The system architecture for the Canadian Interim Mobile Satellite Service (IMSS) which is planned for commencement of commercial service in late 1989 is reviewed. The results of an associated field trial program which was carried out to determine the limits of coverage and the preliminary performance characteristics of the system are discussed

Bossler, D.

Eldamhougy, H.

Gordon, K.

Shariatmadar, M.

Skerry, B.

English

NASA Technical Reports Server

N88-25703
SYSTEM ARCHITECTURE FOR THE CANADIAN INTERIM MOBILE SATELLITE SYSTEM
M. SHARIATNADAR, K. GORDON, B. SKERRY, H. EL-DANHOUGY, D. BOSSLER
Telesat Canada, Canada
TELESAT CANADA
333 River Road
Ottawa, Ontario, Canada
KIL 8B9
ABSTRACT
This paper reviews the system architecture for
the Canadian Interim Mobile Satellite Service (INSS)
which is planned for commencement of commercial
service in late 1989. It also discusses the results
of an associated field trial program which has been
carried out to determine the limits of coverage and
the preliminary performance characteristics of the
system.
1.0 INTRODUCTION
In Canada, there is considerable momentum and commitment building
towards a Mobile Satellite System (MSAT) implementation program;
however there still remains a significant period of time before NSAT
becomes a service reality. Since an immediate need exists amongst
various industries for the types of services that are best delivered
by mobile satellite technology, Telesat Canada is cooperating with
specific user groups to develop a portfolio of mobile satellite
services whose initial implementation will use the Inmarsat MARECS-B
satellite and could last until the launch of a dedicated NSAT.
To this end, Telesat issued a Request for Proposal in July, 1987
which called for a complete hub messaging facility and an initial
consignment of fifteen mobile data terminals which were intended to
prove the IMSS concept prior to making available for procurement
additional volume quantities of mobile terminals to offer commercial
service in 1989. This service would permit fleet dispatchers to have
regular updates of location and vehicle status information as well as
the flexibility of transmitting and receiving general messages. This
paper describes the IMSS system concept and architecture, and outlines
the anticipated range of services and applications, systems design
parameters, and projected system capacity. It also discusses the
results of a pre-implementation field trial program which involved
monitoring the reception of data transmission from the Inmarsat
satellite in Eastern Canada during the fall of 1987.
2.0 SYSTEM CONCEPT
The IMSS uses a star network configuration to provide low-rate
data satellite communication services to the mobile user community as
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OF POOR QUALITY
illustrated in Figure i. The initial coverage of this system is in
Eastern Canada from 30 ° to 15 ° elevation angle as shown in Figure 2.
The frequency of operation covers the entire WARC-87 allocation, i.e.,
1626.5-1660.5 MHz for transmit and 1530-1559 MHz for receive with 5
kHz channel spacing.
In a configuration of over 4,000 mobile terminals, six 5 kHz
satellite channels over the INMARSAT MARECS-B satellite are shared by
the mobile terminals in the IMSB network. One of these channels, the
outbound, is used by the hub to send network control information,
signalling messages, and traffic information to all mobile terminals.
Of the remaining five inbound channels, three channels are used to
send position reports, one channel for two-way messaging and request,
and one for text transfer.
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Figure i: IMSS Service
Illustration
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Figure 2: Canadian Coverage
by MAREC-B
3.0 SERVICES
Three distinct services are planned for IMSS network which are
explained in terms of the type of information, length of messages and
traffic capacity objectives.
Vehicle location is the primary service planned for the system
and is characterized by 7-character messages, representing vehicle
position in terms of longitude and latitude to the second of degree
plus a coded message, sent at periodic intervals of 15 minutes or
multiples, from a population of over 4,000 mobile terminals. Vehicle
position information is determined via the Loran-C receiver which is
integrated with the mobile terminal. A mobile terminal failing to
report its position on the scheduled slot or those whose transmission
