Implementing LTE on FPGAs

The next generation of the 3GPP wireless standard is called long-term evolution (LTE). It provides a leap in performance and a complete move to packet-based processing. In the physical (PHY) level of the LTE specification, specific challenges exist when dealing with higher data throughput rates, as well as the move to OFDM (orthogonal frequency-division multiplexing) for transmission.

Xilinx has developed – or is in the process of developing – several new or revised DSP LogiCORE solutions to meet the demands of the new specification. With such blocks it is critical not only to verify them as stand-alone blocks, but also to validate them in real systems with real-world data. The Xilinx 3GPP downlink reference design provides this validation, as well as providing a reference to customers about how to use the blocks.

The higher data rate in LTE places increased processing demands on all parts of the system: increased DSP hardware processing in the baseband, increased software processing to implement the higher layers of the UMTS protocol stack, and increased I/O communication bandwidth to accept packets and pass data to remote radio-heads.

In this article, I'll review some of the new features of the LTE specification and how Xilinx Virtex-5 FXT devices address the increased processing demands of LTE through its tight integration of microprocessor subsystem, DSP-enhanced FPGA fabric, and high-speed communication links.

The 3GPP LTE Physical Layer
One of the key changes in the Layer 1 (PHY layer) of the 3GPP LTE is the change from CDMA (code-division multiple access) to OFDM (orthogonal frequency-division multiplexing). One of the main benefits of OFDM is that it reduces the problems associated with multiple paths in the radio channel. In CDMA, a significant amount of processing must be devoted to characterizing and tracking the radio channel to compensate for the effects of fading in the channel.

Figure 1 illustrates the structure of an example LTE subframe. The subframe comprises a number of OFDM symbols. Each OFDM symbol provides the data input for an inverse fast Fourier transform (IFFT). In LTE this may be as many as 2,048 input points for in-phase (I) and quadrature (Q) components.

(Click to enlarge)
Figure 1. LTE uses OFDM. A subframe comprises a resource grid, with areas allocated to control, synchronization, and user data. Each column of the grid forms an OFDM symbol that is converted to the time domain by an IFFT.

A subframe can be represented as a resource grid, where each resource element in the grid comprises a single I/Q input point for the IFFT in an OFDM symbol. Resource grids can be layered to provide data to multiple antennas, supporting transmission schemes such as transmit diversity or MIMO (multiple input/multiple output) techniques.

The resource grid is allocated to different purposes. Resource elements are allocated to control channels, data channels, and synchronization signals. The diagram also shows the packetization of data on the channel – different areas of the resource grid are allocated to different users' data as resource blocks. The task of scheduling data transmission and allocating resource blocks to users is performed by the higher software layers in the LTE stack.