Int. J. Communications, Network and System Sciences, 2009, 2, 786-791
doi:10.4236/ijcns.2009.28091 blished Online November 2009 (http://www.SciRP.org/journal/ijcns/).
Pu
Performance Analysis of MAC Protocol for LEO Satellite
Networks
Mingxiang GUAN, Ruichun WANG
Department of Electronic Communication Technology, Shenzhen Institute of Information Technology, Shenzhen, China
Received July 14, 2009; revised August 16, 2009; accepted September 22, 2009
Abstract
Considering that weak channel collision detection ability, long propagation delay and heavy load in LEO
satellite communications, a valid adaptive APRMA MAC protocol was proposed. Different access probabil-
ity functions for different services were obtained and appropriate access probabilities for voice and data users
were updated slot by slot based on the estimation of the voice traffic and the channel status. In the proposed
MAC protocol limited wireless resource is allocated reasonably by multiple users and high capacity was
achieved. Three performance parameters: voice packet loss probability, average delay of data packets and
throughput of data packets were considered in simulation. Finally simulation results demonstrated that the
performance of system was improved by the APRMA compared with the conventional PRMA, with an ac-
ceptable trade-off between QoS of voice and delay of data.
Keywords: LEO Satellites, Adaptive Packet Reservation Multiple Access, MAC Protocol
1. Introduction
Due to various economic and technical constraints, ter-
restrial mobile networks can only provide communica-
tion services with a limited coverage. Recently, in re-
sponse to increasing demand of real-time multimedia
services and the truly global coverage required by per-
sonal communication services, there is a vast research on
non-geostationary orbit (NGSO) satellites systems, espe-
cially on low earth orbit (LEO) satellite constellations
with an altitude between 700 km and 1 500 km. LEO
satellite constellations equipped with inter-satellite links,
such as Iridium, Teledesic, Courier and so on, usually
have onboard switching and onboard routing facilities
and form an independent network in space. Direct con-
nectivity between any pair of satellite mobile users can
be achieved through the satellites and ISLs without any
essential usage of the terrestrial core network. For the
wide application prospect, they have already been the
focus of the research on the satellite communication sys-
tems. This LEO system can provide real time voice and
data traffics in the global range. It is the trend that vari-
ous kinds of traffics will be provided by LEO satellites
system. It is of great importance that an effective me-
dium access control (MAC) protocol will be required to
make full use of limited resource and to provide services
with strict quality for users. MAC protocol is used to
allow many mobile users to share simultaneously a finite
amount of radio spectrum. The sharing of spectrum is
required to achieve high capacity by simultaneously al-
locating the available bandwidth to multiple users. Thus
the appropriate access control protocol will be a key
problem for wireless mobile communications develop-
ment.
LEO satellites will provide not only the real-time traf-
fic such as video and voice but also data traffic with
burst character. The efficiency of resource utilization
will sharply decrease if fixed assignment multiple access
is applied. Also, voice and video traffic will not be sup-
ported sufficiently if competitive multiple access
(ALOHA, CSMA et al.) is used completely [1–3]. Since
PRMA (Packet Reservation Multiple Access) as an ac-
cess protocol for wireless local networks was introduced
by D.J Goodman et al. in 1989 [4], its high efficiency for
voice packet transmission captured much attention, since
then new versions have been proposed to support
multi-media traffic which is very important in the future
mobile system. In literature [5] this protocol was re-
searched profoundly and the author pointed out that
PRMA is competitive protocol with the limit of traffic
and connection number at one time. Three main prob-
lems will be encountered if this protocol is applied in

