Compared with a wired communication network, satellite communications system
can provide users kinds of services worldwide. Satellite channel resources are
often relatively scarce, therefore, this is the core of the study that how to
effectively utilize the limited channel resources to serve as many as possible
users. And multiple access TDMA technology can solve the problem very well (Lu
et al., 2009a).
TDMA (Time Division Multiple Access) is divided time into periodic frames,
each frame is divided into several time slots allocated to ground station for
transmitting signal, the ground station can receive signals from the different
mobile terminals without interference in each slot (Xin
et al., 2011). TDMA multiple access control technology has important
feature of real-time, dynamic allocation, intercept resistance and anti-jamming,
which made TDMA used widely in the military mobile communication network.
TDMA multi-user access is used frequently, in the satellite channel of uplink.
Downlink usually send data using broadcasting mode, Both mode of transmission
use two different radio frequency respectively, to avoid collision (Mohammed
and Le-Ngoc, 1994). In this study, the author take a studies on dynamic
TDMA protocol that is mainly focus on the uplink of satellite network, in the
downlink, satellite broadcast data to ground sites.
SATELLITE-BASED MOBILE COMMUNICATIONS NETWORK ARCHITECTURE
Be different from the traditional planar topology structure of the land mobile
communication network, satellite mobile communication network is a three-dimensional
topology structure. In this network, aircraft nodes keep moving, which make
the whole network topology structure change along with time and with cyclical
regularity and predictability. Satellite mobile communication network can be
flexibly connected by all kinds of land military sites and has high-speed transmission
of information and is a very good extensibility of information system, its structure
diagram as shown in Fig. 1.
||Satellite-based mobile communications network architecture
The first layer is a satellite backbone network, composed of several satellite
nodes which connect each other with inter-satellite link network. Its function
lies in the processing and forwarding data communication between ground sites
terminal and gateway complete sites, is the core of satellite mobile communication
The second layer is the geositesary satellites, has 24 h all-weather communication
ability and two-way communication between the first layers. It is mainly responsible
for the communication with the ground command center.
The third layer is the ground command network, including the command center
and management center (Chen and Zhang, 2009).
PROTOCOL ALGORITHM DESIGN
With TDMA multiple access methods, a key problem of satellite mobile communication
is time slot allocation. In TDMA mode, different user is distinguishing different
time slot. In the case of a certain number of users, the methods, to allocate
the limited channel resources with high response speed and less collision rate,
play important role in the entire communication network. In traditional fixed
TDMA network, Channel allocation strategy is to allocate all time slot according
to the maximum current user demand, before sites communicate with each other.
Owing to this strategy, network can work without Collision detection. But when
the network traffic load is heavy, efficiency and reliability will become so
low that not suitable for military applications.
TDMA dynamic the reservation access strategy design: Dynamic TDMA reservation
access can be achieved by improves the function of the fixed TDMA access mode.
The summary method is that, in a fixed TDMA way, idle time slot contention is
allowed, to improve channel utilization. The Shared channel is divided into
a certain number of frames. The duration of frame must be greater than the channel
propagation delay, the amount of slot in each frame must be greater than or
equal to the nodes, in each slot, only one data packet can be sent. Each node
has its own fixed time slot only one. If there is a data packet to be transmitted,
the node first occupies its slot. If their time slot is not enough, they can
content idle time slot. Until the data packet transmission is completed and
then node releases all time slots into the idle state pool.
This has two principles:
||If node finds its own time slot to be applied by others, then
shouldnt retreat, insist to use this slot. In the corresponding time
slot of next frame, original configuration packet should be retransmitted
||If the node applies for a time slot which not belongs to this node, at
the same time, owner of this time slot apply for. The first node should
immediately retreat and release slot to ensure no longer compete to obtain
this slot in the next frame, but continue to search for other idle slot
In detail of this method, each frame is composed of synchronous time slot,
control slot and data slot. The control slot is located in the head of each
frame and is divided into several equal-length micro slot in a networking system
startup, each site compete to broadcast their ID number in its own micro slot,
when the state of entire network keep convergence all sites will know the whole
micro slot allocation information.
