202 research outputs found
Design of optimal equalizers and precoders for MIMO channels
Channel equalization has been extensively studied as a method of combating ISI and ICI for high speed MIMO data communication systems. This dissertation focuses on optimal channel equalization in the presence of non-white observation noises with unknown PSD but bounded power-norm. A worst-case approach to optimal design of channel equalizers leads to an equivalent optimal H-infinity filtering problem for the MIMO communication systems. An explicit design algorithm is derived which not only achieves the zero-forcing (ZF) condition, but also minimizes the RMS error between the transmitted symbols and the received symbols. The second part of this dissertation investigates the design of optimal precoders which minimize the bit error rate (BER) subject to a fixed transmit-power constraint for the multiple antennas downlink communication channels under the perfect reconstruction (PR) condition. The closed form solutions are derived and an efficient design algorithm is proposed. The performance evaluations indicate that the optimal precoder design for multiple antennas communication systems proposed herein is an attractive/reasonable alternative to the existing precoder design techniques
Adaptive Space-Time-Spreading-Assisted Wideband CDMA Systems Communicating over Dispersive Nakagami-m Fading Channels
In this contribution, the performance of wideband code-division multiple-access (W-CDMA) systems using space-timespreading-(STS-) based transmit diversity is investigated, when frequency-selective Nakagami-m fading channels, multiuser interference, and background noise are considered. The analysis and numerical results suggest that the achievable diversity order is the product of the frequency-selective diversity order and the transmit diversity order. Furthermore, both the transmit diversity and the frequency-selective diversity have the same order of importance. Since W-CDMA signals are subjected to frequency-selective fading, the number of resolvable paths at the receiver may vary over a wide range depending on the transmission environment encountered. It can be shown that, for wireless channels where the frequency selectivity is sufficiently high, transmit diversity may be not necessitated. Under this case, multiple transmission antennas can be leveraged into an increased bitrate. Therefore, an adaptive STS-based transmission scheme is then proposed for improving the throughput ofW-CDMA systems. Our numerical results demonstrate that this adaptive STS-based transmission scheme is capable of significantly improving the effective throughput of W-CDMA systems. Specifically, the studied W-CDMA system’s bitrate can be increased by a factor of three at the modest cost of requiring an extra 0.4 dB or 1.2 dB transmitted power in the context of the investigated urban or suburban areas, respectively
Spatial filtering for pilot-aided WCDMA systems: a semi-blind subspace approach
This paper proposes a spatial filtering technique for
the reception of pilot-aided multirate multicode direct-sequence
code division multiple access (DS/CDMA) systems such as wideband
CDMA (WCDMA). These systems introduce a code-multiplexed
pilot sequence that can be used for the estimation of the
filter weights, but the presence of the traffic signal (transmitted
at the same time as the pilot sequence) corrupts that estimation
and degrades the performance of the filter significantly. This is
caused by the fact that although the traffic and pilot signals are
usually designed to be orthogonal, the frequency selectivity of the
channel degrades this orthogonality at hte receiving end. Here,
we propose a semi-blind technique that eliminates the self-noise
caused by the code-multiplexing of the pilot. We derive analytically
the asymptotic performance of both the training-only and
the semi-blind techniques and compare them with the actual simulated
performance. It is shown, both analytically and via simulation,
that high gains can be achieved with respect to training-onlybased
techniques.Peer Reviewe
Near-Instantaneously Adaptive HSDPA-Style OFDM Versus MC-CDMA Transceivers for WIFI, WIMAX, and Next-Generation Cellular Systems
Burts-by-burst (BbB) adaptive high-speed downlink packet access (HSDPA) style multicarrier systems are reviewed, identifying their most critical design aspects. These systems exhibit numerous attractive features, rendering them eminently eligible for employment in next-generation wireless systems. It is argued that BbB-adaptive or symbol-by-symbol adaptive orthogonal frequency division multiplex (OFDM) modems counteract the near instantaneous channel quality variations and hence attain an increased throughput or robustness in comparison to their fixed-mode counterparts. Although they act quite differently, various diversity techniques, such as Rake receivers and space-time block coding (STBC) are also capable of mitigating the channel quality variations in their effort to reduce the bit error ratio (BER), provided that the individual antenna elements experience independent fading. By contrast, in the presence of correlated fading imposed by shadowing or time-variant multiuser interference, the benefits of space-time coding erode and it is unrealistic to expect that a fixed-mode space-time coded system remains capable of maintaining a near-constant BER
On the performance and capacity of space-time block coded multicarrier CDMA communication systems
Ph.DDOCTOR OF PHILOSOPH
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