M. X. GUAN ET AL. 787
LEO satellites system. They are: 1) The channel collision
detection ability is quite weak. 2) The propagation time
delay is long comparatively to terrestrial communica-
tions system. 3) Heavy load will be supported because of
many users in the coverage of the LEO satellites. The
three characteristics will bring on increase of packet loss
probability, severity of channel congestion and decrease
of QoS (Quality of Service).
More improved PRMA protocols were provided based
on [4–5]. In literature [6] PRMA-HS with re-transmis-
sion character was provided in order to overcome long
time delay problem in satellite communication. But the
access contention becomes serious and performance of
the system degrades under the environment of large
number of users or heavy load. Moreover this protocol
has a changeable channel access time delay and a certain
packet loss probability which are not suitable for services
with strict QoS requirement. In literature [7] IPRMA
(Integrated Packet Reservation Multiple Access) protocol
was proposed for satellite communication. A user can
reserve many slots to improve performance of this pro-
tocol. But it possibly exists that one user occupies the
resource totally. In literature [8] MPRMA (Mini-Packet
Reservation Multiple Access) was provided. In the pro-
tocol an available slot will be divided into many
mini-slots in which competitive packets are transmitted.
From [8] we can see that probability of collision in a
mini-slot decreases. But this protocol can not support a
mass of real-time traffic due to the decline of the effi-
ciency of transmission. In literature [9] the author pro-
vided NC-PRMA (Non-Collision Integrated Packet
Reservation Multiple Access) protocol which adopted
queue model to avoid collision resulted from competi-
tion and performance was improved. But this protocol
is not perfect in the long propagation delay because
implementation of this protocol will be in the environ-
ment of short RTD (Round Trip propagation Delay)
period.
From the analysis above, we can find that access
probability for voice and data is obtained from the same
access function, without considering the different traffic
characteristics and requirements of voice and data users
(voice users require real-time delivery but can accom-
modate higher bit error rates; data users do not need
real-time transmission and can be queued but require low
bit error rates or error-free transmission). Therefore it
will be expected to be more efficient if priority is given
to the transmission of voice, whilst minimizing the ef-
fects on the data packets. Considering that weak channel
collision detection ability, long propagation delay and a
mass of load in communication system in LEO satellites,
an adaptive access control protocol improved form
PRMA based on channel status, quality of service and
estimation of traffic was proposed with priority of voice
traffic referred to [10,11]. Thus in this method, voice
packets access to the channel with a priority and a up-
dated access probability and then, if the resource is
available, data packets can be accepted with an updated
access probability slot by slot. Our simulation results
show that the efficiency is improved by the new adaptive
PRMA protocol.
This paper is organized as follows. In Section 2, two
kinds of access probability functions are derived for
voice and data traffic respectively. Compared to conven-
tional PRMA protocol, three performance parameters of
voice packet loss probability, average delay of data
packets and throughput of data packets are analyzed by
simulation in Section3. Finally system performance and
conclusions are obtained.
2. Protocol Model
2.1. Traffic Analysis
For a voice terminal, the voice source can be character-
ized by a two-state Markov chain model, as shown in
Figure 1. Four parameters are required for the description
of the model. They are: the mean duration of a talk burst
, the mean duration of the silence , the transition
probability from the talking state to the silent state
1
t2 t

and the inverse transition probability

. The parameters

and

can expressed as follows:
1
1exp( /)t



 (1)
2
1exp( /)t



 (2)
where

is the width of one time slot. The empirical val-
ues for and are 1s and 1.35s. The voice terminal
generates one packet per frame which is first composed
of an information field with length of
1
t2 t
s
f
RT , where
s
R
is the bit-rate of voice and is the duration of one
frame, and a packet header with length
f
T
H
bits.
Next data users were concerned. A data terminal has a
discontinuous stream. Denote the average bit rate of the
data terminal by
s
R and a data packet which is the
same as voice packet is generated independently in each
slot with a probability of d

. Hence, the mean bit rate of
a data terminal is:
Figure 1. Two states Markov model of voice.
C opyright © 2009 SciRes. IJCNS

