Abstract
Original language  English 

Qualification  Master Degree 
Awarding Institution 

Place of Publication  Oulu 
Publisher  
Publication status  Published  2010 
MoE publication type  G2 Master's thesis, polytechnic Master's thesis 
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Keywords
 MIMOOFDM
 channel estimation
 superimposed pilots
 FPGA
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FPGA implementation of a channel estimator for a distributed multiple antenna OFDM system : Master's Thesis. / Hiivala, Mikko.
Oulu : University of Oulu, 2010.Research output: Thesis › Master's thesis › Theses
TY  THES
T1  FPGA implementation of a channel estimator for a distributed multiple antenna OFDM system
T2  Master's Thesis
AU  Hiivala, Mikko
N1  CA2: TK810 University of Oulu. Department of Electrical and Information Engineering
PY  2010
Y1  2010
N2  Multiple antenna techniques applied to orthogonal frequency division multiplexing (OFDM) will be featured in future fourth generation wireless transmission systems (4G). The high spectral efficiency of OFDM combined with improved resilience against multipath propagation and fading achieved through multiple antenna diversity techniques make the high transfer rate mobile communications promised by 4G possible. In distributed antenna systems, the antennas centrally attached to base stations are distributed across the cells to enable shorter transmission distances and use of lower transmit power, which reduces interference and enables higher transmission capacity. The use of multiple antennas requires accurate channel state knowledge at the receiver for the demodulation of the received symbols to be possible, necessitating estimation of the channel state. In channel estimators based on transmitting known pilot symbols, an orthogonal pilot set is needed per every transmitting antenna. As the number of transmitting antennas can grow large in distributed antenna systems, transmitting pilots multiplexed with the data symbols can cause considerable overhead and reduce data throughput significantly. By superimposing pilot symbols on data symbols at a fraction of the total transmit power, the effect of channel estimation overhead on data throughput can be avoided. The goal in this work was to study the feasibility of using and implementing channel estimation based on superimposed pilots in a distributed multiple antenna OFDM system. For this purpose, different channel estimation algorithms were studied and their performances were compared by software simulations. Algorithms selected based on the study were converted into a veryhighspeed integrated circuit hardware description language (VHDL) model, targeted to be synthesized onto a field programmable gate array (FPGA) based prototyping platform for realtime simulations. The software simulations indicate that the performance of the superimposed channel estimator is strongly dependant on how effectively the underlying data symbols are removed from the estimates of the pilots, and on the other hand the ratio of transmit power allocated for pilots and data. Based on the algorithm study and software simulations, least squares estimation was chosen as the pilot estimation method, and piecewise linear interpolation in the frequency domain as the channel estimate interpolation method. Averaging the pilot estimates over one OFDM frame in time direction was chosen as the method of removing the data symbols from the pilot estimates. According to the simulation results, the highest modulation order that could be supported by the system is six bits per symbol, resulting in a maximum uncoded channel bit rate of 565 Mbps. The hardware implementation achieved a maximum clock frequency of 112.3 MHz.
AB  Multiple antenna techniques applied to orthogonal frequency division multiplexing (OFDM) will be featured in future fourth generation wireless transmission systems (4G). The high spectral efficiency of OFDM combined with improved resilience against multipath propagation and fading achieved through multiple antenna diversity techniques make the high transfer rate mobile communications promised by 4G possible. In distributed antenna systems, the antennas centrally attached to base stations are distributed across the cells to enable shorter transmission distances and use of lower transmit power, which reduces interference and enables higher transmission capacity. The use of multiple antennas requires accurate channel state knowledge at the receiver for the demodulation of the received symbols to be possible, necessitating estimation of the channel state. In channel estimators based on transmitting known pilot symbols, an orthogonal pilot set is needed per every transmitting antenna. As the number of transmitting antennas can grow large in distributed antenna systems, transmitting pilots multiplexed with the data symbols can cause considerable overhead and reduce data throughput significantly. By superimposing pilot symbols on data symbols at a fraction of the total transmit power, the effect of channel estimation overhead on data throughput can be avoided. The goal in this work was to study the feasibility of using and implementing channel estimation based on superimposed pilots in a distributed multiple antenna OFDM system. For this purpose, different channel estimation algorithms were studied and their performances were compared by software simulations. Algorithms selected based on the study were converted into a veryhighspeed integrated circuit hardware description language (VHDL) model, targeted to be synthesized onto a field programmable gate array (FPGA) based prototyping platform for realtime simulations. The software simulations indicate that the performance of the superimposed channel estimator is strongly dependant on how effectively the underlying data symbols are removed from the estimates of the pilots, and on the other hand the ratio of transmit power allocated for pilots and data. Based on the algorithm study and software simulations, least squares estimation was chosen as the pilot estimation method, and piecewise linear interpolation in the frequency domain as the channel estimate interpolation method. Averaging the pilot estimates over one OFDM frame in time direction was chosen as the method of removing the data symbols from the pilot estimates. According to the simulation results, the highest modulation order that could be supported by the system is six bits per symbol, resulting in a maximum uncoded channel bit rate of 565 Mbps. The hardware implementation achieved a maximum clock frequency of 112.3 MHz.
KW  MIMOOFDM
KW  channel estimation
KW  superimposed pilots
KW  FPGA
M3  Master's thesis
PB  University of Oulu
CY  Oulu
ER 