is lost due to fading conditions will repeat its transmission on a
contention basis after being polled by the hub. This channel has a
capacity of about 4000 position reports per hour.
Two-way general messaging is characterized by the transfer of
messages of variable length with an average length of 32 bytes and
maximum of up to 128 bytes, either from the hub to a particular mobile
terminal or from a mobile terminal to the hub. In the mobile-to-hub
direction, the messages are transmitted on the request/messaging
channel on a reservation basis, where the first packet of each message
is handled with a random access mechanism over the availible
unreserved capacity of that channel. Subsequent packets, each
containing i0 bytes of information, are sent over reserved slots on
164 C- 2)
the same channel. An estimated 330 messages per hour with the average
length of 32 characters can be accommodated on one channel.
Tekt transfer is characterized by transfer of relatively long
messages in the order of 4,000 characters long. This service will be
offered on a demand assignment basis by allocation of a dedicated
inbound channel carrying consecutive 127 byte packets. This channel
has a capacity of about 32.9 characters/second.
4.0 SYSTEM DESIGN PARAMETERS
Communication Modes: The key system parameters are summarized in
Table 1 below. Eight communication modes are used in the system to
provide high system throughput and efficient channel assignment. For
example: (i) the inbound multi-packet modes provide the concatenation
of several fixed length packets into a single message transmission by
combining random and reserved slots in the request/messaging channel;
(ii) the inbound scheduled mode is a low overhead method to send short
messages from mobile terminals to the hub at periodic intervals and is
used for the vehicle location service. Recovery from errors is
performed by the inbound po]ling mode where a new polling command from
the hub is sent tO the mobiles after the expiration of a time out
period.
SYSTEM PARAMETER Values & Applications
Modulation
Symbol Rote
Interleaving
FEC Encoding
Multiplexing
Communication Modes:
1-inbound Unsolicited
2-Inbound Scheduled
3-Inbound Polling
4-Inbound MulUpacket
5-Inbound Demand Asg.
6-Outbound Broadcast
7-Outbound Unsolicited
B-Outbound Demand A_J
BPSK
1200 & 600 bps
(64 R X 162 C)
Rate 1/2 Vfferbi , K-7
TDM,TOMA,S&R-ALOHA
Loq on/off & Request
Position Reports
Lost Position Repo_s
Two-Way Messaging
Text Transfer
Network Control
Two Way Messaging
Text Transfer
OUTBOUND INBOUND
(1200 bps) (600 bps)
Earth Station EiRP (dBW) 60.8 16.0
Path & Absorb. LOSS (dE0 200.7 1Bg.0
Sotelllte C/T (de/K) -1.5.0 -10.6
Upfink C/N0 (dB-Hz) 73.7 45.0
Satellite C/10 (Inter-Mad) (dB-Hz) 55.8 53.7
Satellite EIRP (dBW) 21.6 -21.8
Path • Absorb. Loss (dB) 186.4- 197.1
Earth Station G/T (dB/K) -22 32,0
Do wnllnk C/N0 (dB- Hz) 39.8 41.7
Norn. Unfaded C/N0+I0 (dB-Hz) 39.7 39.8
]nte_erence Loss (Adj. Sat.) (dB) 0.5 0.5
Overall C/N0+I0 (dB-Hz) 39.2 39.3
Overall Eb/N0+I0 (dB) 11.4 14.5
M]n. Unfaded Eb/N0+10 (dB) 4.6 5.5
Modem Implement. Loss (dB) 1.0 1.0
Fade Margin (dB) 5.8 8.0
Table I: IMSS System Parameters Table 2: IMSS Link Budget
Link Budget: With an omni-directional antenna adopted for the
mobile terminal and having a minimum effective G/T of -22 dB/K, the
link budget, Table 2, provides 5.8 and 8.0 dB margins for the outbound
and inbound links, respectively, over the Eb/No needed for a i0 -s
BER considered as the threshold requirement. The lower operation
margin adopted for the outbound link is justified by the use of
interleaving in this link and is supported by the field trial results
to give adequate performance in non shadowed areas. In the inbound
links where the relatively short burst transmission makes the use of
interleaving ineffective, the higher operation margin of 8.0 dB has
been adopted.
165
Frame Structure: Currently, several frame formats and packet
structure options are being considered, of which a potential candidate
is shown in Figure 3. The outbound TDM frame contains 639 information
bytes which are used for network control, signalling and traffic
information from the hub to the mobile terminals. Network control
information broadcast by the hub to all mobile terminals is positioned
within the first 48 information bytes following the frame
synchronization words. The remaining 591 information bytes are
available for fixed and variable length outbound packets. The frames
for the inbound request messaging and reporting channels have a period
