TECHNICAL FIELD

The proposed invention is a multi-user orthogonal frequency division multiplexing with subcarrier number modulation (OFDM-SNM) for the overall spectral efficiency of wireless communication networks enhancement through serving more users by deploying the same OFDM block.

BACKGROUND

There is a recent study named as Orthogonal Frequency Division Multiplexing with Index Modulation (OFDM-IM) proposed in the literature. According to this scheme, the additional data bits are sent by exploiting the indices of subcarriers, which is achieved by letting one portion of the data to determine the indices of subcarriers to be used, while the other portion is transmitted by the corresponding subcarriers by using conventional PSK/QAM modulation. [1 ]-[3]

Another study is named as Orthogonal Frequency Division Multiplexing with Subcarrier Number Modulation (OFDM-SNM) proposed in the literature. According to this scheme, the additional data bits are sent by exploiting the number of subcarriers instead of their indices, like it is done in the IM, which is achieved by letting one portion of the data to determine the number of subcarriers to be used, while the other portion is transmitted by the corresponding subcarriers conventional PSK/QAM (Phase Shift Keying/Quadrature Amplitude Keying) modulation [4]-[5], However, one of the main undesirable features of this scheme, which is also the case with index modulation, is the fact that not all the available subcarriers are utilized for data transmission as some of these subcarriers are made inactive. As a result, due to the above-mentioned disadvantages and the inadequacy of the existing solutions, an improvement in the relevant technical field was required. References given above:

[1] Ba§ar, E. (2015). OFDM with index modulation using coordinate interleaving. IEEE Wireless Communications Letters, 4(4), 381 -384.

[2] Ba§ar, E. (2015). Multiple-input multiple-output OFDM with index modulation. IEEE Signal Processing Letters, 22(12), 2259-2263.

[3] Li, Q., Wen, M., Basar, E., Poor, H. V., Zheng, B., & Chen, F. (2018). Diversity enhancing multiple-mode OFDM with index modulation. IEEE Transactions on Communications, 66(8), 3653-3666.

[4] Jaradat, A. M., Hamamreh, J. M., & Arslan, H. (2018). OFDM with subcarrier number modulation. IEEE Wireless Communications Letters, 7(6), 914-917.

[5] Dang, S., Ma, G., Shihada, B., & Alouini, M. S. (2019). Enhanced orthogonal frequency-division multiplexing with subcarrier number modulation. IEEE Internet of Things Journal, 6(5), 7907-7920.

BRIEF DESCRIPTION OF THE INVENTION

This invention is targeting to design a new communication method that can serve multiple users at the same time and provide higher spectral efficiency. This method can be used in many areas where spectral efficiency is the first priority to be satisfied such as driverless cars in transportation sector, interoperability and connectedness within and beyond smart factories, virtualization and real-time capabilities for process control in industrial automation sector, remote surgery in health sector, virtual reality (VR) and augmented reality (AR) and smart cities.

In this invention, a novel method called “Multi-User Orthogonal Frequency Division Multiplexing with Subcarrier Number Modulation” is proposed by inheriting the capabilities of OFDM-SNM to satisfy such cases that requires multi-user service.

In this invention, it is aimed to serve two users simultaneously, who located in a far position and a near position to the transmitter. While the user, who is located in a far location from the transmitter is served by exploiting the merits of OFDM-SNM, which relies on the idea of activating the subcarriers according to incoming data of far user for the transmission, another user, who is located in a near location is aimed to be served by those subcarriers that are not deployed for the transmission of far user. Such a transmission technique results in to obtain high spectral efficiency and low BER (bit error rate) by avoiding the waste of subcarriers that remain inactive during the operation of OFDM-SNM, and creates a chance to make a use of those inactive subcarriers for another user.

