IEICE TRANS. COMMUN., VOL.E95–B, NO.1 JANUARY 2012

113LETTER

Special Section on Recent Progress in Antennas and Propagation in Conjunction with Main Topics of ISAP2010

Single Antenna Relay System Using De-Noise and Forward Schemefor MIMO Transmission
Kosuke KINAMI†a) , Naoki HONMA† , and Kentaro NISHIMORI†† , Members

SUMMARY
This paper proposes single antenna relay system using Denoise and forward (DNF) scheme for MIMO transmission. In this scheme,
the relay node eliminates the noise by identifying constellation, and retransmits after amplification. DNF does not amplify the noise, and the channel
information is unnecessary in the transmitting side. In this paper, we propose the de-noising scheme for MIMO application. Particularly, DNF canbe used for the multi-stream transmission even though each relay nodes
have single antenna. The simulation demonstrates the proposed scheme
can improve the data transmission quality than the conventional scheme.
key words: MIMO, Relay, DNF

1.

Introduction

MIMO (Multiple-Input Multiple-Output) transmission system has received a lot of attention as way of enhancing spectral eﬃciency [1]. MIMO system enables high-speed datatransmission using spatial multiplexing by using multiple
antennas. However, MIMO is not suitable for long-distancetransmission. Because long-distance transmission limits thenumber of the available paths, and this aﬀects the spatial
multiplexing performance. A MIMO relay transmission system can be one of the candidates that solve this problem [2].
MIMO relay transmission enables enhancing spectral eﬃciency even for long distance data transmission.
AF (Amplify and Forward) and DF (Decode and Forward) have been commonly studied for the MIMO relay
schemes [3], [4]. AF is the easiest transmission scheme inwhich relay node only amplifies received signals since it canbe achieved with a simple configuration. Since this scheme
enhances the noise, it causes deterioration in communication quality. DF is transmission scheme, in which the relay node retransmits after decoding the signal. It does notamplify the noise, but relay node needs multiple antennas
corresponding to number of streams.
DNF (De-noise and Forward) is the transmission
scheme where the relay node eliminates the noise by identifying constellation, and retransmits after amplification [5].
Before the data transmission, the relay node estimates ideal
constellation points that do not contain the noise components. DNF does not amplify the noise, and the channel
information is unnecessary in the transmitting side. ParticManuscript received March 7, 2011.
Manuscript revised July 15, 2011.
†The authors are with the Graduate School of Engineering,
Iwate University, Morioka-shi, 020-8551 Japan.
††The author is with the Graduate School of Science & Technology, Niigata University, Niigata-shi, 950-2181 Japan.
a) E-mail: t2310013@iwate-u.ac.jp
DOI: 10.1587/transcom.E95.B.113

ularly, DNF can be used for the multi-stream transmission
although each relay nodes have single antenna.
In this paper, a single antenna Relay transmission using
DNF is proposed. It is shown DNF scheme can be used
for the multi-stream transmission although each relay nodes
have single antenna. Also, the transmitter does not need
any additional pre-processing like a beam forming, and achannel information is unnecessary at the transmitter. The
simulation demonstrates the proposed scheme can improvethe data transmission quality than AF and DF.
2.

Conventional Relay Method

In this section, conventional relay schemes, AF and DF aredescribed. Figure 1 indicates the relay model in this paper.
H1 and n1 are the channel and noise between the transmitterand the relay nodes. H2 and n2 are the channel and the noise
between the relay nodes and the receiver. G is the gain of
the relay nodes. When the channel from the transmitter tothe receiver defined as H, H can be written as
H = H2 GH1 .

(1)

When the transmit signal is defined as x, the received signal
y can be written as
y = Hx + H2 Gn1 + n2 .

(2)Therefore, AF enhances n1 , and increase error rate.
In the MIMO relay transmission, DF method can berecognized as a multi-user MIMO scheme. In Fig. 1, thechannel from the transmitter to the relay node can be recognized as a down-link, and the channel from the relay node to
the receiver can be recognized an up-link. If the relay node
has no multi-antennas corresponding to the number of thestreams, the relay node can not decode the received signal.

