DESIGN OF MODEMS ON FPGAs USING SIMULINK: A Great Tool for the Signal Processing Engineer

Bradley Comar MITRE Corp, McLean, VA 22102

Introduction: Many interesting research topics exist in the area of digital wireless communications. When studying an emerging field such as MIMO (multiple input, multiple output) space-time coding, the engineer usually conducts investigations using a software simulation tool such as Matlab by The MathWorks. When these simulated investigations are concluded, the engineer may then write a paper and move on to the next project. However, questions will persist about the accuracy of the results. If this system is actually implemented, will it work as well as the simulation? Are the simulated channel models and the quantization assumptions accurate? What pitfalls await the modem designer who tries to implement this system? The solution to these worries could come from having the research engineers actually build their proposed modems. Both Xilinx and Altera offer kits that claim to allow signal processing engineers to easily design projects such as modems using Simulink and implement their designs on FPGA development boards. The Altera DSP Development kit is successfully used to create a modem using a MIMO space-time coding by an engineer with no prior modem design or FPGA experience.

Summary: The challenge that this paper describes is to build a wireless modem which demonstrates the 2x2 Unitary Space-Time Modulation (USTM) scheme. This is a type of MIMO system which uses two transmit and two receive antennas to theoretically achieve better performance than a standard 1x1 SISO system in a dense multipath environment. This should work even when the overall energy per bit for both the MIMO and standard SISO systems is the same. In a line-of-site environment, the MIMO system should still perform at least as well as the SISO system. To test this theory, a modem is built using the QPSK modulation scheme with a bit rate of 1.25Mbps. The early-late gate method of symbol synchronization is chosen. No carrier synchronization is needed because the systems under test use differential keying methods. The platform that is chosen to build these modems is the Altera Stratix line of DSP Development Kits. The DSP Builder software by Altera is used to program the onboard FPGA using special simulink blocks. This development board also contains two A/D converters and two D/A converters. The research engineer has no modem design experience and no FPGA experience at the start of this project. At that time the engineer also has very limited Simulink experience. The task to build the 2x2 USTM modems is achieved. Two Altera Stratix development boards are used. The first board implements the modulation portion of the system while a second board implements the demodulation portion of the system. The first board creates a pseudorandom bit generator which repeats its output after 32Kbits to use in place of data bits. The second board uses this same generator to calculate a bit error rate. Two transmit RF front ends and two receive RF front ends are also designed and built for this demonstration. These front ends operate over the 2.4GHz ISM band. Lab experiments are conducted which show the USTM scheme outperforming the SISO system. See figure 1 for the outline of this system.

tx RF rcv RF Altera front end front end Altera Dev Dev Board Board tx RF rcv RF front end front end

figure 1: Development Boards Used to Verify 2x2 MIMO code

This system takes only a few months to design and build. There are a few pitfalls with using the DSP Builder software which are noted by the engineer. However, the modem design that is implemented and simulated in Simulink does, for the most part, what the FPGA does when the design is compiled and loaded onto the board.

Conclusion: There are tools available to assist the signal processing engineer in designing and building modems on FPGA based boards. The Altera Stratix Development Kit with DSP Builder offers a simple design environment for the engineer without modem design or FPGA experience. These implementations allow the engineer to determine if the research that was conducted is practical. They also help to demonstrate the credibility of the research to the potentially disbelieving customer.

References: 1. Alamouti, Siavash M., “A Simple Transmit Diversity Technique for Wireless Communications,” IEEE Journal on Select Areas in Comms., vol. 16, pp. 1451- 1458, Oct. 1998 2. Hughes, Brian L., “Differential Space-Time Modulation,” IEEE Transactions on Information Theory, vol. 82, pp. 2567-2578, Nov. 2000. 3. Proakis, John G., Digital Communications, 4th Edition, Boston: McGraw-Hill, 2001. 4. DSP Builder, Reference Manual, Prod. Ver. 2.1.3, Doc Ver. 1.0.0 rev. 1, Altera Corp, 2003.