Syed Aon Mujtaba

A low complexity space-frequency BICM MIMO-OFDM system for next-generation WLAN

In this paper, we propose a low complexity space-frequency bit interleaved coded (SF-BICM) multiple-input multiple-output (MIMO) OFDM system as a promising candidate for next generation wireless LANs, expected to operate in excess of 100 Mb/s. A MIMO overlay on a 64-QAM OFDM system can increase computational complexity exponentially if maximum likelihood performance is desired. In this paper, we present two schemes for reducing implementation complexity without compromising PER performance. First, we show that multiplexing OFDM symbols across multiple antennas achieves the best performance, and also leads to the most cost effective implementation. Second, we propose a novel ad-hoc scheme for MIMO soft-symbol demapping that uses hard-decision feedback to perform interference cancellation and maximal ratio combining. Our proposed low-complexity architecture offers a throughput of 108 Mb/s at the comparable SNR of an IEEE 802.11a/g-compliant 54 Mb/s transmission with 1% PER.

On the code-diversity performance of bit-interleaved coded OFDM in frequency-selective fading channels

The IEEE 802.11a OFDM wireless local area network (WLAN) system aims to achieve code diversity in quasi-static frequency-selective fading channels by employing bit interleaved coded modulation (BICM). Since the time variation of the channel during a WLAN packet transmission is small and its frequency selectivity finite, the code diversity is limited by both the degree of the channel variation as well as by the code properties. This paper experimentally investigates the impact of channel frequency selectivity on the code-diversity performance of BICM, with and without 2/spl times/1 Alamouti transmit diversity. Using a simplified model of the finite frequency selectivity, we develop a mathematical framework to show that the code diversity is only probabilistically achieved. Our results indicate that for the frequency selectivity typically seen in indoor WLAN channels, using a channel code with larger d/sub free/ does not improve the packet error rate (PER) performance.

A novel scheme for transmitting QPSK as a single-sideband signal

A novel modulation scheme for transmitting QPSK as a single sideband (SSB) signal is presented. While the theoretical spectral efficiency of the new scheme, SSB-QPSK, is identical to that of QPSK or SSB (i.e. 2 bps/Hz), it is shown that for a Rayleigh-fading mobile radio channel, SSB-QPSK is more robust to equalization imperfections than either QPSK or SSB. Additionally, it is shown that the envelope variation of SSB-QPSK is about 6 dB less than that for QPSK. For a flat Rayleigh fading channel with a one-tap equalizer, it is found that SSB-QPSK is more robust to equalizer phase errors than either QPSK or SSB. The performance in a frequency-selective Rayleigh-fading channel is evaluated with a linear equalizer, in which case it is found that if the equalizer fails to remove all the ISI, the residual ISI (and hence the BER as E/sub b//N/sub 0//spl rarr//spl infin/) is lower for SSB-QPSK compared to QPSK.

A novel scheme for linear multiuser detection in asynchronous DS-CDMA systems with frequency selective fading

A novel scheme for linear multiuser detection (MUD) based on sub-symbol diversity is presented that is applicable to asynchronous CDMA systems with frequency selective fading. Traditional solutions for asynchronous linear MUD result in a prohibitively large decorrelation matrix. The new scheme transforms the asynchronous problem into a synchronous one by creating sub-symbols that are, by design, time-aligned with all other users and their respective multipath signals. The resulting decorrelation matrix is dramatically reduced in size, thus offering a viable solution for implementation. We also present an optimal scheme for combining the sub-symbols that is based on the condition number of the decorrelation matrices.

Performance analysis of a convolutionally-encoded synchronous CDMA system with adaptive beamforming and linear multiuser detection

Beamforming (BF) with multiple receiving antennas significantly improves the bit error rate (BER) performance for a DS-CDMA system in a flat Rayleigh fading environment. However, BF fails to suppress multiple access interference (MAI) completely, which results in an irreducible BER floor. Furthermore, soft-decision Viterbi decoding (VD) with BF further lowers the BER floor. The BER performance difference between the minimum mean square error (MMSE) linear multiuser detector (MUD) and zero forcing (ZF) MUD increases with the eigenvalue spread of the code correlation matrix. BF with MUD performs better than MUD alone, and BF+VD is much more effective at combating MAI than MUD+VD for low to moderate SNRs. In this regime, the performance of BF+VD approaches that of BF+MUD+VD, which clearly offers the best performance for all SNRs.

Performance analysis of coded SSB-QPSK in mobile radio channels

A novel modulation scheme for transmitting QPSK as a single sideband (SSB) signal has been presented by Mujtaba (see, Proc. IEEE Globecom, Sydney, Australia, 1998). In this paper, the performance of SSB-QPSK with convolutional encoding (R=1/2 and K=5) and Viterbi decoding is presented for a mobile radio channel. While the theoretical spectral efficiency of the new scheme, SSB-QPSK, is identical to that of QPSK or SSB (i.e. 2 bps/Hz), it was shown by Mujtaba that for a Rayleigh-fading mobile radio channel, uncoded SSB-QPSK is more robust to equalization imperfections than either uncoded QPSK or uncoded SSB. In this paper, we verify similar results for coded SSB-QPSK. For a flat Rayleigh fading channel with a one-tap equalizer, it is found that SSB-QPSK is more robust to equalizer phase errors than either QPSK or SSB. Performance in a frequency-selective Rayleigh-fading channel is evaluated with a linear equalizer, in which case it is found that if the equalizer fails to remove all the ISI, the residual ISI (and hence the BER as E/sub b//N/sub o//spl rarr//spl infin/) is lower for SSB-QPSK compared to QPSK.