A micro slot node status table can be automatically generated according to
the ID of all the sites, as shown in Fig. 2, sites confirm
its elves' relative position in reservation time slot and send Request packets
in their own micro slot packet.
In the fixed TDMA mode, one micro slot is assigned to one ground sites, micro
slot should be long enough to send an slot-applying packet. If a node has data
to transmit, first, in micro slot it should send slot-applying packet, publishing
that it need a data slot in next frame. All the nodes in the network must be
able to detect the signal channel, in order to accurately understand the network
time slot allocation situation. Including the amounts of site to apply micro
slot, sites ID and sort order.
After ground sites apply time slot successfully, they has gotten their number
of time slot. According to their order of addresses and send data packet once
in a data slot. This frame comes to a close and that cycle repeats. After time
slot is appointed to one site and is fixedly assigned to it until the end of
data transmission. Then this time slot will be retrieved by system, in next
turn, satellite assigns this time slot to ground sites which needed (Lu
et al., 2009b).
Network control principles: In the dynamic TDMA protocol of the network,
the system in centralized control mode, implement the network time slot allocation
function, data is forwarded from one ground site to another directly by satellite,
without the need to adjust the ground sites.
|| Micro slot nodes status table
In the network, there is only one satellite and several ground sites. Network
control information transmitted by the satellite in the tail micro slot of slot-applying
slots which in each frame. For example, there are amounts of M ground sites,
so slot-applying slot will be divided into M+1 micro slot.
||Network boot: With internal timer, Satellite send synchronization
control frame twice and initialize network. Ground sites must receive two
successive, effective synchronous control frames at least. The synchronous
control frame can help ground sites adjust themselves' local timer, in order
to sync with satellite. After all, they form a network
||Time slot application: When a ground site need to apply a new time
slot, just fill its address in the corresponding field of the application
packet and then send to satellite in its own micro slot. When it receives
the reply from the satellite, it means that the application is successful
and in next frame a new time slot can be used
||Time slot retrieve: The last field of the data packet is to identify
transmission over or not. If transmission is over, the satellite retrieve
time slot allocated to the site
||New sites access: In order to achieve time synchronization with
satellite, new site must receive twice successive, effective synchronous
control frame at least, before joins in the network. Satellite allocate
time slot for the new site and scan the slot list. If find idle slots, assign
to the site; If there is no idle time slot, in next turn, this site's slot
application will be priority (Wang, 1992)
Priority-based time slot allocation strategy: Usually the transmission
of intelligence information requires a relatively high real-time and intelligence
information transmission services is an important part of what services the
mobile communication network provide. The packet size is generally a few dozen
bytes, the characteristics of targets is uncertainty in the general, so the
intelligence traffic may mutation. Real-time transmission of intelligence information
is particularly important. This requires the site is a higher priority to send
such information, to avoid the backlog of site data or lost.
The system set up two priority levels: High-priority sites and mobile
communication web sites. The system maintains a polling sequence, to save the
ID number of each site. The assignment of time slot starting from the head of
the queue for ordinary mobile site after the successful of time slot allocation,
its corresponding ID number transplant to the queue tail for high-priority site,
using the different type of polling sequence.
||Priority algorithm flow chart of satellite mobile communications
This cycle until the time slot assignment completed the satellite generates
the reply packet, the packet including the time slot assignment information
and a request sequence of the next frame (included in the structure). The algorithm
is shown in Fig. 3 (Lu et al.,
Within the reservation time slot in each frame, the system according to priority
level of different packets, use different allocation strategies at High-priority
packets, the give high priority site priority allocation of rights and a greater
number of time slot allocation rights. Therefore, the high-priority group site's
requests are able to get a faster response. So, urgent business data can be
delivered in a timely manner, this will supply low priority site other than
the site with the better transmission characteristics.
