CA 02325612 2000-11-09
IMPROVEMENTS 1N AND RELATING TO DATA TRANSMISSION
Field of the Invention
The present invention relates to data transmission systems, and in particular
to
mobile telephone networks employing such systems, and to base stations and
satellite
stations for use in such systems.
Background of the Invention
In a mobile telephone network, there are a plurality of base stations serving
different
I O areas. Each base station is arranged to communicate with a plurality of
mobile telephones
in an associated area.
The base station communicates with the mobile telephones using a discrete
number
of channel frequencies. The mobile telephones are typically arranged in groups
of eight
15 with the base station sending signals to all the members of the group along
a common
downlink channel having one unique frequency allocated to that group. The base
station
receives signals from each of the mobile telephones in the group along an
uplink channel
having a different unique frequency allocated to that group.
20 In operation, the base station sends data to all the mobile telephones of
each group
in a succession of data packets along the common downlink channel. Each packet
of data
consists of two parts. A first part called a header which contains a code ID
identifying a
selected mobile of the group and an uplink state flag (USF;1 for that mobile
to trigger that
mobile to return a packet of data to the base station and a second part
containing a payload
25 of data that the mobile telephone will download and use as required. The
header is -
"transparent", that is it can be read by all the telephones in the group. The
payload is
cyphered a;~d can only be read and downloaded by the telephone whose ID is
contained in
the header. The packets are typically of 20ms in duration and, when a
particular telephone
downloads a packet containing its uplink state flag, that telephone will reply
very shortly
30 after receiving the packet by sending a packet of data back to the base
station along the
common uplink channel.
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If each mobile telephone only receives one uplink state flag (ie is polled)
during
each cycle of eight successive packets of data transmitted by the base
station, it can only
send data to the base station during one 20ms period in every eight.
It is an object of the invention to provide an improved polling rate.
Summary of the Invention
According to one aspect of this invention there is provided a data
transmission
system comprising a base station having a radio transmitter and a receiver and
a plurality of
satellite stations each having a radio transmitter and a receiver, the
transmitter of the base
station and the receivers of the satellite stations all being tuned to the
same downlink
channel, and the transmitters of the satellites and the receiver of the base
station all being
tuned to a common uplink channel and at least one auxiliary uplink channel,
the base
station including control means for generating a succession of data packets
and transmitting
them to the satellites, each data packet comprising a header section and a
payload section,
the header section basis of variable configuration and length and containing a
first uplink
state flag identifying with a first selected satellite which when received by
the first selected
satellite causes that satellite alone to send a data packet back to the base
station along the
uplink channel, and at least one second uplink state flag identifying a second
selected
satellite coupled with the identity of and time slot for, a said auxiliary
uplink channel to
cause the said second selected satellite alone to send a data packet back to
the base station
along said auxiliary uplink channel in said time slot, whereby to effect mufti-
polling of said
satellites using a single packet of data.
According to another aspect of this invention there is provided a base station
having a radio transmitter and a receiver for communication with a plurality
of satellite
stations, the transmitter being tuned to a predetermined downlink channel, and
the receiver
being tuned to a predetermined uplink channel and at least one auxiliary
uplink channel, the
base station including control means for generating a succession of data
packets and
transmitting them to the satellite stations each data packet comprising a
header section and
a payload section, the header section being of variable configuration and
length and
containing a first uplink state flag identifying with a first selected
satellite station and at
least one second uplink state flag identifying a second selected satellite
station coupled with
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.,
the identity of and time slot for, a said auxiliary uplink channel whereby to
effect multi-
polling of said satellite station using a single packet of data.
According to a further aspect of this invention there is provided a satellite
station
with a unique identity having a radio transmitter and a receiver, the receiver
being tuned to
a predetermined downlink channel, and the transmitter being tuned to a
predetermined
uplink channel and at least one auxiliary uplink channel, the satellite
station including
control means for receiving from a base station a succession of data packets,
each data
packet comprising a header section and a payload section, the header section
being of
variable configuration and length and containing a first uplink state flag
carrying the
identity of a first selected satellite station which if that identity
corresponds with the said
unique identity causes the control means to send a data packet back to the
base station
along the predetermined uplink channel, and at least one second uplink state
flag carrying
the identity of a second selected satellite station coupled with the identity
of and time slot
for, a saic: auxiliary uplink channel which if that identity corresponds with
said unique
identity causes the control means to send a data packet back to the base
station along said
auxiliary uplink channel in said time slot, whereby said satellite station is
mufti-polled
using a single packet of data.