LIST OF FIGURES

Figure 1. Transmitter Structure of the Proposed Multi-User OFDM-SNM

Figure 2. A Simple Operation of the Transmission for Far Receiver User and Near Receiver User

DETAILED DESCRIPTION OF THE INVENTION

The transmission structure of the proposed multi-user OFDM-SNM scheme is given in Fig 1. There are two different data streams for two different users in this structure. This structure is based on two different data streams to far receiver user (Y) and near receiver user (Z) transmitted by the transmitter user (X) with four transmit subcarriers. The transmission starts with the far user’s data, which is defined as m bits, to enter the system. These m bits are divided into G groups in which each of them is containing p=p1+p2 bits in the bit splitter. These G groups are deployed to determine the length of the OFDM subblocks, N=NF/G, where the NF is the size of the Fast Fourier Transform (FFT). In each p=p1+p2 bits, p1 represents the SNM mapper, which defines the number of subcarriers that will be activated for the transmission of the data modulated by the conventional M-ary modulation (BPSK), named as p2.

It should be noted that on the contrary of the OFDM-IM, proposed scheme does not dictate any certain number of active subcarriers, K, for the transmission. However, the possible values of K varies according to the p1 = log2(N) out of N number of available subcarriers in the system, which leads the K to be Ke[1 ,2,...,N], Since the K does not have a fixed value and it is dependent on the incoming SNM mapper bits, p1 , the subcarrier activation and the defined value of K may differ for different cases. For such a case where N= 4, Ke[1 ,2,3,4], and p1= Iog2(4) = 2, Table 1 shows the possible values of K according to p1.

Table 1. SNM mapper with p1 =2 & N=4

Thanks to this lookup table, for each subblock the remaining p2=K(log2(M)) bits are modulated by the M-ary signal constellation to transmit the data symbols of the far user over the active subcarriers. Means, transmitting the p2 bits in accordance with the p1 bits that are deployed to define number of active subcarriers for the transmission of far receiver user’s (Y) data.

The transmission of the near user’s (Z) data is initiated at the same time while the transmission of the far user’s data is operated. For each symbol of the p2 bits transmitted for the far user, the remaining antennas are deployed to transmit each symbol of near user’s data, n. Means, transmitting the near receiver user’s (Z) data, n, over the subcarriers that are not deployed by the transmission of m bits. In order to operate this transmission, (N-K) number of subcarriers are deployed to keep the unemployed subcarriers of the OFDM-SNM active to prevent the waste of spectra. Table 1 shows the activation pattern of the subcarriers that are deployed for the transmission of the near user’s data. As each of the symbols of then bits are assigned to be conveyed over (N-K) subcarriers by p1 , n is modulated by the conventional M-ary modulation.

After these procedures are repeated in all of the OFDM subblocks and achieved for far user and near user, the remaining steps are operated same as the conventional OFDM modulation for both users. By taking the inverse fast Fourier transform (IFFT) to XFf and XFn separately, the output vectors xtf and xtn are acquired with dimension NF*1. The cyclic prefix (CP) of length (NCP) is appended on both data to alleviate the inter symbol interference (ISI) effect. After this point, the output signals in time domain over the multipath Rayleigh fading channel by the channel impulse response (CIR), ht= [ht(1 ),ht(2),... ,ht(v)], where v is the length of the CIR, are affected by the additive white Gaussian noise (AWGN) with the noise variance of NO,T in the time domain to obtain the received signals of far user (Y) and near user (Z).

As a sample case the Fig. 2 shows a simple operation of the transmission for far receiver user (Y) and near receiver (Z) user. Since the p1 is 10, the transmission of p2, which is 0, must be operated by three subcarriers. After the transmission of the p2 is achieved by these three subcarriers, the remaining subcarrier is deployed to convey the symbol of the secondary incoming data stream 1 that is sent to near receiver user (Z).

The receiver of the proposed multi-user OFDM-SNM is the reversal of the transmitter, which means removing CP, performing FFT, SNM demapping, for only the far receiver, and detection.

