In the past 20 years, with the rapid development of mobile communication technology and the continuous evolution of technical standards, the fourth generation mobile communication technology (4G) is based on the orthogonal frequency division multiple access technology ( OFDMA ) , and its data service transmission rate reaches 100 megabits or even gigabits per second can meet the needs of broadband mobile communication applications to a large extent in the future. However, with the popular application of intelligent terminals and the continuous growth of new mobile service demands, the demand for wireless transmission rate increases exponentially, and the transmission rate of wireless communication will still be difficult to meet the application requirements of future mobile communication. The IMT-2020 (5G) Promotion Group's "5G Vision and Requirements White Paper" proposes that 5G is positioned as a wireless network with higher spectral efficiency, faster speed, and larger capacity. Compared with 4G, the spectral efficiency needs to be improved by 5 to 15 times.
In order to keep the low cost of reception while achieving good system throughput, orthogonal multiple access technology is adopted in 4G. However, in response to the need to improve 5G spectral efficiency by 5 to 15 times, the industry proposes to adopt a new multiple access multiplexing method, namely Non-Orthogonal Multiple Access (NOMA). In Orthogonal Multiple Access ( OMA ) , only a single wireless resource can be allocated to one user, such as divided by frequency or time, while NOMA can allocate one resource to multiple users. In some scenarios, such as near-far effect scenarios and multi-node access scenarios with wide coverage, especially in dense uplink scenarios, the non-orthogonal access multiple access method using power multiplexing has obvious performance compared with the traditional orthogonal access. Advantages, more suitable for the deployment of future systems. At present, studies have verified the effect of using NOMA in urban areas, and it has been confirmed that using this method can increase the total throughput of wireless access macro cells by about 50%. The capacity limit of non-orthogonal multiple access can only be achieved by combining serial interference cancellation or similar maximum likelihood demodulation. Therefore, the difficulty of technical realization lies in whether a low-complexity and effective receiver algorithm can be designed.
Different from traditional orthogonal transmission, NOMA adopts non-orthogonal transmission at the transmitting end, actively introduces interference information, and realizes correct demodulation through serial interference cancellation technology at the receiving end. Compared with orthogonal transmission, the receiver complexity is increased, but higher spectral efficiency can be obtained. The basic idea of non-orthogonal transmission is to use complex receiver design in exchange for higher spectral efficiency. With the enhancement of chip processing capability, the application of non-orthogonal transmission technology in practical systems will become possible.
Key technologies employed in NOMA:
1. Serial Interference Cancellation (SIC)
At the transmitting end, similar to the CDMA system, the introduction of interference information can achieve higher spectral efficiency, but it also encounters the problem of Multiple Access Interference (MAI). Regarding the problem of eliminating multiple access interference, many achievements have been made in the process of researching the third generation mobile communication system, and serial interference cancellation (SIC) is also one of them. NOMA adopts SIC receiver at the receiving end to realize multi-user detection. The basic idea of the serial interference cancellation technology is to use a step-by-step interference cancellation strategy. In the received signal, the users are judged one by one, and after the amplitude recovery is performed, the multiple access interference generated by the user signal is subtracted from the received signal, and the remaining signals are subtracted. The next user makes a decision again, and so on, until all the multiple access interference is eliminated.
2. Power reuse
SIC eliminates multiple access interference (MAI) at the receiving end. It needs to judge the users in the received signal to exclude the order of the users who cancel the interference, and the basis for the judgment is the power of the user's signal. The base station will allocate different signal powers to different users at the transmitting end to obtain the maximum performance gain of the system and at the same time achieve the purpose of distinguishing users. This is the power multiplexing technology. The power multiplexing technology has not been fully utilized in other traditional multiple access schemes. It is different from simple power control, and the base station follows a related algorithm to perform power allocation.
Of course, the realization of NOMA technology still faces some difficulties. First, the receiver of non-orthogonal transmission is quite complex, and designing a SIC receiver that meets the requirements also depends on the improvement of signal processing chip technology; secondly, the power multiplexing technology is not very mature, and there is still a lot of work to be done.
Foreign research on NOMA has achieved some gratifying results. Japan's NTT DoCoMo company started related research as early as 2010, and has proposed a more systematic plan. In the 5G vision proposed by NTT DoCoMo , many new technologies will be used in various cells ( below ). For example, when using the 800MHz band
The access method of NOMA is adopted in macro cells in the 2GHz band and so on. Previously, only one user could be assigned to a single radio resource (such as a block divided by frequency and time), while NOMA can assign one resource to multiple users. The company has validated the effects of NOMA adoption in urban areas through simulations, and has demonstrated that it can increase the overall throughput of wireless access to macrocells by around 50 percent. In China, there are not many studies on NOMA.
Although the technical standards for 5G have not yet been formulated, it is still unknown whether NOMA will be adopted in 5G. However, due to the development of the mobile communication system up to now, the spectrum resources are already very tight, and the NOMA technology that can improve the spectrum efficiency well is also useless.