An mmWave-Based Adaptive
Multi-Beamforming Scheme for High Speed
Railway Communications
Hui Yin, Rui Jiang, and Youyun Xu
National Engineering Research Center of Communications & Networking
Nanjing University of Posts and Telecommunications, Nanjing 210003, China
Emails: {1017010317, j
ray, yyxu}@njupt.edu.cn
Abstract—Focusing on the increasingly growing demands
for tremendous data rate and reliable mobile services in
the high speed railway (HSR) communication systems, it is
essential to explore an effective solution to enhancing the
system capacity. The directional beamforming combined with
millimeter wave (mmWave) technology remains to be one of
the most potential ways to increase the throughput in the
future HSR networks. Therefore, an mmWave-based adaptive
multi-beamforming scheme for HSR communications is pro-
posed in this paper. In the proposed scheme, multiple beams
with different beamwidth are formed by the base station (BS)
simultaneously to improve the system capacity. The mobile
relays (MRs) are provided with the ability to adjust the
receiver (RX) beams automatically during a transmitter (TX)
beam to enhance the received signal-to-noise ratio (SNR).
The outage probability could be decreased dramatically in
this scheme because that the connection drop occurs only if
the multiple beams miss the communication at the same time.
Unfortunately, when the train moves to the edge region of the
cell, the increasing inter beam interference (IBI) will degrade
the system performance. Thus, a novel adaptive algorithm
adjusting the activated beams in real time is presented in this
scheme to mitigate the IBI and maintain an optimal capacity.
Theoretical analysis and simulations are conducted to verify
that the proposed scheme can improve the throughput with
a lower outage probability in HSR scenarios.
Index Terms—high speed railway (HSR), millimeter wave
(mmWave), multi-beamforming, inter beam interference
(IBI).
I. INTRODUCTION
With the rapid advent of high speed railway (HSR),
the existing global system for mobile communications for
railway (GSM-R) can not satisfy the demands for high
mobility services because that it only provides 200 Kbit/s
with end-to-end quality of service (QoS) [1]. Furthermore,
both the onboard passengers and smart devices in the train
need tremendous date rate and reliable mobile services.
In recent years, the advanced long term evolution for
railway (LTE-R) has not formed a unified standard [2],
which draws increasing research interests especially in
terms of the improvement of data rate and transmission
reliability. Therefore, it is of great significance to enhance
the system capacity for HSR wireless communications to
satisfy the increasingly growing wireless service demands.
Developing new spectrum resources, e.g., millimeter wave
(mmWave) bands [3], is an effective way to raise the
throughput for the reason that the available continuous
spectrum is highly scarce in the lower frequency bands.
However, the large path loss due to the higher frequen-
cy restricts the application of mmWave. To tackle this
problem, directional beamforming is generally taken into
consideration to concentrate the radiant signal energy [4].
In most HSR scenarios, viaduct is one of the typical
terrains, which leads to a much clear line of sight (LOS)
propagation environment with few scatters [5-6]. When
multiple input multiple output (MIMO) technology is u-
tilized in HSR scenarios, the serious interference of sub-
streams transmitted from the antenna arrays degrades the
system performance, which limits the space diversity gain
[7]. Beamforming technical is also a solution to addressing
the above problems.
In HSR wireless communication networks, when the
mmWave beamforming technology is adopted, several
problems such as fast time-varying channel, frequent han-
dovers and Doppler shift [8] should be considered because
of the high mobility. The high penetration loss due to
the well-sealed carriages [9-10] is a critical problem re-
quired to be addressed as well. The Bayesian filter-based
channel estimator [11] presents a good performance in
estimating the rapidly time-varying channels under high
mobility scenarios. From [8], it is observed that the Doppler
compensation and utilization are effective solutions to
overcoming the carrier frequency offset and fast fading.
For the sake of mitigating the penetration loss and solving
the group handover problems, the two-hop architecture is
an appropriate option, in which the mobile relay (MR) is
mounted on the roof of the train [12-13]. It is assumed
that the aforementioned problems can be resolved when
the mmWave beamforming technology is utilized.
A broad beamforming approach was researched in [14],
in which the broad beam is adopted to cover and track
the moving vehicle. Nevertheless, the system performance
would be degraded near the cell edge due to the high path
loss. Reference [15] proposed a C/U-plane decoupled stable
beamforming consisting of long-term TX beamforming
and short-term RX beamforming. In this proposal, the
throughput of the HSR system is kept at a stable level and
the feedback overheads are reduced as well. However, only
single beam used for transmitting signals limits the system
capacity. Multiple beams beamfroming is one of the most
promising ways to increase the throughput of HSR systems.
Dual beams are considered in [16], but this scheme could
not achieve the optimal capacity performance because that
it is absent of real-time adaptability.
In this paper, an mmWave-based adaptive multi-
beamforming scheme for HSR communications is proposed
to improve the throughput. When the single beam scheme
is generalized to that of multiple beams, the inter beam
978-1-5386-6119-2/18/$31.00 ©2018 IEEE