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Description  |
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TECHNICAL FIELD
This invention relates to a method of and a system for communicating data,
in particular communicating data across a radio based communication
system.
DISCLOSURE OF THE INVENTION
The present invention consists in a method of communicating data over a
radio based communication system between a base station and one or more
remote stations wherein the data is transmitted between the base station
and each remote station in a down link comprising one or more down
time-frames of fixed length, and data is transmitted between each remote
station and the base station in an up link comprising one or more up
time-frames of fixed length, each down frame comprising a fixed number of
down slots at least one of which is a general control slot, at least one
other is a down-setup slot, at least one other is a down transfer slot,
and at least one other is a down acknowledgement slot, control data in the
general control slot serving to identify to each remote station the
down-setup slot in which the base station is to announce the transmission
of data for it and identify the down-transfer slot or slots in which said
data is to be transmitted, and each up frame comprising a fixed number of
up sloes at least one of which is identified by control data in the
general control slot as an up-setup slot, at least one other is an up
transfer slot, and at least one other is an up acknowledgement slot, the
up-setup slot being divided into a number of sub-slots in which each
remote station can transmit a request to transmit data to the base
station, and the base station serving to respond to such a request in a
down acknowledgement slot by identifying the up transfer slot or slots in
which the remote station is to transmit said data.
The communication of data on the down link and up link involves two phases:
a setup phase and a data transfer phase, with a separate slot or group of
slots, i.e. data channels, being allocated for each phase. The advantage
of this technique is that the data transfer channel need not be loaded
with data unless the setup phase is successful, and the setup messages can
be very short and hence use very little bandwidth. This is especially
advantageous for transmissions on the up link, since the remote stations,
which may be a mobile fleet of users, are necessarily uncoordinated and
will therefore contend for channel capacity. Contention channels, whether
operating in Aloha- or CSMA- type modes, have to be operated at low
utilisations to avoid instability. By ensuring that contentions take place
only between short setup messages, even though the contention time-slots
have to be operated at low utilisation, this has little impact on the
overall efficiency of channel usage. The base station responds to a
successful up-setup request by allocating capacity on a data transfer
channel, which thereby may be operated in a "scheduled" manner at high
utilisation. Similarly, as packets arrive at a base station to be
transmitted to a particular remote station, the base station first
executes a down-setup to tell the remote station to monitor the down data
transfer channel, then schedules the data itself into the down data
transfer channel.
Preferably, all data transmitted in the down transfer slot is labelled with
a mobile group label by which it is identified by the remote station or
stations to which it is addressed.
The down-setup slot contains data which identifies the mobile or group of
mobiles to which a message is to be sent, i.e. the mobile group label
allocated to that message. The mobile or mobiles then identify that
message simply by reference to the mobile group label as attached to data
in one or more down transfer slots until the complete message has been
received.
In an alternative embodiment of the invention, however, the down-setup
phase may be incorporated in a down acknowledgement slot which the base
station transmits in response to a message from a mobile so that the
mobile is more rapidly setup to receive a message or reply, thereby
shortening the response time. Furthermore, where the reply is so long that
it is divided into a number of separate part messages, each part message
is adapted so as to include the relevant down setup data for the next part
message, again avoiding the need for a separate down setup phase and
thereby shortening the overall response time.
According to a further feature of the invention, any transmission from a
mobile station in response to a message received from the base station may
be delayed in time by at least a minimum number of time-slots; and further
a transmission from the base station in response to a message received
from a mobile may be delayed by at least a minimum number of slots. By
this means the radio subsystem of the mobile need not be capable of
changing from transmit mode to receive mode very quickly; and the time
available for processing signals and protocol messages in the mobile and
base station can be maximised.
According to a further feature of the invention, a mobile which does not
have data to send during a given period of time, need only activate its
receiving and decoding circuits for at least the one down-setup slot in
each frame in which the base station announces the transmission of
messages for the mobiles, and by this means the power consumed in the
mobile may be minimised.
Further reductions in power consumption by a factor of "n" may be obtained
by the mobile only activating its receive and decoding circuits for
down-setup slot of every "n'th" frame, provided that the base station is
aware that this action is being followed and sends transmission
announcements for such mobiles only in the appropriate frames.
