# Analysis of Power Allocation for NOMA-Based D2D Communications Using GADIA
## 4 Principle of NOMA System
### Resource Allocation  
In the conventional OFDMA system, just a single symbol is mapped to a sub-carrier. Distinct characters are sent on different sub-carriers that are orthogonal and do not interfere with each other. Accordingly, the total number of transferred symbols is limited by the number of orthogonal sub-carriers. Figure 3 illustrates a typical diagram of resource allocation carried by user information on the frequency domain and time domain planes of OFDM technology. The smallest unit in the frequency domain represents a sub-carrier with a specific bandwidth. The smallest unit in the time domain can be considered a particular length of the OFDM symbol. Each resource unit is a resource plane composed of one subcarrier and one OFDM symbol. Since each resource unit's user power is the same, the OFDM technology did not develop a power domain in terms of the power domain. It is a two-dimensional planar resource carrying system, and its wireless channel transmission capacity is relatively weak. NOMA can be obtained in a recently emerged new domain, particularly in the power domain. In contrast to the multiple access schemes based on time, frequency, code-domain, or combinations as illustrates in Figure 4. Figure 4 illustrates a diagram of resource allocation in NOMA scheme. The corresponding frequency domain, time domain plane, and OFDM resource plane are the same. The difference is that it has developed the power domain application. Unlike OMA technology that can only allocate a unit resource (i.e., time/frequency) to one user, NOMA can take advantage of allowing different power and assign a unit resource to numerous users. NOMA technology is a three-dimensional resource bearing system, so wireless channels' transmission capacity is much stronger than OFDM. Distinctive signals formed by various users are instantly superimposed on each other after classic channel coding and modulation at the transmitter. Multiple users share the same time-frequency resources, fol  
Figure 4: Resource allocation of NOMA technology. Figure 3: Resource allocation of OFDM technology. lowed by exposure to the receivers by MUD algorithms such as SIC. Accordingly, the SE is magnified at the cost of an enhanced receiver complexity compared to conventional OMA. Furthermore, in the information-theoretic literature, it has been broadly acknowledged that non-orthogonal multiplexing utilizing superposition coding at the transmitter and SIC at the receiver outperforms classic orthogonal multiplexing. Figure 5 illustrates the downlink transmitter and receiver of the NOMA system management process per sub-channel. The system consists of a single base station (BS) and multiple users. The BS transmits the signal to the user set \(u\in 1,2,\ldots,U\) through the sub-channel set \(v\in 1,2,\ldots,V\), and all receivers are equipped with SIC per each sub-channel and single antenna. Assuming that the system bandwidth is \(BW\), each sub-channel bandwidth is \(B_{v}=BW/V\). Assume \(P_{BS}\) denote the transmit power of the BS. Furthermore, let \(P_{v}\) and \(P_{u,v}\), respectively, represent the power of the sub-channel and the power of the \(u\)th user on the sub-channel \(v\). As an example, assume that the base station serves three users denoted as \(UE1\), \(UE2\), and \(UE3\) where their channel responses are, respectively, \(h_{1}\), \(h_{2}\), and \(h_{3}\), and their corresponding signal-to-noise ratios are 20 db, 10 db, and 0 db, respectively. As it can be readily checked, the \(h_{1}\) channel condition is the best one with the highest signal-to-noise ratio. The \(h_{2}\) channel condition is worse than \(h_{1}\), and the \(h_{3}\) channel condition is the worst. Next, we will analyze the signal processing situation at the transceiver end the principle of NOMA. A user over strongest channel should not remove interference from others. The best user decodes the signal containing possible interference and shares the decoded signal with the other users.