Novel OFDM Transmission Scheme Using Generalized Prefix with Sub-carrier Index Modulation

2021 Turkish Journal of Electrical Engineering and Computer Sciences  
4 The cyclic prefix (CP) is a prefix technique widely used in orthogonal frequency division multiplexing (OFDM) 5 systems in order to eliminate the inter symbol interference (ISI) caused by the dispersion of wireless channels. However, 6 CP reduces the number of symbols that can be transmitted in one OFDM symbol. Therefore, CP is one of the bottlenecks 7 of OFDM systems limiting their spectral efficiency (SE). This limitation on the SE of the classical CP-based OFDM 8 system is the main
more » ... is the main motivation for this work to introduce a novel method. In this paper, the design of a new CP structure, 9 which is based on the generalized prefix (GP), is presented. In the proposed structure, extra bits are assigned to the 10 GP structure to increase the SE of OFDM systems. Simulation results show that the proposed method has the same 11 performance as conventional CP-based OFDM but higher spectral efficiency. Specifically, it is shown that the proposed 12 method has 5.5% more spectral efficiency than the classical OFDM system. A simple algorithm is also proposed to detect 13 the additional information bits conveyed by GP. As a result, the proposed method provides both high spectral efficiency 14 and low computational complexity. 15 Key words: Orthogonal frequency division multiplexing (OFDM), cyclic prefix (CP), generalized prefix (GP), index 16 modulation (IM), spectral efficiency (SE), cross-correlation method (CCM). 17 1. Introduction 18 In wireless communication systems, inter symbol interference (ISI) is a significant channel impairment due 19 to frequency-selective fading channels caused by the multi-path effect [1]. Orthogonal frequency division 20 multiplexing (OFDM) has attracted a lot of attention for broadband wireless communication systems due 21 to its robustness against frequency-selective fading channels and its simple implementation [2, 3]. Cyclic prefix 22 (CP) is one of the widely used transmission techniques in the OFDM method to eliminate ISI due to frequency-23 selective fading channels. As CP re-transmits data symbols that have already been transmitted when no new 24 information can be transmitted, it reduces the spectral efficiency (SE). Hence, CP is one of the bottlenecks of 25 OFDM systems limiting their SE. Consequently, the loss in SE of the classical CP-based OFDM system is the 26 main motivation for this work to introduce a novel prefix method. 27 Several prefix approaches such as known symbol padding OFDM, pseudo-random postfix OFDM, zero-28 padding OFDM, unique word OFDM, without guard intervals (NG-OFDM) have been proposed [4][5][6][7][8]. While the zero-padding method is used for frequency-domain channel estimation, the channel matrix of the zero-padding 1 method is not circulant and cannot be diagonalized by the discrete Fourier transform (DFT) [9]. Pseudo-random 2 postfix scheme is used to enable semi-blind channel estimation; known symbol padding and unique word are 3 used for channel, timing and carrier offset estimation [10, 11]. However, the aforementioned methods decrease 4 the spectral efficiency of the classical OFDM system. 5 One of the ways to increase the SE is to use a CP shorter than the length of the channel impulse response 6 (CIR) [12] , but shorter CP degrades the performance of an OFDM system due to ISI. Moreover, these methods 7 either decrease the SE or have a worse bit error rate (BER) performance and also have a high computational 8 complexity. Please note that while CP converts the linear convolution between the channel impulse response 9 and the transmitted signal into a circular convolution, i.e. it diagonalizes the channel matrix, aforementioned 10 prefixes are not able to convert linear convolution to circular convolution, which provides a very simple equalizer 11 structure (single-tap equalization) for practical OFDM systems. As a result, enhancement of spectral efficiency 12 without degradation in BER performance with low complexity (simple equalizer structure) is one of the key 13 objectives of OFDM-based next generation wireless communication technologies. 14 Recently, the generalized prefix (GP) has been proposed instead of CP for especially deep fading channels 15 [13-15]. It is not only reducing the effect of the deep fading channels but also effectively reducing the spatial 16 correlation of the MIMO channel [16]. It was demonstrated that GP has a better BER performance than the 17 other proposed prefix techniques and it also diagonalizes the channel matrix (i.e. it was shown that CP is 18 not the only prefix method with diagonalization property [13]). Hence, GP transforms linear convolution into 19 a symbol-by-symbol circular convolution. GP-based OFDM structure shifts the frequency of the multi-path 20 channel and effectively transforms it into a less fading channel. GP method uses classical CP weighted by a 21 complex coefficient β = e jα where α stands for the amount of shift in the frequency domain of the channel. 22
doi:10.3906/elk-2006-10 fatcat:smdei3nisbaivlumwykgoyt5hi