A further feature of the invention allows the base station to announce
changes to the use being made of slots in the up and down frames using the
general control slots of the down frame, messages announcing such changes
not requiring acknowledgement of successful reception by the mobiles. For
example, the number of up-setup slots divided into sub-slots for mobiles
to request the setting-up of a data transfer channel, may be varied
depending on the degree of load on the system, this change being announced
via the general control slots of the down frame. The number of up-setup
slots divided into subslots may be varied in accordance with the number of
subslots in which collisions between mobile requests take place, and
information contained in the request messages themselves as to the number
of unsuccessful attempts prior to success.
The use of a slotted ALOHA type system allows the use of low-cost,
non-duplex radios.
As with conventional radio based communication systems operating with a
slotted ALOHA type system, in order to ensure efficient operation of the
system it is important that the base station and the mobile are in
alignment with each other. This is achieved using a synchronisation signal
which is regularly transmitted by the base station and which allows the
mobile to know exactly where it is in a frame.
Preferably, the base station supports radio communication with mobiles on
at least one or a multiplicity of duplex information bearers, each
comprising a down link and an up link with a frame structure as described
earlier, where the overall structure is announced and controlled using the
general control slot on one bearer designated a master bearer, all other
bearers being slave bearers; and the synchronising information transmitted
in the down slots of a master bearer carry information allowing a mobile
to rapidly recognise the master bearer, and the synchronising information
of slave bearers carry information allowing a mobile to rapidly retune to
a master bearer in order to receive information on the overall use being
made of down and up slots on all bearers.
DESCRIPTION OF THE DRAWINGS
The invention will now be described by way of example with reference to the
accompanying drawings, in which:
FIG. 1 shows a schematic representation of a communication system for
utilising the present invention; and
FIGS. 2 to 22 show representations of the frames used in the method of
transmitting data in accordance with the present invention, in particular,
FIG. 2 shows the frame formats,
FIG. 3 shows frame related information (down only),
FIG. 4 shows the protocol layer 2 structure (up and down),
FIG. 5 shows the protocol layer 2 structure (up),
FIG. 6 shows details of the upslot timing,
FIGS. 7a and 7b show arrangements of differential encoding,
FIG. 8 shows slot data encoding,
FIGS. 9a and 9b show the encoding of [1/1] and [1/4] slots,
FIG. 10 shows the interleaving process for [1/1] slot,
FIG. 11 shows the base/mobile data flows,
FIG. 12 shows the radio system implementation model,
FIG. 13 shows the down packet set up,
FIG. 14 shows the down packet transfer,
FIG. 15 shows the protocol layer 3 down link slot contents,
FIG. 16a to 16b shows the protocol layer 3 up slot contents,
FIG. 17 shows the down link packet sementation,
FIG. 18 shows the up link packet assembly,
FIG. 19 shows the message segmentation,
FIG. 20 shows a user/host interaction pattern,
FIG. 21 shows the segmentation of an application message in the system of
FIGS. 1 to 20, and
FIG. 22 shows the segmentation of an application message in an alternative
body of the invention.
MODE OF CARRYING OUT THE INVENTION
FIG. 1 illustrates a radio based communication system comprising a fixed
network of radio base stations or sites 1 and one or more mobile stations
2. Each base station has a radio port controller (RPC) associated with it
that predetermines the number of radio frequency bearers that it employs.
Communication bearers are formed by pairs of r.f.channels one for
transmission of data from the base station 1 to mobile station 2
(downlink) and the other of which is for transmission of data from the
mobile station 2 to the base station 1 (uplink). Each channel is therefore
simplex.
The communication system operates according to a protocol which is designed
using a layered approach. Layer 1 defines the basic radio parameters and
is not described in any detail herein. In one embodiment of the system, it
permits the transmission of data at a rate of 6144 bits per second. Layer
2 defines a frame and slot structure that allows the bearers to carry time
multiplexed data between the base stations and the mobile stations, and
also defines a Forward Error Correction (FEC) scheme. Layer 3 defines the
allocation of slots into a selection of data channels. Data is sent as a
connected series of one or more slots called Transmission Sets (TS)
between Radio Multiplexing/Demultiplexing (RMD) means at each end. The RMD
means is responsible for any retransmission of lost slots.
