3GPP Long Term Evolution (LTE) wireless technology is starting to move into deployment, with countries around the world working through the process of rolling out the necessary infrastructure. For CommAgility this meant designing an FPGA-based RF interface for LTE wireless.
There is a requirement for LTE radio frequency (RF) air interface development and testing, which has in turn driven demand for high bandwidth universal radio interface cards that can handle multiple bands and protocols. While CommAgility had already developed baseband LTE hardware modules, it needed to respond to customer demand by also supplying an RF interface for LTE. Having used the Xilinx Virtex-6 FPGA for some of its existing LTE products, it was natural that it looked to this device as the processing heart of the RF interface.
Bringing the RF product to market presented numerous challenges. This article describes the issues involved in RF interface design for LTE, and how the FPGA’s features helped us to overcome them.
The RF module developed, the AMC-RF2x2, is a full-size single-width Advanced Mezzanine Card (AMC) as defined by PICMG (Figure 1). It is designed for use in a Micro Telecommunications Computing Architecture (MicroTCA) chassis alongside other AMCs, allowing a system solution to be created which is configurable at the card level.
Figure 1. AMC-RF2x2
For example, a three-sector base station system may require three RF cards, one for each sector. However, a single processing module, such as the AMC-2C6670, can provide LTE baseband processing for all three sectors. A MicroTCA chassis could thus be populated with one processor module and three RF cards, with a processor AMC (PrAMC) to provide media access and management functionality. Such a system is shown in Figure 2.
Figure 2. Three Sector Base Station Architecture
The MicroTCA backplane allows communication between the AMCs using the Ethernet base fabric for control and the fabric (Serial Rapid IO (SRIO), Ethernet 10 Gigabit Unit Attachment Interface (XAUI), or PCI Express (PCIe)) for data. The MicroTCA Controller Hub (MCH) provides the switching fabric for the MicroTCA chassis, and also supports reliability and availability features such as hot card swapping and card temperature monitoring.
In the system shown in Figure 2, communication between the processor module and the three RF cards is over a Common Public Radio Interface (CPRI) link. This link runs at 4.9152Gbaud and can use either an optical or copper physical interface based on the SFP+ modules used on the cards. The CPRI link carries two channels of IQ data to and from the RF card to support MIMO operation. It is able to support LTE channel bandwidths of 1.4MHz, 3MHz, 5MHz, 10MHz, 15MHz, 20MHz and 40MHz.
The FPGA CPRI interface is implemented using the Xilinx CPRI IP core and is able to support line rates up to 6.144Gbaud. The processor module acts as the CPRI radio equipment controller (REC) and generates the master clock for the CPRI link. This is derived from a GPS receiver on the card, ensuring clock synchronisation between equipment linked across the air interface.
This reference clock is extracted by the Virtex-6 MGT block, acting as the radio equipment (RE), and passed to a phase-locked loop (PLL) for jitter cleaning. The cleaned clock is then used for the air interface timing as shown in Figure 3.
Figure 3. RF Module Clock Synchronisation
The AMC-RF2x2 can also support IQ data transfer and control over the MicroTCA fabric using PCIe, XAUI or SRIO protocols with transfer rates up to 20Gbit/s. The fabric selection can be made automatic using AMC e-keying and the flexible configuration of the Virtex-6 MGT ports to support the protocol choice. This allows the card to be more closely integrated into a chassis based system where IQ data is distributed across multiple cards.
On arrival at the FPGA, the IQ samples are de-interleaved from the CPRI frame according to the 3GPP E-UTRA specification using custom logic. They are then passed on to the Virtex-6 digital up converters (DUCs) as shown in Figure 4.
Figure 4. Digital Data Path
The AMC-RF2x2 uses the Xilinx LogiCORE DUC/DDC Compiler for this operation. This allowed the DUC and DDC design to be implemented and simulated ahead of the interface components being available. The core supports the 5MHz, 10MHz, and 20MHz LTE bandwidths of interest to the RF module, and the Xilinx LogiCORE finite impulse response (FIR) Filter Compiler has been used to develop support for other LTE bandwidths. The use of Xilinx DSP slices allows the DAC/DUC to be efficiently implemented using a minimum of FPGA resources.
