Hanwen Cao

Practical issues in spectrum sensing for multi-carrier system employing pilot tones

Multi-carrier techniques such as OFDM are widely adopted in the physical layer of nowadays radio communication and broadcasting systems. Therefore in cognitive radio, a robust spectrum sensing techniques for such signals is highly desired. Comparing with traditional Cyclic Prefix (CP) based method, newly reported Time-Domain Symbol Cross-correlation based spectrum sensing (TDSC) method [1][2] shows both advantages in sensing accuracy and robustness against shortness of and channel time spread pollution to CP. This sensing method is able to sense all multi-carrier signals with repeated pilot patterns. In this paper, further validations of TDSC-NP (Neyman-Pearson) and TDSC-MRC (Maximal Ratio Combining) methods are presented with practical issues taken into account, e.g. noise uncertainty, oscillator imperfection and computational complexity. The feasibility of applying TDSC for sensing Filter Bank Multi-Carrier (FBMC) signal is validated via simulation. Both simulated signal and captured DVB-T signal with real-world multi-path channel fading are analyzed and the sensing performance of captured signal agrees with that of simulated signal very well.

Key issues towards beyond LTE-Advanced systems with cognitive radio

In this paper, we give an introductory presentation on the state of the art of the key technologies in the field of cognitive radio, including spectrum awareness, cognitive engine, location awareness, digital front-end and baseband spectral shaping. The technical feasibility of opportunistically operating the future evolution of long-term evolution (LTE) over cognitive white spaces is discussed. A platform implementing these new concepts is briefly introduced.

Cognitive Agile Networking testbed

Cognitive radio is a well motivated concept for improving the inefficient spectrum utilization and apparent scarcity that result from traditional spectral regulations. So far, most efforts in the cognitive radio research field focus on techniques for the enhancement of the amount of available spectrum under the stringent constraint of causing no harmful interference to legacy users. But communication reliability also can substantially benefit from the cognitive radio approach by means of agile utilization of transmission opportunities in frequency, time and space. In this paper, we present our Cognitive Agile Networking (CAN) framework that aims at achieving robust low-latency communications by introducing agility in frequency, time and space as well as learning and reasoning for optimizing network operations. In order to respect practical issues of real-world applications, we are building a Cognitive Agile Spectrum Testbed (COAST) for experiments and validations. An initial version of this testbed is introduced here and its spectral agility is demonstrated.

Multi-channel robust spectrum sensing with low-complexity filter bank realization

In this paper, we propose a robust spectrum sensing scheme which can reliably detect signals in multiple channels within short observation time at the cost of low computational complexity. The scheme consists of two parts. The first part is the signal detection algorithm essentially based on the crosscorrelation of the feature sequence generated utilizing the signals periodical pilot structure, which is further enhanced in terms of reducing sensitivity to clock mismatch and mitigating the noise uncertainty problem. The second part is the filter bank realized using the polyphase network (PPN) structure, which enables the simultaneous sensing of multiple wideband channels with low complexity which is favourable for the reduction of implementation cost and power consumption, especially for mobile terminals. In addition to the simulations for performance evaluation under practical conditions, the proposed spectrum sensing scheme is validated using the 8-channel wideband signal captured in UHF TV band. The visualized crosscorrelation values from both measurement and simulation are perfectly matched, which successfully confirms the effectiveness and feasibility of the proposed spectrum sensing scheme in real-world implementation.

Spectrum awareness scheme of the rapidly deployable eNodeB for unexpected and temporary events

In the EU FP7 ICT project ABSOLUTE, a novel network architecture based on the LTE-A aerial and terrestrial rapidly deployable eNodeB (RD-eNodeB) systems has been proposed for emergency communications in unexpected and temporary events. These systems would experience complex spectrum environment with multiple unknown coexisting cellular networks, which needs to be quickly adapted with cognitive radio techniques for rapid deployment. To this end, this paper proposes a spectrum awareness scheme considering initial spectrum sensing on RD-eNodeB, sensing using UEs measurement capability and the radio environment map (REM) during the roll-out and in-service phases of the systems operation. For the initial spectrum sensing, a novel scheme for detecting, classifying and measuring downlink signals from coexisting base stations with different radio access technologies (RATs) is presented, which is invulnerable to the noise uncertainty problem. Encouraging numerical results of effectively detecting and classifying coexisting UMTS signal and LTE signals with different physical layer identities have been obtained via simulation.

