1 Adaptive Roll-off Factor Utilization for FMT-based FBMC Burst Structures Zekeriyya Esad Ankaralı 1 , Alphan S ¸ ahin 1 , and H¨ useyin Arslan 1 1 Department of Electrical Engineering, University of South Florida, Tampa, FL, 33620 Email: zekeriyya@mail.usf.edu, alphan@mail.usf.edu, arslan@usf.edu Abstract—In this paper, we propose an adaptive roll-off fac- tor utilization for filtered multitone (FMT) based filter bank multicarrier (FBMC) burst structures. Conventionally, a single prototype filter which has the same roll-off factor is employed for whole FBMC symbols. Thus, the conventional approach neglects the advantage of using filter adaptation against doubly dispersive channels. Unlike the conventional approach, in this study, different filters in terms of roll-off factors are utilized within the burst and the roll-off factors are adaptively changed by paying regard to the time and frequency dispersions of the channels. Also, by allowing the controlled interference between subcarriers, an average frequency spacing is applied to the burst structure. Therefore, immunity against multipath delay spread or Doppler spread is gained. Additionally, new degree of freedoms, i.e., average frequency spacing, adaptation speed, and filter truncation are investigated for the FMT-based burst structure. Index Terms—Burst structure, FBMC, FMT, roll-off factor, root raised cosine, truncation. I. I NTRODUCTION Consistently increasing number of users along with wide variety of wireless communication applications, and extensive growth on the user demands from the wireless communication systems lead more adaptive, flexible, and efficient future radio access techniques. Especially, over the last two decades, radio access techniques which are able to exploit the multi- dimensional electrospace, e.g. orthogonal frequency division multiplexing (OFDM), have been heavily investigated in the literature. Recently, filter bank multicarrier (FBMC) technique which was introduced by Chang and Saltzberg in 1960s [1], [2], is re-considered as a tempting solution for future radio access technologies because of its flexibility on pulse shapes along with filters well-localized in frequency. Basically, the flexibility of choosing any prototype filter to combat inter- symbol interference (ISI) and inter-carrier interference (ICI) in doubly dispersive channels makes FBMC an attractive choice over OFDM. It is possible to generate FBMC symbols with different modes, i.e., filtered multitone (FMT), staggered multitone (SMT), and cosine-modulated multitone (CMT) [3]. FMT- based FBMC is a multicarrier scheme that has been proposed before for DSL applications [4], [5]. In this mode, subcarriers are separated with guard bands instead of overlapping of adjacent subcarriers as in SMT and CMT modes. Therefore, FMT-based FBMC does not provide the same efficiency of SMT and CMT modes because of guard bands. However, FMT brings flexibility on multiple-input multiple-output (MIMO) . . . . . . Fig. 1: Proposed burst structure. channels which makes an attractive scheme for next generation communications systems. In FBMC, time and frequency characteristics of trans- mitting and receiving filters are the critically important for the performance of the systems. For example, consider an FBMC symbol constructed with root-raised cosine (RRC) filter. Basically, roll-off factor (α) determines the time and frequency characteristics of RRC filter. If small α is applied to the filter, higher sidelobes are obtained in time domain and makes the constructed FMT-based FBMC symbols more susceptible to time varying channels. In the literature, existing methods are based on designing prototype filter to achieve better sidelobe suppression for perfect reconstruction [6], [7]. In [8], the effects of roll- off factor and truncation are also analyzed. These methods are useful when only a single prototype function is applied for whole burst structure. However, use of fixed α factors ignores the adaptations on time-frequency varying channels. In this study, instead of using single prototype filter for the whole burst, α value is changed by ±Δα for the consecutive FBMC symbol by paying regard to the orthogonality loss due the change on α factor as given in Fig. 1. Therefore, desired α on each subcarrier is obtained by changing α factor gradually within the range between α min and α max specified according to transmission quality requirements. In addition to α adaptation, interference between subcarriers is allowed in the proposed burst structure. Frequency spacing, which is directly to related with ICI, is determined according to an