ITA 2008, Information Theory and Applications Workshop, January 28-30 2008, San Diego, USA
In this paper, we discuss stationary time- and frequency-selective channels. No channel knowledge is assumed, neither at the transmitter nor at the receiver. We investigate the
capacity behavior of these doubly selective channels and in particular the role of the key channel parameters delay spread, Doppler bandwidth and channel spread factor (the product
of delay spread and Doppler bandwidth) which govern the capacity behavior at high values of signal to noise ratio (SNR). Different capacity regimes of capacity where the dominant
term of capacity is log(SNR) or log[log(SNR)] and channel conditions (delay spread, Doppler Bandwidth and channel spread factor) are specified which govern in which regime the capacity
will be operating. For critically spread channels (channel spread factor of 1), it is widely believed that the dominant term of the high-SNR expansion of the capacity is log[log(SNR)]
or in other words, the pre-log (the coefficient of log(SNR)) is zero. We give a very simple scheme which shows that even for critically spread channels a non-zero pre-log exists under
certain conditions on Doppler bandwidth and delay spread. Furthermore, we also show that a pre-log may exist even for over-spread channels (channel spread factor greater than 1). For
a specific larger channel spread the log[log(SNR)] regime takes over for which we provide bounds on the pre-loglog.
Invited paper in a conference
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