The DUC filters the baseband data and converts it to the higher sample rate of 61.44MHz required by the RF digital-to-analogue converter (DAC). In the RF module, the DAC itself can then perform a further up-conversion to the required sample rate of 983.04MHz. Additional IP cores can be added to support crest factor reduction and digital pre-distortion which are required for broadcast transmission. However, in the wireless test application under discussion, these features are also left unused.
The RF processing block following the DUC implements gain control. IQ imbalance and DC offset correction are supported in the DAC, and the Virtex-6 acts as a control interface for this DAC functionality. IQ imbalance and DC offset correction are performed automatically in the AMC-RF2x2 whenever the operating mode or operating band are changed.
With the IQ data at the correct rate, it is now passed to the DACs for processing in the analogue domain. On the RF module, the DACs and analogue-to-digital converters (ADCs) use a DDR interface implemented using LVDS signals from the FPGA SelectIO. The clocking blocks within the FPGA allow the interfaces to be synchronised to the main card clocks.
In the receive path, data from the ADCs is sent to an RF processing block. This implements the gain control, IQ imbalance and DC offset correction automatically before data is down-mixed from Low-IF to baseband and then passed to the digital down converter (DDC). The DDC filters digital signal from the ADC and reduces the sample rate from 122.88MHz to the CPRI rate for the receive bandwidth; for example 30.72MHz for 20MHz bandwidth. Finally, the IQ samples pass through a gain control block before being interleaved into a CPRI frame according to the E-UTRA specification.
Control and management
The various on-card devices in the RF chain are managed by a mixture of serial peripheral interface (SPI) busses and general purpose IO (GPIO). The implementation of these busses is subtly different for each device and requires careful inspection of the data sheet timing diagrams. Using the Virtex-6 to support these interfaces allows these variations to be accommodated, while presenting a common control interface.
On-card memory is provided in the form of 128Mbytes of non-volatile Flash memory and 128Mbytes of volatile DDR3 SDRAM. Using the memory interface generator (MIG), it is straight forward to implement support for external DDR3 from the Virtex-6.
Logic in the Virtex-6 is used to present a unified interface to these multiple control busses and to aggregate common functions associated with particular modes of operation. The supported modes of the AMC-RF2x2 are shown in Table 1.
This unified interface is presented to the card support software for control. This software runs on a MicroBlaze soft processor core on the Virtex-6, which connects to peripherals using the Advanced eXtensible Interface 4 (AXI-4) protocol. AXI-4 makes peripheral integration easier by offering a standard interface for all devices.
Capture and play-out of IQ data is supported using internal Block-RAM based storage. Larger buffers are stored by using the Block-RAM to cache data for periodic transfer to the external SDRAM.
A lightweight IP stack is implemented for the MicroBlaze, which allows a telnet session to establish a link with the card. This provides an interface for simple remote control of the card over Ethernet on the AMC base fabric. CommAgility provides a PC-based graphical user interface-(GUI)-based front end for the telnet control, which was implemented using National Instruments LabVIEW.
The CPRI link also includes the Fast Control and Maintenance (Fast C&M) channel. The CPRI IP core Fast C&M interface connects to a Xilinx MII interface supporting an Ethernet interface at speeds of over 100Mbit/s. This allows the Fast C&M channel to support the telnet interface described above. Using the Fast C&M channel, the processor module can control the RF card, for example to read the on-card temperature sensors and support temperature compensation.
Experience with the Xilinx Virtex-6 in baseband designs enabled CommAgility to rapidly develop a highly flexible 2x2 multiple-input multiple-output (MIMO) LTE RF interface module, the AMC-RF2x2. This card covers all LTE bands, bandwidths, and duplex types. Xilinx’s flexible IO interfaces and comprehensive range of wireless IP cores enabled rapid development from concept to delivery in under six months.
Furthermore, the scalability and flexibility of the Virtex-6 range makes possible a straightforward roadmap transition to LTE-Advanced and 4x4 MIMO support. Beyond this, the introduction of Xilinx’s next generation Kintex-7 devices enables power reduction and the possibility to bring some of the baseband processing onto the RF card.
However, the underlying device is only a part of the jigsaw. The provision of Xilinx LogiCORE IP for the full range of building blocks required in the wireless signal chain has accelerated development. Using Xilinx cores with proven operation and optimal resource usage has greatly reduced CommAgility’s development and test time, bringing the card to market in less than six months.