In this paper, we apply angle-domain Doppler compensation for high-mobility wideband massive multi-input multioutput (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 angledomain 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 1/ √ M (M is the number of transmit antennas) when M is sufficiently large. Numerical results are provided to corroborate the proposed scheme.
Abstract-In this correspondence, we propose a new receiver design for high-mobility orthogonal frequency division multiplexing (OFDM) downlink transmissions with a large-scale antenna array. The downlink signal experiences the challenging fast time-varying propagation channel. The time-varying nature originates from the multiple carrier frequency offsets (CFOs) due to the transceiver oscillator frequency offset (OFO) and multiple Doppler shifts. Let the received signal first go through a carefully designed beamforming network, which could separate multiple CFOs in the spatial domain with sufficient number of receive antennas. A joint estimation method for the Doppler shifts and the OFO is further developed. Then the conventional single-CFO compensation and channel estimation method can be carried out for each beamforming branch. The proposed receiver design avoids the complicated time-varying channel estimation, which differs a lot from the conventional methods. More importantly, the proposed scheme can be applied to the commonly used timevarying channel models, such as the Jakes' channel model. Index Terms-orthogonal frequency division multiplexing (OFDM), time-varying channels, high-mobility, carrier frequency offset (CFO), Doppler shifts, oscillator frequency offset (OFO), large-scale antenna array.
The wireless physical channel parameters are recently used to provide secret key. However, the key generation usually suffers from the quantization errors due to the noise, which decreases the key agreement ratio (KAR) between authorized users. Most existing approaches achieve high KAR by discarding some channel parameters which may lower the key generation efficiency and therefore lower the encryption strength. In the frequency-division duplex (FDD) systems, the number of reciprocal parameters, such as the multipath angle and delay, is limited. Therefore how to find a quantization method with high KAR and encryption strength is one of the major problems for secret key generation in FDD systems. In this paper, a robust quantization scheme based on grouping and shifting is proposed, in which all the available parameters are used for key generation. In addition, a key mapping method with error correction based on Chinese remainder theorem (CRT) is proposed to further improve the KAR performance. Simulations demonstrate the effectiveness of the proposed method.
In this paper, we propose a Doppler precompensation scheme for high-mobility orthogonal frequency division multiplexing (OFDM) uplink, where a high-speed terminal transmits signals to the base station (BS). Considering that the time-varying multipath channel consists of multiple Doppler frequency offsets (DFOs) with different angle of departures (AoDs), we propose to perform DFO pre-compensation at the transmitter with a large-scale uniform linear array (ULA). The transmitted signal passes through a beamforming network with high-spatial resolution to produce multiple parallel branches. Each branch transmits signal towards one direction thus it is affected by one dominant DFO when passing over the timevarying channel. Therefore, we can compensate the DFO for each branch at the transmitter previously. Theoretical analysis for the Doppler spread of the equivalent uplink channel is also conducted. It is found that when the number of transmit antennas is sufficiently large, the time-variation of channel can be efficiently suppressed. Therefore, the performance will not degrade significantly if applying the conventional time-invariant channel estimation and equalization methods at the receiver. Simulation results are provided to verify the proposed scheme.
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