2 research outputs found
High-Mobility Wideband Massive MIMO Communications: Doppler Compensation, Analysis and Scaling Law
In this paper, we apply angle-domain Doppler compensation for high-mobility
wideband massive multi-input multi-output (MIMO) uplink transmission. The
time-varying multipath channel is considered between high-speed terminal and
static base station (BS), where multiple Doppler frequency offsets (DFOs) are
associated with distinct angle of departures (AoDs). With the aid of the
large-scale uniform linear array (ULA) at the transmitter, we design a
beamforming network to generate multiple parallel beamforming branches, each
transmitting signal pointing to one particular angle. Then, the transmitted
signal in each branch will experience only one dominant DFO when passing over
the time-varying channel, which can be easily compensated before transmission
starts. We theoretically analyze the Doppler spread of the equivalent uplink
channel after angle-domain Doppler compensation, which takes into account both
the mainlobe and sidelobes of the transmit beam in each branch. It is seen that
the channel time-variation can be effectively suppressed if the number of
transmit antennas is sufficiently large. Interestingly, the asymptotic scaling
law of channel variation is obtained, which shows that the Doppler spread is
proportional to the maximum DFO and decreases approximately as
( is the number of transmit antennas) when is sufficiently large.
Numerical results are provided to corroborate the proposed scheme.Comment: arXiv admin note: text overlap with arXiv:1704.0472
Beamforming Network Optimization for Reducing Channel Time Variation in High-Mobility Massive MIMO
Communications in high-mobility environments have caught a lot of attentions
recently. In this paper, fast time-varying channels for massive multiple-input
multiple-output (MIMO) systems are addressed. We derive the exact channel power
spectrum density (PSD) for the uplink from a high-speed railway (HSR) to a base
station (BS) and propose to further reduce the channel time variation via
beamforming network optimization. A large-scale uniform linear array (ULA) is
equipped at the HSR to separate multiple Doppler shifts in angle domain through
high-resolution transmit beamforming. Each branch comprises a dominant Doppler
shift, which can be compensated to suppress the channel time variation, and we
derive the channel PSD and the Doppler spread to assess the residual channel
time variation. Interestingly, the channel PSD can be exactly expressed as the
product of a pattern function and a beam-distortion function. The former
reflects the impact of array aperture and is the converted radiation pattern of
ULA, while the latter depends on the configuration of beamforming directions.
Inspired by the PSD analysis, we introduce a common configurable amplitudes and
phases (CCAP) parameter to optimize the beamforming network, by partly removing
the constant modulus quantized phase constraints of matched filter (MF)
beamformers. In this way, the residual Doppler shifts can be ulteriorly
suppressed, further reducing the residual channel time variation. The optimal
CCAP parameter minimizing the Doppler spread is derived in a closed form.
Numerical results are provided to corroborate both the channel PSD analysis and
the superiority of beamforming network optimization technique.Comment: Double columns, 13 pages, 10 figures, transactions pape