Layer 2 is described in the following sections 1.1 to 1.4. Layer 3 is
described in terms of a network overview in the following sections 2.1 to
2.10, and in terms of control in the following sections 3.1 to 3.11.
LAYER 2--Data Link Layer
1.1. Frame Format
Each of the bearers transmits data in one or more frames which contains
data as a series of time multiplexed slots, all of which slots are of the
same length, and are grouped into predefined frames, as shown in FIG. 2.
The frame grouping is defined indirectly by the slot trailers, on all
downlinks. Each downslot trailer (DST) comprises three bits, sync a, sync
b and sync c, as shown in FIG. 4 and provides synchronisation for the
individual downslots. A particular sequence of downslot trailers defines
the frame. The sequence of downslot trailers also provides specific bearer
information detailed below.
Uplinks also contain a series of slots that may be transmitted by different
mobiles. Each of these mobiles is required to align its upslot
transmission to the downslot timing. A simpler upslot trailer (UST) is
used on all upslots comprising a single bit sync and a gap, as shown in
FIG. 4. This upslot trailer only provides slot synchronisation, and
carries no frame information.
The number of slots in a frame (the frame format) may lie in the following
range:
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Minimum Maximum
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Slots/frame
14 26 slots even numbers only
Bits/slot 768 768 bits
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All bearers from any one base station or site 1 are required to have the
same frame format, with their frames aligned to within 1 bit in order to
allow mobile stations 2 to switch between slots on different bearers yet
maintain synchronisation.
Frame alignment between different base sites is optional but desirable.
In principle, the protocol allows for different frame formats at different
base sites.
FRAME RELATED INFORMATION (DOWNLINK ONLY)
Frame related information is encoded into all the downlink slot trailers
(DST) of a frame, as shown in FIG. 3. Each Radio Port Controller (RPC)
transmits one master bearer and up to 3 slave bearers. It is important
that mobiles can identify slot 0 of the master bearer as quickly as
possible, and this is therefore the major component of frame related
information. However, the complete frame related information uses all the
encoded bits from every slot of a frame. This information contains:
identification of master or slave bearer
identification of slot 0 of a master or slave bearer
a dotting pattern for synchronisation
definition of the frequency offset from a slave to its master.
All the information bits are encoded in the downslot trailers using the
synchronisation codewords defined below. Each trailer contains 3
concatenated synchronisation codewords, and each of these codewords
encodes one bit of information N=NSYNC as I=ISYNC. The complete frame
related information can either be viewed as 3 encoded bits per slot, or as
three parallel sequences (seq-A, seq-B, seq-C) that repeat once per frame
as shown in FIG. 3. FIG. 3 shows the sequences for the minimum frame
format of 14 slots and for 4 extra slots for an extended frame format.
Specifically, the frame related information is defined by the following
combinations of the encoded bits. Here, the encoded bits are referred to
by their sequence letter:
Master/Slave
Master and slave bearers are clearly distinguished in every slot trailer by
the combination of seq-A and seq-B. The encoded bits of seq-A and seq-B in
each slot of a master have the same polarity, whereas the encoded bits of
seq-A and seq-B in each slot (except slot 0) of a slave have opposite
polarity.
Slot 0
The location of Slot 0 can be immediately identified by the combination of
seq-A, seq-B and seq-C:
An [NSYNC] in all three encoded bits identifies slot 0 on a master bearer
An [ISYNC] in all three encoded bits identifies slot 0 on a slave bearer.
Slot 0 location is used to provide frame synchronisation. It can also be
used as a confirmation of the frame length, since the frame length, given
by the distance between two successive slots 0's, must equal the
predefined value.
Frame Dotting
Frame dotting is introduced to reduce the risk of false frame sync
acquisition. Seq-B always contains a dotting sequence:
[ . . . ,NSYNC,ISYNC,NSYNC,ISYNC, . . . ]
This dotting pattern is a mandatory aspect of successful slot and frame
acquisition.
