Low Cost Remote Antenna Units in CMOS for Fiber-fed Distributed MIMO Systems
Abstract: Massive multiple input multiple output (MIMO) is a revolutionary communication technique, where large numbers of antennas are employed at the base stations to increase the spectral efficiency, reliability and data rate of the system. Moreover, in multi-user MIMO (MU-MIMO) multiple users can be served by the base stations on the same time-frequency resource. Distributed antenna system (DAS) is a form of massive MIMO, where the antennas are distributed over a geographic area instead of being co-located. Optical fibers can be used to connect these remote antenna units (RAUs) to the base station in a low loss and coherent fashion. Since large numbers of RAUs are required it is very important to reduce their cost. This work is focused on designing a low cost RAU intended for a multi-mode fiber (MMF) fed distributed MIMO system. It is also assumed that MMF and vertical cavity surface emitting lasers (VCSELs) would be used to keep the system cost low. A standard 65 nm CMOS technology was used to design the circuits. In paper I, a low cost RAU architecture was presented where a basic downlink signal chain with an integrated photodiode was designed. An intermediate frequency (IF) over fiber approach is chosen, so that an integrated photodiode and low cost optical components could be used. Measurement results indicated that the proposed architecture is promising for implementing low cost RAUs. Paper II presents the design and measurements of a complete RAU, featuring both uplink and downlink signal chains, and frequency generation circuits. The designed RAU also includes a novel scheme which allows the use of an antenna switch instead of a bulky circulator, saving both cost and area. Furthermore, RAU phase synchronization is achieved using reference signal distribution, which is critical for the performance of distributed MIMO systems. In paper III design and measurements of a RF power amplifier driver is presented. The driver was linearized by a triode multiplier, which significantly increased the linearity performance without any power and with minimal area overhead. The design and measurements of a fully integrated RF power amplifier (PA) is presented in paper IV. The PA was linearized by analog pre-distortion, achieved by employing a driver stage biased in class-C. In paper V, design and measurements of different CMOS photodiodes are presented. The obtained results are then used to choose the optimum photodiode structure for the optical receiver. Paper VI presents the design of a transimpedance amplifier, which converts the single-ended current signal from the integrated photodiode into a differential voltage signal. The single-ended to differential conversion was achieved by applying an ac common mode feedback. Furthermore, capacitive cross-coupling was applied to increase the gain of TIA, without affecting its bandwidth or using extra power.
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