Brief Description of the Drawings
A mobile telephone network embodying the present invention, will now be
described, by way of example, with reference to the accompanying drawings, in
which:
Figure 1 is a block diagram of a base station communicating with a plurality
of
mobile telephones; and
Figure 2 is a diagram illustrating the composition of a data packet.
Description of the Preferred Embodiment -
As shown in Figure l, a base station 2 includes a radio transmitter 4 and
receiver 6
for transriitting data simultaneously to a group of eight satellites or mobile
telephones M1
to M8 along a downlink channel of frequency f2 and for receiving data
principally along an
uplink channel of frequency f~.
The signals generated by a control unit 8 and transmitted by the base station
2
consist of a series of packets of a data each of 20ms duration.
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As shown in Figure 2, each data packet includes a header H and a payload P.
The
header includes a mode slot 10 to be described in more detail hereinafter, a
primary uplink
state flag slot 12 containing, for example, the uplink state flag USF2 for
mobile M2 and a
secondary uplink state flag slot 14 containing, for example, the uplink state
flag USF3 for
the mobile M3. The secondary uplink state flag slot is followed by a time slot
16 and a
frequency slot 18 defining, for example, a time slot T3 in an auxiliary uplink
channel of
frequency f3. The payload P then follows.
In operation, when the mobile telephone M2 receives the packet, it responds to
the
uplink state flag USF2 to transmit a packet of data along the default uplink
channel of
frequency fi. When the mobile telephone M3 receives the same packet of data,
it responds
to the uplink state flag USF3 to transmit a packet of data in a specific time
slot TS3 along
the auxiliary uplink channel of frequency f3.
In this way, the base station can receive two data packets from two mobile
telephones in response to the same packet of data sent along the downlink
channel. To
achieve this situation, the duration of the header H will have been increased
at the expense
of the duration of the payload P since, for a given system, standards dictate
that the packets
should all be of the same duration.
While the auxiliary uplink channel has been defined as having a frequency of
f3, it
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will be appreciated, that the base station may have a number of other channels
of different
frequency available for use as an auxiliary uplink channel. The base station
may therefore
select different auxiliary uplink channels on different occasions according to
availability.
Also, while each packet is shown as containing a secondary uplink state flag,
it may
include a tertiary uplink state flag or more each accompanied by its own time
and frequency
slots. However, such additional uplink state flags will reduce the duration of
the payload
and produce progressively diminishing returns.
The above described polling method will be referred to hereinafter as mufti-
polling.
The advantage that pure polling has over contention based access for real time
(RT)
traffic channels lies in the determinism of the access delay. In polling the
access delay is
deterministic whereas in contention based access the access delay is
stochastic and
therefore more damaging to RT traffic. However, as system load increases the
polling
algorithm has to find channel space in order to accommodate positive replies
from the
polled mobiles. Whereas in contention based access there is a known common
pool of
available channels that the mobiles contend for.
Contention based:
As the load increases an increasing number of mobiles contend for a
diminishing
number of spare (unused/multiplexed) channels. This means that the probability
of failure
to acquire a channel will increase as the load increases resulting in dropped
calls - gaps in
speech. The onus on obtaining the channel is down to the mobile, ie the mobile
being
successful. '
Polling based:
As the load increases the problem is not one of contending for spare time
slots but
finding them rapidly enough in order to give the mobile a channel. The effect
of not
finding a channel would be the same as that encountered in the contention
based case. The
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onus on obtaining the channel is down to the base station controller (BSC) ie
the BSC being
successful.
In order to cope with the transitional behaviour required by the polling
algorithm to
cope with increasing load we employ fuzzy techniques that utilise the
following general
rules
If load(t) is low then
Use(Standard GPRS Polling);
Else if load(t) is medium then
Use(Multi Polling);
Else if load(t) is high then
Use(Probabilistic Multi Polling);
End If
The mode slot 10 in the header is used to identify to the mobiles being polled
which, of the
option is being employed by the base station.
When adopting standard GPRS (General Packet Radio Service) Polling - the
standard GPRS MAC (General Packet Radio Service Medium Access Control) header
is
used to poll mobiles, up to two mobiles camped on each time slot.
When adopting mufti polling - the standard GPRS MAC header does not render
sufficient flexibility in order to obtain the multiplexing gains. Thus, mufti
polling permits
more than one mobile to be polled in each time slot with a dedicated reply
channels being
signalled in the MAC header.
Probabilistic mufti polling is adopted when approaching the load limit of the
cell.
