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NEC has an accomplished history of developing OAM mode multiplexing technology, including the successful demonstration of OAM mode multiplexing transmission over 40m in the 80GHz-band in December 2018(3). This time, NEC has succeeded in extending the transmission distance up to 100m, 2.5 times further, and increased the capacity by as much as two times with 16 streams by adding polarization multiplexing to OAM mode multiplexing, as compared with the 80GHz-band demonstration.
NEC plans to apply this technology to its iPASOLINK series of super compact microwave radio systems, mmWave radio products, and mobile backhaul solutions that enable ultra-high capacity for 5G and beyond 5G (B5G) networks.
Due to rapid increases of data capacity in 5G networks, traffic volumes between 5Gbase station aggregation terminals could reach up to 100Gbps. Moreover, an extremely large number of base stations are required for detailed coverage, especially in ultra-dense urban areas, where the cell grid becomes heavily dense. Since it is difficult to connect these high-density cell sites with just optical fibers, expectations for wireless networking, especially in terms of easy and flexible installation, are increasing.
However, it is necessary for traffic between 5G base stations to reach up to 100Gbps, and it is extremely challenging to create sufficient transmission capacity with conventional technologies, such as using wider channels, multi channels or increasing the modulation scheme. Therefore, OAM mode multiplexing technology, which has the potential to support significantly higher capacity due to rich multiplicity, is attracting a great deal of attention.
OAM is one of the physical characteristics of electro-magnetic wave propagation. One feature of an OAM signal is its spiral phase front in the propagation direction. The number of spiral planes in a signal is called an OAM mode, and the shapes of all modes are different related to the rotating direction of the wave; this means that all of the OAM modes are independent of each other. Therefore, multiple OAM modes transmitted on the same channel simultaneously can be separated and demodulated by receivers. This is an OAM mode multiplexing transmission technology as a spatial multiplexing of electro-magnetic waves on the same path. In addition, OAM mode multiplexing is independent of polarization multiplexing as well. The multiplicity can be further increased with the combination of both multiplexing technologies.
NEC announced the development and successful demonstration of a real-time digital signal processing circuit in December 2018. This achieved a wireless transmission over a distance of 40m with a modulation of 256QAM in the E-band (71 to 86GHz) by multiplexing 8 OAM modes, however this was used for just one polarization only.
In the latest demonstration, twice as many modes have been multiplexed for 16 streams of 256QAM modulated signals by adding polarization multiplexing with the OAM mode multiplexing technology. 14.8Gbps (8 modes x dual polarization x 8 bit/symbol x 115 Mbaud) have been transmitted using a symbol rate(4) of 115 Mbaud.
Moreover, NEC achieved transmissions across 100m, 2.5 times the previous demonstration, with almost the same size antenna diameter, suppressing the divergence of OAM mode signals due to propagation by using D-band (130 to 174.8GHz), which is a higher frequency band than E-band.
This demonstration transmitted at an RF frequency of 157GHz through a radio equipped with D-band RF devices developed by NEC.
In order to perform OAM mode multiplexing transmission combined with polarization multiplexing, it is necessary to perform both OAM mode and polarization separation on the receiving side and to retrieve information with high SINR(5).
NEC's newly developed adaptive(6) digital signal processing circuit provides highly accurate extraction of the desired signals, even under conditions in which there is interference with OAM inter-modes as well as cross polarizations due to equipment imperfections or volatilities of the propagation environments. As a result, an extremely high spectrum efficiency of 128bps/Hz(7) has been achieved.
Going forward, NEC plans to further enhance transmission distance, and to realize a transmission capacity of more than 100Gbps with LSI implementing the digital signal processing circuits, and wider bandwidth up to 1GHz. NEC aims to apply these technologies for the backhaul of 5G base stations as well as the fronthaul between CU (Central Unit: aggregation base stations) and DU (Distributed Unit: remote stations).
This research and development was conducted as part of "The Research and Development Project of OAM Mode Multiplexing Radio enabling Ultra High Capacity Transmission in Millimeter wave bands" under a contract with the Ministry of Internal Affairs and Communications, Japan.
(1) According to NEC research.
(2) Multiplexing of electro-magnetic waves in vertical and horizontal directions. As they do not interfere with each other, the technology has been conventionally used to achieve double-capacity in the wireless transmission.
(3) Please refer to the following:
(4) In digital modulation, the transmission symbols are switched at fixed time intervals. The reciprocal of this switching interval is called baud rate (unit = baud).
(5) Signal to interference / noise power ratio. Higher SINR value is required to obtain higher spectrum efficiency.
(6) Ability to change the control automatically with the change of environmental conditions.
(7) The transmission capacity per 1Hz. The higher the value, the higher the efficiency of the radio wave use.