1. Juanjo Noguera Xilinx Research Labs Dublin, Ireland Ahmed Al-Wattar Irwin O. Irwin O. Kennedy Alcatel-Lucent Dublin, Ireland

2.  introduce a new approach to reduce FPGA power consumption.  By exploiting the time varying nature of a systems environment  closely tracking environmental changes and adapting the implementation accordingly using partial reconfiguration

3.  Partial Reconfiguration (PR) allows the reconfiguration of a part of the device while the rest of the FPGA continues operating  there have been multiple hardware enhancements to Xilinx FPGAs to better support partial reconfiguration.

4.  Smaller units of reconfiguration granularity. ◦ From the full device height reconfiguration frames in the Virtex-II and Virtex-II Pro families to the 16- CLB’s high in the Virtex-4 family.  Increased bandwidth in the internal configuration access port: ◦ From 50Mbytes/s in the Virtex-II and Virtex-II Pro families to 400Mbytes/s in the Virtex-4 family  Early Access Partial Reconfiguration (EAPR)

5.  Traditionally, partial reconfiguration has been used to time multiplex multiple mutual exclusive functions, hence reducing cost and static power consumption. ◦ it does not present any benefit in applications where all application functions are required on the FPGA at the same time

6.  use of partial reconfiguration to time- multiplex different implementations of the same function. ◦ reduce the FPGAs dynamic power consumption  specializing the implementation to the current subset of requirements, we can reduce average power consumption.

8.  We have applied this idea of adapting the implementation for power savings to the networking application domain ◦ using a forward error correction core (i.e., Viterbi decoder)

9.  most of the dynamic power dissipation in an FPGA fabric is due to the programmable interconnects and clocking resources  reductions in power consumption by increasing the number of pipeline stages in a FPGA design  Several authors have proposed low-power implementations of the Viterbi decoder

10.  The environment is the stimulus it receives from external sources ◦ e.g. number of users in a system, communication channel conditions, or total throughput. ◦ The number of users in a wireless base-station changes throughout the day. ◦ signal to noise ratio at a wireless base-station changes with the location of the mobile phone ◦ The mixture of voice and data users on a cellular base-station changes throughout the day

12.  Cost of electricity ◦ Google warned that the cost of electricity used to power their equipment could soon be greater than cost of the equipment itself  Reliability ◦ Average heat energy is the greatest determinant of digital electronics reliability  Thermal Engineering ◦ Thermal engineering is concerned with removing excess heat energy from a system.

13.  Application-level partial reconfiguration  Architecture-level partial reconfiguration ◦ the bit width of the data path or the number of pipeline stages in an arithmetic block implementation  Device-level partial reconfiguration ◦ loading the unused function’s FPGA area with the most power efficient idle configuration or directly controlling the FPGA clocking resources (i.e., clock buffers or DCM modules) from the configuration memory

14.  Forward error correction codes such as convolutional codes limit the effects of noise in digital communication  Viterbi algorithm is used for decoding convolutional codes  widely applied in networking applications due to its good noise tolerance

15.  adapting the Viterbi decoder implementation in two ways ◦ changes in the signal to noise ratio ◦ changes in the required throughput  Xilinx provides a Viterbi decoder core in Coregen.

16.  running at 100MHz  dual-port memory blocks (32Kbytes) implemented using on-chip BRAM’s  we connected a power supply with integrated ammeter to the FPGA internal core

18.  The Viterbi algorithm’s constraint length (K) greatly impacts the decoder’s Bit Error Rate (BER) performance  We verified this assumption experimentally using three implementations of the parallel Viterbi decoder with different constraint lengths.  significant impact that the constraint length parameter has on the number of FPGA resources used

22.  The Xilinx Viterbi core has a parameter that enables the user to select among a serial and a parallel architecture

23.  power consumption measurements reveal, that for this example, the parallel architecture is more power-efficient than the serial architecture  sample points for the 8.3Mbps throughput we can observe that there is a difference of 200mW (approx.)

24.  Reducing the number of LUTs and routing resources required to implement a function effectively reduces its capacitance  dynamic power consumption is also proportional to the switching activity of all nodes in the design  The serial architecture requires 12 clock cycles for each decoding process, while the parallel architecture only requires a single clock cycle ◦ Serial average power consumption of 0.7W (approx.), with peaks around 1W. ◦ Parallel average power of 0.5W (approx.) and peaks of 2.5W.