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Beam steering antenna and wideband up/down converters are essential building blocks for the deployment of 5G communication at MMwave bands. In India, a band around 26GHz is likely to be chosen for the purpose. These may be useful to deliver immersive video content to passengers in premium coaches for railways. Key design elements of these building blocks are discussed in this paper.
5G NR (New Radio) FR2 frequency range has been specified from 24.25 to 40GHz by 3GPP standard committee. The maximum allowed RF channel bandwidth in this mmwave band is 400MHz without carrier aggregation. Different bands that are parts of 24.25-29.5 GHz have been fairly confirmed for fifth-generation (5G) mobile cellular communication in different parts of the world . In India, a band around 26GHz is likely to be chosen for the purpose.
An implementation of the transmitter of such a wideband system will involve modulation of RF carrier, centered around few GHz, with high-speed data using OFDM and then upconversion of the modulated RF carrier to the desired mmwave band. The output of the upconverter feeds an antenna element or a phased array antenna which radiates the modulated signal. Most commercial base station antennas use microstrip technology due to its proven advantages of low profile and easy integration with circuits. A wideband linear array without beam steering characteristics for the full 5G bandwidth has been demonstrated in . It is widely accepted that simple microstrip patch arrays have poor scan performance  and have low efficiency due to losses in the substrate and conductors . The use of integrated active antenna arrays can alleviate some of these issues.
Due to the increased path loss at mm-wave frequencies, high gain antennas may be often required which would demand beamforming techniques to be used. Recently, a stripline fed 6×7 microstrip patch array with a gain of about 22 dBi and sidelobe level of 21 dB and beam width of 15° has been demonstrated at 28GHz . These antenna arrays may form a crucial building block in the front end of the access point delivering an immersive experience to passengers in a rail coach delivering on-demand video content. On the receiver side, an antenna or a phased array antenna will receive the signal which is down-converted to RF carrier at a few GHz and then sent to the baseband demodulator to retrieve the original data/signal. A synthesizer is needed to generate LO for up and down converters. This paper describes the implementation of some of the modules/building blocks mentioned above which have been designed and realized for the implementation of mmwave system as part of the 5G testbed at the Indian Institute of Science (IISc), Bangalore.
The following sections describe the design and implementation details of various building blocks mmwave system consisting of the beam steering antenna array, beam control, and up/down converter. An array of the simple rectangular microstrip patch antennas is integrated with active circuits to realize the beam steering antenna discussed below.
Beam steering phased array antennas are used to achieve high gain as compared to what is achievable from the individual antenna elements. The beam direction of the phased array can be changed by programming different phase shifts in the RF path of each antenna element. Recent advancements in the semiconductor industry have enabled the integration of multiple of such Tx/Rx modules in a single MMIC to bring down the overall size. Our design uses 2-dimensional distribution of 2×2 microstrip patch antenna arrays and our antenna design followed this approach.
The design has been realized on a multi-layered PCB, which consists of high-frequency laminates for antenna and MMIC layers and glass epoxy substrate for the inner layers for routing low-frequency signals, such as digital control signals for programming registers for each path and power planes. A 16×4 array has been designed with a power divider circuit at its back as shown in Figure 1 for a fine beam in the elevation plane. Anokiwave AWMF-0108 Ka-Band Tx/Rx IC  is used for beam steering in this active antenna. This can be controlled using an FPGA board.
A Zybo Z7 belonging to the Xilinx Zynq-7000 family with Field Programmable Gate Array (FPGA) logic is used to control the beam steering IC. The radiation characteristics of this antenna is evaluated in a microwave anechoic chamber at the Indian Institute of Science as shown in Figure 2. The measured normalized radiation patterns at indicative scan angles are shown in Figure 3.
The primary aim of the Up/Down conversion block is to translate the IF or baseband IQ 5G NR signal to mmwave band (24-29GHz range) and vice versa, with required gain.
ADMV1013 and ADMV1014 ICs from Analog Devices are used for up and down conversion respectively. The ADMV1013 is a wideband upconverter operating in the 24GHz to 44GHz range, that can take differential IQ baseband signals. The downconverter IC ADMV1014 does the complementary operation.
The complete transmit chain consists of the baseband modulator with IQ baseband or IF signal output up to 6GHz followed by the upconverter which feeds the signal to the beam steering antenna. The receive chain comprises the receiving antenna followed by the downconverter and demodulator. Preliminary characterization of the upconverter was done by varying IF from 1-1.7GHz at different bandwidths in the range of 100-400MHz at a power level of -12dBm. The output RF power centered at lower sideband frequency is shown in Fig. 5. These measurements show that the upconverter can be used for 5G communication.
We have designed and tested some of the key building blocks required for building mmwave communication systems. Design concepts here can be used for building larger systems for 5G communication. It is expected that these components may form a crucial building block in the front end of the access point delivering an immersive experience to passengers in a rail coach delivering on-demand video content. A video on the current capability is available online .
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