The first generation of the Canadian Mobile Satellite (MSAT) system, planned to be operational in the early 1990s, will provide voice and data services to land, aeronautical, and maritime mobile terminals within the Canadian land mass and its territorial waters. The system will be managed by a centralized Demand Assignment Multiple Access (DAMA) control system. Users will request a communication channel by communicating with the DAMA Control System (DCS) via the appropriate signalling channels. Several access techniques for both L-band and SHF signalling channels have been investigated. For the L-band, Slotted Aloha (SA) and Reservation Aloha (RA), combined with a token scheme, are discussed here. The results of Telesat studies to date indicate that SA, when combined with token scheme, provides the most efficient access and resource management tool in a mobile propagation environment. For SHF signalling channels, slim time division multiple access (TDMA) and SA have been considered as the most suitable candidate schemes. In view of the operational environment of the SHF links, provision of a very short channel access delay and a relatively high packet success rate are highly desirable. Studies carried out generally favor slim-TDMA as the most suitable approach for SHF signalling channels

Azarbar, Bahman

Razi, Michael

Shoamanesh, Alireza

English

NASA Technical Reports Server

N88-25734
L-BAND AND SHF MULTIPLE ACCESS SCHEMES FOR THE MSAT SYSTEM
MICHAEL RAZI, ALIREZA SHOAMANESH, BAHMAN AZARBAR
Telesat Canada, Canada
TELESAT CANADA
333 River Road
Ottawa, Ontario, Canada
KIL 8B9
ABSTRACT
The first generation of the Canadian Mobile
Satellite (MSAT) system, planned to be operational
in the early 1990s, will provide voice and data
services to land, aeronautical, and maritime mobile
terminals within the Canadian land mass and its
territorial waters. The system will be managed by a
centralized Demand Assignment Multiple Access (DAMA)
control system. Users will request a communication
channel by communicating with the DAMA Control
System (DCS) via the appropriate signalling channels.
Several access techniques for both L-band and
SHF signalling channels have been investigated. For
the L-band, Slotted Aloha (SA) and Reservation Aloha
(RA), combined with a token scheme, are discussed
here. The results of Telesat studies to date
indicate that SA, when combined with token scheme,
provides the most efficient access and resource
management tool in a mobile propagation environment.
For SHF signalling channels, slim-TDMA and SA
have been considered as the most suitable candidate
schemes. In view of the operational environment of
the SHF links, provision of a very short channel
access delay and a relatively high packet success
rate are highly desirable. Studies carried out
generally favour slim-TDMA as the most suitable
approach for SHF signalling channels.
1.0 INTRODUCTION
Communication Services within urban centres and surrounding areas
are readily available using terrestrial and fixed satellite systems.
Recently, the demand for reliable nationwide mobile communications in
rural areas has significantly increased. This paper briefly
introduces Telesat's baseline networking concept and control
architecture as foreseen for provision of Mobile Satellite (MSAT)
services. The emphasis is placed on the various multiple access
schemes contemplated for interconnecting the network elements as are
presently defined. These schemes are tailored to satisfy the
projected Canadian needs, while being in harmony with the present
state of the relevant technology.
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2.0 BACKGROUND
MSAT is intended to provide cost effective, spectrally efficient
services for voice and data communications across Canada. There will
be a large number of mobile users, each characterized by bursty
traffic and a low activity factor; therefore, the system's available
spectrum will best be utilized by a Demand Assignment Multiple Access
(DAMA) control system.
The DAMA Control System (DCS) will be responsible for network
supervision and providing interconnection among various network
elements such as mobile terminals, base stations, and gateways. It
will also be communicating with the network management system,
responsible for long-term resource planning, overall supervision and
maintenance of the system.
The major categories of service that have been identified to date
as potential applications for MSAT upon its introduction are:
a) Mobile Radio Service (MRS) targetting mobile communities of various
sizes consisting of one or more base stations (dispatch centres) and a
number of MRS terminals; b) Mobile Telephone Service (MTS) destined
for mobile users requiring Public Switched Telephone Network (PSTN)
interconnectivity, via gateway stations; c) MSAT Data Services (MDS)
for users with one-way or two-way data transmission requirements; and
d) MSAT Aeronautical and Marine Services.
To provide these services, two distinct types of channels are
required within the pool of MSAT channels:
- Communication channels used by all the users to transmit and
receive voice or data, and
- Signalling channels used by all the users as well as the DCS
to transmit and receive call requests/responses, commands, etc.
The signalling among the DCS, gateways, and base stations will be
over the SHF-SHF links (or in some cases they may interface using
terrestrial links). For mobile terminals, signalling will be via
L-band-SHF links to the DCS.
It should be noted that packet switched data communications would
be provided by data hubs using slim-TDMA channels. These channels
will be assigned dynamically to data hubs from the pool of MSAT
channels; however, circuit switched data communications would be
treated the same as the voice channels. The study of packet switched
data services will not be addressed here.
3.0 ACCESS SCHEMES
Different access techniques for both L-band and SHF signalling
channels have been investigated. The request and assignment channels
will form the major interface between the user and the MSAT DCS. As a