Fig. 1System model.

c 2012 The Institute of Electronics, Information and Communication Engineers
Copyright IEICE TRANS. COMMUN., VOL.E95–B, NO.1 JANUARY 2012114

As a conventional method, BD (Brock Diagonalization) algorithm [6] is used at the transmitter. The transmitter makeorthogonal beam to each relay node, and the relay node candecode and forward a single data stream. To make orthogonal beam, the transmitter has to acquire the channel information.
3.

DE-Noising Scheme

In this section, the de-noising scheme is described. Figure 2
shows de-noising method in the relay node. First, the relay
node estimates modulation scheme and ideal constellationof the transmit signal. In Fig. 2, ideal constellation point si
is shown as
si = H1 xi ,(3)where xi is training signal from the transmitter. When thenumber of the antennas at the transmitter is set to 2 and the
QPSK modulation is used, si is 16 ideal constellation points
each relay nodes. After that, the relay node receive transmit signal, and calculate a maximum posterior probability.
This process corresponds to selecting the shortest Euclidean
distance point in si . The relay node selects the highest probability point, and reposition the received signal to the ideal
signal. When the received signal at the relay node is defined
as r, r can be written asr = H1 x + n1 .

4.

Simulation

In this section, BER (Bit Error Rate) of the proposed scheme
is simulated. In the simulation, the number of the relay node
is set to m, and each relay node has one antenna. The channel is i.i.d. (Independent and Identically-Distributed), andwhite Gaussian noise assumed. It is assumed the transmitter and receiver do not communicate directly. The numberof the antennas at the transmitter and receiver are set to 2.
In the relay node, the amplification gain G is set equally tothe propagation loss between the transmitter and relay node.
The QPSK modulation is used in this simulation.
The actual example of the de-noising process is shownin Fig. 3. In these figures, circle markers are ideal constellation, and cross markers are the constellation including thenoise. (a) shows the constellation that before de-noising,and (b) is the constellation after de-noising. Like these figures, de-noising is achieved. And as described in the previous section, the relay node can successfully eliminate thenoise. The ideal constellation depends on the channel. If
the ideal points are separated suﬃciently, the relay node cande-noise correctly. To diﬀerent the channel each relay node,increasing the relay node improve communication quality.

(4)

And when n1 is Gaussian, de-noised signal s is shown as
s = arg min{||r − si ||2 }.

(5)That is, the relay node identifies constellation by repositioning the received signal to the nearest ideal point. After that,the relay node transmits after amplification. The received
signal at the receiver y can be written as
y = H2 Gs + n2 .

(6)

As indicated by the (6), the relay nodes de-noise n1 . This
scheme does not amplify the noise, and can be achieved byusing single antenna at the relay nodes.

Fig. 2

De-noising method.

Fig. 3

Example of constellation.

LETTER115

can reduce error rate by increasing the relay node. When
we focus on the SNR at BER =10−3 , the DNF with m = 3
can lower the required SNR by 4 dB compared with the DF
when m is 2.
5.

Fig. 4

BER of AF and DNF.

Conclusion

The relay scheme employing DNF has been proposed in thispaper. In this scheme, the relay node eliminates the noiseby identifying constellation, and retransmits after amplification. When we focus on the SNR at BER =10−3 , DNF canlower the required SNR by 10 dB compared with AF. Also,
DNF can lower the required SNR by 4 dB compared withDF. From the simulation results, it is found that BER can beimproved by proposed scheme, and this indicates DNF withthe single antenna enables high-quality transition even forMIMO systems.
Acknowledgment
This research was partially supported by MEXT KAKENHI(22760261).
References

Fig. 5BER of DF and DNF.

Figure 4 shows BER of AF and DNF when m = 2 and3. By increasing the number of the relay node, the receivercan decode more accurately. When we focus on the SNR at
BER = 10−3 and m is 3, DNF scheme can lower the required
SNR by 10 dB compared with AF.
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