OPPORTUNISTIC COMMUNICATIONS AT THE DOWNLINK OF MOBILE SYSTEMS IN
THE PRESENCE OF INACCURATE CHANNEL INFORMATION
Banafsheh Lashkari
1
, Mehrzad Biguesh
1
, Saeed Gazor
2
1
Wireless Comm. Lab, Elect. Eng. Dept., Shiraz University, Shiraz, Iran
2
Dept. of ECE., Queen’s University, Kingston, Ontario, K7L 3N6 Canada
ABSTRACT
The throughput of a multi-user broadcast opportunistic com-
munications system is highly affected by the accuracy of the
available channel state information (CSI) at the transmitter
side. It is a questionable assumption to have almost perfect
knowledge of the user SNRs at the transmitter due to
the error in the channel measurement, or due to reporting
feedback errors from users to the transmitter. As a result such
errors, the system can not gain its maximum throughput. In
this paper, we investigate the performance of opportunistic
communications system considering imperfect SNR reports.
We show that in order to maximize the throughput, the
transmitter needs to estimate the channel state information.
The imposition of a correlation constraint will prevent the
estimate of channel from fluctuating too widely. Employ
Kalman filter, we estimate the channel state information and
show that the throughput is significantly improved.
I. INTRODUCTION
The quality of wireless communication is impacted by fad-
ing which is the result of multi-path propagation of signals.
Various diversity techniques are proposed to combat fading.
The fundamental idea behind the diversity is to transmit
signals via different and independent dimensions, such as
time, frequency or space [1]. A type of diversity, so-called
multiuser diversity is used in multi-user systems. The multi-
user diversity is best motivated from an information theory
result of Knopp and Humblet [11]. Knopp and Humblet
in [11] focused on the uplink in a single cell system with
multiple users, and showed that the uplink capacity could be
maximized by allowing only one user with the best channel
to transmit the data at each given time. In [6] similar results
are reported for the downlink from base station to the mobile
users.
In practical communication systems, the traffic demand
is generally higher in the downlink communication than
that of uplink. Thus, it is very desirable to maximize the
throughput of the downlink [3]. To achieve this goal, many
algorithms have been proposed which require some accurate
feedback about channel state information from receiver to
the transmitter. Such an accurate feedback is not always
possible in practice, especially when the number of users is
large. As a further consequence of such imperfect feedback,
a significant loss can be imposed on the performance of the
opportunistic communications system.
In this manuscript, we study the robustness of the oppor-
tunistic communication system against the erroneous SNR
feedback reports. We also employ a channel state estimator
in order to enhance the inaccurate reports. Our results show
that the capacity of the system can be significantly improved.
II. SYSTEM MODEL
We consider a multi-user downlink with K single antenna
users and assume that the channels are narrow-band and
that the channel gains are independent and have identical
Rayleigh distribution. The transmitter has to send a private
data to each user in this system. The decision to transmit
data for a user in this system is made based on the channel
state information (CSI) of all users. To measure the channel
quality, base station broadcasts some foreknown training
signal to all users. The base-band received signal by the
kth user at time instant n is written as,
y
k
(n)= h
k
(n)s(n)+ v
k
(n) (1)
where h
k
(n) is the complex Gaussian channel between
transmitter and the kth user, s(n) is the transmitted symbol
selected from constellation C , and v
k
(n) denotes the white
zero-mean complex noise process with variance σ
2
k
. For the
sake of simplicity, we assume that the noise power is the
same for all users, that is σ
2
k
= σ
2
◦
for k =1,...,K.
The average power of transmit symbols is also denoted by
E{|s(n)|
2
} = σ
2
s
.
We assume that h
k
(n) is smoothly time varying and
is almost constant over P successive time instances, i.e.,
h
k
(n) ≈ h
k
[m] for all n ∈{mP, mP +1, ··· (m + 1)P },
where P is a given integer known as the channel coherence
time. Thus, the SNR for the kth user during such time
interval is expressed by
SNR
k
(n) =
E{|h
k
(n)s(n)|
2
}
E{|v(n)|
2
}
=
h
k
[m]
2
σ
2
s
σ
2
◦
. (2)
978-1-4244-1946-3/08/$25.00 ©2008 IEEE 356
24th Biennial Symposium on Communications