![](bg2.png)
S D
Source node Destination node
SER =
1
R
2
R
N
R
ε
SD
Relay nodes
:
:
ε
SR
1
ε
SR
2
ε
SR
n
ε
R D
2
ε
R D
n
ε
R D
1
Figure 1: System model with N relay nodes.
posed protocol elaborately takes advantage of the
powerful error-correcting capability of MDS codes.
Incorporating RP-MDS into PRCSMA may intro-
duce effective performance improvement in accor-
dance with the concept of incremental redundancy
(Pursley and Sandberg, 1989). A destination node
can reinforce an error-correcting capability when it
receives a new frame, even if it includes channel er-
rors. The performance of the proposed protocol is an-
alyzed with the aid of a Markov model. The accuracy
of the model is verified by means of computer simu-
lation.
The rest of the present paper is organized as fol-
lows: Section 2 presents a system model with relay
nodes. PRCSMA is briefly reviewed in Section 3.
In Section 4, after a short reminder of useful prop-
erties of MDS codes, the proposed protocol is de-
scribed. Performance of the proposed protocol is an-
alyzed in Section 5, based on the analysis in (Alonso-
Zarate et al., 2009). Numerical results are presented
in Section 6 in comparison with results obtained from
computer simulation. Finally, Section 7 concludes the
present paper.
2 SYSTEM MODEL
Consider a wireless network consisting of a pair of
source node S and destination node D with N relay
nodes; R
1
, R
2
,. .. , R
N
, as shown in Fig. 1. All chan-
nels are half-duplex, so that a node can not transmit
and receive simultaneously. All nodes are located
within their transmission range. Hence, each node can
overhear ongoing transmission originating from other
nodes. Let ε
SD
, ε
SR
n
, and ε
R
n
D
be the symbol error
probabilities on channels between source node S and
destination node D, between source node S and relay
node R
n
, and between relay node R
n
and destination
node D, respectively, for n = 1,2,...,N.
1
If frame
1
Using the symbol error rate ε, we can evaluate the bit
error rate as 1−
m
√
1−ε when a symbol consists of m bits.
transmission from source node S resulted in erroneous
reception at destination node D and if one or more
relay nodes succeeded in error-free reception of the
frame, then such relay nodes can collaborativelyserve
as supporters for frame retransmission. For effective
use of cooperative communications, we generally as-
sume that ε
SD
> ε
R
n
D
. The duration in which relay
nodes collaborate frame retransmissions is referred to
as a cooperation phase (Alonso-Zarate et al., 2009).
Note that every frame is assumed to include an ap-
propriate header and an ideal Frame Check Sequence
(FCS) for error/collision detection,
2
in addition to the
payload.
3 PERSISTENT RELAY CSMA
(PRCSMA)
PRCSMA (Alonso-Zarate et al., 2009; Predojevet al.,
2012) is a MAC protocol which elaborately re-
solves frame collisions among transmission from re-
lay nodes, based on IEEE 802.11 DCF (IEEE Stan-
dard 802.11, 1999). Similarly to IEEE 802.11 DCF,
each relay node in PRCSMA inserts random back-
off delay before every frame transmission in a dis-
tributed manner according to its own contention win-
dow (CW). Let m denote a message block of k-
symbol length, which is generated at source node S.
A DATA frame consists of a header, payload m, and
FCS. Note that the terms “message block m” and
“DATA frame” are used interchangeably hereafter,
unless ambiguity arises.
The operation in PRCSMA is summarized as fol-
lows. The detailed description can be found in
(Alonso-Zarate et al., 2009). After erroneous recep-
tion of a DATA frame, destination node D broadcasts
a Call For Cooperation (CFC) frame. If one or more
relay nodes receive both the DATA frame and the CFC
frame, then the cooperation phase is invoked. Relay
nodes which join in the cooperation phase is referred
to as active relay nodes. Active relay nodes simulta-
neously start the DCF operation, after the reception of
the CFC frame followed by DIFS (Distributed Inter-
Frame Space). When destination node D correctly
receives a frame, it broadcasts an ACK frame to an-
nounce not only correct reception of the DATA frame
to source node S but also completion of the coopera-
tion phase to all the nodes.
An illustrative operational example with two ac-
tive relay nodes, R
1
and R
2
, is shown in Fig. 2.
Both active relay nodes independently set their back-
2
The term “ideal” implies that the probability of unde-
tected errors can be neglected.
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