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The large bandwidth available with mmWave (millimeter Wave) makes it a promising candidate for 5th generation cellular networks. Proper channel estimation algorithms must be developed to enable beamforming in mmWave systems. In this paper, we propose an adaptive channel estimation algorithm that exploits the poor scattering nature of the mmWave channel and adjusts the training overhead adaptively with the change of channel quality for mmWave cellular systems. First, we use a short training seque
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An adaptive channel estimation algorithm for millimeter wave cellular systems
An adaptive channel estimation algorithm
for millimeter wave cellular systems
LU Wenlü
1,2
, ZOU Weixia
1,2
, LIU Xuefeng
1,2
1. Key Laboratory of Universal Wireless Communications, Ministry of Education, Beijing 100876, China
2. State Key Laboratory of Millimeter Waves, Southeast University, Nanjing 210096, China
Abstract: The large bandwidth available with mmWave (millimeter Wave) makes it a promising candidate for 5th generation
cellular networks. Proper channel estimation algorithms must be developed to enable beamforming in mmWave systems. In this
paper, we propose an adaptive channel estimation algorithm that exploits the poor scattering nature of the mmWave channel
and adjusts the training overhead adaptively with the change of channel quality for mmWave cellular systems. First, we use a
short training sequence to estimate the channel parameters based on the two-dimensional discrete Fourier transform method.
Then, we design a feedback scheme to adjust the length of the training sequence under the premise of ensuring the accuracy of
the channel estimation. The key threshold in the feedback scheme is derived and its inuence on the accuracy of the estimation
results is analyzed. Simulation results confirm that the proposed algorithm can adjust the length of the training sequence
adaptively according to the current channel condition maintaining a stable estimation accuracy.
Key words: millimeter wave, 5G, cellular system, large antenna array, adaptive channel estimation.
Citation: LU W L, ZOU W X, LIU X F, et al. An adaptive channel estimation algorithm for millimeter wave cellular systems[J].
Journal of communications and information networks, 2016, 1(2): 37-44
Journal of Communications and Information Networks
Vol.1, No.2, Aug. 2016
DOI: 10.11959/j.issn.2096-1081.2016.015
Research paper
Manuscript received Jun. 23, 2016; accepted Aug. 3, 2016
This work is supported by The National High Technology Research and Development Program of China (863 Program) (No.2015AA01A703),
The Fundamental Research Funds for the Central Universities (No.2014ZD03-02), The National Natural Science Foundation of China
(No.61571055), fund of State Key Laboratory of Millimeter Wave (No. K201501).
1 Introduction
The data rate requirements of 5G (the next generation)
cellular communication systems will approach or
surpass gigabits per second
[1]
. Such a high data rate
inevitably relies on an enormous available bandwidth
[2]
.
The huge bandwidth available at mmWave frequencies
makes it one of the best candidates for future 5G
cellular systems
[3-4]
. However, extreme path fading
restricts the propagation distance of mmWave.
Directional beamforming with large antenna arrays
appears to be inevitable to support longer outdoor
links and provide sufcient received signal power
[5]
.
Moreover, channel estimation must be performed to
obtain the perfect CSI (Channel State Information)
necessary to support the directional beamforming.
However, the channel matrix can be considerably
large in mmWave systems owing to the large number
of antennas, making the classical approach unfeasible
for the estimation of the mmWave channel
[6,7]
.
Therefore, a major challenge is to develop accurate
and reliable channel estimation algorithms for
Journal of Communications and Information Networks
38
mmWave cellular systems
[8]
.
Considering the high cost and power consumption
of devices and enabling multiple data streams, the
hybrid structure of large scale antennas that enable
hybrid analog-digital beamforming have become a
new trend
[2,9]
. Several estimation algorithms for hybrid
structures have been developed. In Refs.[6] and [10],
a novel multiresolution codebook, which is divided
into analog and digital domains, is designed and a
channel estimation algorithm is proposed. In Refs.
[11] and [12], the authors use a compressed sensing
tool to compress the training overhead and propose
corresponding algorithms. A matrix-block algorithm
(DFT-CEA) based on 2D-DFT (Two-Dimensional
Discrete Fourier Transform ) with a short training
overhead to estimate the channel parameters is
proposed in Ref.[7]. All these algorithms can save
the training sequence, however, they cannot adjust
the training overhead adaptively when the channel
condition (such as SNR) changes.
In this paper, considering that the locations
of the MS (Mobile Stations ) relative to the BS
(Base Station) are random and channel qualities at
different locations differ significantly, we develop a
channel estimation algorithm that adjusts the training
sequence length according to the channel condition.
First, the BS sends a short training sequence; after
computing the 2D-DFT of the received signal, the
MS estimates the path parameters iteratively, namely
path gain, AoA (Angle of Arrival), and AoD (Angle
of Departure ). Then, we design a feedback scheme
that allows the MS to distinguish if the estimated
path parameter is noise or a true path by comparing
with a threshold and feedbacks “Yes/No” to decide
whether to continue to send the training sequence. We
emphasize that the proposed first step refers to the
DFT-ACE method in Ref.[7]; the difference is that
we design an adaptive feedback scheme by which
the system adjusts the training overhead. Numerical
results conrm that the proposed algorithm provides
effective performance in mmWave systems.
Notation: A is a matrix, a is a vector, and a is a
scalar. A
T
, A
H
, A
-
1
, and ||A|| represent the transpose,
conjugate transpose, inverse, and Frobenius norm,
respectively. [y]
a:b
denotes a vector obtained by
extracting elements of vector y from index a to index
b. 0
n×m
denotes a zero matrix of size N×M.
2 System model
We consider the single user mmWave system with
hybrid analog-digital beamforming illustrated in
Fig.1. A BS with M antennas and M
RF
RF (Radio
Frequency) chains communicates with the MS with
N antennas and N
RF
RF chains. The BS transmits M
s
streams to the MS and the MS obtains N
s
streams after
processing; in general, N
S
=M
S
. According to Refs.[2,13],
we know that M
S
≤
M
RF
≤
M and N
S
≤
N
RF
≤
N.
We focus on the downlink transmission. The BS is
assumed to apply an M
RF
×M
S
baseband beamformer
F
BB
followed by an M×M
RF
RF beamformer F
RF
.
Similarly, the MS is constituted of an N
RF
×N
S
baseband combiner W
BB
and an N×N
RF
RF combiner
W
RF
. To simplify the hardware implementation,
each element of F
RF
and W
RF
has unitary magnitude;
however, it may have an arbitrary phase.
H denotes the N×M channel matrix and x
BB
=F
BB
x
and x
RF
=F
RF
x
BB
, where x is the M
S
×1 vector of the
transmitted symbols. We adopt a narrowband block-
fading channel model and y
BB
at the MS is
, (1)
where is the thermal noise with
variance σ
2
. The final processed data at the MS is
y=W
BB
y
BB
.
Considering the limited scattering nature of the
mmWave channel
[14]
, we adopt a geometric channel
model with L dominant scatterers where each
scatterer is assumed to contribute a single propagation
path between the BS and MS
[2]
. L is a statistics mean
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