are looking for a more "practical" profile. Knowledgeable in digital signal processing, experienced in statistical detection and estimation techniques, familiar with PHY and MAC system-level design... tasks offered are for instance:

• Development and evaluation of spectrum management capabilities.
• Development of spectrum sensing techniques.
• Simulations based based on captured signals.
• Integration of complete wireless systems.
• Evaluation of Software Defined Radio (SDR) based technology.
• ...

As we could expect, in the private world the work is much more product oriented than in the academy world, where the practical considerations of implementation and integration are put in a second place. However I find interesting that the profiles required in both industry and academy are somehow similar in terms knowledge and experience.

To finish with, it was unexpected to me the number of openings related with the Department of Defense of the US. While I understand that cognitive radio is a technology with major military applications (reconfigurability, signal detection, adaptability to harsh environmental conditions...), I thought that all these capabilities were already implemented in the military field long time ago.