Protocol for Internetwork Handover between Terrestrial UMTS and
Satellite UMTS networks, using Intelligent systems
1
Aruna Jayasuriya1, John Asenstorfer1, Nirmala Shenoy1, Saman Halgamuge2
Mobile Communications Research Centre, Cooperative Research Centre for Satellite Systems,
Institute for Telecommunications Research, University of South Australia,
Warrendi Road, Mawson Lakes SA 5095, Australia
2
Mechatronics Research Group, University of Melbourne, Parkville, Victoria 3052, Australia
E-mail: aruna@spri.levels.unisa.edu.au, Fax: 61 8 8302 3873
ABSTRACT
1.1 System Architecture
The aim of this paper is to introduce and analyse a
proposed protocol for handover between Satellite
UMTS (S-UMTS) and Terrestrial UMTS (T-UMTS)
networks in the context of 3rd generation mobile
communication systems. 3rd generation mobile
communications systems and the need for lower
handover blocking probabilities are discussed, first.
Then the proposed protocols will be introduced with
information flow diagrams for certain cases of
internetwork handover. Further key functions required
to perform efficient handover is discussed. The paper
concludes with the summary of analysis of handover
performance for the proposed protocol and their
implications.
UMTS architecture is designed to integrate
different wire and wireless networks together at
network level and at system level. Figure 2
highlights different elements of the proposed
UMTS network [6] [7].
MT
BTS
BTS
BTS
SES
CSS
BTS
CSS
SES
1. INTRODUCTION
UMTS, Universal Mobile Telecommunications
System, is the 3rd-generation mobile standard that
links mobile personal communications users into
multi-media,
information-technology
and
telecommunications services and applications
[1][2][3]. To satisfy the predicted traffic demands with
efficient use of radio resources and to support the
concept of global coverage, mixed cell architecture
has been specified in the UMTS standards [3][4]. The
cells namely, pico, micro, macro overlap and mobile
satellites overlapping the terrestrial cells. Mobile
Satellites provides the most effective coverage method
to sparsely populated rural areas and they are planned
to act as umbrella cells; in effect a backup cell, to
macro cells that cover suburban areas.
UMTS is designed to support wide band services,
requiring up to 2 Mbps. However satellite segments
and macro cells will be designed with maximum bit
rates of 144 kbps and 384 kbps, respectively.
PRMA++ media access protocol was developed to
support resource allocation in UMTS and to
accommodate different bit rates of traffic by reserving
required number of slots in a time frame [5].
UMTS will support future multimedia services, which
requires continuos connectivity throughout the entire
length of the call, including roaming between different
types of networks such as T-UMTS and S-UMTS.
This leads to the necessity for an improved handover
technique to support seamless handover between
macro cells and mobile satellite “cells”, with reduced
handover blocking probability.
GSM BSC
IWF
LE
ISDN
BTS-Base Transceiver System
LE-Local Exchange
MT-Mobile Terminal
CSS-Cell Site Switch SES-Satellite Earth Station
IWF-Interworking functions
Figure 1. Integrated UMTS system
1.2 Functional Requirements
A user’s requirements and service profile is
recorded in both networks. The following
functional requirements can be listed as a basis of
location updating process and the internetwork
handover protocols.
1. Users can connect with either Terrestrial or
Satellite networks, with the priority given to
Terrestrial cells due to the cost factor.
2. Users in both types of networks can be
addressed, by referring to the location registration
databases at the LEs.
3. Avoid call dropping due to lack of coverage or
traffic congestion in terrestrial network, by using
satellite network resources.
4. Avoid usage of the Satellite network whenever
terrestrial networks with sufficient resources are
available.
5. Execute the handover procedure at the best
possible time, not only to decrease the probability of
unnecessary handovers, but also to decrease the
probability of call dropping and at the same time with
minimal costs to the user.
1.3 Location Updating
MTs will listen to the broadcast messages in the area
and appropriate location updating will be done at the
LE. The mobile will only be located with an SES,
provided no BTS is available with the required signal
level.
