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Using LTE on FPGAs

Posted: 19 Feb 2009     Print Version  Bookmark and Share

Keywords:3GPP  long-term evolution  Virtex-5 

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 level of the LTE specification, specific challenges exist when dealing with higher data throughput rates, as well as the move to orthogonal frequency-division multiplexing for transmission.

The higher data rate in LTE places increased processing demands on all parts of the system: increased DSP hardware processing in the base band, 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.

This article reviews some of the features of the LTE specification and how Xilinx Virtex-5 FXT devices may address the increased processing demands of LTE through its integration of microprocessor sub-system, DSP-enhanced FPGA fabric, and high-speed communication links.

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.

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Figure 1: LTE uses OFDM. A subframe comprises a resource grid, with areas allocated to control, synchronisation, 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 synchronisation signals. The diagram also shows the packetisation 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.

3GPP LTE downlink processing
Figure 2 shows the processing stages in the base band section of the 3GPP LTE downlink. The processing for both transmit and receive can be split into two main sections: symbol rate processing and sample rate processing.

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Figure 2: 3GPP LTE downlink processing: transmit and receive chain.

Symbol-rate processing is centred around forward error correction, used to add redundancy to the data stream in a bandwidth-efficient manner and to recover data on the receiver. Sample-rate processing is centred around the IFFT/FFT that performs the OFDM part of the base band operation.

Transmit symbol rate processing
The first stage in the Layer 1 (PHY) processing of the LTE downlink takes transport blocks from the media access controller (MAC) layer. Transport blocks have cyclic redundancy checks (CRCs) added, while larger transport blocks may be segmented to ensure that blocks do not exceed a maximum size supported by the forward-error encoder.

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