M. X. GUAN ET AL. 788
Figure 2. Structure of frame and packet.
dds
RRA

 (3)
where is the number of slots per frame and can be
calculated by:
A
int[/ ()]
pf sf
A
RTRT H (4)
p
R is the channel rate before coding.
2.2. Frame Structure
These frames are further subdivided into N time slots as
illustrated in Figure 2(a). Information packets transmitted
from terminals to satellites consist of both a payload
(actual information) and a header (control information)
as illustrated in Figure 2(b). The time slot duration is τ
and T is the duration of a single frame.
2.3. APRMA Protocol
In the beginning of this paper we have got the conclusion
that conventional and improved PRMA protocols sup-
ported mixed voice and data traffic with a low efficiency
in LEO satellites. In the APRMA protocol, is the
access probability of voice and is the access prob-
ability of data respectively. Voice user is given priority
compared with data user. Appropriate access probabili-
ties and are broadcast from the LEO satellite,
which is updated slot by slot based on the estimation of
the voice traffic and the channel status.
v
P
d
P
v
Pd P
The purpose of APRMA protocol is to guarantee
real-time transmission of voice packets by priority trans-
mission compared with data packets. When the channel
load is light, transmitting data packets is allowed. On the
other hand, when the channel load is heavy, transmission
of data packet is postponed. We assume that the LEO
satellites can recognize the total number of users in a cell
and the number of users in reservation mode. Then,
based on the statistical characteristics of the traffic mod-
els, the number of contending voice terminals is esti-
mated and the access probability is calculated. From the
voice model described in 2.1, voice terminals are v
M
and voice terminals in reservation mode are rsv
M
,
probability of n new terminals arrival talk t and
one terminal departing from talk burst can be considered
as a binomial distribution:
(,,)( )(1
kk
Bnk pCp

The
(5)
where
burs
)
(|( ,,)
nk
n
sil sil
silv rsv
p
Pn MBMn
MMM






)

s
il
M
he p
is number of voice terminals in silenode. t m
When topulation of users is large or the probability
of p is small, the binomial model approaches the Pos-
sion model. Thus
s
il
M



is arrival rate of the voice
users and rsv uM


is departure rate of the voice users
respectively. Hence, in the current time slot the estimated
value of rsv
M
can be expressed by:
10
/MM P
1
()
rsv
rsv rsv
OPT RSV
rsv
M
ax MMC
CK K

 

 (6)



 

0r
1
0
(1)(1) (1 )(1)
v
fv v
c
vv v
rcs
PPr
sc M






fv
rc
Br


 

 


ers in reserv
e u
(7)
where e number of voice usation
1rsv
M
n the
is th
mode i previous time slot and 0 P is the access
probability of voice users at current tim slot. OPT K is
the available number of access channel for voicrs.
In the voice system, the balance equation is shown in the
following: Here 1(1)
B
fB
e
se

  , P is the ac-
cess probability. F7),e following
Equation (8) can be found.
rom the Equation ( th
(1
ta syste
) ()
f
vv
rM


  (8)
For the dam, the probability of a
se
cB


data user to
nd a data packet successfully is w:
(1 (1
dd
wpur bM/ ))
d
B

 (9)
where . Theref
d voicr access prob
(1)1
(1) (1)
cb
d
p
 

dv
up
e use
ore, the data
user anability at the balance
point can be shown by the Equation (10):
[ (
(1
1)(1 (1))]
bb
cp r 
 
(1 )
f
cM  10)
where
dd
d
M
B
(
) /(1)
dd
pppp


an
. In the voice, the
n (8) cbe shown in th
system
Equatioe following.
12 v
hch rM

 (11)

M. X. GUAN ET AL. 789
where 1(1/)h

  ，2(1h/ )
f

 . Combination 
he relation betwEquations), teen b and c
is in the following.
(10) and (11
2
1
1
min{(), }
dd d
fv
h
c
bM

M
Mhc




(12)
.3.1. Voice Packet Loss Probability
the ratio of the
2
Packet loss probability is defined as
number of loss packets and the number of the generated
packets at the terminals.
2
2
2
[1 (1
ff
) ](1)
1[1 (1)]1
[1 (1)]
BN
ff
vdrop BB
f
B
f
B
f
vv
Pvv
v
v









 




(13)
where is the maximum time delay of voice packet
t.
)b
(14)
.3.2. Average Time Delay of Data Packet
time from
t is:


N
isand v the successful access probability of voice
packe
1v 1(1 (1(1/)) (1) (1)
c
dd
rbMAppp

 
2
Average time delay is defined as the lasting
packet generated to packet received successfully at the
LEO satellites. When a data packet arrived at a data user,
j data packets were waited for transmission. Average
e delay of this data packet was (1)/jw. Therefore
the average time delay of data packe
tim
1
W

(1 )
ad
d
wp
 (15)
.3.3. Throughput of Data Packet
of the number of
2
Throughput is defined as the ratio
packets received successfully and the number of packets
generated at the terminals in a time unit. The Throughput
of data packet is defined as the proportion of timeslots
that successfully carry information packets.
2
(1) (1
d
b
Tr M
 