2 INTERNETWORK HANDOVER
Special Handover Protocols are to be used in UMTS
network elements. They must be capable of handover
between intra-network as well as inter-network
components. The Quality of Service (QoS) desired in
different networks may be different due to their
different transmission environments, power levels and
allowed bit rates. So these Handover (HO) protocols
must also take into the consideration, the fact that,
different network components are designed to perform
with different QoSs and if the service profile permits,
graceful degradation/upgradation of QoS is allowed
over such network boundaries.
resources from the target BTS. If no resources have
been found during a certain time CSS requests a
SAT measurement from MT via old BTS (M3,M4).
From this measurement (M5) appropriate target
SES is identified and a HO_REQUEST(M6) is sent
to the SES via LE. If SES has resources available a
HO_INFO (M7) packet is sent to the MT with all
timing advance information and frequency
information. Upon receiving this packet MT sends
a HO_ ACCEPT(M8) packet to the system through
the old BTS and starts transmitting via
Satellite(M9) according to the timing information
supplied by the system. After receiving this SES
will provide the synchronisation information(M10)
for further transmission from MT.
MT
M1
BTS
CSS
LE
SES
M2
M4
M3
M5
M5
M6
M6
M7
M7
M7
M7
M8
M8
M8
M9
M9
2.1 Proposed Handover Protocol
M10
The proposed HO protocol distributes the HO
processing over different network elements. However
the amount of processing hardware that can be
included in any component is limited to power and
size considerations of each device, which is covered in
more detail later.
The geographical area under consideration for internetwork HO is boundaries of macrocells, mostly in
rural areas, with radii of about 30-35kms. It has been
shown that signal strengths decrease gradually at these
distances from BTSs [8][9]. This provides enough
time for superior processing and decision-making
schemes to be implemented. So intelligent systems
such as Knowledge Base Systems can be used in internetwork HO processing. This will prevent too early,
too late or wrong HO decisions being made at the
network boundaries.
On the other hand S-UMTS to T-UMTS handover is
not time critical because Satellite network will
continue to provide service for the user, using its “near
global” coverage. However the cost to handle a call
through the S-UMTS network will be higher,
suggesting the transference of Satellite users as soon
as possible to terrestrial networks for attractive tariff
schemes, and best possible service.
Figures 2 &3 describe the proposed HO protocol for
special scenarios.
If the BTS signal measurement (M1) is low BTS
sends a HO_REQUEST(M2) to the CSS requesting
SAT
M10
Figure 2. Information flow for the case BTS to
Satellite Handover (special case of using Satellite
as a backup for BTS-BTS HO)
Satellite to BTS HO protocol uses a similar
information flow and is shown in figure 3. HO is
initiated by SES, when a BTS of sufficient signal is
present, and resources from target BTS(N2,N3) is
requested via the corresponding LE and CSS.
Mobile Terminal (MT)
As transmit and processing power is constrained in
the MT minimal processing is available to perform
a seamless handover. They must be provided with
functionality to perform in both types of networks
via a common protocol. Also they are capable of
responding to the HO commands, issued by
network components in the higher layers, to
perform different HOs like BTS to BTS, BTS to
SAT, inter SAT, intra SAT, to mention a few.
Low Earth Orbit (LEO) Satellites
Payload of LEO satellites intended to be used in the
S-UMTS networks is also power critical and they
will only act as retransmission points as far as
internetwork traffic and HO are concerned.
MT
BTS
CSS
LE
SES
SAT
N1
N1
N2
appropriate CSS, via the LE. The SES will keep
trying until it gets the acceptance from the T-UMTS
network.
N2
N2
2.2 S-UMTS as a backup for macro-macro
cell HO
N3
N3
N3
N4
N4
N5
N6
N5
N5
N7
Figure 3. Information flow for the case Satellite to
BTS Handover
N1 -BTS & Satellite signal measurements
N2 -HO_REQUEST
N3 -HO_INFO (target cell)
N4 -HO_INFO (target cell), timing info
N5 -HO_ACCEPT
N6 -Data transmission according to timing advance set by
the system
N7 -Synchronisation information from BTS
Base Transceiver System (BTS)
BTS initiates all the handovers triggered by
insufficient signal strengths to maintain a call. It sends
a HO_REQUEST packet to the CSS indicating the
best cell for handover. This can be a terrestrial cell or
a satellite cell if, no terrestrial cell with sufficient
signal strength is close by.
