Novel OFDM System Using Orthogonal Pilot Symbols with Subcarrier Index Modulation

Copyright © BAJECE ISSN: 2147-284X http://dergipark.gov.tr/bajece Abstract — In this work, two new transmission schemes are proposed to increase the spectral efficiency of orthogonal frequency division multiplexing (OFDM). In practical OFDM systems, channel estimation is usually performed by employing pilot symbols which is based on inserting known symbols in the time-frequency domain. However, pilot symbol designing is one of the bottlenecks of OFDM systems limiting the increase of spectral efficiency. We apply new pilot design structures, which are use orthogonal Walsh-Hadamard codes. To increase the spectral efficiency of OFDM systems, we assign the extra bits to the index of the each orthogonal Walsh-Hadamard codes. Simulation and theoretical results show that proposed methods have better performance than conventional OFDM with higher spectral efficiency. Moreover, no more energy is spent for additional information carried in the indices compared to classical OFDM systems. As a result, proposed methods provide both spectral efficiency and energy efficiency.


I. INTRODUCTION
RTHOGONAL frequency division multiplexing (OFDM) has attracted a lot of attention for broadband wireless communication systems due to robustness against frequencyselective fading channels and simple implementation [1]. OFDM has been adopted by several popular standards such as long term evolution-advanced (LTE-A), worldwide interoperability for microwave access (WiMAX), local area network (LAN), and digital video broadcasting (DVB) [2][3]. The channel estimation is a crucial and challenging procedure for OFDM systems in practical scenarios especially for mentioned standards above. In [4], OFDM performance is presented for pedestrian and vehicular channel models, i.e., practical channels. Channel estimation (CE) techniques can be roughly divided up into two groups, i.e., using blind-way or pilot symbol aided (PSA) [5,6]. While blind channel estimation methods are based on second or higher order statistics; pilot-assisted YUSUF ACAR, is with STM Savunma Teknolojileri Mühendislik ve Ticaret A.Ş., Ankara, Turkey, (e-mail: yusuf.acar@stm.com.tr).
https://orcid.org/0000-0002-3956-1097 SULTAN ALDIRMAZ COLAK, is with Kocaeli University, Kocaeli Turkey, (e-mail:sultan.aldirmaz@kocaeli.edu.tr). http://orcid.org/0000-0001-7154-0723 Manuscript received June 08, 2019; accepted September 16, 2019. DOI: 10.17694/bajece.588919 channel estimation methods transmit a known signal, pilots, in both transmitter and receiver sides. Blind-way techniques have higher spectral efficiency compared to that of PSA; however their performance are poor due to the data interference. Moreover, their computational complexity is higher than that of PSA techniques. Several PSA-CE schemes for OFDM applications have been investigated [7,8,9,10]. Since pilot symbols are transmitted instead of data symbols, spectral efficiency of PSA-CE methods is lower than blind-way CE techniques. Interested readers are referred to [11] and the references therein for a review of OFDM channel estimation techniques. Spatial modulation (SM), which has been recently proposed for multi-input multi-output (MIMO) systems, is a very distinct and useful modulation scheme [12]. Using the idea in SM, it was demonstrated by sub-carrier index modulation (SIM) (i.e., OFDM-IM) that spectral efficiency could be increased for OFDM systems [13,14,15]. SIM concepts have attracted increasing attention as a promising technique for the next generation wireless communication systems in the last decade. SIM method allows to transmit additional bits to the conventional modulation schemes by mapping the data bits to the indices of the different medias. For more details on this important field, the interested researchers are referred to survey on SIM schemes in [16].
Recently, two blind CE techniques for OFDM systems have been proposed by inspired the SIM technique [17]. The positions of the pilot signals in the frequency domain of the OFDM signal (before IFFT operation) transmit additional data bits in addition to the modulated symbols. Thus, the spectral efficiency of the system is increased.
The Walsh-Hadamard matrix [18] or pseudo-codes are key components of modern information technology such as code division multiple access (CDMA) communication systems. Walsh-Hadamard matrices are square matrices whose entries are -1 or +1, and columns or rows are orthogonal to each other. Assume that we have pp NN  Walsh-Hadamard matrix as follows:  In the proposed methods, to take advantage of orthogonality of Walsh-Hadamard codes, we assign additional bits to the each orthogonal code. For example, for Walsh-Hadamard codes matrix C with length p N = 4, we simply assign the information bits in each row vector as given in Table 1. For instance, if incoming bits for orthogonal Walsh-Hadamard code word is [0 1], we select as a pilot symbols in the related OFDM symbol. These designs also can be extending for higher p N values. Proposed method is capable of enhancing the spectral efficiency without extra energy consumption compared to conventional OFDM by assigning extra bits to the index of orthogonal pilot symbols. As a result, the index of the Walsh-Hadamard codes convey an additional information and complete (realize) channel estimation. Moreover, proposed techniques do not need any prior knowledge at the receiver and extra energy consumption.
This paper is organized as follows. Section II demonstrates the principle of proposed schemes. Section III introduces the spectral efficiency of the proposed schemes. The receiver design and simulation results are given in Section IV and Section V, respectively. Finally, conclusion is presented in Section VI.