  )
throughputd d
d
MA 
(16)
. Results Analysis
ites system have taken CDMA
how the simulation
re
3
ll the LEO mobile satellA
technology except Iridium system (TDMA technology
was taken). LEO satellites adopt multi-beam formation
technology to make full use of the finite radio frequency
resource. Therefore many cells are formed and users
separated by space can re-use the radio channel. In [12]
CDMA channel attenuation model in AGWN was pro-
posed which adopted BPSK modulation technology and
BCH coding (511, 229, 38). Figure 3 shows the access
probability of voice users in APRMA protocol and Table
1 shows the simulation parameters.
Figure 4, Figure 5 and Figure 6 s
sults with equal loads of voice and data traffic. For 2%
1234567891
0
0.00
0.05
0.10
0.15
0.20
0.25
Access probability
Number of users on channel
Figure 3. APRMA voice terminals access probability.
Table 1. Simulation parameters.
Parameter Value
Channel rate 5.3Mbps
CDM rate
Avte
Inforcket
Maximuacket) 2
1.
A information4599Kbps
Channel rate after coding 1022Kbps
Channel rate before coding 558Kbps
Voice rate 16Kbps
erage data ra3.4Kbps
Frame duration 10ms
mation bit per pa160bits
Frame header 69bits
Slots per frame 10
m delay (Voice p0ms
Mean duration of talk burst1
t 1s
Mean duration of silence2
t 35s
60708090100110120130140150160170
10-6
10-5
10-4
10-3
10-2
10-1
100
Number of voice users(Mv=Md)
Packet loss probability
AP
CP
Figure 4. Voice packet loss probability.
C opyright © 2009 SciRes. IJCNS

M. X. GUAN ET AL. 790
packet lothen the ss probability as an acceptable level,
system capacity is improved about 18% by APRMA
compared to CP (Convention PRMA). Furthermore, Ta-
ble 2 shows the comparison between AP and CP protocol.
From the performance comparison in the Table 2, the
60708090100110 120 130 140 150 160 170 180
0
25
50
75
100
125
150
175
200
225
250
Delay of data packets(ms)
Number of voice users(Mv=Md)
AP
CP
Figure 5. Average time delay of data packets.
60708090100 110 120 130140 150160 170180
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Throughput of data packets
Number of voice users(Mv=Md)
AP
CP
Figure 6. Throughput of data packets.
Table 2. Performance comparison.
Data user 100 150 200
Voice user (PLP) 188 108 78
V) oice user (ATDDP282 170 138AP
Voice user (TDP) 260 185 160
Voice user (PLP) 142 85 58
V) oice user (ATDDP240 134 116
CP
Voice user (TDP) 220 150 135
Voice user (PLP) 24.6 21.225.6
V) oice user (ATDDP14.9 21.715.9
Performance
improvement
(%) Voice user (TDP) 15.4 18.915.6
PLP, ATDDP and TD prob amy
and throughput of data packet respectively.
s -
. Conclusions
ent of LEO satellite communication,
. Acknowledgement
his paper supported by the 3rd natural science founda-
. References
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F. Marangoni, “Advanced dy-
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P stand for packet lossability,verage tie dela
ystem performance is improved by the APRMA proto
col in all three cases.
4
ith the developmW
it is the base requirement that various kinds of services
will be provided. Considering that weak channel colli-
sion detection ability, long propagation delay and heavy
load in LEO satellite communication system, a valid
adaptive access control protocol APRMA is proposed.
Different access probability functions for different ser-
vices are obtained and appropriate access probabilities
for voice and data users are updated slot by slot based on
the estimation of the voice traffic and the channel status.
Simulation results demonstrate that the performance of
system is improved by the APRMA compared with the
conventional PRMA, with an acceptable trade-off be-
tween QoS of voice and delay of data. Also the APRMA
protocol will be suitable for HAPS (high altitude plat-
form station) with the character of weak channel colli-
sion detection ability, long propagation delay and heavy
load.
5
T
tion of institute (No. LG-08010) and 2nd doctoral inno-
vation foundation of institute. The author would like to
thank Dr. Zhong Weizhi and Li Lu for their revisions of
the text, and the editor and the anonymous reviewers for
their contributions that enriched the final paper.
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