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- Aabel, Lise, et al.
(författare)
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A TDD Distributed MIMO Testbed Using a 1-bit Radio-Over-Fiber Fronthaul Architecture
- 2024
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Ingår i: IEEE Transactions on Microwave Theory and Techniques. - 0018-9480 .- 1557-9670. ; In Press
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Tidskriftsartikel (refereegranskat)abstract
- We present the uplink and downlink of a time-division duplex distributed multiple-input multiple-output (D-MIMO) testbed, based on a 1-bit radio-over-fiber architecture, which is low cost and scalable. The proposed architecture involves a central unit (CU) that is equipped with 1-bit digital-to-analog and analog-to-digital converters, operating at 10 GS/s. The CU is connected to multiple single-antenna remote radio heads (RRHs) via optical fibers, over which a binary radio frequency (RF) waveform is transmitted. In the uplink, a binary RF waveform is generated at the RRHs by a comparator, whose inputs are the received RF signal and a suitably designed dither signal. In the downlink, a binary RF waveform is generated at the CU via bandpass sigma-delta modulation. Our measurement results show that low error-vector magnitude (EVM) can be achieved in both the uplink and the downlink, despite 1-bit sampling at the CU. Specifically, for point-to-point over-cable transmission between a single user equipment (UE) and a CU equipped with a single RRH, we report, for a 10-MBd signal using single-carrier (SC) 16 quadratic-amplitude modulation (QAM) modulation, an EVM of 3.3% in the downlink, and of 4.5% in the uplink. We then consider a CU connected to three RRHs serving over the air two UEs, and show that, after over-the-air reciprocity calibration, a downlink zero-forcing precoder designed on the basis of uplink channel estimates at the CU achieves an EVM of 6.4% and 10.9% at UE 1 and UE 2, respectively. Finally, we investigate the ability of the proposed architecture to support orthogonal frequency-division multiplexing (OFDM) waveforms, and its robustness against both in-band and out-of-band interference.
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- Olofsson, Frida, 1992
(författare)
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Evaluation of Radio-over-Fiber technologies for Distributed MIMO
- 2024
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The demand for wireless communication systems with better coverage, higher capacity and lower latency grows continuously. At the same time, the number of subscribers is increasing and has already surpassed the global population. In a cellular network, a centralized macro base station is responsible to serve the users within the cell area. However, it is difficult to reach the increasing requirements by the centralized cellular architecture. Instead, distributed multiple-input-multiple-output (D-MIMO) has been proposed as a solution, where several access points or remote radio heads (RRHs) are spread out in a cell. To achieve most gain from D-MIMO the RRHs should be processed coherently, which requires radio-frequency (RF) phase synchronization among them. The hardware implementation of a D-MIMO network is non-trivial and highly dependent on the level of synchronization that is required by the particular deployment. This thesis evaluates the use of radio-over-fiber (RoF) and centralized frequency up-conversion to achieve phase synchronized RRHs in D-MIMO downlink implementations. The first part of the thesis describes the background and motivation. Specifically, the system models for the centralized cellular base station and the D-MIMO architecture are described.The second part of the thesis explains different technologies for modulating an RF-signal onto an optical carrier using RoF: digital-radio-over-fiber (DRoF), analog-radio-over-fiber (ARoF) and sigma-delta-over-fiber (SDoF). As the need for power hungry digital-to-analog converters (DACs) is avoided in ARoF and SDoF, they are proposed as primary candidates in D-MIMO implementations. Moreover, measurement results show that SDoF is more robust than ARoF towards non-linearities in an RoF transmitter and inter-channel interference in an RoF receiver. Finally, a SDoF architecture for D-MIMO with serially connected RRHs is presented. By connecting the RRHs in series the scalability of the system is enhanced, as one fiber link can be used to transmit signals to all RRHs.The serial connection is implemented using wavelength-division multiplexing (WDM), addressing each RRH by a specific wavelength.Through measurement results it is shown that the system with serial connection can perform similarly to one with the RRHs connected in parallel. To conclude, this thesis presents SDoF as a fronthaul alternative for building robust, scalable and energy-efficient downlinks for D-MIMO networks.The results provide insights into the practical implementation of D-MIMO, a promising architecture for wireless communication systems.
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- Olofsson, Frida, 1992, et al.
(författare)
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Investigation of Fiber Length Differences in Distributed MIMO Sigma-Delta-over-Fiber Systems
- 2023
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Ingår i: 49th European Conference on Optical Communications (ECOC 2023). - 9781839539268
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Konferensbidrag (refereegranskat)abstract
- The effect of having different fiber length to each access point in distributed MIMO is experimentally investigated through testbed experiments. It is found that the delay differences can severely degrade the performance of the system. A method for compensating the delay effect is proposed.
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