Overview of Faster-Than-Nyquist for Future Mobile Communication Systems Marwa El Hefnawy, Hidekazu Taoka DOCOMO Communications Laboratories Europe GmbH Landsberger Strasse 312, 80687 Munich, Germany Email: el hefnawy, taoka@docomolab-euro.com Abstract—Mobile communications has become one of the most developed technologies in the last two decades. Strong demand to increase system capacity is still growing dramatically and non-orthogonal transmission schemes are being considered as a potential solution to improve spectral-power efficiency. The non-orthogonal transmission scheme called Faster-Than-Nyquist (FTN) signaling is surveyed in this paper. FTN is analyzed in both time and frequency domains to show the reason behind its higher capacity as compared to the Nyquist case. I. I NTRODUCTION In recent years, 4G (sometimes called 3.9G), also known as Long-Term Evolution (LTE) or Evolved UTRA (EUTRA), was released and the commercial service has started in some countries. In LTE, the peak data rate in the downlink and the uplink has increased to 300 Mbps and 75 Mbps, respectively [1] , [2]. LTE realizes very low transmission latency of less than 5 msec within the radio access network and uses bandwidths from 1.4 MHz to 20 MHz. LTE supports only packet transmission mode, and all the data and voice services are also realized in the packet domain. Intra-orthogonal radio access schemes are used and Orthogonal Frequency Division Multiple Access (OFDMA) was adopted for the downlink while Single Carrier (SC)-FDMA, also called Discrete Fourier Transform (DFT)-Spread OFDMA, is used for the uplink. Multiple-Input and Multiple Output (MIMO) transmission is one of the key features to achieve high peak data rate. Following the LTE Release 8, standardization activity of LTE-Advanced systems began in 2008 to meet the require- ments for the user data rate and system capacity proposed by the International Telecommunication Union-Radio com- munication sector (ITU-R). LTE-Advanced has been devel- oped based on the original LTE radio interface, and various technologies to increase the data rate and spectrum efficiency have been specified such as carrier aggregation (CA), enhanced downlink and uplink MIMO, inter-cell interference coordina- tion in homogeneous and heterogeneous networks, relaying, etc. Although LTE/LTE-Advanced can accommodate the rapid increase of wireless user access for some years, strong demand to increase the system capacity is still growing dramatically. It is estimated that the total traffic of wireless access will increase beyond 500-fold in 2020 as compared to 2010 [3]. Considering the recent increase use of smart phones and tablets, further de- velopment of new radio access techniques is indispensable to enhance the system capacity as well as user data rate for future mobile communication systems beyond LTE-Advanced. The current LTE-Advanced can not accommodate such high traffic without sacrifying user experience which can emerge around 2020. Among the radio access techniques, the transmission signal waveform, multiple access techniques, multiplexing of data, control and reference signals are the representative core techniques in the mobile communication systems although it is becoming more challenging to improve the performance. A potenial solution for this challenge is the non-orthogonal transmission schemes for their improvement of spectral-power efficiency theortically compared to orthogonal transmission schemes. The non-orthogonal transmission scheme Faster- Than-Nyquist (FTN) is one of the approaches being considered for future systems that could improve the spectrum efficiency by increasing the data rate. We will focus in this paper on the FTN concept overview and analysis in the time and frequency domains. This paper is organized as follows: Section II describes the Faster-Than-Nyquist’s concept and the state of the art, Section III analyzes the FTN in the frequency domain showing how we can achieve higher capacities as compared to the Nyquist case, and Section IV describes the system model and gives numerical results. II. FTN BACKGROUND Inter-symbol interference (ISI) is a distortion that occurs to the sent symbols when they overlap partially or totally leading to a degraded detection performance at the receiver. Nyquist’s criterion for zero ISI [4], for the samples x nT of a signal xt is x nT 1 for n 0 and x nT 0 otherwise where 1 T is the symbol (or baud) rate. The FTN concept was introduced by Mazo in 1975 [5] where the signal is modulated faster than the usual rate which introduces intentional ISI at the transmitter side. Fig. 1 shows how the orthogonal transmission of symbols has become non-orthogonal by using FTN. In the Nyquist case, a signal is sent every T seconds while in the FTN case, the signal is sent every τT seconds where τ 1. Mazo showed that sending sinc pulses up to 25% faster doesn’t decrease the minimum Euclidean distance between symbols for an uncoded system using binary modulation. The complexity of FTN lies in the receiver side which is