In this technique we employ two statistical methods in order for the replying
mobile to gain
access in a fair manner to the remaining channels.
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The statistical nature of speech ie its exponential decay gives the
probability
(P) that a channel is active at time t is given by p~t~ = e-a~ and the
probability
(Q) of remaining on at time t is given by ~(tl = ~ _ ~ -a~ . Where B is
2. Contention is then used on the reply channels, ie the mobiles have look for
space (time slots) in different frequencies reply channels and then contend
with other mobiles for it.
In the first case the elapsed time that a call has been in an active phase in
order to
select a channel for reply. In this way the channel for reply under high load
conditions is
taken from those channels that have been in their active phase the longest and
therefore the
most like.y to become inactive the soonest. In the second case the mobiles
reply time slots
are grouped to allow the mobiles to contend for a reply channel selected in
the same
manner described above. In this manner it is thought that the polling will
degrade fairly as
load increases and spare channel become scarce.
In the second case some determinism is being sacrificed for flexibility.
However
the determinism is not removed entirely since the channels for reply are
provided it is just
the manner in which the channels are filled is now different. Rather than
providing a
channel solely for a given mobile to reply on, the system is providing a group
of channels
any one of which the mobile could reply on.
In the GPRS/EDGE (General Packet Radio Service/Enhanced Data Rates for
GPRS) standard the Packet Control Acknowledgement is sent in response to a
Radio
Resource (RR) message. RR messages are signalled to the mobile by setting the
payload
Type field in the MAC header to ~ontrol~ In the MAC header the RRBP (Relative
Reserve
Block Period) field is used to indicate to the mobile when it is expected to
reply to the
system with a Packet Control Acknowledgement signal. The Supplementary Polling
(SP)
bit indicates the validity of this field.
When GPRS/EDGE mobiles request a voice service they are placed into a polling
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group, if in their silence phase, or given a channel, if in their active
phase. The RR
messages that such mobiles would be interested in under these circumstances
are
reassignments of their channels when active. The design of the Multi-polling
header allows
for this in the reply fields (see Reply Encoding section above). For this
reason the RRBP
and SP fields can be removed from the Multi-polling header when more than one
mobile is
polled in a given time slot.
The CSN 1 (Concrete Syntax Notation 1 ) coding for the mufti-polling header is
given below. The header length depends on the value of the first two bits in
the header
since their value selects the subsequent MAC structure. The definitions of the
structures
are also given below.
For the Single Mobile structure the MAC header is the same as it would be for
a
standard GPRS MAC header in so far as it includes the SP, RRBP and Payload
Type fields.
For subsequent structures only the Payload Type is retained from the standard
header. i he reasoning for the removal of the Relative Reserved Block Period
(RRPB) and
Supplementary Polling (SP) bits has been given above. However, the Payload
Type is
necessary to be included since this signals an automatic response to Radio
Resource (RR)
control messaging.
The length of the polling code depends on the number of mobiles being polled.
The
explanation for this has been given in the Polling Code section above. The
length of the
reply field again depends on the number of mobile being polled. The
explanation of the
construction of the reply field is given in the Reply Encoding section.
The possible codes occupying the mode slot 10 in the header is as follows:
<00 : Single Mobile struct>
<O1 : Two Mobile struct>
<10 : Three Mobile struct>
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<11: Four Mobile struct>;
The codes occupying the slot 12 are as follows:
<Single mobile structure>
<POLLING CODE: bit(3)>
<S/P : bit( 1 )>
<RRPB : bit(2)>
<PAYLOAD TYPE : bit(2)>
<Spare Bits>;
Depending upon whether two, three or four mobiles are being polled, the slot
14 is
occupied by one of the following three options:
<Two Mobile struct>::=
<POLLING CODE: bit(5)>
<PAYLOAD TYPE : bit(2)>
{<Reply Field: Reply Field struct>*2}
<Spare Bits;
<Three Mobile struct>::=
<POLLING CODE: bit(6)>
<PAYLOAD TYPE : bit(2)>
{ <Reply Field: Reply Field struct>* 3 }
<Spare Bits>;
<Four Mobile struct>::=
<POLLING CODE: bit(7)>
<PAYLOAD TYPE : bit(2)>
{<Reply Field: Reply Field struct>*4}
<Spare Bits>;
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Finally, when using the two, three or four options, the slots 16 and 18 in the
header are
occupied by:
<Reply Field struct>::=
<USF NEW: bit(3)>
<REPLY TS:bit(3)>
<REPLY FN::bit(2)>;