ESTABLISHMENT OF SIMULATION MODEL
Protocol and packet format frame design
||Frame of protocol design: In the TDMA protocol, the
length of each frame is fixed, each frame is divided into 10 data slots.
The length of each data slot is 1.0 sec. At beginning of the system simulation,
10 data slot are allocated to every ground site. To dynamic TDMA protocol,
the first data slots are as appointment slots and are divided into 10 equal-length
micro slot which of length is 0.1 sec. Micro slot is allocated with fixed
TDMA protocol. No. 9 of micro slot used by satellite to sends a response
to ground sites for the reply their time slot application (Zhu
et al., 2009), as shown in the frame format shown in Fig.
||Data package format design: The network design three different
types of data packets, which means that three different types of data packet,
the three data packets: Data packet, the time slot request packet and satellite
|| Format of dynamic TDMA protocol frame
|| Data package format
|| Application packet formats
The data packet is produced by the source in the note model contains the site
have to transmit the actual data information setting data packet length 416
bit. In the simulation, as shown in Fig. 5, the data packet
contains the following fields:
||Field of dest_address: Destination address of packet
||Field of sour_address: The source address of the packet
||Field of end_transmitting: Flag of send completed, satellite according
to the sign decide whether retrieve data time slot occupied by the current
The application packet is used by ground sites to apply data time slot from
satellite, package includes the number application of time slot, sites
ID and the site index of micro slot. Format as shown in Fig. 6,
the package contains the following fields:
||Field of slots_need: The number of time slots need
to apply for
||Field of tiny_slot: The current site of micro slots index
||Field of sour_address: The source address of the packet, the value
will be as a reply address sent from satellite
||Field of priority: Indicates the priority level of the current
Satellite maintains a list of unassigned time slots, satellite receive a ground
site request packet and then add the corresponding information of the site to
the unallocated list. When all sites has been completed requests, the satellite
will be ordered by the list, the high-priority sites will be moved to the head
of list, so that the satellite can handle its request first, so as to improve
the speed of access of high-priority sites into the network.
Satellite reply packet is generally sent in the micro slot No. 9, the package
include detail of current time slot allocation for each of sites. Allocation
information include in time slot allocation table, embodied in the form of structure,
as shown in the following:
|| Network layer models
Node ID array preserve the requesting site ID, the start array holds position
the slot start which has been assigned to a site. Slot_num array save number
of time slot assigned to one site.
Network layer model: The network model indicates the position of each
node in the network, connecting links between nodes and node-specific configuration
information. Network model is shown in Fig. 7, for the dynamic
TDMA protocol, the first time slot is divided into 10 micro time slot for transmitting
the time slot request packet, the remaining 9 slots as the data slots for transmitting
data packets. Micro slot is allocated according to a fixed TDMA protocol, the
system has been set up at the beginning of the simulation and remains unchanged
in the whole process, once the site needs to send the data packet, it can be
in allocated micro slot transmit request packet (Mitchell
et al., 2004):
||Ground station process model: Ground station
process model is the core of dynamic TDMA protocol; it is responsible for
the realizing function module of distribution according to ones needs.
ground station node model is specific implementation process of the satellite
dynamic TDMA protocol, the model shown in Fig. 8 above
Workflow of ground sites with dynamic TDMA protocol shown in Fig.
8. As can be seen, the method of fixed distribution blend into the dynamic
TDMA protocol. Its mean is that a micro slot is allocated in fixed TDMA
way. Each ground station must send request packet in micro slot specified.
In the model two states should be introduced:
||The status of the request: In data queue, there are
application packets need to send to satellite. According to the fixed TDMA
protocol, ground site keep making micro slot access judge and satellite
make reply information judge. Until application is successful, then ground
sites enter the sending state to send data
||Sending status: After time slot application is succeed.
