Petar Popovski, Editor in Chief of IEEE JSAC
Published: 31 May 2022
The IEEE JSAC issues of April and May 2022 are two parts of the same Special Issue on Next Generation Multiple Access. This is a core topic in the context of a wireless medium shared by multiple users/links, bringing some fresh perspectives, such as the one based on the recent developments in electromagnetic elements and antennas. The paper selected from the April 2022, authored by researchers from Peking University in China, is based on these new developments:
R. Deng, B. Di, H. Zhang and L. Song, "HDMA: Holographic-Pattern Division Multiple Access," in IEEE Journal on Selected Areas in Communications, vol. 40, no. 4, pp. 1317-1332, April 2022.
The main premise of the paper is the use of a Reconfigurable Holographic Surface (RHS). It is an ultra-thin electromagnetic radiation plate and the signal to be radiated is brought through multiple signal feeds at the bottom of the plate. The name holographic comes from the use of a holographic principle, in which a 3-D picture is created based on the interference between a reference wave and a wave reflected from an object. Here the holographic principle is used to figure out the relation between the incident reference signals and the signals coming from the target object. This object is in fact a transceiver to which a wireless link should be established. Based on the interference pattern, the radiation elements of the plate can be adjusted to create a directed beam towards the object and thus direct the transmitted electromagnetic energy to/from the desired transceiver. Note that the Reconfigurable Holographic Surface (RHS) is an electromagnetic transceiver, while a Reconfigurable Intelligent Surface (RIS) is an electromagnetic reflector, which is a significant physical and conceptual difference.
This paper considers a downlink scenario, in which the RHS transmits to multiple users, such that the RHS is treated solely as an electromagnetic radiator. To the main proposal in the paper is holographic-pattern division multiple access (HDMA), a technique to serve simultaneously multiple users in the downlink. To explain the main idea behind HDMA, recall that any multiple access technique in the downlink uses a certain channelization technique, that defines the individual channels to the users. In a Time Division Multiple Access (TDMA) channelization is expressed through orthogonal time slots. In Code Division Multiple Access (CDMA), the channel is represented by a specific code belonging to a given receiver and the signals of different users are superposed into a composite downlink signal. In HDMA, the channelization is done through the holographic patterns that correspond to different users. Then these patterns are superposed and the individual users receive their data through their corresponding holographic pattern that corresponds to a certain directed beam. The resulting holographic pattern is a weighted superposition of the holographic patterns corresponding to different users. This leads to non-orthogonality among the channels (streams) to the different users, which can be mitigate thorough precoding, analogous to multi-user MIMO.
In the sequel we provide some further reflections by the authors on their article.
JSAC: Your work is based on a theoretical model. What do you think would be the biggest obstacle in implementing this concept in practice?
We think the biggest obstacle in implementing this concept is how to deal with the influence of mutual coupling effect among different RHS elements on the HDMA system, which is hard to be modelled precisely in theoretical work. In more detail, the HDMA is realized by the superposition of different holographic patterns configured by the electromagnetic responses of RHS elements. When the working frequency of the RHS grows, the size of the RHS element together with the element spacing becomes smaller. The coupling effect arising among different elements then grows, which influences the electromagnetic responses of RHS elements. Therefore, the degradation of the HDMA system performance brought by coupling effect cannot be ignored over the high-frequency bands. Such mutual coupling among different RHS elements may restrict the density of these elements, and thus, remains to be a challenge in both modelling and implementation.
Up to now, we have implemented an RHS prototype (consisting of 64 elements) supporting HDMA-based transmission at 12GHz. In the future, we plan to explore the influence of mutual coupling effect on the HDMA scheme design.
JSAC: What has been criticized by the reviewers during the review process and how did you address that?
The reviewers’ comments were generally positive. The reviewers are mostly concerned about the advantages of HDMA over traditional SDMA. To address this problem, we compare HDMA and SDMA from two aspects, i.e., system performance and hardware cost. As for system performance, the element spacing of the RHS lifts the half-wavelength restriction of phase arrays owing to the dense meta-material structure. Such a compact element deployment of the RHS enables a larger number of elements to be deployed compared to a phased array of the same physical size, bringing higher diversity gain. Thus, the HDMA system has great potential to outperform the traditional SDMA system in terms of capacity and the maximum number of accessed users. This statement has also been verified by simulations. As for the hardware cost, the traditional SDMA system is realized by phase-controlled beamforming, which highly relies on complex phase-shifting circuits with numerous phase shifters. Differently, in the HDMA system, the RHS elements can be fabricated by loading low-power and low-complexity RF switches such as PIN diodes. Since all components (such as PIN diodes, PCB, and DC control circuit) needed for fabricating the RHS are high volume commercial off-the-shelf parts, the hardware cost to realize HDMA is also lower than the traditional SDMA system.
To sum up, due to the compact element spacing and low hardware cost, the HDMA scheme can serve as a promising alternative to provide high-speed data services and revolutionary mobile connectivity.
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