Frequency Offset
The frequency offset differs from the other information, because it
requires one encoded bit from several trailers (slot 1 to slot 12
inclusive). These 12 encoded bits are shown as F in seq-C of a slave
bearer in FIG. 3. Together these encoded bits define the offset from that
slave bearer to the master bearer (of the same RPC) as a 12 bit 2's
complement value. The most significant bit is at slot 1, the least
significant at slot 12.
1.2 Slot Formats
The full slot length is designed for optimum segmentation of user messages
up to 256 octets long. The protocol also provides an option of subdividing
one or more upslots into [1/4] subslots, as shown in FIG. 5. These
subslots are used on the uplink only to allow for very short contention
subslots.
The slots and subslots supply the following range of data capacity to layer
3 of the protocol:
[1/1] slot 510 bits (10 forward error correction FEC blocks of 51 bits) of
layer 3 data
[1/4] slot 102 bits (2 forward error correction FEC blocks of 51 bits) of
layer 3 data.
Slot Summary
The slot formats are designed to use an exact number of forward error
correction (FEC) blocks in all the slot formats. The contents of each slot
are defined in the following components as shown in FIGS. 4 and 5:
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DOWNLINK UPLINK
______________________________________
Encoded slot data (ESD)
Padding
Down slot trailer Encoded slot data (ESD)
Up slot trailer
Up slot Gap
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Padding
A minimum of 2 bits of padding is provided on all uplink slots, to provide
initialisation for the differential decoding if required. No padding is
added to downslots, since a mobile receiver can initialise its decoding on
the trailer of the previous slot. Padding is placed at the start of each
upslot or upsubslot, and the padding bits are filled with a dotting
pattern. The dotting pattern used is:
[01] on upslots
Transmission of this padding is mandatory. The protocol also allows for
optional transmission of further padding by mobiles.
Optional Uplink Padding
Mobiles are permitted to transmit additional padding bits at three points:
during the carrier attack time.
during the carrier release time.
in the gap between slots, when the mobile is transmitting successive slots.
If transmitted, this dotting must be fully synchronous to the encoded slot
data. Optional padding can only be added in steps of 2 bits (i.e. an odd
number of padding bits is not allowed).
No other data pattern is allowed if this optional dotting is not
transmitted.
Down Slot Trailer
A down slot trailer is transmitted at each slot position. The down slot
trailer DST uses three synchronisation codewords that together provide a
synchronisation sequence. Each 16 bit codeword can be used normal or
inverted, to encode 1 bit of information. Each codeword is used to encode
one bit of one of the frame sequences as follows:
Codeword 2:1=NSYNC:0=ISYNC:(one bit of seq-C)
Codeword 1:1=NSYNC:0=ISYNC:(one bit of seq-B)
Codeword 0:1=NSYNC:0=ISYNC:(one bit of seq-A)
The encoding uses the following complementary sync words:
NSYNC (Normal sync): [1100 0100 1101 0111]
ISYNC (Inverted sync): [0011 1011 0010 1000]
All mobile receivers are expected to be able to ocate slots to an accuracy
of +/-10 bits by simple timing from a previous slot where frame sync had
been acquired.
Up Slot Trailer
An up slot trailer UST is also required at each slot and subslot position.
This contains the single up slot synchronisation codeword:
Codeword 0:USYNC
USYNC (Up sync): [0011 1001 01]
All mobiles are assumed to be able to transmit slots to an accuracy of +/-1
bit by timing (using the down slot trailer to provide slot and frame
sync). The up slot trailer UST is designed to provide synchronisation to
cover three sources of error:
the mobile timing error of +/-1 bit (just described)
a base timing error of +/-1 bit
0 to 2 bits of propagation delay
This gives a total of 6 bits of timing and propagation errors. The overall
synchronisation requirement is therefore +/-3 bits.
Up Slot Gap
The up slot gap is used to allow for time division multiplexing and timing
errors on the uplink. This gap allows for the following elements:
30 bits to provide carrier attack/release time (of approximately 5 msec at
6.144 kbps)
6 bits to allow for timing and propagation errors.
The timing diagram for upslots (relative to downslots) is shown in FIG. 6.
The timing shown is all referred to the base antenna, but the timing
referenced to each mobile antenna will differ depending on its distance
from the base (i.e. the propagation delay).
All mobiles behave as though their antenna is adjacent to the b | | |
