Opportunistic Spectrum Multichannel OFDMA Przemyslaw Pawelczak, R. Venkatesha Prasad and Ramin Hekmat Faculty of Electrical Engineering, Mathematics and Computer Science Delft University of Technology, Mekelweg 4, 2600 GA Delft, The Netherlands Email: {p.pawelczak, vprasad, r.hekmat}@ewi.tudelft.nl Abstract— Opportunistic Spectrum Access (OSA) is being seriously considered for the future spectrum needs. In this paper we propose a simple and effective multichannel multiple access technique for OSA networks. In our design users of an OSA network must contact the OSA Base Station to gain access to the radio resources. In an OSA environment each of the channels can be arbitrarily occupied by the Primary Users (PUs) of the specific band. Thus the OSA nodes should cause least interference to the PUs while exploiting the voids in the PU usage. We analyze the OSA network where many OSA nodes would be competing amongst themselves and with the PU, using fast retrials. We also propose mechanisms to minimize the probability of collisions and interference caused to the PUs, while maximizing the throughput of the OSA network. I. I NTRODUCTION Opportunistic Spectrum Access (OSA) is a novel and not fully explored paradigm for wireless radio communications, where secondary (unlicensed) users utilize spectrum only when it is not exploited by primary users (PU), i.e., licensed users of the specific band [1]. The utilization of PU channels by secondary users in an OSA system has to be performed in such a way that it will cause no degradation in service to the PUs [2]. The natural candidate for the radio layer of an OSA networks is Orthogonal Frequency Division Multiplexing (OFDM). This is mainly due to its flexibility, which allows for simple adaptation of sub-carriers to fast changing conditions in radio spectrum and to duty cycles of PUs. For example, OFDM by narrowband carrier adaptation overcomes frequency selective fading and allows for multiuser diversity. The major challenge is to design a resource reservation multiple access protocol that works in the OSA environment using OFDM. CSMA-type protocols, while used broadly in unlicensed channels, viz. IEEE 802.11 a/b/g, are not completely suit- able for operation in licensed channels or OSA environ- ment, mainly due to “channel efficiencies that use a licensed band” [3]. One of the solutions for multiple access in licensed bands is to use Slotted Aloha or Packet Reservation Multiple Access (PRMA) [4]. Unfortunately PRMA does not perform well in the case of traffic bursts, and does not scale with increasing number of users [5]. However, instead of random access methods mentioned above, a network can assign unique radio resources to each OSA user. For example, in high data rate CDMA systems each user has a unique code assigned This research was carried out in the Adaptive Ad-Hoc Free Band Wireless Communications (AAF) project funded by the Freeband program of the Dutch Ministry of Economic Affairs and Magnet Beyond project funded by the EU. for the uplink. However, to efficiently use the radio resources the network should have a dedicated channel for scheduling in which carriers are opportunistically assigned to the users. Using TDMA for scheduling purposes may result in under utilization of spectrum usage, and thus contention based mul- tiple access protocols for this type of channel are inevitable 1 . To cope with this issue random access MAC can be used, since control messages are shorter compared to data packets. Also, such a hypothetical MAC can be constructed even on channels which are mostly utilized by the PU. Through these channels, an OSA network can later schedule OFDM carriers that are more stable and match the QoS needs of a particular user of the OSA network. For example, a contention-based MAC can serve as a support for distributed carrier assignment algorithms for OSA networks [6], where the carrier is shared by each node on a TDMA basis. In this paper, we extend the simple Aloha-like protocol for OFDM Access (OFDMA) presented in [3] to the case where carriers are randomly occupied by PU. We provide detailed analysis of packet capture in the presence of Rayleigh fading and give bounds on the performance of the system in the presence of radio regulator constraints [1], [2]. We note that there are already many attempts to design MAC protocols for OSA networks. In [7], authors proposed a secondary medium access control, called AS-MAC, working as an underlay in GSM bands. DOSS protocol is presented in [8], where discussions on the physical radio design for spectrum sensing in the MAC context are presented. In [9], HD-MAC is proposed, where groups of secondary users co- ordinate cooperatively to decide which channels to utilize. A recent survey on related multi-channel MAC protocols and their specific modifications can be found in [3, Section I] and [10]. The reminder of the paper is organized as follows. Sec- tion II introduces system model. Collision analysis for dif- ferent packet service disciplines are given in Section III, accompanied with numerical results in Section IV. Finally Section V concludes the paper. II. SYSTEM MODEL Let every Mobile Station (MS) belonging to the OSA network request radio resources from the Base Station (BS). 1 A similar approach is used in IEEE 802.16, where Mobile Stations send control and channel request messages to the Base Station in a random fashion, while collisions are resolved by exponential backoff.