2 research outputs found
The information and wave-theoretic limits of analog beamforming
The performance of broadband millimeter-wave (mmWave) RF architectures, is
generally determined by mathematical concepts such as the Shannon capacity.
These systems have also to obey physical laws such as the conservation of
energy and the propagation laws. Taking the physical and hardware limitations
into account is crucial for characterizing the actual performance of mmWave
systems under certain architecture such as analog beamforming. In this context,
we consider a broadband frequency dependent array model that explicitly
includes incremental time shifts instead of phase shifts between the individual
antennas and incorporates a physically defined radiated power. As a consequence
of this model, we present a novel joint approach for designing the optimal
waveform and beamforming vector for analog beamforming. Our results show that,
for sufficiently large array size, the achievable rate is mainly limited by the
fundamental trade-off between the analog beamforming gain and signal bandwidth.Comment: Presented at ITA, february 201
On Reciprocity in Physically Consistent TDD Systems with Coupled Antennas
We consider the reciprocity of the information-theoretic channel of Time
Division Duplex (TDD) Multi-User-Multiple Input Multiple Output (MU-MIMO)
systems in the up- and downlink. Specifically, we assume that the transmit and
receive chains are reciprocal. We take the mutual coupling between the antenna
elements at the base station and at the mobiles into account. Mutual coupling
influences how to calculate transmit power and noise covariance. The analysis
is based on the Multiport Communication Theory, which ensures that the
information-theoretic model is consistent with physics. It also includes a
detailed noise model. We show that due to the coupling, the
information-theoretic up- and downlink channels do not fulfill the ordinary
reciprocity relation, even if the input-output relation of the transmit voltage
sources and the receive load voltages, i.e., the channel which is estimated
with the help of pilot signals in the uplink, is reciprocal. This is a
fundamental effect that is not considered otherwise. We show via Monte Carlo
simulations that both, using the ordinary reciprocity relation, and not taking
the coupling into account, significantly decreases the ergodic rates in
single-user and the ergodic sum rates in multi-user systems