of 8.64 seconds which is divided into 14 slots, each capable of I0
bytes of information.
2xUWalt. Network_syt_ Control
Message & Signalling Packets
591 Bytes
_' 8.64- Sec,
Rumh J,_= outbound
q
I°+64 Bits
8640 m=
If JI
Messages . Position Reports . Log on/of . ACKs . As._ignrnens
526.7 ms -
Figure 3: Frame and Packet Structure
inbound
5.0 SYSTEM CAPACITY
The estimate of system capacity for the IMSS network is shown in
Table 3. Because of the nature of frame and packet structure, the
capacities are expressed in terms of: (a) reports/second for the
position reporting channel; (b) 10-byte packets/second for the
messaging and request channel; and (c) bytes/second for the text
transfer channel.
The position reporting channel has an available capacity of 1.62
reports/second which is split into i.ii reports/second for the TDMA
scheduled reports and .51 report/second for the retried reports.
Based on this arrangement, the usable capacity of the system,
reflected by the reserved capacity, is 4000 position reports per hour
per channel.
Similarly, the request/messaging channel has a total capacity of
1.62 ten-byte packets/second but its usable capacity is .45 ten-byte
packet/second, of which: (i) 0.09 packet/second is used for the first
packet of the message and transmitted on a random access basis, and
includes 20% random access and 90% ARQ efficiencies; (ii) 0.2
packet/second is the reserved capacity for subsequent packets of the
message and includes only the ARQ efficiency; (iii) the remaining .16
packet/second is used for signalling purposes as part of other
functions. These combinations will allow 330 messages with an average
length of 32 characters plus 576 ten-byte signalling packets per
hour. Should the average message length change, for example, from 32
to 96 characters, one can still accommodate the 330 messages per
hour. However, the signalling capacity for other functions will drop
166
to 144 packets per hour. Requirements exceeding the above limits
would have to be accommodated by adding more channels to the network.
The text transfer channel is basically a demand assigned channel
and can accommodate about 32.9 characters/second.
Inbound Outbound
(Bytes/Sec.)
Request/Messaging (Ten-Byte Packets/Sec.) Position Reports/Sec.
I i T
Reserved Unreserved Heserved Unreserved
Request I Signalling (Reports) (Retried)
32 Char. 96 Chor. 32 Char. 96 Char. 32 Char. 96 Char.
Messoge Messoge Messoge Messoge Messoge Messo9e
0.202 0.789 0.0916 0.0916 0.16 0.04 1.11 0.51
Text Transfer
(8ytes/Sec.)
Assigned
32.95 61.5
Table 3: System Capacity
6.0 FIELD TRIALS
Operating Conditions: To monitor the reception of data
transmissions from the Inmarsat MAREC-B satellite in an actual land
mobile operating environment, a prototype omni-directional mobile
satellite antenna and receiver were procured and fitted into a van
along with associated monitoring equipment. Signal reception was
recorded as the van was driven along typical commercial routes from
Quebec city (20 ° ) to Winnipeg (3°). A personal computer was used to
capture system performance data on a frame-by-frame basis directly to
disk while a printer displayed any messages received.
Data transmission was from the Inmarsat Enhanced Group Call (EGC)
channel which uses coherent BPSK modulation transmitted at 1200 BPS
with rate 1/2 convolutional coding and constraint length 7. The frame
length is 8.46 seconds long and starts with two packets of 40 and 14
bytes which are used to check channel performance and provide
information on the frame, respectively. The rest of the frame
capacity is left for messages, when no messages are sent, binary
zeroes are transmitted instead.
The link budget for the field trials is similar to Table 2 with
the following changes. There is an average gain in the satellite EIRP
of 1.5 dB because the satellite was not operating at full capacity
between i0 a.m. to 4 p.m. Eastern Canada time when daily field trials
were being conducted. Also, there is no interference loss due to
adjacent satellites and the antenna G/T was higher by 1 dB due to half
duplex mode of operation. These changes result in an overall fade
margin of 8.9 dB.
Route Characterization: Figure 4 shows a sample of Frame Success
Rate (FSR) from Quebec City to Sudbury in two hour intervals and as
such characterizes the typical commercial routes within the coverage
limits of the IMSS. The FSR is either 0% or 100% for each frame
received. Given the scale of the graph, one frame in error appears as
a downward 'spike' An outage, that is, no frame received, is