result, it will be critical that their operation satisfies the
criteria imposed by both the user and the system requirements. In
comparing various access schemes, the degree with which the following
design objectives are met by each approach should be carefully
quantified:
- Minimizing the number of signalling channels;
374
- Minimizing the call set-up/take-down delay;
- Ensuring stability of the channels; and
- Minimizing mobile terminal complexity.
Furthermore, there are two more important resource management
issues to be assessed before defining selection criteria.
First, it is highly desirable to assign a communication channel
when the called party goes off hook, as opposed to assigning a channel
upon a call request and waiting for the called party's response. Such
a deferred conununications channel assignment approach will result in
significant savings in the scarce satellite power and bandwidth.
Second, the size of the packets on the signalling channels has a
direct relationship with the number of signalling channels required to
support the %raffic generated by the MSAT users. A packet consists of
information bits, overhead bits, and guard time. Presently, a packet
size of 150 bits[l] appears to he sufficient for MSAT system.
3.1 L-Band SignallinK Channels
For the L-band signalling channels, Slotted Aloha
Reservation Aloha (RA), combined with a token scheme,
investigated and their performances are described here.
signalling channels are divided into three categories:
(SA) and
have been
The L-band
- Request channels will be used by mobile terminals to
communicate with the DCS, and will be in SA or RA mode.
- Token channels will be used by mobile terminals to transmit
on-hook, off-hook, and acknowledgments to messages received
from the DCS.
- Assignment channels will be used by the DCS to respond to
mobile requests and issue commands to mobiles.
The signalling channels will be operating at 2400 bps using
Differential Minimum Shift Keying (DMSK) modulation.
The channels in SA mode will be used solely for transmission of
call request, log-on, log-off, and ranging messages. The
log-on/log-off message transmissions are expected to occur during the
non-busy-hour period. The 2400 bps channel is divided into 16 slots
of 150-bits long for these messages.
The RA scheme is based on a random access scheme (SA) to reserve
a particular slot for transmitting a call request packet. The
Reservation Request Channel (RRC) is divided into two different types
of slots: First, large slots (150 bits) used for transmission of call
request, log-on, log-off, and ranging messages; second, mini-slots
(15 bits) used to transmit a four-bit token in SA mode to reserve one
of the large slots for transmitting call request and log-off
messages. Every large slot is followed by a group of ten mini-slots.
The downlink part of RRC is used to transmit the reservation
response messages from the DCS to the mobiles.
For the token scheme, the 2400 bps channel is also divided into
15-bit slots. During the call set-up procedure (after reception of
call request) the DCS will designate one slot to the L-band
called-party. This slot will be used by the mobile to transmit
four-bit tokens signifying on-hook, off-hook, and acknowledgement to
messages received from the DCS.
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3.2 Link Availability of L-Band Sisnallin_ Channels
The performance of signalling channels will be significantly
dependant on the propagation effects, which in turn depends on the
elevation angle to the satellite as well as the amount and type of
blockage and shadowing predominate in the service area. The baseline
system design assumes that mobiles would be limited to a service area
defined by a minimum elevation angle of 20 ° to the satellite. It is
to be noted that a very large proportion of the potential mobile users
in Canada fall within this limit. Application of measured fade
statistics for areas in which woodlands constitute up to 35% of the
land area[2] indicated that a 99% availability for the signalling
channels is attainable. The probability of packet loss on forward and
reverse links for mobiles is calculated to be limited to 11% and
11.6%, respectively. The probability of token misdetection on the
reverse link is expected to be 0.3%[1].
3.3 Comparison of Access Schemes
For brevity, the channel performance and delay analysis for both
access schemes would not be presented here in detail. Interested
readers may refer to Reference i for a detailed performance analysis
of these schemes. Figures I and 2 show channel performance versus
delay curves for both SA and RA schemes combined with a token scheme.
In Figure I, the percentage of packets transmitted successfully after
m transmissions on the request channel is plotted against the call
set-up delay. Note that call set-up delay accounts for the time from
transmission of first indication of call request from mobile until
successful reception of ringback message (acknowledgement) by the
mobile from the DCS. Table I further compares the performance of the
two schemes:
Table 1 Comparison of SA and RA
Slotted aloha Reservation aloha
Success rate Delay (sec) Success rate Delay (sec)
First try 69.3 0.92 76.3 1.96
Second try 17.6 1.99 1.0 2.72
Third try 7.5 3.06 14.5 3.97
Fourth try 3.25 4.13 5.4 5.89
Total 98.95% 97.2%
In Figure 2, the channel throughput (in terms of number of
packets) is plotted against the average delay on the channel. As
shown, the delay experienced by RA is usually twice as much as that of
SA, since the process involves transmission of two messages from the
mobile to the DCS (one token, one message).
It is interesting to note that increasing the number of
signalling channels in SA scheme improves the performance of the
network, while its effect on RA is insignificant. SA is more
adaptable to environments with various propagation characteristics,
376
while RA is more susceptible to high packet error rates on the link.
In addition, RA uses one more L-band downlink channel than SA for
