High speed communications over broadband wireless channels has emerged as a key feature of future communications systems due in part to the explosive interest in information technology applications, including wireless sensor networks, mobile computing, high-speed mobile internet, and video transmission over wireless channels. The demand for higher information capacity in these and other similar applications has motivated the use of broadband wireless channels in order to provide wider bandwidth and higher data rates. In addition, multi-user communication schemes are also employed in order to allow users to share the same physical channel; thereby contributing to even higher data rates. A key challenge that limits the performance of such multi-user communications systems is the distortion introduced by the coupled communication channels in the form of fading conditions, inter-symbol interference, inter-user interference, and noise. Another challenge is the increasing complexity of the receiver due to the large number of users and the long impulse response of broadband channels.
Among the promising encoding schemes for such scenarios are space-time block codes (STBC) for multi–antenna wireless systems. The codes provide full diversity gains and achieve good performance with simple receiver structures, especially over flat channels. When implemented over broadband channels, STBCs can be combined with Orthogonal Frequency division multiplexing (OFDM) or single carrier frequency domain (SC–FD) transmission schemes to achieve multipath diversity and to decouple the broadband frequency selective channel into independent flat channels. This dissertation develops efficient receiver structures that exploit the STBC structure in order to reduce the computational cost of the channel estimation and equalization steps for multiuser STBC transmissions over broadband channels.
Specifically, in chapter 3, we develop channel–estimate based joint interference suppression and equalization techniques for synchronous multiple user transmissions. We exploit the STBC structure to show that the receiver structure for frequency selective channels collapses to parallel receivers each with the complexity of a flat channel receiver.
In Chapter 4 We develop adaptive multi–user receivers that do not require explicit channel knowledge at the receiver. We employ the block recursive least squares {(RLS)} algorithm and show how to reduce its computational cost in view of the code structure.
In Chapter 5, we study channel estimation techniques. Adaptive implementations using block normalized least mean squares (NLMS) and block RLS are used. Block array RLS algorithms are also derived to improve the finite precision implementation of the adaptive algorithms.
Finally, in Chapter 6, we study the asynchronous multi–user transmission scenario and design a prefiltering technique to synchronize the received signals from different users. The performance of all receiver structures developed in this dissertation is validated by computer simulations.
Acknowledgment This work was supported in part by the National Science Foundation under grants ECS-9820765, CCR-0208573, and ECS-0401188. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.