Cell Site Switch (CSS)
CSS initiates traffic handovers to distribute the load
through all the cells as much as possible when
individual cells’ traffic increase load beyond some
threshold. The appropriate BTS then selects the MTs
that can be handed over to neighbouring cells while
maintaining an acceptable signal level in their target
cell. CSS acts as the switching point for intra CSS HO
and coordinates the HO procedure. If there are no
radio resources available in the target BTS, the CSS
requests a Satellite signal measurement from the MT,
via the old BTS. If the MT receives the sufficient
signal strength from a satellite, the CSS will initiate a
HO to the S-UMTS network.
Local Exchange (LE)
LE acts as the switching point between T-UMTS and
S-UMTS networks and coordinates the HO between
these two networks.
Satellite Earth Station (SES)
SES monitors the signal strengths of BTSs as received
by the MT, under the S-UMTS network. If a MT
receives a BTS signal above the threshold required to
maintain a link, the SES will initiate a HO to the
The S-UMTS network can be used as a backup to
improve the performance of inter BTS handover. If
the user is willing to use the S-UMTS network the
user can be temporarily transferred to the S-UMTS
network when the target BTS don’t have the
capacity. After this transfer the SES will keep
trying to reserve resources from the likely target
BTSs for the MT. The MTs will be immediately
transferred back to T-UMTS network once
resources have been found.
2.3 Timing adjustments for HO
When a user is transferred to a satellite cell from a
BTS, the MT’s transmitter and receiver timing
should be adjusted due to the large difference in
propagation delays for the two networks. This is
essential as a single packet out of synchronisation
can collide with another packet increasing packet
loss. As the SES is aware of the exact position of
the LEO satellites at a given time and the
approximate position of the MT, approximate
propagation delay and hence the correct timing
advance required by the MT can be calculated in
the system. This will be sent to the MT as part of
the HO information and MT will start transmitting
according to these values. Protocols in the LE will
ensure correct switching between T-UMTS and SUMTS circuits.
For a cell with a 30km radius and a LEO orbit of
1000kms the worst case timing misalignment is 6
s. The proposed PRMA++ media access and
transport scheme only allows a guard band of
6.86s which may be not adequate when other
sources of timing errors are taken into account. One
way of resolving this problem is to reserve 2 slots,
at the end of the frame, for HO users, and the initial
3 packets (the maximum time for the
synchronisation to reach the MT after its first
transmission) can be transmitted in the middle of
these two slots to avoid collisions between adjacent
packets. This will require re-allocation or packing
of slots for users already in the system.
Another method is to identify the exact location of
the MT using signal strength measurements and
leading to a more accurate timing hence avoiding
collisions in the first few packets.
Seamless HO from S-UMTS to T-UMTs requires
buffering for up to 2 packets, for the worst case, as
packets from BTS can reach the LE quickly while
packets from S-UMTS network are still in flight. LE
can either increase the transmitter rate temporarily by
using additional slots or can simply discard these 2
packets when a particular service is not sensitive to
limited packet loss.
2.4 Decision-making using Intelligent Systems
Intelligent systems can be used to supervise the
decision making process. UMTS users can be
categorised according to a priority level they requested
from the service provider at their subscription
depending on the type of services the user requests.
Grade of service (GoS) parameters such as HO
blocking guaranteed for each user will depend on this
priority scheme. For users requiring lower HO
blocking probabilities Mobility Modelling and
prediction methods can be used to establish virtual
connections between the most probable target cells.
This will allow to provide a very low, or zero,
blocking probability at a higher cost. Again using the
satellite as a backup for the HO allows low blocking
probabilities but at higher costs.
Knowledge Base Systems such as Multiple Shape
Basis Function Networks (MSBFN) [12] can be used
to model the mobility patterns for required users.
Following information can be used with intelligent
systems to improve the performance of the HO
process.
