Introduction to the NI mmWave Transceiver System Hardware

Service operators around the world have paid billions of dollars for spectrum to service their customers.  The exorbitant auction prices for spectrum below 6 GHz highlights the strength of the competitive market forces but also the scarcity of this precious resource.  As stated above, enhanced data rates and increased capacity are constrained by spectrum according to Shannon. More spectrum yields higher data rates, which enable service operators to accommodate more users while also delivering a consistent mobile broadband data experience.  In contrast, mmWave spectrum is plentiful and lightly licensed, meaning it is accessible to service operators around the world.  The challenges impacting mmWave adoption lie primarily in the unanswered technical questions regarding this unexplored and largely uninvestigated spectrum.

To capitalize on the promise of mmWave for 5G, researchers must develop new technologies, algorithms and communications protocols as the fundamental properties of the mmWave channel are different from current cellular models, and are relatively unknown.  The importance of building mmWave prototypes cannot be overstated especially in this early time frame.  Building mmWave system prototypes demonstrate the viability and feasibility of a technology or concept that simulations alone cannot.  mmWave prototypes communicating in real-time and over the air in a variety of scenarios will unlock the secrets of the mmWave channel, and enable technology adoption and proliferation.

Figure 1: Three high level 5G use cases as defined by 3GPP and IMT 2020

To that end, NI offers the world’s first real time mmWave prototyping system that enables engineers and researchers to quickly prototype mmWave systems.  By combining modular, flexible hardware with powerful application software, the NI mmWave Transceiver System is an SDR for mmWave applications.  Because the mmWave Transceiver is a full SDR, researchers can deploy their designs quickly as both the hardware and software are fully featured and modular, and then they can quickly iterate to optimize the design using software.

There are numerous challenges to building a complete mmWave communications prototype.  Consider a baseband subsystem capable processing a multi-GHz channel.  As most of today’s LTE implementations typically use 10 MHz channels (20 MHz maximally), the computation load increases linearly with bandwidth.  In other words, the computational capacity must increase by a factor of 100 or more to address the 5G data rate requirements.  The algorithms used in today’s LTE turbo decoders are computationally intensive and require high performance computing hardware to process data in real time.  FPGAs provide an ideal hardware solution for these computations, and for ultra-wide bandwidth turbo decoding, FPGAs are essential.

Although FPGAs must be considered a core component to an mmWave prototyping system, programming a multi-FPGA system capable of processing multi-GHz channels presents increased system complexity.  To address the system complexity and software challenge, NI provides an mmWave physical layer in source code that accounts for the fundamental aspects of a mmWave system baseband and also provides abstractions for data movement and processing across multiple FPGAs to simplify the task.  

FPGAs are only one piece of a mmWave prototyping system.  Data needs to be able to flow back and forth between the digital domain where it is processed and the analog domain where signals sent and received over the air.  Improvements in DAC and ADC technologies make it possible to capture between 1-2 GHz.  There are some mmWave frequency ICs on the market today.  These ICs can be connected to the ADCs and DACs in the mmWave Transceiver System to evaluate them and do prototyping work.  However, RFICs do not offer high power output or the quality of RF that is needed for channel sounding and communications prototyping.   To integrate higher power and better quality RF, there an IF stage is used to convert signals to 12 GHz.  Finally, mmWave radio heads are then connected to the IF module.  Each piece of this prototyping system would require a different set of design expertise and significant engineering resources to develop if one wanted to build a prototyping system from scratch.  The hardware design of each piece is non trivial, and the software needed to control and synchronize all of the stages adds an extra layer of complexity to custom system designs.  The mmWave transceiver system offers a complete prototyping solution to help engineers go from concept and algorithm design to prototyping faster.

NI offers four off the shelf mmWave prototyping system configurations which are discussed in more detail below.  Based on the PXIe platform, the mmWave Transceiver Systems are composed of 2 GHz bandwidth baseband processing subsystem, a 2 GHz bandwidth filtered intermediate frequency (IF) stage and LO module, and modular mmWave radio heads that reside external to the chassis. A system diagram can be seen below in Figure 2.

Figure 2: mmWave system diagram

This modular approach creates a flexible hardware platform that can be modified by adding or removing modules to accommodate a wide variety of channels and configurations.  Users may choose to use the full NI mmWave solution, or integrate their own RF into the NI IF and baseband system.  The user can also use the same system to prototype different bands with the same IF and baseband hardware and software.  The system can scale from a unidirectional SISO system for applications such as channel sounding to a bidirectional MIMO system capable of transmitting and receiving in parallel for a full 2 channel 2 way communications link.  The various system components and configurations will be discussed in detail in this paper.  Application specific software is not discussed in this white paper.