It is waiting for their time slot to send data in the data slot of each
frame. The process will judge the data time slot constantly and send data
||Satellite process model: Unlike fixed TDMA protocol,
satellite in addition to forward data, has the time slot allocation and
recovery function. The satellites take centralized control on the slot allocation;
can take a variety of time slot allocation strategy to meet the needs of
different businesses, greatly improving the protocol flexibility. The model
is shown in Fig. 9
||Initial state: Initializing a number of variables, including: Length
and amounts of data time slot, length and amounts of micro slot and slot
||Idle state: Waiting for the stream interruption arrival
then jumping to the next step. When it enters the code area, it needs the
judgment of the micro slot, in number of 0-8 micro slot, satellite need
to record request information of ground stations. The position of Satellite
micro slot is 9. After entering the micro slot, satellite based on request
information of applications data packet received by the first 8 micro slot
data and allocation of time slots in accordance with the time slot allocation
algorithm. After assignment is completed, generate a reply packet and fill
the satellite reply data packet with slot allocation information and broadcasts
the packet (Mohammed and Le-Ngoc, 1994)
|| Flow chart of time slot allocation algorithm
|| Satellite process model
SIMULATION AND RESULT ANALYSIS
In order to analyzing the performance of the dynamic TDMA protocol in detail.
The simulation set up two global statistics: Average of end-to-end delay (end_to_end_delay_handle)
and the amounts of packets sent (Sum_Packets_send).
Average end-to-end delay of the network: The large volume of business,
the site generated packets at a rate faster, date packets rapidly increasing
in the queue.
|| Average end-to-end delay result
|| Setting of nodes table (for verifying priority policy)
Usually sites cannot complete the task transmitted from the queue data packet
in an even number of time slots, as shown in Table 1.
Interval of time between with each two data packages generated by gnd_0~gnd_7
site is 0.0001 sec, these packages have 7 times to be sent completely. In the
fixed allocation, each site can only be assigned one time slot, so 7 frames
have to be used to send.
A less increasing with dynamic allocation can be seen from Fig.
10 in the fixed allocation increasing is very significantly. This is because
when the system is in a busy state, the data packets in queue continue to increase
rapidly. In fixed TDMA, site only be allow transmit data packets in belong its
own time slot (only one), for site without sending, its idle slot do not allow
the busy site to occupy. The idle time slot resources waste so greatly that
increased waiting time of the data packet. For dynamic allocation, the channel
utilization is high, the system assigns some slots for heavy business site and
idle time slot does not exist. It can greatly reduce the waiting time of the
Comparison between priority with not: In the network, setting up gnd_5
site is a high priority. The rest of the site is the normal priority site. Verification
of priority strategy in large volume of business is shown in Table
1. All sites maintain a rapid rate of generating data packets, prompting
each site emit slot application to the satellites. Without use of priority assignment
strategy, satellite record site information according to each site request sequence.
It is Using a first-come first-served principle for all ground station for time
slot allocation when satellite allocate time slot for gnd_5 site. Maybe there
is no idle time slot to be allocated. The gnd_5 site is a high priority sites
although, gnd_5 site for high-priority sites, but can only wait for the re-allocation
in the next frame. After joining the priority allocation policy the high-priority
site requests will be priority disposed and it'll get more time slots than the
From the graph of Fig. 11a and b something
can be seen, in the same simulation conditions, sites of gnd_5 can send more
data packets using priority strategy than nor-priority.
||Number of packets sent in gnd_5, (a) Amounts of packet sent
with priority strategy, (b) Amounts of packet sent without priority strategy
Because of the high level of priority in the allocation and more slots allocated,
this allows high-priority sites rapid access and sends data packets in the network.
In this study, it presents a simulation study on priority TDMA protocol based
on a suitable space-based tactical communications network. It can be seen from
the simulation results; the high-priority sites have relatively low end-to-end
delay and faster access into network. So, it solve the real-time requirements
of tactical communications network and transmission of emergency data. The overall
performance of system, meet the target and requirement.