| Forthcoming wireless cellular systems, such as Wireless Metropolitan Area Networks (WMANs), will have to deliver on promised "wired" bandwidths while overcoming hurdles such as severe channel dispersion, no line of sight between transmitter and receiver, mobility at vehicular speeds, and inter-cell interference arising from shared spectrum scheduling. Moreover, the fading rates that result in time and frequency may prohibit the use of conventional coherent transceiver designs. These issues motivate us to consider spectrally efficient noncoherent communication systems that do not rely on pilot-symbol based estimation of continuously varying channels. Rather; the channel is estimated implicitly, based on the statistics of the received signal and probabilistic models of the fading process. Given the success of belief propagation decoding in a variety of fields, a Bayesian framework for iterative demodulation and decoding is employed to approach the capacity of such channels. With the goal of practical transceiver designs, several reduced complexity implementations, which maintain near Shannon theoretic performance, are proposed. We further introduce a multi-antenna noncoherent eigenbeamforming receiver that adaptively learns the spatial channel to a given mobile with little or no pilot overhead. With the key observation that outdoor channels are characterized by relatively few dominant spatial modes, eigenbeamforming receivers enjoy beamforming gains in Signal-to-Noise Ratio (SNR) that result from scaling up the number of receive elements, while simultaneously reducing the complexity of noncoherent demodulation and decoding, which scales with the number of dominant modes. Finally, a side-by-side comparison of noncoherent and coherent transceivers is performed in the context of a packetized Orthogonal Frequency Division Multiplexing (OFDM) system, such as that in development for the IEEE 802.16 WMAN standard. |