1. Mobility modelling and prediction
2. Traffic intensity in various cells to achieve
uniform utilisation of cells through HO.
3. Provision of services and QoS according to the
user service profile requested at subscription.
4. Improved
processing of signal strength
measurements to avoid unnecessary HO.
5. Identify particular HO patterns in cells and use
them to build a “semi-virtual” connection between
those cells (for example a set of BTS running along a
highway, near a turn off, may have large percentage of
it’s users moving along the highway)
Although the signal strengths are assumed to be
slowly varying in the region of interest, sudden
decreases in the signal strength variations are expected
due to shadowing effects. Various methods have been
investigated to decrease the probability of taking a
wrong decision at such instances [10][11]. Wrong
decisions can be improved using intelligent systems to
identify locations of these sudden variations and hence
accounting for them in processing of signal strength
measurements.
3 SIMULATION AND RESULTS
The above HO protocols, without Intelligent Systems
for decision-making, were modelled and simulated
with 60 users with random trajectories, distributed
evenly in the network. Several simulation sets were
carried out using different random call generation
patterns. Data ignoring transient period, were
averaged to obtain the following results. The
following parameters were used to analyse the
performance of the proposed internetwork handover
protocols.
1.
2.
3.
4.
HO blocking probability
HO interruption delay
Improvement in Blocking probability due to
using satellite as a backup for BTS-BTS HO
(case of congested cells)
Additional time spent in the S-UMTS network
when BTSs are present but congested, delaying
HO to the terrestrial network.
Table 1 summarises the simulation results for
Internetwork HO performance. The satellite is used
as a backup for BTS-BTS HO.
Blocking
probability
0.88%
Avg. system
utilisation
85%
Avg. HO delay
158.5ms
Table 1-Handover performance
Figure 4 – 1 - cumulative density function for HO
interruption time
Figure 4 shows the cdf for HO interruption delay.
The large step around 160ms confirms the average
delay of 158.5 ms obtained above. However the
blocking probability increases rapidly to 6.5% and
HO delay of 280.7ms, when the system utilisation
reaches 88%.
A congested T-UMTS network is used to evaluate the
advantage of using the S-UMTS network as a back
up in BTS to BTS HO. Table 2 compares the two
scenarios. Figure 5 ( histogram of additional time vs.
no of HO) shows that average additional time spent
in S-UMTS network is higher for case 2.
Average length of a call used in the simulation was
180s and results show that on average users spend
2.15% for case1 and 4.1% for case2, of their call time
in S-UMTS network although they are under the TUMTS coverage.
HO block. Prob.
BTS utilisation
SAT utilisation
Additional cost of
using satellite
Without S-UMTS
backup
(Case 1)
With S-UMTS
backup
(Case 2)
8.1%
85%
36%
3.7s
4.5%
87%
45%
7.3s
Table 2- Improvement in performance for HO from
BTS, by using satellite as a backup
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network as a backup. This is as expected as this
effectively increases the resources available for TUMTS handover. This process also results in
achieving high utilisations in the T-UMTS network
without excessive additional load in the S-UMTS
network. This can be thought of as S-UMTS
network acting as a buffer for handover users in TUMTS networks, until resources in BTSs are
available.
It can also be observed that the average additional
time users spend in S-UMTS networks (due to lack
of T-UMTS resources) is somewhat higher for case
2. This highlights the importance of using the user
service profile in HO decisions to make the service
attractive to both users requiring moderate GoS at
low costs and high GoS at high costs.
5 REFERENCES
Figure 5. Additional cost for using S-UMTS network
(additional cost vs. no of HOs made at that cost)
4 CONCLUSION
As a conclusion of the analysis and performance
evaluation of the proposed HO protocols, it can be
stated that the proposed protocols provide blocking
probability of 0.88% and 153.5 ms delay for HO
which is an acceptable GoS in handover between SUMTS and T-UMTS networks. However the
performance must be improved to decrease the
sensitivity to blocking probabilities even under load
conditions greater than 85%.
BTS to BTS HO blocking probability improves
dramatically when users are allowed to use S-UMTS
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