Subblock Aided OFDM with Index Modulation

Recently, orthogonal frequency-division multiplexing (OFDM) with index modulation (IM) has been appeared as a novel method for future wireless communication systems. However, such a mechanism has low spectral efficiency due to some sub-carriers are not activated in order to implicitly convey information. In this paper, a subblock dependent approach, called sub-block aided OFDM-IM (SA-OFDM-IM) technique, is proposed for spectral efficiency enhancement of the OFDM-IM scheme with low complexity. The simulation results illustrate that the proposed SA-OFDM-IM and well known OFDM-IM have the same bit error rate (BER) performance while SA-OFDMIM has 40% more spectral efficiency with low complexity


I. INTRODUCTION
ULTIPLE-ANTENNA techniques constitute a key technology for next-generation fixed and mobile wireless communication systems [1]. Their performance is related to some restriction such as the spacing between transmitter and receiver antennas [2], [3], inter-antenna synchronization (IAS) at the transmitter and also inter-channel interference (ICI) [4], [5]. To circumvent these problems, Mesleh et al. proposed spatial modulation (SM) as a low computational complexity alternative to well known MIMO systems [6]. The SM technique is using the indices of transmit antennas to carry extra data, in addition to the 2 dimensional M-ary signal constellations. To show the potential benefits of SM, several experimental studies have done and it has shown that SM scheme is an alternative to the plain MIMO systems [7]. Consequently, the SM is promising transmission technique that has a very flexible structure and provides low complexity with high spectral efficiency [8].
Orthogonal frequency division multiplexing (OFDM) is being used by wireless communications systems such as the world wide interoperability for microwave access (WiMAX), long term evolution (LTE), IEEE 802.11 (Wi-Fi), etc. [9], [10], [11], [12]. As, OFDM has become the very popular multi-carrier modulation method, it continues to be widely studied. Following this trend, the index modulation (IM) concept which is inspired by SM has draw attention as a promising technique for the new-generation wireless communication systems in the last decade [13]. It has many significant advantages such as reduced peak-to-average power ratio (PAPR), better bit-error-rate (BER), energy efficiency, higher robustness against the inter-carrier interference (ICI) etc. [13], [14], [15].
In [16] and [17] MIMO-OFDM-IM technique is proposed by combining MIMO and OFDM-IM transmission methods. In [18] sub-carrier block interleaving is proposed for OFDM-IM to enhance its error performance. The bit error probability of OFDM-IM is analytically derived in [19]. Recently, generalization of OFDM-IM has been proposed by extending the index domain to include quadrature and inphase dimensions [20]. For more details on these studies, the concerned readers are referred to [21].
In spite of its advantages mentioned above, there is still limitation to the operation of OFDM-IM in wireless communications. The OFDM-IM scheme cause a decrease in data rate due to its unused subcarriers. To solve this difficulty, this paper proposes a new OFDM-IM with improved spectral efficiency and low complexity through integrating a new constellation design using sub-block. It is also shown that proposed SA-OFDM-IM performs same performance with traditional OFDM while SA-OFDM-IM has more spectral efficiency with low complexity.
The remain of this study is given as; Section II demonstrates the principle of proposed SA-OFDMIM scheme. Section III introduces the receiver part of the SA-OFDM-IM. The computational complexity and simulation results are given in Section IV and Section V, respectively. Finally, conclusion is presents in Section VI.

II. PROPOSED SA-OFDM-IM METHOD
In [13], sub-carriers were partitioned into g subblocks which have n sub-carriers. In each sub-blocks only k out of these n sub-carriers are active, and the remains are inactive. The total number of transmitted data bits for OFDM-IM system is calculated as 22 log ( ( , )) log ( ) . C n k shows the binomial number. It is clear that, unused sub-carriers cause the low spectral efficiency. In this work, sub-block aided OFDM-IM (SA-OFDM-IM) is proposed, to gain a high spectral efficiency rate without degrading BER performance and maintaining same power consumption.
Block diagram of the proposed scheme is given in Fig. 1. To take advantage of sub-blocks (not subcarriers), we design the new constellation scheme as shown in Fig. 2. We split all symbols in a pairs. For example, in Fig. 2   We simply assign the information bits in each symbol pairs as given in Tables I-II for proposed constellation diagram. Then, for each sub-block, we used these symbol pairs according to incoming information bits. For instance, if  .1 (b) incoming bit for symbol pairs is [0], we use symbol pair   12 11 , xx in the related sub-block as given in Table I , xx in the related sub-block as given in Table II. Moreover, each symbol has one information bit such as 1 2 x  [1] and 2 2 x  [0] as shown in Fig. 2. These designs also can be extend for M-ary signal constellation.   The total transmitted bits of proposed scheme can be determined as bits. Hence, each sub-block contains b bits. In the each sub-block, the b bits are split into three parts as for different goals. The first part, 1 b bits, is used to determine the sub-carrier index as given in Table III. The second part, 2 b bits, is used by symbol pairs as given Tables I-II

Example 2:
In this example, incoming bits are [1 0 1 0 0 1] for fourth sub-block. The first two bits [1 0] illustrate that in transmission fourth and third sub-carriers will be activated. (1), (2), , ( ) , Then, after applying IFFT, the OFDM symbol given as: x N  At the receiver, the received signal is given as  performance. We also use log likelihood receiver (LLR) for OFDM-IM systems for QPSK modulation to compare with the proposed scheme. As illustrated in Fig. 7, the total number of bits carry by the proposed scheme with M = 2 is SA m = 1280 while OFDM-IM with BPSK is IM m = 1024 bits. As a result, the spectral efficiency of the proposed method is 40% higher than OFDM-IM system, while both of them has the same performance and complexity.

VI. CONCLUSIONS
High spectral efficiency is one of the challenge issue in the next generation wireless communication systems such as 5G, 6G etc. In this work, to obtain the high spectral efficiency of the OFDM-IM method, new constellation diagrams are designed for each sub-blocks. The bit error rate and total number of bits for proposed method are investigated and the computational complexity is given. Results show that using the proposed constellations in each subblock a high spectral efficiency is achieved with same BER performance and tolerable complexity compared to currently known OFDM IM. In future, this work might be extended to multiple-input multiple-output (MIMO) scheme to increase spectral efficiency more.