transmitting the reservation response messages. One of the advantages
of RA scheme is its capability of handling up to seven calls/sec/
channel compared to SA handling only four calls/sec/channel, ensuring
channel stability as shown in Figure 2.
The above study clearly shows that, under the assumptions used,
SA outperforms the RA access scheme.
3.4 SHF SiKnallinK Channels
............................... _vp=_ LUL the first 8enecation
MSAT indicates that the Canadian mobile satellite network will consist
of about 150 base stations and four gateways, each generating varying
amounts of SHF traffic[3]. It is assumed that every station would
transmit a message every 2.3 seconds[3]. With the main objective to
minimize the channel access delay, the following access schemes have
been considered:
- Random access, and
- Fixed assignment.
The only candidate considered for random access is SA due to its
higher performance compared to pure Aloha and its faster response with
respect to RA. Typically, the channel throughput for an SA channel
would be approximately 15% for a reasonable degree of packet
collisions and packet delay. As a result, the number of required
channels would be 30 channels. One of the major drawbacks of this
access scheme is that collision of messages such as off-hook, on-hook,
and acknowledgments could result in long call processing delays and
chaos in signalling channels.
For fixed assignments, an FDMA system with dedicated channels, or
a TDMA system with permanently assigned time slots, have been
investigated. Due to the requirement for transmitting on average one
message every 2.3 seconds, an FDMA system would not be practical.
Even though there would be no channel access delay for this scheme,
the channel throughput would be very low (in the order of 1/(16 x 2.3)
= 2.7% assuming a 2400 bps data rate and 150-bit packets).
In the case of a TDMA system with 2.3 second long frames, and
every station being assigned to a particular time slot in a channel,
the channel utilization will be very high, requiring only five
signalling channels. However, in such a system, the average channel
access delay will be excessively high (about 1.15 seconds long). For
the purpose of analysis, an average channel access time of 300 ms is
assumed to be reasonable. To accommodate such channel access delays,
the frame size could be reduced to 600 ms resulting in approximately
sixteen signalling channels. Note that channel utilization will be
24.5% assuming a 2400 bps data rate. The channel efficiency would be
increased to 26% if the transmission rate is increased to 4800 bps.
The number of required channels in this case will be half of the
previous case, so the SHF spectrum requirement will remain the same.
The assignment channels are similar to broadcast channels. The
DCS would be the only transmitter on these channels. The SHF stations
will monitor them continuously while they are logged on to the system.
377
Telesat's studies indicate that the TDMA access scheme with a 600
ms frame size can satisfy the Canadian system and user requirements.
The channel performance will be the highest and the average channel
access time will be 300 ms. It should be pointed out that the data
transmission rate would not influence the signalling channels'
performance significantly; however, the packet transmission time and
packet processing delay will be reduced.
Note that base stations and gateway stations will share the
signalling channels. A dynamic signalling channel assignment scheme
should be implemented to provide flexibility in the system operation.
The most important phenomenon affecting the SHF link perfoz_ances
is the attenuation of the signal by rain in the earth-to-space paths.
Telesat's studies indicate that a one-way propagation availability
better than 99.99% of time can be achieved for most Canadian users.
This availability could be further improved if uplink power control is
used at the central control station site.
4.0 CALL SET-UP PROCEDURE
Figure 3 provides a time diagram describing how the various
access schemes would be used in setting up/taking down a call. As
shown, a base station initiates a call by transmitting a call request
to the DCS. The mobile party would be informed via L-band assignment
channels. The ringing message would be acknowledged by an Ack token
on the token channel. A second token would indicate an off-hook. At
this point in time, a voice channel would be assigned to both parties.
For call take-down, one of the parties goes on-hook which causes
transmission of a tone over the voice channel to the other party. The
terminal then informs the DCS. In the case of mobile users, a token
would be sent over the a token channel. The DCS sends call
termination messages to both parties and reclaims the voice channel.
5.0 CONCLUSION
Candidate multiple access schemes for L-band and SHF signalling
channels in the Canadian MSAT system have been described here. It was
shown that the most appropriate channel access scheme for the mobile
terminals would be SA combined with a token scheme using L-band
channels. Furthermore, slim-TDMA with short frames was shown to be
the superior approach for the SHF signalling channels.
REFERENCES
[I] "MSAT DAMA System Study", Telesat Canada, prepared for Department
of Communications, Ottawa, Canada, July 1986.
[2] Butterworth, J. "Propagation Measurements for Land-Mobile
Satellite Systems at 1542 MHz", Department of Communications
Report, Ottawa, Canada, August 1984.
[3] "MSAT System and Service Definition", Telesat Canada, prepared
for Department of Communications, Ottawa, Canada, October 1987.
378
65
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Fi_. 2. Total capacity of slotted
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BASE
STATION
CALL RQ.
VOICE CHN.
-- -Ec_ ....
SHF L-BAND
)CS MOBILE
ACK -, _. _ •
OFF- HOOK
CONVE
ON-HOOK OVER
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Fi_ 3. Call set-up/take--
down procedure for
an MRS call
379


L-band and SHF multiple access schemes for the MSAT system

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