2.1

Traditional OFDM system

Fig. 1 shows the baseband processing flow of traditional OFDM system. The transmitter undergoes operations such as coding, modulation, framing, FFT, etc., and then transmits signal through radio frequency. The receiver performs corresponding inverse operations on the signal to achieve successful demodulation. Fig. 2 shows the traditional physical layer frame format, CP is used before both preamble and data to reduce inter-symbol interference. Short preamble is used for capture and synchronization, long preamble is used for channel estimation and equalization. When the received signal is non-orthogonal aliased, if the traditional frame structure is used, long preamble used for channel estimation will also be aliased, which greatly reduces the success probability of SIC demodulation. Thus, designing a new frame format is especially important for non-orthogonal demodulation performance.

Fig. 1.

Baseband processing structure of traditional OFDM system

Fig. 2.

Traditional frame format in OFDM system

2.2

Intra-Slot SIC based on OFDM description

The only way to distinguish different user signals depends on the difference in power. Assuming that two transmitters and one receiver are set, the RX will receive aliased signals. In order to ensure normal demodulation, SIC is required. The specific process of SIC detection at receiver is:

In OFDM system, it is assumed that the data of user 1 is modulated to form a high-power signal s(t), and the data of user 2 is modulated to form a low-power signal c(t), which are signals with different powers but the same frequency at the same time, expressed as follows:

In the above formula, N is the number of subcarriers, while k is the number of OFDM symbols, P_c and P_s are the powers of two signals, f_c is the carrier frequency and Δf and T_s are subcarrier spacing and OFDM symbol period respectively. T_s = T_u + T_cp, T_u = 1/Δf.

As shown in Fig. 3, assuming that the channel between two communicating nodes is an Additive White Gaussian Noise channel, the signal received by user C is expressed as

Fig. 3.

Three-user communication scenario

where n(t) is white Gaussian noise. The receiver firstly demodulates and decodes high-power signal s(t), then subtracts it from the aliased signal, so as to obtain the data of another user as

2.3

Model and Scheme Design of SIC based on OFDM

Fig. 4.

Baseband processing structure of OFDM using intra-slot SIC

Fig. 5.

Transceiver processing flowchart of OFDM using intra-slot SIC

Fig. 6.

Improved frame format in OFDM system

Assuming that the unit power signals sent by two transmitters are respectively expressed as X₁ and X₂, The transmit power P₁ > P₂, Therefore, when channel noise N exists, the aliased signal received by the receiver is:

In order to correctly solve the aliased signal at the receiver, the following conditions need to be met:

Therefore, the receiver needs to decode the first user’s data, then subtract the first user’s signal from the aliased signal.

After that, the second user’s data is decoded according to the normal receiving steps.

2.4

Model and Scheme Design of Intra-slot chunked SIC based on OFDM

Unlike full-band SIC, chunked SIC-OFDM system divides the total frequency band into blocks, selecting the respective available frequency bands according to the frequency patterns of two users, then the receiver solves the data respectively according to their own patterns. The schematic diagram is as follows:

As shown in Fig. 7, after the OFDM data subcarriers are divided into blocks, the subcarriers are divided into 8 independent blocks according to the frequency in the figure. Each transmission selects the carrier block with better channel conditions according to the EVM for data transmission. When the channel conditions of the entire communication band cannot support the full-band SIC, OFDMA combined with chunked SIC is used to expand the communication capacity, and compared with the full-band SIC, although part of the system capacity is sacrificed, the bit error performance will be improved.

Figure 7.

Chunked SIC-OFDM patterns of three nodes

Which subcarrier blocks the sender selects in this transmission is determined by subcar in the demodulation module of receiver after previous transmission, and this variable is determined by the deviation of data received in the previous transmission and the standard modulation point, which is called EVM. Taking QPSK as an example, assuming the data after modulation is set as z=a+bi. The vector coordinates of standard QPSK constellation points are (1, 1), (1, -1), (-1, 1), (-1, -1), then the EVM is calculated as follows:

c and d are the real part and imaginary part corresponding to z, is the average power of modulated signal at the transmitter. The smaller the EVM value, the better the channel quality. So in the next transmission, these blocks can be used to transmit data. If the EVM value of sub-carrier block is large, it means that the data on the sub-carrier block is far from the standard modulation point, and the channel quality is poor, so data will not be sent on this block in the next transmission. At this time, blocks are not used will be filled by other data. At the receiver, blocks with good channel conditions need to be demodulated by SIC, and the ones with poor channel conditions can be demodulated normally (without using SIC), so except for the data modulation and demodulation module, the rest of the processing flow is the same as that of SIC-OFDM.