A Novel Quantize-and-Forward Cooperative System: Channel Estimation and M-PSK Detection Performance
EURASIP Journal on Wireless Communications and Networking
Hindawi Publishing Corporation
A Novel Quantize-and-Forward Cooperative System: Channel Estimation and M-PSK Detection Performance
Iancu Avram 0
Nico Aerts 0
Dieter Duyck 0
Marc Moeneclaey 0
Carles Anton-Haro
0 Department of Telecommunications and Information Processing, Faculty of Engineering, Ghent University , 9000 Gent , Belgium
A method to improve the reliability of data transmission between two terminals without using multiple antennas is cooperative communication, where spatial diversity is introduced by the presence of a relay terminal. The Quantize and Forward (QF) protocol is suitable to implement in resource constraint relays, because of its low complexity. In prior studies of the QF protocol, all channel parameters are assumed to be perfectly known at the destination, while in reality these need to be estimated. This paper proposes a novel quantization scheme, in which the relay compensates for the rotation caused by the source-relay channel, before quantizing the phase of the received M-PSK data symbols. In doing so, channel estimation at the destination is greatly simplified, without significantly increasing the complexity of the relay terminals. Further, the destination applies the expectation maximization (EM) algorithm to improve the estimates of the source-destination and relay-destination channels. The resulting performance is shown to be close to that of a system with known channel parameters.
1. Introduction
As wireless communication networks become more
widespread, new methods are being developed to increase the
reliability of information transfer. In a multipath
propagation environment, the reflected signals can combine both
constructively or destructively at the receiving antenna,
giving rise to Rayleigh fading. This imposes an upper bound
on the reliability of a point-to-point communication system.
One way to overcome this problem is by the use of
multielement sending or receiving antennas [1]. However, due to
size constraints of mobile terminals, this technique cannot
always be applied.
In a cooperative communication system, this problem
is overcome by exploiting the broadcast nature of wireless
communication. Information broadcast by the source is
also received by terminals other than the destination. These
terminals relay to the destination the information sent by the
source, creating additional independent channels between
source and destination. This technique is analyzed from an
information theoretic point of view in [2], where upper
and lower bounds are obtained for the capacity of the relay
channel. In [3], it is shown that in a fading environment,
the spatial diversity introduced by the relay terminals
improves the reliability of a communication system, which
is now determined by the probability that all channels are
simultaneously in fading. By increasing the reliability of the
communication system, higher data rates can be achieved
without increasing the transmitter power. Alternatively, one
can keep the data rate constant and lower the transmission
energy, extending the battery life of portable devices.
The diversity gain of various cooperative strategies is
discussed in [4]. It is shown that the Amplify and Forward
(AF) protocol, in which the relay amplifies the received
signal, indeed introduces spatial diversity. However, when
using half-duplex terminals that cannot transmit and receive
data at the same time, the relay needs to store the received
information, in order to forward it later on. This situation
is depicted in Figure 1. The AF protocol assumes this data
can be stored with an infinite precision. In a more realistic
system, this data is quantized before storage, yielding the
Quantize and Forward (QF) protocol. In [5], upper and
lower bounds on the capacity of the relay channel are
obtained for a relay that quantizes the received data using a
Source
h1
Wyner-Ziv coding scheme. Other quantization methods have
been analyzed in [6, 7]. The QF protocol described in [6] is
attractive for the use in wireless sensor networks, because the
complexity of the individual relay terminals is kept low. This
is done by moving the more computational intensive tasks
to the destination, where typically there is more processing
power available.
While cooperative communication has been well
investigated from an information theoretic point of view, other
aspects also need to be studied in the development of a
practical implementation. The issue of channel coding is
addressed in [8], where low density parity check (LDPC)
codes are designed for the Decode and Forward (DF)
protocol. Channel parameter estimation is discussed in [9] for the
AF protocol, where pilot-based estimates are calculated for
the different channel coefficients involved. Because only the
received pilot symbols are used in [9], the obtained estimates
could be further refined by also using the information about
the channels that is embedded in the re (...truncated)