The mmWave Transceiver System is an SDR platform for building mmWave applications including system prototyping. It gives users access to a flexible hardware platform and application software that enables real-time over the air mmWave communications research.  The software is open to the user and can be modified as research needs change so designed can be iterated and optimized to meet specific goals or objectives.

The NI mmWave transceiver system is comprised of PXIe chassis, controllers, a clock distribution module, FlexRIO FPGA modules, high speed DACs, high speed ADCs, LO and IF modules, and mmWave radio heads. The modules can be assembled in various configurations to address a large number of mmWave applications ranging from channel sounding to MIMO communications link prototyping.  This document provides a detailed overview of the hardware that is used in the mmWave transceiver system and how modules interact with each other.  Detailed performance specifications for the system can be found in the mmWave transceiver system specifications sheet and model page. 

PXI Express Chassis

The mmWave prototyping system is based on the PXIe-1085 chassis.  The chassis houses the different processing modules and provides power supply, interconnectivity and timing and synchronization infrastructure. This 18 slot chassis features PCI Express (PCIe) Generation 3 technologies in every slot for high-throughput, low-latency applications. The chassis is capable of 4 GB/s of per-slot bandwidth and 24 GB/s of system bandwidth.  The PXIe-1085 uses a dual-switch backplane architecture, shown in the system diagram of Figure 3.  Because of the flexible design of PXI, multiple chassis can be daisy-chained together or put in a star configuration when building higher channel-count systems.

Figure 3: 180Slot PXIe-1085 Chassis (a) and System Diagram (b)

High-Performance Reconfigurable FPGA Processing Module

Central to all SDRs is the software and computational elements that compose the physical layer.  The mmWave prototyping system uses single slot FPGA modules to add flexible, high-performance processing modules, programmable with LabVIEW, within the PXIe form factor. The PXIe-7976R FlexRIO FPGA module can be used standalone, providing a large and customizable Xilinx Kintex-7 410T with PCI Express Generation 2x8 connectivity to the PXI Express backplane. The mmWave transceiver system maps the different processing tasks to multiple FPGAs, depending on the particular configuration in a software configurable manner.

Figure 4: PXIe-7976R FlexRIO Module (a) and System Diagram (b)

High performance FPGA for high data throughput applications

The NI PXIe-7902 FPGA module is powerful processing module built with a Xilinx Virtex 7 485T.  The large FPGA makes it ideal for processing heavy applications such as the mmWave physical layer.  This module can transfer data across the backplane of a PXIe chassis with PCIe gen 2x8 speeds.  For applications needing to support faster data rates, the PXIe-7902 also features 6 miniSAS HD front panel connectors composed of 24 Multi Gigabit Transceivers (MGTs).  The MGTs can be connected to other PXIe-7902 modules or to other modules such as a DAC or ADC to enable up to 2 GHz of real-time bandwidth on multi-channel baseband signals.

Figure 5: PXIe-7902R FPGA module (a) and System Diagram (b)

Ultra wideband DAC and ADC

The PXIe-3610 DAC is shown below in Figure 6, and the PXIe-3630 ADC is shown in Figure 7.  They provide access to analog baseband differential I/Q pairs through 4 MCX front panel connectors.  These modules can be connected together to create a baseband loopback test system, connected to the PXIe-3620 IF module, or to 3rd party baseband hardware.  Basic performance information is shown below in Table 1.  Detailed performance information can be found in the mmWave transceiver system data sheet.

Table 1: Basic performance specifications of the PXIe-3610 and PXIe-3630

Figure 6: DAC module and block diagram

Figure 7: ADC module and  block diagram

IF module

The PXIe-3620 LO/IF module is capable of processing one transmit and one receive chain for up to 2 GHz of bandwidth each. The NI PXIe-3620 mixes the input signals with the integrated LO to upconvert the baseband signal to a software programmable IF between 8.5 – 13 GHz.  For receive, the NI PXIe-3620 takes an 8.5 to 13 GHz input IF and converts to baseband.  This module contains internal gain control and is capable of transmitting up to 7 dBm and receiving a 20 dBm signal. The PXIe-3620 also provides the LO reference signal for the NI mmWave radio heads (mmRH modules).  The LO/IF module can optionally accept an external LO signal or can drive the LO signal for other IF modules to synchronize multiple transmit/receiver streams in a MIMO topology.  The differential I/Q pairs are accessible through MXC connections on the device front panel.

Figure 8: PXIe-3620 IF module

mmWave heads

The modular mmWave radio heads provide a high quality RF signal for the mmWave Transceiver System and support the following frequency bands.

