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Implement a 2x2 MIMO OFDM-based channel measurement system (no data yet) at 2.4 GHz

Goals. Implement a 2x2 MIMO OFDM-based channel measurement system (no data yet) at 2.4 GHz Perform baseband processing and digital up and down conversion on Nallatech Xtreme DSP Kits using System Generator tools

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Implement a 2x2 MIMO OFDM-based channel measurement system (no data yet) at 2.4 GHz

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  1. Goals • Implement a 2x2 MIMO OFDM-based channel measurement system (no data yet) at 2.4 GHz • Perform baseband processing and digital up and down conversion on Nallatech Xtreme DSP Kits using System Generator tools • Integrate DSP Kits with National Instruments radios for amplification and analog up and down conversion • Download large amounts of received data to a PC over PCI interface in practical amount of time • Have a fully operational system by September 2004

  2. OFDM Transmitter IFFT Cyclic Suffix Low Pass Filter Symbol Upconvert to 2.4 GHz (using NI radios) Upconvert to IF

  3. OFDM Transmitter 64 point streaming IFFT Adjustable length from 0-64 802.11 compliant 60 tap root raised cosine with 5x oversampling IFFT Cyclic Suffix Low Pass Filter Symbol Upconvert to 2.4 GHz (using NI radios) Upconvert to IF Analog upconversion performed in NI Radios Upconvert to 25 MHz carrier

  4. OFDM Receiver Coarse Sync. Symbol timing Fine timing PLL Downconvert To Baseband Estimate Channel FFT De-Suffix Low Pass Filter

  5. OFDM Receiver Tracks phase of 15 MHz carrier tone, using FIR loop filter Downconvert from 15 MHz carrier Length 48 correlator implemented as FIR Coarse Sync. Symbol timing Fine timing PLL Downconvert To Baseband Estimate Channel FFT De-Suffix Low Pass Filter 60 tap root raised cosine FIR Length 64 streaming FFT Discard cyclic suffix samples

  6. Issues Resolved - Transmitter • Filter • Root raised cosine filter, FCC compliant • Found proper filter order • Performed sufficient oversampling to smooth DAC output. • Cyclic suffix • Able to add cyclic suffix of adjustable length • Synchronized with IFFT to allow for unequal samples in, samples out • Associate FPGA clock with correct carrier frequency (IF) • Clock the FPGA at 100 MHz • Upconvert 20 MHz (RF BW) wide baseband signal to 25 MHz (IF) • Connection between the FPGA and the radio • Connected D/A converter to NI radios • Successful tests verified on spectrum analyzer + Oscilloscope

  7. Issues Pending - Transmitter • 2 antennas need 2 different preambles for individual synchronization • Current implementation only accounts for single transmit antenna • Transmitters are timing synchronized but have nondeterministic phase • We require orthogonal pilot tones • Also require orthogonal short sequences with good correlation properties • Generating frames containing both preamble and measurement signals • Currently beginning integration • Must be able to accommodate long preamble to insure packet detection and synchronization in measurements • Must be able to adjust cyclic suffix length according to symbol and channel type

  8. Issues Resolved - Receiver • Packet detection: detect the presence of a packet by correlating short training symbols • The coarse frequency estimation: use the spacing between correlation peaks to provide a rough carrier estimate to PLL • Design of Phase Locked Loop: robust to 16 kHz offset in 15 MHz carrier and integrated with coarse frequency estimator • FPGA clock: run the FPGA at 60 MHz for 20 MHz wide signal arriving at 15 MHz center frequency

  9. Issues Pending - Receiver • Symbol timing recovery and sampling frequency mismatches • Must be able to advance/retreat periodically in order to match transmitter and receiver sample times to symbol lengths • Good symbol timing is essential to FFT performance • Complex channel estimation: On chip or off? • Complex algorithms on chip allow us to download less data • Performing channel estimation offline allows more thorough analysis • Integrate synchronization with signal processing • Demodulate using PLL output • Estimate symbol timing using short symbol correlation • Estimate Tx sample times from PLL and correlation

  10. Current Stage… • Transmitter • able to transmit single channel input • still need to implement multiple channel inputs • still need to develop measurement signal input • Receiver • still need to integrate coarse synchronization, PLL, and down-stream processor to have a fully functional receiver • still need to tune the parameters of the synchronization block for packet detection and PLL convergence • still need to implement multiple channel outputs

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