Cyclostationary multitone beacon signal for opportunistic spectrum access

Cognitive radio is a promising solution to the problem of spectrum scarcity by means of allowing secondary radio networks access the spectrum opportunistically. One of the most important issues in cognitive radio is how to detect existing over-the-air signals reliably. Not a few literatures have reported that signals could be detected via their inherent or embedded properties. However, these may not be reliable and flexible enough for all kinds of signals with different modulation types. In this paper, we propose a multitone beacon signal carrying cyclostationary signatures, which is able to strengthen the reliability and efficiency of signal detection at low cost of spectrum overhead. This beacon not only can indicate the presence or absence of user signal but also can reveal some other information helpful to opportunistic spectrum access through the bit word modulated in its cyclostationary signatures. It could be applied to device/network identification, indication of spectrum allocation and spectrum rendezvous, both for primary and secondary users. The generation and detection algorithm of the beacon signal are presented firstly in this paper, then an initial simulation analysis and experiments with radio devices are illustrated to discuss the performance.

Power optimal allocation in decode-and-forward opportunistic relaying

To combat the bottleneck effect in the relay channels, this paper proposes a simple power-control algorithm for decode-and-forward cooperative networks with opportunistic relay selection. Taking advantage of the existing channel state information, which is a by-product during the process of relay selection, an optimal power allocation factor can be easily figured out. This factor is quantized into a few control bits and then fed back to the source via the existing feedback channel, i.e., the flag packet. Using the channel capacity and outage probability as metrics, the theoretical analysis and Monte-Carlo simulation corroborate that such distributed power allocation between the source and the best relay can effectively boost the system performance. Most importantly, this performance gain comes at almost no price due to just reusing the existing CSI and flag packet.

Cognitive Radio --- A Current Snapshot and Some Thoughts on Commercialization for Future Cellular Systems

Cognitive Radio (CR) has received tremendous interest during the past decade from almost all research disciplines in wireless communications. Meanwhile, companies across the world in the value chain are showing more and more interest in CR. This is due to the expected significant scarcity of spectrum caused by data-consuming modern smartphones or similar mobile devices. However, partly conflicting interests and severe technical challenges remain for a successful commercialization of CR ideas, in particular for Long-Term Evolution (LTE) cellular systems. The aims of this survey are as follows: 1) we try to limit the various physical parameters (e.g. the carrier frequency range, the cell size) being relevant for cognitive cellular systems either in qualitative or in quantitative terms, 2) a conceptual approach for a performance bound being of practical relevance will be pointed out, 3) different players in the value chain of cellular systems and potential conflict of interests will be discussed, 4) the technical state-of-the-art CR and the remaining major challenges will be highlighted, 5) a current snapshot about the quickly evolving area of regulation and standardization will be provided. Finally, some products, services and prototypes for CR will be presented.

Implementation Aspects of a DSP-Based LTE Cognitive Radio Testbed

One of the key issues of designing radio communication systems is to enhance the efficiency and flexibility of the available radio spectrum. In this context, reconfigurable implementation of the different system layers such as Media Access Control (MAC), Physical (PHY), and RF layers with optimal cross-layer design is driving nowadays research on designing reliable and robust mobile communication systems. This paper presents the software and hardware implementation aspects of a DSP-Based cognitive LTE testbed and addresses the design, the implementation and the realistic verification of the key components of the system.

An MGF-Based Performance Analysis of Opportunistic Relay Selection with Outdated CSI

Up to now, a closed-form expression of the ergodic capacity for amplify-and-forward opportunistic relaying in the presence of outdated channel state information(CSI) is still not available in the literature. That is mainly due to the mathematical intractability in the conventional performance analysis through manipulations of probability density function (PDF). In this paper, therefore, we take advantage of a novel mathematical approach based on moment generating function (MGF) to evaluate the performance. MGF of the end-to-end signal-to-noise ratio for opportunistic relaying over independent and identically distributed Rayleigh channels with outdated CSI is given. Then, the closed-form expressions of ergodic capacity and outage probability are derived directly from MGF without any involvement of PDF. Finally, Monte-Carlo simulations are set up to corroborate the validity of the theoretical analysis.