Next, a simple frequency domain equalizer is applied on the both output data. In the next step, a simple spatial energy-based detector is deployed to extract the pattern of the active subcarriers by selecting an appropriate threshold value for the far user. The reason why this type of a detector is used in the proposed system is to create a simplicity, rather than using complex detectors such as ML (maximum likelihood) or LLR (loglikelihood ratio). After that the number of active subcarriers is defined in each subblock and their corresponding bits are determined by the SNM demapper, which is the reverse process of the SNM mapper at the transmitter to obtain the additional data bits of far user. Now, the active subcarriers obtained at the receiver for each subblock is used for constellation symbols detection. As the last step of the reception of far user all the bits that come from the SNM demapper and the symbol detection are merged for each subblock. By doing this procedure for all the subblocks the reception of the far user’s data is concluded. Since there is no need to demap the SNM for the reception of near user, the constellation detection of the near user’s data is operated conventionally for each subblock and by doing the same procedure for all the subblocks the received data of near user is acquired for the whole OFDM block. By deploying this procedure, both far user’s (Y) and near user’s (Z) data’s demodulation is completed.

Table 2. BER comparison of proposed multi-user OFDM-SNM between Far

User and Near User.

Table 2 shows the BER performances of far user (Y) and near user (Z). It can be seen from this figure, the BER result far user (Y) gets better values in comparison with the near user’s (Z) performance. The reason behind that is in some of the cases all the subcarriers are activated for far user (Y) and near user (Z) is not served at the time.

Table 3. Throughput comparison of proposed multi-user OFDM-SNM between Far User, Near User, and Conventional OFDM In Table 3 the throughput performances of the far user (Y) and near user (Z) is shown in both cases where the CP is included and excluded. In addition to that, the comparison of the simulation results with the throughput result of the conventional OFDM are provided. The table shows that the throughput performance of near user (Z) exhibits a similar performance with the conventional OFDM. The reason behind that is in each of the time, the number of symbols transmitted to near user (Z) is one, whereas the throughput performance of far user (Y) shows better characteristics than the near user’s (Z) performance since it carries the features of SNM.

Table 4. BER comparison of Far User and other schemes in the Literature

It is also important to note that, the transmission of far user’s (Y) data is the first priority in the proposed multi-user adapted OFDM-SNM scheme, and in some of the cases near user (Z) is not served. That’s why there is not any decrease in the BER result of far user (Y). In fact, it shows the same performance as the single user OFDM-SNM. In Table 4 the BER comparison of the multi-user OFDM-SNM with the other schemes in the literature is shown. As it can be seen, far user’s (Y) output result exhibits a similar performance as the OFDM-IM, and outperforms the conventional OFDM as well as the other strong techniques in the literature such as OFDM-SGM and OFDM-DM. This figure also proves that the proposed technique has the capability of serving to another user without degrading the BER performance of the main user.

The method for the overall spectral efficiency of wireless communication networks enhancement through serving more users by deploying the same OFDM block in a case where the transmitter user (X) conveys two different data streams for two different receiver users who are located in a far position (Y), and a near position (Z) can be summarized below;

- Dividing incoming bits of far user (Y), m, into G groups for each subblock,

- Determining subcarrier activation order,

- Transmitting each conventional M-ary symbol in each subblock to the far receiver user (Y) by a specific number of subcarriers that their number is defined by the SNM (subcarriers number modulation) mapper sub-stream,

- Deploying the remaining of the subcarriers that are not used for the transmission of the far user (Y) for the transmission of a secondary data stream that serves to a near receiver user (Z),

- Repeating the same procedure for each OFDM subblock,

- Demodulating the transmitted data of the far user (Y) and near user (Z).

The OFDM subblock comprises two different data streams to far receiver user (Y) and near receiver user (Z) transmitted by the transmitter user (X) with four transmit subcarriers. And the mentioned subcarrier number modulation mapper method’s steps are;

- Putting far user’s (Y) data, m, into bit splitter,

- Dividing m bits into G group for each OFDM subblock that each of them include p number of bits,

- Dividing the p number of bits in each OFDM subblock into two groups as p1 and p2,

- Modulating the p2 bits by the conventional BPSK,

- Transmitting the p2 bits in accordance with the p1 bits that are deployed to define number of active subcarriers for the transmission of far receiver user’s (Y) data,

- Transmitting the near receiver user’s (Z) data, n, over the subcarriers that are not deployed by the transmission of m bits,

- Repeating the same procedure for all of the OFDM subblocks.