indicated by the FSR 'disappearing' over the graph and 'reappearing'
once reception has resumed. The number to the right of each two hour
interval is its average FSR. The results indicate that reception is
167
very good until Sudbury with 15 ° elevation. Beyond that (not shown
for the sake of brevity), system performance degrades due to the lower
elevation angles and the very hilly terrain encountered Just past
Sudbury until the prairies are reached. The FSR is 94% (98% excluding
outages) for greater than 15 ° elevation angles and 75% (97% excluding
outages) for the entire field trials. For greater than 15 ° elevation,
the ratio of the number of frames that were not received, due to
outages, to the number received incorrectly was 2.7. For the entire
area tested, this ratio was increased to 8. In general, outages were
caused by complete blockage of the signal such as by a hill or
building. A few more dB in fade margin would probably not improve
reception in these areas. After blockage of the signal, it is likely
that the receiver loses synchronization with the carrier. This can
make the outage time longer than the blockage time. Thus, the length
of time the receiver takes to reacquire synchronization is critical.
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Elapsed Houri
Figure 4: Propagation Measurements in Terms of Frame Success
Multipath Fading= The effects of multipath fading were studied
in areas with no shadowing. Figure 5 shows a sample of the relative
fade depth for a half-hour period approaching Winnipeg and Quebec
City. The fade depth is relative to the fade needed to produce an
error in a frame and is not identical to the fade margin because the
fade margin is referenced to a BER of 10 -5 Both the prairies and
Quebec City fades are drawn to the same scale with the right vertical
axis showing the number of renormalizations. This is the actual
parameter measured within the decoder and is related to the fade depth
in a non-linear fashion. Clearly, multipath was more pronounced at
the lower elevation angle and the threshold for producing errors was
exceeded a few times. Nea_ Quebec City at 20 ° elevation, multipath is
still present but the overall majority is within 3 dB of the error
threshold.
Fade Margin Sensitivity= Given the importance of fade margin in
system capacity and performance, the sensitivity of the system to a
reduction in fade margin was investigated for elevation angles greater
than 15 ° . Knowing the relative fade depth for every frame, we can
simulate a reduction in fade margin (say by X dB) by assuming that any
frame with a fade depth of X dB or more would have been received in
error. Thus, the total FSR can be calculated for that reduction in
fade margin. This is shown graphically in Figure 6.
168
PRAIRES Elev 4 °
_+2._ l
!o I
>-5
0
QUEBEC CITY Elev 21 °
100
IOOZ
7s
0
so
I °I 2sI,LI, II
o
.5
S
w
OA
O
"5
• .i5' " ._ 6 .i5 " -'s "2 0
Time (Hours) Time (Hours) RELATIVE MARGIN
75%
L
50%--
Figure 5: Effects of Multipath Fading
Near Winnipeg and Quebec
de)
+'2I
Figure 6: Frame Success Rate
Vs. Margin Reduction
This assumes that, when the fade margin is reduced, the fade
depth will remain the same. While this is true for shadowing, it is
hard to predict for multipath fading because the depth of fade is
related to the power of the signal (the exact relationship is
dependent on the carrier to multipath ratio as well as the absolute
level of the carrier). The worst case scenario is assumed that the
fade depth remains the same regardless of a reduction in transmitted
power (i.e. fade margin).
Keeping these limitations in mind, the graph shows that a
reduction of 3.5 dB will reduce the overall FSR by 3%. This enables
us to adopt, for the interim, a fade margin of 5.4 dB for the outbound
channel subject to the second phase field trial results in the spring
of 1988.
7.0 CONCLUSIONS
The interim mobile satellite services are viewed as an important
and integral part of the Canadian mobile satellite program and serve a
real purpose in achieving an eventual domestic MSAT system. Telesat
is currently in the process of securing both space segment and ground
segment facilities to meet the requirement of these services.
8.0 ACKNOWLEDGEMENT
The authors wish to acknowledge the contributions of their
colleagues both at Telesat Canada and at the Canadian Department of
Communications for development of the IMSS services.
169


System architecture for the Canadian interim mobile satellite system

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