Wireless OFDM Systems: How to make them work?Marc Engels Wireless Local Area Networks (WLANs) experience a growing popularity recently. Where WLANs were primarily used for niche applications in the past, they are now deployed as wireless extensions to computer networks. The increase of the datarates from 2 Mbps up to 11 Mbps for roughly a constant price has played a major role in this breakthrough. As a consequence, an even greater success can be envisioned for the more recent OFDM-based WLAN standards in the 5 GHz band, which offer up to 54 Mbps. At IMEC we have realised this potential already several years ago and have established a successful research programme on OF- based WLAN. In 1995, we started our research on wireless OFDM in the frame of a - operation project with SAIT, a Belgian telecom company. The goal of the project was to establish a robust network for industrial environments. This resulted in a first OFDM chip, supporting QPSK, for wireless networking at the end of the project (1999). 1999 was also the start of an intense co-operation with National Semiconductor Inc., which resulted in a second generation ASIC in 2000. This OFDM processor supports up to QAM-64 and has a more optimal channel estimation algorithm. |
Contents
1 Worldwide number of Internet users | 1 |
Understanding the indoor environment | 11 |
8 Time correlation for different models of mobility | 29 |
The OFDM Principle 33 | 32 |
3 Spectrum of an OFDM signal | 36 |
an essential ingredient Summary When people agree on OFDM 4 1 4 2 4 3 WLAN standards HIPERLAN2 Differences between HIPERLAN2 and IE... | 41 |
7 Discretefrequency representation for the Doppler multipath | 44 |
9 OFDMQPSK performance in a multipath channel versus | 47 |
75 | 76 |
1 The OFDM system model | 77 |
5 The ML estimator enables low complexity and timefrequency | 85 |
9 Simplified TimeFrequency grids in OFDM | 91 |
95 | 96 |
2 Principle of the Schmidl and Cox autocorrelation based timing | 101 |
113 | 115 |
151 | 155 |
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Common terms and phrases
algorithm amplitude antenna architecture ASIC automatic gain control bandwidth baseband BER performances bit error BPSK burst carrier frequency offset channel estimation channel impulse response channel model channel response chapter clipping clock complex correlation crest factor cyclic prefix Deneire Doppler equaliser ETSI fading Figure filter FPGA frequency domain front-end effects front-end non-idealities function Gaussian noise guard interval HIPERLAN HIPERLAN/2 I/Q imbalance IFFT IMEC implementation loss impulse response indoor input interference linear LMMSE MAC frame matrix Mbps mobile mode modulation multipath channel multiple OFDM signal OFDM symbol OFDM system orthogonal output parameters path PDU train phase noise physical layer pilot power amplifier preamble propagation protocol PSAM quantization ratio received signal sampling scheme signal processing specific spectral spectrum subcarrier synchronisation training sequences transceiver transmission transmitted uncoded vector wireless LAN WLAN word-length