Enabling Dynamic Spectrum Access using SS-MC-CDMA Sarath, D., Nolan, K.E., Sutton, P.D., Doyle L.E. Centre for Telecommunications Value-Chain Research (CTVR) Trinity College Dublin Rep. of Ireland Email: sarathd@tcd.ie, keithnolan@mee.tcd.ie, suttonpd@tcd.ie, ledoyle@tcd.ie Abstract— The demand for frequency spectrum and diversity of wireless devices accessing this spectrum depends upon the time of day, the characteristics of a frequency band, and the geographical location of the observer/wireless device. This paper highlights the value of employing a novel dynamic spectrum access technology using a highly reconfigurable spectrum access scheme with excellent frequency diversity properties. We name this robust scheme selective subcarrier multi-carrier code division multiple access (SS-MC-CDMA). This paper contains three main contributions. Firstly, the reconfigurability options in a physical layer (PHY) using MC- CDMA and the flexibility in system design of a cognitive radio are presented. Secondly, an application of the novel SS-MC-CDMA system in a dynamic spectrum access (DSA) scenario is developed in order to indicate the value and potential of this system. Finally, based on this scenario, we present some initial key results from both simulation and real-world experiments. I. I NTRODUCTION The demand for frequency spectrum can be dependent on the time of day, desired frequency characteristics e.g. line of sight/non line of sight capability, and the location of the wireless device or network. In order to help cope with the ever- increasing demand for this resource, the development of prac- tical and robust spectrum sharing techniques involving primary licensed incumbents and secondary opportunistic spectrum users is vital. The combination of an agile spectrum access and multiplex system combined with the awareness, decision- making, and learning abilities of a cognitive radio offer a viable solution to help accomplish this objective. This paper focuses on the opportunities that a novel highly reconfigurable spectrum access scheme with excellent frequency diversity properties can offer in this context. Dynamic spectrum access (DSA) techniques aim to exploit the under-utilized frequency spectrum by employing new ad- vancements in the field of spectrum sharing and opportunistic access. Secondary opportunistic users would have the potential to exploit unused or underused frequency bands allocated to primary users. The development of cognitive functionality in wireless communications systems has made the realisation of these concepts possible. Cognitive radio (CR) can be described as a node in a network with the abilities to form an awareness of its environment and context, make decisions and inferences from this information combined with knowledge of the user’s objectives. In addition, a CR can act in a manner that attempts to accomplish the user’s objectives, and learn from these experiences for possible use in the future [1]. This paper primarily focuses on the PHY layer issues. At this layer, multi-carrier spectrum access techniques in- cluding orthogonal frequency division multiplexing (OFDM) and multi-carrier code division multiple access (MC-CDMA) offer a number of significant advantages over single-carrier (SC) systems in volatile wireless channel environments. Multi- carrier systems are inherently more resilient to the effects of inter-symbol-interference (ISI) and multi-path fading [2]. In addition, the spectral efficiency is higher than SC systems due to the closely-packed carriers. Received signals can be equalised in the frequency domain, which reduces the com- plexity of this stage compared to time-domain equalisation techniques. MC-CDMA has emerged as a feasible alternative to OFDM for forward-looking multi-carrier communications sys- tems [3]–[6] by exploiting the flexibility and potential offered by the combination of OFDM and CDMA. In conjunction with the cognitive capabilities of the node, we can choose the subcarriers for allocating data, resulting in the novel SS-MC- CDMA system. We highlight the reconfigurability options of MC-CDMA systems and their variants in a CR scenario in [7]. This paper builds on this work to present and analyse a novel application of SS-MC-CDMA in a practical DSA scenario. Section II presents a brief explanation of a MC-CDMA transceiver, highlights the various reconfigurability options on adopting a CR approach and introduces our SS-MC-CDMA system. Section III details an application of SS-MC-CDMA and illustrates its scope in DSA. Section IV presents some of the key experimental results related to this scenario and highlights a real test scenario. Section V points out the scope for future research and concludes. II. MULTI - CARRIER CDMA AND SS-MC-CDMA A. Multi-carrier CDMA (MC-CDMA) MC-CDMA is an OFDM-based multiple-access spread- spectrum scheme. By combining frequency domain and time domain dispersal of modulated symbols, two main variants of