• 24.25 - 33.4 GHz
• 37 - 43.5 GHz
• 71 - 76 GHz

All of the radio heads support 2 GHz of bandwidth. There is an option of  purchasing a transmitter, receiver, or transceiver. For details on the products, please refer to Table 2 below.The mmWave heads contain attenuators and amplifiers for maximum gain control and noise figure. A detailed RF characteristics are listed in Table 3. The radio heads can be connected to a user provided antenna, such as a horn antenna or a phased array antenna.  

Table 2: Product names of the mmWave radio heads and part numbers for required digital cables for each model

	24.25 - 33.4 GHz	37 - 43.5 GHz	71 - 76 GHz
Receiver
Instantaneous Bandwidth	2 GHz	2 GHz	2 GHz
Analog Gain Range	50 dB	50 dB	55 dB
1 dB Gain Compression1	-13 dBm	-13 dBm	-12 dBm
Noise Figure2	6 dB	6 dB	6 dB
Transmitter
Instantaneous Bandwidth	2 GHz	2 GHz	2 GHz
Analog Gain Range	55 dB	55 dB	55 dB
Output IP32	28 dBm	28 dBm	30 dBm

 

Table 3: Characteristics of each radio head

1. Near minimum gain, For lower gain settings, 1 dB compression is higher than full scale.

2. At maximum gain

 

Figure 9: Left: 24.25-33.4 GHz and 37-43.5 GHz radio head

Right:71-76 GHz mmWave radio head

The NI mmWave transceiver system is meant to be a flexible hardware platform capable of servicing a wide variety of communications needs.  While hardware can be added or removed from a system to create various types of systems, there are 4 base configurations offered to meet the most common use cases:

• Unidirectional SISO
• Unidirectional 2x2 MIMO
• Bidirectional SISO
• Bidirectional 2x2 MIMO

Unidirectional Systems

The 2 unidirectional options consist of 2 PXIe chassis with the transmitter(s) in one chassis and the receiver(s) in the other chassis.  This configuration  is well suited for making channel sounding measurements.  With this configuration, one can physically separate the transmit and receive sub systems to take channel sounding measurements in a wide variety of environments.  Because of the modular nature of this PXIe based system, extra hardware can easily be added to accommodate different research needs such as adding more receive channels for more accurate angle of arrival measurements.  As an alternative to adding extra receive channels for a parallel receive or parallel transmit and receive implementation, an external switch can also be added to the SISO system.  This flexibility allows researchers to choose the hardware configuration that best meets their needs of measurement speed and configuration.  Because the mmWave transceiver system was designed for MIMO architectures, it is easy to share an LO between each channel for phase coherence.  A diagram of the SISO and 2x2 MIMO version of this system can be seen below in Figures 10 and 11.

Figure 10: Unidirectional SISO configuration

Figure 11: Unidirectional MIMO configuration

Bidirectional Systems

Each of the bidirectional system configurations consists of 2 PXIe chassis with one transmit and one receive in each chassis per channel.  These systems are designed for communications prototyping, giving researchers the hardware they need to create a real-time 2 way communications link.  There are many unknowns in mmWave communications research.  Determining how a signals behave in a mmWave channel is important.  Having a well defined channel model will help algorithm developers, but ultimately real-time communications links need to be prototyped to validate their performance in these new frequency spectrums.  Whether trying to validate a new physical layer and air interface or explore how the existing LTE physical layer can be adapted for ultra wide bandwidths like 2 GHz, the NI mmWave transceiver system can be used to prototype their performance in real-time.  Combining NI’s mmWave system with additional FPGA processing and LabVIEW, it is possible to perform modulation, demodulation, coding, and turbo decoding on up to 2 GHz of bandwidth in real time. These systems are designed to be used as a platform for researchers to develop and test communication protocols.  Unlike sub 6 GHz communications, mmWave signals are highly directional, and the protocol needs to ensure that two or more modes are able to locate each other.  Nodes need to be able to exchange control and measurement information, as a part of a beam steering or random access protocol for example.  Figures 11 and 12 show diagrams of these bidirectional system configurations.

Figure 12: Bidirectional SISO configuration

Figure 13: Bidirectional MIMO configuration

NI’s mmWave transceiver system is a modular set of hardware that can be used for a variety of applications from channel sounding to prototyping real-time 2 way communications systems.  This system is build on the PXI platform and provides a flexible set of modules that can be combined in a number of different configurations to meet ever changing research needs.  The mmWave radio heads themselves are also modular and can be replaced with other RF front ends to investigate multiple different frequencies with the same base set of hardware and software to save engineering design time and to get maximum system reuse.  This hardware combined with the power of LabVIEW provides an excellent platform for mmWave communications prototyping and helps engineers innovate faster.

• View the mmWave Product Flyer
• Shop the mmWave Transceiver System
• See the mmWave manuals