II. PROPOSED METHODS
The general system block diagram is given in Fig We proposed two methods which are inserted the selected pilot tones considering the whole OFDM and subgroups. In the first system, only one pilot signal is selected according to the index bits ( 2 b ), while in the second system, OFDM is divided into subgroups, and the pilot signal is selected according to the index bit for each subgroup. This allows more bits to be moved in an OFDM symbol. The proposed schemes are explained as follows:

A) Orthogonal Pilot Code Index Modulation (OPC-IM):
In the proposed Orthogonal Pilot Code Index Modulation 0] are mapped to orthogonal pilot code index, (i.e., the code 1 c will be used for pilot symbols according to the Table II) then the remained bits [1 0 · · · 1 1 0] are used for mapping according to M-ary modulation scheme. Therefore, in the proposed OPC-IM, in contrast to classical OFDM, pilot symbols are used to not only estimate the channel response, but also convey additional data bits through the index domain while maintaining same power consumption (with OFDM system with PSA-CE).

B) Subblock Orthogonal Pilot Code Index Modulation (SOPC-IM):
To increase the spectral efficiency of proposed OPC-IM method, an OFDM symbol is divided into subblocks with length k N s   Fig. 3. As seen from this figure, there are two subblocks and each of them use four orthogonal pilot tones for CE. For example, assume that an incoming bit sequence of is [1 0 0 1 0 1 1· · · 1 1 0]. For the first subblock, the first two bits [1 0] are mapped to orthogonal pilot code index 3 (i.e., the code 3 c will be used for pilot symbols in first subblock) and then incoming bits [0 1] are used for subblock 2 (i.e., the code 2 c will be used for pilot symbols in second subblock) according to Table 1. Therefore, the proposed SOPC-IM method has more spectral efficiency than OPC-IM thanks to independent pilot tone selection for each subblock. Finally, remaining bits [0 1 1· · · 1 1 0] are used for M-ary modulation scheme as usual.

III. SPECTRAL EFFICIENCY
The spectral efficiency of classical OFDM, without taking into account the cyclic prefix (CP) overhead, is given as follows: It is clear that, pilot tones cause the loss of the spectral efficiency. The spectral efficiency of the proposed OPC-IM and SOPC-IM are given in (2) and (3) where () Finally, cubic SPLINE interpolation technique is used in the process of constructing the whole channel response [6].  frequency selective Rayleigh fading channels with length N L = 10. Signal to noise ratio (SNR) is described as E b /N 0 where N 0 is the noise variance and E b is energy per bit. We assume that the receiver perfectly determines the index bits of Walsh-Hadamard codes. The BERs of conventional OFDM and OPC-IM are given in Fig. 4 as functions of the SNR for PSA channel estimation. As shown in from Fig. 4, the proposed OPC-IM method achieves almost same performance of the classical OFDM with different modulation scheme such as BPSK, QPSK, 8-QAM, 16-QAM and 64-QAM. Fig. 5 illustrates the BER of SOPC-IM and conventional OFDM as functions of the SNR. As shown that proposed SOPC-IM method achieves better performance than the conventional OFDM for lower modulation size. Because, at lower modulation size, the percentage of the bits which are carried by the index of orthogonal pilot tones are higher than that of the lower modulation size. ), respectively. As can be seen from Table IV, BER performance of the proposed systems slightly outperforms the classical pilot-aided OFDM system. The performance gap between SOPC-IM system and the classical OFDM system is also seen from Fig. 5.

VI. CONCLUSIONS
In this work, new pilot design approaches are proposed to enhance the spectral efficiency of conventional OFDM system. To obtain the high spectral efficiency, our designs use Walsh-Hadamard codes. Using these codes in each OFDM symbols shows that the proposed methods have higher spectral efficiency, better BER performance and no extra energy consumption with tolerable complexity compared to currently best known OFDM scheme.